A Review on Liquid Pulsed Laser Propulsion

Abstract

Laser propulsion is a new conceptual technology that drives spacecraft and possesses advantages such as high specific impulse, large payload ratio, and low launch cost. It has potential applications in diverse areas, such as space debris mitigation and removal, microsatellite attitude control, and orbital maneuvering. Liquid pulse laser propulsion has notable advantages among the various laser propulsion systems. We review the concept and the theory of liquid laser propulsion. Then, we categorize the current state of research based on three types of propellants—non-energetic liquids, energetic liquids, and liquid metals—and provide an analysis of the propulsion characteristics arising from the laser ablation of liquids such as water, glycidyl azide polymer (GAP), hydroxylammonium nitrate (HAN), and ammonium dinitramide (ADN). We also discuss future research directions and challenges of pulsed liquid laser propulsion. Although experiments have yielded encouraging outcomes due to the distinctive properties of liquid propellants, continued investigation is essential to ensure that this technology performs reliably in actual aerospace applications. Consistent results under both spatial and ground conditions remain a key research content for fully realizing its potential.

1. Introduction

In 1972, American scholar A. Kantrowitz proposed the concept of laser propulsion based on the ablative mode [1]. He explained that a high-power laser focused on a target surface causes its material to melt, vaporize, and ionize, forming a high-speed reverse plasma plume. This process generates thrust opposite to the jet direction and produces a specific impulse far exceeding that of traditional chemical rocket propulsion. Pulsed laser propulsion (PLP) is an advanced propulsion concept. The embryonic concepts related to this idea can be traced back to earlier proposals. Between 1923 and 1924, Russian space pioneer Konstantin E. Tsiolkovsky and German scientist Hermann Oberth proposed the concept of light pressure propulsion, laying the theoretical foundation for what would later evolve into laser propulsion technology. In 1953, German scholar Eugen Sänger introduced the idea of a photon rocket, envisioning propulsion via high-temperature plasma radiation generated by nuclear reactors. After the invention of lasers in 1960, Sänger refined his proposal, suggesting the replacement of conventional light sources with nuclear-pumped lasers—thereby establishing crucial technical groundwork for the development of laser propulsion. Previous research has explored its potential for ground-based laser launches into orbit, laser propulsion vehicles for orbital transfers, laser-driven small spacecraft for short-term missions, and laser micro-thrusters for small satellite attitude control [2,3,4,5,6]. Compared to traditional propulsion methods, such as cold gas, chemical, and electric propulsion, PLP offers significant advantages. It features small impulse bits, strong optical controllability, a wide adjustable thrust range, and precise control.

Extensive research has been conducted on PLP, primarily focusing on propulsive force generation and impulse coupling in laser ablation. In the late 1980s and early 1990s, the advancements in small satellites and laser technologies brought increased attention to laser propulsion systems. In 1983, Professor Myrabo of Rensselaer Polytechnic Institute proposed the concept of light-powered flight [7]. By October 2000, Lightcraft Technologies Incorporated, founded by Myrabo, validated this concept. Using a CO₂ laser with a pulse average power of 10 kW, the team launched a lightcraft with a 12.2 cm diameter and 50.6 g weight at White Sands Missile Range. The craft reached a height of 71 m and flew for 13 s [8]. This experiment highlighted PLP’s potential as a promising propulsion technology with many advantages over conventional methods [9,10]. The replacement of conventional components in chemical propulsion systems—including cryogenic or high-pressure fuel tanks, gas-driven turbopumps, nozzle cooling structures, and auxiliary systems—with lightweight diode lasers or diode-pumped fiber lasers forms the basis of a laser propulsion system. This structural simplification significantly reduces system mass, thereby lowering launch costs. Laser propulsion offers thrust capabilities ranging from μN to N levels, making it well-suited for applications such as microsatellite attitude control (1–40 μN·s impulse bits) and orbital debris removal using ground-based megawatt-class lasers (achieving Newton-level thrust). In terms of specific impulse (I_sp), laser ablation propulsion demonstrates a remarkable range of 500–5000 s (depending on target material) [11], significantly exceeding that of chemical propulsion systems (200–450 s) and approaching the performance of electric propulsion systems (e.g., ion thrusters: 3000–4000 s).

The system enables adjusting the specific impulse through key operational parameters, including the laser intensity on the target surface, focal spot size, and laser pulse duration. Furthermore, thrust magnitude can be independently modulated via laser pulse repetition rate adjustments without compromising specific impulse characteristics. Compared to conventional propulsion technologies, this approach eliminates the requirement for substantial onboard propellant storage (ablative mode requiring only target material) while achieving dynamic thrust control through laser parametric adjustments. This operational flexibility renders it particularly advantageous for extended deep-space missions and precision orbital maneuvers requiring millimeter-level positioning accuracy. However, it is undeniable that chemical and electric propulsion exhibit higher technological maturity and mission adaptability compared to laser propulsion. Chemical propulsion generates instantaneous massive thrust through fuel combustion, boasting strong technical reliability. It has been widely applied in scenarios requiring rapid acceleration, such as rocket launches and crewed spaceflight. Although electric propulsion delivers smaller thrust, its exceptionally high specific impulse and fuel efficiency far exceed that of chemical propulsion. It can operate long-term using solar energy, making it ideal for missions requiring sustained micro-thrust, such as satellite orbit maintenance and deep-space exploration. In contrast, the powerful thrust of chemical propulsion and the high efficiency of electric propulsion have already seen extensive applications in aerospace engineering. Both remain mainstream choices in current space propulsion systems, whereas laser propulsion, as a cutting-edge field, still demands prolonged technological development and engineering validation.

The United States leads international efforts in Laser Propulsion Thruster (LPT) technology through the work of the Phipps team, which has achieved significant milestones. Their developments include a laser plasma micro-thruster for the U.S. Air Force TechSat21 satellite, marking the first engineering application of a laser plasma micro-thruster with a transmissive solid target band mode. Between 2002 and 2007, they designed millisecond pulse-width laser micro-thrusters (ms~μLPT) and nanosecond pulse-width laser micro-thrusters (ns~μLPT) to meet microsatellite power requirements. In 2008, they proposed a kilowatt-level pulse laser engine capable of 6 N thrust with two working modes: nanosecond and millisecond pulse widths. This variable-specific impulse and thrust engine featured specific impulse ratios of 116 s/3660 s and thrust ratios of 57 mN/6.48 N. Early designs utilized semiconductor single-tube lasers as radiation sources, producing low specific impulses (~200 s) but high impulse coupling coefficients (~2000 mN/W), referred to as low-specific-impulse high-thrust mode. Later designs introduced nanosecond pulse lasers with over 10,000 watts of peak power, achieving high specific impulses (1000 s–3000 s) with lower coupling coefficients (~100 mN/W), known as high-specific-impulse low-thrust mode.

Outside the U.S., notable advancements include work by the NAKANO team at the Tokyo Metropolitan Institute of Technology. They developed a laser micro-thruster using laser ignition of solid energetic propellants, achieving a specific impulse of approximately 100 s and an impulse of 60 mN·s. However, a satellite CPU malfunction prevented experimental validation. In 2014, Jian Cai’s team at the University of Science and Technology of China integrated laser rod technology to design a solid-target laser plasma micro-thruster prototype for satellite attitude and orbit control. They tested its propulsion performance using an 808 nm wavelength semiconductor laser. By 2022, Hong Yanji’s team at the Space Engineering University further advanced solid-state laser micro-thrusters, achieving successful in-orbit verification through adjustments in laser pulse width and target layer thickness [12]. They demonstrated that laser micro-thrusters can perform orbital transfer maneuvers using thrust levels in the hundreds of micronewtons at altitudes above 300 km, confirming their operational viability for nanosatellite propulsion in low Earth orbit (LEO) environments. Despite these advancements, further improvements in PLP propulsion performance remain challenging. Propellant characteristics significantly influence laser propulsion’s performance parameters, making propellant selection a critical research focus [11]. Solid propellant-based laser micro-thrusters face complex transmission mechanisms, low total thrust (the thrust generated by using up all the propellant carried), and inefficient utilization of laser-ablated materials. Research is increasingly shifting toward laser ablation of liquid propellants. Liquid laser propulsion, a subfield of laser propulsion, emerged relatively late, mainly gaining attention from the late 1990s to the early 2000s. However, its earliest instance dates back to 1986, when Kurasaki at the University of Tokyo developed and thoroughly tested a water-based laser propulsion thruster under high-vacuum laboratory conditions [13,14]. Additionally, in 1996, NASA launched the Lightcraft Technology Demonstration project to explore the practical applications of laser propulsion, which included experiments using liquid propellant.

Liquid laser propulsion (LLP), championed by Yabe, offers notable advantages over solid targets by generating larger impulse coupling coefficients [15,16,17,18]. Yabe’s team demonstrated a coupling coefficient of 3500 N/mW by irradiating a water film on an aluminum target [19]. Liquid targets exhibit strong laser absorption, efficiently converting laser energy into kinetic energy for propulsion. As a result, liquid propellants achieve high pulse coupling coefficients and energy utilization efficiency [20,21,22]. High impulse coupling coefficients, combined with high specific impulses, make pulsed LLP highly promising for propulsion applications. Liquid propellants also allow flexible supply methods, higher mass flow rates, greater thrust, and adjustable specific impulses, making them an attractive target for ongoing research [23,24,25]. Liquid propellants are green, non-toxic, energy-dense, and easy to store and supply, making them ideal energy carriers. However, challenges such as low ionization rates and excessive fluid splashing hinder progress, and the interaction mechanisms between lasers and liquid propellants require deeper exploration.

With further research on the propulsion performance of different liquid propellants, scholars have discovered that energetic materials have better development prospects as propellants. Energy-containing materials inherently carry high-energy groups. During the interaction between the laser and these materials, the decomposition of these groups releases a significant amount of heat, promoting material breakdown and producing numerous smaller particles. Due to the involvement of chemical energy release during laser ablation, the energy conversion efficiency of laser-ablation micro-propulsion using energetic materials has been improved. This results in both a high impulse coupling coefficient and a high specific impulse. Consequently, energetic materials have garnered extensive attention from scholars in the current research field of laser-ablation propulsion [26,27].

In addition, low melting point liquid-phase alloys have also been explored as propellants in laser micro-propulsion research. In 2011, Sergey A. et al. proposed using liquid Ga–In alloy as an ablative target material instead of solid metal. Their experiment demonstrated a larger specific impulse (approximately 1000 s), although the single pulse impulse was relatively small, at only about 5.6 ± 1.2 nN·s [28,29]. Kurilovich et al. investigated the laser ablation propulsion performance of unconstrained (free-fall motion) eutectic indium–tin alloy (50In–50Sn) [30,31,32]. Although the gasification plume velocity reached 8.5 km/s during their experiment, the small plume mass limited the impulse generated. Therefore, more in-depth work is needed to explore the potential of liquid alloys as ablative working fluids and enhance their performance.

This article first reviews the fundamental theory of the interaction between pulsed lasers and liquid propellants. Then, it introduces the research status of several typical liquid propellants and summarizes their propulsion performance. Subsequently, it elaborates on the progress of existing laser-ablation plasma principle prototypes. Finally, it discusses potential research directions and key scientific issues to address in laser micro-propulsion liquid propellants.

2. Theory of Interaction Between Pulsed Laser and Liquid

The defining feature of laser propulsion when using liquid propellants lies in its momentum coupling coefficient (C_m). This coefficient, representing the ratio of momentum change to incident laser energy, is notably one or even two orders of magnitude higher than in gas or solid propellants [11]. The significant enhancement in C_m for liquid propellants can be attributed to the so-called “projectile effect,” a phenomenon unique to this propulsion method.

LLP operates in several modes—the restricted liquid propulsion mode, the thin-film liquid propulsion mode, and the aerosol propulsion mode—each characterized by distinct thrust-generation mechanisms. In the restricted liquid propulsion mode, a high-intensity laser penetrates the liquid propellant layer, ablating the solid material beneath it. This interaction produces a plasma explosion at the liquid–solid interface, propelling liquid droplets outward and generating thrust in the opposite direction to the splashing liquid. The thin film liquid and aerosol propulsion modes refer to the laser incident on the fluid film or aerosol, causing it to ionize and form a high-speed reverse plasma plume, generating thrust opposite to the jet direction. Figure 1 shows the schematic diagram of LLP. In the transmissive-type LLP, the thrust is opposite to the incident laser, whereas in the reflective-type LLP, the thrust is in the same direction as the incident laser.

Figure 1. Schematic diagram of liquid laser propulsion.

Figure 1. Schematic diagram of liquid laser propulsion.

The laser heats the target material. Therefore, a high-power laser induces melting, vaporization, and ionization. Eventually, material gasification and ionization form plasma. The heated plasma radiates energy outward and produces shock waves. In the following subsection, we will explore the liquid propellants’ breakdown and ionization mechanisms under laser irradiation and discuss the subsequent formation of shock waves in the plasma.

2.1. The Breakdown and Ionization Mechanisms for Liquid Propellants

The high energy of the high-power pulsed lasers is quickly absorbed by the material molecules at the laser’s focal point. When the energy exceeds the material’s threshold, the molecular bonds break down, forming a plasma. The laser-induced breakdown mechanisms in liquids include cascade (avalanche) and multiphoton ionization.

The cascade ionization mechanism relies on the inverse bremsstrahlung radiation to absorb energy, which requires a high density of free electrons as “seed electrons” at the focal point of the incident laser. The thermal excitation of impurities in the liquid generates free electrons. When the initial liquid molecules collide, the avalanche ionization process begins. When a free electron collides with other particles in a liquid, it transfers energy and generates an electron through ionization if the laser energy exceeds the ionization threshold of the molecules. At higher laser energies, the electrons in the liquid continuously absorb energy and repeat the process.

The ionization potential of liquid molecules depends on their band structure, electron density, and particle distribution. The breakdown and ionization of liquid molecules generate plasma radiation spectra, flashes, and liquid shock waves, which can be detected using corresponding sensors. The critical electron density refers to the minimum free electron density generated when a breakdown phenomenon occurs and is a key parameter. Generally, a density of free electrons above 10¹⁸ cm⁻³ produces a significant flash phenomenon in the liquid medium. Multiphoton ionization refers to the absorption of multiple photons by atoms or molecules in a liquid medium through laser irradiation, resulting in a total energy exceeding the ionization potential of the atoms or molecules and causing ionization.

The temporal evolution of plasma density is the basis for determining the threshold of pulsed laser-induced liquid working medium breakdown, represented by the free electron rate equation [33,34]:

$${\displaystyle \frac{\mathrm{d}\rho }{\mathrm{d}t}}={\left({\displaystyle \frac{\mathrm{d}\rho }{\mathrm{d}t}}\right)}_m+{\eta }_{casc}\rho -g\rho -{\eta }_{rec}{\rho }^2$$

(1)

Here, $\rho$ denotes electron density; ${\left({\displaystyle \frac{\mathrm{d}\rho }{\mathrm{d}t}}\right)}_m$ represents the electrons generated through multiphoton ionization; ${\eta }_{casc}\rho$ represents the electrons produced by avalanche ionization, ${\eta }_{casc}$ represents the cascade ionization coefficient; $g\rho$ accounts for the electrons lost due to diffusion outside the laser focus; ${\eta }_{rec}{\rho }^2$ represents the composite loss of ions and electrons, and ${\eta }_{rec}$ represents the recombination coefficient.

In a liquid medium, assuming that the atoms or particles in the medium possess $k$ imaginary energy levels, the absorption of photons causes these energy levels to rise, ultimately bringing the atoms to a true ionized state. Thus, the ionization rate $w$ can be expressed using the following expression:

$$w={\displaystyle \frac{N_i}{p{n}_0{V}_{\tau }}}$$

(2)

In the above formula, $N_i$ represents the number of ions generated by laser irradiation of the liquid; $p$ denotes the initial static pressure at the focal point; $V_{\tau }$ represents the volume at the laser focusing point; $\tau$ represents the duration of laser irradiation; and $n_0$ is the Loschmidt constant.

2.2. The Formation of Shock Waves from Laser Plasma

Plasma shock waves are a phenomenon resulting from the breakdown of liquid by pulsed lasers. As explained above, when a pulsed laser with a power density exceeding the breakdown threshold of the liquid medium interacts with the liquid, a high-temperature, high-pressure plasma is generated near the laser’s focal point. During the plasma’s expansion and absorption of laser energy, a series of compression waves are produced and superimposed at the plasma front. Additionally, the generated high-temperature and high-pressure plasma exceeds the acoustic wave propagation velocity in the liquid, creating a shock wave front characterized by a steep cross-sectional gradient. In the early stages of plasma formation, the laser-induced shock waves exhibit axial symmetry. However, as the shock wave propagates, it transitions from axial symmetry to a spherical wavefront. At the onset of plasma formation, shock waves propagate at supersonic speeds. Over time, as they expand and attenuate, these waves gradually transform into sound waves. The propagation of plasma shock waves in liquids can be mathematically described by the following set of equations [35]:

$$u_1-u_0=\sqrt{(p_1-p_0)}\left({\displaystyle \frac{1}{{\rho }_0}}-{\displaystyle \frac{1}{{\rho }_1}}\right)$$

(3)

$$D-u_0={\displaystyle \frac{1}{{\rho }_0}}\sqrt{{\displaystyle \frac{p_1-p_0}{\frac{1}{{\rho }_0}-\frac{1}{{\rho }_1}}}}$$

(4)

$$E_1-E_0={\displaystyle \frac{1}{2}}\left(p_1+p_0\right)\left({\displaystyle \frac{1}{{\rho }_0}}-{\displaystyle \frac{1}{{\rho }_1}}\right)$$

(5)

In the above equations, $E_0$, $p_0$, $u_0$, and ${\rho }_0$ represent the internal energy, pressure, particle velocity, and density of the undisturbed liquid medium, respectively, while $E_1$, $p_1$, $u_1$, and ${\rho }_1$ denote the corresponding parameters at the shock wave front. $D$ denotes the speed at which the shock wavefront propagates in the liquid.

In actual liquid media, the energy of plasma shock waves dissipates, and abrupt changes in the wavefront and background of the shock wave occur within a thin layer. This thin layer corresponds to the thickness of the shock wave cross-section, denoted as φ. The thickness of this cross-section is of the order of a few molecular free paths, and various physical parameters within this interval undergo continuous and rapid changes. The thickness of the shock wave front is negligible compared to the wavelength of the sound wave.

The energy of the shock wave at a radius R from its center is given by

$$E={\displaystyle \frac{4\pi R^2}{{\rho }_0{v}_0}}\int p^{2}dt$$

(6)

Here, R represents the distance from the measurement point to the center of the shock wave, $p$ represents the pressure at the shock wave front, and $t$ denotes the time elapsed from the start to the end of the shock wave attenuation.

Based on Newton’s second law, $F=ma=m{\displaystyle \frac{\mathrm{d}D}{\mathrm{d}t}}$, where $F=ps$, $m={\rho }_{0}V={\rho }_0\cdot s\cdot \mathrm{d}l$, we obtain

$$ps={\rho }_0\cdot s\cdot \mathrm{d}l\cdot {\displaystyle \frac{\mathrm{d}D}{\mathrm{d}t}}$$

(7)

where $U_p={\displaystyle \frac{\mathrm{d}l}{\mathrm{d}t}}$.

The above equation can be written as

$$p={\rho }_0\cdot U_p\cdot \mathrm{d}D$$

(8)

Integrating with the above equation, we can obtain

$$p-p_0=(D-D_{t=0})U_p{\rho }_0$$

(9)

At the initial moment, the speed of the shock waves $D_{t=0}=0$.

The relationship between the propagation speed of shock waves and pressure is derived as follows [36]:

$$p-p_0=D{U}_p{\rho }_0$$

(10)

In this equation, $p$ and $p_0$ represent the pressures of the shock wave and the liquid medium, respectively; $U_p$ denotes the propagation speed of particles in the liquid medium; and $D$ represents the propagation speed of the shock waves.

The propagation velocity $U_p$ of particles in the liquid medium is approximately linearly related to the velocity $D$ of shock waves, as expressed by the following equation:

$$D=A+B{U}_p$$

(11)

When a shock wave emits spherical radiation, the principle of conservation of momentum establishes the following relationship:

$$4\pi r^{2}D\Delta t\rho U_p=k$$

(12)

In the formula, $\rho$ represents the density of the shock wave precursor medium; $\Delta t$ is the rise time of the shock wave front; and $k$ is a constant. After the laser-induced breakdown of the liquid, the liquid medium is compressed from the breakdown point to the region ahead of the shock wave, causing the density to change from ${\rho }_0$ to $\rho$. By combining the above two equations, the propagation velocity $D\left(r\right)$ of the shock wave at different distances r can be expressed as

$$D\left(r\right)={\displaystyle \frac{A}{2}}+\sqrt{{\displaystyle \frac{A^2}{4}}+{\displaystyle \frac{C}{r^2}}}$$

(13)

The expression for the propagation velocity of this shock wave applies to the calculation of shock waves in both axial and radial directions. Here, C is a constant composed of A, B, and p, which can be determined using weighted linear regression.

The equations for the pressure at the shock wave front and the propagation velocity of particles in the liquid medium can also be derived from the above equation as follows:

$$p=C{\displaystyle \frac{p_0}{B}}{\displaystyle \frac{1}{r^2}}$$

(14)

$$U_p\left(r\right)={\displaystyle \frac{1}{B}}\left(\sqrt{{\displaystyle \frac{A^2}{4}}+{\displaystyle \frac{C}{r^2}}-{\displaystyle \frac{A}{2}}}\right)$$

(15)

It can be observed that the pressure of spherical shock waves is proportional to ${\displaystyle \frac{1}{r^2}}$ and is applicable only in the near-field region of plasma shock waves.

2.3. Definition and Calculation of Propulsion Parameters

The main parameters for evaluating the performance of laser propulsion include impulse $I$, impulse coupling coefficient $C_m$, specific impulse $I_{sp}$, and conversion efficiency $\eta$. When a laser with energy $E$ irradiates a working fluid with mass $m$, the magnitude of the impulse generated by the laser action is denoted as $I$. Consequently, the specific impulse $I_{sp}$, which represents the impulse obtained per unit mass or unit weight of the working fluid, is defined as [3,37]

$$I_{\mathrm{sp}}={\displaystyle \frac{I}{mg}} \mathrm{or} I_{\mathrm{sp}}={\displaystyle \frac{F}{\dot{m}\mathrm{g}}}$$

(16)

In this equation, F represents the average thrust, $\dot{m}$ denotes the average mass flow rate, and g represents the gravitational acceleration. When the exit velocity is v, which represents the average velocity distribution along the thrust direction, and the mass erosion efficiency is considered a parameter, the specific impulse is given by

$$I_{\mathrm{sp}}={\displaystyle \frac{v}{g}}$$

(17)

For pulsed lasers, the $C_m$ is defined as the impulse generated per unit of laser energy [38]. For continuous lasers, it can also be expressed as the ratio of thrust F to incident laser power P, given by

$$\begin{gathered} C_m={\displaystyle \frac{I}{E}}={\displaystyle \frac{mv}{E}} (For\; pulsed\; lasers) \\ ={\displaystyle \frac{F}{P}} (For\; CW\; laser) \end{gathered}$$

(18)

During the ablation process, if the laser energy per gram of the target material ablated is E₁, the following relationship applies:

$$E_1={\displaystyle \frac{E}{m}}$$

(19)

From the above equations, the exit velocity can be expressed as

$$v=C_m{E}_1$$

(20)

Therefore, based on the measured C_m and E₁, the velocity of the particle ejected during laser ablation can be calculated.

The ratio of laser energy to propellant kinetic energy is defined as the energy conversion factor η, which characterizes the efficiency of converting laser pulse energy into the kinetic energy of the ejected particles [39]. This factor is expressed as follows:

$$\eta ={\displaystyle \frac{E_k}{E}}={\displaystyle \frac{1}{2}}{\displaystyle \frac{m{v}^2}{E}}={\displaystyle \frac{1}{2}}{C}_{\mathrm{m}}g{I}_{\mathrm{sp}}$$

(21)

In this context, we consider two conversion efficiencies: energy conversion efficiency ${\eta }_1$, which represents the efficiency of converting electrical energy into laser energy, and thrust efficiency ${\eta }_2$, which represents the efficiency of converting electrical energy into the thruster’s kinetic energy. These are related as follows:

$${\eta }_2={\eta }_1\eta$$

(22)

In addition, the ablation mass m is a crucial parameter in laser propulsion technology, and the mass ablation rate $\dot{m}$ can be expressed using the following equation:

$$\stackrel{·}{\mathrm{m}}={\displaystyle \frac{P_1}{E_1}}$$

(23)

Here, P₁ represents the laser’s average power, and E₁ denotes the laser energy consumed per gram of target material. The mass ablation rate is directly related to the lifespan of the ablative material, which is a critical factor in the design of laser ablation thrusters. This lifespan is associated with the impulse coupling coefficient and specific impulse. If the initial mass of the ablative working fluid is M, the lifespan of the ablative material can be expressed as

$$\tau ={\displaystyle \frac{M}{\stackrel{·}{m}}}={\displaystyle \frac{MP}{E_1}}={\displaystyle \frac{Mv}{P{C}_{\mathrm{m}}}}={\displaystyle \frac{M{I}_{\mathrm{sp}}g}{P{C}_{\mathrm{m}}}}$$

(24)

From this, it is evident that the lifespan of ablative materials is closely related to the carrying capacity of the working fluid, the laser’s average power, and the coupling coefficients of specific impulse and cooling. These factors serve as key indicators for evaluating the operational lifespan of laser plasma thrusters.

3. Current Research on Pulsed Laser Propulsion Using Liquid Propellants

There are various liquid propellants for pulsed lasers, with water being the most common. Water is environmentally friendly, readily available, inexpensive, and classified as a non-energetic propellant. Comparatively, the propulsion characteristics of organic liquids differ significantly from those of water or aqueous solutions. Generally, organic liquids with higher molecular weights exhibit greater viscosity, and their physical properties gradually approach those of solids, resulting in a notable reduction in the sputtering volume. Moreover, most organic compounds are flammable and release chemical energy under laser ablation, which can further enhance the momentum coupling coefficient. Modification methods, such as the mutual doping of liquid propellants or the incorporation of auxiliary materials, are commonly employed to enhance the propulsion performance of liquid propellants. In recent years, liquid metals and alloys have become a major research hotspot. These materials address the issue of focal point movement but also possess lower ablation thresholds compared to solid metals. The following discussion introduces the propulsion performance of liquid propellants across three categories: non-energetic liquid propellants, energetic liquid propellants, and liquid metals.

3.1. Non-Energetic Liquid Propellants

Yabe et al. [19] from the Tokyo Institute of Technology began conducting laser propulsion experiments using water as a propellant in 2002. They covered the ablative material with a layer of transparent material to limit the rapid expansion of the laser ablation plasma. This approach prolonged the duration of the high-temperature and high-pressure plasma, thereby applying greater thrust and impulse to the ablative material. In the studies of laser ablation liquid propulsion performance, propulsion characteristics can be optimized by modifying the geometric configuration and confinement modes of the liquid propellant. In solid-confined liquid (L(S)) or liquid-confined solid (S(L)) configurations, the phase of the thrust-generating medium (solid or liquid) is determined by the laser’s focal position—whether the beam is focused at the solid–liquid interface or within the liquid medium. This article also reports on the propulsion performance in liquid-confined solid (L(S)) and liquid-confined gas (G(L)) modes. In these configurations, although the primary propellant may be a solid or a gas, the presence of a liquid confinement layer, or the combined effects of solid/gas and liquid, significantly enhances propulsion performance, which is also analyzed in this paper. (L, G) denotes (liquid, gas), representing atomized liquid propellant.

Figure 2 presents the statistical results of the momentum coupling coefficient ($C_m$) for various propellants under different confinement modes. Figure 2a illustrates the $C_m$ values for propellants in the solid-confined liquid mode under different laser parameters, where the primary propellant is liquid. Figure 2b shows the $C_m$ for propellants in the liquid-confined solid mode, also under varying laser conditions, where the primary propellant is either solid or gas. As indicated in the figures, the laser technologies commonly used in liquid laser propulsion (LLP) research include short-wavelength lasers (1064 nm and 532 nm) and mid-infrared lasers (10.6 μm). Comparatively, laser ablation using the 10.6 μm wavelength tends to produce lower $C_m$ values than short-wavelength laser ablation. Therefore, to achieve higher momentum coupling coefficients, short-wavelength lasers are recommended for LLP studies.

From Figure 2a, the highest achievable $C_m$ in the current L(S) configuration is approximately 10,200 N/MW. Senegačnik et al. [40] investigated the propulsion effects of water under various geometric constraint shapes by drilling holes of different sizes into the surface of aluminum rods and filling them with varying volumes of water. Experiments were performed on a flat surface (F) as well as on a surface with 2 mm (D2) and 3 mm (D3) wide cylindrical blind holes. The highest coupling coefficient and energy-conversion efficiency are achieved in combinations of narrow hole (geometry D2)—small droplet (V = 4 mm³) and wide hole (geometry D3)—large droplet (V = 10 mm³). The confinement by cavity substantially increases the propulsion effects by shaping the ejected flow of the liquid, while the cavitation bubble, induced inside the water layer, plays a significant role in propulsion efficiency. When the laser is focused at the liquid interface, the system operates in the L(S) propulsion mode. For flat surface (F) configurations, the $C_m$ of the L(S) mode exceeds that of the S(L) mode. However, under D2 and D3 confinement conditions, the L(S) mode exhibits even higher $C_m$ values and achieves maximum impulse coupling. When the focus moves within the solid target, the $C_m$ decreases due to the reduced conversion efficiency of pulse energy into cavitation bubble energy. Conversely, if the focus moves outward, the bubbles migrate toward the liquid–gas interface, failing to eject all the liquid from the target.

Japanese researchers have conducted extensive studies in the laser ablation of liquid propellants. Yabe et al. [19] performed experiments on metal-free water cannon targets (MFWC) and water cannon targets (WC), typical examples of bulk liquid targets. In the experiment with a metal-free water cannon target, the surface of the metal target was coated with a layer of water film. Laser breakdown occurred at the interface between the metal target and the water film. During the expansion and evolution of the laser-supported plasma flow field, numerous water droplets are ejected from the water film, generating impulse through the recoil effect. The impulse was directed perpendicular to the metal target surface, with an impulse coupling coefficient of 3536 N/MW. In the experiment with a pure water bubble target, water droplets directly penetrated the bubble target and were ejected through a small hole on the left side. The thrust direction was opposite to that of the MFWC, and the impulse coupling coefficient reached 2400 N/MW [41]. The study also revealed that as the focal point of the lens approached the outlet and the outlet diameter increased, the impulse coupling coefficient also increased. However, the mass of the ejected liquid grew correspondingly. The specific impulse obtained from the experiment was estimated to be less than 2 s, based on the measured impulse coupling coefficient, laser energy, and mass loss. They also designed a water film cannon (WFC) target and conducted experiments. In the WFC experiment, a layer of water film was above the opening of an acrylic tube filled with air. A lens focused the laser at a specific point in the air and caused the breakdown. The water film splashed along the direction of the pipeline opening, generating impulse through recoil, with an impulse coupling coefficient of 3680 N·s/MJ [41]. This represents a G(L) confinement mode, which also achieves a remarkable impulse coupling coefficient.

The American team at the University of Alabama in Huntsville studied the propulsion performance of propellants such as ethane, ethanol, isopropanol, and water [42,43]. Their research demonstrated that within a laser power density range of 10⁶ W/cm²–10⁹ W/cm², vaporization and sputtering are the two primary physical processes in liquid working fluid laser propulsion. The vaporization process contributes most significantly to impulse generation, while the sputtering process contributes to impulse minimally but consumes approximately 95% of the total mass. Additionally, the study found that liquids with higher laser absorption coefficients yield greater impulse coupling coefficients. However, for the liquid propellants studied, the specific impulse and energy conversion efficiency remained low, with ethane achieving the highest specific impulse of only about 2.3 s. This limitation may be attributed to the high mass consumption during the sputtering process. By comparing propulsion parameter results for hexane, ethanol, and water in quartz versus glass vessels, their study revealed minimal influence of container materials on the C_m of liquid propellants. They further investigated container geometry (cylinder, cone) and liquid surface morphology (flat, concave) effects, concluding that the surface geometry had a significant effect on C_m for surface-absorbing liquids, while for volume-absorbing hexane, there is virtually no change measured in C_m between flat and concave surfaces. Conversely, both ethanol and water-surface-absorbing liquids experience a significant increase in C_m as the surface becomes more concave [44,45]. In addition, considering that sodium tetrafluoroborate (NaBF₄) aqueous solution strongly absorbs radiation in the wavelength range of 8–11 μm, the researchers dissolved NaBF₄ in water and conducted propulsion measurement experiments using a TEA CO₂ laser with a wavelength of 10.6 μm and a pulse width of 300 ns [46]. Unexpectedly, the experimental results showed that adding NaBF₄ to water did not improve the momentum coupling. This is likely due to the relatively small absorption depth of water, which is only 11.5 μm at a 10.6 μm wavelength [47,48]. As absorption is already limited to the near-surface region, the addition of another absorber does not significantly affect the coupling coefficient.

Zhiyuan Zheng’s team from the Institute of Physics at the Chinese Academy of Sciences conducted experimental research on water and ink targets using a YAG laser [49,50,51,52]. In their experiments, the laser beam was transmitted through a glass layer on the target to suppress water splashing in the direction of the lens, ensuring that the sputtering process occurred only along the tail opening. The laser was focused at a specific point within the water, causing breakdown and generating impulse through recoil. When using ink as the target material, the breakdown point was closer to the interface between the glass layer and the target material, resulting in increased sputtering of the target. The experiments produced an impulse coupling coefficient of 2500 N/MW. Laser propulsion of bulk liquid targets typically operates in a bulk absorption mode, where the breakdown point occurs within the target material. By applying constraints to the target material, the sputtering process can be directed along a specific trajectory, reducing material consumption and allowing control over the propulsion direction. This results in a high energy coupling coefficient.

Xiaowu Ni and his team from the Nanjing University of Science and Technology conducted laser propulsion experiments on six different constrained containers in water and air [53,54,55,56]. Their findings revealed that the propulsion performance of all containers was superior in water compared to air, regardless of whether the container had a cavity at its tail (i.e., the laser irradiation surface). The enhanced propulsion effect in water was attributed to the first bubble expansion process, where objects with cavities at their tails absorbed more energy. Furthermore, constrained containers with cavities exhibited better propulsion performance than those without, with hemispherical cavities outperforming 90° conical cavities.

Wang et al. [57] and Cai et al. [58] from the University of Science and Technology of China conducted experimental research on a double-layer target composed of a substrate and a water film using a YAG laser. They obtained the impulse coupling coefficients of double-layer targets with various substrate materials and water films of different thicknesses. When the substrate material was aluminum and the water film thickness was 3 mm, the impulse coupling coefficient of 3.5 mN/W was the highest, two orders of magnitude higher than directly ablating aluminum substrates. Additionally, they found that when the atomic weight of the substrate material closely matches that of water molecules, it promotes complete vaporization of the water, leading to faster vaporization diffusion rates, strong recoil on the condensed target, and higher C_m.

In the United States, Sinko [59] conducted experimental research on liquid film targets, including water, ethanol, and hexane, applied to polyoxymethylene resin (POM) substrates using a TEA CO₂ laser. The experiment utilized an infrared laser with a pulse width of 300 ns and a wavelength of 10.6 μm. Sinko incorporated a small amount of surfactant into the working fluid, promoting droplet dispersion and forming a uniform liquid film at the target. Through their experiments, they discovered that the absorption coefficient of the liquid significantly affects propulsion performance and substrate vulnerability. The POM substrate covered with a water layer remained undamaged, whereas those covered with ethanol and hexane layers showed varying degrees of damage. Moreover, they observed that water and ethanol liquid films had no notable effect on improving propulsion performance. However, hexane liquid films significantly enhanced propulsion performance. Compared to the ablation of pure POM material without a liquid film, the impulse coupling coefficient of hexane-covered targets doubled, reaching 540 N/MW, and the maximum specific impulse was comparable to pure POM material during ablation, reaching 250 s.

The liquid film acts as a constraint on the high-temperature and high-pressure gases generated during ablation. Meanwhile, the target recoils through its sputtering and significantly increases the impulse coupling coefficient. Concerning the bulk liquid targets, film liquid targets significantly reduce target material consumption. However, due to the tendency of sputtering to occur in water-target materials, the impulse coupling coefficient remains nearly unchanged, and the improvement in specific impulse is minimal. In contrast, hexane targets are less prone to sputtering, leading to a decrease in the impulse coupling coefficient but a marked improvement in specific impulse and overall propulsion performance.

Yanji Hong and colleagues from the University of Aerospace Engineering designed a liquid release system to enhance the propulsion performance of pulsed laser ablation of water propellant. This system atomizes the water propellant into droplets with diameters ranging from 40 to 80 μm through low-pressure atomization, a size comparable to the laser absorption depth (approximately 10 μm). Experiments were conducted on the ablation of these liquid droplets using a 10.6 μm pulsed laser with a pulse width of 5 μs [60,61]. The study examined the effects of factors such as average droplet size, pulse repetition frequency, pulse number, focusing position, and nozzle shape on the propulsion performance of the propellants. The experiments achieved a specific impulse of 102 s and an impulse coupling coefficient of 520 N/MW, corresponding to an energy conversion efficiency of 26.1%.

Figure 2. The impulse coupling coefficients for different propellants studied by researchers under various modes [19,40,41,42,43,44,45,46,49,50,52,53,57,58,59,60].

Figure 2. The impulse coupling coefficients for different propellants studied by researchers under various modes [19,40,41,42,43,44,45,46,49,50,52,53,57,58,59,60].

Table 1. Liquid pulsed laser propulsion performance: non-energetic propellants.

Country	Leader	Laser Parameters	Target	Constrained State	Cm (N/MW)	Isp (s)	References
Japan	Yabe	1.064 μm 5 ns	Water (Al)	L(S)	3536	-	[19,41]
		10.6 μm 100 ns	Water (Al)	L(S)	400	-
America	Pakhomov	10.6 μm 300 ns	Water (quartz glass/Al): Cylinder	L(S)	600	-	[44,45]
			Hexane (quartz glass/Al): Cone	L(S)	240	-
			Ethanol (quartz glass): Cylinder	L(S)	560	-
			Ethanol (Al): Cone
			Water (Al)	L(S)	450		[46]
China	Z.Y. Zheng	0.532 μm 7 ns	Water	L(S)	2500	8.9	[52]
Slovenia	Matej Senegačnik	1.064 μm 7 ns	Water: F	L	1800	-	[40]
			Water (Al): D2	L(S)	10,200	-
			Water (Al): D3	L(S)	8700	-
China	Z.P. Tang	1.064 μm 12 ns	Water	L(S)	1790	19	[57]
China	Y.J. Hong	1.064 μm 10 ns	Water droplet	(L, G)	500	100	[60]
Japan	Yabe	10.6 μm 100 ns	MFWC	S(L)	2400	-	[19,41]
America	Pakhomov	10.6 μm 300 ns	POM (Water)	S(L)	600	-	[42,44]
			POM (NaBF4)	S(L)	450	3	[43]
			POM (hexane)	S(L)	570	-	[59]
China	Z.Y. Zheng	0.532 μm 7 ns	Water model car	S(L)	3990	-	[49,50]
China	X.W. Ni	1.064 μm 10 ns	Al (Water)	S(L)	3400	-	[53]
Slovenia	Matej Senegačnik	1.064 μm 7 ns	Al (Water): F	S(L)	6900	-	[40]
			Al (Water): D2	S(L)	10,200	-
			Al (Water): D3	S(L)	5800	-
China	Z.P. Tang	1.064 μm 12 ns	Al (Water)	S(L)	9839	-	[58]
			Fe (Water)	S(L)	7700	-
			Cu (Water)	S(L)	4100	-
			Zn (Water)	S(L)	8100	-
Japan	Yabe	1.064 μm 5 ns	WFC	G(L)	3680	-	[19,41]

summarizes the research findings of multiple teams on the propulsion performance of non-energetic liquid propellants with various shapes. In the table, L(S) denotes the solid-confined liquid mode, S(L) denotes the liquid-confined solid mode, and G(L) denotes the liquid-confined gas mode.

3.2. Energetic Liquid Propellants

Selecting liquid propellants based on performance is a key approach to enhancing the efficiency of liquid propulsion systems. Organic liquids have demonstrated notable advantages, and polymer-based liquid ablative fuels hold significant promise for future applications [62,63,64,65]. Glycine polymer represents a novel high-energy-density material with high specific energy and capacity and an important research focus in liquid laser propulsion. Many researchers have investigated propulsion performance, sputtering behavior, and other characteristics of this working fluid under various conditions, such as differing mass fractions and doping levels of glycerol and GAP. Figure 3 presents the statistical results for C_m (Figure 3a) and I_sp (Figure 3b) of energetic liquid propellants under various modes studied by researchers. Content within parentheses refers to the doping components in the liquid propellants. From the figures, it is evident that short-wavelength lasers are primarily used in the study of energetic liquid propellants. In addition to glycerol and GAP, ADN-based liquids have recently gained interest as energetic propellants. However, due to the difficulty in releasing chemical energy from ADN liquid propellants during laser ablation, their C_m and I_sp values remain relatively low.

Liquid splashing in laser-ablated propellants is a critical factor influencing propulsion performance. Glycerol possesses a certain viscosity, which helps reduce sputtering during laser ablation when used as a propellant. Fujita et al. from Japan examined the propulsion effects of glycerol, reporting single-pulse thrust experimental values ranging from 2.3 to 5.1 × 10⁻⁵ N⋅s [66]. Considering the laser energy deposition characteristics on both the interior and surface of droplets and the thrust generation mechanism, they proposed that achieving higher specific impulse requires optimizing the droplet absorption length about laser irradiation conditions and time evolution. Lippert et al. [67] and Fardel et al. [68] from Switzerland investigated the propulsion performance of GAP and found that when the GAP doping concentration was below 70%, the measured viscosity reached 14 Pa⋅s. At this concentration, the volume of the splashing solution was less than an order of magnitude compared to a solution with a 50% mass fraction.

Zheng et al. [69] investigated the effect of glycerol viscosity on laser plasma propulsion by controlling the ambient temperature. At an ambient temperature of 12 °C, the maximum target momentum reached approximately 6.97 × 10⁻⁴ kg⋅m⋅s⁻¹, while at 40 °C, it was reduced to about 3.53 × 10⁻⁴ kg⋅m⋅s⁻¹. The study also revealed that the influence of ambient temperature on target momentum became more pronounced with increasing laser energy. At a laser energy of 160 mJ, the momentum coupling coefficient corresponding to the maximum target momentum reached 4360 N/MW. These results demonstrate that glycerol exhibits excellent propulsion characteristics under optimized conditions.

Doping solid materials into liquid propellants is a widely used modification method to enhance the propulsion performance. Lippert et al. [67] noted that the specific impulse of a 70% mass fraction GAP solution could reach 680 s. They attributed this improvement to GAP being an energetic material and the inclusion of a specific infrared laser absorber in the solution, which significantly increased the specific impulse due to combined effects.

In addition to infrared dye doping, carbon-based materials, such as carbon nanoparticles, are common dopants. Li et al. from the Space Engineering University enhanced the propellant’s laser energy absorption by doping with varying concentrations of carbon powder. Their research focused on the jet behavior, shock wave velocity, and impulse of GAP after carbon doping. The study demonstrated that carbon doping significantly improved the impulse coupling coefficient and specific impulse of glycerol. A comprehensive analysis identified 1% carbon powder concentration as the optimal doping level for glycerol. At this concentration, the impulse coupling coefficient increased dramatically from 67 mN·s/J (undoped) to 1250 mN·s/J [70,71]. Ye et al. [72,73,74], also from the same team, examined the laser ablation process and working medium sputtering for carbon-doped ethanol, glycerin, gelatin (a hydrazine mixture), and GAP propellants over time. Using a pressure sensor, they recorded the force variation during ablation. Figure 4 compares the flow field images and thrust performance curves during the sputtering. The analysis demonstrated that the primary stage of thrust generation occurs before sputtering. Plasma expansion and propellant gasification contribute most significantly to impulse, and the relative contributions depend on the liquid’s laser absorption characteristics. The contribution of liquid sputtering to impulse was minimal, with later sputtering leading to a substantial loss of liquid mass. This mass migration at low speed reduces specific impulse and ablation efficiency.

Zheng et al. further investigated the sputtering characteristics of carbon-doped glycerol under nanosecond laser ablation [75,76,77]. Their findings corroborated those of Nanlei Li, showing that carbon doping effectively enhances GAP propulsion performance. The specific impulse of carbon-doped glycerol propellants in their experiments reached 840 s, with an ablation efficiency of 98%. The laser wavelength significantly influences the propulsion performance of laser-ablated liquid propellants. Zheng et al. [78] also investigated the effects of 1064 nm and 532 nm lasers on the ablation characteristics of carbon-doped glycerol. At low fluence levels, the coupling coefficient generated by the 1064 nm wavelength laser is higher than that produced by the 532 nm laser. At a laser fluence of approximately 10⁹ W/cm², the primary absorption mechanism is reverse bremsstrahlung radiation, and the shielding effect is stronger for longer wavelengths. Compared to excitation at 532 nm, plasma excited at 1064 nm is more likely to reach the critical electron density. Consequently, for the 1064 nm wavelength laser, the momentum generated by the propellant decreases as laser fluence increases. Additionally, the results indicate that at high laser energy densities, the propellant achieves a greater specific impulse under 532 nm laser ablation. By optimizing the laser wavelength, carbon content, and liquid properties, an ideal coupling coefficient and specific impulse can be achieved.

In the 21st century, with the development of propulsion technology and the continuous evolution of aerospace fuels, propellants such as glycerol and GAP are increasingly insufficient to meet the performance demands of modern aerospace applications. As a result, innovative fuels like ADN, methylimidazole (1-allyl-3-methylimidazole dinitrile amine salt), and tetrahydrodicyclopentadiene have garnered significant attention due to their numerous advantages, including environmental friendliness, high energy density, and rapid chemical reactivity. Among these, ADN stands out as a green aerospace fuel, offering a high specific impulse and combustion rate. As a strong oxidant, ADN is a viable alternative to ammonium perchlorate (AP) [79]. Compared to hydrazine, ADN provides superior quality and energy per unit volume while being less sensitive to water and oxygen. Moreover, its combustion and decomposition products are pollution-free, indicating that it is environmentally friendly [80,81].

Pan Yue and colleagues, under the supervision of Lizhi Wu at the Nanjing University of Science and Technology, conducted a comprehensive investigation into the laser absorption, laser ablation, and propulsion characteristics of ADN-based liquid propellants [82]. Their findings demonstrated that ADN-based energetic liquids are highly effective as liquid propellants, with an absorption coefficient of 354.05 cm⁻¹ and an absorption depth of 28.24 μm. At a laser energy of 63.97 mJ, ADN-based liquid propellants achieved optimal propulsion performance, yielding an impulse of 53.31 μN⋅s, an impulse coupling coefficient of 0.8757 mN⋅s/J, a specific impulse of 281.59 s, and an energy conversion efficiency of 122.40%. During the ablation process, these energetic liquids decompose, releasing additional energy and achieving energy conversion efficiencies exceeding 100%, significantly enhancing laser utilization. These results underscore the potential of energetic liquid propellants as a key direction for the future development of laser-based propulsion systems.

The team further explored the laser propulsion performance of a mixed solution comprising ADN and the ionic liquid 1-allyl-3-methylimidazolium dicyandiamide under microcavity confinement on an aluminum alloy substrate [83]. To enhance laser absorption efficiency, the ADN propellant was doped with infrared dyes. The experiments recorded a maximum specific impulse of 84.14 s under a laser energy density of 21.51 J/cm², corresponding to an impulse coupling coefficient of 1066.73 N/MW. Their analysis revealed that the maximum contribution of chemical energy to the kinetic energy of ADN-based liquid propellants was 93.93%.

Doping modification is a commonly employed method for improving the performance of liquid propellants. Wu Lizhi et al. investigated the laser ablation and propulsion characteristics of HAN doped with nano-aluminum particles [84]. Their findings revealed that doping HAN with nano-aluminum significantly improved its maximum laser absorption rate by over 80%. However, the absorption depth decreased markedly, reaching a minimum value of 277 μm. Additionally, including nano-aluminum reduced sputtering behavior during laser ablation, enhancing impulse generation for the HAN propellant. At a laser power of 25 mJ, the kinetic energy of the HAN propellant increased by 56%. Compared to undoped propellants, the doped propellant demonstrated significantly improved propulsion performance.

Baosheng Du from the Space Engineering University investigated the laser plasma propulsion performance of ADN liquid propellants optimized with near-infrared dyes and ammonium perchlorate (AP) solid powder under an incident laser mode [85]. They found that adding 2% AP solid powder alters the propellant’s ablation volume and energy characteristics, leading to an increased enthalpy change and enhanced combustion rate. Through the optimized laser ablation of the 2% AP-enhanced propellant, they achieved the following optimal propulsion parameters in a 200 μm-scale combustion chamber: single-pulse impulse I~9.8 μN⋅s, specific impulse I_sp~234.9 s, impulse coupling coefficient C_m~62.43 dyne/W, and energy conversion efficiency η~71.2%. Figure 5 presents Schlieren images of the laser ablation process for liquid propellants at different exposure times, comparing the 40%-AAD sample (acetone + 0.4 mL ADN solution + 0.6 mL dye solution) with the 40%-AAD + AP propellant sample. Adding AP to the liquid propellant resulted in faster plume ejection velocities and a larger impulse [86,87].

Figure 3. The impulse coupling coefficient and specific impulse for energetic liquid propellants under different modes studied by researchers [66,67,68,69,72,73,75,78,82,83,84,85].

Figure 3. The impulse coupling coefficient and specific impulse for energetic liquid propellants under different modes studied by researchers [66,67,68,69,72,73,75,78,82,83,84,85].

Figure 4. Sputtering flow field images and thrust performance curves [72] [Reproduced with permission from the Chinese Journal of Lasers, published by the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and the Chinese Optical Society, 2017].

Figure 4. Sputtering flow field images and thrust performance curves [72] [Reproduced with permission from the Chinese Journal of Lasers, published by the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, and the Chinese Optical Society, 2017].

Figure 5. Schlieren images of laser ablation of liquid propellants at different exposure times for 40%-ADD sample and 40%-AAD +AP propellant samples [85] [Reproduced with permission from Baosheng Du, Micromachines, published by Baosheng Du, 2023].

Figure 5. Schlieren images of laser ablation of liquid propellants at different exposure times for 40%-ADD sample and 40%-AAD +AP propellant samples [85] [Reproduced with permission from Baosheng Du, Micromachines, published by Baosheng Du, 2023].

Chentao Mao, Luyun Jiang, and their colleagues from the Space Engineering University conducted experimental and simulation analyses on the laser ablation of ADN-based propellants, focusing on the underlying mechanisms. Chentao Mao specifically investigated the effects of laser specific energy, chemical specific energy, and liquid propellant mass on the impulse generated by the vaporization plume of propellants. He analyzed the vaporization plume impulse based on the principle of energy conservation and the environmental gas impulse grounded in point explosion theory. By considering the impulses generated by the vaporization plume and the surrounding gas, Mao analyzed the influence of laser energy and liquid propellant mass on impulse, specific impulse, impulse coupling coefficient, and energy conversion factor. Theoretical predictions indicated that the impulse generated by the propellant is proportional to the square of the absorbed energy, while the impulse generated by air is directly proportional to the absorbed energy. Experimental validation demonstrated that these findings are applicable in atmospheric conditions, and the analysis of impulses generated by liquid propellants in a vacuum remains reasonable [88]. Luyun Jiang researched the complex effects of initial temperatures, pressures, and propellant composition ratios on key parameters, including temperature, combustion rate, and heat release, in ADN-based propellants using straight nozzle simulation models. The research revealed that ADN decomposes and releases heat, producing nitrogen-containing molecules such as N₂O and NH₃. An increment in the initial temperature and the ADN ratio significantly accelerates the thermal decomposition rate of ADN [89].

Table 2. Liquid pulse laser propulsion performance: Energetic propellants.

Country	Leader	Laser Parameters	Target	Dopant	Cm (N/MW)	Isp (s)	References
Japan	Fujita	1.064 μm 10 ns	GAP	-	232	-	[66]
Switzerland	T. Lippert	1.064 μm 6 ns	GAP	infrared dye	-	680	[67,68]
China	Zhiyuan Zheng	0.532 nm 10 ns	Glycerine	-	4360	6	[69]
		0.532 nm 10 ns	Glycerine	-	3000	8	[75,76,77,78]
		1.064 nm 10 ns	Glycerine	-	3750	9.5
China	Jifei Ye	1.064 nm 8 ns	GAP	carbon	1493	140	[73]
			Glycerine	carbon	1470	-	[72]
			ADN	infrared dye	624.3	234.9	[85]
China	Lizhi Wu	1.064 nm 6.5 ns	ADN	infrared dye	875.7	281.6	[82]
			ADN	1-allyl-3-methylimidazolium dicyandiamide	1070	84.14	[83]
			HAN	aluminum nanoparticles	640	189	[84]

below summarizes the research conducted by various teams on the laser propulsion of energetic liquid propellants.

3.3. Liquid Metal Propellants

Metal targets as ablative propellants in laser propulsion offer a high specific impulse [90], but using solid metal materials as ablation targets necessitates a specialized mechanical system to precisely control the positioning of the metal within the focal area of laser irradiation [91]. This limitation can be mitigated by employing liquid-phase alloys as propellant targets. Liquid metal targets can self-recover fast under the influence of surface tension, eliminating the need for complex mechanical systems. Moreover, compared to solid targets, liquid metal working fluids are expected to have a lower ablation threshold and higher efficiency [92,93]. Sergey A. and collaborators utilized capillary tubes to supply liquid Ga–In alloy, ablating the liquid alloy with a laser at the capillary tube port. The size of the liquid alloy in the ablation area using this method is comparable to that of droplet-shaped target materials. Meanwhile, the alloy droplets remain constrained by the solid wall surface and the liquid inside the tube [28], enhancing propulsion performance. The plasma generated by the laser ablation of liquid alloys propagates at a relatively narrow angle (30–35°), aligning closely with the behavior of solid-state targets [94,95,96,97]. An analysis of flight time and mass energy revealed that the average ion velocity at the core of the plasma flow ranges between 25 and 30 km/s. The impulse per single pulse measured in their experiment was 5.6 ± 1.2 nN⋅s, with an I_sp of approximately 1000 s. Compared with the ion I_sp-ion, the specific impulse value is three times lower, but it remains within an acceptable range for LPT. Thus, capillary-based target units filled with liquid metal propellants present a promising approach for LPT applications, simplifying the propellant supply mechanism to the laser radiation focal area while maintaining favorable thrust characteristics [98,99,100,101].

In 2016, Dmitry Kurilovich et al. investigated the thrust performance generated by the laser ablation of liquid indium–tin droplets during free-fall motion under gravity [23]. They observed that while the propulsion mechanism of metal droplets fundamentally differs from that of water droplets, the fluid dynamics response remains similar. They found the maximum velocity of the droplet after laser ablation to be 350 m/s, corresponding to a specific impulse of approximately 36 s. The laser energy applied to the droplet was around 50 mJ, with an impulse coupling coefficient of less than 5 N/MW and an energy conversion efficiency of less than 0.08%. Under Kurilovich’s experimental conditions, the propulsion performance of laser-ablated droplets was suboptimal. This result may be due to the absence of constraints on the metal droplets, which led to incomplete conversion of the vaporization plume force into thrust along the droplet’s movement direction after laser ablation. Instead, significant deformation occurred perpendicular to the droplet’s movement. The droplet primarily underwent velocity and position changes within 0.4 μs after laser irradiation, with deformation occurring between 0.4 and 9 μs [30]. Additionally, while the velocity of the vaporized plume reached a relatively high value of 8.5 km/s, the plume’s mass was too small to generate substantial impulse. According to the principle of impulse conservation, this resulted in a lower velocity for the droplet. Their research demonstrated that the thrust of liquid metal droplets remains highly stable during extended operation, suggesting that the target’s lifespan is primarily limited by the available liquid supply.

4. Current Status of LPT Prototype Development

In 2018, S. A. Popov et al. [102] proposed a hybrid plasma source designed to serve as a micro-satellite thruster, utilizing a liquid gallium–indium alloy as the propellant. The plasma source operates in two distinct modes: (1) generation of laser ablation plasma and (2) generation of plasma within a laser-triggered, low-voltage vacuum-arc discharge. In the first mode, the target for the laser beam is the working liquid meniscus. When no voltage is applied to the discharge cell, only laser-ablation plasma is generated, as described in [28], and the thruster functions as a standard LPT. In the second mode, with voltage applied to the electrodes, the laser ablation process initiates a high-current vacuum discharge. In this configuration, thrust is due to the cathode plasma recoil impulse, operating similarly to conventional arc jet thrusters that utilize a solid-state cathode with arc currents of several tens of amperes [103]. This hybrid thruster concept integrates a laser propulsion mode capable of generating ultra-low thrust pulses with high precision. Additionally, the researchers found that the probability of discharge ignition increases significantly when the laser beam is adjusted to ensure that the laser plasma jet radially points toward the anode.

In 2019, Baoyu Xing et al. designed and developed a micro-flow system that utilizes pulse signals to control piezoelectric valves and piston cylinders, with liquid GAP as the propellant [104]. The system feeds the liquid in micro-flows of approximately 50 μg per pulse, with the supply frequency synchronized with the laser frequency to ensure optimal ablation. The laser ablation occurs in a micro-thruster, where the plasma products expand through a micro-nozzle, achieving a nominal specific impulse of 600 s. The integrated propulsion system incorporates a piezoelectrically actuated valve module synchronized with a spring-driven piston–cylinder assembly, achieving pulsed operation through programmable electronic control signals. The liquid GAP propellant is precisely transported through a microfluidic delivery system into the micro-propulsion chamber. Upon droplet arrival, a focused laser beam penetrates the optical window to initiate controlled ablation of the fuel droplets.

LLP technology has not yet achieved on-orbit verification or practical application. Although LLP offers high specific impulse potential for space missions, its implementation is still limited by several core technical challenges. One key issue is the trade-off between specific impulse and efficiency. While liquid propellant, such as water or nanofluid micro-droplets, can achieve relatively high impulse coupling coefficients (up to 350 dyne/W), liquid splashing effects significantly reduce the achievable specific impulse, typically to below 1000 s. This performance is far lower than that of ion thrusters (over 3000 s) or Hall thrusters (above 1000 s). Moreover, converting laser energy into kinetic energy via a liquid propellant involves thermal losses during propellant evaporation and plasma formation, which limits the overall energy conversion efficiency. Another major challenge is propellant storage and delivery. In microgravity environments, the flow and atomization of liquid propellants are difficult to control, often requiring auxiliary gases or microfluidic technologies. These systems, however, are sensitive to vibrations and thermal expansion, reducing reliability. Additionally, liquid propellants are prone to volatilization or freezing in the vacuum and extreme temperatures of space, demanding complex thermal control and sealing solutions. These requirements increase system mass and design complexity. Thermal management and overall system integration also pose significant hurdles. LLP systems must integrate laser sources, propellant chambers, reflectors, and control units, all within a compact and lightweight structure suitable for spacecraft. Sustained high-power laser operation further intensifies the need for efficient heat dissipation, which is particularly challenging under strict mass and volume constraints. In summary, for LLP to become viable for space applications, it must overcome key bottlenecks in efficiency, working medium management, and system complexity.

5. Laser Technology for Laser Propulsion

The core technology behind laser propulsion is laser ignition. This process uses a high-energy laser beam to rapidly heat the surface of a propellant, initiating pyrolysis, vaporization, and plasma formation. Compact laser systems—with low power consumption, high precision, and modular design—have significantly advanced the practical implementation of efficient laser propulsion. Notable examples include more compact and reliable solid-state lasers, as well as diode-pumped disk lasers, fiber lasers, and other technologies that continue to push the limits of efficiency, output power, and beam quality.

At present, the lasers used in laser micro-thrusters mainly focus on two types: semiconductor lasers and solid-state lasers. The technological development of these two types of lasers is relatively mature, allowing the integration of spaceborne and miniaturized applications without the need for considerable cooling and power systems. The drawback is that under the constraints of volume, mass, and power consumption, it is challenging to adjust the energy, frequency, and pulse width of solid-state and semiconductor lasers on a large scale, which limits the optimization design of the laser ablation capability of liquid propellants on spacecraft. In recent years, rapid advancements have been made in 1064 nm small-scale semiconductor lasers, with notable progress in energy output, power, size, and mass. A 1064 nm semiconductor laser diode can now achieve high output power in nanosecond pulse mode, reaching up to 2 W. These lasers typically use butterfly packaging or open modular designs, which significantly reduce their footprint. The HiPoLas Gen-V laser ignition system utilizes integrated ceramic laser rod technology (combining Nd: YAG and Cr: YAG) to achieve a compact structure with a diameter of only 23 mm. The system operates at a 1064 nm wavelength with a 2 nanosecond pulse width and delivers pulse energies of approximately 35–45 millijoules. It is focused over a distance of 5–40 mm using a sapphire lens to ensure reliable ignition [105,106]. Additionally, the system features Kovar alloy packaging and a sapphire window, offering temperature resistance above 1000 °C and pressure resistance greater than 100 bar, allowing it to operate reliably in extreme environments (such as cryogenic liquid hydrogen conditions). This makes it highly suitable for integration into rocket engines as well as laser propulsion systems. The advancement of miniature laser technologies has made the implementation of liquid laser propulsion (LLP) systems in practical aerospace applications increasingly feasible.

6. Conclusions and Outlook

In this review, we briefly described the development history and the characteristics of liquid laser propulsion in Section 1. Section 2 reviewed the interaction between pulsed lasers and liquid propellants, focusing on the breakdown ionization mechanism in liquid media, the generation of laser plasma waves, and performance indicators of laser ablation propulsion. Section 3 introduced three types of propellants currently used in liquid laser propulsion technology: non-energetic liquids, energetic liquids, and liquid metals. We analyzed the propulsion performance generated by laser ablation of liquid propellants such as water, GAP, HAN, and ADN. Section 4 summarized the liquid laser propulsion prototypes and their performance. Next, we will outline the future directions and challenges facing pulsed LLP.

Laser propulsion is a new propulsion technology to drive spacecraft. It has the advantages of enabling small impulses, high specific impulse, large thrust span and easy adjustment, large payload ratio, and low launch cost. It demonstrates various potential applications in many fields and can be widely used in the launch of small satellites in low Earth orbit, debris removal in Earth orbit, laser-driven spacecraft for orbit transfer, and small satellite attitude and orbit control. Liquid propellants have aroused interest among scholars due to their unique advantages. The research results obtained in the laboratory still face significant challenges in practical applications. Therefore, we look forward to the following future research directions:

• The impulse coupling coefficient of liquid propellants is high, but their specific impulse is low. This is because the ionization rate of propellants is low under laser ablation, and another reason is that the sputtering of propellants is relatively severe. Therefore, it is necessary to study the coupling mechanisms between laser parameters such as pulse width, energy distribution, spot size, and liquid propellants and to design a working mode that optimizes the interaction between lasers and liquid propellants over time. Especially for energetic liquid targets, reasonable laser parameters should be designed to achieve the release of chemical energy.
• Research on composite propulsion targets, such as multiple types of targets and multiphase targets. For example, hexane gun targets and high-viscosity solutions have the propulsion characteristics of both solid and liquid targets, while atomized water droplets belong to composite targets composed of liquid and gas targets. Composite target materials can combine the performance advantages of various target materials and have good propulsion performance, which is an optional development direction for liquid target laser propulsion in the future.
• The plasma emission angle and velocity of liquid-phase metals or alloys during ablation are consistent with solid metals, enabling high specific impulse and easier flow control than viscous liquids. It is a highly promising liquid propellant.

At present, liquid laser propulsion technology mainly faces the following key technical challenges:

• Research on the mechanism of pulsed laser ablation of liquid propellants. The first consideration is the efficiency of laser energy deposition. An effective deposition of laser energy in the liquid can ensure that the laser effectively ablates the liquid. However, most liquid propellants have weak light absorption characteristics in the pulse width range of spaceborne lasers. Thus, the propellants require further doping, especially under high specific impulse and microscale conditions, where efficiency is crucial. In addition, the dissociation mechanism of liquid propellants by lasers is still not completely understood due to the complex chemical reactions and the variability of their states and phases. The synchronous release mechanism of laser energy and chemical energy of energetic propellants is crucial for improving the propulsion performance of liquids. The loading process of laser energy and the release process of chemical energy need to be synchronous, which helps prevent liquid from splashing out in the combustion chamber. Both energetic processes must quickly release high temperature and pressure to produce a good propulsion effect.
• Research on the design and supply technology of high-performance liquid propellants. Matching laser parameters through the design of liquid propellants is currently the most effective way to improve the performance of laser ablation liquid propulsion. Therefore, the new liquid propellants are a key research issue in LLP. In addition, the supply of liquid propellants and the coordinated operation of high-frequency lasers are also challenging technologies. The products of laser ablation are complex, and there are multiphase flows such as plasma, liquid, and gas, which can cause liquid backflow at the supply end and interfere with the supply state. Therefore, ensuring a stable supply of liquid propellant during laser ablation is a key technology for producing stable and efficient propulsion effects.
• Laser thruster’s integration and optimization. With the rapid advancement of microelectronics and micromachining technology, laser micro-thrusters are also progressing towards miniaturization and high integration. Developing laser micro-thrusters with compact size, low power consumption, reduced mass, and exceptional performance remains a significant challenge. The structural design of a micro-thruster involves the collaborative work of the laser module, power control module, and storage and supply modules. Additionally, considerations must include protecting the laser from contamination by ablation byproducts.

Overall, liquid target propellants exhibit high impulse coupling coefficients and low specific impulses, enabling precise thrust generation in the order of μN. This capability expands their applicability to ultra-static platforms, such as gravitational wave detection. Shortly, liquid propellant laser micro-thrusters are expected to achieve higher total thrust, presenting substantial potential for attitude and orbit control, space resource development, and deep space exploration using micro- and nano-satellites. Research should focus on conducting fundamental theoretical studies on LLP, addressing critical technological challenges, and rapidly transitioning from laboratory experiments to in-orbit verification on platforms such as micro- and nano-satellites and spacecraft. These efforts will help demonstrate the capabilities of laser propulsion technology and expand the practical applications of laser micro-thrusters for future space missions.