A Comprehensive Review of Application Techniques for Thermal-Protective Elastomeric Ablative Coatings in Solid Rocket Motor Combustion Chambers

Abstract

Elastomeric ablative coatings are essential for protecting solid rocket motor (SRM) combustion chambers from extreme thermal and erosive environments, and their performance is governed by both material composition and processing strategy. This review examines the main elastomer systems used for SRM insulation, including ethylene propylene diene monomer (EPDM), nitrile butadiene rubber (NBR), hydroxyl-terminated polybutadiene (HTPB), polyurethane (PU), silicone-based compounds, and related hybrids, and discusses how their rheological behavior, cure kinetics, thermal stability, and ablation mechanisms affect manufacturability and in-service performance. A comprehensive assessment of coating technologies is presented, covering casting, molding, centrifugal forming, spraying, automated deposition, and emerging additive-manufacturing approaches for complex geometries. Emphasis is placed on processing parameters that control adhesion to metallic substrates, layer uniformity, defect formation, and thermomechanical integrity under high-heat-flux exposure. The review integrates current knowledge on how material choice, surface preparation, and application sequence collectively determine insulation efficiency under operational SRM conditions. Practical aspects such as scalability, compatibility with complex chamber architectures, and integration with quality-control tools are highlighted. By comparing the capabilities and limitations of different materials and technologies, the study identifies key development trends and outlines remaining challenges for improving the durability, structural robustness, and ablation resistance of next-generation elastomeric coatings for SRMs.

1. Introduction

Combustion chambers of solid rocket motors are subjected to extreme thermal loads, with combustion products reaching high temperatures and elevated pressures, accompanied by pronounced erosive attack from entrained solid particles [1,2,3]. To ensure structural integrity under such conditions, internal thermal protection systems are essential, among which elastomeric ablative coatings represent a key solution. These coatings rely on a multimodal thermal-protection mechanism that integrates endothermic pyrolysis, mass transport, and the formation of a thermostable carbonaceous char layer [1,4,5]. The high elasticity of the polymer matrix enables these materials to accommodate thermomechanical stresses arising from fluctuations in temperature and pressure, thereby mitigating cracking and interfacial debonding of the insulation. The validity and effectiveness of this approach are supported by extensive structural and thermophysical data reported for elastomer-based ablators [1,6,7,8]. Significant advances have been made in formulations based on EPDM, NBR, HTPB, and silicone elastomers. However, these advances do not fully define the performance of the ablative layer. The final behavior also depends on the application route and processing conditions. Application routes influence thickness, porosity, adhesion, and stability during thermal-pressure cycling [9,10].

In recent decades, there has been a sustained increase in interest in elastomeric ablators for the internal insulation of SRMs. Existing reviews generally indicate that EPDM-based composites remain the technological benchmark, while alternative systems such as NBR, PU, and others continue to gain momentum [2]. For EPDM materials, several studies have demonstrated enhanced thermal stability and increased char yield through the incorporation of multiwalled carbon nanotubes (MWCNTs), along with notable improvements in ablative performance [11,12]. For silicone rubbers, intensive research efforts are directed toward reducing thermal conductivity through the use of hollow microspheres and aerogel fillers [13,14]. The behavior of such coatings is governed by the formation of three distinct functional zones: a carbonized char layer, a pyrolysis zone, and a virgin layer. This stratified behavior is extensively detailed in dedicated reviews and in specialized chapters addressing ablative materials and their testing methodologies [1,5].

Alongside formulation, recent studies increasingly focus on specific application routes and surface-preparation procedures, as these directly affect layer uniformity, adhesion, and process reproducibility. For the liner-insulation adhesive interface in SRMs, systematic studies have shown that surface energy, diffusion of low-molecular-weight species, and surface-modification approaches influence interfacial strength [9]. Recent work further reports improved adhesion following multifunctional activation of the insulation surface [15]. Beyond classical manual techniques (sheet layup, vulcanization), the field has adopted mechanized methods such as spraying and thermal spraying, extrusion, and centrifugal molding, as well as automated approaches including prepreg and tape winding. For nozzle-component winding, control of compaction force and defect formation has been demonstrated [16], while modern methodological reviews are available for spray-based deposition [17]. In recent years, additive approaches within polymer and composite systems have expanded rapidly, including those developed for aerospace applications and for tailoring the thermal-conductivity anisotropy in printed composites [18,19]. For EPDM-based ablators, recent studies have outlined a technological pathway toward layered architectures (additive and hybrid manufacturing) that improve thermal conductivity while preserving ablative performance, directly linking structural design and forming method to overall thermal efficiency [20]. In parallel, hybrid processes (such as multimaterial extrusion and co-extrusion) are advancing and provide broader control over geometry and property gradients [21]. Despite this progress, most publications concentrate either on formulation or on isolated deposition methods, whereas comprehensive comparative studies of ablative layer manufacturing technologies remain relatively limited [1,2,5]. This makes it challenging to determine an optimal solution that meets protective performance requirements alongside manufacturability, scalability, reparability, and the operational constraints of SRMs. The present study aims to address this gap.

2. Requirements and State of the Art

Gas temperatures in the combustion zone can reach ~3000 °C. The chamber wall experiences heat fluxes of 2.3–3.0 MW/m². Laboratory tests often apply higher loads, reaching up to 4.5 MW/m² to assess coating durability [22,23,24]. The operating pressure is typically ~8 MPa [25]. Under these conditions, the thermal-insulation layer must ensure controlled ablation, promote the formation of a stable carbonaceous char, and retain mechanical integrity under thermal cycling [1,5,26].

The functional behavior of an ablative coating depends on the thermal stability of the matrix, filler architecture, and the ability of the formed char to withstand heat-flux and erosion. Key requirements include high thermal stability, low linear ablation rate, strong adhesion to the motor case, low thermal conductivity, resistance to vibrational and mechanical loading, manufacturing feasibility, and compatibility with chamber geometry [1,2,6,9]. Elastomeric materials are commonly employed as matrices due to their ability to generate a carbonaceous char while maintaining good processability, including EPDM, HTPB, NBR, PU, and silicone rubbers. Their comparative characteristics are summarized in

Table 1. Thermally resistant elastomeric matrices used in ablative coatings.

Matrix	Decomposition Temperature (°C)	Ablation Exposure Temperature (°C)	Tensile Strength (MPa)	Elongation %	Hardness Shore A	Thermal Conductivity, W·m−1·K−1 at 25 °C	Ref.
EPDM	~460	~2500	7–20	300–600	50–80	0.20–0.30	[27,28,29,30,31,32,33,34]
HTPB	~445	~2000	0.5–5	400–800	40–80	0.18–0.25	[35,36,37,38,39,40,41,42,43]
NBR	~440	~1800	10–25	150–400	60–90	0.20–0.25	[44,45,46,47,48,49,50,51,52,53]
PU	~360	~2200	10–40	200–700	70–95	0.18–0.25	[54,55,56,57,58,59,60,61,62,63,64,65,66,67]
Silicone rubbers	~500	~2100	5–12	200–900	20–80	0.20–0.25	[68,69,70,71,72,73,74,75,76,77,78,79]

.

Based on Table 1 and the reported literature, EPDM is traditionally regarded as the most effective matrix for SRM internal insulation, whereas PU serves as a more technologically versatile alternative due to its processability and compatibility [1,2,54,55,56]. For EPDM-based systems, numerous strategies have been demonstrated to enhance thermal stability and ablative resistance through formulation modifications, including nanofillers, MWCNTs, and fibrous reinforcement [11,12,80]. For silicone systems, active research is directed toward reducing thermal conductivity and improving thermal protection via the incorporation of aerogels and hollow microspheres, as well as through the development of organosilicon ablators [10,13,14,68]. For HTPB, extensive data have been accumulated on decomposition kinetics and its use as a liner and binder [35,36,81]. In contrast, NBR-based systems are often limited by filler compatibility and vulcanization conditions, which directly affect adhesion and interfacial bonding [44,45].

To enable quantitative cross-study comparison of material systems and deposition routes, this review uses a unified set of SRM-relevant reporting metrics extracted from the source studies. Primary metrics include ablation rate, recession depth, the demonstrated heat flux range or heat flux limit, and thermal conductivity under thermal load when reported. Each extracted value is reported together with the boundary conditions, including heat flux, chamber pressure, exposure time, test configuration, and whether the flow is particle-laden. When boundary conditions are missing or not comparable across sources, the value is treated as a qualitative indicator and flagged as “test conditions not directly comparable” in

Table 2. Experimental conditions and ablation performance metrics of polymer-based thermal protection materials under high-heat-flux testing.

Material System	Process Route	Geometry	Test Type	Heat Flux, MW/m2	Time, s	Ablation Metric	Ref.
EPDM + Kynol + SiO2 + perlite	Hot-press cure	15 × 15 × 15 mm	OAT	5.0	40	Mass loss and thermal response, LAR N/A	[29]
EPDM insulator	N/A	80 × 40 × 10 mm	Ground firing	N/A	5.51 and 4.65	Recession 2.78 mm and 6.71 mm. LAR 0.505 mm/s and 1.443 mm/s	[86]
Silicone	Laminate panel	10.2 × 10.2 × 1.27 cm on steel	SRM flame impingement	7.094 and 12.48	12	Mass loss and peak erosion, LAR N/A	[87]
Silicone + SiO2 + carbon fiber	Molded cure	30 × 10 mm	OAT	4.0	30	LAR 0.064 mm/s best	[88]
HTPB	Compounding and cure	100 × 100 × 3 mm sheet	OAT	N/A	N/A	LAR 0.652 mm/s and 0.431 mm/s	[89]
NBR + silica	Compounding	N/A	OAT	2.5	15	Relative LAR reduction: 29–41%	[90]
PU + carbon fiber + biomass powder	Cast or mold	100 × 7 mm	OAT	N/A	30	LAR 0.401 to 0.068 mm/s	[91]

Note: OAT, oxy-acetylene torch test; LAR, linear ablation rate.

[1,82,83,84,85].

Ablation of elastomeric insulation in SRM-relevant environments is governed by coupled heat transfer, polymer decomposition, transport of pyrolysis products through porous char, and mechanically driven removal [92,93]. Under SRM-type chamber conditions, temperature, pressure, wall heat flux, and particle loading can shift the relative contributions of thermochemical ablation and mechanical erosion, so these boundary conditions and the test configuration should be reported with performance data to support cross-study interpretation [82,94,95]. In aluminized systems, sub-scale tests near the nozzle exit report micron-scale alumina particle size distributions, and some datasets report distributions centered around 2–3 μm depending on propellant and sampling approach [96,97].

For predictive SRM design and cross-study interpretation, mechanistic statements should link measurable inputs and boundary conditions, including heat flux, pressure, gas composition, particle loading, and test configuration, to outputs such as recession depth and ablation rate [82,94]. A practical mechanistic framework can be formulated in terms of four interacting subprocesses that can be parameterized from experiments and used in coupled material-response models [98].

First, pyrolysis kinetics sets decomposition onset, mass-loss and gas-generation rates, and an endothermic sink term in the transient energy balance of charring ablators, so material-response calculations typically require kinetic descriptors such as Arrhenius parameters or model-free isoconversional functions extracted from thermal-analysis data [99,100,101,102]. Second, char formation and evolution control insulation effectiveness through char yield, microstructure, and permeability, which jointly govern apparent thermal transport, structural continuity, and pyrolysis-gas transport through the porous layer [101,103]. Pyrolysis-gas outflow into the boundary layer can reduce convective heat transfer by surface blowing, but the magnitude is conditional on char permeability and pressure-driven internal flow [93,98,104].

Third, gas–solid interaction at the hot surface includes heterogeneous oxidation and related reactions that consume carbonaceous char and contribute to recession [105,106]. Because recession driven by surface reactions is sensitive to local pressure and species composition, cross-facility generalization is only valid when these boundary conditions are reported together with the performance data [94,107]. Fourth, thermo-chemo-mechanical coupling links thermal gradients, pyrolysis-gas-driven pore pressure, and evolving char stiffness to stress concentrations that may drive cracking, spallation, and delamination [108,109]. Predictive SRM models therefore benefit from coupling thermal and kinetic inputs with temperature-dependent mechanical properties of both the virgin material and the char, plus an explicit interface damage or debonding treatment for critical interfaces such as propellant–insulation or insulation–structure [82,110,111]. This mechanistic structure can be used to label which reported performance values are transferable across studies and which remain conditional on the test environment [98].

Alongside formulation, significant attention is given to how coatings are applied. Different application routes influence achievable thickness, layer uniformity, adhesion, and porosity, which together define the quality of the protective layer [1,2,6].

Table 3. Requirements and processing considerations for elastomeric ablative coatings.

Parameter	Primary Requirements	Ref.
Adhesion strength (insulation, case, propellant)	≥0.7–1.0 MPa as an acceptable baseline, up to 2.0–2.6 MPa with optimization, in some cases, reaching 10–15 MPa	[112,113,114,115,116]
Surface preparation	Metal surface roughness 1.5–3.0 μm	[117,118,119]
Coating thickness (insulation)	1–10 mm	[92,120,121]
Ablation performance	Ablation rate 0.015–0.2 mm/s, density 0.8–1.5 g/cm3, thermal conductivity 0.1–0.5 W/m·K, high specific heat 1000–2100 J/kg-K	[2,20,92,122,123]
Cure profile and thermal gradient control	Degree of cure ≥ 0.90 through thickness, avoid surface overcure and core undercure. For a 20 mm slab, undercure to reversion risk window 44–77 min at 150 °C and ≈4 min at 160 °C. Through thickness cure index ≤ 0.0142, optimized ≈0.011. Thermocouples and validated heat transfer plus cure kinetics model	[124,125,126,127]

summarizes the key requirements and processing considerations relevant to selecting an appropriate manufacturing route.

The parameters listed in Table 3 reflect the operational constraints that any insulation system must satisfy. They describe adhesion, porosity, thickness range, and the acceptable application temperature. Additional criteria are compatibility with the application temperature, suitability for cylindrical or conical geometries, a preference for automated routes, and sufficient reparability and resistance to thermomechanical loads. Together, these factors ensure the reliability of the coating under combustion chamber operating conditions.

A comprehensive evaluation should take into account both the material formulation and the practical constraints of the selected processing route. It should consider thermal stability, elasticity, and material compatibility. It should also account for technological and structural constraints. This combination allows for the required insulation and mechanical performance to be achieved. The chosen processing route affects uniformity of the layer, interfacial bonding, defect formation, and the stability of the coating during operation.

Adhesion durability of elastomeric ablative coatings depends not only on surface preparation and primer chemistry, but also on residual stresses accumulated during cure and subsequent thermal exposure. These stresses can drive cracking and interfacial debonding even when initial adhesion appears adequate [128,129,130]. Cure-induced volumetric shrinkage in constrained substrate-bonded polymer layers generates in-plane tensile stresses and promotes interfacial shear stresses. The magnitude and evolution of these stresses are strongly process dependent [129,131,132].

Under service-relevant thermal transients and thermal cycling, thermal expansion mismatch between the metallic case and polymer insulation adds additional interfacial shear and peel components. This increases the propensity for microcracking and progressive debonding, especially near stress concentrators such as free edges and geometric discontinuities [133,134,135]. Interfacial defects associated with poor wetting and bondline porosity reduce fatigue resistance and interfacial fracture energy, accelerating debond growth under cyclic thermomechanical loading [136,137]. Accordingly, cure schedule optimization, including multi-step cure strategies shown to reduce residual stress, and routine verification of interface integrity using nondestructive evaluation and monitoring methods should be treated as design-relevant variables for SRM insulation systems [138,139,140].

The next section further examines existing deposition methods for ablative coatings, including their processing features and applicability under thermal and mechanical loading.

3. Methods for Applying Ablative Coatings

To systematize current approaches in this field, a review and comparative analysis of ablative coating fabrication methods reported in recent scientific studies was carried out. The analysis shows that several factors influence the choice of a processing route, including the type of polymer matrix, allowable temperature, and required deposition precision [6,45,141,142,143,144,145,146,147,148]. Manual application methods, despite their simplicity and low cost, exhibit limited reproducibility and poor thickness control, which restricts their applicability in regions subjected to high heat fluxes [1,2,5,17,149].

Mechanized technologies, including molding, hot pressing, and prepreg winding, remain the most widely used in industrial practice due to their high reliability, strong automation potential, and the consistent performance of the resulting coatings. However, these methods require sophisticated equipment and precise control of viscosity and curing, and they exhibit notable limitations when applied to components with complex geometries [1,2,6,16,17,45,141,142,143,144,145,146,147,148,149,150,151,152,153].

Additive and hybrid manufacturing approaches have become viable alternatives for forming complex insulation geometries, enabling precise thickness control and multilayer architectures. These approaches enable the fabrication of graded and multilayer structures, improve the precision of coating-thickness and geometry control, and allow the thermal-protection layer to be integrated with structural elements of the combustion chamber without the need for subsequent machining. Several studies demonstrate successful implementations of hybridized processing routes for producing multifunctional coatings that combine structural and thermal-protection capabilities [4,10,17,18,20,21,149,154,155].

Based on a synthesis of the literature and an engineering-focused applicability assessment, this review examines nine key methods selected according to their technological maturity, adaptability to rocket-motor requirements, operating-temperature range, and potential for integration with SRM components. These methods are grouped into three categories (Figure 1), reflecting the evolution of ablative-coating deposition technologies from traditional to advanced approaches [1,2,5,6,17,149].

The presented classification illustrates both traditional and modern technological approaches that enable the required functional performance of the protective layer. Each method shown in the scheme offers potential applicability to different regions of an SRM structure, depending on geometry, operating conditions, and processing constraints. In the following subsections (Section 3.1, Section 3.2 and Section 3.3), each method is examined in detail with emphasis on thermal resistance, reproducibility, achievable layer thickness, geometric complexity, manufacturing flexibility, and cost efficiency, accompanied by a comparative assessment based on published experimental data [1,2,5,6].

3.1. Manual Application Methods

The simplest methods to implement are manual application methods, which use brushes, rollers, or spray guns to deposit the coating. Brush application is used primarily for local repair, touch-up operations, or coating surfaces with complex geometries. Although the method is considered relatively simple, it is characterized by limited reproducibility: the resulting layer thickness depends strongly on operator skill, and regions of nonuniform material distribution may occur, along with sagging and cracking in areas of excessive application. Such operations typically employ thixotropic or paste-like materials with high wettability and a stable internal structure to prevent sagging, flow, or phase separation during storage [1,2,5,6,45,156].

Brush application is particularly effective for applying epoxy-based coatings to porous or irregular substrates. Although the method allows surface irregularities to be corrected, its productivity is low, and it is therefore employed mainly for small-area applications [157].

Roller application provides better control over layer thickness. In laboratory tests, a conventional roller used for epoxy resins produced relatively high thickness variability, ~214 µm, whereas a more recent roller system with material feed from an airless pump reduced material losses and enabled more uniform coating deposition. Therefore, in repair scenarios, a conventional roller is often chosen, as it provides a practical compromise between application speed and coating uniformity [158,159].

Spray application is increasingly used for forming uniform thermal-protection layers. NASA specifications for Type B silicone ablative coatings indicate that the material must be applied exclusively by spraying. The components are mixed and delivered through an airless spray system, enabling the deposition of a 3–5 mm layer without prior surface priming [81,160]. A European study proposed an external thermal-protection coating based on silicone rubber that was applied using a spray gun equipped with a 2 mm nozzle and operated at 0.4–0.8 MPa. The resulting 3–5 mm coating, cured at room temperature, withstood high-temperature exposure and maintained a tensile strength of at least 3 MPa [161]. Spray deposition provides the most uniform coating among manual methods, especially over large surface areas, but requires specialized equipment and controlled environmental conditions (humidity and temperature).

The adhesion quality of an ablative coating to its substrate depends strongly on standardized surface preparation, as illustrated in Figure 2. The internal surface of the motor case is first cleaned of contaminants and oxides through sanding or abrasive treatment, followed by degreasing with organic solvents such as acetone or alcohols. This procedure provides the required cleanliness and microscale roughness for subsequent bonding, typically corresponding to Ra 1.5–3.0 μm. A primer containing chemically active groups that promote bonding between the metal and polymer is then applied to the dry surface, followed by intermediate drying at 60–80 °C to remove volatiles [9,15]. The ablative material is then applied manually. The final stage involves thermal curing under prescribed conditions, ensuring the complete formation of a robust thermal-protection lining [45,141,142,162].

Different formulations are used for each manual application method and are primarily distinguished by their viscosity, service-temperature range, and curing requirements. For brush or roller application, thixotropic, paste-like materials with a stable internal structure are preferred [6,45]. Spray application relies on aerosolizable formulations and therefore imposes stricter requirements on ventilation, surface preparation, and process control [17,149].

Table 4. Curing regimes of polymer matrices for manual application in rocket thermal insulation.

Materials	Curing Temperature, °C	Curing Time	Applied Method	Yield Stress or Thixotropy	Off Window Defects	Ref.
EPDM compound	150/25	120 min/7 days	Brush	Min 1 Pa, target 0.6–1.8 Pa, recovery seconds to minutes	Sagging, runs, brush marks, poor leveling, bubbles	[1,2,45,163,164,165,166,167,168,169]
Silicone paste	200/20	30 min/room temperature	Roller	40–212 Pa, 539–3385 Pa, thixotropy needed	Slump, sag, ribbing, streaks, pores	[1,6,142,170,171,172,173,174,175,176,177,178,179,180,181]
PU system	80 or self-vulcanization	60–120 min	Spray	Target 1.2–1.9 Pa, leveling 30–300 s	Runs, sag, orange peel, dry spray, clogging	[6,28,141,163,182,183,184,185,186,187,188]
NBR compound	160/25	60 min/24 h	Spray	Target 0.44–1.38 Pa, thick film 2.2–2.3 Pa	Sagging, runs, cobwebbing, orange peel, clogging	[27,45,189,190,191,192,193]

summarizes the main types of polymer matrices used across these manual routes, and is extended to connect each matrix–process pair to the expected rheology window trend and the dominant defect risks when the formulation is off-window. In practice, deposition quality in manual routes is governed by three rheological descriptors measured at the application temperature: viscosity, an indicator of yield stress or thixotropy, and gel time or pot life. Low viscosity improves wetting and spray atomization but increases the risk of sagging and thickness loss on vertical surfaces, while high viscosity improves shape retention yet raises the probability of entrapped air, incomplete leveling, and local void formation. Likewise, an excessively short pot life increases the likelihood of nozzle clogging and inter-pass discontinuities, whereas an excessively long pot life can promote filler settling and compositional inhomogeneity.

Figure 3 presents a visual comparison of the performance characteristics of different polymer systems used in thermal-protection coatings [1,2,6]. The diagrams illustrate two key parameters: the material viscosity at 25 °C, which determines ease of application and forming [10,45,189], and the operating-temperature range, which reflects thermal stability under combustion-chamber conditions [54,55,56].

As shown in diagram (a), the PU system exhibits the lowest viscosity, making it suitable for extrusion and 3D-printing processes, although its usable thermal range is comparatively limited [6,54,55,56]. The NBR compounds demonstrate the highest viscosity, which restricts their applicability in deposition methods that require high feed rates [45]. Diagram (b) indicates that NBR and silicone-based coatings provide the widest operating-temperature range, whereas PU and EPDM formulations are suitable primarily for moderate thermal environments [6,10,189]. These differences must be taken into account when selecting a matrix for a specific application method and operational scenario [1,2,6].

Manual application methods remain relevant for local repairs, initial material evaluation, and surfaces with limited accessibility. Despite their dependence on operator skill, they offer flexibility in applying a wide range of elastomeric formulations and enable adaptation to complex geometries. When higher productivity, layer uniformity, and deposition precision are required, mechanized methods become preferable, and these are discussed in Section 3.2 [1,2,5,6].

3.2. Mechanized Methods

Modern mechanized methods for producing thermal-protection coatings provide high reproducibility, uniform material distribution, and precise control over processing parameters. Depending on the combustion-chamber design, the required coating thickness, and the material type, several approaches are employed, including automated spraying, molding (including centrifugal casting), hot pressing, and vulcanization [1,2,6,17]. These methods differ in the required equipment, their application domains, the complexity of surface preparation, and the characteristics of the resulting layer. A comparative analysis of these technologies is presented in

Table 5. Comparison of mechanized methods for applying thermal-protection coatings.

Parameter	Automated Spraying	Formation in Shape	Hot Pressing	Vulcanization
Application Area	Large surfaces, internal liners	Axisymmetric components, chambers	Pressed panels, inserts	Complex shapes, hollow components
Layer thickness	0.1–0.5 mm	up to 10 mm	1–5 mm	1–5 mm
Equipment	Robotic systems	Centrifugal machines	Molds, vulcanizers	Ovens
Material Types	PU, Polydimethylsiloxane, PF (resol)	EPDM, PF, silicones	EPDM, NBR	Room and high temperature vulcanizing materials, PF, PU
Features	Requires viscosity control and degassing	High adhesion, requires precise casting	High density and stability	Suitable for complex geometries
Rheology control sensitivity	Very high, atomization depends on viscosity	High, filling, and venting limited by viscosity and pot life	Medium, press flow set by compound plasticity	High, window set by scorch time and cure time

Note: Phenol formaldehyde (PF).

[149,194,195,196,197].

Mechanized routes impose narrower rheology windows than manual application because deposition rate and quality are controlled by equipment parameters. In automated spraying, viscosity and pot life govern atomization stability, layer build, and the onset of defects such as dry spray, overspray porosity, and sagging. In molding and centrifugal casting, viscosity and pot life govern mold filling and degassing efficiency, which control trapped air and bulk porosity. In hot pressing, viscosity and shear stability govern flow under pressure and consolidation, influencing void closure and interlayer bonding. Accordingly, Table 4 and Table 5 are extended to map viscosity, yield stress, or thixotropy, and gel time or pot life to route specific defect formation thresholds and practical quality control indicators.

In recent decades, considerable attention has been devoted to the development and refinement of methods for forming ablative coatings for the combustion chambers and nozzle assemblies of rocket engines [5]. Mechanized technologies are regarded as the most promising, as they provide high reproducibility, geometric control, and resistance to thermomechanical loads [198]. The literature describes various approaches to applying and forming protective layers based on thermosetting polymers, elastomers, and phenolic systems [2,6]. The following sections present the principal technological methods most commonly employed in the fabrication of thermal-protection elements for rocket systems, with consideration of their practical applicability to specific engine designs [1].

Automated Spraying: This approach is based on depositing the composite material as an aerosol stream using stationary or robotic spraying systems. It is widely employed for treating large or complex-profile surfaces where coating uniformity and minimal defect formation are critical. Automated spraying is particularly effective for applying internal insulation layers, external thermal-protection shells of solid rocket boosters, and for repairing or reinforcing regions subjected to high thermal loads. The method enables controlled coating thickness, provides reliable adhesion to the primed substrate, and is well-suited for multistage deposition with intermediate curing between layers. Epoxy, phenolic, and silicone suspensions or pastes are typically used in such processes [6,17,149].

Molding in closed forms is used to produce geometrically precise and dense thermal-protection components in cases where high thickness, structural integrity, and minimal porosity are required. A plastic or liquid formulation is placed into a press or casting mold, followed by thermal curing. This method is employed in the fabrication of end insulators, throat-insert components, sealing rings, and segments of the internal insulation liner [150,198].

Centrifugal Casting, as a variant of molding, is employed for axisymmetric components such as insulation sleeves, internal cylindrical liners, and nozzle sections. The rotation of the mold enhances material uniformity and density, improves adhesion to the substrate, and results in protective layers with reduced porosity, which is particularly characteristic of phenolic and silicone matrices reinforced with fillers [199,200,201].

Hot pressing compacts and cures the composite material under controlled pressure and temperature, forming a monolithic, thermally resistant structure. This method is effective for pre-prepared components such as sleeves, spacers, and plates, as well as for multilayer assemblies providing combined protection (thermal barrier plus erosion resistance). In studies on ablative composites, including phenolic systems, hot pressing is employed for the fabrication of liners and nozzle-throat components [198,202,203,204].

The method is particularly suited for processing prepregs based on phenolic resins, silicones, or polyimides, as well as for vulcanizable rubber-based composites containing carbon or ceramic fillers [5,203,204,205].

Vulcanization is employed to cure thermosetting and elastomeric compositions (EPDM, NBR, silicones), which are widely used as internal insulation in SRMs. The process can be carried out under hot-press conditions, in an autoclave or oven, or at ambient temperature. Typical applications include elastic damping and thermally resistant layers within the combustion chamber, propellant-case liners, and end seals. Vulcanization enhances the material’s thermo-oxidative stability, its capacity to absorb heat flux, and its ability to mitigate thermally induced stresses [6,27,28,45,141,142,189].

Schematic representations of the aforementioned coating fabrication methods are shown in Figure 4, highlighting the main stages and physical processes associated with each approach.

The selection of a matrix material for a thermal-protection coating depends on the engine’s design features and the chosen application technology. Each processing route, whether automated spraying, molding, hot pressing, or vulcanization, imposes specific requirements on the viscosity, thixotropic behavior, uniformity of distribution, and curing conditions of the polymer formulation.

In practice, thermosetting and elastomeric binders are most commonly used, including phenol-formaldehyde resins, PU and silicone formulations, as well as EPDM and NBR-based elastomers. These systems differ in thermal stability, chemical resistance, and process compatibility with various fillers, including carbon fibers and ceramic powders. Such differences influence layer uniformity, adhesion quality to the substrate, and material resilience under thermomechanical loads in critically stressed regions, such as the combustion chamber, nozzle area, and internal liners of solid propellant rocket motor cases [1,2,6,13,14,27,28,45,54,55,56,68,143,144,145,189,206,207,208].

Table 6. Viscosity and curing regimes of polymer matrices used in rocket thermal insulation.

Materials	Curing Temperature, °C	Curing Time	Applied Method	Rheology (η at 25 °C), Pa·s	Pot Life, Gel, Cure Time, min	Ref.
EPDM compound	150–180	30–60 min	Vulcanization, pressing	Mooney 25–60 MU	ts2 0.5–54 min, t90 2–118 min	[2,27,28,45,141,142,209,210]
Silicone system	120–180	10–30 min	Spraying, vulcanization	29.6–48.3 Pa·s at 600 s−1	full cure ~4 min 170 °C	[10,13,189,211,212,213]
PF (resol)	130–180	20–40 min	Pressing, casting	0.275–0.600 Pa·s	gel 6–10 min 130–150 °C	[143,144,145,203,204,214]
PU system	60–120	10–40 min	Spraying, molding	NA Mooney 28–48 MU	t90 3–17 min 160 °C	[6,54,55,56,215,216,217]
NBR compound	150–180	30–60 min	Vulcanization, pressing	0.03–2.0 Pa·s	pot life up to 420 min, fast gel < 0.33 min	[44,45,141,142,218]

Note: t₉₀ is the optimum cure time, ts₂ is the scorch time, and Mooney ML (1 + 4) is a compound processability viscosity index. All are strongly dependent on measurement temperature and the test method.

presents the main types of matrices used across various mechanized application methods.

The visualized characteristics of various elastomeric formulations used in ablative coatings are presented in Figure 5. Diagram (a) illustrates the viscosity of the polymer systems at 25 °C as a function of the mechanized application method employed (automated spraying, molding, centrifugal casting, hot pressing, vulcanization). Diagram (b) shows the corresponding service temperature range for these systems, allowing their thermal stability to be compared under conditions relevant to thermal-protection coatings. The visualized characteristics presented for EPDM [6], silicone [10], PF (resol) [203,204,214], polyurethane [54,55,56], and NBR-based compounds [45].

Diagram (a) demonstrates that the viscosity of the formulations varies significantly depending on the matrix type and the selected application method. The lowest viscosities are observed for PU and silicone systems during automated spraying, whereas the highest values occur for NBR-based formulations under hot pressing, imposing limitations on material feed and necessitating careful control of rheological properties [45,54,55,56,150,189].

Diagram (b) indicates that PF resol formulations exhibit the highest thermal stability range. In inert atmospheres, they remain stable up to ~400 °C before the onset of significant decomposition and chemical reactions, making them most suitable for extreme operating conditions. PU and NBR-based systems are limited to roughly 300 °C and should be selected with consideration of the specific application area [45,54,55,56,143,144,145,202,203,204,214].

Mechanized methods for applying thermal-protection coatings offer a wide range of technological solutions for producing ablative layers with varying thicknesses, densities, and geometries. Their applicability is determined by component geometry and matrix properties, including viscosity, thixotropy, and curing conditions. A synthesis of the data indicates that phenolic systems provide the broadest operating-temperature range (for both protective coatings and press-molded composites), whereas PU and NBR systems are more suitable for moderate-temperature applications. For high-viscosity NBR formulations, sensitivity to feed conditions increases, particularly under hot-press processing. Method selection should consider operational requirements, geometry, available equipment, and curing conditions [1,2,6,17,149].

Modern requirements for coatings with complex geometries and their integration with structural components necessitate more flexible approaches. This has driven the active development of additive and hybrid technologies, which are discussed in the following Section 3.3 [18,21].

3.3. Additive (3D Printing) and Hybrid Methods

In recent years, additive and hybrid technologies have been increasingly employed for the fabrication of thermal-protection coatings in rocket-engine components. They provide precise geometric control, enable property gradients, and allow the processing of complex shapes without the need for subsequent machining [21,219]. The literature reports approaches based on extrusion 3D printing, layer-by-layer prepreg layup, and co-extrusion, which enable the fabrication of multilayer elastomeric coatings with designed thermal stability and ablative resistance [1,2,6]. A comparison of the key additive and hybrid methods is presented in

Table 7. Comparison of additive and hybrid methods for applying thermal-protection coatings.

Parameter	Extrusion 3D Printing	Layer-by-Layer Prepreg Layup	Co-Extrusion	Ref.
Application Area	Internal surfaces of chambers, complex geometries	Axisymmetric components, cylinders, shells	Multilayer coatings, thermal protection with property gradients	[4,5,21,198,219,220,221,222]
Layer Thickness	0.1–2 mm per pass	0.2–0.5 mm per pass	1–3 mm (entire structure)	[4,5,21,219,223,224,225]
Required Equipment	Ablative material thermal-protection system, 3D printer with extruder	Winding machines, heated chambers	Extrusion die with multi-channel feed	[4,5,21,198,219,221,222,226]
Material Types	EPDM, Polydimethylsiloxane, room-temperature vulcanizing silicones	PF resins, epoxies, bismaleimide resin	Multi-component formulations	[4,5,20,21,198,219,223,225,227]
Features	Suitable for small-batch production	High strength and thermal stability, requires curing	Simultaneous formation of functional layers without assembly	[4,5,21,198,219,220,228,229]

.

Extrusion 3D printing: Material is deposited layer-by-layer through a nozzle, enabling the formation of coatings with complex geometries without the need for tooling. Both thermoplastics and reactive pastes cured in situ are used. Advantages include automation, precise thickness control, and the ability to integrate the coating with combustion chamber components [21,219]. For reactive formulations, rheological properties and curing conditions are critical to prevent porosity [21,219].

Extrusion-based 3D printing (Figure 6) enables tool-free, layer-by-layer deposition through a nozzle to fabricate complex geometries from rheology-tailored inks or pastes [230,231,232]. Depending on the feedstock and process route, material extrusion may involve thermoplastic deposition or reactive formulations that solidify via in situ gelation or curing [233,234]. Dimensional stability and interlayer quality depend on coordinated control of volumetric flow rate, nozzle travel speed, and layer height, as well as toolpath-related bead packing, which also affects void formation [235]. For reactive systems, the printable time window is additionally limited by pot life and cure kinetics, requiring simultaneous tuning of rheology and curing conditions to prevent shape collapse and to reduce porosity and interlayer voids [234,236,237].

In a recent study, an experimental small SRM chamber liner was fabricated using selective laser sintering from polyamide-12 reinforced with a filler. During testing, the material exhibited stable ablative behavior. The ablation rate was approximately 0.2 mm/s and increased toward the nozzle region [18,19,219].

The main limitations of the technology are the restricted selection of compatible polymers and the need for precise tuning of their rheological properties. Thermoplastics generally exhibit lower thermal resistance compared to thermosetting matrices. The use of reactive pastes requires multi-axis robotic deposition systems and carefully controlled curing conditions to prevent porosity formation [21,143,144,145,147,219].

Nevertheless, the development of 3D technologies offers the potential for broader adoption of this method in the fabrication of ablative coatings by reducing labor intensity and tooling costs [18,21].

Layer-by-layer prepreg layup: A classical and widely used industrial method for fabricating ablative coatings involves the layup of fiber-reinforced prepreg layers impregnated with a polymeric binder. Both woven and nonwoven fibers (carbon, aramid, glass, etc.) are used as reinforcing components, pre-impregnated with phenol-formaldehyde, epoxy, or elastomeric matrices [5,146,147,148,198,202,203,204,214].

Figure 7 illustrates the layer-by-layer prepreg layup process, which involves sequentially placing layers with a specified fiber orientation onto the internal surface of the chamber or nozzle, followed by thermal treatment in an oven, autoclave, or in situ. Through chemical curing (cross-linking), the polymeric binder forms a robust char and stabilizes the reinforcing structure, providing thermal resistance to the coating [1,5,143,144,145].

Since the mid-20th century, the layer-by-layer prepreg layup method has been actively employed in the fabrication of nozzle-block and SRM chamber liners. The resulting fiber-reinforced composites exhibit high mechanical strength and thermal stability [5,198,205]. For example, an EPDM-aramid fiber combination exhibits a linear ablation rate of approximately 0.01–0.1 mm/s and high resistance to prolonged heat fluxes [11,80].

However, the method is not without limitations. Manual layup requires a high level of skill, and the use of adhesive formulations increases the risk of defects, including thickness variations and the presence of interlayer joints. Additionally, curing of thick-walled components is accompanied by shrinkage and gas evolution, necessitating strict control of temperature profiles and degassing procedures [5,6,9,198].

Current developments in this method include automation (tape-laying machines, robotic manipulators) and the design of new binders with mild curing profiles that minimize porosity [16,142,147,151].

Co-extrusion is a method in which multiple polymer formulations are simultaneously extruded through a common die, producing a multilayer structure with tailored functionality. In rocket thermal-protection systems, this approach enables the integration of ablative and insulation layers within a single processing cycle [4,21].

The co-extrusion process, illustrated in Figure 8, is based on the principle of forced multilayer formation. Polymer streams, such as different EPDM-based formulations with fillers, are separated within the die, repeatedly layered, and exit the die as a consolidated multilayer ribbon. The resulting preform is then cooled and stabilized through vulcanization or matrix curing [4,21,45,143,147].

The advantages of this method include high uniformity, precise control of individual layer thickness (on the order of tens of micrometers), and improved overall performance of the coating [4,21]. In one experiment, a multilayer EPDM composite with 40 layers exhibited a back-face heating temperature 20% lower (76 °C versus 96 °C) than a homogeneous material, while showing a comparable level of erosion [4,20].

However, the technology is currently limited in terms of achievable component geometry, with production generally restricted to flat or cylindrical panels that require subsequent assembly, while ensuring uniform through-thickness vulcanization without interlayer defects remains a critical challenge. In the future, as equipment and feed control algorithms advance, co-extrusion could be adapted for forming more complex geometries, including cylindrical liners with graded structures, or for direct deposition onto the combustion chamber surface [4,18,21,45].

One of the key factors determining the suitability of a specific matrix in additive and hybrid thermal-protection coating methods is the curing parameters. The temperature and duration of thermal or chemical polymerization directly affect adhesion, structural integrity, and the thermal resistance of the final coating. To ensure process compatibility with various methods (extrusion, prepreg layup, co-extrusion), it is essential to consider the matrix’s temperature sensitivity and its ability to form a uniform structure during multistage deposition. A summary of the curable systems used in elastomeric thermal-protection coatings is presented in

Table 8. Curing regimes of different matrix types used in rocket thermal insulation.

Material	Curing Temperature, °C	Curing Time	Ref.
PF (resol)	150–180/25	60–120 min /7 days	[143,145,204,238,239,240]
Epoxy resin	120–180	2–4 h	[146,147,241,242]
Bismaleimide resin	180–220/post-curing at 230	3–5 h	[243,244,245,246,247]

.

The data indicate that PU and epoxy systems exhibit the lowest curing temperatures, making them compatible with layer-by-layer deposition and low-temperature extrusion methods [6,147]. At the same time, phenol-formaldehyde resins and, in particular, polyimide formulations require higher curing temperatures (up to 230 °C) and dwell times of 5–7 h, which limits their use in rapid and automated processing [143,244]. When selecting a matrix, it is important to consider not only the operating temperature but also the thixotropic behavior, resistance to shrinkage, and the ability to cure uniformly under multilayer assembly conditions [1].

To compare the processing characteristics of different matrix types used in additive and hybrid coating fabrication methods, comparative graphs are presented in Figure 9. Diagram (a) shows the viscosity of prepregs at 25 °C, indicating their suitability for feeding, extrusion, and uniform material distribution [248,249,250,251]. Diagram (b) illustrates the operating-temperature range of the resulting coatings, reflecting the thermal stability of the system during service in rocket engines [219,252,253,254].

Comparative analysis showed that PU and epoxy formulations exhibit the lowest viscosity at 25 °C, facilitating their processing in additive manufacturing operations [147,155]. At the same time, formulations based on high-molecular-weight components exhibit higher viscosity but provide an extended operating-temperature range of up to 400–450 °C [244]. This makes them preferable for critical regions where maximum thermal resistance and heat-flux tolerance are required. Selecting a polymer matrix necessitates a balanced approach that considers not only operational requirements but also the technological constraints of the specific application method.

Additive and hybrid methods for fabricating elastomeric thermal-protection coatings provide high geometric fidelity, enable integration with combustion-chamber structural elements, and allow the implementation of gradients in thermophysical properties without the need for subsequent machining [21,219]. These technologies demonstrate significant potential for the development of multifunctional systems with enhanced thermal resistance, adhesion, and wear resistance, which are particularly relevant in rocket engineering. The effectiveness of each method is determined by the rheological properties of the polymer systems, curing conditions, material delivery methods, and component geometry [4,219]. Current research focuses on process automation, the implementation of intelligent control systems, and the development of new composite matrices tailored to the extreme thermogasdynamic conditions of solid rocket motors [21,255].

For practical SRM use, coating performance is commonly constrained by a small set of operational failure modes that are not adequately represented by ablation rate alone [92,256,257]. A primary mode is erosion localization, in which recession accelerates near thickness non-uniformities and geometric discontinuities such as edges, steps, and joints [256,258,259]. This localization can be seeded by process-induced porosity and inter-pass discontinuities, and is amplified under near-wall shear and particle-laden flow conditions [260,261,262].

A second mode is char spallation or exfoliation, where portions of the carbonaceous layer detach under combined thermal gradients and flow-imposed mechanical loading, converting a stable charring response into accelerated recession by repeated exposure of fresh material [109,257,263]. A third mode is interfacial delamination between the coating and the metallic case or primer. Delamination propensity increases with cure shrinkage–driven residual stress, thermal expansion mismatch, and cyclic thermomechanical loading [264,265,266]. It can be further accelerated when connected porosity or cracks provide gas-transport pathways and enable pressure-driven blistering or debond growth at the interface [267,268,269]. Accordingly, in the following sections, recession behavior is interpreted together with defect sensitivity, interfacial durability, and char-layer integrity, rather than as an intrinsic property of the polymer matrix.

A comparative analysis of all three groups of methods presented in Section 3 enables the formulation of overarching conclusions regarding their prospects for application in current and next-generation rocket-engine designs.

3.4. Challenges and Emerging Trends

Over the last 3–5 years, thermal-protection manufacturing research has increasingly moved from material-only comparisons toward process-centric qualification, where repeatability, inspection readiness, and scalability are treated as design constraints. This trend is accelerated by complex internal geometries, multilayer and graded concepts, and the need to reproduce coating integrity under SRM-relevant conditions. Consequently, recent work emphasizes process windows, quality control indicators, and reporting discipline that enables cross-study benchmarking and industrial translation [270,271,272].

A persistent gap is the weak comparability of ablation datasets. Recession or backside temperature is often reported without a consistent boundary-condition set (heat flux or enthalpy, pressure, gas composition and particle loading, exposure time, and specimen geometry), which blurs true process effects such as porosity, cure gradients, and interface degradation. Progress here requires SRM-relevant fire-testing practices plus a minimum reporting checklist linking processing parameters to measurable structural indicators [1,273,274].

Interface durability remains a high-risk issue because defects can nucleate during cure and then grow under thermal gradients and cycling. Cure shrinkage and thermomechanical mismatch generate residual stresses that can trigger microcracking and progressive debonding even when initial adhesion appears acceptable. Current directions therefore prioritize controlled cure kinetics and rheology evolution during deposition, monitoring where feasible, and acceptance criteria sensitive to early interfacial damage rather than pull-off strength alone [128,129,131].

Key emerging trends across application routes include the following:

-

Process-window engineering via coupled control of viscosity, yield stress or thixotropy, and pot life, mapped to defect risks [231,272];

-

Automated thickness and geometry control for internal surfaces using robotics and controlled tooling routes to reduce variability [271,275];

-

Expansion of additive and hybrid manufacturing for multilayer or graded architectures, with focus on void suppression and interlayer bonding [276,277];

-

Wider adoption of nondestructive evaluation and in situ diagnostics for porosity, delamination, and cure-gradient detection to support qualification workflows [270,272,277,278];

-

Design maps and process–structure–performance relationships to guide technology selection for typical matrices, instead of one-off demonstrations [272,277].

4. Summary

This comprehensive review synthesizes manual, mechanized, and additive or hybrid routes for producing elastomeric ablative thermal protection in solid rocket motor combustion chambers. The focus is placed on manufacturability, repeatability, and technology readiness, and on how application route and processing window translate into coating performance through coupled effects of rheology, curing, interfacial adhesion, and defect formation. The review also highlights the need to interpret ablation metrics together with SRM-relevant boundary conditions to enable meaningful cross-study comparisons.

Manual routes remain indispensable for local repair, small-batch production, and highly contoured or hard-to-access regions, but they exhibit the highest operator sensitivity and the lowest reproducibility. Mechanized routes such as spraying, molding, centrifugal casting, and hot pressing provide improved thickness stability and scalability, making them the most production-ready options for repeatable liners. Additive and hybrid routes, particularly 3D printing and prepreg winding, offer the strongest pathway to complex multilayer and graded architectures and to automation, but they are currently limited by material compatibility, qualification maturity, and the absence of unified manufacturing standards for SRM thermal protection.

Key priorities for future research include establishing quantitative process–structure–performance relationships under SRM-relevant conditions and developing design maps for representative matrices such as EPDM, NBR, polyurethane, and silicone systems. Progress also requires standardized fire-testing protocols, robust thickness and porosity control, and in situ monitoring of interfacial degradation to support qualification and lifetime prediction.

Table 9. Recommended application methods based on task requirements.

Selection Criterion	Recommended Application Methods
Complex geometry + high scalability	Spraying, centrifugal casting, hot pressing
Complex geometry + low scalability	3D printing, prepreg winding
Simple geometry + cost-sensitive	Brush, roller (trowel), pouring
Simple geometry + flexibility and integration	Co-extrusion
High material flexibility required	3D printing, co-extrusion
Moderate material flexibility required	Spraying, prepreg winding
Flexibility not required (rigid coating)	Centrifugal casting, hot pressing

provides a concise method-selection guide based on geometry and scalability constraints and links task requirements to suitable application routes.

Overall, no single method is universally optimal, and selection should be made through a trade-off between scalability, cost, and coating-formation precision under SRM-relevant constraints. The framework summarized in Table 9 can be used as a practical starting point to define process windows and qualification plans tailored to a given chamber design, including the required level of automation, inspection strategy, and acceptance criteria.