Experimental Research on Reconstruction Techniques for Instantaneous Regression Rate of Hybrid Rocket Motor with Single-Port Wagon Wheel Fuel Grain

Round 1

Reviewer 1 Report

The subject of the research is relevant for the field of Hybrid Rocket Motors (HRMs) where the authors present their contribution. In particular, the accurate measurement / estimation of the burning rate of a propellant is quite fundamental to characterize those propellants intended for this class of propulsion. The idea of checking the fitting of various recession rate correlations available in the literature against experimental values measured by the authors with the proposed method is interesting. 

Said that, one major drawback of the manuscript is that the authors do not identify well the goals of their investigation in the Abstract, neither in the presented Conclusions , which instead of a "Conclusions section" looks like a continuation of the "Results section". The work should clearly emphasize within the right sections what has been attained with the conducted research. And this is missing in the manuscript. Then a new elaboration of various sections should be done.

The manuscript is well structured but many expressions are confusing and should be corrected:

Title: "Experiment Research on..."? vs. "Experimental Research on..."

Abstract:

line 14: rebuilding -> building

line 15: ...experiment in laboratory -> ...an experiment in laboratory

line 17: in several places the authors write "HTPB based fuel (including Al)..." and this term should be improved by defining a shorten version, i.e to introduce the notation "HTPB/Al", and then to use it across the text. 

line 19: other performance parameters of HRM duting the firing test -> other performance parameters of the HRM measured during the firing test. 

line 21: a maximum error of lower than 5.75% -> needs rewording

line 24: CT is not defined, so it is not clear what the authors refer to.

line 25-26: The error of average regression rate... -> needs rewording

line 27: ...regression rate is practicable to characterize... -> needs rewording

line 28: "Overall, this paper..." -> poor way to emphasize the idea

line 30 ...rate of HRM. -> ...rate of the HRM. OR even better: ...rate of HRMs.

Keywords:

Really, is it necessary to use such long terms? Too many long definitions in the list!

1. Introduction

Line 38: the authors cite [1-4] in the context of being HRMs attactive for military and commercial applications. But references [1-4] are specific scientific references themselves; and not the kind of studies that the cite seems to indicate. Then, it looks like a cosmetic one.

Line 112: again the "HTPB based fuel (including Al)..." sentence is included. This should be solved in a better way.

Line 118-120: rewording needed.

2. Experimental setup:

Line 131: which is teh reason of implementing such a small expansion ratio of 3?

Line 133: The high concentration hydrogen peroxide ... -> The hydrogen peroxide... 

Line 140: Table 1: Main structure parameters of HRM -> Table 1: HRM configuration parameters.  (it should mantain coherence with Figure 1). And this also for the heading of Table 1, where "Structure of the body" should be changed (i.e.: HRM configuration).

Line 148-150: the parameters are already defined in Table 2, so are redundant. It is not needed to write them twice. Lines 150-151 refer to Table 2 which included their definition.

Line 157: 2.3 Energy characteristic propellant -> needs rewording

Line 158: "HTPB-based fuel (included Al)..." -> again the same expression commented before.

Line 161: the authors use the subindex "the" to refer to "theoretic" evaluation of the characteristic velocity, thrust coefficient, etc; and it would be clearer to use a more accepted index notation, for example "id" for "ideal".

Line 165: eliminate the bold notation for the NASA-CEA code

LIne 178: In the footnote of Figure 5, it should be included that the plots are built with the CEA code.

Line 196: the specification of the sensor CYB-602S should appear in the text before Line 191 for clarification purposes.

3. Methodology

Line 227: the CEA program has been already introduced in in Line 165. The clarification of "chemical equilibrium program" should be written in its first appearance in the text, not here.

Line 235: needs rewording.

Lines 284 - 303: the wording is quite redundant. For example: Pc is defined in line 281 and Ae/Ag somewhere before. And in (3) (Line 290-291) it is written: "...combustion chamber pressure Pc and nozzle expansion Ae/Ag..." when it is enough to state "...Pc and Ae/Ag...". It is not adequate to introduce a parameter definition and not to use it as it is in what follows. The wording should be compacted across these sentences. The same use of definitions occurs in Line 382. This use of definitions should be revised across the entire manuscript.

Line 306: why the absolute threshold 0.001 and not a relative one?

4. Results

Figure 9: Which is the reason of so much unburnt propellant after the firing as seen in the photo? Some explanation regarding this point should be given in the text.

Line 476: erosion of front part? -> needs rewording

 

 

 

 

 

 

 

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

In this paper, the authors apply a reconstruction technique to a hybrid rocket with a single-port star-shaped fuel grain to calculate the fuel regression rate history. The reconstruction technique the authors employed has been introduced previously. They used theoretical specific impulse instead of theoretical characteristic exhaust velocity. Still, the two have no essential difference since theoretical specific impulse can be calculated from the calculated results even when theoretical exhaust velocity is used. Rather, the method using the theoretical specific impulse is inferior to the original method because it becomes inapplicable if flow separation occurs in the nozzle. With the single-port star fuel geometry, the port cross-sectional area significantly changes during firing, so obtaining the fuel regression rate data shown in Figure 15 with only one firing test is attractive. However, the authors still need to verify the reliability of the calculated fuel regression rate history. For example, they can conduct a combustion experiment for 10 seconds, calculate the fuel regression history, and compare the data 5 seconds after ignition with the data of a 5-second firing under the same conditions. In conclusion, the reviewer does not recommend publishing this paper in an academic journal. Other concerns follow.

1. L51-53, "not by the rate of chemical transformations, but rather by the transport of the reagents to/from reaction zone by diffusion [12–14],  which leads to the low fuel regression rate of the hybrid rocket motor.": The authors employ Marxman's equation as the fuel recession rate equation, which assumes that the heat transfer rate from the flame to the solid fuel determines the fuel gasification rate. This explanation is inconsistent with Marxman's model.
2. Much of the text in the Introduction is identical to the Introduction in Reference 25 and is close to plagiarism. Please refer appropriately to what Reference 25 has already pointed out.
3. Although the authors use the expression "combustion efficiency," it is commonly called c* efficiency. Please use the term c* efficiency since it is a well-established technical term.
4. L390-391: Equation 18 is the equation proposed by Marxman, not the equation proposed by Ref. 34. Please refer to the appropriate literature.
5. L417-418: Equation 19 is closer to the equation proposed by Marxman. Reference 35 does not modify Marxman's equation to be more appropriate.
6. L420-421: Equation 19 should use the local flow density, but Figure 15 uses the value at the most downstream for the entire region. Why is the variation smaller than in Figure 14 despite this? Please add a discussion.
7. L462-463: Since the reconstruction technique uses the oxidizer history and total fuel consumption as input data, it is unsurprising that they agree, as Table 7 shows. These matches do not indicate that the histories obtained by the reconstruction technique are reliable.
8. L522-524: The fact that the fuel regression rate increases with increasing flow density downstream is a well-known fact that Sutton's Rocket Propulsion Elements also points out, and it is not the first time Reference 38 has pointed this out.

Author Response

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I do not see in the Conclusions section a clear presentation of what is achieved by the authors by proposing their method.

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

I am not opposed to accepting this paper as the authors have appropriately responded to the previous review opinions.

Author Response

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Round 3

Reviewer 1 Report

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