Study on the Ablation Mechanism of the First Pulse Insulation Layer in a Double-Pulse Solid Rocket Motor
Round 1
Reviewer 1 Report
The paper presents the results of a combination of numerical simulations and experiments for the study of the ablation behavior of the first pulse insulation layer in a double-pulse solid rocket motor. First, the authors defined a numerical model for the simulation of the two-phase fluid dynamics inside the thrust chamber of the rocket, to identify the main parameters affecting the ablation behavior. Then, they carry out experiments in a lab-scale environment, to perform ablation measurements on EPDM ablator samples in different conditions. Finally, a thermo-chemical ablation model is defined and used to reproduce the experimental results.
The overall quality of the work is acceptable, and the research topic is interesting, but some major issues need to be fixed before it can be published. In general, the authors are encouraged to provide more quantitative details about the results of simulations and tests, and the used methods. Moreover, I would recommend providing a better connection among the different sections. In particular, the simulations of sec. 2 seem only loosely related to the rest of the work. Following are more specific comments:
1. Is the model in sec. 2 validated? Do the authors employ a commercial software or an in-house code?
2. A grid independence analysis should be carried out.
3. How is the propellant regression rate calculated in the simulations of sec. 2?
4. Page 7, line 188: the authors state that the particle velocity is low, but no quantitative simulation results are reported in this regard. Please include a contour, a plot or at least some representative values.
5. The results of Figure 5 are too qualitative, although the flow field structure is useful. What is the gas velocity? Are the tests presented in the next section representative of a realistic condition? Could you estimate the shear stress or the heat flux to the wall? This is a major point to justify the presence of sec. 2 and, in general, the relevance of the research.
6. How are the velocities used in the experiments calculated? Are they also measured during experiments?
7. What are the dimensions of the test apparatus?
8. How are two pulses realized? I only see one propellant grain in the picture.
9. Is the simulation of Fig. 7 carried out by means of the sec. 2 model?
10. What propellant are you burning?
11. Is the geometry 2D planar? How do you ensure that only one face of the specimen in Figure 8 is exposed to the flow? I don’t exactly get how the high- and low-speed sections are shaped.
12. What is the accuracy of the ultrasonic thickness measurement? Is it a local measurement? Did the authors assess if there was any non-uniformity in the ablation along samples length?
13. Please give more details about the tested materials, and define the acronym EPDM.
14. Does it make sense to peel off the carbonized layer in group A specimens? Is this realistic or somehow interesting for the research? As I understand, the peeling procedure is not taken into account in the simulations of sec. 4.
15. Tables reporting ablation rates should include the standard deviation of the measurements, besides the average value.
16. Aren’t there any data about Group B samples ablation after 1st pulse? It would have been very useful to strengthen the discussion.
17. Is the ablation rate calculated by the model of sec. 4 constant over time? Could you provide profiles in time and/or in space?
18. If the simulations of sec. 4 are unsteady, as I understand, at which time are the results reported in the figures? Is there any evidence of relevant unsteady phenomena?
19. The authors are encouraged to expand the discussion of results, with a more detailed and quantitative comparison with the experimental outcomes.
20. Is it possible to use the model in sec. 4 to study re-ignition after the first pulse? Would the regression rate be affected?
Here are also some minor suggestions:
21. Please define all quantities used in the equations. Some are missing.
22. Page 4, line 145: include a reference for the Rosin-Rammler distribution.
23. Indicate the units of measurements of the plotted quantities in the caption of each Figure, and not only in the text.
24. Improve readability of Fig. 4 colormap.
25. Page 5, Table 2: are the values of radial and axial lengths of the interlayer switched?
26. Page 11, Table 8: correct the header of the first column (it says “Group A” instead of “Group B”).
Finally, following are some minor language corrections:
1. Page 2, line 56: include the name of the authors before citing ref. [11].
2. Page 2, line 75: replace “was basically coincides” with “basically coincided”.
3. Page 2, lines 76-77: replace “reflux” with “recirculation” (2 occurrences).
4. Page 8, line 236: replace “peeling” with “peeled”.
5. Page 10, line 298: replace “the ablation erosion was obviously light” with “the ablation rate was obviously lower”.
6. Page 12, line 344: replace “cp” with “ρ” and “T” with “cp”.
7. Page 15, line 412: replace “of the flow field Most” with “of the flow field, most”.
Author Response
Dear Reviewer:
Manuscript ID: aerospace-1903379
Type of manuscript: Article
Title: Study on the ablation mechanism of the first pulse insulation layer in a double-pulse solid rocket motor
Authors: Kaining Zhang, Chunguang Wang *, Weiping Tian
Received: 22 August 2022
E-mails: [email protected], [email protected], [email protected]
We greatly appreciate the thorough and thoughtful comments provided on our submitted article. We made sure that each one of the reviewer comments has been addressed carefully and the paper is revised accordingly.
The details of the revisions to the manuscript and our responses to the referees’ comments are shown in the appendix. Please see below, in blue, for a point-by-point response to the reviewers’ comments and concerns. At the same time, the response to each reviewer are uploaded to the attachment.
Sincerely yours
Dr. Chunguang Wang
Xi’an Jiaotong University, 710049, P.R. China
Appendix
Responds to the Reviewer 1：
The paper presents the results of a combination of numerical simulations and experiments for the study of the ablation behavior of the first pulse insulation layer in a double-pulse solid rocket motor. First, the authors defined a numerical model for the simulation of the two-phase fluid dynamics inside the thrust chamber of the rocket, to identify the main parameters affecting the ablation behavior. Then, they carry out experiments in a lab-scale environment, to perform ablation measurements on EPDM ablator samples in different conditions. Finally, a thermo-chemical ablation model is defined and used to reproduce the experimental results.
The overall quality of the work is acceptable, and the research topic is interesting, but some major issues need to be fixed before it can be published. In general, the authors are encouraged to provide more quantitative details about the results of simulations and tests, and the used methods. Moreover, I would recommend providing a better connection among the different sections. In particular, the simulations of sec. 2 seem only loosely related to the rest of the work. Following are more specific comments:
Question 1:
Is the model in sec. 2 validated? Do the authors employ a commercial software or an in-house code?
Response 1：
Thank you for all your comments about our work. In this paper, the shape of grain at different time under the Ⅱ pulse working condition is selected for simulation analysis. The simulation model for calculation is shown in Figure 2. Analysis of grid has been added to section 2. Fluent is used to simulate in section 2.
Question 2:
A grid independence analysis should be carried out.
Response 2：
We agree with your suggestion. Analysis of grid has been added to section 2.
Question 3:
How is the propellant regression rate calculated in the simulations of sec. 2?
Response 3：
We are sorry for the unclear description. The calculation parameters are based on hydroxyl terminated polybutadiene (HTPB) propellant. The results in this section are accurate for the HTPB propellant. For other propellants, the results may be different, because the composition and content of particle phase of different propellants may be different. Relevant description has been added to the paper.
Question 4:
Page 7, line 188: the authors state that the particle velocity is low, but no quantitative simulation results are reported in this regard. Please include a contour, a plot or at least some representative values.
Response 4：
We agree that no quantitative simulation results are reported in this regard. In fact, the description of particle velocity is not necessary, so we delete the relevant description. In this paper, the influence of different gas phase velocity is the focus of analysis.
Question 5:
The results of Figure 5 are too qualitative, although the flow field structure is useful. What is the gas velocity? Are the tests presented in the next section representative of a realistic condition? Could you estimate the shear stress or the heat flux to the wall? This is a major point to justify the presence of sec. 2 and, in general, the relevance of the research.
Response 5：
We agree with your comments and thank you for your questions.
Q: The results of Figure 5 are too qualitative, although the flow field structure is useful.
A: In this paper, the purpose of Figure 5 is to get a qualitative conclusion: the main factor causing the difference of the ablation of the I pulse insulation layer is the difference of the gas phase velocity in the recirculation zone of the I pulse combustor. The quantitative analysis of the engine is not the focus of this paper, which will be analyzed in detail in the subsequent research.
Q: What is the gas velocity?
A: We are sorry for the unclear description. The gas velocity has been modified to the gas phase velocity.
Q: Are the tests presented in the next section representative of a realistic condition?
A: Sub-scale engines are used in the experiments in the section 3. In the section 2, the simulation analysis is carried out based on the real model. Relevant description has been added to the paper.
Q: Could you estimate the shear stress or the heat flux to the wall? This is a major point to justify the presence of sec. 2 and, in general, the relevance of the research.
A: The shear stress and heat flux of the wall can be calculated, but these two variables are determined by the gas phase velocity. This paper only discusses the influence of gas phase velocity. The shear stress and heat flux of the wall are the focus of the next study.
And the second section is part of the overall study. In the section 2, the simulation analysis is carried out based on the real model, and a conclusion is obtained: the main factor causing the difference of the ablation of the I pulse insulation layer is the difference of the gas phase velocity in the recirculation zone of the I pulse combustor. To further analyze the influence of gas phase velocity on ablation, a sub-scale engine was established for experiments in the third section. The model is only to establish an environment with different gas velocities to study the influence of gas velocities on ablation. Relevant description has been added to the paper.
Question 6:
How are the velocities used in the experiments calculated? Are they also measured during experiments?
Response 6：
We are sorry for the unclear description. The velocities used in the experiments are pre-calculated in the software based on the sub-scale engine model. Figure 7 shows the gas phase velocity at different positions of the sub-scale engine.
Question 7:
What are the dimensions of the test apparatus?
Response 7：
The experimental device is a sub-scale engine with a maximum diameter of 300 mm and a length of 200 mm. Relevant description has been added to the paper.
Question 8:
How are two pulses realized? I only see one propellant grain in the picture.
Response 8：
We are very grateful for your questions. The II pulse working conditions was realized by two artificial ignition. After the first ignition, the ablation rate of group A specimens was measured. Relevant description has been added to the paper.
Question 9:
Is the simulation of Fig. 7 carried out by means of the sec. 2 model?
Response 9：
We are sorry for the unclear description. Figure 7 shows the gas phase velocity at different positions of the sub-scale engine. Sub-scale engines are used in the experiments in the section 3. And in the section 2, the simulation analysis is carried out based on the real model. Relevant description has been added to the paper.
Question 10:
What propellant are you burning?
Response 10：
We are sorry for the unclear description. HTPB propellant was used in the experiment. Relevant description has been added to the paper.
Question 11:
Is the geometry 2D planar? How do you ensure that only one face of the specimen in Figure 8 is exposed to the flow? I don’t exactly get how the high- and low-speed sections are shaped.
Response 11：
We agree with your suggestion and thank you for your questions. The specimen is 3D object. We only protect the bottom. The sides of the specimen are exposed to the fluid at the same time, but the middle part is slightly affected. Therefore, we do not measure at the boundary of the specimen.
In Figure 7, it is found that there are high-velocity section and low-velocity section in the sub-scale engine. In the experiment, the working conditions of high velocity and low velocity are obtained by placing specimen s at corresponding positions.
Question 12:
What is the accuracy of the ultrasonic thickness measurement? Is it a local measurement? Did the authors assess if there was any non-uniformity in the ablation along samples length?
Response 12：
We are grateful for your questions. The accuracy of the ultrasonic thickness gauge is 0.001 mm. Ultrasonic thickness measurement is not a local measurement. After the specimen is taken out and the carbonized layer is removed, the thickness is measured using an ultrasonic thickness gauge. Several groups of data were measured and the average value was calculated. The sides of the specimen are exposed to the fluid at the same time, so we do not measure at the boundary of the specimen. Standard deviation is added to tables reporting ablation rates. Relevant description has been added to the paper.
Question 13:
Please give more details about the tested materials, and define the acronym EPDM.
Response 13：
We are sorry for the unclear description. Ethylene propylene diene monomer (EPDM) is used as insulation layer because of its low density, high thermal decomposition temperature and thermal resistance.
Question 14:
Does it make sense to peel off the carbonized layer in group A specimens? Is this realistic or somehow interesting for the research? As I understand, the peeling procedure is not taken into account in the simulations of sec. 4.
Response 14：
We agree with your suggestion and thank you for your questions. In this paper, the influence of carbonized layer on ablation is studied, so the carbonized layer of group A needs to be peeled off to form a control experiment with group B. This is realistic for the research. Because the carbonized layer is quickly peeled off by the high gas phase velocity under Ⅱ pulse working condition. The simulation results in section 4 are the final results after re-ignition, under Ⅱ pulse condition. Relevant description has been added to the paper.
Question 15:
Tables reporting ablation rates should include the standard deviation of the measurements, besides the average value.
Response 15：
We agree that standard deviation is necessary in tables reporting ablation rates. Standard deviation is added to tables.
Question 16:
Aren’t there any data about Group B samples ablation after 1st pulse? It would have been very useful to strengthen the discussion.
Response 16：
We are sorry for the unclear description. The ablated thickness is obtained by subtracting the original thickness from the thickness after peeling the carbide layer. Therefore, the carbide layer must be peeled off to measure ablation data. However, ablation data of Group B after I pulse cannot be obtained because of the overlying carbide layer.
Question 17:
Is the ablation rate calculated by the model of sec. 4 constant over time? Could you provide profiles in time and/or in space?
Response 17：
We are grateful for your questions. The ablation rate calculated by the model is not constant, it is related to the gas phase velocity and the thickness of the carbonized layer. Working conditions are added in this section. The simulation results are the results of the final time after re-ignition under the condition of Ⅱ pulse.
Question 18:
If the simulations of sec. 4 are unsteady, as I understand, at which time are the results reported in the figures? Is there any evidence of relevant unsteady phenomena?
Response 18：
We agree with your suggestion and thank you for your questions. The simulation in this section is a transient simulation. And the simulation results are the final results after re-ignition, under Ⅱ pulse condition.
Question 19:
The authors are encouraged to expand the discussion of results, with a more detailed and quantitative comparison with the experimental outcomes.
Response 19：
We take your suggestion very seriously. The discussion of the results has been expanded.
Question 20:
Is it possible to use the model in sec. 4 to study re-ignition after the first pulse? Would the regression rate be affected?
Response 20：
We are sorry for the unclear description. The simulation results in this section are the final results after re-ignition, under Ⅱ pulse condition. The working conditions are consistent with the experimental working conditions in Section 3. This results are consistent with ablation experimental results under Ⅱ pulse condition.
Question 21:
Please define all quantities used in the equations. Some are missing.
Response 21：
We are grateful for your suggestion. The undefined quantities used in the equation are defined in detail again.
Question 22:
Page 4, line 145: include a reference for the Rosin-Rammler distribution.
Response 22：
We are grateful for your suggestion. The description and formula of Rosin-Rammler (R-R) distribution have been added, and references have been added.
Question 23:
Indicate the units of measurements of the plotted quantities in the caption of each Figure, and not only in the text.
Response 23：
We are grateful for your suggestion. The necessary units of the plotted quantities are indicated in the figure.
Question 24:
Improve readability of Fig. 4 colormap.
Response 24：
We are sorry for the unclear figures. The size of Figure 4 has been reduced due to formatting issues, which is why it cannot be seen clearly. We tried to make it bigger, but we don't know if the current state of Figure 4 is good. If it's still difficult to read, we'll deal with it further.
Question 25:
Page 5, Table 2: are the values of radial and axial lengths of the interlayer switched?
Response 25：
We are sorry for our mistake that confuse you. the values of radial and axial lengths have been switched in the paper.
Question 26:
Page 11, Table 8: correct the header of the first column (it says “Group A” instead of “Group B”).
Response 26：
We are sorry for our mistake. The header of Table 8 has been corrected.
Finally, following are some minor language corrections:
- Page 2, line 56: include the name of the authors before citing ref. [11].
- Page 2, line 75: replace “was basically coincides” with “basically coincided”.
- Page 2, lines 76-77: replace “reflux” with “recirculation” (2 occurrences).
- Page 8, line 236: replace “peeling” with “peeled”.
- Page 10, line 298: replace “the ablation erosion was obviously light” with “the ablation rate was obviously lower”.
- Page 12, line 344: replace “cp” with “ρ” and “T” with “cp”.
- Page 15, line 412: replace “of the flow field Most” with “of the flow field, most”.
Response：
We appreciate your professionalism and seriousness. The above 7 language problems have been corrected, and we have reviewed the full text.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper is a well-written work in insulation degradation in a solid rocket motor environment. The approach and m any limitation of the analysis in regard to the stripping away of the charred layer.
Author Response
Dear Reviewer:
Manuscript ID: aerospace-1903379
Type of manuscript: Article
Title: Study on the ablation mechanism of the first pulse insulation layer in a double-pulse solid rocket motor
Authors: Kaining Zhang, Chunguang Wang *, Weiping Tian
Received: 22 August 2022
E-mails: [email protected], [email protected], [email protected]
We greatly appreciate the thorough and thoughtful comments provided on our submitted article. We made sure that each one of the reviewer comments has been addressed carefully and the paper is revised accordingly.
The details of the revisions to the manuscript and our responses to the referees’ comments are shown in the appendix. Please see below, in blue, for a point-by-point response to the reviewers’ comments and concerns. At the same time, the response to each reviewer are uploaded to the attachment.
Sincerely yours
Dr. Chunguang Wang
Xi’an Jiaotong University, 710049, P.R. China
Appendix
Responds to the Reviewer 2：
The paper is a well-written work in insulation degradation in a solid rocket motor environment. The approach and m any limitation of the analysis in regard to the stripping away of the charred layer.
Reviewer 2 did not give detailed comments.
Author Response File: Author Response.pdf
Reviewer 3 Report
See the notes in the attached pdf file.
Comments for author File: Comments.pdf
Author Response
Dear Reviewer:
Manuscript ID: aerospace-1903379
Type of manuscript: Article
Title: Study on the ablation mechanism of the first pulse insulation layer in a double-pulse solid rocket motor
Authors: Kaining Zhang, Chunguang Wang *, Weiping Tian
Received: 22 August 2022
E-mails: [email protected], [email protected], [email protected]
We greatly appreciate the thorough and thoughtful comments provided on our submitted article. We made sure that each one of the reviewer comments has been addressed carefully and the paper is revised accordingly.
The details of the revisions to the manuscript and our responses to the referees’ comments are shown in the appendix. Please see below, in blue, for a point-by-point response to the reviewers’ comments and concerns. At the same time, the response to each reviewer are uploaded to the attachment.
Sincerely yours
Dr. Chunguang Wang
Xi’an Jiaotong University, 710049, P.R. China
Appendix
Responds to the Reviewer 3：
Question 1:
Page 1, line 34 (Original Version): replace “is consisted by” with “consists of”.
Response 1：
We are sorry for our mistake. The language problem has been corrected, and we have reviewed the full text.
Question 2:
Page 1, line 35-36 (Original Version): replace “Two....device (PSD), which share a nozzle.” with “Two....chambers, which share the same nozzle, are connected...”.
Response 2：
We are sorry for our mistake. The language problem has been corrected, and we have reviewed the full text.
Question 3:
Page 2, line 56-57 (Original Version): I suppose that the punctuation is missing/lacking in these sentences.
Response 3：
We are sorry for the unclear description. This sentence has been revised in the new version.
Question 4:
Page 2, line 75 (Original Version): replace “was basically coincides” with “coincided”.
Response 4：
We are sorry for our mistake. The language problem has been corrected, and we have reviewed the full text.
Question 5:
Page 3, line 89 (Original Version): replace “The” with “the”.
Response 5：
We are sorry for our mistake. The language problem has been corrected, and we have reviewed the full text.
Question 6:
Page 4, line 145 (Original Version): please provide a reference about the Rosin-Rammler distribution.
Response 6：
We are grateful for your suggestion. The description and formula of Rosin-Rammler (R-R) distribution have been added, and references have been added.
Question 7:
Page 5, line 153 (Original Version): please add x and y axis in the figure 2.
Response 7：
We are sorry for the unclear figures. The x and y axis have been added in figure 2. And we modified Figure 2 to improve its readability.
Question 8:
Page 5, line 155 (Original Version): please provide a reference / explaination / source for these data.
Response 8：
We are very grateful for your questions. Reference to gas parameters has been provided.
Question 9:
Page 5, Table 2 (Original Version): how did you choose these time points for the simulation?
Response 9：
We are sorry for the unclear description. After PSD is turned on, stable pressure is established in the engine within 2s. Simulation analysis is performed at three time points: the start state (2s), the intermediate state (7s) and the end state (14s).
Question 10:
Page 5, Table 2 (Original Version): please provide a ref for this geometry.
Response 10：
We are sorry for the unclear description. The radial length and axial height of the interlayer are shown in Figure 2. We have improved the readability of Figure 2.
Question 11:
Page 5, Table 2 (Original Version): assuming that y is the vertical axis, Ny>0 means that is it pointing downward?
Response 11：
We agree with your suggestion and thank you for your questions. In the new version, the positive direction of y points upwards. And we have corrected " Ny=1g" to " Ny=-1g", which means that the engine is subjected to gravity load.
Question 12:
Page 5, line 158-160 (Original Version): did you perform preliminary calculations in order to select this cell size / number for this work / previous works on the topic? Please provide an explanation / parametric analysis / reference?
Response 12：
We are grateful for your questions. Analysis of grid has been added to section 2.
Question 13:
Page 5, line 162 (Original Version): this and subsequent figures (3 4 5 6 7) should fit the text width, in order to improve their readability. Please modify the .tex accordingly.
Response 13：
We are sorry for the unclear figures. The size of figures has been reduced due to formatting issues, which is why it cannot be seen clearly. We tried to make it bigger, but we don't know if the current state is good. If it's still difficult to read, we'll deal with it further.
Question 14:
Page 7, line 182 (Original Version): please move this information to the caption of the figure.
Response 14：
We are grateful for your suggestion. The necessary units of the plotted quantities are added to caption of the figure.
Question 15:
Page 7, line 211 (Original Version): please move this information to the caption of the figure.
Response 15：
We are grateful for your suggestion. The necessary units of the plotted quantities are added to caption of the figure.
Question 16:
Page 7, line 212-213 (Original Version): why? Please explain the reasons of this choice
Response 16：
We are sorry for the unclear description. After the PSD of the II pulse engine is destroyed, the channel sizes are different, resulting in inconsistent gas phase velocity in the I pulse combustion chamber. Therefore, the ablation of the I pulse insulation layer is different. We focus on the influence of different gas phase velocities on the ablation.
Question 17:
Page 7, line 216 (Original Version): how did you choose this pressure?
Response 17：
We are grateful for your questions. The average value of the pressure curve under the operating conditions of the II pulse engine is 7MPa. We used average pressure conditions.
Question 18:
Page 9, Table 3 (Original Version): please add [mm/s] in the header.
Response 18：
We are sorry for the unclear description. The necessary units have been added in all header of tables.
Question 19:
Page 10, Table 4 (Original Version): please add [mm/s] in all the headers.
Response 19：
We are sorry for the unclear description. The necessary units have been added in all header of tables.
Question 20:
Page 10, line 298 (Original Version): replace “light” with “lighter”.
Response 20：
We are sorry for our mistake. The language problem has been corrected, and we have reviewed the full text.
Question 21:
Page 11, Table 6 (Original Version): please add [mm/s] in all the headers.
Response 21：
We are sorry for the unclear description. The necessary units have been added in all header of tables.
Question 22:
Page 12, line 339: please make this figure and the text inside it larger.
Response 22：
We are grateful for your suggestion. We have made this figure and the text inside it larger.
Question 23:
Page 12, line 339: please center this and subsequent figure and make their width match the text one.
Response 23：
We are sorry for the location and size of figures. The picture is not centered due to format problems. We tried to modify it, hope this works. In addition, in order to match the width of the text, we have adjusted the size of figures.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The paper presents the results of a combination of numerical simulations and experiments for the study of the ablation behavior of the first pulse insulation layer in a double-pulse solid rocket motor.
The paper has been revised with respect to the previous version, but I still have a few recommendations to improve the quality of the work.
1. The statement that HTPB properties are used for the calculations in sec. 2 is a little generic. How did you compute exactly the regression rate?
2. Although you only wanted only a qualitative visualization of the flow field in sec. 2, I think values of gas phase velocity are needed to justify your choice of experimental conditions. I understand that you are basically limited to the capabilities of your sub-scale test engine, and this is fine, but still I believe it is needed to provide a QUANTITATIVE estimation of the gas phase velocity values expected in a real-scale application (as that simulated in sec. 2) to understand how much your results are representative of that.
3. n Fig. 7 you show the velocity distribution of a simple geometry without the test specimens. But, for what I can understand from the scheme in Fig. 6, the specimens affect the shape of the duct, so they may have an influence on the gas phase velocity. Since you have no direct measurement of this fundamental quantity, please ensure that your estimations are correct. Anyway, please clearly state in the manuscript that velocities are only calculated and not measured.
4. I still don’t get how you manage to do two tests at an interval of 50 s. Do you need to replace the propellant grain?
5. You reported a very nice description of the ablation rate measurement method in the response to my comments. Please include all the details also in the manuscript.
6. Also include the response to Question 16 (about the need to peel off the carbonized layer for the thickness measurement) in the manuscript.
7. Which is the initial condition of the simulations reported in sec. 4?
8. I still think a quantitative discussion of the results in sec. 4 is missing. It is required to include an estimation of the ablation rate calculated by the model of sec. 4 and compare it with the experimental measurements.
Author Response
Dear Reviewer:
Manuscript ID: aerospace-1903379
Type of manuscript: Article
Title: Study on the ablation mechanism of the first pulse insulation layer in a double-pulse solid rocket motor
Authors: Kaining Zhang, Chunguang Wang *, Weiping Tian
Received: 22 August 2022
E-mails: [email protected], [email protected], [email protected]
We greatly appreciate the thorough and thoughtful comments provided on our submitted article. We made sure that each one of the reviewer comments has been addressed carefully and the paper is revised accordingly.
The details of the revisions to the manuscript and our responses to the referee’s comments are shown in the appendix. Please see below, in blue, for a point-by-point response to the reviewer’s comments and concerns. At the same time, the response to each reviewer are uploaded to the attachment.
Sincerely yours
Dr. Chunguang Wang
Xi’an Jiaotong University, 710049, P.R. China
Appendix
Responds to the Reviewer 1：
The paper presents the results of a combination of numerical simulations and experiments for the study of the ablation behavior of the first pulse insulation layer in a double-pulse solid rocket motor.
The paper has been revised with respect to the previous version, but I still have a few recommendations to improve the quality of the work.
Question 1:
The statement that HTPB properties are used for the calculations in sec. 2 is a little generic. How did you compute exactly the regression rate?
Response 1：
Thank you for all your comments about our work. We are sorry for the unclear answers in the last version. We selected three time points for simulation, as shown in Figure 2. The grain size is inconsistent due to combustion. The basic data of HTPB is unchanged, as shown in Table 1. And the change in grain size will lead to a change in pressure in the combustion chamber, as shown in Table 2.
Question 2:
Although you only wanted only a qualitative visualization of the flow field in sec. 2, I think values of gas phase velocity are needed to justify your choice of experimental conditions. I understand that you are basically limited to the capabilities of your sub-scale test engine, and this is fine, but still I believe it is needed to provide a QUANTITATIVE estimation of the gas phase velocity values expected in a real-scale application (as that simulated in sec. 2) to understand how much your results are representative of that.
Response 2：
We deeply agree with your suggestion. In sec. 2, we added the simulated gas phase velocity to verify the choice of test conditions to a certain extent, as shown in Figure 6. The relevant description has been added to the paper.
Question 3:
Fig. 7 you show the velocity distribution of a simple geometry without the test specimens. But, for what I can understand from the scheme in Fig. 6, the specimens affect the shape of the duct, so they may have an influence on the gas phase velocity. Since you have no direct measurement of this fundamental quantity, please ensure that your estimations are correct. Anyway, please clearly state in the manuscript that velocities are only calculated and not measured.
Response 3：
We are sorry for the unclear description. The specimen size is much smaller than the sub-scale engine size, and positions for placing the specimen are reserved inside the sub-scale engine. Therefore, the influence of the specimen on the gas phase velocity can be ignored. A description is added to the paragraph describing the size and shape of the specimens.
The temperature is too high to measure the gas phase velocity inside the sub-scale engine under working conditions, so the gas phase velocity is calculated by numerical simulation to estimate the test velocity. A description has been added to the paragraph introducing Figure 8 in the new version.
Question 4:
I still don’t get how you manage to do two tests at an interval of 50 s. Do you need to replace the propellant grain?
Response 4：
We are sorry for the careless description in the manuscript. After the I pulse, a new HPTB grain is loaded into the sub-scale engine. The "time interval between two pulse ignition is the 50s" in the original text refers to the real engine. But in this test, this is unrealistic. So this sentence is deleted.
Question 5:
You reported a very nice description of the ablation rate measurement method in the response to my comments. Please include all the details also in the manuscript.
Response 5：
We agree with your comments and thank you for your suggestions. After taking out the specimens and removing the carbonized layer, the residual thickness of the insulation layer is measured by an ultrasonic thickness gauge with a resolution of 0.001 mm. The relevant description has been added to the paper.
Question 6:
Also include the response to Question 16 (about the need to peel off the carbonized layer for the thickness measurement) in the manuscript.
Response 6：
We are sorry for the unclear description in the manuscript. After peeling the carbide layer, the ablated thickness is obtained by subtracting the original thickness from the residual thickness. The ablation rate of group A is measured after the I pulse and the Ⅱ pulse, respectively. And the ablation rate of group B is measured only after the Ⅱ pulse. According to the test settings, the carbonized layer of group B needs to be retained after I pulse, so the residual thickness of group B after the I pulse cannot be measured. Relevant description has been added to the paper.
Question 7:
Which is the initial condition of the simulations reported in sec. 4?
Response 7：
We are very grateful for your questions. The initial condition of the simulations are shown in sec. 4.4. We are sorry that you did not notice the description of the initial conditions due to the misleading title of sec. 4.4. Therefore, we change the title from "calculation results" to "Numerical simulation and results of ablation".
Question 8:
I still think a quantitative discussion of the results in sec. 4 is missing. It is required to include an estimation of the ablation rate calculated by the model of sec. 4 and compare it with the experimental measurements.
Response 8：
We are very grateful for your suggestions. We are sorry to mislead you. We have discussed the results quantitatively in Sec. 4.4. In this version, we added experimental values to make the comparison clearer.
Author Response File: Author Response.pdf
Round 3
Reviewer 1 Report
The authors have satisfactorily answered the raised points and now the manuscript is ready for publishing.
