Multi-Objective, Multi-Disciplinary Design Optimization and Multi-Attribute Evaluation of Hybrid Rocket Motors Used for Manned Lunar Lander

Round 1

Reviewer 1 Report

The manuscript presents a methodology for design optimization of lunar landers descent stages. Several systems are taken into account, and also variables such as cost, weight and performance. There is merit in the research, but following are some questions and suggestions to help improving the article.

 

The abstract would be enhanced if the authors could add the most significant results, like the increase in performance of the HRM.

 

On line 61, wouldn't it be "research" or "study", instead of "incident"?

 

UDMH (Unsymmetrical dimethylhydrazine) is not described in the text. People unfamiliar with the acronym could be confused.

 

On Figure 2, the grain outer diameter is marked as "D", while in the text it is "Dp". Also, "hp" is not described in the text.

 

Were Fig. 4 data obtained by the authors? If not, reference(s) is(are) needed.

 

Line 203 - "respectively" should be added after "nozzle".

 

On line 339, "is" must be replaced by "are".

 

On Figs. 8 to 13, it is difficult to read the "Velocity increment" axis title.

 

On line 477, the correct number of the Table is 7.

 

Overall, the manuscript is well written, and the methodology seems robust, with interesting results showing a more efficient way of choosing the different systems. However, the selection of propellants for HRM is not clear. Hydrogen peroxide and HTPB are a good pair, but there are several other possibilities of oxidizers and/or solid fuels, like N2O, liquid O2, paraffin, HDPE etc. The same happens for the LRM, as hydrazine is chosen as fuel; although the performance is quite high, the toxicity is unacceptable, and there are other options and possibilities. What was the criteria of the authors when choosing the propellants?

Author Response

Thanks for your professional work. We have carefully considered the comments , which help us to improve the manuscript substantially. Any revisions to the manuscript are marked up using the “Track Changes” function. We also provide a point-to-point response to your comments with references to the changes made in the text. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The reviewer believes this is an ambitious study that incorporates and compares various evaluation axes comprehensively. However, the authors still need to assess the validity of the comparative study methodology used. For example, Operability, Manufacturability, Flexibility, and Safety/environmental protection strongly depend on the designer's choice of the six systems. On the other hand, Total Mass and Cost also rely on the design. In other words, these evaluation indicators are a mixture of those that largely depends on the system selection and those that change in the design process after the choice. The authors need to quantify the extent to which they can determine the evaluation ranking when they decide on the selection and how much the evaluation value can change by the subsequent design.   Some minor points follow.

1. p1, L31, "Nov. 16": Please include the year, as this is an academic paper they read over a long period, not a newspaper article.
2. p4, L156: What is Ab?
3. p8, L273-274: There are manufacturing methods that did not exist then, such as 3D printing, so there is a need to discuss how much of an impact this will have.
4. p21, L506, "The total mass of the grain mass" --> "The total mass of the grain"
5. p21, L516-517, "total pulse" --> "total impulse"

Author Response

Thanks for your professional work. We have carefully considered the comments , which help us to improve the manuscript substantially. Any revisions to the manuscript are marked up using the “Track Changes” function. We also provide a point-to-point response to your comments with references to the changes made in the text. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 3 Report

This paper proposes a novelty technique in conceptual design stage of a lunar lander, which is found the hybrid rocket motors (HRMs) are an alternative choice for the task using comparative study. This manuscript proposed an interesting idea with very well written, but the proposed technique is lacked in analysis result in comparative study of Pareto front among six candidates. The sound of discussions and conclusions try to convince the reader to believe the HRM is an alternative choice when compare with the liquid rocket engines (LREs). In my opinion, I cannot recommend this paper to publish in this journal at present form. It needs a revision and reconsiders before accepting. The following comments can make the paper clearer.
- It should be adding results go beyond the state-of-the-art in the abstract.

- The word “pretty good” in introduction part should be change to formal word such as “quite” or “fairly”.

-The typos in Figure 3 (Ispave) need to be corrected.

-In part 2, all of the design constraints are necessary to completely define the limits before assigning in Eq. (16, 17).

-In MODO part: NSGA is needed some reference, number of population and iteration, and computer specs.

- In comparative study of the Pareto solution sets needs some performance indicator such as c-metric, hypervolume etc. The results of comparative by performance indicator can help the author to convince the reader that is why the author chooses the best one of six propulsion schemes. If the author chooses the best one by observation, MODO solution is not clearly comparing.

- In discussion part of the table 6 is lacked to convince the reader due to lack of supporting in the state “the Pareto frontier solutions of several HRMs do not differ much from it, indicating that the HRMs have some alternative possibilities on the manned lunar lander”. The clearer comparing needs a performance indicator.

Author Response

Thanks for your professional work. We have carefully considered the comments , which help us to improve the manuscript substantially. Any revisions to the manuscript are marked up using the “Track Changes” function. We also provide a point-to-point response to your comments with references to the changes made in the text. Please see the attachment.

Author Response File: Author Response.docx

Reviewer 4 Report

1. How can the author guarantee the result's accuracy? was there any validation process for the simulation?

2. Detailed information is missing on internal ballistics, including the regression rate model.

3. How were different grain shapes considered for heat transfer estimation? also for the regression rate?

4. 'Thermodynamic calculation' of figure 3 might require chemical equilibrium calculation, details of which are missing. Additionally, detailed equations used for the algorithm should be listed.

5. Figures should be stand-alone. Figure 4 has missing information, regarding what the pressure and eplison means.

6. (a)Mixing ratio is unclear. Is it oxidizer to fuel ratio or fuel to oxidaer ratio? (b)Is it mass based or volume based? (c)How was it calculated? (d) What does the author mean by vacuum isp? What was the nozzle expansion ratio considered and what was the nozzle exit pressure?

7. The authors should double check if all variables that appear in the manuscript had been defined before they were used.

8. Detailed information is missing for equation 7, regarding the way it was calculated for each variable. The way it was estimated should be addressed clearly for the six variables of equation 7 and each value should be listed in a table.

9. Where was equation 9 from? meaning of St is missing.

10. Figure 5 does not make sense as there is a start box but it also started from another box. The variables might be familiar to the authors but it would be hard to be understood by potential readers without any description on their meanings. The variables in figure 6 as well have the same issue.

11. Details on the Expert grading method used should be added.

12. Cost model used seems to be an old version. How was the cost difference because of time difference considered?

13. Better to put Isp performance variation too during operation of the hybrid rocket due to OF shifting, which would not occur for the liquid rocket system and make a significant difference between them. 

14. Internal ballistics simulation result for the hybrid rocket should be added. For example, Isp, thrust, chamber pressure, and temperature with respect to time of operation.

Author Response

Thanks for your professional work. We have carefully considered the comments , which help us to improve the manuscript substantially. Any revisions to the manuscript are marked up using the “Track Changes” function. We also provide a point-to-point response to your comments with references to the changes made in the text. Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

In my opinion, the authors addressed almost of my comments, the paper is recommended to publish in current form.