Hybrid Rocket Engine Noise: Measurements and Predictions of Acoustic Environments from Horizontal Static Fire

Round 1

Reviewer 1 Report

This work presents a interesting method proposed by Eldred for predicting rocket jet noise, which is validated using experimental data collected during a firing test campaign. Overall, I find the paper to be worthy of publication with minor clarifications and small improvements that can further enhance its quality. Congratulations to the authors.

Section 1: The introduction provides a brief overview, but I suggest expanding it to provide a broader contextualization and emphasize the importance of this type of study in the current space market. You can refer to [R1] for inspiration on these issues, as it provides valuable insights that could benefit the authors.

Section 2: When introducing equations in the methodology section, it would be beneficial to provide references to the literature where these equations can be verified.

Section 3.1:

• Line 157-158: Please provide further explanation as to why the sound can be assumed to be roughly constant. If possible, include a reference that supports this assumption.
• For the items listed between lines 168 and 176, I recommend the authors provide a better justification for using the closed-closed and closed-open settings. Explain the reasoning behind these choices in more detail.

Conclusion: The conclusion section is currently too short, and I believe it can be improved. I suggest clearly answering the research question or aim of the study in this section. Additionally, it is important to mention the limitations of the study to provide a balanced assessment of the work.

Reference:

R1: Pessoa Filho, J. B. (2021). Space age: Past, present and possible futures. Journal of Aerospace Technology and Management, 13, e3421.

Author Response

• All suggestions have been implemented. The introduction now better explains the main concept of the paper (Eldred's model and its limitations and how to overcome them), hybrid rocket engine overview and acoustic coupling due to combustion instabilities;
• Where possible, I have included references to the equations in the article, except where the equation is trivial (e.g., a simple application of logarithmic scaling to convert sound power to sound power level, or basic concepts of acoustics);
• The computational fluid dynamics analysis carried out as part of the same project revealed a slight difference between the average temperature of the chamber and that of the post-combustion chamber (chamber temperature is approximately 3 percent higher than post-combustion chamber);
• With reference to the investigation of combustion instabilities due to resonant coupling for standard engines, the choice of acoustic boundary conditions has been justified. The reasons for these choices are explained in more detail in the article and in some of the references cited in it;
• The conclusion section has been made longer, clearer, more detailed, more technical and more straightforward. The applicability of the results of this work, in terms of Eldred's model, has been discussed. However, as far as the results of the simple acoustic study are concerned, further research is needed.

Reviewer 2 Report

The current work deals with improving the understanding of the acoustics of low-thrust hybrid rocket engines. The topic is important and interesting to the readers and appropriate for the scope of the Applied Sciences journal. The authors have contributed to presenting a semi-empirical acoustics model of hybrid propulsion systems. The introduction provides sufficient background and references.   The research design and the methods are well described. The results are clearly presented. Overall, the paper is worthy of publication. However, the manuscript needs effort to improve its quality and meet a journal standard. Below are some comments to help the authors to improve the paper's quality.

1.       The introduction should focus on a hybrid rocket engine noise. I suggest the authors add references and rewrite the introduction by identifying the novelty/gap between this study and current research trends, including discussing previous methods of jet noise prediction and the advantages of these methods compared to other methods. The paper must offer a distinct novelty to the reader to be considered for publication.

2.       I suggest adding an experimental apparatus schematic diagram, including the measurement system's specification, accuracy, and uncertainty analysis.

3.       The experiment results need more physical discussion about the effectiveness of the proposed methods. Moreover, the results and discussion should be presented more profoundly and scientifically and need to compare to previous studies.

4.       The limitation of the proposed methods should be presented. Is it valid only for the particular rocket engine?

5.       The Conclusion needs to be written more technically.

Author Response

• The introduction now focuses on the main concept of the article, namely Eldred's model and its limitations regarding hybrid propulsion (a brief overview and references on hybrid engines are also given) and how to overcome them. In addition, some space is devoted to resonant acoustic coupling, given its importance;
• As suggested, the schematic diagram of the experimental equipment has been added. In addition, details of the specifications (model, performance, ...) of the equipment have been given in a table. Unfortunately, I did not have the opportunity to do an uncertainty analysis for this paper, but looking at other Eldred papers based on experimental results, no such analysis has ever been reported;
• There has been an improvement in the discussion about the effectiveness of the proposed methods. The results and discussion are now presented in a more technical and scientific manner (there are no references to Eldred's application to low-thrust hybrid rocket engines). In addition, further references have been provided, in particular to justify the assumptions behind the simple acoustic analysis (previous studies on conventional engines);
• As stated in the paper, since a generic hybrid propulsion system has more or less similar characteristics to the one analyzed here, the results obtained are quite general. Therefore, Eldred's model can be reasonably applied to a generic low-thrust hybrid rocket engine;
• The conclusion section has been made longer, clearer, more detailed, more technical and more linear.

Round 2

Reviewer 2 Report

The authors have accommodated reviewer comments. The paper can be accepted in its present form.