Preliminary Design of Regional Aircraft—Integration of a Fuel Cell-Electric Energy Network in SUAVE

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

The manuscript under consideration presents research of considerable relevance and timeliness on the integration of hydrogen-based propulsion systems into regional aircraft, with particular emphasis on distributed electric propulsion (DEP). The paper is well structured, provides an adequate bibliographic review, and is generally clear in its objectives. The work effectively demonstrates how higher-fidelity subsystem models within the SUAVE framework enhance the analysis of component interactions and system synergies.

In order to enhance the quality of the manuscript prior to publication, the following points must be considered:

- The references must be verified and completed. For instance, Reference [11] in the manuscript appears incomplete or may require additional information to ensure clarity and traceability. The text must be checked for typographical errors and terminology. In particular, the misuse of 'Tabular' (e.g. in line 194) where 'Table' would be used must be addressed.

- The integration of fuel cell technology and DEP carries with it potential electrical risks, and the authors are invited to provide additional information regarding these risks. The authors are also invited to clarify whether preventive measures or safety strategies have been considered to mitigate these risks, and if so, how they might influence the overall design and operation of the aircraft.

The manuscript's topic is of current interest, and the methodological rigor and clarity of presentation are impressive. The paper will be enhanced by addressing the minor issues identified, clarifying certain technical and safety aspects, and verifying the references.

Author Response

 

Comments and Suggestions for Authors

Dear Authors,

The manuscript under consideration presents research of considerable relevance and timeliness on the integration of hydrogen-based propulsion systems into regional aircraft, with particular emphasis on distributed electric propulsion (DEP). The paper is well structured, provides an adequate bibliographic review, and is generally clear in its objectives. The work effectively demonstrates how higher-fidelity subsystem models within the SUAVE framework enhance the analysis of component interactions and system synergies.

Dear Reviewer, thank you for your kind feedback and highlighting the broad spectrum of topics covered in our paper on hydrogen-powered aircraft.

In order to enhance the quality of the manuscript prior to publication, the following points must be considered:

• The references must be verified and completed. For instance, Reference [11] in the manuscript appears incomplete or may require additional information to ensure clarity and traceability. The text must be checked for typographical errors and terminology. In particular, the misuse of 'Tabular' (e.g. in line 194) where 'Table' would be used must be addressed.

Thank you for pointing this out. We fully agree with this comment and have amended the source and corrected “tabular” in 2.2 Method calibration Page 5 line 197 to “table”.

• The integration of fuel cell technology and DEP carries with it potential electrical risks, and the authors are invited to provide additional information regarding these risks. The authors are also invited to clarify whether preventive measures or safety strategies have been considered to mitigate these risks and, if so, how they might influence the overall design and operation of the aircraft.

Thank you for your constructive feedback. Our study focuses on the overall aircraft design, which is why detailed analyses of specific electrical risks were not a primary objective. However, we acknowledge the importance of these aspects and have addressed potential risks and preventive measures in our paper.

To enhance the depth of discussion, we included additional references in the revised version of our manuscript that specifically examine electrical risks and safety strategies in integrating fuel cell systems and distributed electric propulsion in 5. Conclusion, Page 20 line 525- 540.

The manuscript's topic is of current interest, and the methodological rigor and clarity of presentation are impressive. The paper will be enhanced by addressing the minor issues identified, clarifying certain technical and safety aspects, and verifying the references.

Thank you for your valuable feedback.

 

 

Submission Date

17 February 2025

Date of this review

23 Feb 2025 17:49:20

 

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

Title: 
Preliminary Design of Regional Aircraft – Integration of a Fuel Cell-Electric Energy Network in SUAVE

An interesting work is described regarding the distribution of electric propulsion (DEP) in aircraft configurations using hydrogen-based fuel cells.

It is a very extensive and complete work. However, some improvements are considered before the work is published.

The abbreviation SUAVE should be included in the abstract, as it appears in the title.

The description of the elements in equations 1 and 2 is a bit confusing. It is recommended to describe the respective elements after each equation so that the reader can follow the development.

For example, section 2.4.1 can be followed very well due to the descriptions and values mentioned up to eq. 3.

But there is some confusion later on, for example, between eqs. 5 and 6, it seems that the last division of eq. 6 could be T(t) from eq. 4?.

p.9, L. 312 load factor.The...

this equation is further explained in the appendix.  Do you describe 21 equations in the appendix?

Is it possible to describe the values for the figures in A1? It would help the reader understand the size of the components.

At what pressure is it considered that the hydrogen will be operated? I have this question.

Author Response

 

Comments and Suggestions for Authors

Title: 
Preliminary Design of Regional Aircraft – Integration of a Fuel Cell-Electric Energy Network in SUAVE

An interesting work is described regarding the distribution of electric propulsion (DEP) in aircraft configurations using hydrogen-based fuel cells.

Dear Reviewer, thank you for your kind feedback and highlighting the broad spectrum of topics covered in our paper on hydrogen-powered aircraft.

It is a very extensive and complete work. However, some improvements are considered before the work is published.

• The abbreviation SUAVE should be included in the abstract, as it appears in the title.

Thank you for pointing this out. We agree with this comment. We have, therefore, amended the abbreviation in the abstract, Page 1 line 9.

• The description of the elements in equations 1 and 2 is a bit confusing. It is recommended to describe the respective elements after each equation so that the reader can follow the development. For example, section 2.4.1 can be followed very well due to the descriptions and values mentioned up to eq. 3.

Thank you for pointing this out. We agree with this comment. Therefore, we have split the formulation description in 2.3 Impact of New Propulsion Architecture, Page 6, from lines 219 to 233.

• But there is some confusion later on, for example, between eqs. 5 and 6, it seems that the last division of eq. 6 could be T(t) from eq. 4?.

For this network setup, where the system throttle is directly applied to the fuel cell, the load factor used in the equation indeed equals the throttle from equation 4. This has now been stated in section 2.4.2, page 10, line 321 to avoid confusion. However, the original formulation of the load factor has been maintained in equation 6, as it represents an universal surrogate model for the fuel cell that is independent from the definition of the throttle setting in the network model.

• 9, L. 312 load factor. The...this equation is further explained in the appendix.  Do you describe 21 equations in the appendix?

Only the first section of the appendix deals with the derivation of equation 6, this has now been clarified with a direct reference in line 321.

• Is it possible to describe the values for the figures in A1? It would help the reader understand the size of the components.

For better understanding, all values of the figures have now also been explained in the text of appendix A.1 (page 23, line 595 to page 24, line 621). However, as only the physical dependencies shall be illustrated here, numerical values are not included on purpose.

 

• At what pressure is it considered that the hydrogen will be operated? I have this question.

The values used for the tank calculations refer to a pressure of 1.2 to 1.4 Bar_a, as stated by Brewer et al. The pressures and the thermodynamic behavior of LH₂ over time are not considered in detail. This aspect is accounted for through the allowances.

We have incorporated this clarification in Section 2.4.1 Integration Aspects of the Energy Network (Page 7, Line 270 to 273). The equations used for the fuel cell in the network model do not include the pressure either, the chosen parameters from literature reflect an optimistic performance scenario. Those issues can only be addressed by higher fidelity models in the future, which has been added as a statement to the conclusion, page 21, line 545.

 

 

Thank you for your valuable feedback.

 

 

 

 

 

 

Author Response File: Author Response.docx

Reviewer 3 Report

Comments and Suggestions for Authors

Review Report

Journal: Aerospace-MDPI.

Submission number: aerospace-3509101.

Article title: Preliminary Design of Regional Aircraft – Integration of a Fuel

Cell-Electric Energy Network in SUAVE

Authors: Jakob Schlittenhardt, , Yannik Freund, Jonas Mangold1 , Richard Hanke-Rauschenbach, and Andreas Strohmayer

 

This paper examines an electric propulsion configuration with fuel cells and distributed electric propulsion, utilizing a reverse-engineered ATR 72-500 as a reference model to calibrate the methods and ensure accurate performance modeling, as part of the Synergies of Highly Integrated Transport Aircraft project and research. Furthermore, a reference configuration is integrated, featuring a turboprop engine with the same service entry, enabling a meaningful performance comparison.

In general, this manuscript is well-written and presents valuable insights suitable for publication in Aerospace MDPI. However, certain aspects require further refinement. Some minor revisions are recommended:

• Although the study focuses on a specific mission design, real conditions can be diffetent. Is it possible to obtain the system’s performance using some flight scenarios? 
• Since hydrogen thermal management may be more challenging in cold climates, could an analysis be integrated to obtain how the system's efficiency and range vary under these conditions?"
• The study relies on the SUAVE software for aircraft performance modeling; however, could the authors validate the results through experiments?

 

Comments on the Quality of English Language

The English could be improved.

Author Response

 

Comments and Suggestions for Authors

This paper examines an electric propulsion configuration with fuel cells and distributed electric propulsion, utilizing a reverse-engineered ATR 72-500 as a reference model to calibrate the methods and ensure accurate performance modeling, as part of the Synergies of Highly Integrated Transport Aircraft project and research. Furthermore, a reference configuration is integrated, featuring a turboprop engine with the same service entry, enabling a meaningful performance comparison.

In general, this manuscript is well-written and presents valuable insights suitable for publication in Aerospace MDPI. However, certain aspects require further refinement. Some minor revisions are recommended:

Dear Reviewer, thank you for your kind feedback and highlighting the broad spectrum of topics covered in our paper on hydrogen-powered aircraft.

• Although the study focuses on a specific mission design, real conditions can be different. Is it possible to obtain the system’s performance using some flight scenarios? 

 

Thank you very much for your valuable comments. We fully agree that the classical approach of evaluating performance only at the design point or a single specific off-design point is insufficient for a comprehensive assessment.

Our new approach considers a matrix of off-design missions rather than isolated points to address this limitation. To generate Figure 9, all off-design missions within a relevant range (a minimum range of 250 nautical miles and a minimum payload of 3000 kg) were analyzed with adapted mission profiles. The key insight from this analysis is that energy efficiency per kilogram of payload per kilometer deteriorates with decreasing payload and shorter mission distances. Thus, we have taken an initial step toward a more comprehensive assessment. In the future, the weighting of off-design points will be refined by incorporating real-world economic data.

The influence of climatic environmental conditions was not included in this study. However, we acknowledge the significance of this aspect and will further explore this promising approach in future work by incorporating it at higher fidelity levels. To this end, we have expanded the discussion on the importance of increasing fidelity levels to better integrate real-world conditions in 5. Conclusion, Page 20 Line 541 to Line 549.

 

• Since hydrogen thermal management may be more challenging in cold climates, could an analysis be integrated to obtain how the system's efficiency and range vary under these conditions?"

Integration and operation of the thermal management system can indeed be more challenging under warmer conditions. We have employed a simplified model representing the average flight profile assuming standard atmosphere conditions. This has been clarified in Section 2.4.2 Energy Network Model (Page 11, Line 355).

In future work, the model fidelity will be refined and extended to incorporate real-world operating conditions better, particularly to assess the impact of different climates on system efficiency and range. This point has been further emphasized in Section 5. Conclusion, Page 20 Line 541 to Line 549.

• The study relies on the SUAVE software for aircraft performance modeling; however, could the authors validate the results through experiments?

As this research operates within a low Technology Readiness Level (TRL) context of Overall Aircraft Design (OAD), the approach enables early-stage exploration and sizing of subsystems. The study primarily relies on the SUAVE software for aircraft performance modeling, which is based on well-established empirical formulas, such as those from Raymer, derived from real-world data. These formulas are refined using real-world data by calibrating the reference aircraft.

Higher-fidelity models will be developed based on this preliminary dataset, which will be integrated into the OAD framework. Specifically, the results of this study will contribute to the refinement of a detailed surrogate model for the thermal management system (TMS), enabling the incorporation of realistic real-world requirements and operational constraints.

We contextualized Section 1.2 State of the Art and Motivation (Page 3, lines 111 to 113) and Section 5 Conclusion (Page 20, lines 541 to 549).

The formulation of the energy network is grounded in fundamental physical principles/experiments, while the surrogate models are developed through physically and analytically derived methodologies. Values from literature are used for model parameters, as listed in Appendix A.2, A.3 and A.4. An explanation has been added in section 2.4.2, page 9, line 307.

Comments on the Quality of English Language

The English could be improved.

We proofread the manuscript thoroughly and adapted the English language to a shorter and more precise form in lines: 7; 13; 59-63; 172; 176.

Thank you for your valuable feedback.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Author Response File: Author Response.docx