Energy Transition in Aviation: The Role of Cryogenic Fuels
Round 1
Reviewer 1 Report
Line 44: "Of course, this is proportional to...": it is not proportional due to among others increasingly stringent emission standards for new aircrafts, improved aircraft efficiency and improvements in ATM. Emissions savings are outpaced by traffic/activity growth, as correctly stated in Lines 57-61. Please amend to nuance the "of course..." statement.
Lines 66 and following: please consider adding regerences to the EU ReFuel Aviation initiative [https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12303-ReFuelEU-Aviation-Sustainable-Aviation-Fuels] and to ICAO CORSIA based on ICAO global aspirational goal of carbon-neutral growth from 2020.
Line 127: Please provide source for Figure 6
Lines 160-61: add considerations on jet fuel taxation exemption (Chicago Convention)
Lines 263: RED 2018/2001 does not provided jet-specific values and does not provide a fossil jet fuel baseline: road diesel is used as proxy. Conversely, ICAO CORSIA has developed such values albeit with different sustainability criteria and different treatment of ILUC values: these considerations may be worth adding as it would be worth mentioning this distinct treatment of alternative jet fuels in EU and uncer CORSIA.
Lines 345 and following: very interesting, well presented, and informative.
Conclusions could be "beefed up" from discussion identifying more specifically lines of action for R&D and specific support measures to the production and uptake of cryogenic fuels.
Author Response
irst of all we are very thankful to the reviewer for giving us valuable comments. We have addressed most of the comments from the reviewer. The action taken for each of the comment is provided in the text below.
Comment: Line 44: "Of course, this is proportional to...": it is not proportional due to among others increasingly stringent emission standards for new aircrafts, improved aircraft efficiency and improvements in ATM. Emissions savings are outpaced by traffic/activity growth, as correctly stated in Lines 57-61. Please amend to nuance the "of course..." statement.
Reply: The statement has been changed in accordance to the comment of the reviewer.
Comment: Lines 66 and following: please consider adding regerences to the EU ReFuel Aviation initiative [https://ec.europa.eu/info/law/better-regulation/have-your-say/initiatives/12303-ReFuelEU-Aviation-Sustainable-Aviation-Fuels] and to ICAO CORSIA based on ICAO global aspirational goal of carbon-neutral growth from 2020.
Reply: The reference to ICAO CORSIA has been added.
Comment: Line 127: Please provide source for Figure 6
Reply: The figure is original, based on in-house calculations about aircraft performance and public data of automobiles, therefore there are no additional sources.
Comment: Lines 160-61: add considerations on jet fuel taxation exemption (Chicago Convention)
Reply: The reference to Chicago convention has been added.
Comment: Lines 263: RED 2018/2001 does not provided jet-specific values and does not provide a fossil jet fuel baseline: road diesel is used as proxy. Conversely, ICAO CORSIA has developed such values albeit with different sustainability criteria and different treatment of ILUC values: these considerations may be worth adding as it would be worth mentioning this distinct treatment of alternative jet fuels in EU and under CORSIA.
Reply: This is a good point that the reviewer has raised. The text has been modified in accordance with the suggestions given by the referee and a reference to ICAO CORSIA chapter 6 which mentions the details of Corsia eligible fuels has been added in the text.
Comment: Lines 345 and following: very interesting, well presented, and informative.
Reply: Thanks for your appreciation
Comment: Conclusions could be "beefed up" from discussion identifying more specifically lines of action for R&D and specific support measures to the production and uptake of cryogenic fuels.
Reply: The conclusions have been modified.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Dear Authors,
thank you very much for submitting your article. After going through the review process I can say, that it is a well written review article. The formatting is very good, language is excellent, it is easy to read, the figures and tables are of good quality. The number of references are perhaps a bit short, but acceptable.
What I was slightly concerned with is the fact, that the topic of cryogenic fuels is not novel. The aspects reviewed can all be found in other literature, and no significantly new analysis or point of view was presented. However, it is a very good, comprehensive review of the topic and combined with the presentation and insights learned from the various aircraft design exercises makes the paper acceptable for publication in my opinion.
One discussion point I missed from the paper is the state and potential application of other methods of hydrogen storage, for example in slush form or using some absorption/chemical storage technology. While they are not all necessary cryogenic technologies in the strictest sense, their discussion would have provided some additional much needed novelty for the paper.
All in all, I recommend the paper for publication.
Author Response
First of all we are very thankful to the reviewer for giving us valuable comments. We have addressed most of the comments from the reviewer. The action taken for each of the comment is provided in the text below.
Comment: thank you very much for submitting your article. After going through the review process I can say, that it is a well written review article. The formatting is very good, language is excellent, it is easy to read, the figures and tables are of good quality. The number of references are perhaps a bit short, but acceptable.
Reply: Thanks for your comments. We sincerely thank the reviewer to his/her comments.
Comment: What I was slightly concerned with is the fact, that the topic of cryogenic fuels is not novel. The aspects reviewed can all be found in other literature, and no significantly new analysis or point of view was presented. However, it is a very good, comprehensive review of the topic and combined with the presentation and insights learned from the various aircraft design exercises makes the paper acceptable for publication in my opinion.
Reply: We agree that the topic of cryogenic fuels is not new. There has been several attempts in the past which we have tried to cover in our paper. The novel insight of the paper is to present that there is no “silver bullet” and it is better to go for a dual fuel option rather than sticking to only one type of fuel.
Comment: One discussion point I missed from the paper is the state and potential application of other methods of hydrogen storage, for example in slush form or using some absorption/chemical storage technology. While they are not all necessary cryogenic technologies in the strictest sense, their discussion would have provided some additional much needed novelty for the paper.
Reply: Thanks for the comment. We have added a section in H2 storage.
Comment: All in all, I recommend the paper for publication.
Reply: Thank you!
Author Response File: Author Response.pdf
