This NASA research paper on H2 engines includes a diagram of Wesley Kuhrt's 1956 design for a hydrogen-fuelled engine, which has no heat exchanger:

LH2 engine sans heat exchanger

(Image by United Technologies Corporation, via NASA.)

But, later on, it also has two diagrams showing hydrogen-fuelled engines with heat exchangers for warming the hydrogen before use; first, a Pratt & Whitney JT3C (J57) modified to burn hydrogen instead of kerosene:

hydrogen-fuelled JT3C

(Image by Pratt & Whitney, via NASA.)

followed by the Pratt & Whitney 304, an engine designed from the start to run on hydrogen:

P&W 304, running on hydrogen, with the heat exchanger

(Image by Pratt & Whitney, via NASA.)

In a jet engine using liquid hydrogen for fuel, does the hydrogen have to be warmed (presumably using a heat exchanger) and converted to a gas before it can be introduced into the combustion chamber, or can it be pumped in and burned straight as a liquid?

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    $\begingroup$ Please edit to put the question in the actual question, not just the title. Also, the image is too small to see well, can you link a larger copy? $\endgroup$
    – Zeiss Ikon
    Jun 4, 2021 at 16:25
  • $\begingroup$ Closely related, maybe a dupe? $\endgroup$
    – Pondlife
    Jun 4, 2021 at 17:57
  • $\begingroup$ Also, please cite the full details of the NASA paper. $\endgroup$ Jun 4, 2021 at 18:09
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    $\begingroup$ In order to mix with oxygen and burn, hydrogen must be gaseous. If it is injected as a liquid, it must evaporate first, needing a longer combustion chamber. $\endgroup$ Jun 4, 2021 at 19:48
  • $\begingroup$ And what did the NASA research paper say or not say specifically that led to you asking this question? $\endgroup$ Jun 4, 2021 at 21:07

1 Answer 1


Hydrogen has to be in gaseous state to burn. The boiling point of hydrogen is ~20 kelvins, while oxygen boils at about 90 kelvins and freezes to solid at ~54 kelvins. It is therefore impossible to mix these two for combustion if hydrogen is in liquid state.

A non scientific answer to pumping liquid hydrogen directly to the combustion chamber is, that it will not work, because the evaporization of hydrogen would bind too much energy from the compressed air, thus cooling it and lowering the pressure to a state that no longer can support efficient combustion.

However, as elements might work in surpising ways under pressure, I'm not 100% sure.


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