It's tough to find data about turbofan efficiency running on hydrogen.

The Tupolev Tu-155 has been used as a test bed for exotic fuels right before the fall of the soviet union, without much follow-up.

Is there any spec available on these engines? (like:)

  • Rated power
  • How much hydrogen used per second
  • Durability
  • Thermal efficiency

Matthieu Thomas of Airbus wrote an article stating an hydrogen turbofan could be more efficient than current kerosene ones because of:

  1. Wide flammability range of hydrogen
  2. High auto-ignition temperature

But as the piece is oriented pro-hydrogen, the author is probably not willing to admit any performance-wise downside of hydrogen when used in a turbofan. So a subsidiary question would be: Is there any foreseeable decrease of turbofan performance when switching to hydrogen?

  • 2
    $\begingroup$ There're two separate questions here (what were the NK-88/89's engine specs? / is there a possibility that switching a turbofan from kerosene to hydrogen could decrease its efficiency?); you might want to consider splitting one of those out into a separate question. $\endgroup$
    – Vikki
    Mar 2 at 23:58
  • 2
    $\begingroup$ @Vikki-formerlySean you're right, I marked the efficiency decreased part as a secondary question and removed the highlight. I keep it there as the intended topic is hydrogen turbofan efficiency; the second part suggests a direction to go the extra mile $\endgroup$ Mar 3 at 9:48

Based on my reading, the biggest disadvantage of hydrogen as an engine fuel is its low density. Even when stored as a cryogenic liquid, it take up more than ten times the space as the same mass of kerosene, which means the same size tanks will hold only about a third as much energy (even though hydrogen has about three times the energy content per kilogram as kerosene).

Given these qualities, an engine suitably redesigned to burn hydrogen ought to be able to run more efficiently in terms of specific power output (energy produced per mass of fuel) or specific fuel consumption (same thing only upside down -- fuel consumed per energy produced), but even ignoring the weight of insulation and the fuel cost of boil-off in flight (or before takeoff), an aircraft the same size would have roundly a third of the range. This is usually taken to mean that hydrogen fueled aircraft would have the same issues as hydrogen fueled rockets: they have to be very large to carry an economically useful amount of fuel.

There are other methods of storing hydrogen (developed for automobile prototypes); as I recall, the highest density of storage (kilograms of usable hydrogen per volume) has been with metal hydrides; these require neither very high or very low temperatures, and operate at reasonable pressures (rather than hundreds of atmospheres as common pressure bottles do) -- but they barely exceed the density of liquid hydrogen, and there's a penalty in the weight of an "empty" tank, which is still filled with a solid material when no more hydrogen can be drawn from it.

Beyond qustions of range, hydrogen is difficult to work with in terms of materials (it causes most metals to become brittle just from being in contact with hydrogen), it leaks through almost everything, and is so flammable that these unavoidable leaks are very prone to become invisible conflagrations (because a hydrogen fire contains nothing that glows from heat, the flame is practically invisible -- the bright flames of the Hindenburg were due to the envelope and its coating, not the hydrogen). This is obviously irrelevant to engine efficiency, but very relevant to whether such an engine can last long enough in service to be economical to operate.

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    $\begingroup$ I agree on everything you say. tbh, I'm looking at efficiency figures to run calculations and show with figures how unpractical hydrogen is anyway, for those who don't comprehend what you mentioned. For instance, replacing kerosene with hydrogen in an A350-900 cargo would eat up all the storage room the plane has to offer and still lack 260 cubic meter to store all the hydrogen needed to pack as much energy as kerosene does. In the process, you'd save 70 tons of weight. With engine efficiency figures, I could more precisely calculate how much hydrogen you'd need to bring for the same flight. $\endgroup$ Mar 3 at 9:37

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