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Since hydrogen is lighter than air, would hydrogen fuel tanks help provide lift for the airplane?

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    $\begingroup$ The amount of lift generated would be tiny for all the trouble and expense involved. Look how much hydrogen was required to lift the Hindenberg! $\endgroup$ – abelenky Aug 11 '15 at 20:40
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    $\begingroup$ @abelenky: Actually the lift would not change. Only the weight could. $\endgroup$ – Jan Hudec Aug 12 '15 at 9:57
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Let's do some quick math. Air has a density of about 1.3 g/L and hydrogen has a density of about 0.09 g/L. The best case for lifting force would be storing the hydrogen at the same pressure as the surrounding air. This means that every L of hydrogen can displace 1.21 g of air, reducing the density of the aircraft. The amount of air displaced by the aircraft (and therefore the buoyancy) would not change, but the density of the aircraft in that space would decrease. This would decrease the lift needed, but let's see by how much.

The greatest amount of free volume in an aircraft (that isn't part of the pressurized section of the fuselage) is the fuel tanks. An A380 has a fuel capacity of about 323,000 L. If we consider filling half of that with hydrogen, this would result in a total reduction of about 190 kg. An A380 has a max takeoff weight of 575,000 kg. This change would help but would be very small compared to the total lift needed.

As David Richerby points out, hydrogen needed as fuel would need to be pressurized (if not liquified) to fit enough onboard. This would increase the density above that of any displaced air, which would instead add weight to the aircraft.

Perhaps the hydrogen could be produced instead of stored. Newer aircraft do have something called an OBIGGS, or onboard inert gas generating system. This system generates nitrogen which is used to fill the space in fuel tanks after the fuel is used. Being an inert gas, nitrogen can lower the amount of oxygen in the space below what would be required for ignition of the fuel vapors.

However, this system splits nitrogen out of air, considering that our atmosphere is mostly nitrogen. Hydrogen would have to come from somewhere else, usually water, which would add more weight. Another problem with hydrogen is that when added to air it can be explosive all by itself, which would make the situation even worse. Considering the minor benefit, this would not be a system worth installing on an airliner.

Regarding the feasibility of a hydrogen powered aircraft, NASA did a study and decided that this is not a good source of power for an aircraft. The volume and weight required to store even liquid hydrogen would be too much for the aircraft to be very useful.

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    $\begingroup$ The buoyancy is going to be even less because if you wanted to use that hydrogen as fuel, you'd have to pressurize it to get enough on board. $\endgroup$ – David Richerby Aug 11 '15 at 21:54
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    $\begingroup$ Also, I'm not sure what the relevance of OBIGGS is. You couldn't inert your hydrogen buoyancy tank because the inerting gas would significantly increase its mass. And you certainly couldn't inert it if it was intended to be used as fuel! $\endgroup$ – David Richerby Aug 11 '15 at 21:55
  • $\begingroup$ I'm just making the point that sometimes aircraft do get filled with gas, but you are right that hydrogen makes no sense in that context. $\endgroup$ – fooot Aug 11 '15 at 22:29
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    $\begingroup$ As long as the aircraft shape does not change, the neither does the amount of displaced air! $\endgroup$ – Jan Hudec Aug 12 '15 at 9:55
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No. If you want to use hydrogen as fuel, you need a lot of it, which means you need to pressurize it. Hydrogen is less dense than air when it's at ordinary atmospheric pressure but, as you start to pressurize it (i.e., fit a greater mass of hydrogen into a fixed volume), that advantage decreases. Worse, if your fuel is pressurized, you need to store it in a strong tank and strong means heavy.

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    $\begingroup$ Taking the numbers from @fooot's answer: Air is 13.3 times denser that hydrogen. As consequence, hydrogen in a tank at at a pressure of already 13.3 bar has the same density as the ambient air. That common gas bottles everybody knows are initially filled with 200-300 bar. So, that advantage decreases really fast. $\endgroup$ – sweber Aug 12 '15 at 6:42
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Buoyancy (a.k.a. static lift) depends on the volume of the object. If you put something else in the tanks, the buoyancy does not change.

What can change, however, is the weight. If you wanted to use it as fuel, you'd have to take such amount as to get the same combustion heat as you get from kerosene. Hydrogen has lower heating value 119.96 KJ/kg and kerosene has ~43 MJ/kg, so you would need almost three (2.79) times less hydrogen by weight. This is totally unrelated to its density, but it is the only way how it can offer advantage as fuel.

However:

  • You'd still need to squeeze a lot of it into the aircraft, which means either huge pressure or very cold.
  • Even liquid hydrogen has density of only 70.85 kg/m³ (gaseous at 300 bar has only 26.96 kg/m³) compared to 800 kg/m³ for kerosene, so even for three times less weight you'd still need four times larger fuel tanks and they would come with higher form drag and higher weight. They would also have slightly higher buoyancy, but the drag penalty would be higher than any gain from that.
  • Dealing with stuff colder than −253 °C is complicated and it takes a lot of energy to keep it cool it and keep it cold. For spacecrafts the weight advantage is so important that its worth it, but for aircraft it isn't.
  • Compressed hydrogen is not really an option. The tanks would be heavier than what you could save in the weight of fuel.

So overall hydrogen does not have any advantage to offer for aircraft. It only has it for spacecraft that leave the atmosphere quickly so they don't care about aerodynamic drag much, but care about weight a lot.

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  • $\begingroup$ See also here. $\endgroup$ – Jan Hudec Aug 12 '15 at 14:09
  • $\begingroup$ Your first paragraph is correct, assuming that the volume of the aircraft doesn't change. However, simply making the aircraft much larger and using uncompressed (or only lightly compressed) Hydrogen gas is an option that actually would actually increase buoyancy. Of course, it's not feasible for airliners due to the massive drag increase and the risk of the whole thing going up in a big fireball (which, at least after the first few seconds, yields a dramatic penalty to lift.) $\endgroup$ – reirab Aug 12 '15 at 15:48
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It would increase lift if it were unpressurized. Of course, it would also increase volume by a few orders of magnitude if it were going to produce enough lift to matter in the context of a significantly-sized aircraft, which would lead to form drag penalties that far outweighed the lift benefit for craft that fly anything close to the speeds used in modern airliners. This can, of course, work for much slower aircraft like airships, though such use can have undesirable consequences.

If it's pressurized, it will be more dense than air and, therefore, will not create lift.

The aforementioned undesirable consequences look something like this:
Undesirable consequences of using Hydrogen for lift generation.

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    $\begingroup$ A picture is worth a thousand words. $\endgroup$ – Terry Aug 12 '15 at 6:31
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    $\begingroup$ Only three words in this case. $\endgroup$ – voretaq7 Aug 12 '15 at 16:28
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    $\begingroup$ Bringing emotional arguments into this are we $\endgroup$ – digitgopher Aug 12 '15 at 16:48
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    $\begingroup$ @jjack Not sure I follow you. Clearly, the aircraft in that incident was storing hydrogen. This is the primary reason that, after that point, airships started using helium instead of hydrogen, even though helium is much more difficult to acquire. The "We don't like to die in a large fireball" argument generally carries over quite well to modern aircraft and is a driving consideration for several modern aircraft design decisions, including the lack of storing large amounts of hydrogen on board. $\endgroup$ – reirab Dec 29 '17 at 20:01
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    $\begingroup$ @jjack Because of the problem that caused the Hindenburg disaster, modern airships are filled with helium, not hydrogen, even though helium is much more difficult to acquire. Still not quite sure I understand your argument. What do you mean by the argument (hydrogen burns very easily in the presence of oxygen which is plentiful in our atmosphere) does "not really carry over to storing hydrogen on aircraft?" $\endgroup$ – reirab Dec 29 '17 at 21:29

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