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This is inspired by How can adverse maneuverability due to a fuel tank behind the rear pressure bulkhead be mitigated in Airbus ZeroE hydrogen plane? .

I was thinking, with the cryogenically stored hydrogen (and required equipment) being heavy and so far away from the centre of lift of the craft in Airbus's design, it might be possible to trim the aircraft by releasing some of the hydrogen into a chamber where it can be lighter than air, reducing the nett weight of the tail, rather than trimming aerodynamically or with a movable weight.

I'm not good enough to do the math, but I suspect the size of the chamber would have to be colossal to make enough of a difference to matter - with the added bonus of being essentially an explosion waiting to happen.

Would there be a sweet spot for the average temperature of the stored hydrogen that balances the weight (including weight of corresponding equipment) against the volume of fuel stored, or is it going to be a case of coldest/most dense is the most efficient and trim needs to occur the traditional way?

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    $\begingroup$ I'm not sure this platform is meant for "please do the maths for me". It would be courteous to include your attempts at the problem and ask for help when you get stuck. $\endgroup$ – Sanchises Sep 28 at 6:32
  • $\begingroup$ @Sanchises I think this is reasonable question to ask that could maybe be answered by intuition by those experienced without diving deep into mathematics. At least an opinion if such a design is at all possible $\endgroup$ – Erik Sep 28 at 11:40
  • $\begingroup$ While pressurized Hydrogen can be stored relatively safely, low-pressure hydrogen, like that used to make an aircraft buoyant, is dangerous. $\endgroup$ – zymhan Sep 28 at 16:07
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    $\begingroup$ @zymhan: Low pressure hydrogen is not dangerous itself. What's dangerous is having hydrogen mix with air. $\endgroup$ – jamesqf Sep 28 at 17:12
  • $\begingroup$ Thanks @Sanchises, I'm not after the math itself, if someone wrote out an equation, I'll be honest, I probably wouldn't even understand half of it. As Erik mentioned, I'm more after testing my intuition - I am thinking that the space to expand the gas into would have to be a size so large that it would no longer fit into a practical aeroplane shape. $\endgroup$ – James D Sep 28 at 22:13
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The buoyancy of hydrogen is not as high as expected, and is even lower at high altitude due to the lower ratio of the density of hydrogen and air.

Just for comparison:

The tail tank of the A380 has a capacity of 23698 l.

When this tank is filled with kerosene / air / hydrogen, the content weights:

Substance           | Density [kg/l] | Weight of 23698 l [kg]
--------------------+----------------+-- -----------------
Kerosene            | 0.8            | 18958
Air (sealevel)      | 0.00122        | 28.91
Air (10km)          | 0.00041        | 9.72
Hydrogen (sealevel) | 0.00009        | 2.13
Hydrogen (10km)     | 0.00003        | 0.72

At sealevel, the lift of this tank filled with hydrogen is 28.91kg-2.13kg=26.78kg.
And at altitude, it's just 9.72kg-0.72kg=9.00kg

This is nothing compared to the 18958kg of kerosene, and shows that it makes no sense to fill any volume in an aircraft with hydrogen for trim.

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  • $\begingroup$ Thanks! Really good point about the reduced ratio between density in the tank and outside air as altitude increases, it hadn't crossed my mind but in hindsight it makes perfect sense. $\endgroup$ – James D Sep 29 at 22:06
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It is customary to pump liquid fuel for this purpose, as a given trim change requires far less volume than moving gas around.

Exchanging the actual gas requires large volumes, and also some way to keep the hydrogen and air separate as their relative volumes change. Airships typically have an inner gasbag called a ballonet for this purpose. In the case of an airship, gas is pumped in or out of the surrounding gasbag. The ballonet is vented to the air and shrinks or expands to maintain constant pressure and prevent the outer dimensions of the gasbag from changing.

If you created enough volume in a plane, it would make it noticeably larger and that would add both weight and drag. Filling large containers with hydrogen or any other fuel vapour would be as unacceptably dangerous as it is for all airships, so you would have to use a separate supply of inert lifting gas such as helium.

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