Airbus have recently announced their commitment to building a hydrogen-powered plane, which, with typical aviation pace, will maybe enter service after 2035. Hydrogen as a fuel has questionable benefits for many applications, but for aircraft it actually makes a lot of sense, because although it is bulky, it doesn't weigh very much, and when building planes, lightness is rather important.

However, they have missed an obvious trick. Carrying enormous tanks of hydrogen everywhere would bring to the table the possibility and numerous advantages of a deployable balloon. Consider the following.

Co-Pilot: Oh no, we're going to crash!

Pilot: Calm down Roger, pull the emergency brake!

[Balloon inflates, plane comes to a gentle stop in mid air.]

Co-Pilot and passengers: We're saved!

Another potential benefit: Vertical takeoff by inflating an enormous hydrogen balloon. Once the aircraft reaches 70,000ft, it releases the balloon and drops into a dive, levelling off at cruising speed and altitude. Think of the fuel savings! Possibly best suited for transport of cargo and those who enjoy rollercoasters.
-- mitxela, Mar 01 2022

Good for disasters soon after takeoff; less useful for disasters at or near the end of the journey.

Also, releasing a balloon filled with hydrogen doesn't exactly save fuel now does it?
-- pocmloc, Mar 01 2022


As per [pocmloc], pump the hydrogen from the balloon back into the fuel tanks (the transition from ballooning to flying may be complicated...).
Also, how much hydrogen is required to lift an airliner (up past 250T)?
-- neutrinos_shadow, Mar 01 2022


// how much hydrogen is required to lift an airliner (up past 250T)? // Good question, let's figure it out.

At sea level, hydrogen has a lift capacity of about 1.2kg per m3. Let's be overly cautious and say that to lift our 250T plane we'd need 250,000m3 of hydrogen gas.

250,000 cubic metres would correspond to a spherical balloon of 78 metres in diameter. Not absurdly unreasonable! For context the Hindenburg was 245m long, but thin, its volume was around 200,000m3.

Admittedly, although the lifting capacity doesn't change, at higher altitudes the diameter of our balloon would increase significantly due to the lower pressure.

A more pressing question is how much liquid hydrogen could we reasonably expect to be on board. At STP, 1 mol of gas is 24 litres, so our balloon, at 250 million litres, would contain 1.04e7 mol. Conveniently 1 mol of hydrogen weighs 1 gram, so about 10,400 kg of liquid hydrogen is needed to fill the balloon.

Hydrogen potentially has a higher energy density by mass than regular jet fuel, but most jumbo jets' fuel capacity is about half their maximum takeoff weight (an A380 at full tank carries over 250,000kg of kerosene), so 10T of hydrogen to fill the emergency balloon is really quite reasonable.
-- mitxela, Mar 02 2022


I was just reading about the "Doomsday Plane" that is flying over Nebraska today, and somehow that brought me here, to this idea, with a thought of my own, or at least I hope so...

Because after reading this idea and thinking about it for a mere moment it occurred to me like an anvil atop my head, that what this place needs desperately, is a "Doomsday Hullaballon" of its own, and it should be known only as "The Bristolz", in memory of her creator.

Of course, there would be a beautiful memorial prior to her maiden, and seemingly only voyage when a roster of those bakers who have left this plane already, via a whole different sort of "doomsday plane", names would be lovingly read, and their memory honored for their service as halfbakers.
-- blissmiss, Mar 02 2022


// how much hydrogen is required to lift an airliner (up past 250T)? //

// Good question, let's figure it out.//

In none of those equations did I see jettisoning anything extraneous to survival of passengers.
The weight of the wings? Useless.
The weight of the tail section. Useless.
The weight of the baggage? Useless.
Weight of the cockpit? Useless... sans recording equipment.
Weight of first class housing? useless.
Weight of engines? Useless.

Subtract all of those things from your equations and then say the same stuff.
-- 2 fries shy of a happy meal, Mar 03 2022


I keep reading this as "Pork in The Sky"
-- xenzag, Mar 03 2022



random, halfbakery