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Comment by mikeash | Hacker News Reader

Dirigibles require huge volumes, because the lifting power of gas is small. Those huge volumes make for a relatively weak, slow, and difficult-to-maneuver craft. A big reason why airships disappeared in favor of airplanes was that airships had a really tough time handling or avoiding bad weather because of these limitations.

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I have a blurry recollection of seeing an aircraft design that was a combination of an airplane and a dirigible. It looked like a poofy manta ray, an inflated rigid lifting body or flying wing. It wasn't quite VTOL & needed a very short runway, but it also gained more maneuverability in flight. Maybe it was too much of a compromise though.

Perhaps you're thinking of the infamous 'Deltoid Pumpkinseed'[1][2] ?

[1] https://en.wikipedia.org/wiki/AEREON_26

[2] https://www.amazon.com/Deltoid-Pumpkin-Seed-John-McPhee/dp/0...

You mean this one?

http://mashable.com/2017/05/24/airlander-butt-plane-successf...

There's actually been more than one of these types of aircraft (known as Lighter Than Air - or LTA Aircraft), either as an idea or prototype throughout history.

Could it be this one [0], or its predecessor [1]?

[0] http://www.latimes.com/business/la-fi-lockheed-martin-aircra... [1] https://en.wikipedia.org/wiki/Lockheed_Martin_P-791

That type of vehicle is called a hybrid airship!

[1] https://en.wikipedia.org/wiki/Hybrid_airship

The lifting power of helium is small, as opposed to hydrogen. Also, helium has supply problems.

Is it not possible to have a modern airship with hydrogen as its primary lifting agent?

Hydrogen doesn't buy you that much additional lifting power.

Air is about 1.2kg/m^3 at STP, so that's the theoretical maximum you can lift with a cubic meter of gas bag. (And you can only get that value if you could somehow hold a vacuum, of course.)

Hydrogen is about 0.09kg/m^3 at STP, so you can lift about 1.1kg/m^3. Helium is about 0.18kg/m^3, so you can lift about 1kg/m^3. Helium is twice as dense, but that only costs you about 10% of your lifting power since both are so light compared to air.

I see no reason you couldn't use hydrogen, but I don't know if anyone would want to take the risk. (Specific flaws of the Hindenburg aside, hydrogen is explosive across a distressingly wide range of mixture ratios with air.) If you did use hydrogen, I think the main motivation would be avoiding the use of scarce helium, not the minor increase in lift capacity.

If it's an unmanned drone-blimp, the safety issue because somewhat less critical.

But still critical enough to make it impractical. How would you do maintenance on a hydrogen airship without risking the people doing the maintenance? And you'd never be able to fly over populated areas.

It is possible but we all know what happened to the Hindenburg. Using hydrogen is asking for a fire to happen.

https://en.m.wikipedia.org/wiki/Hindenburg_disaster

Far more helium airships have wrecked than hydrogen airships, even counting only before the Hindenburg.

Wrecked, yes, but they probably didn't burn like a torch with people jumping out of them to avoid burning alive.

What you saw was the engine-fuel burning on the ground; the hydrogen went straight up in flames and burnt no-one.

Keep in mind that the Hindenburg was supposed to be helium filled, but due to import restrictions they couldn't get enough.

Vacuum still has the best lifting power of them all..

We just need a material as light as a balloon but rigid enough to hold its shape and maintain a huge vacuum at 1 atmosphere outside pressure.

Perhaps graphene aerogel, with a nonpermeable graphene casing?

I'm not sure of what its behavior would be with all its air sucked out, though.

like a hair full of nothing..

I assume huge volumes are also easily detectible?

Seems reasonable. All else being equal, your radar cross section is proportional to your actual cross section. Plus it's probably easy to spot a blimp with your eyes.

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