[D R Avery]

D R Avery asked the Naked Scientists:
   
Hydrogen is less dense than helium and it seems would offer a higher lifting power... and with the immense strides in technology over the last few years I've wondered about...

Can a vacuum Airship be made possibly from carbon struts that are stronger than steel and lighter than most metals?  It seems the skin of the ship could probably be similar to today's ships and might look a bit funny where the skin is pulled concave between struts... but the vacuum won't ignite the way hdrogen does and the internal density of the ship would be about as light as can be made... providing even more lift than either hydrogen or helium.

The question intrigues me and I'd love to have an answer.

D R Avery
Trumball St
Whittier, CA 90604

What do you think?

[Geezer]

Interesting idea, but I suspect the mass of the envelope would always turn out to be greater than the mass of a gas filled envelope with the gas it contained. The other issue is that any mechanical failure of the envelope could lead to a catastrophic collapse under atmospheric pressure.

[CliffordK]

The collapsing pressure would be pretty extreme, and failure of your structure would be just as deadly as the explosion of the Hydrogen.

I would think you could fill your blimp with a mix of hydrogen and helium, then evacuate it somewhat, but not trying to get a perfect vacuum.  The H₂/He mix would keep in mind the UEL/LEL of hydrogen to limit the explosiveness of the mix.

One of the most unique substances that has been created in recent years is evacuated silica aerogel.

http://en.wikipedia.org/wiki/Aerogel#Silica

The density of the evacuated aerogel is about 1mg/cm³ which is less than air at 1.2mg/cm³, so it would be a solid that would float in air.  It is still more dense than Hydrogen and Helium, but it would be a pretty extraordinary construction material.  It is hygroscopic, and the evacuated version may also absorb air, so it would need to be covered with sheeting. 

Perhaps one could make an evacuated aerogel structure that looked like Swiss Cheese, and thus you could have the structural strength of the aerogel, and the lightness of the voids.

[Geezer]

The collapsing pressure would be pretty extreme, and failure of your structure would be just as deadly as the explosion of the Hydrogen.

BTW - in hydrogen's defense, the Hindenburg did not explode. It actually burned. Had the envelope been filled with a large amount of oxygen as well as hydrogen, it could have exploded, but in that case, it's unlikely it would been airborne at the time.

There is some conjecture that the biggest problem with the Hindenburg was the highly flammable nature of the material that the outer envelope was made from. If you watch the film of the disaster, you can see that it did burn very quickly.

[CliffordK]

BTW - in hydrogen's defense, the Hindenburg did not explode. It actually burned. Had the envelope been filled with a large amount of oxygen as well as hydrogen, it could have exploded, but in that case, it's unlikely it would been airborne at the time.

If you had it hydrogen filled with a partial vacuum like I had suggested, then a small leak might be more likely to give you a hydrogen/oxygen mix, with some extraordinary results.

The key for a hydrogen blimp would be to control the air/hydrogen interface, preferably with a non flammable,non conductive material with a very high heat resistance.

If Helium is a fossil fuel, are we at risk of depleting our stores?

[SeanB]

Helium is a product which is made in nuclear reactions inside the earth, basically any nuclear reaction or decay that emits an alpha particle. The only reason it is hard to get is that it rises fast, and is a very small part of the atmosphere. Most is obtained from making liquid natural gas, as a leftover that does not condense with the rest of the gas, unless you cool it down to liquid nitrogen temperatures and compress it further. The only reason it is in the natural gas at a useable level is that the hard rock that captured the rising gas in the first place is also less permeable to He than the rocks it is rising through. Helium, along with Hydrogen, tends to diffuse through solid steel quite well, it is not something you are able to store as a high pressure gas for long periods without losing a portion, though you do not have the problem of hydrogen reacting with the material you store it in and making it brittle with time.