On building an airship

I have recently decided that I want an airship.

If you have seen Up, you know what I'm talking about.  The idea of a powered lighter-than-air transport device allowing for leisurely exploration, equipped with a full cabin and living quarters is incredibly freeing.  To be able to roam the Earth above the clouds, descending wherever you choose to land, reaching places untouched by any other vehicle... the idea sounds incredible.

Unfortunately, it's pretty much still only in the realm of fantasy - or perhaps possible for a crazed billionaire like Sir Richard Branson.

The single biggest issue with an airship is the most obvious - getting it off the ground.  Currently there are two options, neither of which is especially appealing:

1. Hot air.  This is how hot air balloons work.  It's cheap, which is great.  On the downside, hot air (at least, in balloons) lifts about 0.3 ounces per cubic foot.  That's not a lot.  Just to lift me would take 6,500 cubic feet of hot air.  Add in the carriage, heater, propulsion source, and, not insignificantly, the actual containment bag, and we're talking about an incredibly massive apparatus.  I'd like for my airship to be smaller than the city I park it over.

2. Helium.  Currently, this is how airships like the Goodyear blimp get off the ground.  And it has much more lifting power than hot air, at nearly an ounce per cubic foot.  So you only need 1/3 the bag size as using hot air.  But the caveat with helium is price: about $0.60 per cubic foot.  Now, that's not too bad for birthday balloons.  But for two 150-lb people and a 200-lb structure, we're talking nearly $3 grand in helium costs.  And that isn't a one-time purchase; helium dissipates, and the dirigible must be refilled every now and then.

There is one more option that isn't on here: hydrogen.  Hydrogen provides about 7% more lift than helium, but it is also very, very explosive.  Hindenburg ring any bells?  In addition, hydrogen airships are illegal in many countries, simply due to the ever-present risk of danger.  There's also a wide range of concentrations where hydrogen is still very flammable, which means even mixing it with air doesn't do the trick of preventing your airship from disappearing in a very loud "bang."

So for lift, I'm currently going with helium, but it's not a great solution.  But let's put a pin in that for now and turn to the other side of the coin: weight.

Humans are fairly dense.  Food is heavy.  Fuel is heavy.  And engines are heavy.  All of these things need to somehow get up in the air, which means a lot of lift.  Fortunately, in this area there are a few useful innovations that could be beneficial.

#1: Aerogel.  This substance is basically a gel, but instead of containing liquid between the matrix of solid molecules, it contains air.  Very expensive to make (currently more expensive per volume than gold), but it is also incredibly light.  And unlike "gel" suggests, it's quite solid and durable.  Building many of the airship structural parts out of aerogel, especially internal struts and supports, could help drastically reduce the weight.

#2: Stirling engines.  These are engines that run based on a temperature differential.  For example, they can run when placed on a hot surface in a cooler environment.  And given the fact that hot air is providing lift on its own, using that heat for propulsion would be a great idea as well.  Perhaps most important with a Stirling engine is that, because its "fuel" is the temperature differential, there's no need to carry around heavy, flammable liquids like gasoline.

One option that uses Stirling engines is to have a black airship; in the day, the sun would heat up the interior of the structure, creating a temperature gradient to power the Stirling engines.  Of course, this would be fairly ineffective on cold days or at night, but it would help lessen other fuel demands.

#3: Carbon fiber.  Carbon fiber seems to be everywhere now (I can buy a carbon fiber wallet and clipboard online now, if I had any inclination to do so), but it is a very useful material.  Incredibly strong and quite light, this would be ideal for the structure of the airship and cabin.  Synthesizing it is expensive, of course, but costs are constantly dropping, further aided by the use of 3D printers.  A question to consider: could a Stirling engine be build mainly of carbon fiber parts?

Unfortunately, lightening the load only goes so far.  Even if the structure was weightless, there's still the weight of people, cargo, food, and so on.  We're still talking about a couple thousand cubic feet of substance for lift.  So until a better method of achieving lift is discovered, this will be a frustrating concept.

Still, the end result is incredibly appealing.  What a retirement that would be...