# Vacuum Lifting Cells

One option for ultra-tech zeppelins is to do away with lifting gas altogether and use vacuum lifting cells. Since vacuum has no mass, it provides even greater buoyancy than hydrogen.

The only problem is that (as you probably realise) you're going to need extreme ultra-tech materials or force fields to maintain a vacuum cell under the external atmospheric pressure.

Air density at sea level is about 1.2 kg/m^3, so each cubic metre of vacuum gives us 1.2 kg (times gravity) of lift. This is only about 10% greater than hydrogen, and 20% greater than helium, but with tens of thousands of cubic metres of cell volume this can be a substantial gain in lifting capacity.

Assume a roughly cylindrical vacuum cell, say 100m long and 10m radius, that holds 31,400 m^3 (or 1.1 million cubic feet) of vacuum. That generates 37,700 kg (or about 83,000 pounds) of lift. Surface area of the cell is 6911 m^2, so your vacuum cell structural material is limited to a maximum mass of about 5.4 kg/m^2.

The density of steel is 7800 kg/m^3, so the maximum thickness of steel plate you can use is 0.7 millimetres (1/36 inch). Obviously this isn't going to be anywhere near strong enough! And it needs to be even thinner if you want any appreciable usable lift (as opposed to lift merely used to keep the vacuum cell itself aloft).

To get an idea of the strength required from an ultra-tech material, the vacuum cell has an air pressure equivalent to just over 10 tonnes on every square metre of its surface. This is like taking a sheet of that 0.7 millimetre thick steel, using it to bridge a 100 metre wide chasm, parking 500 cars on it (you'd have to stack them about 10-high to fit them on), and expecting it to not even *bend* appreciably - let alone collapse.

The mind boggles at a material with this sort of structural strength! Such a material is likely to be available only at TL12 or above - individual GMs will need to make a judgement call.

In order to gain an advantage over helium lifting gas, the structural material (or force field generators) required to maintain a vacuum cell must have a mass of less than 0.2 kg per cubic metre of cell volume greater than that required for a helium cell of the same size. (The limit is 0.1 kg per cubic metre for an advantage over hydrogen, but it can easily be argued that safety considerations make hydrogen less desirable!)

One possible way to estimate the structural strength required for a vacuum lifting cell is to use the "crush depth" rules from p133 of GURPS Vehicles, 2nd Edition. One atmosphere of pressure difference is equal to 33.9 feet of water overhead... round up to 12 yards to be conservative. Also assume a vacuum cell wouldn't have a "submersible hull" construction. Then, according to the given rules, even DR1 of armour is overkill. Taking the lowest DR (1), add 10 (11), multiply for hull strength of the cell (call it medium, so 11 still), multiply by 10 yards (110), and divide by 2 for lacking a submersible hull (55 yards, 165ft). A very light frame would reduce that to 13.75 yards, which is more than enough.

Even though the official GURPS rules produce this result, I am sceptical about DR 1 being strong enough to withstand one atmosphere of pressure differential over as large an area as a typical zeppelin gasbag. A submarine is a lot smaller and makes good use of the arch principle by transferring some of the external pressure to compression within the structural material itself. With a large gasbag, doing this is likely to buckle the material. I also suspect the crush depth formula in Vehicles is more realistic for greater depths, and may break down somewhat for very low DR values. After all, who's gonna design a submarine with DR 1? Cloth is DR 1!

In fact, from the calculations above, I think that realistically you will need extreme ultra-tech materials before you can approach the required lightness, strength, and above all stiffness to maintain a usable vacuum cell. I would hazard the guess that such materials wouldn't be available until about TL12, if ever. You'd need something like scrith from Larry Niven's Ringworld.

The other option does away with concerns of structural strength and armour. Force fields can reasonably maintain the required vacuum cell integrity with relative ease. Forcefields are TL11 - by this point, light, long-endurance power sources (fusion, anti-matter, even fission) are available - so forcefields make a good choice.

You could have a vehicle much like a regular car or truck, with a forcefield generator that can be flicked on to generate a huge, invisible, vacuum-filled force-bag attached to the roof, and it would simply float up into the sky. :-) Who needs contragravity?!

Some of this material is based on posts to the Usenet newsgroup rec.games.frp.gurps by Mike Miller and M. A. Lloyd.
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