[Spaceships 7] Homebrew: Expanding on Gasbags
 

Last night (while I was trying to sleep) I deduced that the lifting gas used in standard Gasbags (SS7 p.9-10) was Helium. How? Analyzing the Lift row of the Gasbag table, I realized that each ton of system provided 6 tons on lift. This meant it had to displace 7 tons of atmosphere (lift+load). Given that the temperature and pressure of the atmosphere and lifting gas are the same and a standard Earth atmosphere has a mean molecular weight (MMW) of 28.97 (see the NASA Factsheet), the effective molecular weight of the lifting gas should be 4.14. The mean atomic weight (and as a mono-atomic gas its molecular weight) of Helium is 4.002602 (from Wikipedia). I think it is safe to assume the difference is caused by the weight of the envelope itself (65.7 lbs./ton or 3.285%) which would have negligible volume.

This brings about a discussion of other possible lifting gases. Assuming the lifting gas has a pressure, MMW and temperature of P₁, M₁ and T₁ °K respectively and the atmosphere has a pressure, MMW and temperature of Pₐ, Mₐ and Tₐ °K, the lift provided by one ton of lifting gas = (Pₐ×Mₐ/Tₐ)/(P₁×M₁/T₁) - 1 tons.

Examples: The same gasbag using Hydrogen (MMW 2.016) at atmospheric temperature would produce 28.97/2.016 - 1 = 13.37 tons per ton of H₂. Subtract 0.03285 tons for the gasbag, we get 13.34 tons net lift. This would be 66.7% of vehicle's mass. Another half gasbag system would provide more than neutral atmospheric buoyancy.

Of course in Earth's atmosphere, this would be a volatile system. On Mars with its 95% CO₂ atmosphere (MMW 43.34) it would be a different story. The same gasbag would produce 43.34/2.016 - 1 = 40.324 tons lift per ton of H₂. Subtract 0.03285 tons for the gasbag, we get 40.29 tons net lift. This is over twice the vehicle's mass, so a half gasbag system provides more than enough buoyancy.

This works well enough for the inner planets with their relatively heavy atmospheres, although on Venus (MMW 43.45) you might want to use a heavier gas like methane (MMW 16.04) since H₂ at that temperature would probably diffuse through the envelope. (Mercury effectively has no atmosphere.) The outer gas giant planets have a different problem; their atmospheres are mostly H₂ to begin with, so an H₂ gasbag at ambient temperatures would have little or no lift. The solution is to heat the lifting gas to reduce its effective density (see the formula above) and thus increase its lift. This would make it a high-energy system, although how much lift 1PP could generate I'll leave to someone else.

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

I'm fairly sure that if hydrogen had twice the lifting power of helium (as suggested by your numbers) we'd use it for gasbags more, flammable or not.

however, spaceships does things by weight, and that's throwing me for a loop when I try to compare the numbers online with the numbers on the internet: people are always comparing volumes.

which means that you can probably save weight by using hydrogen, but you should probably be paying twice as much for the system because the expensive part (the gasbag) is twice is big, even if the whole things weighs about half as much.

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

I see the actual system consisting of a fuel tank (containing the liquefied gas), an expander, a compressor and an envelope. And in Spaceships, all fuel tanks containing the same mass of fuel cost the same, regardless of their contents or their density. (Not realistic I know, but this is a simplified system.)

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

you're probably right about the three parts. Two of them should scale in cost with volume. With the tanks, you're not actually using fuel tank systems, you're just using equipment storing lifting gas. That may or may not scale with volume, but I think its more than fair to make the systems cost double and take the same volume for providing double the lift.

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

According to this website Helium weighs about twice as much as Hydrogen, however the important factor isn't their relative weights to each other but rather their relative weight to the Air, which primarily consists of Nitrogen and Oxygen. Due to impurities in both gases this the lifting power of Helium is between 88%-93% of the same volume of Hydrogen. This is why airships pretty much switched to Helium - you only get about 10% less lift, but the risk of flammability is drastically reduced and necessity to prevent all leaks means you can even save a little weight on materials.

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

Yes, but working in weights makes it confusing. That ton of hydrogen has roughly twice the volume at the same temperature and pressure as a ton of helium, so it displaces twice as much air. You also need more envelope to hold it.

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

The problem with the website's figures was that they are centered around the volume of the gasbag rather than the mass of the lifting gas, and Spaceships systems are centered on mass. On a ton-for-ton basis, hydrogen is much superior to helium for lift. One thing I didn't address was the cost; I have no idea what sort of multiplier I should use. The hardware used for each gas would be different, and since the lifting gas isn't expended, it can be considered part of the hardware. Leaks would be replenished as a part of regular maintenance. (Note: if hydrogen is relatively cheap, there might be a venting allowance as gas recovery might be more expensive.)

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

Liquefying hydrogen or helium is not trivial to do.

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

But water is pretty common, so as long as you have lots of power, you can refuel pretty easily on nearly any large body in the solar system.

Re: [Spaceships 7] Homebrew: Expanding on Gasbags
 

Another issue never addressed in Spaceships 7 are the velocity limits of a propelled gasbag. (Apparently they were never meant to be for more than simple aerostats.) The massively increased surface area of the craft would create a very high drag coefficient and the fragility of the envelope would require a fairly low top air speed. Also, the propulsion systems mentioned in Spaceships suitable for atmospheric work (jets and rockets) are high-temperature devices; not something I'd care to have near a gasbag, flammable contents or not. What is needed is an aerial propeller system, but sadly there doesn't seem to be one. :-(