[Spaceships 7] Homebrew: Expanding on Gasbags

DaltonS · 03-21-2017, 12:17 PM

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.

Dalton “interest, comments, advice?” Spence

03-21-2017, 12:17 PM	#1
DaltonS Join Date: Aug 2004 Location: Hamilton, Ont. CANADA	[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. Dalton “interest, comments, advice?” Spence Last edited by DaltonS; 03-22-2017 at 08:03 AM. Reason: Added pressure to the equation. Not needed for non-rigid airships.