03-21-2017, 11:17 AM | #1 |
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.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 07:03 AM. Reason: Added pressure to the equation. Not needed for non-rigid airships. |
03-21-2017, 12:42 PM | #2 |
Hero of Democracy
Join Date: Mar 2012
Location: far from the ocean
|
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.
__________________
Be helpful, not pedantic Worlds Beyond Earth -- my blog Check out the PbP forum! If you don't see a game you'd like, ask me about making one! |
03-21-2017, 01:02 PM | #3 |
Join Date: Aug 2004
Location: Hamilton, Ont. CANADA
|
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.)
Dalton “now how do you propel a blimp?” Spence
|
03-21-2017, 01:50 PM | #4 |
Hero of Democracy
Join Date: Mar 2012
Location: far from the ocean
|
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.
__________________
Be helpful, not pedantic Worlds Beyond Earth -- my blog Check out the PbP forum! If you don't see a game you'd like, ask me about making one! |
03-21-2017, 05:10 PM | #5 | |
Join Date: Aug 2004
Location: Binghamton, NY, USA. Near the river Styx in the 5th Circle.
|
Re: [Spaceships 7] Homebrew: Expanding on Gasbags
Quote:
__________________
Eric B. Smith GURPS Data File Coordinator GURPSLand I shall pull the pin from this healing grenade and... Kaboom-baya. |
|
03-21-2017, 05:23 PM | #6 |
Night Watchman
Join Date: Oct 2010
Location: Cambridge, UK
|
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.
__________________
The Path of Cunning. Indexes: DFRPG Characters, Advantage of the Week, Disadvantage of the Week, Skill of the Week, Techniques. |
03-22-2017, 06:54 AM | #7 | |
Join Date: Aug 2004
Location: Hamilton, Ont. CANADA
|
Re: [Spaceships 7] Homebrew: Expanding on Gasbags
Quote:
Dalton “who will be updating the equation to include pressure” Spence
|
|
03-22-2017, 01:28 PM | #8 | |
Night Watchman
Join Date: Oct 2010
Location: Cambridge, UK
|
Re: [Spaceships 7] Homebrew: Expanding on Gasbags
Quote:
__________________
The Path of Cunning. Indexes: DFRPG Characters, Advantage of the Week, Disadvantage of the Week, Skill of the Week, Techniques. |
|
03-22-2017, 01:33 PM | #9 |
Untagged
Join Date: Oct 2004
Location: Forest Grove, Beaverton, Oregon
|
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.
__________________
Beware, poor communication skills. No offense intended. If offended, it just means that I failed my writing skill check. |
03-23-2017, 07:22 AM | #10 |
Join Date: Aug 2004
Location: Hamilton, Ont. CANADA
|
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. :-(
Dalton “any ideas how to get around this?” Spence
|
Tags |
gasbag, spaceships 7 |
Thread Tools | |
Display Modes | |
|
|