[Spaceships 7] Homebrew: Expanding on Gasbags

[ericbsmith]

Originally Posted by ericthered

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.

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.

[johndallman]

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.

[DaltonS]

Originally Posted by ericbsmith

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.

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.)

Dalton “who will be updating the equation to include pressure” Spence

[johndallman]

Originally Posted by DaltonS

(Note: if hydrogen is relatively cheap, there might be a venting allowance as gas recovery might be more expensive.)

Liquefying hydrogen or helium is not trivial to do.

[Flyndaran]

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.

[DaltonS]

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

[Flyndaran]

A very weird omission, I agree. I think way back when I wanted to create a steam punk-ish dirigible, I had to use Vehicles from 3rd ed. to get something usable numbers. I know I created a thread asking about it, but can't remember specifics.

[DaltonS]

Originally Posted by Flyndaran

A very weird omission, I agree. I think way back when I wanted to create a steam punk-ish dirigible, I had to use Vehicles from 3rd ed. to get something usable numbers. I know I created a thread asking about it, but can't remember specifics.

I checked out that thread. A lot of interesting points were raised, but as this was a TL5 Dirigible they didn't seem directly applicable to this situation, although using my lift equations for your hydrogen gasbags could reduce their number and leave space for more systems. As for aerial propeller systems, I'm thinking the best design would be as dedicated power plants that provide thrust instead of power points. (Yes, that would be one for each type of power plant. ;) ) Figuring out just how much thrust is the tricky part. When I was translating Gravity Screens from GURPS Vehicles Expansion 2, the size of a standard energy point I used came from here.

Originally Posted by Anthony

We can derive it from two sources: power output of weapons, and power output of electric drives.
...
Overall, I'd assume 1 ep is somewhere around 100 kW/ton, which is on a par with a reasonable performance ground car (though said car probably isn't spending only 5% of its mass on its power plant).

I worked out the following figures from The "Airscrew Table" on VE2 p.33. With the airscrew having a mass of 2-3% of the power plant, I think it's safe to ignore it. The "Additional Cost" is per ton of system; add it to the cost of the power plant used.

                          Accel. Additional
TL      Type             (in Gs)  Cost/ton
5-6     Aerial Propeller  0.00625   $300
Late 6  Aerial Propeller  0.0075    $200
7+      Aerial Propeller  0.00875   $800
7       Ducted Fan        0.01     $2,400
8+      Ducted Fan        0.01     $1,600

What the 3E TLs translate to in 4E is problematical, as is top airspeed (500 mph for regular streamlined aircraft, 150 for dirigibles?).

Dalton “who wonders why he didn't work this out before” Spence