Empty Cargo Holds
 

Hello, this has probably already been answered somewhere but I can't find anything related through Search. Maybe there's a FAQ I don't know about?

In Spaceships, if I have empty cargo holds, my ship has less mass. Is it reasonable to have each empty cargo hold "count as" a fuel tank for the purposes of delta-V increase (GURPS Spaceships p. 17) ?

Re: Empty Cargo Holds
 

I believe your logic is sound, but I'm not entirely sure treating an empty cargo hold as a fuel tank is the right way to go about it. Isn't there something in a later chapter for when fuel tanks start to get empty?

Re: Empty Cargo Holds
 

It's probably easiest to just recalculate the acceleration and delta based on a lower ship mass . . . I think.

Re: Empty Cargo Holds
 

It certainly seems logical - treating the empty cargo hold as an empty fuel tank only for the purposes of the delta-v multiplier - since the delta-v multiplier is intended to represent the effect of decreasing mass due to the empty fuel tanks.

SM+8/TL10: Fusion Torch (15mps on Hydrogen), 5 Fuel Tanks (250 tons of fuel, 75mps Total); 4 Cargo Bays - where 2 are empty.
Treating the empty cargo bays as empty fuel tanks raises the Delta-V by 1.2 to 90mps.

This would be the same calculation for if you had 7 fuels tanks giving 126mps; If you burn 36mps of fuel, you've emptied 2 fuel tanks, and have 5 fuel tanks of hydrogen left. i.e. 2 empty fuel tanks has the same state as 2 empty cargo bays - just without that initial 36mps of fuel!

Calculating the actual effect requires the rocket formula which is somewhere here on the forums, but it does not seem necessary.

Re: Empty Cargo Holds
 

Count as an empty fuel tank. Yes, I really like this as a rule. It also lets you do things like run empties back on a cargo run or dump cargo to gain delta V.

That should only count for Cargo Hold modules, and a damaged cargo hold won't count until you've actually taken the time to empty the ruined cargo from it. Empty Hangars... well, I suppose you could say that every 2 empty Hangars could count as one empty fuel tank for delta V purposes. But that's starting to get fiddly. I'd absolutely not allow that for steerage cargo you get from a habitat module.

Re: Empty Cargo Holds
 

You could also have jettisonable modules for passengers and non-essential crew.

(Alert lighting comes on. Internal blast doors seal.)

"This is the captain speaking. We are being pursued and will not escape unless we can increase our acceleration now. Therefore I regret to inform you that we are about to jettison non-essential personnel. You have five minutes to make your final prayers, or put on a vacc suit. Sorry. We have no other option. Captain out."

;->

Re: Empty Cargo Holds
 

'Count as an empty fuel tank' isn't particularly simple. If you've got 15 'fuel tanks' but 9 tanks full of reaction mass, you do not have 9/15ths of the delta-V you'd have at 15 tanks. You have more than that, because the last few drops are the ones that do the most.

Re: Empty Cargo Holds
 

Thank-you for your answers.

I've also noticed that fuel for the mass driver (e.g. "powdered rock") can be stored in cargo holds or fuel tanks, so there's that too.

Re: Empty Cargo Holds
 

I can only see the chart under the Fuel Tank section which says that performance improves as the tanks are emptied. It abstracts this as a general multiplier to dV based on the total number of tanks (irrespective of if they are full or not, since I think it represents the effective dV over the course of all the fuel being used.)[/I]

I'm wondering if the math is correct If we say, well I have 8 fuel tanks and two full cargo holds, my effective dV is x1.2. On the return trip I have two empty cargo holds, my effective dV is x1.4 (the next step on that table, 10 tanks).

Writing it out now, I have a feeling it's not correct. Per what Ulzgoroth said.

Re: Empty Cargo Holds
 

Yeah, it's not. What you actually have is 8/18ths of your mass being fuel, or 20*8/18 = 8.88... fuel-tank equivalents.

Which, um. Should give a better number than your way, but doesn't. (It gives one-tank delta-V x 8*10/9*1.2 = 8*1.33..., if one applies the rules.) Probably to do with multiplier table breakpoints, I can check the rocket equation math later.