[Spaceships] Is This Legal?

[ericbsmith]

Originally Posted by vierasmarius

That second entry should read "0.45 mps (TL9)". Must be errata in your copy. You should re-download the pdf from e23.

Huh, this is strange. This appears to be a case of an error creeping into the PDF from... I don't know where. My original POD copy of the book lists it as:

"Each fuel tank of hydrogen gives a delta- V of 0.3 mps (TL7-8) or 0.45 mps (TL9+)."

While the current PDF lists it as:
"Each fuel tank of hydrogen gives a delta- V of 0.3 mps (TL7-8) or 0.45 mps (TL8)."


Now, I read that line three times, once after you pointed out the error, before I realized my current PDF copy was listing TL8 twice. My guess is that they intended to change it to "0.3 mps (TL7) or 0.45 mps (TL8+)." but lacking some official statement I'd go by the original printed version which was "0.3 mps (TL7-8) or 0.45 mps (TL9+)." It's also possible this was just a random editing snafu.

EDIT: The POD is version 1.2 November 6, 2007.

[vierasmarius]

Originally Posted by ericbsmith

EDIT: The POD is version 1.2 November 6, 2007.

Yeah, looks like I was the one who forgot to update. =P

[MatthewVilter]

Originally Posted by vierasmarius

Yeah, looks like I was the one who forgot to update. =P

To the busted version. =P


Hum, well I guess I will submit an errata tomorrow. Never done that before. :)

[Varyon]

Originally Posted by scc

No it isn't, this is from Spaceships 7 on the subject of smallar systems:

The appropriate reversal of that is that a larger Fuel Tank would get 3x the Delta-V. You know, the same thing as getting 3 Fuel Tanks.

If all you're looking for is a little more Delta-V, I suggest drop tanks. I don't think there's a RAW source for those, but they're pretty easy to write in - make a space"ship" that is streamlined with a single armor module (as normal), give it a normal sized External Clamp, and have everything else consist of Fuel Tanks. Boom, extra fuel.

You'll need to do some calculations, of course. Let's say you've got an SM+10 vessel with 6 fuel tanks, and you toss on an SM+10 drop tank. Your total amount of fuel is 12,000 tons, and the combined total weight of the vessel is 20,000 tons. That marks your fuel as equivalent to 12 Fuel Tanks (it's 60% of the vessel's mass). However, the multiplier on page 17 is a result of lost mass, and you're going to drop that drop tank as soon as it's empty. That means you're effectively losing 1 additional space, so use the 13 tank entry - a multiplier of 1.6. So, assuming you drop the drop tank as soon as it's empty, you'll end up with a delta-v of 2.88 mps.
This probably has some error margins in it, but should be close enough.

[Ulzgoroth]

Originally Posted by Varyon

The appropriate reversal of that is that a larger Fuel Tank would get 3x the Delta-V. You know, the same thing as getting 3 Fuel Tanks.

If all you're looking for is a little more Delta-V, I suggest drop tanks. I don't think there's a RAW source for those, but they're pretty easy to write in - make a space"ship" that is streamlined with a single armor module (as normal), give it a normal sized External Clamp, and have everything else consist of Fuel Tanks. Boom, extra fuel.

You'll need to do some calculations, of course. Let's say you've got an SM+10 vessel with 6 fuel tanks, and you toss on an SM+10 drop tank. Your total amount of fuel is 12,000 tons, and the combined total weight of the vessel is 20,000 tons. That marks your fuel as equivalent to 12 Fuel Tanks (it's 60% of the vessel's mass). However, the multiplier on page 17 is a result of lost mass, and you're going to drop that drop tank as soon as it's empty. That means you're effectively losing 1 additional space, so use the 13 tank entry - a multiplier of 1.6. So, assuming you drop the drop tank as soon as it's empty, you'll end up with a delta-v of 2.88 mps.
This probably has some error margins in it, but should be close enough.

Doing the delta-V figuring piecewise is probably both more accurate and more useful.

Figure the delta-V for the ship+drop tank combo going through the fuel in the drop tank. Separately figure the delta-V for the ship without the drop tank going through its internal fuel. The ship with the drop tank will do those two things sequentially...

[Varyon]

Originally Posted by Ulzgoroth

Doing the delta-V figuring piecewise is probably both more accurate and more useful.

Figure the delta-V for the ship+drop tank combo going through the fuel in the drop tank. Separately figure the delta-V for the ship without the drop tank going through its internal fuel. The ship with the drop tank will do those two things sequentially...

I had initially done it that way, but I'm not certain how to calculate when the ship has gone through all the fuel in the tank, because that fuel doesn't account for exactly 3/4 of the overall delta-v, unless I'm horribly mistaken. If I am, then instead of overall delta-v of 2.88 you instead have initial delta-v of 2.52 mps, then once you've used up 3/4 of the delta-v (1.89 mps, for 0.63 mps remaining) you drop the drop tank and end up with 1.26 mps remaining, for total overall delta-v of 3.78 mps.

[Ulzgoroth]

Originally Posted by Varyon

I had initially done it that way, but I'm not certain how to calculate when the ship has gone through all the fuel in the tank, because that fuel doesn't account for exactly 3/4 of the overall delta-v, unless I'm horribly mistaken. If I am, then instead of overall delta-v of 2.88 you instead have initial delta-v of 2.52 mps, then once you've used up 3/4 of the delta-v (1.89 mps, for 0.63 mps remaining) you drop the drop tank and end up with 1.26 mps remaining, for total overall delta-v of 3.78 mps.

I'm not exactly sure what you're doing, but it sounds like you're working too hard.

The ship+tank combination has 9 system-equivalents of fuel in the drop tank. (18 systems divided by 2 since the combination is double sized.) That gives you 9*1.4*dV = 12.6*dV where dV is your drive's per-tank delta-V. (I don't know what engine you're assuming here.)

After you've burned that the external tank is empty and you jettison it. All the internal tanks are still full.

At that point you've got 6 systems internal fuel, for another 6*1.2*dV = 7.2*dV. (You also are no longer limited to half your nominal acceleration.)

Total delta-V is thus 19.8*dV, which falls into the large gap between 16.8 for 12 tanks and 20.8 for 13 tanks.

[Anthony]

The correct way to compute delta-V, given the way Spaceships rates systems, is 19.5*(delta-V per tank)*ln( full weight / empty weight). If full weight is 20 spaces, you'll match the standard numbers.

[Ulzgoroth]

Originally Posted by Anthony

The correct way to compute delta-V, given the way Spaceships rates systems, is 19.5*(delta-V per tank)*ln( full weight / empty weight). If full weight is 20 spaces, you'll match the standard numbers.

Not with the drop tank, because you lose an extra chunk of dead weight at a particular point in the middle of the process.

If you just did that, it would be equivalent to calling it 12 tanks and disregarding the dropping of the drop tank.

[Anthony]

Originally Posted by Ulzgoroth

Not with the drop tank, because you lose an extra chunk of dead weight at a particular point in the middle of the process.

Spaceships already assumes that fuel tanks are weightless, so drop tanks are totally useless.