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Old 02-04-2016, 01:16 AM   #11
Marasmusine
 
Join Date: Feb 2005
Location: Doncaster, UK
Default Re: Empty Cargo Holds

What about the mass driver/powered rock example, in which fuel tanks and cargo holds are one and the same? The rocket equation surely wouldn't care if it's a fuel tank being emptied of fuel, or a cargo hold being emptied of fuel, its still a reduction in mass?

Related question: Does the dV multiplier hold true when looking at partially full tanks? e.g. If I have 6 fuel tanks in total, but only one is full (e.g. 3 mps of argon), do I still get the x1.2 multiplier (I get 3.6 mps)? Sorry for being dumb.

Last edited by Marasmusine; 02-04-2016 at 01:42 AM.
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Old 02-04-2016, 02:05 AM   #12
SCAR
 
Join Date: Oct 2004
Location: Yorkshire, UK
Default Re: Empty Cargo Holds

Spaceships is an intentionally simplified system, and the dV multiplier table is a simplification of what the rocket equation would give for reduced mass from empty fuel tanks if you did the calculation.

If you really wanted to be accurate, you would need to be recalculating the delta-v per mass of fuel every time the mass of the spaceship change - from burning fuel, firing missiles, taking damage, jettisoning cargo, being boarded, launching vessels from hangers, etc. - which would be no fun (for most people).

For quick play at the table, treating empty cargo holds as empty fuel tanks for the dV multiplier is going to be close enough for game purposes and most people.
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Old 02-04-2016, 02:16 AM   #13
Marasmusine
 
Join Date: Feb 2005
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Default Re: Empty Cargo Holds

Yeah... I am wanting to keep it abstract.
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Old 02-04-2016, 02:39 AM   #14
Ulzgoroth
 
Join Date: Jul 2008
Default Re: Empty Cargo Holds

Quote:
Originally Posted by Marasmusine View Post
What about the mass driver/powered rock example, in which fuel tanks and cargo holds are one and the same? The rocket equation surely wouldn't care if it's a fuel tank being emptied of fuel, or a cargo hold being emptied of fuel, its still a reduction in mass?
Yes, obviously, anything that is serving as a fuel tank should be treated as a fuel tank.
Quote:
Originally Posted by Marasmusine View Post
Related question: Does the dV multiplier hold true when looking at partially full tanks? e.g. If I have 6 fuel tanks in total, but only one is full (e.g. 3 mps of argon), do I still get the x1.2 multiplier (I get 3.6 mps)? Sorry for being dumb.
Properly? No. Spaceships lets you calculate the delta-V when your tanks are full. It does not really give you tools to convert between delta-V and fuel at any points other than full tanks and empty tanks.

As seen before, what empty tanks are equivalent to is making your ship less massive. So with 1 full tank and 5 empty tanks, your ship is only 3/4s its rated mass. So your one tank is equivalent to 4/3s of a tank, and should provide delta-V according to that. (Which is slightly better than 1.2x one tank, again. Because the last bit of reaction mass you use is the most effective.)
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Old 02-04-2016, 03:03 AM   #15
scc
 
Join Date: Mar 2013
Default Re: Empty Cargo Holds

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Originally Posted by Ulzgoroth View Post
Properly? No. Spaceships lets you calculate the delta-V when your tanks are full. It does not really give you tools to convert between delta-V and fuel at any points other than full tanks and empty tanks.

As seen before, what empty tanks are equivalent to is making your ship less massive. So with 1 full tank and 5 empty tanks, your ship is only 3/4s its rated mass. So your one tank is equivalent to 4/3s of a tank, and should provide delta-V according to that. (Which is slightly better than 1.2x one tank, again. Because the last bit of reaction mass you use is the most effective.)
Actually that depends upon the exact circumstances. If he left port with 6 full tanks and only has one left then he does get the multiplier, otherwise he doesn't.
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Old 02-04-2016, 03:13 AM   #16
Ulzgoroth
 
Join Date: Jul 2008
Default Re: Empty Cargo Holds

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Actually that depends upon the exact circumstances. If he left port with 6 full tanks and only has one left then he does get the multiplier, otherwise he doesn't.
That's clearly wrong. Delta-V is state-based, it doesn't care about how your ship came to mass X tons and have Y tons of reaction mass on board.

Also, how would you ever leave port with 6 full tanks and have 1 left? You can't tell when you've spent 5 tanks of reaction mass.
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Old 02-04-2016, 03:27 AM   #17
scc
 
Join Date: Mar 2013
Default Re: Empty Cargo Holds

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Originally Posted by Ulzgoroth View Post
That's clearly wrong. Delta-V is state-based, it doesn't care about how your ship came to mass X tons and have Y tons of reaction mass on board.
The rocket equation doesn't work like that but, it works on a fraction.

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Also, how would you ever leave port with 6 full tanks and have 1 left? You can't tell when you've spent 5 tanks of reaction mass.
I'm pointing out how things are wrongly applied in Spaceships
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Old 02-04-2016, 03:57 AM   #18
Ulzgoroth
 
Join Date: Jul 2008
Default Re: Empty Cargo Holds

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Originally Posted by scc View Post
The rocket equation doesn't work like that but, it works on a fraction.
Yes. A fraction of your current mass.
Quote:
Originally Posted by scc View Post
I'm pointing out how things are wrongly applied in Spaceships
Do you have a cite for where Spaceships actually does this wrong application? I didn't think it addressed the situation, rightly or wrongly.
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Old 02-04-2016, 05:22 AM   #19
munin
 
Join Date: Aug 2007
Location: Vermont, USA
Default Re: Empty Cargo Holds

A (naive) application of the rocket equation says that:
delta-V'/delta-V = ln(m0'/mf')/ln(m0/mf)
Empty cargo holds reduce both m0' and mf', but reducing them by the same amount causes m0'/mf' to increase, increasing delta-V'. Increasing the number of fuel tanks will cause the empty cargo holds to have a greater impact on the change in delta-V.

Plugging that into a spreadsheet with a variety of fuel tank and empty cargo hold counts results in delta-V increasing by 6-9% for each empty cargo hold at the beginning of the trip.

Code:
delta-V'/delta-V

fuel\holds     1     2     3     4     5     6
1          1.054 1.114 1.182 1.258 1.345 1.445
2          1.056 1.118 1.188 1.267 1.358 1.463
3          1.057 1.122 1.195 1.278 1.373 1.484
4          1.059 1.126 1.202 1.289 1.390 1.508
5          1.062 1.131 1.211 1.302 1.409 1.536
6          1.064 1.137 1.220 1.318 1.432 1.569
A delta-V increase of 7.5% per empty cargo hold seems like a reasonable approximation for Spaceships.
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