10-03-2017, 03:19 AM | #1 |
Join Date: Feb 2006
Location: Not in your time zone:D
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[Spaceships] simply fuel math...
RAW Super Fusion Torch, 50G 450mps but, if you allow use of an SM-4 section to get only 0.5G, how far will that tank of fuel take you - 45,000mps?
This is either one of those math things so simple you can't believe your own answer or it's Costello's rent arrears and I'm the landlord:/ I mean, I can't believe my own answer so, I need someone else to either say "Yup, it's really that simple." or swat me upside the head and show me the actual math that proves it's really all rocket science. TQ in antici...
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10-03-2017, 03:23 AM | #2 | |
Join Date: Jun 2006
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Re: [Spaceships] simply fuel math...
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-- MA Lloyd Last edited by malloyd; 10-03-2017 at 03:27 AM. |
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10-03-2017, 04:31 AM | #3 |
Join Date: Sep 2008
Location: near London, UK
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Re: [Spaceships] simply fuel math...
Looked at from a different angle, adding more engines doesn't get you any less delta-V from the single fuel tank, so removing fractions of an engine shouldn't get you any more.
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10-03-2017, 05:00 AM | #4 |
Join Date: Aug 2004
Location: Binghamton, NY, USA. Near the river Styx in the 5th Circle.
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Re: [Spaceships] simply fuel math...
Delta-V - the change in velocity - is a measure of engine efficiency and does not change with engine size (at least, not on the scale of engines mounted on spaceships). A larger engine uses fuel just as efficiently as a smaller engine does.
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Eric B. Smith GURPS Data File Coordinator GURPSLand I shall pull the pin from this healing grenade and... Kaboom-baya. |
10-03-2017, 05:35 AM | #5 |
Join Date: Feb 2006
Location: Not in your time zone:D
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Re: [Spaceships] simply fuel math...
Thank you, all.
Brain off day. I think part of my scribbled notes were what if you "super" the rocket and scale from there and then I've wandered off into Lalaland:D
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"Sanity is a bourgeois meme." Exegeek PS sorry I'm a Parthian shootist: shiftwork + out of country = not here when you are:/ It's all in the reflexes |
10-03-2017, 07:47 PM | #6 |
Join Date: Aug 2004
Location: Buffalo, New York
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Re: [Spaceships] simply fuel math...
Ok, piggy backing on this thread...
Suppose you decide to design a spaceship with the following in mind: You have a SM 9 ship. A single "reaction Engine" for this ship will produce 3G acceleration. However, using the Swapping out rule from page 5 of Transhuman Space ships... Suppose I decide to use one SM 8 rocket engine to produce 1G extra, allowing for a 4G acceleration? Further suppose that I use the other two modules for Fuel. Those two fuel will be 2/3rds of a fuel tank for a SM 9 ship. So what happens to the Delta-V increase (page 17 of the First GURPS SPACEBOOKS rules)? It states that if you have 6 to 8 tanks, multiply your delta-V by 1.2. If you have 9 to 12, multiply by 1.4 Now, if we have 2/3rds of a tank, we have 2/3rds of the delta-V that a normal SM9 tank would have. No question there. But what happens to the Delta-V multiplier when you're at the "threshold" level between the multiplier values? In other words... if you have 5 and 2/3rds of a tank, does this get the full 1.2 modifier? If you have 8 and 2/3rds of a tank, do you get the full 1.4 multiplier? How would you resolve that issue? |
10-03-2017, 08:16 PM | #7 | |
Join Date: Jul 2008
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Re: [Spaceships] simply fuel math...
While the initial question is well answered, I'd add a thought - the smaller engine will take 100 times as long to burn through the reaction mass...but since it's producing 1/100 times as much acceleration, naturally the delta-V comes out the same.
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(B) Assuming I was using the by-the-book delta-V rule rather than using the rocket equation to smooth it, I'd use it by the book - 5 and 2/3s is not 6 to 8, it is less than 6, so you don't get that multiplier. 8 and 2/3s is slightly more of a problem, but for consistency I'd put it with the lower group rather than the upper group.
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. Last edited by Ulzgoroth; 10-03-2017 at 08:21 PM. |
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10-03-2017, 09:54 PM | #8 | |
Join Date: Jan 2014
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Re: [Spaceships] simply fuel math...
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Aside: The multipliers given in the book exist to approximate the rocket equation's effect on Delta V (i.e. higher proportion of propellant, higher delta V). However, they do seem to error on the low (but occasionally the high). For example, 19 tanks would have a multiplier of 3.15 according to my equations, but it gets a multiplier of three. On the other hand, 6 tanks should have a multiplier of 1.18 according to my equations, but it gets a multiplier of 1.2. |
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10-03-2017, 10:27 PM | #9 | |
Join Date: Jul 2008
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Re: [Spaceships] simply fuel math...
Quote:
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I don't know any 3e, so there is no chance that I am talking about 3e rules by accident. |
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10-03-2017, 10:43 PM | #10 | |
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Join Date: Sep 2004
Location: Southeast NC
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Re: [Spaceships] simply fuel math...
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