Steve Jackson Games Forums [Spaceships] simply fuel math...
 Register FAQ Calendar Search Today's Posts Mark Forums Read

 Notices Our primary server had issues at our hosting provider and had to be restored from backup. Posts made after 5/20/2018 11:59PM CDT (UTC -5) and before 5/22/2018 3:00pm CDT may be missing and are not expected to return. Posts made after 5/22/2018 3:00pm CDT (UTC -5) and before 5/23/2018 10:00am CDT may be missing from view until new posts are made to that thread.

 10-03-2017, 03:19 AM #1 jacobmuller     Join Date: Feb 2006 Location: Not in your time zone:D [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... __________________ "Sanity is a bourgeois meme." Exegeek PS sorry I'm a Parthian shootist: shiftwork + out of = not here when you are:/
10-03-2017, 03:23 AM   #2
malloyd

Join Date: Jun 2006
Re: [Spaceships] simply fuel math...

Quote:
 Originally Posted by jacobmuller 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:/
It's simple the other way around. It gets you 450 mps. No matter what size the engine is, the final velocity depends on the size of the fuel tank relative to the entire ship, not relative to the engine. A smaller engine just takes longer to get there - 40 hours instead of 24 minutes to burn your tank dry.
__________________
--
MA Lloyd

Last edited by malloyd; 10-03-2017 at 03:27 AM.

 10-03-2017, 04:31 AM #3 RogerBW MIB     Join Date: Sep 2008 Location: near London, UK 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. __________________ Podcast: Improvised Radio Theatre - With Dice Gaming stuff here: Tekeli-li! Blog; Webcomic Laager and Limehouse Buy things by me on Warehouse 23
 10-03-2017, 05:00 AM #4 ericbsmith     Join Date: Aug 2004 Location: Binghamton, NY, USA. Near the river Styx in the 5th Circle. 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. __________________ Eric B. Smith GURPS Data File Coordinator GURPSLand The future keeps telling us what the past was about. You make the past mean different things by what you do with the time that comes after.
 10-03-2017, 05:35 AM #5 jacobmuller     Join Date: Feb 2006 Location: Not in your time zone:D 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 __________________ "Sanity is a bourgeois meme." Exegeek PS sorry I'm a Parthian shootist: shiftwork + out of = not here when you are:/
 10-03-2017, 07:47 PM #6 hal   Join Date: Aug 2004 Location: Buffalo, New York 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
Ulzgoroth

Join Date: Jul 2008
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.
Quote:
 Originally Posted by hal 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?
So (A) there's no actual reason to involve the smaller rocket in this, it has no bearing on the question.

(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.
__________________
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.

10-03-2017, 09:54 PM   #8
TGLS

Join Date: Jan 2014
Re: [Spaceships] simply fuel math...

Quote:
 Originally Posted by hal 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?
OK, if you want something more accurate, but more fiddly, then I think this is right. Take the natural logarithm of 20 divided by 20 minus the number of fuel tanks and divide by 0.05 (ln(20/(20-tanks))/0.05). This gives you the effective number of fuel tanks for that particular drive, so calculate your delta V from there.

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.

10-03-2017, 10:27 PM   #9
Ulzgoroth

Join Date: Jul 2008
Re: [Spaceships] simply fuel math...

Quote:
 Originally Posted by TGLS OK, if you want something more accurate, but more fiddly, then I think this is right. Take the natural logarithm of 20 divided by 20 minus the number of fuel tanks and divide by 0.05 (ln(20/(20-tanks))/0.05). This gives you the effective number of fuel tanks for that particular drive, so calculate your delta V from there.
It's probably simpler to describe "divide by 0.05" as "multiply by 20".
__________________
I don't know any 3e, so there is no chance that I am talking about 3e rules by accident.

10-03-2017, 10:43 PM   #10
RyanW

Join Date: Sep 2004
Location: Southeast NC
Re: [Spaceships] simply fuel math...

Quote:
 Originally Posted by TGLS OK, if you want something more accurate, but more fiddly, then I think this is right. Take the natural logarithm of 20 divided by 20 minus the number of fuel tanks and divide by 0.05 (ln(20/(20-tanks))/0.05). This gives you the effective number of fuel tanks for that particular drive, so calculate your delta V from there.
If you want to be really pedantic, divide by ln(20/19) instead of 0.05. Assuming the table is accurate for exactly one tank, that will give you 1 = 1. It's only about a 2.5% difference, though.
__________________
RyanW
The goal of a mentor should be to run out of answers before the pupil runs out of questions
Check out my GURPS (et al) wiki, Sweetcandle. Latest addition: Avatar style bending in Fate Core

 Tags spaceships

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Fnords are Off [IMG] code is Off HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Illuminati Headquarters     SJ Games Discussion     Daily Illuminator     Forum Feedback and Help     MIB Demo Events Warehouse 23     Warehouse 23 General Discussion     Warehouse 23 Digital     Pyramid Munchkin     Munchkin 101     Munchkin     Munchkin Collectible Card Game     Other Munchkin Games Roleplaying     Dungeon Fantasy Roleplaying Game         DFRPG Resources     GURPS         GURPS Resources     Transhuman Space     Traveller     The Fantasy Trip     In Nomine     Roleplaying in General     Play By Post Board and Card Games     Car Wars     Ogre and G.E.V.         Ogre Video Game         Ogre Scenarios     Board and Dice Games     Card Games     Miniatures The Network     GURPS Character Assistant     GURPS Vehicle Builder     GURPS Character Builder The Gnomes of Zurich     The Industry     Conventions     Trading Post     Gamer Finder

All times are GMT -6. The time now is 04:07 AM.