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Old 11-04-2017, 03:08 PM   #1
hal
 
Join Date: Aug 2004
Location: Buffalo, New York
Default Question regarding GURPS SPACESHIPS formulas

Hello Folks,
Was working on some space craft for my Cyberpunk campaign - thinking I'd give GURPS SPACESHIPS a whirl to see what it could do. Knowing that my buddy doesn't much like dealing with Math during gaming, I thought I'd put together a spreadsheet that has all of the Formulas from the first book GURPS SPACESHIPS on it, and I'd simply enter the values as necessary.

Under "Space Travel with Reaction Drives" page 37, it talks about deltaV and moving between Mars and Earth. More specifically, it talks about accelerating for a bit, and then having to drop down to a final velocity of 2.1 miles per second to reach orbit around Earth.

But my question is this:

The ship starts off with a velocity equal to that required to escape the Mar's velocity (or .93 MPS). If it accelerates to 25 MPS, it would have needed an additional 25 MPS (its target speed) less that it already had, or 24.07 MPS.

So, in the example given - shouldn't the formula have been working of off the 24.07 MPS rather than 25? Likewise, if the ship needs to slow down to 2.1 MPS, it requires a delta V change of 25-2.1 or 22.9 MPS which would require its own calculation?

"Example: Since Princess of Helium has an acceleration of 0.5G and wants to reach 25 mps, it spends 25 x 0.0455/0.5 = 2.3 hours accelerating to cruising speed near Mars. In the process, the occupants experience a gentle one- half of an Earth gravity acceleration. The ship will need to decelerate for the same time at the other end of its trip."

Just checking to insure that I'm doing this right, which is why I'm following the examples given in the book - step by step so that my answers agree with what is in the book.

Thanks.

Hal
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Old 11-04-2017, 03:47 PM   #2
Ulzgoroth
 
Join Date: Jul 2008
Default Re: Question regarding GURPS SPACESHIPS formulas

Well, the reality is more complicated than that, since Earth and Mars are in motion relative to one another, and the ship will pick up some extra speed as it drops from martian to terrestrial orbit...

But if you're treating the planets as stationary, as the book seems to, what you're saying seems to make sense. There is an additional consideration of gravitational acceleration and deceleration when near the planets, but if you can do your burns close enough to periapsis it won't much matter.
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Old 11-04-2017, 03:54 PM   #3
hal
 
Join Date: Aug 2004
Location: Buffalo, New York
Default Re: Question regarding GURPS SPACESHIPS formulas

So, I should essentially be adding the orbital velocities of the staring planet and the ending planet to the equation?
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Old 11-04-2017, 04:10 PM   #4
MrTim
 
Join Date: Jul 2005
Location: Albuquerque, NM
Default Re: Question regarding GURPS SPACESHIPS formulas

That assumes that the planets are moving straight toward each other, though, which requires very carefully planned (and very small) launch windows.
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Old 11-04-2017, 07:04 PM   #5
Fred Brackin
 
Join Date: Aug 2007
Default Re: Question regarding GURPS SPACESHIPS formulas

Quote:
Originally Posted by hal View Post
So, I should essentially be adding the orbital velocities of the staring planet and the ending planet to the equation?
If you care that much about absolute accuracy. For a ship with 50 miles per second or more of Delta-V it's not critically important. If the ship had only 5 mps of Delta-V or so it would be more important.
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Old 11-04-2017, 09:16 PM   #6
Anthony
 
Join Date: Feb 2005
Location: Berkeley, CA
Default Re: Question regarding GURPS SPACESHIPS formulas

In general the rules on SS37 should only be used for ships with enough delta-V that escape velocity is a rounding error; the actual interaction between planetary escape velocity and transfer velocity is complex and dependent on the acceleration of the craft (for example, to go from low earth orbit to escape velocity actually ranges from about 2.1 to 4.9 mps delta-V, depending on thrust).
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Old 11-04-2017, 09:41 PM   #7
hal
 
Join Date: Aug 2004
Location: Buffalo, New York
Default Re: Question regarding GURPS SPACESHIPS formulas

Quote:
Originally Posted by Anthony View Post
In general the rules on SS37 should only be used for ships with enough delta-V that escape velocity is a rounding error; the actual interaction between planetary escape velocity and transfer velocity is complex and dependent on the acceleration of the craft (for example, to go from low earth orbit to escape velocity actually ranges from about 2.1 to 4.9 mps delta-V, depending on thrust).
Oddly enough, this only started as a "create a spreadsheet and make sure it matches the concepts and numbers". If the example says one thing, and the concept says another, then I question it a little.

I guess that's why I'm going to look at the Halfway to Anywhere article, as it has a bit more to the formula than is in GURPS SPACESHIPS. Originally, I had considered the prospect of simply using GURPS TRANSHUMAN SPACE ship building rules, but thought "hey, I paid the money for the rules, I should at least give it a whirl!".

Thanks Folks.
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