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Old 10-04-2017, 03:39 PM   #17
hal
 
Join Date: Aug 2004
Location: Buffalo, New York
Default Re: [Spaceships] simply fuel math...

Quote:
Originally Posted by Anthony View Post
Formula for delta-V is ISp * ln( loaded/dry ). However, if ISp and delta-V are not in the same units, you need a conversion factor. ISp has historically been described in pound-seconds per pound (where the first pound is a lb force, the second pound is a lb mass), or just Seconds.

The conversion from Seconds to mps is ~0.0061 mps/Second, I don't know where you're getting 0.003 from.
From page 189 of TRANSHUMAN SPACE that reads under the heading of "Realistic Delta-V"

"For more realism, use this formula: Delta-V (in mps) = 0.003 ´ ISP ´ ln [Loaded Mass (in tons)/Dry Mass (in tons)]. The symbol “ln” means “natural logarithm”; a natural logarithm key, marked “ln,” can be found on most calculators."

My guess is that the .003 is a conversion factor required for translating the normal units Isp is given as, into mps values.

THS lists Oxygen/Kerosene as having an ISP of 514 - which is better than the other chemical rocket Isp values. I guess that's why I wondered at the calculated 974.79 value.

Granted, I only used ONE fuel tank for the calculations, but then again, one fuel tank's worth of Delta-V was given in GURPS SPACESHIPS, which seemed to me that I was talking Apples to Apples here. THS uses Delta-V in mps units, just like GURPS SPACESHIPS does.
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