Chemical Drive: fuel capacity versus velocity

Shadow Shack's picture
Shadow Shack
June 10, 2009 - 2:15am

Over in the Historic Adventures project, I am working on some mechanics for a pf:100 period adventure/campaign. Since this setting is going to predominantly entail system ships, I needed some definitions for chemical drive operation as the canon rules were rather vague, citing little more than fuel costs versus journey but not addressing performance (which the rules also state as ADF of 1 for chemical drives).

So I whipped up a prototype table that cites fuel capacity and establishes a maximum velocity, seeing as the bulk of a system ship's travel time will be spent coasting at zero G I figured there would need to be some definition here. Naturally this will require some play testing, but this is a prototype that I feel is a good basic formula (which may be subject to tweaking yet).

It also illustrates the (unlikely) possibility that a system ship, given enough fuel, could attain jump velocity...hinting at a possible earlier historic point where such a venture may have been attempted prior to the advent of ion drives.
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TerlObar's picture
June 10, 2009 - 2:43pm
When I get a chance I'll do some number crunching on this.  I actually think it will be quite hard to get anywhere near 1% c with chemical drives.  As you try to push the velocity up the amount of fuel goes up expoentially as you have to push all the mass of all the additional fuel you're carrying as well.

For reference, the fastest spacecraft we've ever launched, got up to 16km/s from it's rockets (that 0.0053% c) relative to the earth.  It's total velocity peaked at 46 km/s relative to the sun since it got a boost from the earth's 30 km/s orbital velocity around the sun but that is still only 0.015% c.  Now granted, it was launched from the earth's surface, starting the same rocket in orbit would have probably doubled or maybe tripled its speed (I'll have to do the math) but the entire space craft only weighed just over 1000 pounds and would fit in the back of a pickup truck.  Launching a big spaceship would take even more fuel.
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Shadow Shack's picture
Shadow Shack
June 10, 2009 - 10:23pm
Yeah, the 1% was something I tried to avoid permitting. Using the figures from the KH book (12,000,000kph) comes out to 200 hexes per turn and the highest figure I've gotten so far with the formulas was 40 or 50 hexes per turn (assuming having enough fuel to slow back down to zero). But I've yet to try every possibility...

Like I've always said, given enough fuel the chem drive could become void capable. I just tried to keep far enough away from that to encourage the ion drive purchase (both for modern or pre-history gaming).
I'm not overly fond of Zeb's Guide...nor do I have any qualms stating why. Tongue out

My SF website