Onno Meyer wrote: >> 6 fuel tanks for a fusion drive would get you 29 mps (46 km/s) of >> Delta-V for a TL9 fusion rocket using water, and an acceleration of >> 0.015G per engine system. An average trip would involve roughly three >> days of burn-time overall (during which your average speed would be >> 7mps), and an indefinite number of days of coasting time (during which >> your average speed would be 15 mps). During the burn-time, you'd >> cover 0.02AU; during cruise time, you'd cover 0.015AU/day. For ships >> that take multiple drive systems, divide the burn-time by the number >> of systems taken, and divide the burn-time distance by the square of >> that. > > I was thinking of old G3E/VE2nd TL10, because the new rules > don't allow me to mix-and-match-and-refit between different > size classes.
FWIW, I've developed some house rules for GURPS Spaceships involving decimal Sizes and the use of percentages instead of system slots. I originally developed them so that I could use Carriers, Fighters, and Mecha to build mecha rather than spacecraft-with-legs; but there's nothing that limits them to that use. > I tried to convert old TL11 assault ships into > new TL11^ and it worked nicely, so call it TL10^ under the > new rules. > > So what about G4E TL10^ with the Fusion Torch as default for > freighters and pirates? The Fusion Rocket has a much lower > accel, so it is out. The Antimatter Plasma Torch would be a > real competition, but AM is expensive ... A Fusion Torch using water as reaction mass produces 1.5G of thrust per system when fully loaded, with a base delta-V of 5 mps for 5% fuel tanks. That should be workable, right? In comparison, a TL10 Orion Drive produces 2G of thrust per system, with a base delta-V of 8 mps. Or you could go with the high-thrust model, which would provide 4G per system with a base delta-V of 4 mps. OTOH, water effectively free ($20/ton); nuclear pulse bombs cost $250,000 per ton. Finally, the Antimatter Plasma Torch using antimatter-boosted water produces 3G of thrust per system, with a base delta-V of 40 mps; its fuel costs $12,000,000 per ton. Of course, what really matters is cost per mps of delta-V. In terms of base delta-V, the Fusion Torch costs $4 per mps to operate; the orion drive costs between $30,000 and $60,000 per mps to operate; and the antimatter plasma torch costs at least ten times as much as the Orion drive to operate. (There's the additional fact that you get more mps/ton as the fuel tank's overall percentage goes up; but this is only likely to significantly affect the Fusion Torch, which is already insanely cheap to operate.) >> Instead of lightsails (0.0001G per system), go with magsails (0.001G >> per system); > > I was thinking of lightsails for some civilian applications. I figured as much. However, magsails cost the same as solar sails in Spaceships, and they provide 10 times as much acceleration at 1 AU; and while Spaceships doesn't explicitly cover this point, magsails don't lose effectiveness with distance nearly as quickly as lightsails do (they scale inversely with AU^(4/3) rather than AU^2), and they can be used to produce acceleration in directions other than directly outbound (albeit at reduced effectiveness; ISTR a figure of 10% thrust when operating at right angles to the solar wind, though it may have been a bit higher). Conversely, they don't gain effectiveness from being close to the star as quickly (but you'd have to be within 0.03 AU before lightsails are the better option), and they have an outbound top speed (but it's 375 mps, so you can probably ignore it in most cases). >> rapid-response ships might even be equipped with a >> Mag-Orion combination, generally using the magsail to get around when >> there's no urgency or to speed up the so-called cruise time; such a >> system would generate approximately 0.54 mps per day of mostly free >> delta-V per magsail system. One week, and such a system would match >> the total delta-V of one fuel tank. Indeed, such a ship could plot an >> "intercept course" that involves using up all of its fuel to get to a >> ship in transit, moving at an average speed of 0.022AU/day (50% faster >> than normal) before accounting for the mag-sail, and then rely >> entirely on its magsail to get home afterward. > > For a warship, I would leave the mag-sail at home (5% of > spaces is quite a lot) and rely on salvage tugs. Since the magsail generates its own delta-V as you go, you can easily justify that 5% in the mass saved by having a smaller fuel tank. It has the added advantage of providing radiation shielding, which is useful for ships with radioactive exhaust. >> In fact, the mag-sail >> is useful enough that civilian ships might invest in it, too. And if >> the average travel time is over a week, it would probably be >> worthwhile to replace one of the fuel tanks with a magsail system: >> same or better effective delta-V, probably an improved travel time, >> and more of a safety net if something goes wrong. Just don't lose the >> magsail. > > Might be true for TL9^, but not TL10^, I think. The above figures were based off of drives with a base delta-V of 4 mps. For the Fusion Torch (5 mps), it should take closer to 10 days for a magsail to pay for itself in terms of reaction mass saved. OTOH, I originally forgot to account for the fact that a smaller fuel tank would result in a lower initial cruising speed; so magsail-enhanced trips would tend to take a bit longer (eyeballing it, I'd guess as much as 50% longer) - which makes it more likely that the magsail would be worth its weight in water, but runs into the economic maxim that time is money. >> > * FTL is by hyperdrive. Hyperdrive works only far away from planets >> > and suns (to the order of 10 million miles / 0.1 AU). Hyperdrives >> > are fairly precise, very hard to detect, but come with an >> > inconvenient calculation/preparation time. >> >> A commercial ship would take roughly six days to reach a 0.1-AU >> threshold, and then another six days to approach the destination. >> Minimum interplanetary travel time looks like it would be closer to >> two weeks. > > I was thinking of calculation time/FTL engine preparation on top > of travel times. OK. That would mean that you can't even begin the calculations or preparation until you're at least 0.1 AU away from the planet. So matters would take twelve days plus however long the FTL prep work takes. >> > * Pirates are up to 60% reaction mass, three times the delta-V, and >> > they go to the trouble to refuel in the outer system before a >> > hunt. >> >> Note that military ships can do likewise, and with vastly superior >> acceleration. > > Yes, except that it isn't "vastly" superior any more, merely > superior. True. With a thrust-optimized antimatter plasma torch, a military ship could manage four times the acceleration of a pirate ship with comparably sized fusion torch engines. OTOH, the potential ten-fold increase in available delta-V would enable a warship to close in on the pirates much more quickly: if it takes a merchant ship five days to get from the planet to the edge of the exclusionary bubble, it would take a warship with an equivalent amount of reaction mass six hours to make the same trek. With antimatter propulsion, the main limiting factor wouldn't be whether or not the pirate hunters can get there in time; it would be whether or not they could justify the expense of doing so. If you limit the pirate hunters to nuclear pulse drives, expense is still a major concern (though not nearly as much of one), and they'd also have to worry about slow response times: with 60% fuel tanks, they'd take a day getting from a planet to the Edge. >> The pirate is going to have to burn a considerable amount of delta-v >> matching vectors with the target, since said target will probably be >> moving outbound from the planet at 0.015 AU/day, or more. The good >> news (for the pirate) is that the target probably won't have the >> delta-v needed to evade. >> Jonathan "Dataweaver" Lang > > I was thinking of intercepts on the inbound leg. > > * Outbound, the freighter has spent 25% of its delta-V on an > outbound vector, with 75% left to dodge pirates, assuming > that the freighter will spend every last drop to evade. > Patrols, if any, will know the flight plan and adjust their > patterns if required. Hmm. Once again, we appear to have made different assumptions. You appear to be assuming that an FTL ship would have to stop relative to the source planet before jumping, and would arrive motionless with respect to the destination planet. I was assuming that the jump would be initiated while the ship is still coasting away from the source world at half of its delta-V, and would finish the jump heading toward the destination world at the same speed; it would only use delta-V at the departure and arrival points. Going with what I think are your assumptions, the travel times would be approximately twice what I've previously indicated. Incidently, this significantly improves the chances that a magsail will be useful. (It also says some interesting things about the principles by which the FTL drive works.) -- Jonathan "Dataweaver" Lang _______________________________________________ GurpsNet-L mailing list <[email protected]> http://mail.sjgames.com/mailman/listinfo/gurpsnet-l
