Onno Meyer wrote: > Jon replied to me: > > Check to see if some sort of space sail can be used to improve fuel > > efficiency; at the very least, a mag-sail should be able to save on > > fuel by acting as a braking system. The question is if it will save > > enough fuel to be worth its mass. > > Good idea, but I don't think the numbers work out. To brake a > 15,000 ton ship from 0.02 c in less than a decade, I need 63 > tons of magsail thrust -- more than 10,000 tons of sail. That > mass is better spent on reaction mass for the existing drives.
IIRC from the original Roleplayer article that first introduced me to magsails, the magsail thrust increases with the relative speed of the solar wind, which is why a magsail can't have an outbound velocity that's faster than the solar wind. And if you're roaring into a system at 2% of lightspeed, that means that those magsails are going to have incredibly good thrust-to-mass ratios for much of the braking process. It also helps that they don't lose thrust with distance from the primary all that quickly. > The new Spaceships rules are much more friendly, but I want my > design to work in both editions. I really don't trust either edition's numbers. Some time ago, I did some number-crunching based on the Roleplayer article's equation, combined with the fairly reasonable assumption that a magsail would have a mass-to-energy ratio comparable to a superconducting loop. The performance statistics that I came up with bore little resemblance to either set of published magsail statistics. > > If you're going for a generation ship, consider the Habitat modules > > from VX2 (I think). You might even go with a small metallic asteroid > > reshaped into a hollow cylinder: sure, it's a significant amount of > > mass; but it's also a wide-open space, which will benefit the > > colonists. > > Those modules are from GURPS Traveller, IIRC, not part of the > official Vehicles 'canon' ... I don't have the book on hand; but I'm pretty sure that habitat modules appeared in one of the Vehicles Expansions. I'm also pretty sure that they _didn't_ appear in GURPS Traveller; they don't fit the setting all that well. > Back to Jon: > > GURPS Space gives advice on the minimum population for a start-up > > colony; your generation fleet (it needn't be one ship, and probably > > shouldn't be) should probably have a bit more than that. After all, > > the fleet itself is essentially a colony already. > > 4E Space suggests 10,000 colonists. My tentative numbers come > in at $20M per person. Ugh. Nobody ever said that interstellar travel was cheap - at least, not without positing some sort of superscience. > And HK Piter wrote: > > You might want to check out GURPS Biotech for 4e. One of the > > sample game backgrounds details a kind of Generation fleet that > > journeys to Draconis or some star like that. They used hollowed > > out asteroids hollowed out and fitted with fusion engines for > > the ships, and a comet mounted with fusion engines for spare > > volatiles. Since you want to make a ship, make the ship, and > > just have a spare asteroid and/or comet mounted with engines as > > a mini fleet. > > Two objections to that. The Biotech fleet was written to make > a Biotech background, with some iffy reasoning -- why would > faster growth make the fleet run out of heavy metals, but not > out of volatiles which were ALSO required for braking? And > those ships are on the scale of towns, not on the scale of > vehicles. If you're going to be traveling between the stars for decades or more, you'll _want_ something the size of small towns. It's why I suggested the habitat modules earlier. And that way, your life support needs won't need to deal with providing food or air, just light and heat: a single farm module is capable of handling the food and air needs for a hundred people easily; and IIRC, that was considered to be a conservative estimate during the playtest. -- Jonathan "Dataweaver" Lang _______________________________________________ GurpsNet-L mailing list <[email protected]> http://mail.sjgames.com/mailman/listinfo/gurpsnet-l
