Spacecraft

While many spacecraft are specially tailored to do a particular job, there are some standard designs which are mass-produced and then modified as needed. This document contains some general notes on spacecraft design.

In general, there are two main types of spacecraft: Those which are capable of landing on a terrestrial-type world (Landers), and those which are not. Landers need to have Superior or better streamlining, and generally have some armor -- a civilian lander will have about 80 DR on its underside and 20 or so on the rest, most of this being fireproof ablative (heatshields). Most landers do not actually have wings, but make do with a lifting-body configuration because this is much cheaper -- civilian vehicles shouldn't need to pull sharp maneuvers, anyway. Military landers usually have wings, since it makes them much more agile while in atmosphere (important in case you're going to fly in potentially hostile airspace). Most landers only have limited life support, but those intended for extensive "frontier" missions are usually equipped with full life support and a very durable power plant (either an RTG or a fission reactor, depending on its size and planned mission).

The most commonly used high-thrust propulsion system on spacecraft is the lovely antimatter-thermal rocket. The high-thrust fusion rockets on p. V036 do not exist, but the "optimized fusion rocket" does exist, and is sometimes used for very slow missions (unmanned probes to the far reaches of a solar system, etc). Antimatter-thermal rockets use water as reaction mass, and manage to spew this out at an exhaust velocity of about 83 km/s, which (along with the excellent thrust/weight ratio) is why they're used on practically all landers (well, all landers that are intended to be able to get back up into orbit, anyway). Landers are generally equipped with an air-breathing propulsion system (either jets or ducted fans), since antimatter-thermal rockets aren't very nice to be around (the vast majority of all the energy in the antimatter is wasted, which means the rocket creates a continual "explosion" behind it -- gamma rays, shock wave if in atmosphere, etc). In fact, the energy "wasted" is equivalent to about one ton of TNT per second per ton of thrust, and most landers have quite powerful rockets (generally several tens or even hundreds of tons of thrust). For this reason, the rockets aren't normally fired until the lander has taken off and flown as high as it can get on "normal" propulsion.

To calculate the total delta-v a reaction-drive spacecraft can get (that is, its maximum change in velocity), you need to know how much the spacecraft masses, the total amount of reaction mass used, and the exhaust velocity of that reaction mass. The formula is then quite simple:

delta-v = Ve * ln (M/m),
where Ve is the exhaust velocity, M is the mass of the spacecraft including the reaction mass before you begin firing the rockets, and m is the mass of the spacecraft after you've used all the reaction mass you're going to use. Since the delta-v you need to get into orbit around an Earthlike world is about 8 km/s, it turns out that about 10% of the takeoff mass of a lander should be reaction mass. (Designs usually include a bit of a margin for use on worlds with higher escape velocities, and for maneuvring in orbit, and that sort of thing). Most landers also include a small chemical-fuel rocket system, so they can safely make small maneuvers when they're in the immediate vicinity of other craft (docking with a mothership or a space station, for instance).

As for ships intended for longer missions, most big ships aren't streamlined at all, but instead carry smaller landers on board. Antimatter-thermal rockets are the most common propulsion system also for nonlanding ships, giving the best performance for the price in most types of mission. With this kind of rocket, most ship designs are capable of fairly rapid transit from one orbit to another; however, interplanetary and interstellar travel is still something that takes days and weeks, or even months. No "push the button and it goes" sort of thing here, no "next episode drive". Long-duration ships are generally built with quite spacious interior accommodations so the passengers don't suffer too much discomfort, and they are spun up to simulate gravity in most cases. Rocket burns are either so weak that they can go on continuously without bothering anyone, or they happen only when planned (and everyone is strapped down, or should be anyway) and only last for a few minutes at a time. Some ships use solar sails, but most of these are unmanned freighters (although there are some manned ships which use solar sails).


Last modified: Fri Mar 14 21:22:24 PST 1997