Maneuver Drive

In the period before gravitic drives were introduced, a number of systems were used, often for very different purposes.

Resistojet

The earliest and simplest form of propulsion for space vehicles. They are akin to huge teakettles heating quantities of water into high-pressure vapor-which is then released to create thrust. Although simple and inexpensive, they not very inefficient. Miniature resistojets are often used for station keeping and attitude change by much more advanced craft.

Solid Fuel Rocket

Usually associated with the propulsion of unmanned missiles. Solid rockets play an important role in the early stages of spaceflight. They can be dangerous to use since, once ignited, they cannot be turned off. As a result, any failures within solid rocket systems have a high probability of catastrophic results. However, the solid rocket retains the advantage of a very high power-to-weight ratio.

Liquid Fuel Rocket

The basic start-up technology most civilizations use to get off their planet and into space. The high power of these engines makes them good bootstraps, but also makes them voracious fuel eaters. While some energy can be gained from rockets, this is usually ignored or used to charge batteries since rocket operation is usually very short. The rockets most frequently encountered in known space (98% of the time) are cryogenically fueled (liquid hydrogen and liquid oxygen). Others use hydrocarbons, although the costs and environmental disadvantages of using that fuel type generally outweigh any conceivable advantages.

Ion Drive

Becomes practical for manned vehicles at TL7. The thrust in this system is created by electronically reducing the fuel to a stream of charged particles (ions) which creates a very low thrust. The ion engine indicated on the chart is actually comprised of more than 100 separate 5O-centimeter thrusters. The primary advantage of this system is its endurance, low power requirements and reliability. However, the low velocities generally relegate vessels of this type to short range runs taking weeks or even months. The fuels used for this are known as ’ionizates. ”This term includes mercury, d u m and a variety of liquefied noble gases (argon, neon, krypton, etc.).The values given for this fuel represent an average since some of these substances would be more than indicated and others lighter. Ionizates are found as trace elements on most Earth-like atmospheres, but are more frequently gathered from certain gas giants and their moons, which occasionally boast large concentrations of noble gases.

Mass Drivers

MDs use electromagnetic repulsion (the principle used by the weapons of the same name) to generate thrust. Wily, just as firing a gun will impart acceleration to the firer in Zero-G, so will the electronic firing of rocks-which in this case are propelled in (and discharged from) an endless treadmill of steel containers. The primary darwback to such systems is that they require tremendous amount of raw mass as propellant. Poor prospectors find this vice to be a virtue-can put a pressure dome on a small asteroid (very small), emplace one or two mass drivers and begin firing pieces of the rock for propulsion. Another major use of this system is to propel promising asteroids out of a belt and toward mining vessels which can then reduce the asteroids to useable ores.

Nuclear Thermal Rockets

These need a dedicated, on board nuclear power plant in order to function. They use the heat from the reactor to excite liquefied gases for a high-pressure release. This gives them the nickname “nuclear teakettles.” Nuclear thermal rockets are very efficient, but very expensive, systems-they require the addition of a separate power plant. They are most effective when using liquefied hydrogen, although most non-oxidizing atmospheric gases will do in a pinch. This gives the vehicle an excellent self-refueling capability-which is handy since it is still a fairly voracious consumer of fuel.

MPD

Magneto-plasma-dynamic drives utilize hydrogen plasma to create thrust. This is, in effect, a crude, very low temperature plasma gun. With this technology, true in system commerce can begin to flourish freely and easily.

Fusion Rockets

Identical to those in COACC, but given the importance of space exploration and utilization, this technology is pressed into intrastellar use long before it becomes common in COACC military craft. Generally, after first being used as an energy source, fusion technology is next employed in spacecraft because of its excellent power-to-thrust ratio and because it produces considerable power to run the ship’s other systems. Even after gravitic drives arrive on the scene, many designers prefer fusion because these drive-unlike gravitics do not experience decreased performance when they venture more than 100 diameters from a significant gravity source. Of course, this limitation to gravitic drives disappears at TL11, when fusion drives lose their last tangible advantage. Experimental fusion rockets (more properly called plasma rockets) are used at the very end of TL8. With fusion moving just past the break-even stage, they represent the first application of this immature technology in a spaceflight role.

Source: Hard Times