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The only widely used space propulsion system at present is the rocket engine, which takes many forms. A rocket can work in a vacuum because it does have something to push against: the reaction mass it expels to the rear, typically by burning a fuel mixed with an oxidizer. The faster the reaction mass is ejected, the less reaction mass is needed, and this becomes very important when you consider the high velocities needed to achieve Earth orbit (about 18,000 miles per hour and up) or escape velocity (over 25,000 miles per hour) . Remember, the rocket has to carry fuel to accelerate the payload, fuel to accelerate the engine, tanks, etc., fuel to accelerate the fuel...and fuel to accelerate the fuel that accelerates the fuel, and so on. This is why space travel is so expensive, payloads are so small relative to the size of the rockets, and rocket engineers must resort to staging, using a large rocket to accelerate a smaller one.
The first rockets, built centuries ago in China, used solid propellants where the fuel (charcoal and sulphur in the very first ones) was combined with the oxidizer (potassium nitrate). Solid fuel rockets, relatively simple, reliable, and inexpensive, are still in use; they were greatly improved by researchers such as the late and mysterious Jack Parsons. Typically today they use a fuel like powdered aluminum, and an oxidizer like potassium perchlorate, held together by a binder. The exhaust velocities (called specific impulse by rocket engineers and measured in seconds) of such fuels are rather low, and the solid fuel rockets are heavy, since the entire rocket has to withstand the pressures created by burning the fuel.
Liquid fuel rockets were first proposed by the Russian theoretician and visionary Konstantin Tsiolkovsky. The American Robert Goddard built and flew the first one, using liquid oxygen as an oxidizer and gasoline as fuel. Liquid fuels have high specific impulses; for example, lox (liquid oxygen) and kerosene have a specific impulse of 289 seconds. Red fuming nitric acid and hydrazine (H2N2H2, a corrosive and toxic fuel) provides 276 seconds because nitric acid, whle liquid at room temperatures and therefore easily storable, is a less efficient oxidizer than lox. Lox and hydrazine give 303 seconds, and lox and liquid hydrogen (difficult to store and handle due to its extremely low temperatures) gives 381 seconds.
It was realized early on that since the first part of a spacecraft's flight is in our atmosphere, whose resistance must be overcome, it makes sense to use that air, with a hybrid system of some sort. The early rocket planes, like the X-1, X-2, and X-15 used an "air breathing" first stage (a B-29 or B-52 aircraft) and a rocket second stage. Both stages were manned and fully reusable. Very high velocities can be attained using certain types of jet engines. A ramjet has no turbine or compressor, and must be accelerated to several hundred miles per hour using a booster, such as a solid fuel rocket. However, even though a conventional ramjet slows the incoming air to subsonic speed, the engine can accelerate up to mach five (five times the speed of sound). At such speeds it can scoop up a lot of air in a short time, allowing it to continue to accelerate at very high altitudes. A supersonic combustion ramjet, or scramjet, allows some of the air from the intake to continue at supersonic speeds within the engine. This involves some very difficult engineering, but such an engine could, in theory, accelerate to a substantial fraction of orbital velocity before a rocket engine would have to take over. Even more difficult to engineer is a pulse detonation engine, which uses carefully timed detonations, rather than continuous burning like a regular ramjet, creating supersonic pressure waves. Such an engine can take off from a stationary position with no booster and achieve extremely high velocities and altitudes and still have fairly good fuel efficiency. Theoretically, it is possible to build a scramjet that carries no fuel, but gets energy by recombining the monatomic oxygen in the atmosphere above 100 kilometers into ordinary O2, the stable form.
Nuclear rocket engines have been tested though never flown, using very high temperature fission reactors to heat a propellant like hydrogen (its low molecular weight allows a higher specific impulse at any given temperature than any other propellant...around 800-900 seconds). It is also theoretically possible to build a nuclear ramjet. Howver, people are understandably a little concerned about high speed reactors flying over their heads. An alternative, if other means are used to reach low Earth orbit, is to use solar energy to heat the hydrogen, allowing as good a specific impulse as a nuclear rocket, but typically with a lower thrust.
And then there are ion engines, which are a kind of rocket engine using solar or muclear power to produce electricity to produce electric or electromagnetic fields to accelerate a propellant to extremely high velocites, although with a very low thrust. Robert Goddard experimented with this technology many decades ago, and European rocket pioneer Herman Oberth wrote about it. Electrostatic versions use the coulomb force, and can be gridded (typically using an inert gas like xenon as a propellant) or Hall effect thrusters using xenon or bismuth. Electromagnetic ion rockets use the Lorentz force to accelerate propellants like hydrogen, argon, ammonia, or nitrogen. Ion rockets can achieve specific impulses on the order of 3,300-10,000 seconds.
From Earth orbit, it is even possible to reach escape velocity and beyond with no rocket at all, using the minute pressure exerted by sunlight to push against large but very thin and light sails. These are most efficient if they are highly reflective, and only work effectively above 800 kilometers from the Earth's surface; below that altitude residual atmospheric drag renders them ineffective . The solar wind, high velocity charged particles streaming out from the Sun, also can push a sail, and it is possible to make a solar wind sail with a grid of extremely fine wires, electrically charged. Such a sail can be controlled by varying the charge. A conventional solar sail also receives some thrust from the solar wind. Such sails, immense in size and flimsily constructed to keep their mass low, pose formidable engineering problems and have such low thrust that a spacecraft can take many months to build up a useful velocity.
It is also theoretically possible to reach Earth orbit with no rocket, using a space elevator. Geosynchronous communications satellites are on a west to east orbit over 22,000 miles above the equator, so that they orbit the Earth every twenty four hours and remain stationary relative to any point on the Earth's surface. If a cable of some incredibly strong material like "Bucky tube" carbon, 100 times stronger than steel, was extended from an orbiting weight a little above this height to the Earth's surface and securely anchored there, the angular momentum of the weight would hold the cable taut. Researchers have proposed maglev linear induction motors to propel vehicles up and down these cables, but this could be difficult to achieve, as there is no room for any additional weight other than the cable and the vehicles themselves, and there is the question of a power source for the vehicles, the incredibly tricky engineering to construct the system (using conventional rockets) and the fact that no one yet knows how to mass produce Bucky tubes of the required length.
All the above systems are relatively conventional and well known. But there may be other possibilities. In 1999 a NASA shuttle did the tethered satellite experiment, extending a miles long cable behind the shuttle, ostensibly to generate electricity as the shuttle and attached cable flew at over eighteen thousand miles per hour through Earth's magnetic field. NASA gave no reason for wanting a new source of electricity, and did not explain why they didn't simply use more solar or fuel cells. In addion, once the circuit was closed, the cable's induced electromagnetic field would react with Earth's field, creating drag that would bring the shuttle down out of orbit. In my book, published in 2000, I pointed out that, since the astronauts do like to come home safely, this would actually be useful, and really was a form of propulsion, which I called electromagnetic propulsion. I further pointed out that the most basic physics tells us that such a generator, putting kinetic energy in and getting electrical energy out, can be reversed and turned into an electric motor by extending the cable ahead of the shuttle and putting electricity (from a solar cell array) into it. This would react with Earth's magnetic field to create a weak thrust that could slowly accelerate a spacecraft to escape velocity. Away from Earth, the system could react with the Sun's magnetic field, allowing a craft to cruise around the inner Solar System, although, with such low thrust, it couldn't land anywhere. No one slse seems to have noticed this, but in 2000 "Discover" magazine and "Scientific American" both ran articles on tethered satellites and admitted that they could be used for propulsion, which they described precisely as I had said. Why NASA lied about this in 1999 and why no more has been heard on the subject is a mystery.
And this brings us to the most controversial possibilities, such as gravity control. Since I have addressed this topic at some depth in another article I will only touch upon it here. A number of inventors over a span of many years have claimed to have achieved at least some degree of control over gravity. One of these was the late Townsend Brown, who pioneered electrogravitics, using optimally shaped and highly charged capacitors to generate a thrust (which many believe indicates gravity control) from the negative to the positive plate. Other inventors, like John Searle, have used spinning permanent or electromagnets. Richard Hoagland has pointed out, discussing what he calls hyperdimensional physics, that Dr. Bruce DePalma has found that a spinning ball shot upward with the same energy and mass as a non- spinning ball will rise higher and faster and fall faster.
Often confused with gravity control are reactionless space drives, which claim to violate the well established law of conservation of momentum, by, typically, trying to create an imbalance in centrfugal force...which is really not a force at all, but angular momentum. One of the most famous of these was the Dean drive of the nineteen fifties, but evidence of its efficacy is ambiguous at best. I myself designed, but never built or tested, such a device in 1986. My version was essentially a kind of rocket that expelled a solid reaction mass to the rear, using a pulsing electromagnet powered by, for example, solar energy. The reaction mass would be a permanent magnet, with the like poles of the magnet and electromagnet facing one another. I theorized that it might be possible to capture and reuse the same reaction mass over and over by having it enter a closed circuit coil riding on bearings on rails, converting much of the magnet's kinetic energy into electricity, hopefully thereby reducing its momentum and allowing a momentum imbalance. Needless to say, it is highly improbable that my invention would work.
But it was fun dreaming about it.
William B Stoecker