One of the most important factors in writing space based science fiction is: “How do my characters get from point A to point B?”
That’s an easy answer if point A and B are on the same planet. But when point A and B grow farther apart, even if it’s only as far from Earth to the Moon, the answer gets a lot more complicated if scientific accuracy matters.
Engines that exist.
There are currently, in the real world, four types of rocket engines that have been created.
Solid fuel. Once started, cannot be stopped then restarted.
Liquid fuel. Easy started and restarted, but very volatile and complex to make.
Cold-gas. This is simply a bottle under pressure being opened. Very low thrust.
Ion. Low thrust but high impulse, this rocket is versatile but complex to make.
Future Engines, probable and improbable.
The ideal engine puts out large thrust and high impulse, can be restarted, and is easy on fuel. Some fictional (as of yet) engines that may or may not be feasible. When I write sci-fi, I personally try to stick to probable science as much as possible. I believe we will conquer fusion, and I believe it very likely we’ll get around the solar system with such engines in the future.
Solar Sails. Certainly a viable method in the inner solar system, but far away from a star these thin and vulnerable membranes may not be sufficient to move ships in a timely manner.
Plasma Propulsion. This exists, but hasn’t been tested in space. Ad Astra Rocket Company in Texas is developing the VASIMR drive, which could potentially move a spacecraft to Mars in 39 days. Now we’re talking. It works by using magnetic currents and the electric potential of reactive material to accelerate ions of that material out the back.
Thermal Fission. Simple, heat the propellant to insane temperatures and let it explode out the back of the spacecraft as a result of thermal pressure. As long as nuclear energy has the taboo it does, probably not going to happen on a spaceship launched from Earth.
Continuous Fusion. (Caveat: I use this in my sci-fi books.) Hundreds of times more efficient than chemical rockets, fuel superheated to hundreds of millions of degrees fuses and is then vented directionally into space. It is likely this technology could move large ships long distances in a short time.
Pulsed fusion/fission. The Orion Project was an actual study by the US Government to see if atomic bombs could be used to propel a starship. It’s likely it could be done, but it is both enormously expensive and very dangerous. It works by exploding the bombs against a plate on the back of the ship, using the energy released to push the ship forward.
Antimatter. The be all/end all of potential reality based rocket propulsion. 100% efficient at converting material to energy, this method uses the least fuel for the most go. Basically a stream of matter and an equal amount of antimatter meet in a reaction chamber, explode in a massive energy release, which is then directed out the back of the spacecraft. 40% of the energy potential of the matter/antimatter reaction becomes thrust. No other method even approaches that. The problem: In all of history so far, humanity hasn’t been able to produce enough anti-matter to boil 8oz of water. Production is hard, production is limited, so this tech is very far into the future.
What this means to us authors.
If an author is writing science fiction and is willing to use handwavium to create a propulsion system, the sky is literally… no, fictionally the limit. From the impulse engines of Star Trek to the reactionless ion drives on the TIE fighters of Star Wars, a lot of handwavium happens in sci-fi. If we don’t sometimes disregard the realities of physics, the story can bog down with the times involved, the realities of having to carry enough fuel to power the USA for a year… it’s a problem.
But sci-fi has a long and now respected history of using whatever magic drive systems the author sees fit to meet his/her needs. FTL drives (some of which may even be possible, but that’s a different article) are a prime example. We have ideas on how to move faster than light, but nothing is even remotely close to being mathematically proven, let alone put into action.
The truth is it’s called science fiction for a reason. If an author can, rely on feasible technology. That’s why my ships use fusion drives. But if the author can’t, then concoct the best handwavium technology possible. If a society has a galactic empire, for example, there must almost certainly be FTL travel unless the members of that society are immortal and travel does not spend lifespan. The story comes first, and as a result sometimes the author must rely more on the fiction than the science in science fiction.