Space travel makes the difference between a plot stuck on Earth and a plot stuck in the unfathomable immensity of the cosmos. Needless to say I like rockets. A lot. I spend most of my time thinking about them. I help design and build them. My favorite quote about the difficulty of rocket science comes from John Carmack; famous (and rich, I might add) for designing computer games and now for running a small rocket company that competes for challenge prizes and contracts from NASA. In an acceptance speech for a prize that they thought they would have won 3 years earlier and for a million dollars cheaper he said:
… I have to modfy that position a little bit now in that it [rocket science]
still really isn’t that complicated, it is simple compaired to a lot of things
that we do now and days in the modern world … but the aerospace work, while
it may be simple, is not easy.
Which I think is advice to live by. As I’ll explain later, the only way to move in space is by expelling mass out one end your craft. It always comes down to that. It’s simple to understand, but hard to actually make work. I feel like if I understood why this was the case I might have some profound thing to say about the human condition, but I just know it to be true through experience.
Exchanging Momentum
A fundamental, immutable law of physics is that you can’t get something for nothing; conservation of energy. I have talked about this before, and sure will mention it again because in governs everything around us. In order to change your current course (note that standing still counts as a current course, just one with a speed of zero) you have to push against something. On earth we have air and water and ground to propel us, but what about space? In space you have to bring your own thing to push against.
Momentum is a quantity representing a mass and how fast that mass is moving. The bigger you are and the faster you move the more momentum you have. This value, like energy, is conserved (at least in most cases). Starting out with some mass and some speed you can sacrifice some of you mass for more speed. This is, in a nutshell, a rocket.
Sacrificial Mass
What I mean by sacrificing mass is you have to have fuel of some kind. There is no way around this. Every rocket has to take some mass and toss it away, never to come back, in order to gain speed. Conservation of momentum will not have it any other way, though there are some tricks for making the most of your fuel.
A proper look at the math involved will reveal that faster you expel your fuel the faster you can go. In fact rocket scientists use a funny little term called ‘specific impulse’ or ‘Isp‘ to refer to the efficiency of a rocket and it is directly tied to the speed of the rocket exhaust. Most rockets used today actually burn, as in fire, their fuel. This is a simple and surefire way to go somewhere. Burning fuel turns it into a gas that is really hot and moving potentially thousands of miles an hour out the back of the rocket. So far it’s the only way humans have figured out to get off the surface of our planet. But it’s really not that great. For reference an average professional rocket has an Isp of about 270 s. (the units of Isp work out to be seconds. Yes, I know that makes no sense. But just bear with it; everyone uses it because everyone has always used it.) The very best rockets ever built by mankind, the most amazing, powerful engines you can imagine — that lift hundreds of tones of cargo into space — have an Isp of about 400. This is not very good. That’s why rockets today are large, they’re so inefficient that they have to be mostly fuel to have enough umph to get off the ground.
A Better World
We can do better than just burning something. Right now there are a handful of satellites using a special kind of rocket engine called an ‘ion engine’. Instead of burning fuel it’s more like a particle accelerator. It ionizes atoms and sends them through a magnetic chamber that accelerates them to 100,000 mph and shoots them out. These engines are therefore incredibly efficient, with an Isp of nearly 10,000! Using only a few pounds of fuel they can accelerate a satellite thousand miles an hour! But here too there is a trade off, because of the power required to run the engine they can only accelerate a little bit of fuel at a time and so can’t create enough force to counter-act the gravity at Earth’s surface, in other words they only work once you’re already in space. Bummer.
But there is no reason this has to stay true, or even that this is the best kind of engine. CERN’s Large Hadron Collider experiment accelerates protons to very nearly the speed of light, while using as much energy as a tiny country. Imagine if one could build something similar in space (hopefully with a fraction of the mass) and use it as a rocket. It would be amazingly effective. If you can imagine a way to power such a craft then it is very possible to cross huge distances (several light years) in human timescales (tens of years) without messy things like wormholes and warp fields and other things that probably don’t exist (at least usefully) in the real world.
Today’s Rockets
Conventional rockets use either solid fuels, liquid fuels, or some combination. Solid fuels are the simplest. They have an oxidizer in a sold form (usually ammonium perchlorate, if you’re curious) ground up into little (~150 microns) pieces along with powdered aluminum and often trace amounts of modifiers like carbon or magnesium all bound together with a plastic epoxy. Once lit it will burn until it runs out of fuel. It works in space, underwater, doesn’t matter. While it is simple, it has low Isp.
Liquid fuel rockets on the other hand can use much move volatile fuels, like liquefied oxygen and hydrogen. Why not gas? Because gases take up a lot of space and you would have to make your fuel tanks really big. Liquids have enough density to fit a lot of fuel on a relatively cramped rocket. The fuel/oxidizers are then mixed in a chamber and burned and expelled out of the nozzle of the rocket. This gives them the advantage of being flexible. Like a sink, you can turn them on and off as you need them and throttle the amount of fuel you are burning to get precise control of a vehicle. The downside is the plumbing.
The number one problem with liquid fueled rockets is getting enough fuel to the engine. Big engines require turbo pumps, which need their own fuel and pressure source. The end result is a complicated system always on the edge of failure. For fun, and a glimpse into the complexity of designing a fuel burning rocket engine that burns hot and fast enough to be useful without blowing itself up search for diagrams of rocket engines. You will quickly see how it gets it’s reputation of being so hard to get right.
Clarion Blog 
“the only way to move in space is by expelling mass out one end your craft. ”
Actually… no. There are two other methods. (1) Light (photons) have no mass but they DO carry momentum. Light can either be absorbed on a sail and add its momentum to the spacecraft or it can be reflected off the sail, resulting in transferring twice its momentum to the spacecraft. NASA actually used this technique to conserve fuel on the Mariner 10 mission to Mercury, using the solar panels as their solar sails. In theory you could use a laser to push a spaceship, or even a laser as a “rocket” engine — but you are not expelling mass, only energy.*** (2) You can fall towards a gravity well in a gravitational slingshot maneuver which will add kinetic energy and change your direction.
Also, technically, once one is moving in space one continues to move in space – -Newton’s 1st law. So these three methods are ways to CHANGE your motion in space. However, your basic point that one usually has to carry a large percentage of your mass as fuel to (a) get up into space and (b) accelerate to a velocity that will get you somewhere, is largely correct.
Dr. Phil
*** And of course you can use the Einstein mass-energy relation E = mc² to use mass to create the energy to power the laser.
I think there was a pretty clear ‘rocket’ context going on. You have expanded that to ‘space craft’ and then picked your nits… but I can do the same, expanding your implied space craft of the conventionally envisaged sort to a more general cover-all term for things that move in space. Having done that I can say, but no, there are other ways. To move in space my space craft could haul itself along a space elevator, or be hauled. Somebody could fire soft projectiles at it which are agregated to the space craft and cause it to accelerate. There are plenty of ways to move in space… it could be pulled along by a magnetic field, blown by a solar wind…
You did say “craft” in what I quoted and not “rocket”, so it sounded to me like a general statement. Second, I did include the idea of a laser or photon engine acting like a rocket, in that there was momentum transfer, but that technically one would be expelling energy and not mass. If one uses the mass-energy relationship, then the quibble, as it were, disappears.
Apologies if this came off as needlessly pedantic. I teach this stuff and also endeavor to incorporate reasonable Physics in my SF writing, so I was trying to contribute to the conversation and not just picking nits to score points. Not only is your point about needing considerable mass for fuel a good one for rockets in general, the numbers become outrageous when one posits relativistic speeds. The constraints of Physics tend to be ignored by many writers — and just about everyone in Hollywood — or else one has to postulate some new Physics.
This is not always going to be a complaint. One can have a really rip roaring story even when the details go to hell. Writing strictly scientifically accurate and well designed engineering into a SF story can be very difficult.
Dr. Phil