Generally speaking, a rocket is a rocket, and rocket science is really just a matter of controlling and harnessing a fast expansion of gas to turn it into propulsion. But there are different kinds of propulsion for different kinds of rockets, and some have benefits over others depending on where you’re going.
Before getting into the different kinds of propulsion, let’s start with the most basic misconception. It’s not the case that a rocket rises off the ground because the exhaust is physically pushing against the ground. Rocket propulsion takes advantage of Newton’s third law of motion, that for every action there is an equal and opposite reaction. The force of the hot gas – the exhaust – shooting out one end of a rocket propels it in the opposite direction. This is how a rocket launches and also how it’s able to adjust its velocity in the near-vacuum of space where there’s nothing for it to push against.
The most common kind of propulsion because it’s the one that’s powerful enough to get a rocket off the ground and its payload into orbit, is chemical propulsion.
A chemical rocket gets its thrust from a chemical reaction like burning a fuel. There are different mixtures, but lets take the first stage of the Saturn V, the S-IC, for an example. The first stage burned a refined form of kerosene called RP-1 stored at room temperature with super cold liquid oxygen (LOX) as the oxidiser. Both liquids were stored in separate tanks, pressure fed into a combustion chamber by turbopumps, and ignited. The burning gas expanded until it became too big to be contained in the combustion chamber and was directed out of the rocket nozzle. This gave the whole stack thrust to get off the ground.
There are a few different types of chemical propulsion depending on the fuel. Among the most common are cryogenic propellants use both fuel and oxidiser stored in super cold liquid states; a common mixture is liquid hydrogen (LH2) with LOX. We also see hypergolic reactions that use fuel and oxidiser that ignite on contact without the presence of a flame, a combination like dimethyl hydrazine and nitrogen tetroxide that got the Titan II rocket off the ground for the Gemini program. As of course solid fuel rockets like the space shuttle used. These have both fuel and oxidiser are packed as a powder in one tank, and once it starts burning it can’t be stopped.
A totally different kind of propulsion is electric propulsion like ion engines. Inside an ion engine, electrons are stripped from atoms and these charged ions are sped up in an electric field before being shot out the end of the engine providing thrust. Then there’s nuclear propulsion. A Nuclear Thermal Rocket (NTR) generates thrust by using cryogenic hydrogen to cool a fission reactor core. The gas is heated and expands, and the expanding gas is directed through the nozzle to generate thrust.
So what’s the best for of propulsion? Well, there’s no magic kind of rocket that does all the things we want. Instead, we need to look at what those rockets have to do before picking a propulsion system.
The RP-1 and LOX powered chemical rockets that leave the Earth are fairly complex systems owing to the turbopumps and plumping that feed the propellant and oxidiser into the combustion chamber. But since they leave from a launch pad where engineers are on hand, they’re easy to maintain. Easier to maintain, at least, than a rocket in deep space propelling a spacecraft to another planet.
For deep space missions, we’ve seen a lot of hypergolic rocket engines, like the Apollo command module’s Service Propulsion System. Because these ignite on contact, getting a crew into and out of lunar orbit was as simple as opening a valve, which is great because there was little room for error.
But the specific impulse of chemical propellants — specific impulse is a measurement of an engine’s efficiency taking into account a rocket or spacecraft’s changing mass as it burns its fuel — is lower than the specific impulse of both ion and nuclear engines. Ion and nuclear engines have much weaker, lower thrust, but they produce more power per mass of a spacecraft than chemical propulsion. You need a lot less fuel for these engines to get the same change in velocity, or delta V. So while they can’t get a rocket into orbit, ion and nuclear propulsion are ideal for long-duration deep space missions.
Dawn is a great example. The spacecraft’s ion engine accelerates ions from xenon fuel 7-10 times faster than the exhaust of chemical engines. Without ion propulsion, Dawn would have needed ten times more fuel just to go to Vesta, which would have made the spacecraft heavier and demanded a much larger launch vehicle. And it would never have had enough fuel to also visit Ceres. The downside of ion engines is that it takes a lot more time to make a course correction so any mission needs plenty of space (literally) to be effective. They also need huge solar panels to gather electricity to ionize fuel as the spacecraft flies.
Nuclear engines have the similar benefit of a small amount of fuel providing a lot of power to get the spacecraft to a far away destination. But a reactor core is an extremely complex device, so these engines counter longevity with extreme complexity. And then there’s the political difficulty of launching anything nuclear into space. At some point that rocket will fly over a populated area, and if there’s an anomaly during ascent – engineer speak for if it explodes – it could be disastrous for the humans in the area. But we do launch nuclear material into space, typically as the power source for spacecraft in the form of radioisotope thermoelectric generators, like the one that powered the Apollo ALSEP experiments and is still powering the Curiosity rover.
These challenges behind electric and nuclear propulsion is why we still see chemical propulsion in deep space, and because nothing has the power of a chemical reaction it’s why we still see these rockets launching missions into orbit in the first place. That hasn’t stopped some people from proposing methods to harness the power of alternate propulsion methods going back to the mid 20th century with programs like NERVA and Orion. But ultimately these lofty concepts never gained enough traction to overcome hurdles, and we’re still in an era dominated by chemical rockets. At least until someone figures out the space elevator.
A huge thank you to Scott Manly for helping me get my non-rocket scientist brain around this rocket science! Sources: Northwestern University; Northwestern University; NASA’s Dawn mission page; NASA; NASA on Rocket Propulsion; NASA.