How do rocket engines work? Very well, usually. Thanks for asking.

Ba-dum Tiss.

Seriously though, it might not be a bad idea to talk about the basics of how rockets work so that other, more detailed information about spaceflight makes sense. I’m most definitely not an expert, but I’m happy to teach what I know, and I’m even happier to be taught, so if you know of something I’ve written that’s wrong, just shoot me a line.

The very basics:

Newton’s laws are pretty clear. An object at rest stays at rest, and an object in motion stays in motion (unless acted upon by an outside force). Here on Earth, we get kind of a biased intuition about those things, because

A) we are surrounded by air, and air is something, so objects are acted upon by it, and

B)We’re attached to a giant rock, which has gravity, and gravity is an outside force that we don’t ever see but take for granted. So that skews our perception of what “rest” and “motion” mean (even though they’re all relative anyway).

If you were in some kind of environment in microgravity, like the ISS, and you had managed to sneak aboard with one of those compressed-air t-shirt launching guns that cheerleaders use at football stadiums (like the one that killed poor Mrs Flanders on The Simpsons), and went outside on a space walk and you brought the launcher, you’d get in a lot of trouble. But before you got in trouble, you could have some fun. If you shot a t-shirt, you would get propelled backwards as the t-shirt got propelled forwards. That’s because the t-shirt pushed as hard on your gun (and you) as your gun did on the t-shirt. Equal and opposite reactions.

If you had a large amount of compressed air and t-shirts, you could drive yourself around the ISS (or anywhere else in microgravity, for that matter). Granted, it would be slow, because while the t-shirt is shot out with enough force to hurl it to the top of the stands, you have a lot more mass (and therefore more inertia) than the shirt, so you won’t go as fast.

Sadly, though, you don’t have infinite t-shirts or infinite compressed air. You only have what you can hold, so eventually you’ll run out. Alas, it’s probably for the best that you don’t have so many, because t-shirts and compressed air might be (virtually) weightless in microgravity, but they still have mass, just like you do, so they have inertia, just like you do. The more t-shirts and gas you carry (and thus the more mass you have), the less effect each launched t-shirt has on you. More t-shirts gives you more fuel to push with, but it also loads you with more mass, so you have to push more anyway to get the same result.

This relationship is called the “tyranny of the rocket”, and it’s a very real problem that rocket scientists have to deal with.

So we’ve figured out that you have a number of t-shirts and an amount of compressed air, and that you and your fuel together have a certain amount of mass. As it turns out, we can use this information to figure out how fast you can go, as long as we know how hard each t-shirt is pushed away from you. Like this:

Suppose each t-shirt has a kilogram of mass (these are really big t-shirts — 2.2lbs!). We know that when you shoot a shirt, it comes out of the barrel at 1 kilometer per minute, or 60 kilometers per hour. We have also checked, and the gun uses 100 grams (0.1kg) of air for each shot. You, your space suit, and the gun together have a mass of 100kg, so you should eat something when you’re done with this exercise, maybe.

When we start, you’ve got 10 shirts, and 1 kg of compressed air. You have enough air for 10 shots, and when you’re out of air, you’ll be out of shirts. This is a balanced fuel mixture. That’s good, because any extra of either is wasted mass, and wasted mass is wasted fuel.

Now, you shoot the first shirt. It goes flying out of your barrel at 1km/min, your mass goes down by 1.1kg (because of the t-shirt and the air), and you go flying backwards…but by how much? Lets find out.

If you started out with 100kg, and a shirt is 1kg, and the shirt moves at 60km/hour, you will have 1/100th the effect, so you’ll start moving backwards at 0.6km/hr, or 0.01km/m. That’s pretty pokey. But wait, you don’t have 100kg of mass anymore. You’ve got 98.9 because the shirt and gas have been expended, so really, you’re going at 0.606km/hr, just a bit faster than we thought. And when you fire again, you’ll only have a mass of 97.8kg, so you’ll be accelerated by 0.613km/hr. And since you were already going backwards at 0.606km/hr, you’re now headed backwards at 1.219km/hr. Pretty soon, we’ll be talking about some real speed!

Hall Effect thruster. A special electrical engine that relies on repelling xenon ions. Read more at http://en.wikipedia.org/wiki/Hall_effect_thruster

This change in velocity that we’re talking about is called DeltaV (Delta for the greek letter Delta, is the symbol for change, and the V is for velociraptors, as far as I can tell. Someone else told me that it was velocity, but who are you going to trust, me or some person I don’t even remember?). The amount of DeltaV a rocket has is measured in speed, and its value is determined by the relationship of the mass of the rocket and the mass (and efficiency) of the fuel.

So, if we fire every t-shirt in our arsenal, and we’re going backwards at 6.39km/hr, then that’s the DeltaV for our “rocket” — 6.39km/hr.

Remember back to when I mentioned air? If you had done this exercise in an environment with an atmosphere, like inside the ISS, you would end up going slower than 6.39km/hr, because the shirt would still be pushing you, but you would also be pushing against the air. Since you’re not at all aerodynamically shaped (sorry, you’re just not), it’s hard to tell exactly how fast you’d be going, but being shaped smoothly and tapered like a rocket helps decrease atmospheric drag, and increases your maximum speed in the end.

How much does it increase it? That depends on a lot of things. How much air we were dealing with, for instance. If we had the equivalent of sea level air pressure, that’s a lot more air than if we had the equivalent of an airplane cabin, most of which are pressurized at 10,000ft or so). And even THAT’S a lot more pressure than if you were outside of the airplane cabin at the 40,000ft cruising altitude of a lot of jetliners.

Which, again, is more than you get at the Karman Line, the internationally-recognized beginnings of space. Even though it’s still “space”, it’s not a perfect vacuum. In fact, the atmosphere of the Earth stretches REALLY FAR OUT. Hundreds and thousands of kilometers, past the ISS, past the Hubble Space Telescope, and sometimes out past theVan Allen Radiation Belts.

Anyway, this “pushing away mass in order to go faster in the opposite direction” style-acceleration is called a mass-reaction engine. It’s what basically every rocket engine has used, ever since the Chinese invented rockets centuries ago. In fact, if you think about it, every single method we know of to change our velocity relies on one thing: pushing off of something else.

If you’re in a car, you’re not using the internal combustion engine’s exhaust to push you. Instead, you’re using the explosive force to turn a crankshaft attached to wheels, and the wheels move you forward by relying on friction and pushing off the ground in the direction you want to go. This is why, even if you had wings on your car, you wouldn’t be able to fly…the wheels would have no ground to push on.

Let’s switch to propeller-driven aircraft or a propeller-driven boat. You have an internal combustion engine that’s turning a screw (or, if you’re on a big nuclear vessel, you’ve got a nuclear reaction producing heat that’s boiling water, and the pressure from that steam is turning the screw), and the propeller is pushing off of water or air to move the craft forward. This is why airplanes (and boats) won’t move in space. There’s no medium for them to push against.

Consider also jet-driven boats and airplanes. Both engines take in the fluid (either air or water), and accelerate it (though a turbo pump or through burning fuel), and expel it out the back faster than it came in. By pushing it out faster than it came in, the vessel is pushed forward faster, in an equal and opposite reaction. We’re getting closer now to rocketry, but if you take a jet engine into space, there’s no air or water for it to accelerate, so it doesn’t work.

So since there’s no medium that we can (or at least, that we know how to) push off in the vacuum of space, we have to bring our own mass, and throw it away from us. In the previous example, we used t-shirts and compressed air, and we said that we could accelerate the t-shirt to 60km/hr. What if we could change that to 600km/hr? Because of the whole “equal and opposite” thing, we would also need to change the other side of the equation, and we would find ourselves moving at over 6km/hr from a single shirt!

It’s probably not possible for 100g of compressed air to do this, sadly, so this isn’t realistic, but what you can see from this relationship is that the faster you expel the mass in the back, the faster you move forward, at least when taking into account the mass of the fuel and so on.

Here’s a table of the more common fuel mixtures that are used in rocketry, along with the energy that they can produce. T-shirts and compressed air aren’t listed, clearly because they haven’t caught on yet. The table is from a Rocket and Space Technology page.