Happy Star Wars Day! And May the Fourth be with you.
It is the tradition of my people—physics bloggers—to commemorate the date by posting some type of Star Wars analysis.
Since we just finished season 3 of The Mandalorian, I think it's appropriate to take a look at the iconic "jetpack." Just as a refresher, Mandalorians are a group of people in the Star Wars universe originally from the Mandalore system. They are best known for their armor, and many of them also use jetpacks. If you haven’t seen the show, these are back-mounted devices with two rocket nozzles that shoot out exhaust trails. (You can see a supercut of jetpack scenes from season 2 here.)
Of course, the first time we saw one of these jetpacks in action was when Boba Fett used one in Episode VI: Return of the Jedi. Since then, we have seen quite a few Mandalorians flying around—enough that we can get some data and try to figure out how these things work.
Everyone calls these flying machines "jetpacks"—but do they work as a jet or a rocket?
To learn the difference, let's start with rockets, like the RS-25 engines used on NASA's Space Launch System (SLS). All rockets work by shooting mass out the back of the engine. For its propellant, the RS-25 uses a chemical reaction between liquid oxygen and liquid hydrogen. When you combine oxygen and hydrogen you get water vapor plus a whole bunch of energy, which is used to shoot the water vapor out as exhaust.
Why does this move the rocket forward? Consider the change in momentum of this water vapor. Momentum is the product of mass and velocity. The water vapor created by the reaction between the oxygen and hydrogen is initially at rest inside the rocket, but it ends up moving out the back at a very high speed. Newton's third law says that if the rocket engine pushes on the water vapor, the vapor pushes back on the rocket. Pushing the water vapor back and out of the engine creates forward-pushing thrust. (Or, in the case of a rocket headed to the moon, upward-pushing thrust.)
Other types of rockets might use other liquid fuels, like methane, or a solid fuel. (For example, the space shuttle’s solid rocket boosters used powdered aluminum mixed with oxygen.) But the principle is the same.
You know what's really great about a rocket engine? It creates a thrust force that doesn't depend on the rocket’s surroundings. You can use a rocket in outer space, where there's no air, or even underwater.
But there is a disadvantage too. All of the fuel must be contained inside the rocket. If you want an engine powerful enough to lift the rocket off the surface of the Earth, you need a lot of fuel. And if you need a lot of fuel, you need a bigger rocket. You can see the problem this leads to. If you want to get into orbit or all the way to the moon, you need a very large rocket. The SLS is 212 feet tall. SpaceX’s Super Heavy rocket is 390 feet. (At least it was until it exploded after launch a few weeks ago.)