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.

Jetpack vs. Rocket

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.)

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Let’s say you don’t need to fly quite so far. What about a jet engine? These are the things you mainly see on commercial airliners, but very small jet engines can also be used to make a real-life jetpack.

Just like rockets, jet engines produce thrust by shooting mass out the back, which is mostly just air. The energy comes from combusting jet fuel, which is similar to kerosene and is made from petroleum. The increase in the momentum of this ejected matter produces a forward-pushing force.

There is a big difference though: The jet engine sucks air in through the front of the engine. The oxygen in this air is used in a combustion reaction with the fuel to provide energy that increases the exiting speed of the air-fuel mixture. This means that the jet engine only needs to carry fuel, and not oxygen. However, this also means that the jet engine can only work in an environment that has its own oxygen. It won't work in outer space; it won't work underwater.

Well, what about the Mandalorian jetpacks—are they jet engines or rocket engines? I'm going to say they are rockets. First, you need to bring air in for jet engines to work, and you don't really see an air intake on the top of the jetpack. (Maybe it's just super small.) Second, we have seen that these jetpacks work underwater, like when Bo-Katan went underwater to save Din Djarin in the Living Waters on Mandalore. That rules out jet engines.

So, I'm declaring these jetpacks to actually be rocket packs. But since “jetpacks” sounds cool, we can continue to use the term, even though we know it's wrong.

Rocket Thrust

Let's make some approximations in case we ever want to actually make a jetpack like we see in the Star Wars universe. We can look at scenes in *The Mandalorian* to see how these flying machines perform.

The first thing you are going to want to do with a jetpack is to just hover above the ground. I mean, what better way to demonstrate your superiority over other people than to just rise above them and stare down as they stand helplessly below you? In this type of move, you would have an acceleration of zero meters per second per second. Newton's second law says that the net force is equal to the product of an object's mass and its acceleration. So, an acceleration of zero means the net force must also be zero.

For a hovering Mandalorian, there would be two forces. There's the downward-pulling gravitational force that we can calculate as the mass (m) multiplied by the gravitational field (g). Then there's the upward-pushing force from the jetpack (the thrust). So, if we just estimate the mass and gravitational field, that will give us the thrust force needed for hovering.

The mass seems like a simple estimation. A typical adult human would have a mass of about 75 kilograms. Of course, a Mandalorian wears armor *and* a jetpack. Let's just say this other stuff has a mass of 25 kg, for a total of 100 kg, which is a nice number.

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But what about the gravitational field? This is a value that depends on both the size and mass of the planet that you are on. The value on the surface of the Earth is 9.8 newtons per kilogram. I’m afraid we have no measurements for the value of gravity on the planet Mandalore. But since everything in *The Mandalorian* looks like it's on Earth (because it's filmed on Earth), let's just use the same value. With these estimations, the rocket would need a thrust of at least 980 newtons to allow someone to hover.

Of course, a real Mandalorian wouldn't want to just hover. If you want to do more than float there, you will need to accelerate as you take off. Let's say you want to accelerate upward at 9.8 meters per second per second. (This is the same as the downward acceleration you would have if you were falling.) In order to move upward like that, the *net* force would have to be 980 newtons. But remember, there's that downward gravitational force of 980 newtons. The only way to get this to work would be to have the rocket thrust equal *twice* this value, at 1,960 newtons.

OK, now what if the Mandalorian wants to swoop down and save someone who’s falling? (This actually happens in the series.) In that case, they’re going to need to accelerate upwards again—but their effective mass will be larger because the jetpack must now move two people instead of just one. Just to cover all emergency situations, let’s estimate that a maximum force of 4,000 newtons might be needed. The nice thing about liquid-fuel rockets is that you can adjust how fast the fuel gets used, which will change the thrust force. So in this case the Mandalorian would have to increase the thrust (and use more fuel) in order to stop a friend from falling.

Of course, this has consequences. The more thrust you produce, the shorter time you have to fly. A bigger tank would help, but that means more mass—and that would be unwieldy for something you have to carry on your back. So there are limits to how often you can rescue your friends.

OK, what if the Mandalorian wants to fly some distance to catch up with a giant dragon thing that has kidnapped a child? (This happens too.) It is a little difficult to calculate how much thrust the rocket would require—but don't worry, we can get a rough estimate.

Suppose the Mandalorian is flying horizontally with a constant velocity. Since the acceleration is zero, the net force must also be zero. There are really just three forces to consider: the downward gravitational force (mg), the thrust from the rocket (FT), and some type of interaction with the air. Although the human body doesn't really make a great airplane wing, the interaction between air and the body still produces an upward-pushing lift force (FL) as well as a backwards-pushing drag force (FD). Here's a diagram showing these forces:

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Since the lift force and drag force are really part of the same interaction with the air, there is a relationship between their magnitudes—it's called the lift-to-drag ratio (L/D). This is also called the glide ratio, and it describes how much a flying object without any kind of propulsion will move forward for every meter of drop. For comparison, a soaring bird has a high glide ratio, with a value of 100:1 This means the lift force will be 100 times as large as the drag force, and the bird will move forward 100 meters for every 1 meter of drop.

However, the human body doesn't fly well. A human (or Mandalorian) flying through the air will have a much lower ratio, something like 0.6:1. That means the person would move forward 0.6 meters for every 1 meter of drop. That’s not exactly the same as plunging straight down, but it’s close.

On top of that, we can model the magnitude of this drag force (and thus the lift force) as something that is proportional to the square of the flying velocity (kv2). Finally, if I estimate the angle of the thrust (θ), I can break that force into horizontal (x) and vertical (y) components. All of this stuff gives me the following two equations:

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These look like they are a mess. But actually, there are only two variables that I can't get values for: I don't know the thrust force (FT), and I don't know the velocity (v). However, I have two equations with these two variables, and that means there should be a solution.

Let's use a thrust angle of 25 degrees and a drag coefficient of k = 0.186 kilogram × meters, based on the drag coefficient of a falling skydiver. With that, I get a flying speed of 70.4 meters per second (157.6 miles per hour) and a thrust of 1,014 newtons. If you want to fly faster, you would need to increase the thrust, and this would mean the flier would be tilted forward into a more horizontal position.

Rocket Fuel Consumption

Now that I have the rocket thrust needed to fly, we can look at fuel consumption.

Remember that rockets work by shooting mass out of the back. It's this change in the momentum of the exhaust that produces the force. The momentum principle says that the force will be equal to the rate of change of momentum (p = m × v). Instead of thinking about the change in velocity for one tiny little molecule of the exhaust, we can just assume all the ejected gas is moving with some velocity (v) and then create an expression for the rate at which the mass is ejected.

Let’s use the flight in *The Mandalorian*, Chapter 20, in which Din Djarin and some other Mandalorians are using their jetpacks to chase a large flying creature. I have already calculated the thrust to fly horizontally. We can also get a fairly good value for the total flight time (Δt) at about 45 seconds. Now if I just estimate the mass of the fuel, I can calculate the exhaust velocity.

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All of that fuel has to be contained in the jetpack, and I can't see the mass of the fuel being over 10 kilograms, or 22 pounds. (I’m basing my rough estimate on how much water you could carry in a backpack.) I mean, the Mandalorians move around like the jetpacks are just made of plastic, so their mass can’t be huge. With a 10 kg mass lasting for 45 seconds, we get a mass flow rate of 10/45 = 0.22 kilograms per second. I already know the thrust (1,014 N), so that means the ejected exhaust would have a velocity of 4,563 meters per second. That's over 10,000 miles per hour.

Now, the Mandalorian *himself* isn’t going 10,000 miles per hour. That’s because, although the momentum of the exhaust is equal to the momentum of the Mandalorian, the two have very different masses, and that affects their speed. The exhaust has a very low mass but a very high speed. The Mandalorian has a much higher mass, so he would produce the same momentum at a lower velocity. If he was flying in space, where there’s no air, he would keep increasing in speed. But in the Mandalorian atmosphere—which we’re assuming is a lot like Earth’s atmosphere—air drag prevents that from happening. So he ends up moving at a much lower speed.

Is 10,000 miles per hour for the exhaust velocity a reasonable value? Well, there were real rocket packs built in the 1960s that could let pilots fly around for about 30 seconds. However, the main difference compared to the Mandalorian packs was the size: These were bigger than any backpack you could imagine and used 30 liters of hydrogen peroxide as fuel. With a density of 1,450 kilograms per cubic meter, 30 liters of hydrogen peroxide would have a mass of 43 kilograms. A flight time of 30 seconds means this rocket has a mass flow rate of 1.45 kg/s and an exhaust velocity of 699 m/s (or 1,563 mph). This exhaust speed produced enough thrust to lift both the person and all the fuel—and was also enough thrust to actually let a couple of guys fly around during the 1967 Super Bowl halftime show.

That's quite a bit less powerful—but what the heck. Surely the Mandalorians have figured out a way to make more efficient rockets than the ones we had in the 1960s.

Want More Star Wars Physics?

Here are some of my favorite articles from the past:

How fast is a blaster bolt?Why does R2-D2 fly like that?Calculating Yoda’s massAnd finally, an analysis of all the Jedi jumps (including Jar Jar’s).

If you need *even more,* I have a Death Star-sized list right here.