How to Chose a Rocket Engine and Propellant

You can find rocket engines using ethanol, kerosene, hydrogen, xenon and now recently methane. What decides the propellant used?

Okay, so this probably isn’t a regular predicament for you, but perhaps when reading about various rocket companies like SpaceX, ULA, Arianespace you will see talk of different kinds of rocket engines and fuels like RP-1 (kerosene), hydrogen, methane etc. I will try to answer why a particular engine and fuel is typically chosen.

If you don’t know how a rocket engine works, look at my article explaining the operations of a typical rocket engine.

I will ignore solid fuels here, as they are rather irrelevant for reusable rockets and are too inflexible to be used in outer space. Solid fuel rockets are rather primitive and uninteresting. They are dumb tubes filled with fuel, like a new years rocket, which you ignite and then burn until the end. There is no ability to control the thrust, turn it off etc. That makes them dangerous for manned missions, which is my main interest. Anyway the important question is:

What is the Best Rocket Engine?

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Busek Bit-7 ion thruster

For instance ion thrusters are super efficient, but really weak, while e.g. the F1 rocket engine used on the Saturn V moon rocket is extremely powerful but rather inefficient. An analogy would be between a car which is able to go fast (powerful engine) vs a car which can drive far (efficient engine, good milage).

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F1 rocket engine used on the Saturn V

Efficiency is measured by how much a rocket engine will change its momentum per unit of fuel, called specific impulse. Momentum is the product of mass and velocity.

So high specific impulse means less fuel is needed to change the velocity of a rocket engine. F1 has a specific impulse in vacuum of 304 seconds and 263 seconds at sea level. In contrast Ion thrusters are over 3000 seconds. That means one unit of fuel will produce roughly 10x (3000/304) as much thrust with an Ion thruster. Or stated in a different way, you need 10x less fuel to change the direction or orientation of your rocket when using an Ion thruster.

Why not Just Use the Most Efficient Rocket Engine?

So to get off the planet, you want your initial engines to be really powerful, so you spend as little time as possible trapped in the gravitational field. Once you are in outer space you don’t need powerful engines anymore. Then you’d want to efficient engines so you don’t run out of fuel quickly.

What is the Best Rocket Fuel

For this reason first stage rocket engines which need to escape the gravity of a planet, are better off using big high trust rocket engines with kerosene, while upper stages being used in outer space can be smaller and use hydrogen.

Also hydrogen is a highly problematic fuel. If we used it in the first stage, it would make the rocket enormous because the fuel tanks need to be so big. Additionally it needs to be cryogenically cooled and the bigger the tanks are, the thicker he walls need to be, making everything heavier. This makes hydrogen a poor choice for first stage.

Fuel Rich or Oxygen Rich Engines

Not Quite Done

Methane is a fuel, in between kerosene and hydrogen. More efficient than kerosene, but not as dense. That means it is not well suited as fuel for the first stage in comparison to kerosene. Nor is it better than hydrogen in the second or third stage.

But here is the kicker. It is a great fuel for reusable rockets. Unlike kerosene, methane burns cleanly not leaving lots of nasty soot in the rocket engine, which needs to be cleaned out. That means longer lasting engines and less maintenance. Consequently methane was not interesting in the past, since nobody pursued reusable rockets.

Methane Advantages

  1. Much easier handling than hydrogen. Don’t need active cooling, passive cooling is sufficient.
  2. Don’t need sophisticated metallurgy like hydrogen to avoid hydrogen embrittlement of metals getting in touch with the hydrogen.
  3. Thanks can be much lighter since pressure is lower and less insulation is needed. Hence since you got less to lift, they end up equally efficient as rockets using hydrogen.
  4. Better for reuse than kerosene due to clean burning, avoid wearing out engines (coking).
  5. Easy to synthesize on Mars. Which is important for Elon Musk if he wants to return from Mars.
  6. Methane turbo-pumps are simpler than those for hydrogen, and turbo-pumps is a big part of the complexity of a rocket engine.
  7. Methane can be vaporized to pressure the tank, so you don’t have to use helium to pressurize the tank as it gets drained like RP-1.

While the kerosene hydrogen combination looks optimal, by using a high trust fuel for the first stage and high efficiency for the top stages, it adds a lot of complexity, which increases the cost of the rocket, since you have to deal with different fuel system, different tanks and different rocket engines. That is why the Falcon 9 rocket from SpaceX uses Kerosene for all stages. While not optimal, it is cheaper. Fuel costs is a tiny fraction of the cost of launching a rocket, so it is rather irrelevant if you waste fuel.

But this is why Methane is such a great fuel. It is a better all around fuel. It is a better fuel for the later stages than kerosene, being more efficient, while still not that much worse for the first stage.

Written by

Geek dad, living in Oslo, Norway with passion for UX, Julia programming, science, teaching, reading and writing.

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