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?
As you probably guessed, the short answer is: it depends. With rocket engines there is typically a major conflict between power and efficiency. Really powerful rocket engines tend not to be as efficient as weaker ones.
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).
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?
The problem with weak engines like ion thrusters is that they take a long time to get up to speed. While on a planet surface, this is a big problem, as the engine has to work against gravity pulling it down. The engine might simply not have enough thrust to work against gravity. And even if it did, a weak engine would spend very long time trapped in a planets gravitational field, wasting lots of fuel just fighting gravity.
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
Again it depends. The story is much the same as for rocket engines. Do you want high trust of high efficiency? Hydrogen gives high efficiency, or more specifically high specific impulse, but Kerosene gives more thrust. Why is that? Kerosene is 18 times denser than hydrogen which means more fuel can get pumped through the turbo-pump and into the combustion chamber. So that gives lots of thrust but it also means you burn it up faster. Hydrogen gives more energy relative to weight, so you can keep going for longer. In other words it gives better milage.
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
When reading about rocket engines you might see this mentioned. It can get confusing because sometimes people will talk about fuel rich engines as something good and other times oxygen rich as good. Again it all depends on the fuel itself. It is more efficient to use small molecules coming out of the rocket engine. That is partly why hydrogen is such an efficient fuel. Fuel rich, means not all the fuel is burned. But that is okay, because it will still get hot from burned fuel and accelerate out of the engine giving thrust. For a hydrogen and liquid oxygen (LOX) engine it is best to have excess of hydrogen as hydrogen atoms are smaller than oxygen atoms. But if you burn kerosene it is the opposite. You want oxygen rich burning in the engines as the oxygen molecules are smaller than the hydrocarbon molecules in kerosene.
Not Quite Done
Still this isn’t all there is to choosing fuels and engines. Lately both Blue Origin and SpaceX have started developing engines using Methane as fuels. Why are they doing that, and why did nobody do that in the past?
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
A great reddit thread here and stack exchange discussing all the various advantages of Methane as a rocket fuel, but I’ll try to summarize.
- Much easier handling than hydrogen. Don’t need active cooling, passive cooling is sufficient.
- Don’t need sophisticated metallurgy like hydrogen to avoid hydrogen embrittlement of metals getting in touch with the hydrogen.
- 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.
- Better for reuse than kerosene due to clean burning, avoid wearing out engines (coking).
- Easy to synthesize on Mars. Which is important for Elon Musk if he wants to return from Mars.
- Methane turbo-pumps are simpler than those for hydrogen, and turbo-pumps is a big part of the complexity of a rocket engine.
- 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.
