One of the many propellants you frequently hear me mention in different stories is hydrolox. Hydrolox is a combination of liquid hydrogen and liquid oxygen, which is slightly more hydrogen than oxygen in a typical mix to keep the temperature in the combustion chamber lower.

Hydrolox is considered the most efficient rocket propellant combination ever, with an average specific impulse of around 450 seconds. Average specific impulse is basically how much energy the fuel has per one unit of propellant, or how the rocket goes “brrr”. For comparison, kerolox engines use Rocket Propellant 1 (RP-1), a form of kerosene, and liquid oxygen. They have an average specific impulse in the mid 300s.

The Merlin 1D on SpaceX’s Falcon 9 uses RP-1/LOX. What this means is that you can get more velocity from the same amount of hydrolox propellant, because hydrogen molecules are very small and easy to accelerate quickly out of the throat of your rocket engine.

This sounds great, right? Well it’s more complicated than that. This is rocket science after all. The thing that makes hydrogen a good propellant, its small molecular size, also makes it really difficult to store. It simply doesn’t want to stay where you put it and will leak super easily.

The super-cold liquid hydrogen is also problematic to store in fuel tanks for long periods of time. Keep them filled for too long and the tank walls will become brittle and crack. This isn’t good for something that needs to survive the mechanical and thermal stresses of passing through the atmosphere at high speed, like a rocket going into space.

Special care is taken to load and empty liquid hydrogen from a rocket tank a limited number of times. The space shuttle external tank was only capable of thirteen fill and drain cycles and the Space Launch System Core Stage is capable of 22. This seems like a lot until you realize how much processing the SLS goes through that involves tanking and detanking. Remember, in early 2021 the SLS Core Stage was on the B2 test stand at Marshall doing its Green Run full duration hold down fire test. That test ended 67 seconds into a planned 480 seconds.

In post-test communication, NASA implied that the 67 second burn would have to suffice because the core stage could only be filled nine times and it had already been filled three times in previous testing. There were no more spare cycles left. The remaining cycles were reserved for further propellant testing at the launch pad and the launch itself. Later NASA confirmed that a second Green Run needed to be completed and in March 2021 a full 480 second Green Run was conducted with no apparent problems.

Another disadvantage to using hydrogen is it is very not dense. For such a small molecule, it takes up a whole lot of space. To hold rocket-usable amounts of it, you need a very large tank, which is why the SLS Core Stage and the Shuttle external tank are so large.

More Information

• Rocket Propellants (Robert A. Braeunig)
• Lox/LH2 (Astronautix)
• Lox/Kerosene (Astronautix)
• Falcon 9 info page (SpaceX)
• NASA analysis of shuttle fuel tank enters final stages (Spaceflight Now)
• Green Run Update: Hot Fire Met Many Objectives, Test Assessment Underway (NASA)
• Green Run Update: Data and Inspections Indicate Core Stage in Good Condition (NASA)
• Green Run Update: Engines Igniting as Hot Fire Gets Underway (NASA)