I was reading an article about how France is looking into creating a reusable rocket engine powered by Lox/CH₄. (Methane)

I was wondering why they would go with Methane as a fuel, so I did some reasons.

These days, there are 4 basic options for launcher fuel, Liquid Hydrogen, Kerosene (RP-1), Hydrazine, and Methane.

Hydrazine has fallen out of favor for boosters, though it is still used in thrusters of various sorts. It has low impulse, and it’s toxic, but the fact that it can be used as a monopropellant means that it is convenient to use for orbital maneuvering, since it requires half the parts, and you don’t need to make sure that the flow of a separate fuel and oxidizer are synchronized for short the “blip” that would be needed for an orbital rendezvous or station keeping.

The commonly used propellants are LH₂ and RP-1 each have distinct advantages:

• Hydrogen has the highest impulse (fuel economy).
• RP-1 is denser, and requires smaller tanks.
• RP-1 can be stored at room temperature.

Methane falls in between Hydrogen and RP-1. It’s less dense than RP-1, and more dense than LH₂, and is more fuel efficient than RP-1 and less so than LH₂.

Methane is also a lot easier to handle than LH₂, with hydrogen condensing at -252.9°C, while liquefies at a relatively balmy -161.6°C, much closer to the boiling point of LOX. (-183°C)

Additionally, for reusable and restartable engines, Methane has the advantage that it does not coke up, so recycling the engine for another use is more straightforward than RP-1.

Additionally, if you want to go to Mars or the outer planets, it is relatively trivial to manufacture or extract Methane, while manufacturing LH₂ would be extremely difficult, and manufacturing RP-1 would be nigh impossible.

So, now you know more than you want to about why a number of rocket manufacturers are looking into Methane as a propellant.