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At its base are 33 engines, burning a mix of liquid methane and liquid oxygen.

During launches, the external tank carried propellants — liquid oxygen and liquid hydrogen — that powered the space shuttle’s three main engines to help bring the shuttle into orbit.

But Swedish submarines have liquid oxygen stored in tanks on board to run the diesel engines underwater to recharge the batteries, so they can stay submerged for longer and reduce the risk of detection.

Relativity is among several companies building rockets to launch into orbit using liquid oxygen and liquid methane as propellants.

Its Terran 1 rocket is scheduled to lift off as early as March 8 from Cape Canaveral, Fla. Relativity’s rocket is made mostly with 3-D printing and uses liquid oxygen and liquid methane as propellants.