If an engine must operate in an environment without atmospheric air, like on Mars, what modification is mandatory for its oxidizer supply?

Answer

Carrying its own oxidizer supply

For operations outside Earth's atmosphere where ambient air is insufficient or nonexistent, the engine must be designed like spacecraft propulsion, carrying its necessary oxidizer onboard.

If an engine must operate in an environment without atmospheric air, like on Mars, what modification is mandatory for its oxidizer supply?

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