The Propellant Module (PM) is an important vehicle in order to achieve the reuse portion of reuse and commonality.
In the past, NASA had envisioned a propellant depot in Earth orbit (Image A).
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| Image A; a 1970s era Liquid Hydrogen Fuel Depot in Earth orbit |
Our vision is slightly different in that we need LO2 as well as LH2. So a propellant depot in orbit is not feasible.
Instead, our plan is to refill a propellant module (PM) and bring it up to orbit. Once emptied, the PM returns to Earth for refurbishment, refill, and relaunch.
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