Deep space missions will require the placement of large quantities of hydrogen in low earth orbit for use as spacecraft propellant. Procuring hydrogen in low earth orbit from the surface of the earth via chemical rocket booster can be relatively expensive.
The Orbital Hydrogen Collector (OHC) offers an alternative method of obtaining hydrogen propellant in low earth orbit. The OHC ‘scoops’ the hydrogen that exists in minute amounts in the near vacuum of the upper atmosphere at altitudes of low earth orbit. This hydrogen propellant can then be accessed by docking spacecraft to the OHC.
FIGURE 1 (not to scale) shows the basic design of an Orbital Hydrogen Collector. The direction of orbit is from right to left, as shown. The ‘flow’ of atmospheric hydrogen is thus from left to right.
Hydrogen molecules enter the scoop (a) and are compressed into higher-pressure gaseous and/or liquid form by the compressor unit (b). The hydrogen is then pumped into storage tanks (c). Spacecraft can access the stored hydrogen by docking with the propellant transfer boom (d).
The collection of atmospheric hydrogen causes the OHC to lose momentum and experience orbital decay. To compensate, the OHC can utilize electric propulsion to provide orbital boost. In the figure, photovoltaic panels (e) provide electric power to engine (f) while propellant is pumped from storage tanks (c) to engine (f).
The operational exhaust velocity of the engine must be higher than orbital velocity, or the engine will not be able to compensate for momentum lost to atmospheric drag. Plasma engines are the logical candidates for OHC propulsion.
At $5000/kg to ship payloads into orbit using expendable rocket boosters, it could cost tens to hundreds of billions of dollars to place into orbit the hydrogen propellant needed for just one human expedition to Mars. Informal calculations indicate that the OHC could collect the same amount of hydrogen in orbit for as little as one percent of that cost.