One of the incredible mentions in the Elon Mars concept was a thousand spacecraft in orbit ready for the Mars launch window to open. I’m not sure how many launch windows down the road this would be, I assume several decades. Whether it is next decade or next century though, an expensive asset like a spacecraft that is only capable of being used once every other launch window is a massive investment that is mostly idle.
I suggest an alternate concept for having a thousand vehicles heading toward Mars during one launch window. Each vehicle is an inert barge with a homing beacon and barely enough structure to house the cargo during thrust and coast. No engines, electricity, shielding, or other frills. Â These barges carry onlyÂ items that store well. Machinery, provisions, propellant, clothing, etc.
The orbital gathering place for these barges is a high Earth orbit above the radiation belts but below Lunar orbit. The storage orbit keeps however many barges are heading out during each launch window in Â parking lot adjacent to a refueling facility that is stocked up between launch windows. The third item is skeletal booster tugs with no frills like ability to reenter or handle gravity.
There is a certain limited amount of time in a Mars launch window when the Hohman transfer orbit uses minimum propellant. There are periods of time on both sides of the ideal window that still get you to Mars, just at the expense of additional propellant. Total available time in the window can be a few months depending on available propulsion.
At the first opportunity, a tug with a dry mass of perhaps ten tons, a propellant load of two hundred tons, and a hundred ton barge, does a short burn to drop its’ perigee to just outside noticeable atmospheric drag. At perigee it is at nearly escape velocity when it does a strong (~4km/sec)Oberth effect burn to place the whole assembly on a Mars trajectory. Immediately after reaching the required velocity, the tug separates and a short retro burn to place the light tug back into an eccentric orbit with an apogee equal to the barge parking lot. The orbital equivalent to the F9 boostback.
Back at the parking lot, the tug does aÂ short burn to match velocities and goes for docking. Refuel, clamp onto another barge, and go again on intervals of one to four days. Depending on assumptions, each tug could send as many as a hundred barges per window to be caught on the other end.
So I can see the possibility of a hundred thousand tons of vessels heading to Mars during one launch window. The main hardware investments being launch vehicles, depots, and tugs that are kept employed at other tasks in the meantime between windows. People launch separately in vehicles suitable.
The main strength I would see in a scenario such as this is that the expensive hardware would be constantly available for use for other tasks. This is important for those of us that don’t see that much value in Mars as the next step out. The same equipment would be useful for asteroid missions or sending a Pluto lander. A heavyweight to Europa or a close solar corona Â investigation. Or more immediately useful support for Lunar and NEO missions.