It is fairly obvious that there are many benefits to having refueling capabilities in space. Perhaps not to the monster rocket fanatics with blinders and a few others, but to most of us it approaches no-brainer territory. The problem is getting to the first egg. The first bird we would accept as a chicken came from an egg, but that egg did not have to be laid by a bird we would accept as a chicken. Mules do not have mules for parents. The first refueling capability doesn’t have to have depots in the business plan, perhaps a buddy tank for starters.
The problem with getting refueling capability in space is often called a classic chicken and egg problem. Without a depot there is no refueling capability or reason to build vehicles to use it, and without the vehicles to use it there is no reason to build a depot. Dual launch architectures try to get around this by designing missions around a propellant launch and a mission launch. So far, this hasn’t worked well either as the mission planners try really hard to avoid boxing themselves into a corner where if either launch fails, the mission fails.
One possibility hinted at in comments a while back is to have the primary mission fully capable of success if the primary launcher works properly. Then have enhanced secondary capability if the propellant launch succeeds and can top off the tanks in the primary. It took me a while to wrap my thoughts around the concept, after which I started working it over to see what I could think of. I can’t give due credit to the original thinker because I didn’t really grasp it until days later, just that it was somewhere in the 40 depot posts by Jon Goff and hundreds of comments associated with them. The originator can chastise me in comments for not digging back for is name.
So this is my take on the idea. A vehicle arrives in LEO with payload for LEO. It hits the refueling vehicle either before or after placing its’ primary in the correct orbit. It off loads any excess propellant it managed to bring up if it is going to deorbit, or if it simply has excess for the secondary planned mission(s). Alternately it accepts enough more propellant to accomplish other missions that are non-critical to the primary mission.
The secondary missions suggested were along the lines of deorbiting various dead satellites. Surveying satellites with minor issues with the intent to design possible repair missions. Rendezvous with satellites in useless orbits and boosting them to the proper orbits. Creating the capability of the upper stage herding dead GEO sats after a GTO burn for the primary customer. Testing servicing options that would be too expensive for a dedicated launch.
The whole point would be to have some capabilities in place and in use for the cost of one launch that did not have to satisfy any customers other than the company that launched it. It would be replenished with excess propellant in vehicles that didn’t need all they hauled up. It would refuel various company vehicles that were scheduled to do extra work for revenue. The refueling wouldn’t have to be a maximum top off, just a calculated amount for individual missions. By using the capability in house, the problem of selling to and satisfying outside customers goes away. By not putting the capability in the critical path, the primary customers should have very few concerns about endangering their ROI.
It would seem that a capability could be started in the two digit millions. It could be very crude with perhaps LOX only to start, or LOX/Kero, or hypergolics, just as long as it fit that particular companies’ business plan.
Later on it might be possible to put this in the critical path of missions because a solid experience base and track record had been built up. Then after that it should be much simpler to get true multi-propellant depots up and running with whatever the market expresses a need for.