Autonomous GTO space tug architectures

[sevenperforce]

A lot of the discussions centered around lowering the cost of access to space deal with the difference between GTO mission and LEO mission requirements. A reusable SSTO or near-SSTO design might be able to achieve LEO more cheaply than existing LV architectures, but it would almost certainly not be able to match the GTO performance of something like the Falcon 9's expendable second stage.

The Skylon concept advances a high-ISP reusable kick stage that is supposed to go up as part of the payload, push the payload onto GTO while Skylon loiters in LEO, then returns to Skylon and is carried back down to Earth for the next flight. It could work, I suppose, but I'm interested in other options.

A more flexible idea would be an LEO propellant depot with a high-isp, low-thrust autonomous tug that remains in orbit permanently. In theory, any reusable launch vehicle could rendezvous with the depot and deliver a payload, then depart; a tug would fuel up, push the payload to GTO or even GEO, then return. All autonomously. The existence of such a system would be quite useful and could enable a lot more SSTO-style concepts to become economically viable.

What would such a system look like on a nuts and bolts level? What propellant would be used? Would it be SEP or some sort of high-isp low-thrust chemical rocket? What would it take to put the depot in LEO, and how often would it need to be refueled (or would it be designed in such a way that the LV could carry the fuel up)? Would the tugs be able to aerobrake out of GTO, or would they need enough extra propellant to lower their orbit to LEO?

Finally, what other options are out there? Could you build an induction coil launcher that would literally shoot payloads into GTO from LEO?

[Archibald]

You can build a ballistic SSTO (either with internal or external tanks - the later is easier but not a true SSTO - kind of 1.5STO) and refuel it in LEO so that it can deliver a payload to GTO - see Philip Bono ROMBUS of 1963 Bono draw plans for Moon and Phobos landings.)

Delta-v wise, Earth surface to Earth orbit is 9 km/s. By comparison, GTO 2.5 km/s are nearly trivial. If a SSTO can do 9 km/s, it can certainly do 2.5 km/s with partial refueling.
Of course you have to get down from GTO, but aerobraking is propellant-free.

[sevenperforce]

You can build a ballistic SSTO (either with internal or external tanks - the later is easier but not a true SSTO - kind of 1.5STO) and refuel it in LEO so that it can deliver a payload to GTO - see Philip Bono ROMBUS of 1963 Bono draw plans for Moon and Phobos landings.)

Delta-v wise, Earth surface to Earth orbit is 9 km/s. By comparison, GTO 2.5 km/s are nearly trivial. If a SSTO can do 9 km/s, it can certainly do 2.5 km/s with partial refueling.
Of course you have to get down from GTO, but aerobraking is propellant-free.

Aerobraking from GTO also has 2.3x the peak heat loading of aerobraking from LEO.

As I've mentioned before, if I was going to go SSTO to GTO, I would do a 1.5STO approach using strap-on boosters to increase launch dV, rather than trying to worry about on-orbit refueling.