can they help that earth launch system become reusable?
only standard chemical propulsion and ion propulsion (current).
Quote from: turbopumpfeedback2 on 11/06/2015 08:01 pmcan they help that earth launch system become reusable?Quote from: turbopumpfeedback2 on 11/06/2015 07:52 pmonly standard chemical propulsion and ion propulsion (current).By "standard/current", do you mean... ?"Provide enough fuel in LEO, at low enough cost, to allow a full propulsive deorbit burn that zeros velocity before re-entry, allowing simply subsonic re-entry"or"Build a rotovator (rotating tether) with a tip velocity low enough (relative to the ground) to allow a reusable (suborbital) SSTO to transfer cargo to orbit"
Rotating tether is ok as long as the material used was used in real applications with lengths of at least few meters.
I have always liked the idea of a magnetic rail in orbit that goes around the entire earth. Sort of an orbital loop but without the tower. It could be used for accelerating suborbital vehicles up to orbital velocity and also doing the reverse, negating the need for a deorbit burn while also negating the need for station keeping propellant.
If you suppose some more LEO infrastructure, you could actually fly your second stage on a suborbital trajectory and provide the final circularization burn with on-orbit propellant.
Suppose there was cheap refuelling available in LEO, does your second stage rocket now need heat shielding for descent? Assuming ~5 km/s of delta-v on your second stage, retropropulsion could make reentry as effortless as a suborbital flight. If you suppose some more LEO infrastructure, you could actually fly your second stage on a suborbital trajectory and provide the final circularization burn with on-orbit propellant. This allows your first stage to be bigger - and easier to reuse. A lot of the problems of a fully reusable two-stage rocket go away if there's cheap propellant and relevant infrastructure in LEO. Not to mention that a LEO-to-elsewhere infrastructure makes it possible to focus the reusable launch system on the ground-to-LEO leg.
Remember that the axis of rotation during the magnetic acceleration of payloads, is on the opposite side of the ring from the point of acceleration, not at the centre. Therefore the ring will be constantly pushed out of orbit (even more than the inherent instability issues with a rigid ring.)
Is the "more infrastructure" an orbital ship which drops into an eccentric orbit to intercept the incoming payload and accelerate it back to orbit before it hits the atmosphere?
um.. I want to add a new one but then I will need to rationalise it fits into the OP..I have always liked the idea of a magnetic rail in orbit that goes around the entire earth. Sort of an orbital loop but without the tower. It could be used for accelerating suborbital vehicles up to orbital velocity and also doing the reverse, negating the need for a deorbit burn while also negating the need for station keeping propellant.The reason I think this might fit in the OP whereas the rotovator might not is that we already do have magnetic rails that move things at pretty high velocity and we use them daily. It is just that they are sitting on the earth. I argue it is only the absurd scale that would push this even further into scifi-land than a rotovator in most people's minds. A rigid ring would not be stable in orbit so this would require some new approach that is harder to fit in the OP. We could start much smaller though, perhaps a rail of a few hundred km, and maybe we just use it for internal commuting, dropping garbage (and station-keeping), helping just a bit with deorbiting vehicles at first. There could be a natural evolution between exploiting magnetic rail just a bit and the whole monstrous orbital ring thingy.
Quote from: Paul451 on 11/07/2015 02:36 amIs the "more infrastructure" an orbital ship which drops into an eccentric orbit to intercept the incoming payload and accelerate it back to orbit before it hits the atmosphere?That's what I was thinking of, yes. There's other, more creative methods too.
In what way, exactly, do you have in mind that an orbital ring is unstable?
A rotating ring isn't orbiting. Eventually tidal/gravitational perturbations will cause it to drift until one side brushes the planet, destroying the ring. A ring will therefore need constant maintenance.
Looks to me like a reverse bomber would need (back of envelope) twice the delta-V it delivers to the payload. It needs to first slow to match the payload, then to accelerate again to a stable orbit. And at higher than high ISP engine acceleration. Unless the reverse bomber is getting propellant cheap from somewhere other than the ground this doesn't work out economically.