Quote from: ChrisWilson68 on 04/09/2016 12:43 amRemember, the delta-V to get back to a landing from orbit is equal to the delta-V to get to orbit. So orbit and back requires twice the delta-V to get to orbit. Some of that you get from drag, but not too much or your vehicle burns up.Untrue. Drag offers lots of delta V,
Remember, the delta-V to get back to a landing from orbit is equal to the delta-V to get to orbit. So orbit and back requires twice the delta-V to get to orbit. Some of that you get from drag, but not too much or your vehicle burns up.
and don't forget that going up you are pushing the second stage and most of the propellant in the first stage. Coming down, the stage only need stop enough propellant to stop the itself.
I think this all boils down to - how can a booster deorbit without burning up or using fuel for most of the dV (like from Mach 25 to Mach 6)It has been speculated a few times that the reentry burn SpaceX does is not a delta-V manouver but an active plasma-heatshield manouver.If that's the case, the booster would basically conduct atmospheric breaking, but detach the shock front from the rocket surface by a layer of exhaust (still hot but less hot than reentry shock front) gas by a few but crucial inches to prevent melting, while at the same time widening the shock front to increase friction and as such deceleration.The physics behind that are anything but trivial.We do know that Falcon 9 S1 currently cannot land beyond SES-9 speeds. But the reason for that is mainly that it will deplete its fuel supply to reach those speeds (with payload) and wouldn't have any left for reentry and landing burns. (as I understiid it, SES-9 even used a shortened reentry burn for that reason, despite the higher speed, thus SpaceX worry it might not survive reentry, but it did)We know a single core F9 S1 could (barely) make SSTO.If that's the case, then a tri-core FH with no 2nd stage and very light payload would also make it - especially if the side cores deplete early and RTLS, leaving the center core with spare of propellant for its orbital insertion. enough spare for the landing manouver(it's basically a single core being given a push)but of course even such a rocket would never have enough dV for a propulsive deceleration. It might however have enough fuel to make a 3 engine burn to create a plasma reentry shield during an aerobreaking manouver. (plus landing burn)The main issue I see is, I know no way to calculate how much fuel is needed for this propulsively-shielded reentry manouver. It stands to reason it needs a lot less than a fully propulsive landing.what could be calculated is how much spare fuel a FH center core would have if doing a CSTO (center stage to orbit, it's not really SSTO if you stage the side cores) if that's not enough for a regular landing burn plus around two minute of reentry-fire then you can forget the entire thing.
Indeed, this idea was a non-starter from the moment OxCartMark pointed out that the MVac nozzle can't handle atmospheric stresses.Otherwise it might have been useful to re-run Stan-1967's numbers with a triamese approach rather than a biamese approach, just to see whether the payload reaches F9 levels with sufficient remaining fuel for re-entry. Might also have been useful to see whether a virtual-heatshield re-entry burn as suggested by CorvusCorax would work.But without MVac capability this concept is dead in the water.