This is about US reusability and the long posts above arguing that US recovery is so difficult because mass mass mass.
Quote from: meekGee on 08/30/2017 12:16 amThis is about US reusability and the long posts above arguing that US recovery is so difficult because mass mass mass.Mass (or rather mass estimating the parts) is only part of the problem. As an earlier poster noted the first stage landing gear weighs about the same as a Tesla. Would you have the US come in with the same landing speed, and therefor likely need a landing gear of proportionate size? Or slower? Or faster? But (per the title of this thread) you're going for a lifting entry, so high angle of attack while structure is bottom heavy. Note the term "drag" fins. Good at slowing down, not so good at generating lift. And AFAIK SX have never put any on a US to test the atmospheric conditions at those altitudes, which will be significantly different. This is both a formidable structures and a formidable control problem. Mass is a significant part of it but it's a long way from being all of the problems they will face. Keeping most of the stresses along the booster made booster recovery a lot easier. Once they go off axis that stage will get a lot heavier.
No it's not, as noted above.Even though mass penalty for a second stage is 1:1, for LEO missions the payload is so much heavier than the US that even a doubling of second stage empty mass (which is very pessimistic) is perfectly acceptable.GEO is a different matter, but US reusability is for high flight rate LEO missions.
Quote from: meekGee on 08/30/2017 03:08 pmNo it's not, as noted above.Even though mass penalty for a second stage is 1:1, for LEO missions the payload is so much heavier than the US that even a doubling of second stage empty mass (which is very pessimistic) is perfectly acceptable.GEO is a different matter, but US reusability is for high flight rate LEO missions.I think I see where your logic is going wrong. You seem to be thinking that the recovery mass will scale with the size of the stage. It doesn't. In order to keep the numbers simple I'll assume the booster separates at 1/2 orbital speed and 1/4 orbital altitude. Both stages have both kinetic and potential energy. To land both stages you have to lower both their KE and PE to zero. I'll think in terms of a single Kg of mass. So S1 KE + PE = 1/2 m v(staging)^2 + mg x height_of_stagingS2 KE + PE = 1/2 m v(orbital)^2 + mg x orbital_heightBut if v(orbital) = 2x v(staging) and orbital_height is 4x height_of_stagingThat means total energy is 4x per unit mass from orbit to that of 1/2 stagingThat either means a) The US TPS has to have an erosion rate 4x less than the the booster TPS or 4x more of it will be ablated away. Yes I think I'm beginning to see why Musk called it "uneconomic"And that's with all things being equal.Except they are not because a) The Merlin Vac nozzle is much bigger than it's first stage counterparts and b) This assumes the trajectory that worked to lose the energy for the booster (but longer) can work for the US. There's a lot of devils in those details. c) Per the title of this thread people don't want to reenter like the first stage, they want to try out ways to do so like ITS IE a side on lifting entryBut if SX could do that wouldn't they have already fitted it to their boosters?BTW regarding PICAX GW Johnson makes some interesting comments on PICAX, saying SX is in their 3rd generation and their goal is lowering the price of it. IDK Wheather SX can make a PICAX 4.0 that's 4x better than the current grade. I guess it depends how much "stretch" PICAX has left in its thermal properties. Science can't be rushed, and TPS development involves a lot of science. So no TPS mass does not scale down with stage size. Couple that with having to protect a bigger nozzle and wanting to try an ITS landing instead (with it's much more complex control issues) and this is not going to happen anytime soon.
Nobody suggest that US recovery would be similar to boosters recovery.If anything, it is similar to Dragon recovery.You need a heat shield, some structural reinforcement, some propellant, something like grid fins, and a parachute.If it were up to me, I'd eject the radiative nozzle.These components do not add up to near the empty weight of the stage, and so are fine for LEO recovery.
Quote from: meekGee on 08/30/2017 10:51 pmNobody suggest that US recovery would be similar to boosters recovery.If anything, it is similar to Dragon recovery.You need a heat shield, some structural reinforcement, some propellant, something like grid fins, and a parachute.If it were up to me, I'd eject the radiative nozzle.These components do not add up to near the empty weight of the stage, and so are fine for LEO recovery.Dropping the nozzle would aid simplify recovery but would negatively impact the refurbishment for re-use. Perhaps acceptable for some test flights, but it won't get you a system that closely mirrors the final destination. We do already know that the Vacuum Raptor will be actively cooled all the way to the edge of the vacuum nozzle (3m diameter), which suggests that they do not want to dump any nozzles in the long term. SpaceX wants to expend as few pieces as possible.
Nobody suggest that US recovery would be similar to boosters recovery.
If anything, it is similar to Dragon recovery.
You need a heat shield, some structural reinforcement, some propellant, something like grid fins, and a parachute.
If it were up to me, I'd eject the radiative nozzle.
These components do not add up to near the empty weight of the stage, and so are fine for LEO recovery.
But if you look at speed, mass, aspect ratio - you see that the US is a lot closer to Dragon than it is to the bottom, that's all.
Quote from: meekGee on 08/31/2017 02:26 pmBut if you look at speed, mass, aspect ratio - you see that the US is a lot closer to Dragon than it is to the bottom, that's all.Explain how it is and how you plan to make US recovery work. and how (given the title of the thread) this relates to developing ITS better/faster/cheaper.
https://forum.nasaspaceflight.com/index.php?topic=43374.msg1707753#msg1707753As far as how it may apply to the ITS, the re-entry attitude and TPS strategies would likely be similar.
You've chosen to design US recovery like booster recovery, and then spend all this time showing how difficult it is.But if you look at speed, mass, aspect ratio - you see that the US is a lot closer to Dragon than it is to the bottom, that's all.
Anything that does not match the pattern of the operating stage means the stage will need modifying.
Quote from: john smith 19 on 08/31/2017 11:07 pmAnything that does not match the pattern of the operating stage means the stage will need modifying.On the last RTLS we have seen the stage with a significant angle of attack. That was a first stage, long and fragile. A second stage is much shorter. It can reenter with a very small angle of attack initially then gradually increasing angle when speed goes down. It will need flaps at the engine end to provide drag, steering and protection for the engine. Just like ITS and like IXV.
You really don't see how this is quite different to an actual stage, with a mass fraction that Musk says is nearer 3%?
On the last RTLS we have seen the stage with a significant angle of attack. That was a first stage, long and fragile. A second stage is much shorter. It can reenter with a very small angle of attack initially then gradually increasing angle when speed goes down. It will need flaps at the engine end to provide drag, steering and protection for the engine. Just like ITS and like IXV.Looking up the IXV shows it's empty weight is 480Kg and its loaded mass 1900https://en.wikipedia.org/wiki/Intermediate_eXperimental_Vehicle
first lifting body to make reentry from orbital speed.