Quote from: Hop_David on 11/26/2012 07:53 pmI want you to acknowledge recovering a 2nd stage is much more difficult than recovering a Dragon capsule. That recovering a Capsule with a 25% propellant mass fraction does nothing to demonstrate the same can be done with a stage having a 90% propellant mass fraction.Nope! Because the vast majority of the volume of a Dragon capsule is empty, and you're talking about landing and empty stage, not a full one.If all we did was fill up a Dragon capsule with propellant (for the way up... and considering how lightweight we can make propellant tanks, this shouldn't require reinforcements that have significant mass requirements), it would increase its propellant mass fraction to around 75%.
I want you to acknowledge recovering a 2nd stage is much more difficult than recovering a Dragon capsule. That recovering a Capsule with a 25% propellant mass fraction does nothing to demonstrate the same can be done with a stage having a 90% propellant mass fraction.
Quote from: Robotbeat on 11/26/2012 08:25 pmBecause the vast majority of the volume of a Dragon capsule is empty, and you're talking about landing and empty stage, not a full one.You can't be suggesting that the empty stage be a compact truncated cone. The stage is a cylinder, and it is much more "empty" than the capsule. Not only is the mass fraction different, the aerodynamics are different.Clearly, we recover capsules "all the time"; not so with empty stages.
Because the vast majority of the volume of a Dragon capsule is empty, and you're talking about landing and empty stage, not a full one.
Don't forget a pressurized vessel is much stronger than an essentially unpressurized one, like a capsule at splashdown. Thus the capsules need more structure than a pressurized stage would under same situation.
Quote from: Robotbeat on 12/13/2012 07:16 pmDon't forget a pressurized vessel is much stronger than an essentially unpressurized one, like a capsule at splashdown. Thus the capsules need more structure than a pressurized stage would under same situation.You see the 2nd stage re-entering as a pressurized volume?How much propellant do you imagine will still be in the tanks just before re-entry?
Structure is only part of the deal though. My understanding for the Falcon was that the major issue was/is instability during entry. The TPS is on the end of the tanks, yet when it hit atmosphere the stages want to automatically come around to an engine first atitude because of the mass distribution. All the structure AND pressurization won't help if you're tumbling till you hit the materials limit of the vehicle.
Because I'm sure SpaceX hasn't thought about reentry stability for their next-gen Falcon 9 reusability attempt.
Quote from: RanulfC on 12/19/2012 05:38 pmStructure is only part of the deal though. My understanding for the Falcon was that the major issue was/is instability during entry. The TPS is on the end of the tanks, yet when it hit atmosphere the stages want to automatically come around to an engine first atitude because of the mass distribution. All the structure AND pressurization won't help if you're tumbling till you hit the materials limit of the vehicle.Won't this only be a problem if the vehicle starts tumbling? Until then it sounds like the old balancing an upright stick problem.
What about the actual moment of turn over? What speed is it going then?
QuoteWhat about the actual moment of turn over? What speed is it going then?From the moment it hits the atmosphere from what I gather, and speed is either around Mach-6-10 for the fist stage and Mach-24 for the second. So that's what you have to work with at the moment.
So both sides accept that there is a current problem with stability of the second stage? What is the evidence?
If you put fins on the second stage, that might keep it pointed nose first. Still there would be sidewall heating issues, even in that case....
Ok I will leave it to RanulfC to explain.
(also I didn't understand your answer RB. Are there a different set of rockets for landing??)
Quote from: Warren Platts on 12/20/2012 05:35 pmIf you put fins on the second stage, that might keep it pointed nose first. Still there would be sidewall heating issues, even in that case....Those inflatable heat shields also look promising, and way more extreme than we would probably need. Perhaps you could have a moderate inflating section to change the form a little. I still don't really understand the argument that there is a problem to deal with though. The tumbling doesn't mean anything unless they have been trying not to tumble and failing. A longer body could make balance easier. I thought the reasons for the typical capsule shape were because sharp cones get hotter rather than because entering flat end first was particularly stable. The second stage isn't really 'sharper' if it has less density and the same mass per shield area. Im also not sure how unbalanced it would really be. It still has 15% (or something like that) of fuel remaining. Isnt that still quite significant compared to the weight of engines etc?(consider this a list of all the things I know I don't know rather than an opinion on reentry vehicle design )
Several factors differ for the second stage which of course the biggest is probably the entry speed will be much higher even IF all the remaining propellant is used to try and slow the stage down. The "fact" that at this point SpaceX is supposed to be looking at terminal retro-thrust and landing means the entry speed will probably remain higher rather than lower. Now you "definatly" have to address the stability issue both during entery and free-flight.Anyway you look at it, with the current "design" the stage is going to WANT to fly "engines-first" for entery and landing and the only way to get them to NOT do so involves extra mass in propellant, RCS, and probably other systems as well. To get the heating down to a point where the engine bell(s) are going to be able to take the punishment you either are going to end up trying to propulsivly slow down the vehicle, provide a "propulsive" aerospike by running the engine at a lower "power" during the entry, of find a way to make the entire vehicle more "fluffy" to spread out the heating pulse.(And keep in mind the Merlin can only be "throttled-down" to 70% of full thrust as currently designed)Randy