As a very basic first step toward second-stage reuse, SpaceX could simply equip one of the two standard payload adapters with spray-on or stick-on TPS, spin-stabilize the stage for re-entry, and let it come down in a ballistic trajectory. It would still impact the ocean at terminal velocity, but it would be a demonstration of re-entry.Chutes could be added for a survivable splashdown...they would only run around 50 kg...but I don't know that there's any space for them to fit. Maybe around the engine, if there's room? There's not much room there.
Quote from: sevenperforce on 04/19/2017 03:01 pmAs a very basic first step toward second-stage reuse, SpaceX could simply equip one of the two standard payload adapters with spray-on or stick-on TPS, spin-stabilize the stage for re-entry, and let it come down in a ballistic trajectory. It would still impact the ocean at terminal velocity, but it would be a demonstration of re-entry.Chutes could be added for a survivable splashdown...they would only run around 50 kg...but I don't know that there's any space for them to fit. Maybe around the engine, if there's room? There's not much room there.If you are going with this approach, why not allow the heat shield to drop down on retaining straps (similar to Mercury) allowing landing bags to inflate and deploy for a soft landing on land, rather than a splash down and salt water incursion...
Quote from: Rocket Science on 04/19/2017 03:24 pmQuote from: sevenperforce on 04/19/2017 03:01 pmAs a very basic first step toward second-stage reuse, SpaceX could simply equip one of the two standard payload adapters with spray-on or stick-on TPS, spin-stabilize the stage for re-entry, and let it come down in a ballistic trajectory. It would still impact the ocean at terminal velocity, but it would be a demonstration of re-entry.Chutes could be added for a survivable splashdown...they would only run around 50 kg...but I don't know that there's any space for them to fit. Maybe around the engine, if there's room? There's not much room there.If you are going with this approach, why not allow the heat shield to drop down on retaining straps (similar to Mercury) allowing landing bags to inflate and deploy for a soft landing on land, rather than a splash down and salt water incursion...Wind is a problem there. The second stage can come down on chutes, splash down, and tip over without being damaged because it is far shorter than the first stage, but if it came down on land it would likely cartwheel due to residual horizontal velocity and rip itself apart. Plus, there's the issue of the landing site; bringing down a spent stage on land requires a very large exclusion zone because there's really no way to control exactly where it comes down.
A deploy-able tripod gear may stop a tip-over if you release the the chutes and a landing can be made at one of the many dry lakes sites out west... (quick sketch below)
Quote from: Rocket Science on 04/19/2017 03:57 pmA deploy-able tripod gear may stop a tip-over if you release the the chutes and a landing can be made at one of the many dry lakes sites out west... (quick sketch below)If you have large enough landing legs and you cut the chutes at precisely the right moment, then you might be able to land it without tipover. If wind speed is low enough.The dry lakes out west have pretty high winds, usually, and you still need a large exclusion zone in case re-entry ends up lower than expected.
The stock S2 can spin up to 5 rpm. Not sure if that's enough, or if it can maintain that all the way through re-entry though:
Here's some modest and steps you could follow to investigate S2 return across four test missions:+ On a Dragon mission*, add Pica-X to the nose of S2, spin it up and see how close to Earth it gets.+ If it gets low enough, add grid fins to the next flight, confirm they can null the spin and fly to the pickup co-ordinates.+ If that works, add a parafoil and see how it flies.+ If all is well, attempt to capture the next one with a helicopter._______* I say Dragon mission because the payload mount for a satellite needs to be dealt with. I guess you could build up the Pica-X shield and add a hinged trap door that closes over the payload mount. But that's beginning to sound too complicated if we're just proving the concept.
Quote from: adrianwyard on 04/19/2017 05:28 pmThe stock S2 can spin up to 5 rpm. Not sure if that's enough, or if it can maintain that all the way through re-entry though:I'm sure it could spin faster if it wanted to; that limit is for customers. The cold gas thrusters could spin up the stage well beyond its structural capacity if that was desired.QuoteHere's some modest and steps you could follow to investigate S2 return across four test missions:+ On a Dragon mission*, add Pica-X to the nose of S2, spin it up and see how close to Earth it gets.+ If it gets low enough, add grid fins to the next flight, confirm they can null the spin and fly to the pickup co-ordinates.+ If that works, add a parafoil and see how it flies.+ If all is well, attempt to capture the next one with a helicopter._______* I say Dragon mission because the payload mount for a satellite needs to be dealt with. I guess you could build up the Pica-X shield and add a hinged trap door that closes over the payload mount. But that's beginning to sound too complicated if we're just proving the concept.Elon said he'd like to test it out on the Falcon Heavy inaugural launch, which will definitely feature a payload fairing and adapter rather than the Dragon. But the payload fairing is pretty tough; if you attached Pica-X panels to the surface and painted the adapter attachment points with ablative paint, it might be enough. Recall that peak heating is on the edge of a heat shield; the center gets less heat than any other point.
If they wanted to go full-propulsive-landing on the F9 second stage, the only way to do it without significant modifications to the outside of the stage would be something like this:Basically they would be designing a replacement payload adapter with all the recovery hardware on it, including expendable payload attachment adapter, heat shield, landing legs, bipropellant tanks, pressurant tanks, and SuperDracos. Note the high cosine losses on the SuperDracos.Might be simpler to jettison the heat shield completely, though that goes against rapid reuse.
If the changes to S2 are just Pica-X around the payload mount hardware and spinning up for entry, I guess there's no harm in trying. My guess is it would eventually conduct too much heat to internal structure, but even if it failed it could be useful to know how long it survived.
If spin stabilization is indeed the answer to S2's CG/CP challenges, then we can imagine grid fins maintaining that spin (windmilling) once in the sensible atmosphere. They'd be deep in the wake, so wouldn't get too hot.
Legs can go straight through the heatshield, see Dragon 2. So can the landing thrusters, they just need to blow off a tile when they start up. Shouldn't need to add more than 1/2 a stage diameter to the height of the stage to fit everything in.
EDIT: back of the envelope...terminal velocity for S1 is slightly subsonic...S2 masses less than 25% of S1...estimating similar drag...grid fin L/D for S2 would be 4:1...
Quote from: sevenperforce on 04/20/2017 04:40 pmEDIT: back of the envelope...terminal velocity for S1 is slightly subsonic...S2 masses less than 25% of S1...estimating similar drag...grid fin L/D for S2 would be 4:1...Erm, that's not how it works. You're working off mass and drag. But you need lift and drag.An object with a L/D of 4:1 is essentially a crude glider. Think Shuttle orbiter, or jump chute. It doesn't matter how many grid fins you stick on the back of a rocket, it's not going to turn into a glider.
Have been thinking about an approach similar to this, however perhaps everything necessary for a S2 landing could be contained within one self contained 'standalone' module (minus the PICA heat shield) fixed between a standard S2 and the payload adaptor, also with grid fins fitted to this same module, first using the Merlin 1D/grid fins for a stable re-entry similar to S1, then only once the stage has significantly slowed (perhaps after another short Merlin 1D burn) inverting the S2 for a RTLS Super Draco powered landing.
... Block 5, supposedly capable of getting an L/D ratio of 1:1 on the first stage ...
the new grid fins should be capable of taking a scorching and being fine. And they'll also have significantly more control authority, so, that should improve reusability of the rocket. It will improve the payload to orbit by being able to fly at a higher angle of attack and use the aerodynamic element of the rocket to effectively glide like a big cylinder. It does have an L/D of roughly 1 if flown at the right angle of attack, but you need control authority, particularly pitch control authority, that's higher than we currently have to achieve that.