Saw this on YouTube today, thought I'd post it. Didn't hear anything new for the regulars, but may be good for the non-engineers.
Quote from: Norm38 on 04/23/2018 04:41 pmSaw this on YouTube today, thought I'd post it. Didn't hear anything new for the regulars, but may be good for the non-engineers.NahI would bet against each of this guy’s guesses and interpretations. It’s not “obvious”. It doesn’t “have to be”. SpaceX is not going to rig up some complex structure like HIAD or whateverThey can’t surround the second stage in airbags. It’s too big and they couldn’t be well distributedAll of his borrowed video snippets are irrelevant.edit: typos
NahI would bet against each of this guy’s guesses and interpretations. It’s not “obvious”. It doesn’t “have to be”. SpaceX is not going to rig up some complex structure like HIAD or whateverThey can’t surround the second stage in airbags. It’s too big and they couldn’t be well distributedAll of his borrowed video snippets are irrelevant.
But the exact details are very tricky and clearly SX have learned some new science since 2014 when Musk last visited this issue.
Quote from: Comga on 04/29/2018 02:03 pmNahI would bet against each of this guy’s guesses and interpretations. It’s not “obvious”. It doesn’t “have to be”. SpaceX is not going to rig up some complex structure like HIAD or whateverThey can’t surround the second stage in airbags. It’s too big and they couldn’t be well distributedAll of his borrowed video snippets are irrelevant.True. Just the KE alone (27 mega m/s Vs 5 mega m/s) makes it 29x harder in energy dissipated before we get to the potential energy of the increased altitude. I comes down to this. 1) You want the engines in the rear.2) A rear heavy object wants to flip heavy end forward.3) Too much side load and the (very) lightweight tank walls collapse. How you keep the loads along the stage, and the engines pointing backward, are the crux of the problem.Now a really big skirt (with minimum TPS) just above the engine bay cana) Radically increase drag in the high (near vacuum) atmosphere so the stage is moving much more slowly before it descends to the thicker low atmosphere. big deceleration. Small(ish) heating.b) Keep the light end (with a thick, simple PICX heatshield) pointing into the airstream.As long as it last long enough to get into the air density where the grid fins can work then your home dry.But the exact details are very tricky and clearly SX have learned some new science since 2014 when Musk last visited this issue.
With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
An inflatable radically changes the COP/COM equation, the engine location becomes less critical.With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.
Quote from Elon Musk in a comment below the recent Instagram photo of the fairing descending under a parachute: "No, the upper stage engine is designed for vacuum operation only. We will either bring it in hot and fast Dragon style with a heat shield on front or slower with a giant party balloon."I guess they are still in the concept phase of design on this.
Quote from: envy887 on 04/30/2018 03:40 pmAn inflatable radically changes the COP/COM equation, the engine location becomes less critical.With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage. To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet) in width.That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.
Quote from: john smith 19 on 05/02/2018 07:32 amQuote from: envy887 on 04/30/2018 03:40 pmAn inflatable radically changes the COP/COM equation, the engine location becomes less critical.With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage. To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet) in width.That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.A toroidal ballute around the engine would probably be easier than a skirt, but a simple spherical towed ballute hanging off the bottom of the stage would probably be easiest.
Quote from: envy887 on 05/02/2018 01:27 pmQuote from: john smith 19 on 05/02/2018 07:32 amQuote from: envy887 on 04/30/2018 03:40 pmAn inflatable radically changes the COP/COM equation, the engine location becomes less critical.With a 100x increase in ballistic coefficient, you don't need PICA-X. Stagnation temperatures drop to ~800 C or lower and easily into the working range of COTS ceramics and metals or a coating of SPAM.Certainly shifts the CoP. Not so sure about the Centre of Mass. AFAIK the consensus remains the engine bay will be the heaviest section of the stage. To lower the ballistic coefficient 100x needs a skirt about 16.5m (54 feet) in width.That's a pretty substantial unfurling task. Not impossible, but tough. If it can be made like those emergency slides aircraft use you could get quite a lot of area in a fairly small space, using inflatable tubes to stiffen it.A toroidal ballute around the engine would probably be easier than a skirt, but a simple spherical towed ballute hanging off the bottom of the stage would probably be easiest.Like a sort of space "sea anchor" ?One problem with that would be the mounting. I'd think you'd want to put it as close to the center as possible but that's occupied by the engine. So off center you've got a force tugging the off its line, or you need say a 3 or 4 point attachment arrangement around the nozzle to equalize loads. Again, tricky to deploy reliably. A smaller ballute, one per mounting point, might be easier, but now you need all of them to inflate or you get the the uneven loading. Just remembered. 800c is probably in the range of the Dunlop developed woven metal "airmat" technology. Unfortunately I don't know of any plastics that go above 400c (IIRC 300-350c is pretty challenging for regular use). OTOH probably well within the range of a layer of flexible PICAX, which SX should have access to.
Does anybody have any calcs about what the g-force is for the trailing balloon idea. I assume because it has a higher ballistic coefficient it decelerates faster.
Quote from: rsdavis9 on 05/03/2018 07:01 pmDoes anybody have any calcs about what the g-force is for the trailing balloon idea. I assume because it has a higher ballistic coefficient it decelerates faster.To a first order, it does not.It decelerates at the same rate, just where the atmosphere is 100* less dense.The deceleration rate is in fact slightly different because the atmosphere scale height at the entry interface (the amount the atmosphere is changing due to height) varies due to a temperature discontinuity in the atmosphere).
Quote from: speedevil on 05/03/2018 07:35 pmQuote from: rsdavis9 on 05/03/2018 07:01 pmDoes anybody have any calcs about what the g-force is for the trailing balloon idea. I assume because it has a higher ballistic coefficient it decelerates faster.To a first order, it does not.It decelerates at the same rate, just where the atmosphere is 100* less dense.The deceleration rate is in fact slightly different because the atmosphere scale height at the entry interface (the amount the atmosphere is changing due to height) varies due to a temperature discontinuity in the atmosphere).I think that rsdavis9 misspoke. Yes, they decelerate at the same rate, they are tied together after all. But the balloon experiences a much greater drag force than the first stage, that is its whole purpose. The drag force will hold tension on the lines attaching the two. How great can this drag force be expected to be? If the balloon increases the deceleration by 0.1 g above that of stage 1 alone, then the drag force would equal about 2 tons of force. So the question boils down to this, "What would be the optimum and maximum increase of deceleration of the assembly be between balloon deployment and balloon release?That is, increase over that of stage 1 alone.