SpaceX News

[Skipjack]

Why would it have to be the same shape as a F9 first stage? I don't think an F9 based SSTO makes much sense. The payload mass ratio would be too low. What I could see is an SSTO based on the upcoming Raptor engines. If the Merlins are any indication, we might see an engine that combines an extremely high thrust to weight ratio with an Isp that is quite a bit higher than that of the Merlin. I would take these engines and combine them with a vehicle design similar to Gary Hudson's Phoenix. That solved the weight distribution and TPS problem.

[DeltaV]

Something that might work along these lines is to have a translating interstage section that slides back to reveal a PICA-covered nose. The engine end is hopeless in this regard.

How would you manage the pitch over maneuver once in lower atmosphere in preparation for landing? Seems like a lot of potential stress...

I wasn't presenting a preferred configuration, rather, exploring Skip's idea of PICA use on F9 stage 1. Simulation would answer the question of whether a path through state space could be found where qbar stays below structural limits. If the ballistic coefficient was low enough there might be a way. But I don't like the idea.

[DeltaV]

Weight of the engines makes it difficult getting anything similar to the Falcon9 first stage to reenter at any attitude other than tail first. One option for keeping the engines cool is running an expander cycle transpiration system: fluid through nozzle coolant passages, hot fluid powering a turbopump, then dumping through the engine to exit subsonic as a shield layer. This may need a separate turbopump for the reentry coolant cycle, running a single fluid at lesser volume. Running the engine at minimal throttle would use more propellant, but works without the extra systems.

The old balance a broom on fingertip problem. A challenge, but not that different control-wise from what is already done. But, yeah, not a preferred configuration. While the exhaust plume scheme might work, transpiration cooling would do nothing for the unavoidable hot spots in the geometrically-complex engine surroundings.

[DeltaV]

Why would it have to be the same shape as a F9 first stage? I don't think an F9 based SSTO makes much sense. The payload mass ratio would be too low. What I could see is an SSTO based on the upcoming Raptor engines. If the Merlins are any indication, we might see an engine that combines an extremely high thrust to weight ratio with an Isp that is quite a bit higher than that of the Merlin. I would take these engines and combine them with a vehicle design similar to Gary Hudson's Phoenix. That solved the weight distribution and TPS problem.

I thought the discussion was about ways that SpaceX might return an F9, or derivative thereof. I have to speed read since I have very little free time these days.

You already know that my preferred space hopper is a Polywell-powered waverider with 8 embedded ball-valve VTOL/cruise propulsors + REB for orbit (preference subject to modification upon new Woodward-effect data):
viewtopic.php?f=4&t=2317&p=60161#p60161

[ladajo]

Something that might work along these lines is to have a translating interstage section that slides back to reveal a PICA-covered nose. The engine end is hopeless in this regard.

How would you manage the pitch over maneuver once in lower atmosphere in preparation for landing? Seems like a lot of potential stress...

I wasn't presenting a preferred configuration, rather, exploring Skip's idea of PICA use on F9 stage 1. Simulation would answer the question of whether a path through state space could be found where qbar stays below structural limits. If the ballistic coefficient was low enough there might be a way. But I don't like the idea.

Fair enough, thanks for clarifying.

[Skipjack]

Why would it have to be the same shape as a F9 first stage? I don't think an F9 based SSTO makes much sense. The payload mass ratio would be too low. What I could see is an SSTO based on the upcoming Raptor engines. If the Merlins are any indication, we might see an engine that combines an extremely high thrust to weight ratio with an Isp that is quite a bit higher than that of the Merlin. I would take these engines and combine them with a vehicle design similar to Gary Hudson's Phoenix. That solved the weight distribution and TPS problem.

I thought the discussion was about ways that SpaceX might return an F9, or derivative thereof. I have to speed read since I have very little free time these days.

You already know that my preferred space hopper is a Polywell-powered waverider with 8 embedded ball-valve VTOL/cruise propulsors + REB for orbit (preference subject to modification upon new Woodward-effect data):
viewtopic.php?f=4&t=2317&p=60161#p60161

My input was merely about the fact that most people over estimate the weight of the heat shield. The penalty from those is generally not the prohibitive factor for SSTO. PICA-X is very lightweight with less than a quarter gram per ccm3. As Gary Hudson will tell you, lightweight structures are more important than Isp. With the upcoming Raptor methane engines, we will have engines with a good enough Isp and a fantastic T/W ratio. That T/W ratio is the key. SSTO RLVs would have been possible 30 years ago and I am sure today it is even more possible than ever.

[hanelyp]

Weight of the engines makes it difficult getting anything similar to the Falcon9 first stage to reenter at any attitude other than tail first. One option for keeping the engines cool is running an expander cycle transpiration system: fluid through nozzle coolant passages, hot fluid powering a turbopump, then dumping through the engine to exit subsonic as a shield layer. This may need a separate turbopump for the reentry coolant cycle, running a single fluid at lesser volume. Running the engine at minimal throttle would use more propellant, but works without the extra systems.

The old balance a broom on fingertip problem. A challenge, but not that different control-wise from what is already done. But, yeah, not a preferred configuration. While the exhaust plume scheme might work, transpiration cooling would do nothing for the unavoidable hot spots in the geometrically-complex engine surroundings.

A nose first reentry works, so long as the attitude control system works. Attitude control fails, even briefly, and the stage is back to tail first. The best feature of the Soyuz capsule is fault tolerance, bringing passengers back alive, if not comfortable, in the event systems like attitude control malfunctions.

Transpiration through the engines would leave hot spots, but I don't see it doing =nothing= to cool even the hot spots. The transpired gas not only carries away heat, it pushes outwards on the hypersonic air stream. Of course whatever heat shield the engines poke through matters.

[DeltaV]

Regarding hot spots, I was thinking more of the plumbing, wiring and structure surrounding the engines. Looking at a photo of a landed F9, however, they seem to be sealed up better than I had thought.



But those seals are not good enough for tail-first reentry from LEO, even with transpiration. Need either a large, all-enveloping plume or multiple smaller plumes with additional PICA (overlapping spherical-cap shells for gimbals). That brings up the issue of the acoustic tolerance of PICA at launch. Probably not very good at those intensity levels.

[DeltaV]

Attitude control fails, even briefly, and the stage is back to tail first. The best feature of the Soyuz capsule is fault tolerance, bringing passengers back alive, if not comfortable, in the event systems like attitude control malfunctions.

Modern airliner doctrine is undersized control surfaces to reduce drag. A decent pilot will be unfazed by the drop from Cadillac mode to Shopping Cart mode, but the ones who skip simulator time for another round of golf may upset a few passengers.

Since F9 stage 1 has no passengers, an occasional reentry control loop failure may be an acceptable business expense.

[kunkmiester]

It's awfully tall and narrow to be flipping around in atmosphere though. I had the impression this was one of the problems they were having when still trying to recover with just parachutes.

[paperburn1]

Once again In my mind it is still critical how much energy you can shed before the actual contact atmosphere above the Kármán line at an altitude of more than 100 km. Factors that we have to consider is this is not a shuttle return where we are basically dead stick landing the brick with wings. We have rockets, and fuel so we can shed a huge amount of velocity in space and not be depending on Aerobraking to bring us down to a speed where we can safely land. If we shed all this velocity outside the atmosphere then basically all were doing is a Jeff Bezo landing à la Microsoft. This means no real heavy-duty shields, no weight penalties for TPS devices other than those that probably already exist to protect the rocket bottom from the motors. The only real limit to this is the fact that you will have almost no real payload to orbit. I suspect this is the real reason why they have not been a demo flight of this by space X. Elon musk himself has estimated the total payload at 2 to 3% given the weight of the rocket. So I have very high confidence in the figures as I stated earlier. Of course it would not be the first time I was ever wrong

oops edit for clarity
25:1 fuel to hardware ratio doesn't mean it can put 4% of its liftoff mass into LEO, it means it can put 4% of its liftoff fuel mass into orbit. The first stage had a fuel mass of 116,881 kg, and 4% of that is 4,675 kg. The empty mass of the stage is 4,319 kg, so the 25:1 ratio, if accurate would imply only around 500kg of payload goes into orbit. Given super chilled fuel and stripping everything extra off of the ship including the two redundant motors. It should give enough to recover the SSTO.

[paperburn1]

Space X has a new vehicle

They call it "The dragon wagon "
https://imgur.com/a/ps05P

[Tom Ligon]

Musk has tweeted:

"Most recent rocket took max damage, due to v high entry velocity. Will be our life leader for ground tests to confirm others are good."

The press has squeezed several articles out of this. The rocket survived but will never fly again. Which honestly is not bad considering they knew this return was going to be rough, and thought the odds of it succeeding were slim.

[krenshala]

It's awfully tall and narrow to be flipping around in atmosphere though. I had the impression this was one of the problems they were having when still trying to recover with just parachutes.

This is why the first stage does its flip right after second-stage separation/ignition, does its boost-back burn, then immediately flips back to engines-first (its now on a retrograde suborbital flight toward the launch pad). At that point, even though its still in atmosphere, the dynamic pressure (even laterally) is easily managed by the structure of the rocket. By the time dynamic pressure is high enough to potentially be a problem, its well on its way to its targetted landing site.

[Tom Ligon]

Heat-shielding any long skinny projectile is problematic. Some friends of mine an I have been tinkering with an adaptation of NASA's IRVE inflatable heat shields. Our variation is to inflate them with polyisocyanurate foam rather than gas, making them less aeroelastic. Getting the ballistic coefficient of a beach ball instead of a javelin is a big help. A little dynamic pressure over a large area slows you down fast, at higher altitude.

A fairly small bag (made of a refractory fabric such as 3M Nextel, used on IRVE) backed with a couple of inches of polyisocyanurate foam (deployed quickly by mixing two liquids, can set in 30 seconds), would add heat shielding to the tail section. I presume the nozzles can take quite a bit. We tested the foam and were able to stand minutes of propane/compressed air torch fire, with heat flux similar to a shuttle re-entry. The foam does degrade and char, but holds its shape.

What SpaceX is doing is essentially an inflatable re-entry shield without the envelope. They're just expelling the gas rather than inflating a bag. The problem of heating is not so much from "friction" with the air as it is with compression in a shock wave not quite in contact with the heat shield. The shock wave can compress to incandescence. Using rocket exhaust to move that away is pretty clever, but sustaining the effect is expensive.

I presume their engine nozzles are cooled by propellant. Do they cool the entire tail section this way?