Author Topic: MCT Speculation and Discussion Thread 4  (Read 604154 times)

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2480 on: 07/28/2016 07:34 PM »
It could be, but it's not going to. At least not before MCT flies.

I started at least one thread speculating about a Raptor-based reusable upper stage for Falcon 9/FH, given that hint from the Air Force (which is not new, we've known about that for a quite long while). But now we know from Musk that they're not going to pursue that right now.

That leaves the questions of if and when wide open...
Sure, the future is wide open. But a post-MCT world is going to be weird and difficult to analyze before we see MCT and how well it flies.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Impaler

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Re: MCT Speculation and Discussion Thread 4
« Reply #2481 on: 07/29/2016 02:18 AM »

I don't think it can at all. It will break up from heat load on reentry unless heavily shielded - at which point it's basically a reusable upper stage, which Musk specifically said they aren't working on. The velocity change for a orbital reentry is about 4 to 5x greater, and the max heating rate 20 to 25x greater than what the F9 S1 sees on entry. Even if it's possible to orbit enough fuel retroburn through the peak heating phase (and I STRONGLY doubt that it is), why would they? The stage has a low ballistic coefficient, lots of area to dissipate heat, and slow terminal velocity: shield the front and part of one side, enter nose-first, and keep all the sensitive, expensive parts out of the hypersonic flow and well away from the bow shock. This is exactly how SpaceX envisioned S2 reuse... and they aren't pursuing it.

The same goes for BFR. I'm extremely doubtful that orbital reentry using primarily retropropulsion for shock standoff and cooling is possible and more optimal than nose-first entry for a S2 type vehicle. If there's any evidence to the contrary please point it out.

First SpaceX and Must throw out ideas without being slavishly committed too them, they generally stop talking about them once they find out they don't work.  They though 1st stages would land on parachutes so any notion that first choice methods are the ONLY options and if they fail the whole effort will fail is flat nonsense. 

Second even at the time of the release of that infamous video it was obvious that the 2nd stage recovery shown was Holly Wood nonsense and nothing more then a placeholder, nose first re-entry is impossible because it is totally unstable, the engine is most massive part of a 2nd stage and this will dictate an engine first entry.  Second it was clear that the nose of a second stage can't do the basic job of attaching a payload if it is a smooth heat shield.

The only feasible entry is engine first, which means either a shield that moves to cover the engine or a retro propulsive burn during entry can push the shock front away from the engine and keep it safe.  Entry can be short and at high G because it is an unmanned vehicle.

What I'm proposing is basically a Test bed, it gets valuable mid-air ignition experience on the Raptor (impossible to replicate on the ground) while delivering higher payloads and then gets some test data.  No actual modifications for re-use are required beyond developing a Raptor upper stage for F9, it's just plunges into the atmosphere to see how long it takes to disintegrate, the data is all that matters, none would ever be recovered.  Just as Grasshopper did not go to space but gathered valuable data.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2482 on: 07/29/2016 03:29 AM »
There are two feasible reentry positions, not just one. Side and engines-first. Both have been tested to varying degrees by SpaceX, Shuttle, DC-X, Blue Origin, as well as simulated.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2483 on: 07/29/2016 09:28 AM »
Isn't the mVac nozzle much too flimsy to withstand an engine first re-rentry? Or even a relatively low altitude/low speed (100's kmh) environment?

Offline Crispy

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Re: MCT Speculation and Discussion Thread 4
« Reply #2484 on: 07/29/2016 09:30 AM »
Isn't the mVac nozzle much too flimsy to withstand an engine first re-rentry? Or even a relatively low altitude/low speed (100's kmh) environment?
Yes, it would be torn to shreds. You'd either have to stow it, or have most of it be disposable.

Offline JamesH65

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Re: MCT Speculation and Discussion Thread 4
« Reply #2485 on: 07/29/2016 12:47 PM »
Isn't the mVac nozzle much too flimsy to withstand an engine first re-rentry? Or even a relatively low altitude/low speed (100's kmh) environment?
Yes, it would be torn to shreds. You'd either have to stow it, or have most of it be disposable.

So amy S2 powered recovery will require either an engine that can cope with its nozzle being discarded, or have alternative propulsion/navigation.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2486 on: 07/29/2016 01:00 PM »
Or retractable nozzle. Am I the only one who remembers the Falcon 9 reuse video?
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline docmordrid

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Re: MCT Speculation and Discussion Thread 4
« Reply #2487 on: 07/29/2016 03:10 PM »
Or, re-enter horizontally like a biconic then either rotate or land on minimalist skids. Perhaps small flush folding winglets. Like an elongated version of the early Kliper or Blues SV.
« Last Edit: 07/29/2016 03:26 PM by docmordrid »
DM

Offline matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2488 on: 07/29/2016 06:10 PM »
There are two feasible reentry positions, not just one. Side and engines-first. Both have been tested to varying degrees by SpaceX, Shuttle, DC-X, Blue Origin, as well as simulated.

Is there not a third? Reenter top first, and then do a swoop maneuver to land engine side down, propulsive, on legs? Or maybe a fourth, unlikly because of CG issues, Reenter top first and land upside down, propulsive, on legs?

Matthew

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2489 on: 07/29/2016 06:21 PM »
There are two feasible reentry positions, not just one. Side and engines-first. Both have been tested to varying degrees by SpaceX, Shuttle, DC-X, Blue Origin, as well as simulated.

None of those vehicles reenter engines-first from orbital velocity. They are either side/nose first from orbit (Shuttle, DC-X) or engine-first from sub-orbital velocity (SpaceX, BO).

Retracting or discarding the radiatively cooled nozzle extension is an absolute must for engine-first retro-propulsive entry from orbit for both aerodynamic and cooling issues.

But the biggest issue is orbiting enough propellent to run an engine during reentry. To get from orbit to a safe heating speed in atmosphere (below Mach 5) requires losing 6500 m/s of velocity: even at 10 g average deacceleration that's 65 seconds of retro burn, during which a Merlin will consume up to 17.5t of propellent and a Raptor would consume some 3x that. Even with throttling that's a massive payload reduction.

Retropropulsion decreases drag, so getting as high as 10g of deceleration is not all that likely. And fast entries increase the heating rate along with the structural loads, since the total dissipated energy remains constant.

Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2490 on: 07/29/2016 07:11 PM »
Retropropulsion often reduces drag, actually. But that's fine because the thrust of retropropulsion may be much greater than the thrust.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline matthewkantar

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Re: MCT Speculation and Discussion Thread 4
« Reply #2491 on: 07/29/2016 08:41 PM »
If second stages are going to be reused and deliver worthwhile payloads, I think most of the braking will need to be done with friction and not propulsion.

Matthew

Offline BobHk

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Re: MCT Speculation and Discussion Thread 4
« Reply #2492 on: 07/29/2016 08:56 PM »
So can anyone make a 2nd stage with a clamshell fairing that opens, delivers payload to orbit, closes [and acts as a heatshield for reentry (if it even needs to)]...then the stage flips, the bell retracts its vulnerable bit and that single engine lands the stage on some legs attached to the side?

Added bits (what kind of uphill penalty will this incur?):
 Landing legs
 X-Wing gridfins
 Actuators for clamshell payload fairing
 Fuel for re-entry

SpaceX seems to have chewed this through and come up with? nothing yet.

Can you design a fairing that can act like a seed wing in atmosphere to bleed velocity and take pressure off the engine to slow the thing down?    You people are a lot smarter than me.  Why no epiphanies?




Online Robotbeat

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Re: MCT Speculation and Discussion Thread 4
« Reply #2493 on: 07/30/2016 12:31 AM »
Sure, but SpaceX isn't going to take that approach. There are also several reasons that is suboptimal.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2494 on: 07/30/2016 05:08 AM »
If second stages are going to be reused and deliver worthwhile payloads, I think most of the braking will need to be done with friction and not propulsion.

Matthew

How about like this?

Offline GORDAP

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Re: MCT Speculation and Discussion Thread 4
« Reply #2495 on: 07/30/2016 08:45 AM »
...
Second even at the time of the release of that infamous video it was obvious that the 2nd stage recovery shown was Holly Wood nonsense and nothing more then a placeholder, nose first re-entry is impossible because it is totally unstable, the engine is most massive part of a 2nd stage and this will dictate an engine first entry.  Second it was clear that the nose of a second stage can't do the basic job of attaching a payload if it is a smooth heat shield.
...

Impaler, could/would nose first reentry be stable if there were grid fins near the engine?  I think they could be configured to provide significant drag at that end, right?  And stability would be helped if there were a (fairly heavy) TPS up at the nose of the 2nd stage.

And I think attaching a stage with a smooth heat shield to something else is a solved problem.  Else how does the Dragon capsule mate to the current 2nd stage?

Seems like a reusable Raptor 2nd stage could be done if it entered and landed nose first.  Add upside down landing legs near the top of the stage, TPS around the 'nose' (perhaps even a bit oversized diameter, to protect the body of the stage), some Super Dracos for landing assist, and grid fins near the engine.

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2496 on: 07/30/2016 02:45 PM »
...
 it is totally unstable, the engine is most massive part of a 2nd stage and this will dictate an engine first entry.
...

False. On a Dragon to ISS mission the residual propellent outweighs the entire second stage, and it only takes 1% of the initial 100 tonne propellent load to outweigh a Merlin.

At atmospheric drag will settle the remaining propellent in the nose long before it's significant enough to overpower the RCA thrusters, resulting in a quasi-stable configuration that cold be controlled by either RCS or small active aerodynamic surfaces during re-entry.

« Last Edit: 07/30/2016 02:51 PM by envy887 »

Offline envy887

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Re: MCT Speculation and Discussion Thread 4
« Reply #2497 on: 07/30/2016 03:08 PM »
...it was clear that the nose of a second stage can't do the basic job of attaching a payload if it is a smooth heat shield.

The only feasible entry is engine first, which means either a shield that moves to cover the engine...

These two statements are entirely inconsistent. Either it's possible to have a moveable heatshield to cover either the engine or the payload adapter, or it's not possible to have either. More likely, both are possible but both are too complicated and heavy to be feasible.

Bot attachment through the headshield isn't necessary. Dragon already attaches around the perimeter of the heatshield. The second stage - interstage interface is also completely around the perimeter of the stage. Attaching the payload adapter and fairing in a similar manner would likely require the least development work and mass penalty, and allow a smooth continuous heatshield on the stage 2.

The payload adapter would have to be discarded before re-entry.

Online OneSpeed

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Re: MCT Speculation and Discussion Thread 4
« Reply #2498 on: 07/31/2016 12:48 PM »
Seems like a reusable Raptor 2nd stage could be done if it entered and landed nose first.  Add upside down landing legs near the top of the stage, TPS around the 'nose' (perhaps even a bit oversized diameter, to protect the body of the stage), some Super Dracos for landing assist, and grid fins near the engine.

I agree that a second stage could enter and land nose first. However, given the difficulty of accurately predicting the landing zone of a GEO second stage, the odds are that it would land in the ocean. Why not design for this eventuality? If the drag coefficient was made sufficiently high, perhaps using a HIAD, retro propulsion for EDL would not be necessary, hence eliminating the need for extra fuel. The TPS around the nose could also be an inflatable, simplifying staging, and reducing mass. Grid fins would also not be required.

Online RonM

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Re: MCT Speculation and Discussion Thread 4
« Reply #2499 on: 07/31/2016 03:53 PM »
A reusable second stage would be actively guided it its landing zone, just like the first stage. No need to drop it in the ocean. It could land back at the launch site or on a barge.

BTW, this is the MCT speculation thread, so why are we having what seems to be a F9/FH discussion?

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