Author Topic: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION  (Read 1087829 times)

Offline ugordan

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #540 on: 02/09/2016 05:15 pm »
Besides the difference between re-entry heating at 3km/s and 1.5 is 4 times!

Like I posted earlier, there already is precedent for a stage surviving a 4x higher dynamic pressure, Asiasat 6. I say pressure, not heating because I believe dynamic pressure is the big issue, not heating on an engine section that carries hefty TPS anyway. Musk also quotes psf, not temperatures.

It doesn't prove all such entries would work and it doesn't prove F9FT with the grid fins and legs would fare as good and certainly actually guiding a stage toward a certain landing spot is more challenging. It's apparently not an outright death sentence for a stage, though.
« Last Edit: 02/09/2016 05:19 pm by ugordan »

Offline mme

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #541 on: 02/09/2016 05:31 pm »
In terms of PR, is it worthy to try recovery of with very low chances of success? big ground explosions are no good for SpaceX reputations, they would be flown by "fourth barge attempt explosion in a row" headlines everywhere.

One alternative is that they don't mention the recovery attempt, i.e. use the "The F9 for this flight will be in expendable mode", and then attempt the landing anyway, ...

And if it succeeds, then well, it's a win win and they can release some glorious footage...

That's the Blue Origin way not the SpaceX way.  Musk and Shotwell both love to tell people that if you aren't failing at experiments, then you aren't pushing the boundaries enough.  No way does Musk stay silent about trying something crazy.
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Offline whitelancer64

Disregard energy management for a moment. In addition to that basic function, the boostback/slowdown and reentry burns also serve as trajectory correction maneuvers. Without them, there is a possibility that the uncertainty in the state vector at 1st stage cutoff would be too big for just the fins to compensate for during reentry phase.

In short, I think there has to at least be a short reentry burn to aim toward the general area of the ASDS and then to rely on the fins for finer adjustments before landing burn.

The grid fins deploy while the 1st stage is still well above the Karman Line, so they should have good control all through the descent. The stage would be on an entirely ballistic trajectory prior to atmospheric reentry, so the trajectory uncertainty should be low. Whether or not the stage survives reentry without a deceleration burn is probably the bigger question.
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Offline ugordan

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #543 on: 02/09/2016 05:49 pm »
The stage would be on an entirely ballistic trajectory prior to atmospheric reentry, so the trajectory uncertainty should be low.

Why should/would it be low? That ballistic trajectory will have basically been set up several minutes earlier when staging happened so any state vector difference at that point to what the preflight staging predict was would have quite a bit of time to propagate into a bigger downrange impact point uncertainty. Of course I have no actual numbers to back any of this up.

Offline whitelancer64

The stage would be on an entirely ballistic trajectory prior to atmospheric reentry, so the trajectory uncertainty should be low.

Why should/would it be low? That ballistic trajectory will have basically been set up several minutes earlier when staging happened so any state vector difference at that point to what the preflight staging predict was would have quite a bit of time to propagate into a bigger downrange impact point uncertainty. Of course I have no actual numbers to back any of this up.

Well, not to sound unhumble, but something that it turns out we've generally been pretty good at over the past ~50 years or so is calculating ballistic trajectories (See: Apollo, Voyager and misc. other probes, etc.). Where the uncertainty lies is the amount of drag that the rocket will encounter, since the atmosphere (especially at high altitudes) is so variable that that's difficult to pin down with precision.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
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Offline ugordan

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #545 on: 02/09/2016 06:01 pm »
No, no, I'm not talking about us not being able to *propagate* a state vector. I'm talking about the uncertainty of the starting point, at staging. In your interplanetary example, think about injection errors.
« Last Edit: 02/09/2016 06:02 pm by ugordan »

Offline georgegassaway

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #546 on: 02/09/2016 06:18 pm »
Disregard energy management for a moment. In addition to that basic function, the boostback/slowdown and reentry burns also serve as trajectory correction maneuvers. Without them, there is a possibility that the uncertainty in the state vector at 1st stage cutoff would be too big for just the fins to compensate for during reentry phase.

In short, I think there has to at least be a short reentry burn to aim toward the general area of the ASDS and then to rely on the fins for finer adjustments before landing burn.

The grid fins do provide a pretty good amount of aerodynamic steering.  It would have to be pretty far off ballistically to be too far away from the glide cross-range capability of the grid fins.

For this kind of flight/landing attempt, you would expect them to locate the ASDS right where a ballistic descent  of the booster would be projected to splash down.  But as you imply, there could be some variability in that at cutoff.  I do not know how many off-nominal miles downrange such an error might be, or how many miles glide cross-range the grid fins can provide (I know, it’s not a very good glide. But it starts being effective from a high altitude).

But let's say that the error is too far for the grid fins to handle  One possibility for that would be during turnaround, only pitch about 90 degrees to the proper vector attitude direction needed and then do a lateral (perpendicular to flight path) course correction burn to put the projected ballistic descent path  back inside the ballpark for the grid fins to be able to glide it in to home plate later.  Would not require much of  burn to do that kind of course correction, zero wasted on retro, 100% for nudging the descent path close enough to the barge for the grid fins to handle.  Of course any burn in this case uses fuel that should be saved as much as possible for landing. But there’s not much point soft-landing on water, been there, done that.

Anyway, I sort of agree that if the ballistic path is off nominal and  happened to be beyond what the grid fins can handle (which is a lot), that there might be a need for a “short burn”.  But not a re-entry burn. An incredibly brief  90 degree attitude course-correction burn as soon after staging as it can calculate for, maneuver,  and  do it.  Because the sooner it does it, the less delta-v required and thus less propellant.

All  the above of course presumes the vehicle withstands such a high-speed re-entry and still works properly, I'm only referring to grid fin glide range and a possible course correction option.

Would be a hell of a great thing if after 3 failures to land safely on an ASDS, this one made it despite lack of a boost-back burn (which even the ASDS landings have had to an extent) and lack of retro burn.

- George Gassaway
« Last Edit: 02/09/2016 06:25 pm by georgegassaway »
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Offline notsorandom

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #547 on: 02/09/2016 06:39 pm »
Correct me if I'm wrong but cross range depends on not only the ability to control attitude but also the ability to generate lift to change the direction of the velocity vector. Pointing in a certain direction in of itself without lift does no good to change that vector. Are Falcon's fins big enough to generate enough lift or does it use the rocket's body to help do so? If that is the case there is likely a limit to how big the angle of attack can be before the rocket fails due to heating or becomes uncontrollable.
« Last Edit: 02/09/2016 06:39 pm by notsorandom »

Offline cscott

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #548 on: 02/09/2016 06:49 pm »
The rocket's body generates lift.  The grid fins just maintain it at the proper angle-of-attack to do so.  Stall typically occurs between 15 and 20ish degrees.  The grid fins will likely keep it below stall to minimize the forces on the body.
« Last Edit: 02/09/2016 06:57 pm by cscott »

Offline Lar

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #549 on: 02/09/2016 06:57 pm »
@Lar if the attempt fails, they are likely going to arrive nowhere near the ASDS.  Arriving close enough to damage the ASDS means that the landing attempt basically worked.
The only scenario where that would *not* be the case is where it "almost worked" and the engine ran out early (or failed to start due to some damage from the additional heating it normally doesn't experience?)  but the grid fins did a great job of guidance so the stage smacked right into the ASDS dead center at a good clip...

But of course that is where the guidance computer is supposed to divert to miss... so that probably won't happen either.
« Last Edit: 02/09/2016 07:13 pm by Lar »
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Offline cscott

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #550 on: 02/09/2016 06:59 pm »
@Lar if the attempt fails, they are likely going to arrive nowhere near the ASDS.  Arriving close enough to damage the ASDS means that the landing attempt basically worked.
The only scenario where that would *not* be the case is where it "almost worked" and the engine ran out early (or failed to start due to some damage from the additional heating it normally doesn't experience?)  but the grid fins did a great job of guidance so the stage smacked right into the ASDS dead center at a good clip...
We've been over this again and again in the ASDS threads.  The barge so outmasses the rocket that not much would happen, collision-wise, even of you were to manage an impact at stage terminal velocity.  Think light aluminum soda can hitting massive steel wall.  The ASDS is also ballasted with water during landing, adding further to its mass.  It also has multiple sealed compartments, if you are worried about a Titanic situation.

Edit: and again, managing to hit the ASDS implies that the rocket is intact which implies that the atmosphere has already succeeded in slowing the stage down to terminal velocity.
« Last Edit: 02/09/2016 07:12 pm by cscott »

Online Robotbeat

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #551 on: 02/09/2016 07:18 pm »
Okay yeah, but 20 tons of just about anything moving at hundreds of miles per hour would damage the barge significantly.

But that's what the barge is for and why it's steel and not fiberglass. Weld the hole shut when you get back to shore.
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Offline CraigLieb

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #552 on: 02/09/2016 07:20 pm »
Okay yeah, but 20 tons of just about anything moving at hundreds of miles per hour would damage the barge significantly.

But that's what the barge is for and why it's steel and not fiberglass. Weld the hole shut when you get back to shore.
And the Video would be EPIC! but here's to hope we don't see one like that.
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Offline Lar

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #553 on: 02/09/2016 07:26 pm »
Okay yeah, but 20 tons of just about anything moving at hundreds of miles per hour would damage the barge significantly.

But that's what the barge is for and why it's steel and not fiberglass. Weld the hole shut when you get back to shore.

I don't think it's a likely outcome anyway but where I could see it causing issues is debris from the impact hitting equipment, Not so much damaging the deck itself. And the equipment is now protected behind walls (IIRC).

A terminal velocity impact is different than a low speed impact. So the splash paths of debris might be different, and it may be more (or less) energetic. But I expect more.

Review the footage of straws driven through boards by tornadoes... aluminum cans might well damage containers (if not for the blast walls) despite being flimsy.

Anyway this is fun to talk about and I'm excited. I hope they go for it and will be rooting for them. Fail forward. If you're not falling you're not skiing hard enough...
« Last Edit: 02/09/2016 07:28 pm by Lar »
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline sdsds

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #554 on: 02/09/2016 07:45 pm »
@ugordon: Do we have any evidence that on prior braking burns F9 used its current state as an input into the burn direction or duration? Is it possible all prior braking burns have been "open loop" i.e. with pre-determined directions and durations? Conceivably just doing a very brief but "navigated" burn might get them onto a ballistic trajectory with better accuracy on this attempt than on prior attempts....
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Offline Johnnyhinbos

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #555 on: 02/09/2016 07:46 pm »
Not to open a can of worms, but could they unzip the booster if it's determined to be more of an anti ASDS weapon rather then a lovely companion rocket, or can only the Range trigger the FTS (and if so, would they initiate it in order to spare excessive damage to the barge).

By the way - it could very easily do significant damage to the barge and systems. Look at the damage that the original JRTI sustained on the first landing attempt. I've attached an image just in case you forgot. Those are steel containers that house the diesel generators for the thrusters, among other things. Crumpled by that flimsy tin can. That kind of damage could really put a crimp in an aggressive launch (recovery) manifest.
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Offline Kabloona

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #556 on: 02/09/2016 07:52 pm »
Quote
Not to open a can of worms, but could they unzip the booster if it's determined to be more of an anti ASDS weapon rather then a lovely companion rocket, or can only the Range trigger the FTS

Only Range can send a destruct command.

Quote
By the way - it could very easily do significant damage to the barge and systems. Look at the damage that the original JRTI sustained on the first landing attempt.

That's why they added blast walls.
« Last Edit: 02/09/2016 07:54 pm by Kabloona »

Offline Semmel

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #557 on: 02/09/2016 08:23 pm »
Not to open a can of worms, but could they unzip the booster if it's determined to be more of an anti ASDS weapon rather then a lovely companion rocket, or can only the Range trigger the FTS (and if so, would they initiate it in order to spare excessive damage to the barge).

Also I think I heared "FTS is safed" on the radio in last missions prior to the landing attempt. I think its noted around the reentry burn. So no more FTS near the barge in any case.

@edit: thx mheney, I saved that change :P
« Last Edit: 02/09/2016 08:44 pm by Semmel »

Offline mheney

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #558 on: 02/09/2016 08:39 pm »
Not to open a can of worms, but could they unzip the booster if it's determined to be more of an anti ASDS weapon rather then a lovely companion rocket, or can only the Range trigger the FTS (and if so, would they initiate it in order to spare excessive damage to the barge).

Also I think I heared "FTS is saved" on the radio in last missions prior to the lading attempt. I think its noted around the reentry burn. So no more FTS near the barge in any case.

"Safed", not saved. 

Offline Raj2014

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Re: SpaceX Falcon 9 FT - SES-9 - March 4, 2016 - DISCUSSION
« Reply #559 on: 02/09/2016 08:52 pm »
Is SpaceX attempting another landing again? If yes, where will they land the Falcon?

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