SpaceX FH: STP-2 : LC-39A : June 25, 2019 - DISCUSSION

[Asteroza]

I guess a 2 out of 3 engine landing wasn't in the cards then. Not enough time left to do a longer burn after an emergency center engine shutdown, but with two engines they had enough thrust to abort, so logically landing was possible? I guess that means they might have been able to land with two, but crippled TVC is too scary to try, thus the abort...

[Lars-J]

If the TVC failed, how did the computer intentionally divert?

How do you know it diverted? The TVC failed just before landing (since it got so close), and it seems to have done a hard turn. It just may just have looked like an intentional diversion. With TVC out, there is no control. And the other engines cannot pick up the slack.

EDIT: update after Musks tweet where he assumes it diverted.

But with no TVC control, this may have been an intentional/controlled divert just as much as you can divert a car where the steering wheel locks up. Not much control left. (The center engine has much more gimbal range than the two others)

[Robotbeat]

If the TVC failed, how did the computer intentionally divert?

How do you know it diverted? The TVC failed just before landing (since it got so close), and it seems to have done a hard turn. It just looked like an intentional diversion.

Musk seems to think it probably did.

https://twitter.com/elonmusk/status/1143690145255841797?s=21

[XenIneX]

I'll rephrase my question: if you are lined up with the asds why divert? Just continue regulating thrust. If not lined up then you can't call that intentionally diverted.

Landing is an inherently unstable process, which requires active, precision maneuvering.  Successfully doing after an engine has spontaneously decided to start flapping in the breeze is exceedingly unlikely.

[CJ]

Do we know exactly what kind of a 3-engine landing burn the center core was attempting?

The reason I ask is we've seen (first FH side core landings) a 1-3-1 burn for landing. *If* that is the "3 engine landing" the center core was attempting, and the center core TVC was out, the core could not land suvivably. Either it diverted, or, shut down the two outer engines, leaving an off-center center engine running, an unintentional divert.   

[ThatOldJanxSpirit]

Do we know exactly what kind of a 3-engine landing burn the center core was attempting?

The reason I ask is we've seen (first FH side core landings) a 1-3-1 burn for landing. *If* that is the "3 engine landing" the center core was attempting, and the center core TVC was out, the core could not land suvivably. Either it diverted, or, shut down the two outer engines, leaving an off-center center engine running, an unintentional divert.   

It still has throttle authority, so even with no effective directional control it could still have attempted some kind of abort.

[lrk]

I guess a 2 out of 3 engine landing wasn't in the cards then. Not enough time left to do a longer burn after an emergency center engine shutdown, but with two engines they had enough thrust to abort, so logically landing was possible? I guess that means they might have been able to land with two, but crippled TVC is too scary to try, thus the abort...

Two engines was probably too much thrust to land, as the rocket usually cuts to 1 engine just before landing and even then the TWR is much greater than 1.  Not to mention an engine-out capability for landing is not a trivial thing to add to the control logic, even if theoretically doable. 

[Steven Pietrobon]

twitter.com/elonmusk/status/1143686587877691392

Yes, but we couldn’t take a chance on 2nd stage failing it’s 4th maneuver. This mission was more complex than anything I’m aware of in history of rockets. RIP center core, you did your duty well.

I believe Proton-M/Briz-M missions are more complicated. The Briz-M does five burns plus a tank separation (parking orbit insertion, intermediate orbit insertion, transfer orbit insertion 1, tank separation, transfer orbit insertion 2 and geosynchronous transfer orbit insertion). Mission duration is 11 hours. In comparison, the FH second stage only did four burns and had a mission duration of 3.5 hours.

http://www.russianspaceweb.com/mexsat1.html

[Semmel]

twitter.com/elonmusk/status/1143686587877691392

Yes, but we couldn’t take a chance on 2nd stage failing it’s 4th maneuver. This mission was more complex than anything I’m aware of in history of rockets. RIP center core, you did your duty well.

I believe Proton-M/Briz-M missions are more complicated. The Briz-M does five burns plus a tank separation (parking orbit insertion, intermediate orbit insertion, transfer orbit insertion 1, tank separation, transfer orbit insertion 2 and geosynchronous transfer orbit insertion). Mission duration is 11 hours. In comparison, the FH second stage only did four burns and had a mission duration of 3.5 hours.

http://www.russianspaceweb.com/mexsat1.html

Yeah, he overtrumped that statement quite a bit. This was a complex mission, and maybe SpaceXes most complex mission. But there are plenty more complex missions. Even if you restrict it to launch vehicles only.

[HeartofGold2030]

twitter.com/elonmusk/status/1143686587877691392

Yes, but we couldn’t take a chance on 2nd stage failing it’s 4th maneuver. This mission was more complex than anything I’m aware of in history of rockets. RIP center core, you did your duty well.

I believe Proton-M/Briz-M missions are more complicated. The Briz-M does five burns plus a tank separation (parking orbit insertion, intermediate orbit insertion, transfer orbit insertion 1, tank separation, transfer orbit insertion 2 and geosynchronous transfer orbit insertion). Mission duration is 11 hours. In comparison, the FH second stage only did four burns and had a mission duration of 3.5 hours.

http://www.russianspaceweb.com/mexsat1.html

So the most complex mission done with cryogenic propellants then? Briz-M is a hypergolic tug, so multiple relights with coasts in between isn’t really that impressive because hypergols don’t suffer from the same problems cryos do when stored on orbit for lengthy periods e.g. boil off. These aforementioned problems are what makes STP-2 such a complex mission.

[rsdavis9]

twitter.com/elonmusk/status/1143686587877691392

Yes, but we couldn’t take a chance on 2nd stage failing it’s 4th maneuver. This mission was more complex than anything I’m aware of in history of rockets. RIP center core, you did your duty well.

I believe Proton-M/Briz-M missions are more complicated. The Briz-M does five burns plus a tank separation (parking orbit insertion, intermediate orbit insertion, transfer orbit insertion 1, tank separation, transfer orbit insertion 2 and geosynchronous transfer orbit insertion). Mission duration is 11 hours. In comparison, the FH second stage only did four burns and had a mission duration of 3.5 hours.

http://www.russianspaceweb.com/mexsat1.html

So the most complex mission done with cryogenic propellants then? Briz-M is a hypergolic tug, so multiple relights with coasts in between isn’t really that impressive because hypergols don’t suffer from the same problems cryos do when stored on orbit for lengthy periods e.g. boil off. These aforementioned problems are what makes STP-2 such a complex mission.

And don't most(all?) hypergols have bladders?
So no fuel settling.

[kdhilliard]

What happened to the spent second stage?  There is no mention of it’s final disposition in the press kit:  https://www.spacex.com/sites/spacex/files/stp-2_press_kit.pdf

Propulsive depassivation and it has been left out in a graveyard orbit.

Does that imply a fifth engine start, with burn to depletion?

[ugordan]

Anyone know what the inclination of the DSX payload was supposed to be in the end? Preflight orbit target showed it to be 43 deg, whereas it actually ended up in a 42.2 deg inclination.

Since both the apogee and perigee are right on target in the TLEs (within expected accuracy), the inclination discrepancy of 0.8 deg seems to be too significant, unless that was the plan all along.

[ZachS09]

Would it have been possible for the center core to be expended while the side boosters still returned to LZ-1 and LZ-2?

My logic is so that we wouldn’t have to worry about a potential landing going awry. But then again, maybe SpaceX wanted to recover the center core to reuse it.

[ugordan]

Would it have been possible for the center core to be expended while the side boosters still returned to LZ-1 and LZ-2?

Why wouldn't it be possible? What the center core does after BECO is of no consequence to booster RTLS, at that point it would behave just like an expendable F9 launch and just provide more margin to the 2nd stage.

My logic is so that we wouldn’t have to worry about a potential landing going awry. But then again, maybe SpaceX wanted to recover the center core to reuse it.

Of course they wanted to recover it. If not to reuse it, at least to inspect for potential weak points during high energy returns.

[ClayJar]

What happened to the spent second stage?  There is no mention of it’s final disposition in the press kit:  https://www.spacex.com/sites/spacex/files/stp-2_press_kit.pdf

Propulsive depassivation and it has been left out in a graveyard orbit.

Does that imply a fifth engine start, with burn to depletion?

Judging by the term and what it looked like in CGI in the pre-mission video, it looked more like venting through the engine (i.e. out the nozzle).  I wouldn't suggest we always use the videos as canonical references, of course, but in this case it looks like the propulsive passivation is basically venting the tanks using the Merlin as a cold gas thruster with a really large nozzle.

I don't know what the I_sp is for an MVac as a cold gas thruster with propellant consisting of helium pressurant and what LOX may boil, but I suppose it's something.

[ThatOldJanxSpirit]

What happened to the spent second stage?  There is no mention of it’s final disposition in the press kit:  https://www.spacex.com/sites/spacex/files/stp-2_press_kit.pdf

Propulsive depassivation and it has been left out in a graveyard orbit.

Does that imply a fifth engine start, with burn to depletion?

Judging by the term and what it looked like in CGI in the pre-mission video, it looked more like venting through the engine (i.e. out the nozzle).  I wouldn't suggest we always use the videos as canonical references, of course, but in this case it looks like the propulsive passivation is basically venting the tanks using the Merlin as a cold gas thruster with a really large nozzle.

I don't know what the I_sp is for an MVac as a cold gas thruster with propellant consisting of helium pressurant and what LOX may boil, but I suppose it's something.

Last parameters update I saw had the rocket body and DSX payload in orbits with just a few km difference in apogee and perigee, so no indication of a fifth burn yet.

[Vettedrmr]

Do we know exactly what kind of a 3-engine landing burn the center core was attempting?

The reason I ask is we've seen (first FH side core landings) a 1-3-1 burn for landing. *If* that is the "3 engine landing" the center core was attempting, and the center core TVC was out, the core could not land suvivably. Either it diverted, or, shut down the two outer engines, leaving an off-center center engine running, an unintentional divert.   

I think it was a 1-3-1 landing burn, primarily based on the symmetry of the plume on the video as the stage pitched away.  As far as it being an intentional maneuver, I don't think Musk *knows*, I think he believes it *could* have.  A landing stage is really unstable, to the point you need two active control systems (RCS and engine gimbal) to keep things under control.  So if the TVS failed for whatever reason (my personal is that hydraulic system was breached and depleted right before touchdown), it's going to pitch over really dramatically, and really quickly.

Have a good one,
Mike

[Alexphysics]

What happened to the spent second stage?  There is no mention of it’s final disposition in the press kit:  https://www.spacex.com/sites/spacex/files/stp-2_press_kit.pdf

Propulsive depassivation and it has been left out in a graveyard orbit.

Does that imply a fifth engine start, with burn to depletion?

It's what NASA defines it to be a propulsive depassivation but I don't certainly know if they ended up doing it. If you look at the difference between the FH S2 orbit and the DSX orbit, there is not much difference so either they determined it was not safe to run the engine again or the tanks literally had no more fuel to make the engine run and just vented off gasses, the ACS gas and all of that and drained the batteries (which is a standard passivation maneuver).

[eeergo]

twitter.com/elonmusk/status/1143686587877691392

Yes, but we couldn’t take a chance on 2nd stage failing it’s 4th maneuver. This mission was more complex than anything I’m aware of in history of rockets. RIP center core, you did your duty well.

I believe Proton-M/Briz-M missions are more complicated. The Briz-M does five burns plus a tank separation (parking orbit insertion, intermediate orbit insertion, transfer orbit insertion 1, tank separation, transfer orbit insertion 2 and geosynchronous transfer orbit insertion). Mission duration is 11 hours. In comparison, the FH second stage only did four burns and had a mission duration of 3.5 hours.

http://www.russianspaceweb.com/mexsat1.html

So the most complex mission done with cryogenic propellants then? Briz-M is a hypergolic tug, so multiple relights with coasts in between isn’t really that impressive because hypergols don’t suffer from the same problems cryos do when stored on orbit for lengthy periods e.g. boil off. These aforementioned problems are what makes STP-2 such a complex mission.

And don't most(all?) hypergols have bladders?
So no fuel settling.

Briz-M uses ullage thrusters, and has the toroidal APT crossfed to the central stage which has to be jettisoned.

Concerning the cryo storage, Centaur has achieved many 3h+ missions with a fully (not semi-) cryogenic stage. It's true that, as far as I'm aware, the stage has only demonstrated 3-burn missions, but cryo loiter times have been much longer, up to 5h. Delta IV's also fully-cryo DCSS has multi-start capability for <7h missions.

Maybe it can be argued STP-2's upper stage made the most challenging multi-burn profile of a semicryo upper stage ever, and indeed landing attempts has been the most complex behavior of lower stages to date for a while now, not just on this mission - but that's kind of different than "most complex mission in the history of rockets".