SpaceX Falcon 9 : SES-12 : June 4, 2018 : Discussion

[AC in NC]

Maybe it has a big party balloon so it doesn't need a orbit burn.

Maybe now that the legs and fins are removed from the booster, it stages faster leaving the upper stage with a few more seconds of fuel.

The only ways for the engine to burn longer are to either reduce mass flow through the engine or make more mass available for the engine to use. Changing the staging time does neither, and since Block 5 MVac has a 5% thrust increase, the mass flow through the engine is likely increased.

However, the faster LOX load in the Block 5 upper stage means that more LOX mass can be loaded since it stays colder and denser.

Could be that the first stage is burning longer, using propellant that would have been used for landing.  That would reduce the second stage first burn length to reach its parking orbit, which would allow for a longer second burn.

Falcon 9 has already lifted at least one heavier payload to a higher orbit. 

 - Ed Kyle

This is likely the answer.  The liklihood of a casual reference matching the technical accuracy required for the dissections that are prone to occur on NSF is exceedingly low.  In would make perfect sense that there was an implicit "... than necessary to achieve the otherwise acceptable orbit" and not that it was meant the 2nd stage engine was burning longer.

[RDMM2081]

I don't think I saw anyone suggest that the first stage deploys higher than normal due to not needing recovery fuel, then the second stage burns at a lower throttle level for longer and it ends up as a net gain because less gravity losses due to the higher/faster stage sep?

[AC in NC]

Not sure I understand that point.

Am I missing something?  How does the first stage not "deploy higher[/faster] than normal" while providing the 2nd stage the ability to burn longer due to "higher/faster stage sep"?

[RDMM2081]

Not sure I understand that point.

Am I missing something?  How does the first stage not "deploy higher[/faster] than normal" while providing the 2nd stage the ability to burn longer due to "higher/faster stage sep"?

You are probably right that it probably doesn't make sense, I have a very weak grasp of orbital mechanics.  The only point really was to explain this longer second stage burn by throttling the second stage engine for part of the burn, but usually when people here talk about engines running throttled down they say it isnt worth it due to the gravity losses.  I was trying to make the point that the gravity losses might be mitigated by getting extra speed to the second stage by using the first stage landing fuel, but I still don't know how that works and sorry for rambling nonsensical gibberish.

[Comga]

The blurb on a USA Today article says

"SpaceX is planning to launch the SES-12 satellite next week, taking extra time in final preparation for the launch of a previously fired rocket"

As usual in the popular media, sloppy language leads to false implications.
They aren't taking more time in  'preparation for the launch of a previously fired rocket" because the extra time was needed to replace a valve on the second stage, and second stages are never previously flown.
Second stages are not even "previously fired".

[AC in NC]

Not sure I understand that point.

Am I missing something?  How does the first stage not "deploy higher[/faster] than normal" while providing the 2nd stage the ability to burn longer due to "higher/faster stage sep"?

You are probably right that it probably doesn't make sense, I have a very weak grasp of orbital mechanics.  The only point really was to explain this longer second stage burn by throttling the second stage engine for part of the burn, but usually when people here talk about engines running throttled down they say it isnt worth it due to the gravity losses.  I was trying to make the point that the gravity losses might be mitigated by getting extra speed to the second stage by using the first stage landing fuel, but I still don't know how that works and sorry for rambling nonsensical gibberish.

Ah yes.  I think that is a plausible alternative and was considering mentioning it.

To summarize succinctly, I think there are two possibilities (and like you I'm not well versed):

1)  Higher/Faster stage sep gets the 2nd Stage to the contracted orbit sooner permitting the final seconds of its nominal full throttle burn length to boost it higher; -or-

2)  Higher/Faster stage sep gets the 2nd Stage provides the 2nd Stage the ability to throttle down somewhat and burn longer.

I just don't know whether that 2nd one is actually worthwhile given some complex trades.

[cppetrie]

The blurb on a USA Today article says

"SpaceX is planning to launch the SES-12 satellite next week, taking extra time in final preparation for the launch of a previously fired rocket"

As usual in the popular media, sloppy language leads to false implications.
They aren't taking more time in  'preparation for the launch of a previously fired rocket" because the extra time was needed to replace a valve on the second stage, and second stages are never previously flown.
Second stages are not even "previously fired".

Aren’t second stages test fired at McGregor? They just don’t have the vacuum nozzle extension, right?

[TrueBlueWitt]

Not sure I understand that point.

Am I missing something?  How does the first stage not "deploy higher[/faster] than normal" while providing the 2nd stage the ability to burn longer due to "higher/faster stage sep"?

You are probably right that it probably doesn't make sense, I have a very weak grasp of orbital mechanics.  The only point really was to explain this longer second stage burn by throttling the second stage engine for part of the burn, but usually when people here talk about engines running throttled down they say it isnt worth it due to the gravity losses.  I was trying to make the point that the gravity losses might be mitigated by getting extra speed to the second stage by using the first stage landing fuel, but I still don't know how that works and sorry for rambling nonsensical gibberish.

Ah yes.  I think that is a plausible alternative and was considering mentioning it.

To summarize succinctly, I think there are two possibilities (and like you I'm not well versed):

1)  Higher/Faster stage sep gets the 2nd Stage to the contracted orbit sooner permitting the final seconds of its nominal full throttle burn length to boost it higher; -or-

2)  Higher/Faster stage sep gets the 2nd Stage provides the 2nd Stage the ability to throttle down somewhat and burn longer.

I just don't know whether that 2nd one is actually worthwhile given some complex trades.

Maybe this one they're running the Block 5 Vacuum Merlin at full thrust not reduced like the first Block 5 flight? that means less grav losses and earlier throttle down at the G limit.  Which could add a few seconds.. Right?

Plus staging higher and faster also means S2 is closer to orbital velocity, so yes it seems that would also mean less overall grav losses after staging.

[John Alan]

Gravity losses during the last few minutes of S2 burn are very, very small...
The forward velocity in the orbital direction is canceling most of it due to the centripetal effect...

Remember that gravity losses are greatest when cutting all thrust would result in an immediate downward motion.
At liftoff... well duh... HUGE losses...
At MaxQ... it would loft up a while before arcing over...
At MECO... it will go quite a ways before arcing over...
And losses continues to drop very quickly as speed then builds from there...

The big difference is this is an expendable flight... And the last 10 seconds (90 engine seconds) of thrust off S1 with it's near empty tanks has a profound kick in shoving S2 higher, or faster, or a bit of both...

In summary... S2 Mvac running at even 10% more thrust has little if any effect on cutting overall gravity losses...
My 2 cents on subtopic above...   

[Semmel]

The only reason to reduce thrust at S2 during its first burn is to get to a higher initial orbit. This allows the stage to ascend further before circularizing its orbit.

[rsdavis9]

So I know the 2nd stage engine is tested at mcgregor and then sent back to hawthorne for integration with the 2nd stage. But is the complete 2nd stage fired at mcgregor(minus the nozzle extension)?

[S.Paulissen]

Yes.

[speedevil]

At some point also being able to throttle deeper would allow a few seconds of extra burn due to not exceeding the spacecraft G limit.
I have doubts this would be the case for a large comsat launch.

[LouScheffer]

At some point also being able to throttle deeper would allow a few seconds of extra burn due to not exceeding the spacecraft G limit.
I have doubts this would be the case for a large comsat launch.

The Merlin engine is sufficiently large that they still need to throttle.  At full thrust, it produces a force of 914 kN (perhaps more with Block 5).  In this case, at the end of the burn, it would be pushing a mass of about 10,000 kg (about 4.7t for the stage and 5.3t for the satellite).  At full throttle, this would be more than 9Gs (90 m/s).  Most satellites are designed to about 5 Gs, so they need to throttle back.

You can see this clearly on the graph of acceleration for InMarSat, an even heavier satellite.

EDIT:  Also, there are some indications they don't use full throttle on GTO insertion burns, even at the start of the burn.  The speculation is that it's not really needed, and running at lower throttle will reduce stresses.

[cppetrie]

So I know the 2nd stage engine is tested at mcgregor and then sent back to hawthorne for integration with the 2nd stage. But is the complete 2nd stage fired at mcgregor(minus the nozzle extension)?

Yes. I’ve seen them extracting it from the test stand when I drove down to see what I could see at McGregor from public vantage points. 

[speedevil]

At some point also being able to throttle deeper would allow a few seconds of extra burn due to not exceeding the spacecraft G limit.
I have doubts this would be the case for a large comsat launch.

The Merlin engine is sufficiently large that they still need to throttle.  At full thrust, it produces a force of 914 kN (perhaps more with Block 5).  In this case, at the end of the burn, it would be pushing a mass of about 10,000 kg (about 4.7t for the stage and 5.3t for the satellite).  At full throttle, this would be more than 9Gs (90 m/s).  Most satellites are designed to about 5 Gs, so they need to throttle back.

However, for light satellites, the total mass may drop low enough that at minimum throttle you have to cut the engine or exceed the G limits.
Being able to throttle to 30, not 50%, for example, might get you several seconds of extra thrust for light satellites that can't take high G.
Sorry my point was unclear.

[DrRobin]

Having followed the progress of satellite electric propulsion for quite some time, I am interested in the capabilities of SES-12 -cited as the largest all-electric satellite yet to launch- in this regard. I see it uses the SPT-140D thruster, rated at 280mN and 1770 sec at 4.5 kW (https://iepc2017.org/sites/default/files/speaker-papers/iepc_2017_ssl_electric_propulsion.pdf), but I couldn't find how much total Xenon propellant it carries relative to its total mass, so it wasn't clear to me how much delta-V it had available. In a practical sense, if it suffered an early failure of a critical component, could it bring itself down to the ISS for repair (perhaps aided by Magellan-style gentle aerobraking)? In a fanciful sense, if SES just decided to take off into the Wild Black Yonder, where could it go? LLO? Earth's quasi-moon, 2016 HO3? Diemos?

[Nomadd]

Not sure I understand that point.

Am I missing something?  How does the first stage not "deploy higher[/faster] than normal" while providing the 2nd stage the ability to burn longer due to "higher/faster stage sep"?

You are probably right that it probably doesn't make sense, I have a very weak grasp of orbital mechanics.  The only point really was to explain this longer second stage burn by throttling the second stage engine for part of the burn, but usually when people here talk about engines running throttled down they say it isnt worth it due to the gravity losses.  I was trying to make the point that the gravity losses might be mitigated by getting extra speed to the second stage by using the first stage landing fuel, but I still don't know how that works and sorry for rambling nonsensical gibberish.

You're doing fine. Running this stuff through your head is how you start, and there's no better place on the planet to do it than here, where we have a great bunch to break things down and steer you in the right direction.

[GWR64]

from the SES12-Mission Press-kit:
MECO: 02:44
the only Block 4 launch, in which the first stage  no fuel for reentry and landing burn left
no legs and fins . I am curious about the speed on the stage-separation.

(Hispasat 30W-6; booster should originally land; MECO: 02:35)

[TrueBlueWitt]

from the SES12-Mission Press-kit:
MECO: 02:44
the only Block 4 launch, in which the first stage  no fuel for reentry and landing burn left
no legs and fins . I am curious about the speed on the stage-separation.

(Hispasat 30W-6; booster should originally land; MECO: 02:35)

Let see.. an extra 9 seconds at 5G? So subtract say 1/2G grav loss at that point?  That's ~45 m/s^2 acceleration.. maybe another 405 m/s at MECO. Even at a full 1G subtracted you've still got another 360 m/s at MECO