SpaceX Falcon 9 : GPS III SV01 : SLC-40 : Dec. 23, 2018 - DISCUSSION

[LouScheffer]

Trajectory notes:

First stage cutoff was at 9550 km/hr = 2653 m/s.  Typical GTO recoveries are at about 2300 m/s, so the gain from going expendable is about 350 m/s, as expected.

Second stage cutoff was at 28272 km/hr, or 7853 m/s, relative to launch, at 168 km altitude.  For this inclination, the help from the Earth's orbit should be about 261 m/s, so an inertial speed of 8114 m/s or so.   By my calculation, this gives about at 168 x 1300 km orbit, which seems low.   Also such an orbit has only a 1.6 hour period, so waiting a full hour for the second burn seems excessive.  Maybe my estimates are wrong, or ground coverage is needed, or something else.

Also, the webcast stated the second stage would be safed between the second burn and satellite release.  If so, how are they going to make the second stage re-enter?  By my calculations, re-entering from a 1300 x 20000 orbit would take about 160 m/s.  The RCS seems unlikely to provide that much, even with an empty second stage, and presumably RCS is also vented as part of safing.  So what's the plan?

[Lar]

Also, the webcast stated the second stage would be safed between the second burn and satellite release.  If so, how are they going to make the second stage re-enter?  By my calculations, re-entering from a 1300 x 20000 orbit would take about 160 m/s.  The RCS seems unlikely to provide that much, even with an empty second stage, and presumably RCS is also vented as part of safing.  So what's the plan?

Is there an intermediate kind of safing that doesn't vent RCS? Or maybe the webcast was confused? Because that orbit will last a while if there isn't some lowering...

[hektor]

If they safe the second stage before separation how do they ensure proper attitude at separation ?

Also would the safing be visible some way on the webcast images ?

[deruch]

Trajectory notes:

First stage cutoff was at 9550 km/hr = 2653 m/s.  Typical GTO recoveries are at about 2300 m/s, so the gain from going expendable is about 350 m/s, as expected.

Second stage cutoff was at 28272 km/hr, or 7853 m/s, relative to launch, at 168 km altitude.  For this inclination, the help from the Earth's orbit should be about 261 m/s, so an inertial speed of 8114 m/s or so.   By my calculation, this gives about at 168 x 1300 km orbit, which seems low.   Also such an orbit has only a 1.6 hour period, so waiting a full hour for the second burn seems excessive.  Maybe my estimates are wrong, or ground coverage is needed, or something else.

Also, the webcast stated the second stage would be safed between the second burn and satellite release.  If so, how are they going to make the second stage re-enter?  By my calculations, re-entering from a 1300 x 20000 orbit would take about 160 m/s.  The RCS seems unlikely to provide that much, even with an empty second stage, and presumably RCS is also vented as part of safing.  So what's the plan?

Reversibly safed, is that a thing?  Or maybe some imprecise/inaccurate comments or maybe SpaceX uses such terminology a little bit differently than how it might generally be understood by others?  etc.

[gongora]

They said they would purge the engine of RP-1.  I'm not sure that's the same as safing.

[edkyle99]

31,424 km/hr at 1,200 km altitude, not including the contribution of earth's rotation.

1,200 x 18,700 km x 55 deg, roughly?

If correct, wouldn't this be underperformance?  I'll note that depending on the SpaceX velocity versus altitude numbers has seemed in the past to provide underestimates of the actual orbit.

[rickl]

Why is there such a long delay between MECO-2 and spacecraft deploy?


That said, this is one of the best webcasts I've seen.  It's waaaay up there now.

[deruch]

Why is there such a long delay between MECO-2 and spacecraft deploy?


That said, this is one of the best webcasts I've seen.  It's waaaay up there now.

Stated reason is that the Air Force wants the deployment to be in an area where they have ground coverage.

[DistantTemple]

Nice to see everyone at SX is on Xmas break, and the office behind them is empty and the lights are out! Even the cheering seemed to be from very few enthusiastic souls.   

[quasarquantum]

Around 1:35:00 there was a callout AOS South Texas, is that one of the Boca Chica dishes?

[whiztech]

Other than Falcon Heavy launch, has SpaceX ever showed view from 2nd stage from higher than low earth orbit?

[deruch]

Around 1:35:00 there was a callout AOS South Texas, is that one of the Boca Chica dishes?

I think so.  They sounded a bit more excited about that one than for the other call outs.

[ncb1397]

By my count, first stage burn was about 171 seconds. 2nd stage first engine burn was about 325 seconds.  2nd stage 2nd burn was about 50 seconds for a total of 375 seconds.

For comparison, the Eshail 2 launch with ASDS recovery was 155 seconds on the 1st stage burn. 324 seconds on the 2nd stage first engine burn and 56 seconds on the 2nd stage 2nd engine burn for a total run time of 380 seconds.

So, first stage was using about 10% longer burn time than an ASDS recovery, but second stage might have been able to run 5 seconds longer+ or ~1+% longer. The extra couple of seconds could be needed to assist for the different requirements for disposal burns.

[gongora]

Other than Falcon Heavy launch, has SpaceX ever showed view from 2nd stage from higher than low earth orbit?

They released a still picture from the DSCOVR launch.
https://forum.nasaspaceflight.com/index.php?topic=36765.msg1330330#msg1330330

[gongora]

By my count, first stage burn was about 171 seconds. 2nd stage first engine burn was about 325 seconds.  2nd stage 2nd burn was about 50 seconds for a total of 375 seconds.

For comparison, the Eshail 2 launch with ASDS recovery was 155 seconds on the 1st stage burn. 324 seconds on the 2nd stage first engine burn and 56 seconds on the 2nd stage 2nd engine burn for a total run time of 380 seconds.

So, first stage was using about 10% longer burn time than an ASDS recovery, but second stage might have been able to run 5 seconds longer+ or ~1+% longer. The extra couple of seconds could be needed to assist for the different requirements for disposal burns.

I think the AF usually asks for more precise orbital insertion.  On some of the GTO launches the requirement seems to be a minimum apogee and if they can do a little more that's great.

[deruch]

Sort of interesting (and maybe just a bit of a goof), but the commentators called out a "thank you" to the FAA for licensing the launch.  Maybe just force of habit or a bit of script editing that fell through the cracks, but the FAA shouldn't have been the "licensing" authority for this launch.  As this was an NSS launch procured through the USAF's EELV program, the Air Force was oversight. 

[LouScheffer]

31,424 km/hr at 1,200 km altitude, not including the contribution of earth's rotation.

1,200 x 18,700 km x 55 deg, roughly?

If correct, wouldn't this be underperformance?  I'll note that depending on the SpaceX velocity versus altitude numbers has seemed in the past to provide underestimates of the actual orbit.

Perigee seemed to occur a while after SECO at 1213 km and 31426 km/hr, which makes sense since it was on the way down. That gives 8729 m/s, plus Earth rotation contribution of 261 m/s, for a total of 8990 m/s.  This gives a 1213 x 18,900 km orbit.  According to the specs for this launch, it was supposed the have a perigee of 1200 +- 20 km, which it does, and an apogee of 20,181 km +- 100, which it does not.  On the other hand, they announced "nominal orbital insertion", and they should know.  My particular guess is that the government put the maximum possible they might want into the RFP, then amended it later to the value the mission planners really wanted.  Another possibility might be that the safing operations they talked about between SECO and release might add a few m/s.

EDIT: fix units on first speed

[LouScheffer]

Another possibility for the lower apogee:  They deliberately target an apogee below the GPS orbit.  This way, if the de-orbit burn fails, the second stage is left in an orbit that does not threaten the GPS satellites.  If this is the case, an orbit slightly below 19000 km is best, since it is below all the nav systems (the lowest is GLONASS at 19,100 km).  Galileo and Beidou are higher.

[LouScheffer]

With the final orbits in, it seems a recoverable F9 could have done the job easily.  Let's compare the delta V from LEO:

 +158 m/s for 55 degree inclination
 +278 m/s to go from 175x175 to 175x1200 (8081 m/s - 7803 m/s)
+2064 m/s to go from 175x1200 to 1200x20181  (9052 m/s - 6988 m/s)
 +156 m/s to dispose of second stage (1200x20181 to 100x20181, 2581 m/s - 2425 m/s)
-----
2656 m/s total
The final number will be slightly less since the disposal burn does not need to decelerate the satellite, and hence will be about twice as efficient.   So maybe 2600 m/s total.

F9 recoverable can put about 5500 kg to GTO (about 2450 m/s from LEO).  Reducing the payload to 3900 kg provides 490 m/s more, or about 2940 m/s total.  Even assuming an over-fueled (4400 kg) GPS like the current one, there's an extra 327 m/s for a total of 2777 m/s.  In any case it has more performance than needed.

[gongora]

Is this the GPS sat?

1 43873U 18109A   18357.69183462  .00000088  00000-0  00000+0 0  9992
2 43873  55.0049 198.4368 5566277 269.9967  91.9277  3.89174632    03

55.0deg 1199km x 20205km