Why did this fairing separation happen at such a high altitude? (SpaceX Iridium-6/GRACE-FO)

Question

I noticed in the video of the recent Iridium-6/GRACE-FO launch that the fairing separation happened at an altitude of about 143 kilometers. This is significantly higher than any altitude I can remember. I first started watching this last year and noted the altitude for several launches here.

@awksp's excellent answer explains the relative constancy of the altitude as likely originating from a standardized spec. The answer is thorough and worth reading, but here is a snippet from there:

...after the 3-sigma high theoretical free molecular heating for a flat plate normal to the free stream drops below 1135 W/m$^2$ (360 Btu/hr ft$^2$) based on the 1962 United States Standard Atmosphere.

After seeing the 143 km, I went back and checked several more recent SpaceX launches and compiled more numbers that can be compared with the table in the previous question. It seems Iridium-6/GRACE-FO stands out with a uniquely and significantly higher altitude for fairing separation than any other recent launch as well, including two other Iridium launches.

Question: Why did the fairing separation here happen at such a high altitude compared to so many other SpaceX launches? Solar activity-induced atmospheric heating? Abundance of caution for this payload combo? Anomaly? Just to see if we're watching? Something else?

MISSION                speed  altitude  post 2nd stage 
                       km/hr    km      ignition (sec) 
-------                -----    ----    --------------  
Iridium-6/GRACE-FO     8754     143          28 
Bangabandhu Sat- 1     8978     111          53 
TESS                   7055     107          24 
Iridium-15             8052     110          42 
HISPASAT 30W-6         8979     111          48 
PAZ Mission            6509     119          20 
CRS-14                 8158     115?         29 
GovSat-1               9222     111          52  
FH Test Roadster      10196     115          28 
Iridium-4              7578     111          30 
CRS-13                 5982     106?         32 

Screenshot Click for full size.

Answer 1

While the rocket needs to be in space for the fairing to be jettisoned, the altitude is actually not the factor on which separation timing depends, it is staging.
As you can see on your spreadsheet, while the altitude changes greatly between different launches, the number of seconds after S2 stratup is pretty constant, and was actually relatively short on this launch.

The reason the fairing is only opened after S2 startup are multiples :

• Fairing reuse : On this launch SpaceX attempted to catch the fairing in their effort towards fairing reuse. Passing next to 9 Merlin engines at full thrust could result in pretty substantial damage.

• Stage one collision : the stage one being 2.5 times longer than S2, it would be possible for one of the rotating fairings to collide with it when horizontal compared to the rocket.

• Strucural load : ejecting while under S1 acceleration would result in around 9 times more stress, and it may very well not be able to handle that.

Moreover, each mission has a different trajectory, it would be interesting to compare to other polar and Iridium launches to see what was the separation altitude, or maybe this was a uniquely steep trajectory for the GRACE-FO payload, which was reelased just after S2 first shutdown. But without those numbers we can simply compare Main Engines CutOff (MECO) times and see what changes :

MISSION                MECO   altitude     
                        m:s     (km)           
-------                -----    ----    
Iridium-6/GRACE-FO     02:48    143          
Bangabandhu Sat- 1     02:31    111           
TESS                   02:29    107           
Iridium-15             02:34    110           
HISPASAT 30W-6         02:35    111          
PAZ Mission            02:29    119           
CRS-14                 02:45    115?          
GovSat-1               02:38    111            
FH Test Roadster       02:40*   115           
Iridium-4              02:33    111           
CRS-13                 02:33    106?          

*Actually BECO (B for Booster) More on FH launch profile here.
Data extracted from r/spaceX launch and updates threads.

As you can see, Iridium-6/GRACE-FO sticks out by having the latest MECO time of all recent launches. But it might be a bit of a stretch to say that S1 burn time is directly correlated to vertical speed.

Let's instead search for the launch profile : Since u/veebay hasn't done the complete launch profile of this launch yet, we'll just compare with the latest release :
The "1:1 ascpect ratio 1st stage flight profile" graph is what interests us.

There is two general groups : GTO and above launches which have a very flat trajectory, almost at 30°. Meanwhile LEO launches have a very steep trajectory, up to 60°. Since S1 always burn for the same duration, a steep profile results in a shutdown, separation, and fairing release at a higher altitude. All the payloads have orbits at around 750 km of altitude, while GRACE-FO is even lower at 500 km.

We can think that the GRACE-FO specially low orbit resulted in a very steep launch, and thus a very high fairing release.

Answer 2

There are four "Notice(s) to Airmen" (NOTAMs) that seem to cover the fairing landing zone: A1987/18, A1954/18, A1167/18 and A1165/18.

WARNING AREA W291 ACT EAST OF 121 DEGREES WEST LONGITUDE AND SOUTH OF 30.5 DEGREES NORTH LATITUDE. THIS MAY NOT BE THE ONLY AIRSPACE ACTIVATION WITHIN W291. OPERATORS ARE ADVISED THAT ADDITIONAL NOTAMS MIGHT EXIST FOR THE CLOSURE OF OTHER PORTIONS OF W291. SFC - FL800, 22 MAY 13:30 2018 UNTIL 26 MAY 01:00 2018. CREATED: 19 MAY 01:00 2018

W291 is a large area in the ocean south and east of Southern California. The named area is MISR-1W and MISR-1E in the bottom right of this image:

(From page 1-15 of this doc)

Several of the TMA areas (north and west of the MISR area) are already reserved for other activities at overlapping times, so the further-south regions may be all that SpaceX was able to reserve for this launch. (Commercial space is not the highest priority user for any of this, and TMA regions get a lot of use)

Vandenberg is at 34.74° N, 120.57° W; the trip south to 30.5° N is about 480 km. That's actually a long way. A flight of 3 minutes 23 second (from the screen shot) at 8726 km/hr peak, maybe 5000 km/hr average is only about 300 km downrange. That's short of the top end of the warning zone, but the fairing will travel some distance further as it falls back to Earth.

So, without inside information, it looks like SpaceX was keeping the fairing on longer/higher to make sure it impacted where it was allowed to.