Main Engine Cut Off

Yesterday was a great day for SpaceX, with the beautiful and seemingly-flawless launch of Inmarsat–5 Flight 4.

They hit their 2 week turnaround target between this launch and NROL–76, as they did between EchoStar 23 and SES–10 previously. (The longer gap between SES–10 and NROL–76 was due to delays on the payload side, but we’ll never get more information on that because it’s the NRO.)

They’ve got CRS–11 slated for flight in just over two weeks (with a previously-flown Dragon), then BulgariaSat–1 two weeks after that (on a previously-flown core), and something slated just about every two weeks through the rest of the year.

SpaceX’s tempo has been on target and smooth, thanks in no small part to the upgraded infrastructure at 39A. They’ve had very little work to do between flights, which otherwise would be a time and attention hog. The rebuilt infrastructure at SLC–40 will probably be very similar—except for the lack of Falcon Heavy support—which means once that pad is back online, we can expect a similar tempo from its launches.

There were two extra interesting bits from yesterday’s launch: fairing recovery work, and new fueling procedures.

Chris Bergin noticed quite a bit of extra hardware inside the fairing which points to continued testing of fairing recovery, or at least component testing for fairing recovery.

Joey Roulette‏, who was at the launch and talked to the SpaceX spokesperson, had some interesting info on later loading of LOX:

He meant 10 minutes later in the count, at T–35 instead of the published T–45.

SpaceX was flying new hardware on this mission, at least in the upper stage, but maybe in both stages. It’s probably not a bad assumption to make that “newer tech” means the redesigned COPVs which will allow faster loading of LOX and helium without another Amos–6 incident.

That’s a good sign that SpaceX is progressing towards Block 5, the final version of Falcon 9. We know the currently-flying Falcon 9 vehicles are Block 3, but we have no clue what Block 4 actually contains, other than “whatever comes between 3 and 5.” There are some indications that Inmarsat–5 Flight 4 flew with a Block 3 first stage and a Block 4 upper stage.

Interestingly, Joey Roulette had some info on the next flights in this regard:

They wouldn’t be this sure about switching to the new hardware within a month if this was a test to make sure everything worked as designed before committing to the changes. It seems like an intentional decision to skip CRS–11—which makes sense given that NASA prefers to fly on hardware that has flown 3 times previously—and BulgariaSat–1.

BulgariaSat–1 makes less sense to skip if these really are only changes to the upper stage, but makes more sense to skip if the entire vehicle is moving to Block 4.

My assessment: a Block 4 upper stage was flown on Inmarsat–5 Flight 4 because they wanted every bit of performance they could get for such a heavy payload. That lets them get a flight of Block 4 hardware under their belt, and move forward with confidence to Block 4 vehicles after BulgariaSat–1.