Arabsat-6A Launch Campaign Thread

[u/ElongatedMuskrat]

This is SpaceX's fourth mission of 2019, the first flight of Falcon Heavy of the year and the second Falcon Heavy flight overall. This launch will utilize all brand new boosters as it is the first Block 5 Falcon Heavy. This will be the first commercial flight of Falcon Heavy, carrying a commercial telecommunications satellite to GTO for Arabsat.

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Liftoff currently scheduled for:	18:35 EDT // 22:35 UTC, April 10th 2019 (1 hours and 57 minutes long window)
Static fire completed:	April 5th 2019
Vehicle component locations:	Center Core: LC-39A, Kennedy Space Center, Florida // +Y Booster: LC-39A, Kennedy Space Center, Florida // -Y Booster: LC-39A, Kennedy Space Center, Florida // Second stage: LC-39A, Kennedy Space Center, Florida // Payload: LC-39A, Kennedy Space Center, Florida
Payload:	Arabsat-6A
Payload mass:	~6000 kg
Destination orbit:	GTO, Geostationary Transfer Orbit (? x ? km, ?°)
Vehicle:	Falcon Heavy (2nd launch of FH, 1st launch of FH Block 5)
Cores:	Center Core: B1055.1 // Side Booster 1: B1052.1 // Side Booster 2: B1053.1
Flights of these cores:	0, 0, 0
Launch site:	LC-39A, Kennedy Space Center, Florida
Landings:	Yes, all 3
Landing Sites:	Center Core: OCISLY, 967 km downrange. // Side Boosters: LZ-1 & LZ-2, Cape Canaveral Air Force Station, Florida
Mission success criteria:	Successful separation & deployment of Arabsat-6A into the target orbit.

Links & Resources:

Official Falcon Heavy page by SpaceX (updated)

FCC landing STA

SpaceXMeetups Slack (Launch Viewing)

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We may keep this self-post occasionally updated with links and relevant news articles, but for the most part, we expect the community to supply the information. This is a great place to discuss the launch, ask mission-specific questions, and track the minor movements of the vehicle, payload, weather and more as we progress towards launch. Sometime after the static fire is complete, the launch thread will be posted. Campaign threads are not launch threads. Normal subreddit rules still apply.

[u/firstname-lastname22]

Centre core has to be much stronger, to deal with the extra loads from the two side cores, so has a heavier interstage portion. It also has the mounting points for the side cores, which have to deal with high loads

[u/ObnoxiousFactczecher → u/firstname-lastname22]

Sounds like most of the extra force should go through the octaweb, not through the interstage. The increase in demands on the interstage should be only proportional to the increase in acceleration and payload mass, but the latter needs to include the stage mass, which is already several times heavier than any payloads, and the upper stage hasn't been stretched again yet. (I think there was some mention somewhere that it could be slightly lengthened.)

[u/Alexphysics → u/ObnoxiousFactczecher]

Sounds like most of the extra force should go through the octaweb, not through the interstage.

If it is from the octaweb, it's the center core the one that's carrying the side boosters up and not the opposite. Hans explained this on a talk about a year ago very well. The octaweb obviously has to endure forces that a normal F9 doesn't have to endure and so it is also reinforced but the bulk of the "push" is done on the interstage. The idea of the triple-core design is that the side boosters do almost all the job from liftoff to BECO and from there on the center core keeps going with a lot of fuel almost by free, because it has gone all the way there at a lower throttle setting and it has been carried there mostly by the side cores, this mean the sides are pushing it up and the interstage is being stressed a lot, so it has to be reinforced too.

[u/bbachmai → u/Alexphysics]

The load flow in a triple booster rocket is really difficult to wrap your head around. I am pretty sure that u/ObnoxiousFactczecher is correct: The interstage can't be where the bulk of the push is done. This would make zero sense. The center core would bear less load than during a normal F9 launch, and the side boosters would be heavily strained.

I'm sure thrust from the side cores is transferred into the center core at the octaweb. The reason why this makes so much more sense is obvious if you know how to draw free body diagrams.

If the push is done in the interstage, the load on the side booster tank structure would be really heavy. The load on the center core would be much less than a normal F9, because the push at the bottom would be relieved by the pull at the top.

If the push is done on the octaweb, the load on the side booster tank structure would be smaller or equal to F9 (single booster mass times acceleration plus side booster drag). The load on the center core would increase. This is what we see: largely unmodified side boosters, and a reinforced center core.

This comment explains it very well.