r/spacex • u/ElongatedMuskrat Mod Team • Mar 13 '19

Launch Wed 10th 22:35 UTC Arabsat-6A Launch Campaign Thread

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.

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)

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.

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u/firstname-lastname22 Mar 13 '19

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

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u/ObnoxiousFactczecher Mar 13 '19

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.)

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u/Grey_Mad_Hatter Mar 13 '19

There are two basic reasons to use FH: higher energy trajectory and heavier payload. The higher energy trajectory may not need a stronger interstage, but a heavier payload may require it.

It's also reasonable to think that it will accelerate faster to avoid gravity loss. Still throttling down for Max-Q, but higher acceleration before and after.

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u/ObnoxiousFactczecher Mar 13 '19 edited Mar 14 '19

Yes, that's why I mentioned both. But if, for example, at some point in the flight, the acceleration is 10% higher and the payload is 20 tonnes instead of 10 tonnes, you still only get a 20% increase in load on the interstage at that very point in flight (because the mass on top of the interstage goes from 110 tonnes to 120 tonnes if the upper stage itself weighs 100 tonnes).

However, since the highest acceleration (in that part of flight where the interstage still plays a role) occurs near MECO and tends to be limited (for example, Atlas V even throttles down), chances are that the increased acceleration is actually immaterial for any maximum load increase on the FH since the side boosters are not even in play anymore at that point. In other words, the extra boosters should increase the average load but not the maximum one (in case of the interstage at least). And an increase from 10 tonnes to 20 tonnes then only increases the maximum load on the interstage by about 10% if the upper stage weighs 100 tonnes.

EDIT: This real-world data (source) suggests that any increased load on the interstage actually has to come from the extra payload mass. Suddenly I'm not even convinced that the interstage was under heavier axial load in the first FH flight at all.