Falcon Heavy’s 3 first stage cores have all completed testing | Hacker News Reader

Falcon Heavy’s 3 first stage cores have all completed testing

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Will they do a hold-down test with all three boosters strapped together? As that may incur that risk of non-synchronized ignition "side-effects".

Isn't the rocket usually strapped down to the launch pad until all engines are confirmed working properly? What would these side effects be?

If one engine starts even a fraction of a second too late, it will suck up the exhaust from the other engines with predictably mission ending consequences.

I might totally wrong, but I don't think the engine are brought up to full throttle in a normal launch while it is still strapped down. In a hold down test, they are. I am just wondering if they are going to to risk possible synchronization issues in a hold down test, which effectively doubles the chance they will encounter a problems vs doing it once at launch.

Falcons are held down at full throttle for three seconds prior to launch, to ensure that everything is working perfectly before letting go. (They've aborted during this period in the past, although that was a long while ago.) As far as I know, this is unique among launch vehicles. I presume (but do not know for certain) that they will do the same for the heavy.

Well, they're held down as they are throttling up. They aren't at full throttle the whole time.

Additionally... Many liquid fueled rockets can abort on the launch pad after ignition, but before the clamps release. The shuttle, for instance, had 5 aborts after main engine start (once the SRBs lit though, the shuttle was going up, one way or another).

> If one engine starts even a fraction of a second too late, it will suck up the exhaust from the other engines with predictably mission ending consequences.

It's even more precise than that. The engines will be started in pairs, a few dozen ms apart, in order to minimize the risk of sympathetic vibrations tearing the rocket apart.

>suck up the exhaust

Wat. It's a rocket, it doesn't 'suck up' anything except what's coming through the turbopumps. The engine starts are staggered to minimize stresses on the thrust structure. For the Saturn V:

"At 8.9 seconds before launch, the first stage ignition sequence started. The center engine ignited first, followed by opposing outboard pairs at 300-millisecond intervals to reduce the structural loads on the rocket. When thrust had been confirmed by the onboard computers, the rocket was "soft-released" in two stages: first, the hold-down arms released the rocket, and second, as the rocket began to accelerate upwards, it was slowed by tapered metal pins pulled through dies for half a second."

Maybe "blown" is a better word, but the point is the only thing stopping exhaust going up another nozzle is it's own pressure. I read about it here I think:

https://www.space.com/38016-spacex-falcon-heavy-rocket-maide...

> If one engine doesn't ignite internally as it should, you will get a flame front that travels back into the engine."

> "That creates a pressure spike, and that can cause what we call 'rapid unscheduled destruction' — and that's bad,"

From playing KSP (so not an expert of course), having either of the boosters out of sync would cause the center of mass to shift to one side (due to imbalance of fuel), which will induce torque on the rocket. That being said, this should be easy to compensate for by simply reducing the trust on one of the boosters.

Yes, they will be doing at least one (and likely more than one) full-up static fire, as they have done for all their single core launches in the past.

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