Could the staggered start have something to do with not loading the cross members all at once? At what stage is the thrust from the booster imparted into the core? It surely can not all be imparted at the release of the hold downs?
This article :
https://www.nasaspaceflight.com/2017/11/spacex-aims-december-launch-falcon-heavy/has good pictures and explanations of the compression bridges that arch across the reaction frame and support the bottom mating points of the cores. So for example, fully loaded but before ignition, the centre core rests all its weight on its usual four points, its two hold-downs and the pads at the top of the two compression bridges.
However, after reaching full thrust but before clamp release, the centre core is held down by its two hold-down clamps and possibly by some force at the mating points with the side boosters, or possibly not; maybe that's a lifting force by then, it's hard to estimate because it depends on how everything flexes. (Note that the compression bridges are called that because they only support weight, they don't hold down at all.)
I imagine there will be strain gauges everywhere. Perhaps the staggered start is simply to give them time to react if the strains differ from their predictions; if they ramp up the thrust gradually, they can shut everything down the moment something goes off-nominal, before the hardware gets permanently bent out of shape.
https://twitter.com/thesheetztweetz/status/955966079066656768
Michael Sheetz @thesheetztweetz
Falcon Heavy static fire planned for tomorrow (1/24) at noon ET with a 6 hour window.
KSC source says that SpaceX is targeting February 6 for the launch, if the static fire goes well.
I don't know how credible he is, but it would place the launch 13 days after the static fire. Previously, Gwynne Shotwell said that they need two weeks, so it seems plausible.
It would be great if they could launch on the 6th since we could face another government potential shutdown on the 8th...
Even if they do a launch attempt on the 6th, but scrub, they have the 7th and 8th as backups before the possible shutdown at midnight, so there is a chance they can get it launched before the shutdown if the NET launch date sticks.
A rectangle is weaker than a square in torsional rigidity... Just to clarify, I'm speaking about the loads to the reaction frame...
Edit to add:
There is no rectangle. There are cross members between the boosters in the reaction frame Those cross members also support the vehicle.
However those cross members do not hold down the vehicle. Once the engines reach a TWR>1 the center core only has 2 hold downs keeping it down, which as you noted is not stable, so it is relying on the side boosters for stability. So the cores are not independent.
It would be great if they could launch on the 6th since we could face another government potential shutdown on the 8th...
Even if they do a launch attempt on the 6th, but scrub, they have the 7th and 8th as backups before the possible shutdown at midnight, so there is a chance they can get it launched before the shutdown if the NET launch date sticks.
.....and before the Olympics opening ceremony on the 9th, not ideal to be watching from a motel room if free to do that much. Brings to mind the sleepless nights of trying to choose between Siegfried & Roy
However those cross members do not hold down the vehicle. Once the engines reach a TWR>1 the center core only has 2 hold downs keeping it down, which as you noted is not stable, so it is relying on the side boosters for stability. So the cores are not independent.
That doesn't mean it comes off the side members
However those cross members do not hold down the vehicle. Once the engines reach a TWR>1 the center core only has 2 hold downs keeping it down, which as you noted is not stable, so it is relying on the side boosters for stability. So the cores are not independent.
That doesn't mean it comes off the side members
Indeed. But the two hold-downs create a pivot axis for the center octaweb, and unless the thrust is balanced on both sides of that pivot the combination of compression bridge ("cross member") and side octowebs will have to apply a torque to keep the center octoweb from twisting about the two holddowns.
However those cross members do not hold down the vehicle. Once the engines reach a TWR>1 the center core only has 2 hold downs keeping it down, which as you noted is not stable, so it is relying on the side boosters for stability. So the cores are not independent.
That doesn't mean it comes off the side members
Indeed. But the two hold-downs create a pivot axis for the center octaweb, and unless the thrust is balanced on both sides of that pivot the combination of compression bridge ("cross member") and side octowebs will have to apply a torque to keep the center octoweb from twisting about the two holddowns.
Don't forget that there are also cross members up on top, they are high enough that they should be able to easily handle the center core trying to 'rotate' sideways.
I think some people are misunderstanding each other in the discussion. Some are talking about load paths though the thrust frame to the stack and other may feel that it has not been accounted for and will self destruct. SpaceX has experience for Falcon 9 and computer simulated the loads on the Heavy. The test will just confirm the fidelity of the sim. I like to hearken back to the Shuttle days and it's famous "twang" at main engine ignition. FH will have it's own version of "multiple twangs" during the staggered ignition of 27 engines where some engines are timed to act as dampeners... Stay tuned...
Does someone have a comparison between Delta IV Heavy and FH first launch preparations? My search skills couldn'the locate anything yet.
Delta IV Heavy first went vertical Dec. 10th 2003 and launched Dec. 21st 2004. So Delta IV Heavy preparations took over a year. And the payload did not deploy to the correct orbit.
That's a fair comparison. Two cases of a three-barrel where the individual barrels are already known.
Clearly not a trivial issue.
Indeed. But the two hold-downs create a pivot axis for the center octaweb, and unless the thrust is balanced on both sides of that pivot the combination of compression bridge ("cross member") and side octowebs will have to apply a torque to keep the center octoweb from twisting about the two holddowns.
the bridge is enough, since the vehicle is sitting on both.
I think some people are misunderstanding each other in the discussion. Some are talking about load paths though the thrust frame to the stack and other may feel that it has not been accounted for and with self destruct. SpaceX has experience for Falcon 9 and computer simulated the loads on the Heavy. The test will just confirm the fidelity of the sim. I like to hearken back to the Shuttle days and it's famous "twang" at main engine ignition. FH will have it's own version of "multiple twangs" during the staggered ignition of 27 engines where some engines are timed to act as dampeners... Stay tuned...
Nobody is suggesting that SpaceX didn't account for something and it will self destruct.
It's actually the opposite, SpaceX is worried enough to change the usual starting sequence and us armchair engineers (and non engineers) are trying to understand what they are worried about.
But I do think we've gone around in circles enough times, I'll sleep just fine without knowing the answer (and I hope we never find out, if we do find out it would likely be because something went kaboom)
Does someone have a comparison between Delta IV Heavy and FH first launch preparations? My search skills couldn'the locate anything yet.
Delta IV Heavy first went vertical Dec. 10th 2003 and launched Dec. 21st 2004. So Delta IV Heavy preparations took over a year. And the payload did not deploy to the correct orbit.
That's a fair comparison. Two cases of a three-barrel where the individual barrels are already known.
Clearly not a trivial issue.
Actually not. Delta IV Heavy used three unique cores than are not used on any other versions. Their early trade studies and design decisions, along with under performance really f'ed up the "common" core concept. Atlas V Heavy would have avoided many of these issues. The Boeing EELV scandal and Air Force wrongfully award of the majority of the missions to Boeing really messed up the EELV program and what it could have achieved.
I think some people are misunderstanding each other in the discussion. Some are talking about load paths though the thrust frame to the stack and other may feel that it has not been accounted for and with self destruct. SpaceX has experience for Falcon 9 and computer simulated the loads on the Heavy. The test will just confirm the fidelity of the sim. I like to hearken back to the Shuttle days and it's famous "twang" at main engine ignition. FH will have it's own version of "multiple twangs" during the staggered ignition of 27 engines where some engines are timed to act as dampeners... Stay tuned...
Nobody is suggesting that SpaceX didn't account for something and it will self destruct.
It's actually the opposite, SpaceX is worried enough to change the usual starting sequence and us armchair engineers (and non engineers) are trying to understand what they are worried about.
But I do think we've gone around in circles enough times, I'll sleep just fine without knowing the answer (and I hope we never find out, if we do find out it would likely be because something went kaboom)
What "usual starting sequence"? This is the first static fire of the stack on the pad...
I think some people are misunderstanding each other in the discussion. Some are talking about load paths though the thrust frame to the stack and other may feel that it has not been accounted for and with self destruct. SpaceX has experience for Falcon 9 and computer simulated the loads on the Heavy. The test will just confirm the fidelity of the sim. I like to hearken back to the Shuttle days and it's famous "twang" at main engine ignition. FH will have it's own version of "multiple twangs" during the staggered ignition of 27 engines where some engines are timed to act as dampeners... Stay tuned...
Nobody is suggesting that SpaceX didn't account for something and it will self destruct.
It's actually the opposite, SpaceX is worried enough to change the usual starting sequence and us armchair engineers (and non engineers) are trying to understand what they are worried about.
But I do think we've gone around in circles enough times, I'll sleep just fine without knowing the answer (and I hope we never find out, if we do find out it would likely be because something went kaboom)
What "usual starting sequence"? This is the first static fire of the stack on the pad...
The "usual" starting sequence used on F9.
I think some people are misunderstanding each other in the discussion. Some are talking about load paths though the thrust frame to the stack and other may feel that it has not been accounted for and with self destruct. SpaceX has experience for Falcon 9 and computer simulated the loads on the Heavy. The test will just confirm the fidelity of the sim. I like to hearken back to the Shuttle days and it's famous "twang" at main engine ignition. FH will have it's own version of "multiple twangs" during the staggered ignition of 27 engines where some engines are timed to act as dampeners... Stay tuned...
Nobody is suggesting that SpaceX didn't account for something and it will self destruct.
It's actually the opposite, SpaceX is worried enough to change the usual starting sequence and us armchair engineers (and non engineers) are trying to understand what they are worried about.
But I do think we've gone around in circles enough times, I'll sleep just fine without knowing the answer (and I hope we never find out, if we do find out it would likely be because something went kaboom)
What "usual starting sequence"? This is the first static fire of the stack on the pad...
The "usual" starting sequence used on F9.
That has nothing in common across 27 engines, I thought that is implicit... That's why we test...
Does someone have a comparison between Delta IV Heavy and FH first launch preparations? My search skills couldn'the locate anything yet.
Delta IV Heavy first went vertical Dec. 10th 2003 and launched Dec. 21st 2004. So Delta IV Heavy preparations took over a year. And the payload did not deploy to the correct orbit.
That's a fair comparison. Two cases of a three-barrel where the individual barrels are already known.
Clearly not a trivial issue.
Actually not. Delta IV Heavy used three unique cores than are not used on any other versions. Their early trade studies and design decisions, along with under performance really f'ed up the "common" core concept. Atlas V Heavy would have avoided many of these issues. The Boeing EELV scandal and Air Force wrongfully award of the majority of the missions to Boeing really messed up the EELV program and what it could have achieved.
Good to know, thx.
However, in the grand scheme of things, how far off are the individual DIV barrels from the single stick one?
It's the same engine, similar (at least) tanks, similar (at least) avionics, no?
Just sounds like as with SpaceX they found out that the heavy is more complex then they thought, and ended up not where they thought they would.
Atlas may have worked out better - I don't have the information to judge.
SpaceX ended up with dissimilar cores too, right? Plus the added interactions, (which I'm sure are more significant after release) and there it is, getting debugged on the pad.
At least they have two pads...
What "usual starting sequence"? This is the first static fire of the stack on the pad...
"The usual starting sequence [of a single stick F9]". Of course it doesn't have a commonality but it's the only thing we have to compare against and it leads to some conclusions (as the other poster stated (para-quoted): "There is some reason for SX to have chosen a different start)
Restating outloud the circumstances that lead to this thread of discussion:
1) A F9 can handle the startup stresses of all engines starting simultaneously (eg: the usual startup sequence)
2) FH has staggered startup which it's reasonable to presume is for some physical reason rather than simply caution.
3) The FH cores are reasonably independent with respect to thrust but perhaps varyingly less so due to other "forces" (eg: acoustic, pressure, vibration, ...)
4) We're interested in knowing if there's a FH System reason for the stagger and what that might be or whether it's simply the total systemic shock of such an ignition to the Pad, GSE, Trench, etc... or whether it's simply prudence
Have faith, Keep calm, Go SpaceX...
SpaceX ended up with dissimilar cores too, right?
The center core is unique, the side cores are not. (And are close enough to regular F9 boosters that they could be converted). AIUI all DIV Heavy cores are different from regular DIV cores.
Also AIUI, Atlas V Heavy would not have required any unique cores. So FH falls in-between the two, but falling much closer to the AV Heavy than the DIV Heavy in that regard.
SpaceX ended up with dissimilar cores too, right?
The center core is unique, the side cores are not. (And are close enough to regular F9 boosters that they could be converted). AIUI all DIV Heavy cores are different from regular DIV cores.
Also AIUI, Atlas V Heavy would not have required any unique cores. So FH falls in-between the two, but falling much closer to the AV Heavy than the DIV Heavy in that regard.
That's my understanding too, but as I think is clear, it's not just about a 1:1 core comparison.
Anyway - home stretch now, we hope.
