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#1300
by
AC in NC
on 23 Jan, 2018 19:34
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a: But WHY the staggered start? if there is no interaction between the cores, then why is this start different than any other F9 start? (as has been discussed several times already, but I've yet to see an explanation that I understood)
Distribute the ignition forces that the overall system must otherwise absorb instantaneously.
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#1301
by
mn
on 23 Jan, 2018 19:39
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a: But WHY the staggered start? if there is no interaction between the cores, then why is this start different than any other F9 start? (as has been discussed several times already, but I've yet to see an explanation that I understood)
Distribute the ignition forces that the overall system must otherwise absorb instantaneously.
Each core is absorbing the forces generated by itself as it has done numerous times.
Can you explain what additional force is now being generated but not already being handled.
And the official reason given was related to torque stress on the octaweb, so you get extra points if your explanation fits into that pigeon hole.
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#1302
by
Space Ghost 1962
on 23 Jan, 2018 19:44
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You'd like to start the engine opposite pairs closest to the outside clamps (not the end clamps on the outboard boosters) first on each core.
Last would be the ones closest to the end clamps.
(The center engines likely last, could be first as well. They don't matter as much.)
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#1303
by
Jim
on 23 Jan, 2018 19:46
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a: But WHY the staggered start? if there is no interaction between the cores, then why is this start different than any other F9 start? (as has been discussed several times already, but I've yet to see an explanation that I understood)
Distribute the ignition forces that the overall system must otherwise absorb instantaneously.
Each core is absorbing the forces generated by itself as it has done numerous times.
Can you explain what additional force is now being generated but not already being handled.
And the official reason given was related to torque stress on the octaweb, so you get extra points if your explanation fits into that pigeon hole.
Likely flame duct and overpressure related
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#1304
by
AC in NC
on 23 Jan, 2018 19:48
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Each core is absorbing the forces generated by itself as it has done numerous times.
There are more forces than just the thrust. And no core has ever experienced those forces from another core much less two.
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#1305
by
Rocket Science
on 23 Jan, 2018 19:50
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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:
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#1306
by
Jim
on 23 Jan, 2018 20:12
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Each core is absorbing the forces generated by itself as it has done numerous times.
There are more forces than just the thrust. And no core has ever experienced those forces from another core much less two.
What forces from the other cores? There are none, the cores are all independently restrained on the pad
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#1307
by
Jim
on 23 Jan, 2018 20:14
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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.
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#1308
by
AC in NC
on 23 Jan, 2018 21:08
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There are more forces than just the thrust. And no core has ever experienced those forces from another core much less two.
What forces from the other cores? There are none, the cores are all independently restrained on the pad
Acoustic, Pressue, Vibration, IDK probably others as well. Pad restraints don't eliminate those. Do I need to apologize for some transgression by calling these "forces" instead of something else?
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#1309
by
Rocket Science
on 23 Jan, 2018 21:14
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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.
Hi Jim, yes I know that there are diagonal members in the frame. I was comparing the Falcon 9's load concentration in comparison across the longer Falcon Heavy frame being distributed during staggered ignition... Thoughts?
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#1310
by
the_other_Doug
on 23 Jan, 2018 21:23
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If Jim is insistent that there can be no torques generated between the cores, maybe Chris G. needs to have a talk with him, as it was his article, IIRC, on the FH staggered start that quoted SpaceX as stating this was being done to minimize one core's engine start-up torque from affecting the stack as a whole. Since Jim is convinced this is not possible, maybe Chris (or someone) needs to publish a retraction.
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#1311
by
woods170
on 24 Jan, 2018 09:36
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If Jim is insistent that there can be no torques generated between the cores, maybe Chris G. needs to have a talk with him, as it was his article, IIRC, on the FH staggered start that quoted SpaceX as stating this was being done to minimize one core's engine start-up torque from affecting the stack as a whole. Since Jim is convinced this is not possible, maybe Chris (or someone) needs to publish a retraction.
Chris G got his info from SpaceX. Given the fact that Jim is not a SpaceX employee, and therefore has only limited insight into the "ins-and-outs" of Falcon Heavy, I think it is a safe bet that SpaceX knows something that Jim doesn't.
Just sayin'
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#1312
by
Elthiryel
on 24 Jan, 2018 10:35
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https://twitter.com/thesheetztweetz/status/955966079066656768Michael 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.
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#1313
by
MaxTeranous
on 24 Jan, 2018 11:10
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Of course the 3 cores impact each other in some ways. They're less than a meter apart for goodness sake.
If they didn't impact each other at all then it would be perfectly safe to stand 1 meter away from a Falcon 9 launch
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#1314
by
docmordrid
on 24 Jan, 2018 11:16
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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.
Michael Sheetz is with CNBC.
https://www.cnbc.com/michael-sheetz/
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#1315
by
jak Kennedy
on 24 Jan, 2018 11:30
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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?
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#1316
by
kevinof
on 24 Jan, 2018 12:09
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I believe it's more to do with the acoustic shock of all engines cranking up at once. By staggering the startup you should be able to reduce this impact. The outside observer wont notice it though - I think it's only a matter of a couple of hundred milliseconds between ignitions.
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?
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#1317
by
MP99
on 24 Jan, 2018 13:24
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a: But WHY the staggered start? if there is no interaction between the cores, then why is this start different than any other F9 start? (as has been discussed several times already, but I've yet to see an explanation that I understood)
Distribute the ignition forces that the overall system must otherwise absorb instantaneously.
Each core is absorbing the forces generated by itself as it has done numerous times.
Can you explain what additional force is now being generated but not already being handled.
And the official reason given was related to torque stress on the octaweb, so you get extra points if your explanation fits into that pigeon hole.
ISTM that F9 may rotate slightly in its mounts while starting up, but that this doesn't really matter.
If FH's three cores do the same, then they will rotate against each other where they join, and presumably these don't want to have play during flight. Therefore, they limit the torque that the core-to-booster connections have to put up with.
Cheers, Martin
Sent from my GT-N5120 using Tapatalk
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#1318
by
Rocket Science
on 24 Jan, 2018 13:30
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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...
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#1319
by
ugordan
on 24 Jan, 2018 13:34
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It would be great if they could launch on the 6th since we could face another government potential shutdown on the 8th...
By now, I figured everyone would have figured out that that tentative launch date is just another pie in the sky.
They haven't fired her up once yet and it's a good bet that today's static fire won't be that and let's not even talk about any likely additional issues after the test.
