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#600
by
wannamoonbase
on 29 Dec, 2017 14:09
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I think the probability of success is pretty high, but not as high as a F9 of course.
SpaceX has experience, latest design and modeling technology. Things can happen but I don’t think there are many unknowns and that SpaceX will be conservative on this first one, as much as they can be anyway.
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#601
by
mn
on 29 Dec, 2017 14:28
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torque stresses
So I've seen this mentioned several times but I never saw an explanation.
Can someone please explain what this is about, and in particular why it would be more difficult on 3 cores vs 1 core.
(My simple mind would think that whatever stress is induced by the engines is local to the core, so you don't have 3 times the stress, you just have the same stress 3 times)
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#602
by
Johnnyhinbos
on 29 Dec, 2017 14:40
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torque stresses
So I've seen this mentioned several times but I never saw an explanation.
Can someone please explain what this is about, and in particular why it would be more difficult on 3 cores vs 1 core.
(My simple mind would think that whatever stress is induced by the engines is local to the core, so you don't have 3 times the stress, you just have the same stress 3 times)
Generally speaking single propeller boats can only back down in one direction - this is because the propeller is rotating in one direction, thereby imparting a rotational torque on the boat (visualize a prop "walking" along the ground as it turns - that will show you the direction of the torque.) In forward this torque is offset by the prop wash hitting the rudder and the rudder counter acting this torque, but in reverse the wash runs in the opposite direction and therefore the rudder loses it's effectiveness.
Twin prop boats deal with this torque issue by spinning the props in opposite directions.
The FH is a 27 prop boat... with three groups of nine props, each group arranged in a circle, and the three groups of circles all in a line. That line will get torqued as the three circles want to rotate.
So what imparts the torque within each engine? I'll leave that for you to answer... :-)
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#603
by
mhenderson
on 29 Dec, 2017 14:46
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What are the logistical factors involved in sourcing fuel, liquid oxygen, helium, and igniter hypergolics in the quantities required for a Falcon Heavy static fire test or launch?
1) Many of these are no biggie to keep on hand. I would expect RP1 fuel to be stable for storage, probably already on site for a static fire test. I would expect Helium to be stable, but prone to leakage losses. Hypergolics are probably nasty to keep around (toxic and corrosive), but stable to store under the proper conditions. Am I correct? And do they fully tank all of these fluids during a static fire or do they "cheat" and just load a lesser amount sufficient for a brief firing? (I assume they top each of them off completely for a valid test run, but hey, you guys are the experts.)
2) My expectation is that LOX is both dangerous to handle and the most costly to store for any period of time. FH requires ~3X the amount of an F9 launch ... how far in advance of a static fire or launch does the supplier need to gear up production? Are those quantities substantial to a big liquid air supplier? i.e. Is it even produced as a big special run or is it made in a more routine and steady fashion and stored? LOX has a boiling point of -297.3°F = -183°C = 162°R = 90K, but SpaceX uses it at a much colder temperature to get the benefit of packing more oxidizer into the tanks. Is it stored in bulk at the subcooled temperature? Or do they store it at a temperature close to the boiling point and 'finish' it during the transfer/loading process? Here's a NASA doc on subcooling on the run:
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050203875.pdf
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#604
by
zhangmdev
on 29 Dec, 2017 15:10
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#605
by
nacnud
on 29 Dec, 2017 15:21
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#606
by
Paul_G
on 29 Dec, 2017 15:25
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What are the logistical factors involved in sourcing fuel, liquid oxygen, helium, and igniter hypergolics in the quantities required for a Falcon Heavy static fire test or launch?
1) Many of these are no biggie to keep on hand. I would expect RP1 fuel to be stable for storage, probably already on site for a static fire test. I would expect Helium to be stable, but prone to leakage losses. Hypergolics are probably nasty to keep around (toxic and corrosive), but stable to store under the proper conditions. Am I correct? And do they fully tank all of these fluids during a static fire or do they "cheat" and just load a lesser amount sufficient for a brief firing? (I assume they top each of them off completely for a valid test run, but hey, you guys are the experts.)
2) My expectation is that LOX is both dangerous to handle and the most costly to store for any period of time. FH requires ~3X the amount of an F9 launch ... how far in advance of a static fire or launch does the supplier need to gear up production? Are those quantities substantial to a big liquid air supplier? i.e. Is it even produced as a big special run or is it made in a more routine and steady fashion and stored? LOX has a boiling point of -297.3°F = -183°C = 162°R = 90K, but SpaceX uses it at a much colder temperature to get the benefit of packing more oxidizer into the tanks. Is it stored in bulk at the subcooled temperature? Or do they store it at a temperature close to the boiling point and 'finish' it during the transfer/loading process? Here's a NASA doc on subcooling on the run:
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20050203875.pdf
1. I believe that tanks are fully loaded - essentially the hot fire is as if the rocket were launching, except they turn off the engines after x seconds, and don't release the hold downs. The hypergolic for engine startup come from the pad, not from tanks on the stage - the stage stored hypergolic are used for engine restarts after stage separation.
2. I read recently that the Lox is stored at regular lox temperatures, and chilled down as part of the loading process.
Rgds
Paul
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#607
by
Rocket Science
on 29 Dec, 2017 15:41
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Allow me to borrow some lines from the film Marooned:
-"Yes or no, will the bird fly?"
-"It'll fly..."
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#608
by
mn
on 29 Dec, 2017 15:43
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torque stresses
So I've seen this mentioned several times but I never saw an explanation.
Can someone please explain what this is about, and in particular why it would be more difficult on 3 cores vs 1 core.
(My simple mind would think that whatever stress is induced by the engines is local to the core, so you don't have 3 times the stress, you just have the same stress 3 times)
Generally speaking single propeller boats can only back down in one direction - this is because the propeller is rotating in one direction, thereby imparting a rotational torque on the boat (visualize a prop "walking" along the ground as it turns - that will show you the direction of the torque.) In forward this torque is offset by the prop wash hitting the rudder and the rudder counter acting this torque, but in reverse the wash runs in the opposite direction and therefore the rudder loses it's effectiveness.
Twin prop boats deal with this torque issue by spinning the props in opposite directions.
The FH is a 27 prop boat... with three groups of nine props, each group arranged in a circle, and the three groups of circles all in a line. That line will get torqued as the three circles want to rotate.
So what imparts the torque within each engine? I'll leave that for you to answer... :-)
Thanks for explanation, but I'm still stumped by the 1 vs 3 core?
Whatever torque issues are on one core they have obviously solved, so what happens when you have 3 cores side by side? as I said in my question: why is it a larger problem and not the same problem 3 times (with the same solution applied 3 times)?
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#609
by
nacnud
on 29 Dec, 2017 15:50
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Because of the longer momentum arm the torque is applied on?
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#610
by
AncientU
on 29 Dec, 2017 15:57
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Fast. Barn to vertical.
And back to barn... one work day for fit-up*!
* assuming there isn't a second day planned in the future before roll-put for static fire.
Falcon Heavy on its way back to the HIF:
source
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#611
by
abaddon
on 29 Dec, 2017 16:08
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The Falcon 9 single stick clearly doesn’t have any net torque when it’s flying (the very first Falcon 9 did have some unplanned roll immediately after liftoff). I would assume the three boosters will be the same once flying. However there might be torque in the startup sequence before the hold-down clamps are released. I’d imagine any torque concerns are in this part of the timeline, not in flight.
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#612
by
cscott
on 29 Dec, 2017 16:12
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Thrust asymmetries during startup cause torques. With 27 engines, more of the engines are further from the centerline, and so the worst-case torques are higher. With FH there's the additional problem of torques applied across the long moment arm of the booster, causing the nose of the booster to move in relation to the core.
Since SpaceX is doing a staggered start, we know that there is some combination of startup transients that would "break things". The staggered start is intended to ensure that the worst-possible worst-case transients can't add together in the bad way. But of course the staggered start of all 27 engines hasn't been tested yet. There are some unknown unknowns there that could cause the "break things" result.
And with respect to the "are the tanks full" question: the weight of the propellants in the tanks is part of what keeps the rocket on the ground during the static fire. If you were to fire with lower fuel quantities, you'd either have to beef up the hold down to handle the greater thrust loads or (as they do in McGregor) fit a weighted "beanie cap" on top of the rocket to apply an equivalent gravity load.
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#613
by
yokem55
on 29 Dec, 2017 16:13
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I think the probability of success is pretty high, but not as high as a F9 of course.
I think 50-50 is about right. That's what Elon suggested, if I recall correctly.
There are certainly some untested aspects of this vehicle, especially base heating, max-q transonic aerodynamic stresses, pogo, etc. History is a guide. The first Delta 4 Heavy failed, as did three of the first six Titan IIIC missions and one of the first two Saturn 5 launches, though only one of those five combined failures was an outright Fail to Orbit.
As Elon said, we'll all be happy if this thing gets off the pad and over the Atlantic without mishap. I believe that the odds of at least that happening are pretty good.
- Ed Kyle
You might be able to add the first shuttle flight to that list.
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#614
by
mme
on 29 Dec, 2017 16:15
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...
Thanks for explanation, but I'm still stumped by the 1 vs 3 core?
Whatever torque issues are on one core they have obviously solved, so what happens when you have 3 cores side by side? as I said in my question: why is it a larger problem and not the same problem 3 times (with the same solution applied 3 times)?
If you watch the the liftoff of the first Falcon 9, it clearly rotates. For a single stick, that's not really a big deal. But now bolt three of them together and rather than rotate freely in space they will be torquing the connections.
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#615
by
mn
on 29 Dec, 2017 16:21
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Now I remember where I first saw this mentioned:
https://www.nasaspaceflight.com/2017/11/spacex-aims-december-launch-falcon-heavy/Due to thrust torque (a thrust-induced rotation) scenario that could destroy or severely damage the octawebs at the base of each Falcon core that house the Merlin 1D engines, the Falcon Heavy’s 27 engines will not be lit simultaneously like the Falcon 9 engines
So it seems we are dealing with an issue local to each core (worry about damage to the octaweb). so hence my question, why are 3 cores different than one core in this respect.
Suppose we stood 3 cores side by side without any hardware connecting them (obviously would need 3 TELs) would igniting them be different than usual?
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#616
by
NX-0
on 29 Dec, 2017 16:25
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If you watch the the liftoff of the first Falcon 9, it clearly rotates. For a single stick, that's not really a big deal. But now bolt three of them together and rather than rotate freely in space they will be torquing the connections.
Waves and waves of nostalgia there.
F9 V1.0 is such a different animal from what we are seeing now.
'stubby' looking even, especially with no legs
3x3 engines looks freaky, now.
Did Octoweb help with some of the roll issues?
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#617
by
mn
on 29 Dec, 2017 16:28
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...
Thanks for explanation, but I'm still stumped by the 1 vs 3 core?
Whatever torque issues are on one core they have obviously solved, so what happens when you have 3 cores side by side? as I said in my question: why is it a larger problem and not the same problem 3 times (with the same solution applied 3 times)?
If you watch the the liftoff of the first Falcon 9, it clearly rotates. For a single stick, that's not really a big deal. But now bolt three of them together and rather than rotate freely in space they will be torquing the connections.
The issue they are worried about is during ignition, not during flight.
(and we've seen many launches since then, I don't think they have a problem here)
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#618
by
Gotorah
on 29 Dec, 2017 16:46
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The gimbal of the engines is fully capable of eliminating or inducing any roll wanted or unwanted.
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#619
by
Rocket Science
on 29 Dec, 2017 17:04
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I'm not concerned by any flight control guidance authority issues, just potential pneumatics and unlatch hangs at booster sep...
