Author Topic: Falcon Heavy Separation Method  (Read 62411 times)

Offline rpapo

Re: Falcon Heavy Separation Method
« Reply #80 on: 11/05/2017 07:12 pm »
You forget one thing: only three of the nine engines are set up for restart.
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Online Herb Schaltegger

Re: Falcon Heavy Separation Method
« Reply #81 on: 11/05/2017 11:05 pm »
So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

Both the nosecones and the octawebs have pneumatic pusher mechanisms, and the octawebs also have a beefy connection point at the hold-down lugs. Whether or not that pivots before full separation is to be seen though.

I am curious if the center engine(s) on the boosters will ever actually shut down. On RTLS missions we see the booster light back up seconds after separation and use the main engine TVC to aid in a speedy flip into the boost back burn. I can see using the center engine to help guide the boosters away from the center stage. After all, they have the most control authority over the stage while the engines are on.

Seems likely to me. I am trying to imagine the center core continuing to thrust during booster separation, you will not want it to have to pull along the side boosters when they shut down. I would think that by providing just enough thrust on the side booster to zero out the forces on the attach points before separation would be the way to go.

Do you think Delta IV Heavy shuts down the core for booster separation? STS didn’t do it either. There’s no magic involved, just timing and establishing a good separation rate.
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Offline Jcc

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Re: Falcon Heavy Separation Method
« Reply #82 on: 11/06/2017 12:30 am »
So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

Both the nosecones and the octawebs have pneumatic pusher mechanisms, and the octawebs also have a beefy connection point at the hold-down lugs. Whether or not that pivots before full separation is to be seen though.

I am curious if the center engine(s) on the boosters will ever actually shut down. On RTLS missions we see the booster light back up seconds after separation and use the main engine TVC to aid in a speedy flip into the boost back burn. I can see using the center engine to help guide the boosters away from the center stage. After all, they have the most control authority over the stage while the engines are on.

Seems likely to me. I am trying to imagine the center core continuing to thrust during booster separation, you will not want it to have to pull along the side boosters when they shut down. I would think that by providing just enough thrust on the side booster to zero out the forces on the attach points before separation would be the way to go.

Do you think Delta IV Heavy shuts down the core for booster separation? STS didn’t do it either. There’s no magic involved, just timing and establishing a good separation rate.

Good point, but I didn't say anything about the core shutting down. The question is, are the DIVH side boosters completely shut down at separation or is there some residual thrust, and likewise the Ariane, Shuttle, and others. If separation is timed just right, perhaps there is enough residual thrust as those boosters are in the process of shutting down, to not place a load on the attach points.

Online aero

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Re: Falcon Heavy Separation Method
« Reply #83 on: 11/06/2017 01:25 am »
So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

Both the nosecones and the octawebs have pneumatic pusher mechanisms, and the octawebs also have a beefy connection point at the hold-down lugs. Whether or not that pivots before full separation is to be seen though.

I am curious if the center engine(s) on the boosters will ever actually shut down. On RTLS missions we see the booster light back up seconds after separation and use the main engine TVC to aid in a speedy flip into the boost back burn. I can see using the center engine to help guide the boosters away from the center stage. After all, they have the most control authority over the stage while the engines are on.

Seems likely to me. I am trying to imagine the center core continuing to thrust during booster separation, you will not want it to have to pull along the side boosters when they shut down. I would think that by providing just enough thrust on the side booster to zero out the forces on the attach points before separation would be the way to go.

Do you think Delta IV Heavy shuts down the core for booster separation? STS didn’t do it either. There’s no magic involved, just timing and establishing a good separation rate.

Good point, but I didn't say anything about the core shutting down. The question is, are the DIVH side boosters completely shut down at separation or is there some residual thrust, and likewise the Ariane, Shuttle, and others. If separation is timed just right, perhaps there is enough residual thrust as those boosters are in the process of shutting down, to not place a load on the attach points.

We know that the boosters are deliberately shut down, they don't run dry. So they are shut down simultaneously to avoid torquing the stack. Booster thrust drops to zero as the chamber pressure drops and the core continues to accelerate away. And yes, the boosters will shut down, they are now to low mass to chance not shutting down completely.

 How much time is needed for the core to clear the now ballistic boosters? Not very much, so can't the boosters just wait it out before starting to maneuver for boost back? If so, then a simple (large) coathanger-like hook with a flange to nudge the booster away while the core departs, might work well enough. (Four or more hooks.) SpaceX likes pneumatics, so replace the flange with pneumatic pushers, push harder on the top than the bottom to start the booster rotating toward the boost back orientation. Timing is everything but it is not difficult to calculate the time delay needed for the core to clear the booster thruster plumes. Trickier is the problem of when the will the booster clear the expanding plume of the core engines. That depends on whether or not the "push" from the pneumatics is strong enough.
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Offline Cherokee43v6

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Re: Falcon Heavy Separation Method
« Reply #84 on: 11/06/2017 02:30 am »

Good point, but I didn't say anything about the core shutting down. The question is, are the DIVH side boosters completely shut down at separation or is there some residual thrust, and likewise the Ariane, Shuttle, and others. If separation is timed just right, perhaps there is enough residual thrust as those boosters are in the process of shutting down, to not place a load on the attach points.

One thing I will point out as a reminder.  The final Falcon 1 failure was due to 'residual thrust' causing a collision between the booster and the second stage.  I would therefore think that 'institutional history' at SpaceX would call for a complete shutdown to avoid any potentially similar issues.
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Offline intrepidpursuit

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Re: Falcon Heavy Separation Method
« Reply #85 on: 11/06/2017 04:40 am »

Good point, but I didn't say anything about the core shutting down. The question is, are the DIVH side boosters completely shut down at separation or is there some residual thrust, and likewise the Ariane, Shuttle, and others. If separation is timed just right, perhaps there is enough residual thrust as those boosters are in the process of shutting down, to not place a load on the attach points.

One thing I will point out as a reminder.  The final Falcon 1 failure was due to 'residual thrust' causing a collision between the booster and the second stage.  I would therefore think that 'institutional history' at SpaceX would call for a complete shutdown to avoid any potentially similar issues.

SpaceX is very familiar with the performance of the M1D by now. The boosters need to push away from the stage not down, so the same problem would not have the same result. They don't seem to be the type of company that thinks learning from their mistakes means applying every fix to every problem no matter the relevance.

Online Semmel

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Re: Falcon Heavy Separation Method
« Reply #86 on: 11/06/2017 06:51 am »
Assuming separation occurs in vacuum or near enough. When the top connection is released and the centre booster fires, the force of center acceleration will pivot the side boosters outwards. Now if the side boosters are still firing on low throttle of the center engine, the would push them self's away once the bottom connection is released. No pusher needed at all.
If there is still atmosphere to worry about, the top hinges would need enough force and range to overcome the atmosphere until the pressure between the cores and central booster is large enough to prevent a recontact. Not sure that any magic is required.

Offline rakaydos

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Re: Falcon Heavy Separation Method
« Reply #87 on: 11/06/2017 04:25 pm »
Just spin the rocket and let the boosters go. :-)
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Offline Eerie

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Re: Falcon Heavy Separation Method
« Reply #88 on: 11/06/2017 05:02 pm »
Just spin the rocket and let the boosters go. :-)
https://xkcd.com/1244/


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Offline Kaputnik

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Re: Falcon Heavy Separation Method
« Reply #89 on: 11/07/2017 11:36 am »
So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

Both the nosecones and the octawebs have pneumatic pusher mechanisms, and the octawebs also have a beefy connection point at the hold-down lugs. Whether or not that pivots before full separation is to be seen though.

I am curious if the center engine(s) on the boosters will ever actually shut down. On RTLS missions we see the booster light back up seconds after separation and use the main engine TVC to aid in a speedy flip into the boost back burn. I can see using the center engine to help guide the boosters away from the center stage. After all, they have the most control authority over the stage while the engines are on.

Seems likely to me. I am trying to imagine the center core continuing to thrust during booster separation, you will not want it to have to pull along the side boosters when they shut down. I would think that by providing just enough thrust on the side booster to zero out the forces on the attach points before separation would be the way to go.

Do you think Delta IV Heavy shuts down the core for booster separation? STS didn’t do it either. There’s no magic involved, just timing and establishing a good separation rate.

Good point, but I didn't say anything about the core shutting down. The question is, are the DIVH side boosters completely shut down at separation or is there some residual thrust, and likewise the Ariane, Shuttle, and others. If separation is timed just right, perhaps there is enough residual thrust as those boosters are in the process of shutting down, to not place a load on the attach points.

Visually at least, the shuttle's SRBSs seemed to be still thrusting a little at sep.
Which when you think about it, and you think about the inherent uncertainties of controlling/predicting the thrust rate of a solid, just makes the fact that they made the shuttle work at all even more amazing.
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Offline StuffOfInterest

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Re: Falcon Heavy Separation Method
« Reply #90 on: 11/07/2017 01:00 pm »
Since everyone else is throwing out ideas, might as well add mine in.

Just before separation, boosters cut all but the center engine.  That engine is pitched away from the rocket.  Bottom attachment point is on a rotating joint that won't slip away from the center until the top of the booster has moved a few degrees out from center.  As the engine is pitched and the booster slightly angled, once it does separate it will immediately move away from the center core and start swinging around for boostback.

I don't recall off the top of my head if the boostback is single engine or three engine.  If it is a single engine, you are all set.  If it is three engine, the other two engines can fire up as soon as the booster finishes its 180 degree swing or even earlier if there isn't a concern about plume interaction with the other bodies still nearby.

A single engine still firing on the booster should have much less thrust than the center core so it will naturally want to fall away.  The top shouldn't need any pusher mechanism because the angled engine will cause the booster to naturally rotate on the bottom attachment.  Having the booster under power while separating will move the boosters far enough apart to avoid any unpleasant interaction between them during the return.
« Last Edit: 11/07/2017 01:02 pm by StuffOfInterest »

Offline rpapo

Re: Falcon Heavy Separation Method
« Reply #91 on: 11/07/2017 01:40 pm »
I don't recall off the top of my head if the boostback is single engine or three engine.
Boostback starts with one motor, then adds in two more, then shuts down in reverse order.  Just like the entry burn.
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Offline Mike_1179

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Re: Falcon Heavy Separation Method
« Reply #92 on: 11/07/2017 02:49 pm »

Visually at least, the shuttle's SRBSs seemed to be still thrusting a little at sep.
Which when you think about it, and you think about the inherent uncertainties of controlling/predicting the thrust rate of a solid, just makes the fact that they made the shuttle work at all even more amazing.

Booster sep was commanded for the time when thrust from the SRB drops below the drag from them. The thrust from the solids trails off as they run out of prop so you wait until the moment when they're not helping but actually slowing you down and you drop them.

But, like Delta IVH and Titan, you didn't just "drop" them, there were small solid motors that pushed them away and would overcome any aero loads that might have wanted to push them back into the vehicle. Without knowing what the CFD is telling them about these aero loads at separation (shockwaves are weird) it's all just hand-waving. The thrust of the SRBs wasn't used to get them away from the vehicle, but it was accounted for to ensure there was no recontact.

Offline the_other_Doug

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Re: Falcon Heavy Separation Method
« Reply #93 on: 11/07/2017 03:56 pm »
And what kind of aeroloads will FH be seeing at side booster separation?  Will the trajectory be lofted simply to get out of most of the sensible atmosphere before booster sep?
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Online envy887

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Re: Falcon Heavy Separation Method
« Reply #94 on: 11/07/2017 04:15 pm »
And what kind of aeroloads will FH be seeing at side booster separation?  Will the trajectory be lofted simply to get out of most of the sensible atmosphere before booster sep?

It will be faster and probably higher than F9 booster separation, so there will be very little atmosphere. Perhaps not negligible, but much less than at the Shuttle SRB separation at about 45 km altitude.

If you watch some F9 launches like NROL-76, the booster and the upper stage coast along for several seconds with the booster broadside to the direction of travel, apparently under control of the cold gas thrusters. FH boosters will be higher, and the atmosphere density and drag halves about every 3 km at those altitudes.

So IMO, they will only need pneumatic pushers at top and bottom of the boosters, and cold gas thrusters at the top, to get a clean separation.

OneSpeed's FH sim shows a flattish trajectory, 69 km staging, with a dynamic pressure at staging about 1% of MaxQ.
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« Last Edit: 11/07/2017 04:30 pm by envy887 »

Offline Lar

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Re: Falcon Heavy Separation Method
« Reply #95 on: 11/07/2017 09:01 pm »
If you watch some F9 launches like NROL-76, the booster and the upper stage coast along for several seconds with the booster broadside to the direction of travel, apparently under control of the cold gas thrusters. FH boosters will be higher, and the atmosphere density and drag halves about every 3 km at those altitudes.
I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...
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Offline old_sellsword

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Re: Falcon Heavy Separation Method
« Reply #96 on: 11/07/2017 09:06 pm »
If you watch some F9 launches like NROL-76, the booster and the upper stage coast along for several seconds with the booster broadside to the direction of travel, apparently under control of the cold gas thrusters. FH boosters will be higher, and the atmosphere density and drag halves about every 3 km at those altitudes.
I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

How can the boosters stage lower if they’re the same size as F9 S1? Assuming roughly equivalent thrust levels and throttling profiles (only FH center throttles down), they’ll stage at about the same time.

Offline Lars-J

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Re: Falcon Heavy Separation Method
« Reply #97 on: 11/07/2017 09:11 pm »
If you watch some F9 launches like NROL-76, the booster and the upper stage coast along for several seconds with the booster broadside to the direction of travel, apparently under control of the cold gas thrusters. FH boosters will be higher, and the atmosphere density and drag halves about every 3 km at those altitudes.
I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

That assumption is correct for crossfeed, but not without it.

The boosters will likely burn about the same time as an F9 S1, but since the mass they are lifting only has one upper stage ( ~110t? - most part of the mass), it will end up higher. This will be slightly offset by needing more propellant to get back and the center core throttling down, but I think the total effect is that they would stage higher. (and that simulation seems to agree) The center core will certainly stage much faster and higher, though.
« Last Edit: 11/07/2017 09:16 pm by Lars-J »

Offline Wolfram66

Re: Falcon Heavy Separation Method
« Reply #98 on: 11/07/2017 09:12 pm »
If you watch some F9 launches like NROL-76, the booster and the upper stage coast along for several seconds with the booster broadside to the direction of travel, apparently under control of the cold gas thrusters. FH boosters will be higher, and the atmosphere density and drag halves about every 3 km at those altitudes.
I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

Also if you note that as the CGT firing rotates the S1 that the base of the S1 pivots to the left of what would be the main core S1 centerline... {boom} . Therefore, you will need to be CGT firings at the top and the base of each side booster as seen in STS SRB and Ariane 5 booster separation sequences. IDK if CGT at top + S1[l/r] center M1-D steering provides sufficient rotational clearance at base of stack.. opinions?

Offline hkultala

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Re: Falcon Heavy Separation Method
« Reply #99 on: 11/07/2017 09:16 pm »
If you watch some F9 launches like NROL-76, the booster and the upper stage coast along for several seconds with the booster broadside to the direction of travel, apparently under control of the cold gas thrusters. FH boosters will be higher, and the atmosphere density and drag halves about every 3 km at those altitudes.
I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

It depends on how much the center core is throttled down.

If the engines were not throttled down at all, the engines would burn for same time than in F9, for all three cores.

But because there are 3x more engines and 3x more first stage weight but same second stage weight, the T/W would be higher and in the same time it would have gained higher velocity and higher altitude.


So practically:

If the (weighted) average T/W is higher than F9 (weighted) average T/W, the boosters will separate higher.
If the (weighted) average T/W is lower than F9 (weighted) average T/W, the boosters will separate lower.


To minimize gravity losses, at last in the early part of the flight all engines will be running at normal thrust. And on those launches where center core is going to fly back to the launch site, they also want it to stage as fast as possible so that the distance to fly back is shorter (in order to reach same velocity, accelerating quickly means the velocity is reached closer to the launch site) so they also do not want to throttle it much.

Only when core stage is expendable or landing to barge that's far out in the ocean will they want to throttle the first stage deeply.


So, for most flights, boosters will stage higher than F9.

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