Author Topic: Falcon Heavy Separation Method  (Read 25965 times)

Online Lars-J

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Re: Falcon Heavy Separation Method
« Reply #20 on: 07/03/2017 06:58 PM »
Looking at the FH core octaweb image, there is definitely 3 connections. There is a new joint that does not exists on the left or right. So I imagine the 3 connection points roughly like this: (see image)

Online Lars-J

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Re: Falcon Heavy Separation Method
« Reply #21 on: 07/03/2017 07:03 PM »
Crazy question: could you help the boosters separate by spinning the rocket along the axis of flight?

Oh you mean like an aircraft roll? I guess it is theoretically possible, but it seems risky. I expect the separation to be done using a combination pushers (like stage separation) followed by nitrogen thrusters to assure separation. Some have speculated that the center engine of the boosters will keep firing to provide additional control.

Online Jdeshetler

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Re: Falcon Heavy Separation Method
« Reply #22 on: 07/03/2017 07:56 PM »
Click on black box to run GIF.

PR animation by SpaceX.

Offline octavo

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Re: Falcon Heavy Separation Method
« Reply #23 on: 07/04/2017 06:14 AM »
Crazy question: could you help the boosters separate by spinning the rocket along the axis of flight?

Oh you mean like an aircraft roll? I guess it is theoretically possible, but it seems risky. I expect the separation to be done using a combination pushers (like stage separation) followed by nitrogen thrusters to assure separation. Some have speculated that the center engine of the boosters will keep firing to provide additional control.
I do this often in KSP if I have recontact issues during sep. Induce stable roll, stage, then allow the boosters to spin away, then de-roll. Works like a charm
« Last Edit: 07/04/2017 06:14 AM by octavo »

Offline Eerie

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Re: Falcon Heavy Separation Method
« Reply #24 on: 07/04/2017 06:59 AM »
Crazy question: could you help the boosters separate by spinning the rocket along the axis of flight?

Oh you mean like an aircraft roll? I guess it is theoretically possible, but it seems risky. I expect the separation to be done using a combination pushers (like stage separation) followed by nitrogen thrusters to assure separation. Some have speculated that the center engine of the boosters will keep firing to provide additional control.
I do this often in KSP if I have recontact issues during sep. Induce stable roll, stage, then allow the boosters to spin away, then de-roll. Works like a charm


Possible concern is that there may be still too much atmospheric resistance at the altitude where boosters separate, and such tricks are risky.

Or maybe there is just no problem with booster separation, and we are fretting here over nothing.

Offline TomH

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Re: Falcon Heavy Separation Method
« Reply #25 on: 07/04/2017 07:34 AM »
Didn't the shuttle SRB's have some residual thurst at separation that sent them to higher trajectories?

IDK what the thrust was, but obviously you want to jettison while T/W is still slightly >1, otherwise they are a drag on the core. After jettison, T/W drops to <1, but even then, any thrust is partly offsetting gravity losses, therefore, yes, their thrust does still affect their trajectory to some small degree.

No, the SRB's were jettison when they could no longer carry their own weight.  That is why they drop away

When I said T/W slightly >1, I meant something like 1.001. Not being able to carry their own weight would be the moment when T/W= 0.99999... The amount of time that passes between T/W=1.001 and T/W=0.99999 has to be almost unmeasurable.  That difference may matter when modeling a nuclear explosion, but not when jettisoning an SRB. 

After jettison when T/W is slightly <1, that number is still >0, so I know I am correct when I say that tapering thrust still offsets some of the gravity losses that are acting on the booster, thus that residual thrust does have a very small impact of the trajectory of the SRB. As for dropping away, we both know they continue a ballistic upward trajectory and initially only appear to be dropping away because the orbiter is under somewhere around full thrust from the main engines. Obviously their thrust continues dropping to zero fairly quickly and also they reach their zenith and do begin falling. 

Edit/Lar: Some softening. You don't step on Superman's cape, you don't spit into the wind, and you don't mess around with Jim. (leave that to the mods)
« Last Edit: 07/06/2017 07:44 PM by Lar »

Offline Hotblack Desiato

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Re: Falcon Heavy Separation Method
« Reply #26 on: 07/04/2017 11:43 AM »
Didn't the shuttle SRB's have some residual thurst at separation that sent them to higher trajectories?

IDK what the thrust was, but obviously you want to jettison while T/W is still slightly >1, otherwise they are a drag on the core. After jettison, T/W drops to <1, but even then, any thrust is partly offsetting gravity losses, therefore, yes, their thrust does still affect their trajectory to some small degree.

No, the SRB's were jettison when they could no longer carry their own weight.  That is why they drop away

But that would open up an easy method for the separation:

Shortly before separation, 8 of the 9 engines throttle down and then shut off. Just the 2 engines (one one each side) closest to the central core stays lit. Then, the connections between center and side booster disconnect, causing the side boosters to rotate away, because now they have an offcenter thrust.

Just as the side-stages start spinning, these 2 engines shut off aswell. They are barely running a second longer than the other 2x 8 side booster engines, just to provide some off-center thrust.

EDIT: alternatively, the central engine stays lit, and uses gimballing. Might even provide more controlabillity.
Then, after 180, the cold gass thrusters catch the rotation and one of the boosters starts the boost-back burn. 5 seconds after the first booster started, the second booster starts its boost-back burn. This way, the boosters are no longer together and the risk of the boosters of smashing into each other is gone.

EDIT2: my method in KSP involves shutting everything of, and sliding between the side boosters until the central stage is free. Then fire it up again in order to reach the orbit. Doesn't seem to be such a good idea for SpaceX, because it involves reigniting all 9 engines (also means, that all of them need to be vacuum-restartable).
« Last Edit: 07/04/2017 11:47 AM by Hotblack Desiato »

Offline jak Kennedy

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Re: Falcon Heavy Separation Method
« Reply #27 on: 07/04/2017 12:42 PM »
What about using the grid fins to pull the nose of the boosters away? Or is the atmosphere too thin at separation or are the likely to exert too much force?  (tried finding the altitude of the booster separation but not so easy, I see discussion or the core MECO at around 100km as a guess)

Offline Welsh Dragon

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Re: Falcon Heavy Separation Method
« Reply #28 on: 07/04/2017 01:13 PM »
Why are people coming up with all these Heath Robinson separation schemes when the booster CBCs on Delta IV have a perfectly sensible system? Boosters burn out, Booster Separation Rocket Motors (BSRM), located near the nose, fire, boosters tumble away from core. What do they Falcon Heavy boosters have conveniently located near the nose? Ah some, N2 thrusters which we know can flip a stage around in no time.....

Online GWH

Re: Falcon Heavy Separation Method
« Reply #29 on: 07/04/2017 01:17 PM »
Because there aren't any N2 thrusters at the base of the rocket, so the booster wouldn't be capable of lateral movement only rotational. Rotating both boosters about their center of gravity only would cause the tails to colide in the absence of any lateral movement.

One interesting thing I noticed on CRS11 footage is the engines appear to be relit before the stage has completed its flip. EDIT: On reviewing the video the first appearance of engines lighting is probably actually the S2 plume hitting the gridfins, engine ignition seems to be around 70 degrees of rotation but still before the full 90.
« Last Edit: 07/04/2017 01:44 PM by GWH »

Offline jak Kennedy

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Re: Falcon Heavy Separation Method
« Reply #30 on: 07/04/2017 01:25 PM »
Why are people coming up with all these Heath Robinson separation schemes when the booster CBCs on Delta IV have a perfectly sensible system? Boosters burn out, Booster Separation Rocket Motors (BSRM), located near the nose, fire, boosters tumble away from core. What do they Falcon Heavy boosters have conveniently located near the nose? Ah some, N2 thrusters which we know can flip a stage around in no time.....

For the same reason that SpaceX uses spring or some other inert pushers to separate S1 and S2 instead of explosive bolts or rocket motors. They also have grid fins conveniently located near the nose  ;)

Online matthewkantar

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Re: Falcon Heavy Separation Method
« Reply #31 on: 07/04/2017 01:34 PM »
The grid fins not are going to do anything before or during booster separation.

Matthew
« Last Edit: 07/04/2017 01:34 PM by matthewkantar »

Offline Welsh Dragon

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Re: Falcon Heavy Separation Method
« Reply #32 on: 07/04/2017 01:42 PM »
For the same reason that SpaceX uses spring or some other inert pushers to separate S1 and S2 instead of explosive bolts or rocket motors. They also have grid fins conveniently located near the nose  ;)
Where in using the N2 thrusters are explosive bolts or rocket motors involved? Also, how are you planning on unfolding the grid fins while the boosters are still attached? They take several second to unfold after all, and you'd need their control authority immediately following booster staging. Also, at what altitude is booster staging? Is there even enough atmosphere left for the grid fins to have any control authority? That I very much doubt that

Offline old_sellsword

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Re: Falcon Heavy Separation Method
« Reply #33 on: 07/04/2017 01:52 PM »
Because there aren't any N2 thrusters at the base of the rocket, so the booster wouldn't be capable of lateral movement only rotational. Rotating both boosters about their center of gravity only would cause the tails to colide in the absence of any lateral movement.

...

This is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).

Online GWH

Re: Falcon Heavy Separation Method
« Reply #34 on: 07/04/2017 01:56 PM »
Crazy question: could you help the boosters separate by spinning the rocket along the axis of flight?

Oh you mean like an aircraft roll? I guess it is theoretically possible, but it seems risky. I expect the separation to be done using a combination pushers (like stage separation) followed by nitrogen thrusters to assure separation. Some have speculated that the center engine of the boosters will keep firing to provide additional control.
I do this often in KSP if I have recontact issues during sep. Induce stable roll, stage, then allow the boosters to spin away, then de-roll. Works like a charm

I would think the challenge with this is ensuring that the connection points don't interfere with the rocket separating and are clear of the boosters moving away tangentially to the direction of rotation, since they will move in that direction and not straight away.
So the boosters, connecting as this:
o-o-o  won't move away like this o< -o- >o 
But rather like this, direction of rotation counter clockwise:
 o<
 |
 o
 |
>o
In the case of the mechanisms shown in the wind tunnel model that wrap around each booster, they would need to retract on one side of each booster first to allow for clean separation tangent to the point of release.  Whether or not that would cause any problems with fuel sloshing or the loads pulling away from the core during the roll would be dependent on roll rate, and how fast the boosters are to be pulling away.

Offline jak Kennedy

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Re: Falcon Heavy Separation Method
« Reply #35 on: 07/04/2017 02:01 PM »
For the same reason that SpaceX uses spring or some other inert pushers to separate S1 and S2 instead of explosive bolts or rocket motors. They also have grid fins conveniently located near the nose  ;)
Where in using the N2 thrusters are explosive bolts or rocket motors involved? Also, how are you planning on unfolding the grid fins while the boosters are still attached? They take several second to unfold after all, and you'd need their control authority immediately following booster staging. Also, at what altitude is booster staging? Is there even enough atmosphere left for the grid fins to have any control authority? That I very much doubt that

I was talking about S1 S2 separation and how SpaceX does things differently. You don't need all grid fins deployed to be useful. Yes, you are probably right about the amount of control authority that is why I used a question mark. I did try and find the altitude of boosters separation but come up short.

Online GWH

Re: Falcon Heavy Separation Method
« Reply #36 on: 07/04/2017 02:01 PM »
Because there aren't any N2 thrusters at the base of the rocket, so the booster wouldn't be capable of lateral movement only rotational. Rotating both boosters about their center of gravity only would cause the tails to colide in the absence of any lateral movement.

...

This is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).

Yeah those two struts at the bottom of the wind tunnel model could definitely have a pusher component to them, Would simplify the whole arrangement a lot.  Do you know this for fact or are you speculating?

Edit, 2nd windtunnel model photo added.
« Last Edit: 07/04/2017 02:04 PM by GWH »

Offline old_sellsword

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Re: Falcon Heavy Separation Method
« Reply #37 on: 07/04/2017 02:02 PM »
Because there aren't any N2 thrusters at the base of the rocket, so the booster wouldn't be capable of lateral movement only rotational. Rotating both boosters about their center of gravity only would cause the tails to colide in the absence of any lateral movement.

...

This is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).

Yeah those two struts at the bottom of the wind tunnel model could definitely have a pusher component to them, Would simplify the whole arrangement a lot.  Do you know this for fact or are you speculating?

I know it for a fact.

Offline Welsh Dragon

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Re: Falcon Heavy Separation Method
« Reply #38 on: 07/04/2017 02:28 PM »
Because there aren't any N2 thrusters at the base of the rocket, so the booster wouldn't be capable of lateral movement only rotational. Rotating both boosters about their center of gravity only would cause the tails to colide in the absence of any lateral movement.

...

This is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).
Yes, I was taking a bottom pusher or pivot mechanism as a given. Obviously you need something there.

Offline jak Kennedy

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Re: Falcon Heavy Separation Method
« Reply #39 on: 07/04/2017 03:10 PM »
Great photo of the wind tunnel model and with a wider fairing although probably already discussed. Which forum was this originally posted in?

Thanks

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