Author Topic: Falcon Heavy Separation Method  (Read 24495 times)

Offline dcporter

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Re: Falcon Heavy Separation Method
« Reply #60 on: 07/06/2017 01:35 AM »
This thread is weird. SpaceX knows exactly how the separation mechanism is designed, so why all the guesswork? Someone just drop them a note and ask and save us from the Heath Robinson/Rube Goldberg/Gyro Gearloose guesses.

You must be new round these parts

Offline intrepidpursuit

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Re: Falcon Heavy Separation Method
« Reply #61 on: 07/06/2017 04:39 AM »
This thread is weird. SpaceX knows exactly how the separation mechanism is designed, so why all the guesswork? Someone just drop them a note and ask and save us from the Heath Robinson/Rube Goldberg/Gyro Gearloose guesses.

If we get enough wild theories maybe it will annoy someone in the know enough that they weigh in to end the madness. Seems like a valid strategy.

Offline meekGee

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Re: Falcon Heavy Separation Method
« Reply #62 on: 07/06/2017 05:04 AM »
??? Jim, taking cryptic comments to the next level.

Not everything that's undecipherable is cryptic.
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Online envy887

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Re: Falcon Heavy Separation Method
« Reply #63 on: 07/06/2017 12:52 PM »
??? Jim, taking cryptic comments to the next level.

Not everything that's undecipherable is cryptic.

We've finally driven Jim mad...

Offline Perchlorate

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Re: Falcon Heavy Separation Method
« Reply #64 on: 07/06/2017 01:21 PM »
??? Jim, taking cryptic comments to the next level.

Not everything that's undecipherable is cryptic.

We've finally driven Jim mad...

I suspect the only thing that made it cryptic is that he left off the suffix "-less" and meant to say

Meaningless numbers.  Does not factor in time

I couldn't replicate Lou's "free body diagram" physics if my life depended on it, but it looks to me like he DID "factor in time"...3 seconds of it...at the very beginning.

Offline LouScheffer

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Re: Falcon Heavy Separation Method
« Reply #65 on: 07/06/2017 01:30 PM »
This thread is weird. SpaceX knows exactly how the separation mechanism is designed, so why all the guesswork? Someone just drop them a note and ask and save us from the Heath Robinson/Rube Goldberg/Gyro Gearloose guesses.
You can ask, but based on experience you won't get an answer, because:
  (a) It's proprietary,
  (b) It could help someone else design a missile, so it's covered by ITAR and they can't legally say, and
  (c) They've got better things to do with their time than explain to curious strangers the details of their engineering.

This applies to almost all aspects of rocket engineering, not just this thread.  So this forum is filled with guesses based on experience, guesses based on physics, guesses based on intuition, and wild speculation based on nothing whatsoever.  Caveat lector.


Offline meekGee

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Re: Falcon Heavy Separation Method
« Reply #66 on: 07/06/2017 02:23 PM »
This thread is weird. SpaceX knows exactly how the separation mechanism is designed, so why all the guesswork? Someone just drop them a note and ask and save us from the Heath Robinson/Rube Goldberg/Gyro Gearloose guesses.
You can ask, but based on experience you won't get an answer, because:
  (a) It's proprietary,
  (b) It could help someone else design a missile, so it's covered by ITAR and they can't legally say, and
  (c) They've got better things to do with their time than explain to curious strangers the details of their engineering.

This applies to almost all aspects of rocket engineering, not just this thread.  So this forum is filled with guesses based on experience, guesses based on physics, guesses based on intuition, and wild speculation based on nothing whatsoever.  Caveat lector.
(d) it was just an opinion or rumor.

The line between "some circles assume" and "some guys said" is awfully thin.

The track record of this type of information is not so good.

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Online AncientU

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Re: Falcon Heavy Separation Method
« Reply #67 on: 07/06/2017 02:57 PM »

Thousands of pounds?
Well, we can guess the thrust of the nitrogen thrusters.  From the NROL-76 mission, we see they fire for about 3 seconds to start the first stage rotating.  The rotation reaches 90 degrees, more or less, in 7 seconds.  So one revolution every 28 seconds, or 0.224 radians/sec.  To acquire this rate in 3 seconds means an angular acceleration of 0.075 radians/sec^2

Let's make the crude assumption that the booster rotates around the engines, since that's where most of the mass is located (engines + remaining fuel).  We know the empty stages masses about 27t.  9 engines mass about 7t, so let's assume the rest is a 20t cylinder, and that the moment of inertia of the cylinder dominates (the rest of the mass, engines and fuel, is close to the axis of rotation).  Rotating a cylinder around its end has a moment of inertial of mL^2/3.  Using a length of 47 meters and a mass of 20t, this gives a moment of inertia of 14,800,00 kg x m^2.

The torque to accelerate this at 0.075 radians/sec^2 is about 1.1M N x m.  Assuming a lever arm of 47m, that's a thrust of 23400 N, or 2400 kg-force, or 5250 lb-force.  At a typical ISP of 73 for cold gas nitrogen thrusters, that's a flow of 32 kg/second.

So the cold gas thrusters can generate thousands of pounds of force.  On the other hand, separation rocket motors can generate even more force.  The shuttle boosters had 8 motors per booster, each generating 20,000 lb-f for 1.2 seconds.  Each motor massed 80 kg.

Thank you Lou, I really like the use of Math and Physics instead of hand waving and authority. More of that please!

Meaning numbers.  Does not factor in time

Does also not factor in the mass of the steel casings of Shuttle boosters (90-100t each?) or their top-mount, puller design.  Aerodynamics of the STS stack is also vastly different than the FH.
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Offline JamesH65

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Re: Falcon Heavy Separation Method
« Reply #68 on: 07/06/2017 04:25 PM »

Thousands of pounds?
Well, we can guess the thrust of the nitrogen thrusters.  From the NROL-76 mission, we see they fire for about 3 seconds to start the first stage rotating.  The rotation reaches 90 degrees, more or less, in 7 seconds.  So one revolution every 28 seconds, or 0.224 radians/sec.  To acquire this rate in 3 seconds means an angular acceleration of 0.075 radians/sec^2

Let's make the crude assumption that the booster rotates around the engines, since that's where most of the mass is located (engines + remaining fuel).  We know the empty stages masses about 27t.  9 engines mass about 7t, so let's assume the rest is a 20t cylinder, and that the moment of inertia of the cylinder dominates (the rest of the mass, engines and fuel, is close to the axis of rotation).  Rotating a cylinder around its end has a moment of inertial of mL^2/3.  Using a length of 47 meters and a mass of 20t, this gives a moment of inertia of 14,800,00 kg x m^2.

The torque to accelerate this at 0.075 radians/sec^2 is about 1.1M N x m.  Assuming a lever arm of 47m, that's a thrust of 23400 N, or 2400 kg-force, or 5250 lb-force.  At a typical ISP of 73 for cold gas nitrogen thrusters, that's a flow of 32 kg/second.

So the cold gas thrusters can generate thousands of pounds of force.  On the other hand, separation rocket motors can generate even more force.  The shuttle boosters had 8 motors per booster, each generating 20,000 lb-f for 1.2 seconds.  Each motor massed 80 kg.

Thank you Lou, I really like the use of Math and Physics instead of hand waving and authority. More of that please!

Meaning numbers.  Does not factor in time

Does also not factor in the mass of the steel casings of Shuttle boosters (90-100t each?) or their top-mount, puller design.  Aerodynamics of the STS stack is also vastly different than the FH.

That because he's talking about the F9 first stage, not the Space Shuttle....

And yes, he does have time in there.

Online Norm38

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Re: Falcon Heavy Separation Method
« Reply #69 on: 07/06/2017 06:55 PM »
You can ask, but based on experience you won't get an answer, because:
  (a) It's proprietary,
  (b) It could help someone else design a missile, so it's covered by ITAR and they can't legally say

Unless ITAR demands that they not show video of the separation in action and not show the vehicle on the pad, (but it can be seen from the ground with a good enough camera) we will know how separation is accomplished.  We will see gas thrusters or solid motors firing.  We will see which engines are running and when.
They may not be in much of a mood to say anything but "watch this" until it actually happens.
But I bet a six pack you all will be able to reverse engineer the system 5 minutes after you see the first launch.

That doesn't mean that SpaceX has to release all their notes for how they determined the best trade-offs, but this isn't something that can be hidden from North Korea.
« Last Edit: 07/06/2017 07:01 PM by Norm38 »

Offline Mike_1179

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Re: Falcon Heavy Separation Method
« Reply #70 on: 07/06/2017 07:03 PM »

That doesn't mean that SpaceX has to release all their notes for how they determined the best trade-offs, but this isn't something that can be hidden from North Korea.

To be fair, your your ICBM needs to be three-boosters strapped together, you probably need to work on making the payload smaller before you worry about aeroacoustic loads on cross-beams.

Offline Semmel

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Re: Falcon Heavy Separation Method
« Reply #71 on: 07/06/2017 07:10 PM »

That doesn't mean that SpaceX has to release all their notes for how they determined the best trade-offs, but this isn't something that can be hidden from North Korea.

To be fair, your your ICBM needs to be three-boosters strapped together, you probably need to work on making the payload smaller before you worry about aeroacoustic loads on cross-beams.

The four side boosters of the sojus rocket originally were designed for ICBMs.

Online Lar

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Re: Falcon Heavy Separation Method
« Reply #72 on: 07/06/2017 07:52 PM »
There's a fine line between trying to work things out for ourselves (so we collectively learn more) and coming up with crazy rube goldberg ideas for how things work.

There's also a fine line between cryptic but useful comments that give clues or share experience and just saying "you're wrong".

Finally (ha!) there's a fine line between jocular banter that lubricates social discourse and just trying to be funny for its own sake (we have a party thread for that)

Walking all these lines at once? not easy. But amazingly, most of you do it most of the time. So you're mostly awesome that way.  Thank you.

I of course want you all to be awesome all the time. :)
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"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Online georgegassaway

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Re: Falcon Heavy Separation Method
« Reply #73 on: 07/11/2017 06:21 AM »
Is it totally crazy to separate a booster using only rocket power, and to pivot on an attachment point while under power? It meets the simplicity requirement SpaceX seems to prefer, but they are as limited by physics as everyone else. It seems like the stress put on that bottom attachment joint would be incredibly high and it would have to pivot as well.

Well, the Russian R7 and descendants have been sort of doing that since 1957. Although the outer boosters do not pivot in that way, but they sep without any use of rockets or thrusters.



FWIW - For a flying 1/72 shuttle model, I worked up a design to sep the SRB's in a manner that would work under thrust, if need be (The model had a single engine in the ET (offset towards the orbiter to maintain the parallel thrustline thru the model CG).  Anyway, the model SRB's were pushed upwards by the aft attach rings, and the forward attach fitting was used for a spring to pitch the SRB nose away once unlatched.  The design of the aft attachment was such that once the SRB pivoted outwards 15-20 degrees, the aft attachment let go of the SRB so that it could separate cleanly. Worked great. In normal flights, the SRB's were not sepped until 1/2 second after the ET rocket motor burned out. But, two times there was  major problem at launch, so I sepped the orbiter by R/C, and the Flight Computer in the ET detected the emergency sep to abort the flight, and automatically sepped the SRB's which at that time were being pushed upwards by the still burning engine. They sepped cleanly, and landed safely, as did the ET as the F.C. fired to eject its' chutes 1/2 second later.

THere's details (and photos) about the model on my web page:

http://georgesrockets.com/GRP/Scale/Shuttle-G/modeldetails.htm

Here's a merger of two images from there,  to show how the SRB sepped by pivoting outwards, and details of the mechanism (SRB Aft Ring Pivot portion along the right side of the middle of the drawing).



Photo showing the SRB Aft Ring pivoted out (Cast part with wire reinforcement inside the half-round shafts, angled almost enough to pry loose from the corresponding half-round shafts (aluminum) on the ET for separation.



Now, in no way am I saying this design would be suitable for FH (especially since my model SRB's never "push" the stack, they are only pulled along for the ride). And I could afford overkill design and strength that would be pretty heavy for a full size vehicle, not the finesse that I expect FH to have...whatever the design mechanics. But I am showing a pivoted aft release that comes free after "X-degrees" of pivoting.

And FWIW - footage of the above in action.  (warning, volume is a bit loud)

« Last Edit: 07/11/2017 06:28 AM by georgegassaway »

Offline vanoord

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Re: Falcon Heavy Separation Method
« Reply #74 on: 07/11/2017 11:12 AM »
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.
Sounds to me like this basically answers my original question. Pushers at the bottom and N2 at the top. With the attachment method being at the bottom of the booster they can pivot a bit before the pushers fire. Together with engine gimballing and throttling it sounds like clearing the center core is sorted. Suggestions of using the grid fins seem ridiculous considering they are designed to be used while flying backward.

Anyone know if the side boosters will be firing at separation. I'd assume they will be firing 1-3 engines to match acceleration and then for boost back.

The trick with separation is going to be to push the two boosters away, but effectively keep them flying in formation but slowly dropping behind the centre core.

A separation method which causes the boosters to spin isn't conducive to easy recovery - and recovery is the difference with this compared to other rockets using a pair of boosters.

However, the fact that the boosters still have fuel; and have engines that can be throttled enables this - but it's going to be a bit of ballet to separate them, fly them away from the centre core (which will accelerate away) and then when the two boosters are far enough apart from each other, flip them round and fly them home. But, like landing on the ASDS, it's ultimately down to software - and SpaceX have proved they can resolve that.

Slightly OT, but will there be any need to have the two booster cores be aware of each other's relative position post-separation?

My suspicion would that it would just add unnecessary complexity; and that as long as they are programmed to fly different courses, they'll stay clear of each other; and that AFTS would deal with off-course errors before problems occurred.

Offline laszlo

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Re: Falcon Heavy Separation Method
« Reply #75 on: 07/11/2017 12:46 PM »
...FWIW - For a flying 1/72 shuttle model...

That is quite a model. Very well done, and the video is spectacular, too. The orbiter approach part of that video is enough to make the space conspiracy nutjobs start telling us that there was no shuttle program, either.

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #76 on: 07/11/2017 01:19 PM »

Well, the Russian R7 and descendants have been sort of doing that since 1957. Although the outer boosters do not pivot in that way, but they sep without any use of rockets or thrusters.


It does use thrusters.  GOX at the tip.

But as stated before R7 is not a relevant example.  It pushes from the tip of the booster with a ball in a socket.


Offline Eerie

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Re: Falcon Heavy Separation Method
« Reply #77 on: 07/11/2017 03:55 PM »
Just spin the rocket and let the boosters go. :-)

Offline intrepidpursuit

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Re: Falcon Heavy Separation Method
« Reply #78 on: 11/03/2017 02:32 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.

Online Jcc

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Re: Falcon Heavy Separation Method
« Reply #79 on: 11/05/2017 01:36 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.

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