Author Topic: Falcon Heavy Separation Method  (Read 24966 times)

Offline old_sellsword

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Re: Falcon Heavy Separation Method
« Reply #40 on: 07/04/2017 03:17 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

The two pictures ([1], [2]) were originally posted by the official SpaceX Facebook page. It was linked on r/SpaceX, but then the original post on their Facebook page was removed.

I wouldn't be surprised if NSF picked up on it before it was deleted either.

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #41 on: 07/04/2017 03:49 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.....

not the same thrust levels

Offline Welsh Dragon

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Re: Falcon Heavy Separation Method
« Reply #42 on: 07/04/2017 03:57 PM »
That's a good point Jim. I was going by the fact that the turning rate of S1 after separation is pretty speedy, seems to be comparable to the tumble rate of the Delta IV boosters at separation. Is there any quantitative data on the N2 thrusters vs normal booster separation motors? (Guessing proprietary data and/or ITAR raise their ugly heads here).

Offline GWH

Re: Falcon Heavy Separation Method
« Reply #43 on: 07/04/2017 04:53 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

The two pictures ([1], [2]) were originally posted by the official SpaceX Facebook page. It was linked on r/SpaceX, but then the original post on their Facebook page was removed.

I wouldn't be surprised if NSF picked up on it before it was deleted either.

What I posted came from google search today, so it's in the public sphere now regardless.

Offline Lars-J

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Re: Falcon Heavy Separation Method
« Reply #44 on: 07/04/2017 06:32 PM »
Great photo of the wind tunnel model and with a wider fairing although probably already discussed.

No, that looks like the same size fairing that F9 flies with now. It is merely the optical distortion that makes it look larger. Compare with the view of the same model from the rear.
« Last Edit: 07/04/2017 06:34 PM by Lars-J »

Online envy887

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Re: Falcon Heavy Separation Method
« Reply #45 on: 07/04/2017 10:25 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.....

not the same thrust levels

Pneumatic pushers can generate a large thrust over a short time, plus GN2 thrusters with a smaller thrust over a longer time. Used in concert, they should be sufficient.

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #46 on: 07/04/2017 10:32 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.....

not the same thrust levels

Pneumatic pushers can generate a large thrust over a short time, plus GN2 thrusters with a smaller thrust over a longer time. Used in concert, they should be sufficient.

Thousands of pounds?

Online envy887

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Re: Falcon Heavy Separation Method
« Reply #47 on: 07/04/2017 10:44 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.....

not the same thrust levels

Pneumatic pushers can generate a large thrust over a short time, plus GN2 thrusters with a smaller thrust over a longer time. Used in concert, they should be sufficient.

Thousands of pounds?

They have helium available at thousands of psi. Just need enough square inches, and a good flowrate of helium.

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #48 on: 07/05/2017 01:14 AM »
He isnt used at high pressure directly.   It is regulated down
« Last Edit: 07/05/2017 01:16 AM by Jim »

Online envy887

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Re: Falcon Heavy Separation Method
« Reply #49 on: 07/05/2017 04:36 AM »
Works fine for second stage separation.

Offline intrepidpursuit

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Re: Falcon Heavy Separation Method
« Reply #50 on: 07/05/2017 05:12 AM »
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.

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.

Online meekGee

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Re: Falcon Heavy Separation Method
« Reply #51 on: 07/05/2017 03:47 PM »
Then spent boosters are much lighter than spent solids, and inertia matters too.

Once they achieve some angle, the air stream should help separation instead of inhibit it.

Where is the source of information that there are issues with the separation?
ABCD - Always Be Counting Down

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #52 on: 07/05/2017 03:57 PM »
Works fine for second stage separation.

Not the same requirements. 

Online guckyfan

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Re: Falcon Heavy Separation Method
« Reply #53 on: 07/05/2017 05:46 PM »
My understanding is that it is derived from the concerns about grid fins not being effective due to the aerodynamic cone on top of the side boosters.

My non expert opinion is that the grid fins are not involved in separation so this discussion is off on the wrong track.

Edit: There may be a cause for concern that the side boosters may not make it back to LZ-1 due to the grid fin issue. Not a huge loss.

Grid fins will not be used in the separation, and they will not be opened until after boost-back.

And what grid fin issue on FH boosters would prevent them from making it back?

Removed my message.

Online LouScheffer

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Re: Falcon Heavy Separation Method
« Reply #54 on: 07/05/2017 07:18 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.....

not the same thrust levels

Pneumatic pushers can generate a large thrust over a short time, plus GN2 thrusters with a smaller thrust over a longer time. Used in concert, they should be sufficient.

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.


Online Semmel

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Re: Falcon Heavy Separation Method
« Reply #55 on: 07/05/2017 07:40 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!

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #56 on: 07/05/2017 09:29 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

Offline Lars-J

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Re: Falcon Heavy Separation Method
« Reply #57 on: 07/05/2017 09:32 PM »
 ??? Jim, taking cryptic comments to the next level.

Online envy887

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Re: Falcon Heavy Separation Method
« Reply #58 on: 07/05/2017 09:42 PM »
There's no inherent physical limitation to how large an impulse a cold gas thruster or pnumatic pusher can generate, or how fast it can deliver that impulse.

It's merely a question of how heavy those systems are relative to a high thrust SRM and whether the ability to test and reuse those systems overcomes their mass penalty.

Offline laszlo

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Re: Falcon Heavy Separation Method
« Reply #59 on: 07/05/2017 10:59 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.

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