Author Topic: Falcon Heavy Separation Method  (Read 13956 times)

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 406
  • Orlando, FL
  • Liked: 249
  • Likes Given: 173
Falcon Heavy Separation Method
« on: 06/30/2017 07:49 PM »
As we are getting close to FH debut, do we know anything about how the boosters will separate from the central core? The boosters already have N2 thrusters at the top, but not at the bottom. Will the N2 thrusters be enough?

My guess is that there is a pneumatic or hydraulic pusher in the separation system along with the N2 thrusters and that at least the center engine will remain lit for a moment after separation to assist in guiding the stages safely away. The pushers are perhaps unnecessary if the N2 is enough to guide the top of the stage.

Any thoughts or sources on how the separation will occur?

Offline GWH

Re: Falcon Heavy Separation Method
« Reply #1 on: 06/30/2017 08:10 PM »
Moving this over from the L2 thread but in an edited format:

Using the excellent FlightClub simulation, you can obtain propellant mass and acceleration at the point of booster seperation of T+142
https://www.flightclub.io/results/?id=f18e450d-6562-4a57-ab0b-334977993d3a&code=FHD1

I did the rough math in the L2 thread, but based off a sim that is stored there, the Flight Club sim is slightly different and means my old numbers aren't exact. Approximately though, it would have the center core throttle to about 70%, and the boosters shutting off all but 3 engines which are throttled to 70%.

So, theoretically with the boosters matching core acceleration and being "unweighted" the top end of the boosters could push off pivoting about the lower attachment point, before gimballing inward towards the core while releasing to rotate the aft end of the booster away from the core while the core simultaneously throttles up to get away.

Or that's all crazy and would never work, IDK not a rocket scientist.
« Last Edit: 06/30/2017 08:12 PM by GWH »

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 406
  • Orlando, FL
  • Liked: 249
  • Likes Given: 173
Re: Falcon Heavy Separation Method
« Reply #2 on: 07/03/2017 04:42 AM »
Moving this over from the L2 thread but in an edited format:

Using the excellent FlightClub simulation, you can obtain propellant mass and acceleration at the point of booster seperation of T+142
https://www.flightclub.io/results/?id=f18e450d-6562-4a57-ab0b-334977993d3a&code=FHD1

I did the rough math in the L2 thread, but based off a sim that is stored there, the Flight Club sim is slightly different and means my old numbers aren't exact. Approximately though, it would have the center core throttle to about 70%, and the boosters shutting off all but 3 engines which are throttled to 70%.

So, theoretically with the boosters matching core acceleration and being "unweighted" the top end of the boosters could push off pivoting about the lower attachment point, before gimballing inward towards the core while releasing to rotate the aft end of the booster away from the core while the core simultaneously throttles up to get away.

Or that's all crazy and would never work, IDK not a rocket scientist.

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.

Offline GWH

Re: Falcon Heavy Separation Method
« Reply #3 on: 07/03/2017 04:47 AM »
The loads would be less than flight loads at the bottom connection point. If the connection point is a pin and clevis arrangement then the pivot is already there. Side boosters can throttle to where they are just matching core acceleration, so the loading during the pivot shouldn't be high at all since the booster is self supporting.  No other boosters have the thottle range present in 1-9 merlins.

It takes what Musk has said about "flying 3 rockets together in unison" and applies that to seperation.
« Last Edit: 07/03/2017 04:59 AM by GWH »

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 406
  • Orlando, FL
  • Liked: 249
  • Likes Given: 173
Re: Falcon Heavy Separation Method
« Reply #4 on: 07/03/2017 05:07 AM »
The loads would be less than flight loads at the bottom connection point. Side boosters can throttle to where they are just matching core acceleration, so the loading during the pivot shouldn't be high at all.  No other boosters have the thottle range present in 1-9 merlins

I'm not worried about vertical force, I'm thinking about lateral force. If the pivot point is useful it is supporting lateral force, in this case inward force I believe.

I just realized that on the official rendering the bottom connection point is not at the octaweb. It is higher up, presumably near the center of mass of a nearly empty stage. Also, the bottom view appears to show an attachment mechanism that would allow an X axis pivot. That being said, it certainly doesn't look like there is much room for the bottom of the booster to swing toward the center core. Tightly choreographed timing is kinda what they do anyway, so a fraction of a degree of tilt before the bottom points detach is totally possible. A quick quiver of the TVC timed with the upper and lower attachment releases does seem plausible.

This method is just so different than what anyone else does that it seems very risky. Getting those engines to stay plenty clear of each other at separation seems like a tall order without separation motors.

Offline GWH

Re: Falcon Heavy Separation Method
« Reply #5 on: 07/03/2017 05:13 AM »
I wouldn't use the renderings to try and analyze this.

Online yokem55

  • Full Member
  • ***
  • Posts: 393
  • Oregon (Ore-uh-gun dammit)
  • Liked: 202
  • Likes Given: 12
Re: Falcon Heavy Separation Method
« Reply #6 on: 07/03/2017 06:40 AM »


The loads would be less than flight loads at the bottom connection point. Side boosters can throttle to where they are just matching core acceleration, so the loading during the pivot shouldn't be high at all.  No other boosters have the thottle range present in 1-9 merlins
This method is just so different than what anyone else does that it seems very risky. Getting those engines to stay plenty clear of each other at separation seems like a tall order without separation motors.

Didn't the shuttle SRB's have some residual thurst at separation that sent them to higher trajectories?

Offline TomH

  • Full Member
  • ****
  • Posts: 1972
  • CA
  • Liked: 652
  • Likes Given: 195
Re: Falcon Heavy Separation Method
« Reply #7 on: 07/03/2017 06:58 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.

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 406
  • Orlando, FL
  • Liked: 249
  • Likes Given: 173
Re: Falcon Heavy Separation Method
« Reply #8 on: 07/03/2017 08:29 AM »
I wouldn't use the renderings to try and analyze this.

We don't have much else. That's why I'm so curious about the separation method. It seems like a significant engineering task.

Offline old_sellsword

  • Full Member
  • ****
  • Posts: 515
  • Liked: 419
  • Likes Given: 373
Re: Falcon Heavy Separation Method
« Reply #9 on: 07/03/2017 01:05 PM »
I wouldn't use the renderings to try and analyze this.

We don't have much else. That's why I'm so curious about the separation method. It seems like a significant engineering task.

The connection point is not where that render shows it to be. There are three connections on the octaweb and two on the nosecone for each booster.

Offline gospacex

  • Senior Member
  • *****
  • Posts: 3030
  • Liked: 528
  • Likes Given: 604
Re: Falcon Heavy Separation Method
« Reply #10 on: 07/03/2017 01:25 PM »
I would design the connection so that it detaches boosters and give them slight outward nudge on the nose, without any active mechanisms, when booster acceleration falls below core accel (and stays below - of course, the design needs to be resistant to vibration). IIRC R7 boosters do this.

A very simple example is any ordinary door. Most doors would fall off the frame if you turn the frame upside down, reversing gravity's acceleration force on the door.
« Last Edit: 07/03/2017 01:28 PM by gospacex »

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
Re: Falcon Heavy Separation Method
« Reply #11 on: 07/03/2017 02:46 PM »
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

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
Re: Falcon Heavy Separation Method
« Reply #12 on: 07/03/2017 02:47 PM »

Didn't the shuttle SRB's have some residual thurst at separation that sent them to higher trajectories?

It wasn't the thrust but the velocity that they were traveling at that kept them going higher

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
Re: Falcon Heavy Separation Method
« Reply #13 on: 07/03/2017 02:49 PM »
I would design the connection so that it detaches boosters and give them slight outward nudge on the nose, without any active mechanisms, when booster acceleration falls below core accel (and stays below - of course, the design needs to be resistant to vibration). IIRC R7 boosters do this.


R7 boosters are not applicable example.  They push from the very tip and it is like a ball and socket joint and they just fall out.

Offline StuffOfInterest

  • Full Member
  • ***
  • Posts: 368
  • Just interested in space
  • McLean, Virginia, USA
  • Liked: 78
  • Likes Given: 55
Re: Falcon Heavy Separation Method
« Reply #14 on: 07/03/2017 04:19 PM »
How do the Delta IV Heavy side boosters detach currently?  That seems to be the closest layout to what Falcon Heavy will use.

I'm curious regarding something a little more unique to the Falcon Heavy.  If the side boosters are going to do a boost back to the landing site, does it make sense to shut the engines down completely, separate, flip, and then fire three engines back up?  Maybe the side boosters can detach with three engines still running, perhaps just throttled down, flip and boost back.  Avoiding a shutdown and restart has to have some benefit for reliability, but could the separation be done safely with thrust still happening on the sides?

Offline Mader Levap

  • Full Member
  • ****
  • Posts: 909
  • Liked: 366
  • Likes Given: 358
Re: Falcon Heavy Separation Method
« Reply #15 on: 07/03/2017 06:33 PM »
I wouldn't use the renderings to try and analyze this.
We do not have anything better (until we see actual FH photos), so...
Be successful.  Then tell the haters to (BLEEP) off. - deruch
...and if you have failure, tell it anyway.

Offline matthewkantar

  • Full Member
  • ****
  • Posts: 635
  • Liked: 397
  • Likes Given: 390
Re: Falcon Heavy Separation Method
« Reply #16 on: 07/03/2017 06:44 PM »
I wouldn't use the renderings to try and analyze this.
We do not have anything better (until we see actual FH photos), so...

We do have pictures of actual Falcon Heavy boosters on the test stand at Mcgregor.

Matthew

Online Lars-J

  • Senior Member
  • *****
  • Posts: 3294
  • California
  • Liked: 2561
  • Likes Given: 1568
Re: Falcon Heavy Separation Method
« Reply #17 on: 07/03/2017 06:45 PM »
I wouldn't use the renderings to try and analyze this.
We do not have anything better (until we see actual FH photos), so...

But we do. We know that there will be a connection between the octawebs. (we have seen modified FH core octaweb). We also know that there will be a connection in the nose cone and insterstage area. So that already contradicts those renderings.

EDIT: Added two images...
1. The FH wind tunnel model, showing the connections (3 points at the top of the boosters, 3 or 2 points at the octaweb)
2. A picture of the strengthened FH core octaweb, and one of the side booster connection point. (at 2:30pm clockwise)
« Last Edit: 07/03/2017 06:50 PM by Lars-J »

Offline old_sellsword

  • Full Member
  • ****
  • Posts: 515
  • Liked: 419
  • Likes Given: 373
Re: Falcon Heavy Separation Method
« Reply #18 on: 07/03/2017 06:50 PM »
I wouldn't use the renderings to try and analyze this.
We do not have anything better (until we see actual FH photos), so...

We do have pictures of actual Falcon Heavy boosters on the test stand at Mcgregor.

Matthew

Sure, but those don't help us obtain information about the separation mechanisms since the hardware isn't installed or even visible on the test stand.

We do however have at least three really good pictures of a bare FH center core octaweb, from when 1027 was sitting outside Hawthorne last summer. You can see two integrated plates on each side for the pusher mechanisms, and the hold-down lugs on the "sides" (90 and 270) of the octaweb are different than standard ones on an F9 octaweb, implying connection to the side boosters' octawebs.

Offline Eerie

  • Member
  • Full Member
  • ****
  • Posts: 740
  • Liked: 104
  • Likes Given: 6
Re: Falcon Heavy Separation Method
« Reply #19 on: 07/03/2017 06:56 PM »
Crazy question: could you help the boosters separate by spinning the rocket along the axis of flight?

Online Lars-J

  • Senior Member
  • *****
  • Posts: 3294
  • California
  • Liked: 2561
  • Likes Given: 1568
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

  • Senior Member
  • *****
  • Posts: 3294
  • California
  • Liked: 2561
  • Likes Given: 1568
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

  • Full Member
  • ****
  • Posts: 515
  • Silicon Valley, CA
  • Liked: 1506
  • Likes Given: 1025
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

  • Member
  • Posts: 19
  • Liked: 2
  • Likes Given: 18
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

  • Member
  • Full Member
  • ****
  • Posts: 740
  • Liked: 104
  • Likes Given: 6
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

  • Full Member
  • ****
  • Posts: 1972
  • CA
  • Liked: 652
  • Likes Given: 195
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

  • Full Member
  • ***
  • Posts: 336
  • Austria
  • Liked: 62
  • Likes Given: 36
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

  • Member
  • Posts: 84
  • Liked: 13
  • Likes Given: 28
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

  • Full Member
  • *
  • Posts: 196
  • Liked: 185
  • Likes Given: 25
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.....

Offline 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

  • Member
  • Posts: 84
  • Liked: 13
  • Likes Given: 28
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  ;)

Offline matthewkantar

  • Full Member
  • ****
  • Posts: 635
  • Liked: 397
  • Likes Given: 390
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

  • Full Member
  • *
  • Posts: 196
  • Liked: 185
  • Likes Given: 25
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

  • Full Member
  • ****
  • Posts: 515
  • Liked: 419
  • Likes Given: 373
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).

Offline 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

  • Member
  • Posts: 84
  • Liked: 13
  • Likes Given: 28
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.

Offline 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

  • Full Member
  • ****
  • Posts: 515
  • Liked: 419
  • Likes Given: 373
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

  • Full Member
  • *
  • Posts: 196
  • Liked: 185
  • Likes Given: 25
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

  • Member
  • Posts: 84
  • Liked: 13
  • Likes Given: 28
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

Offline old_sellsword

  • Full Member
  • ****
  • Posts: 515
  • Liked: 419
  • Likes Given: 373
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
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

  • Full Member
  • *
  • Posts: 196
  • Liked: 185
  • Likes Given: 25
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.

Online Lars-J

  • Senior Member
  • *****
  • Posts: 3294
  • California
  • Liked: 2561
  • Likes Given: 1568
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 »

Offline envy887

  • Senior Member
  • *****
  • Posts: 2310
  • Liked: 1004
  • Likes Given: 674
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
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?

Offline envy887

  • Senior Member
  • *****
  • Posts: 2310
  • Liked: 1004
  • Likes Given: 674
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
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 »

Offline envy887

  • Senior Member
  • *****
  • Posts: 2310
  • Liked: 1004
  • Likes Given: 674
Re: Falcon Heavy Separation Method
« Reply #49 on: 07/05/2017 04:36 AM »
Works fine for second stage separation.

Offline intrepidpursuit

  • Full Member
  • ****
  • Posts: 406
  • Orlando, FL
  • Liked: 249
  • Likes Given: 173
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.

Offline meekGee

  • Senior Member
  • *****
  • Posts: 7489
  • N. California
  • Liked: 3818
  • Likes Given: 803
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
Re: Falcon Heavy Separation Method
« Reply #52 on: 07/05/2017 03:57 PM »
Works fine for second stage separation.

Not the same requirements. 

Offline guckyfan

  • Senior Member
  • *****
  • Posts: 6240
  • Germany
  • Liked: 1547
  • Likes Given: 1324
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.

Offline LouScheffer

  • Full Member
  • ****
  • Posts: 1517
  • Liked: 1667
  • Likes Given: 190
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.


Offline Semmel

  • Full Member
  • ****
  • Posts: 1002
  • Germany
  • Liked: 695
  • Likes Given: 2062
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
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

Online Lars-J

  • Senior Member
  • *****
  • Posts: 3294
  • California
  • Liked: 2561
  • Likes Given: 1568
Re: Falcon Heavy Separation Method
« Reply #57 on: 07/05/2017 09:32 PM »
 ??? Jim, taking cryptic comments to the next level.

Offline envy887

  • Senior Member
  • *****
  • Posts: 2310
  • Liked: 1004
  • Likes Given: 674
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

  • Full Member
  • *
  • Posts: 136
  • Liked: 153
  • Likes Given: 25
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.

Offline dcporter

  • Full Member
  • ****
  • Posts: 860
  • Liked: 222
  • Likes Given: 387
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

  • Full Member
  • ****
  • Posts: 406
  • Orlando, FL
  • Liked: 249
  • Likes Given: 173
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

  • Senior Member
  • *****
  • Posts: 7489
  • N. California
  • Liked: 3818
  • Likes Given: 803
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.
ABCD - Always Be Counting Down

Offline envy887

  • Senior Member
  • *****
  • Posts: 2310
  • Liked: 1004
  • Likes Given: 674
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

  • Member
  • Posts: 87
  • Southeastern Virginia
  • Liked: 97
  • Likes Given: 106
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

  • Full Member
  • ****
  • Posts: 1517
  • Liked: 1667
  • Likes Given: 190
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

  • Senior Member
  • *****
  • Posts: 7489
  • N. California
  • Liked: 3818
  • Likes Given: 803
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.

ABCD - Always Be Counting Down

Online AncientU

  • Senior Member
  • *****
  • Posts: 4520
  • Liked: 2659
  • Likes Given: 3757
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.
"If we shared everything [we are working on] people would think we are insane!"
-- SpaceX friend of mlindner

Offline JamesH65

  • Full Member
  • ****
  • Posts: 576
  • Liked: 327
  • Likes Given: 7
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.

Offline Norm38

  • Full Member
  • ****
  • Posts: 909
  • Liked: 303
  • Likes Given: 442
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

  • Member
  • Full Member
  • ****
  • Posts: 535
  • New Jersey
  • Liked: 209
  • Likes Given: 45
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

  • Full Member
  • ****
  • Posts: 1002
  • Germany
  • Liked: 695
  • Likes Given: 2062
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.

Offline Lar

  • Fan boy at large
  • Global Moderator
  • Senior Member
  • *****
  • Posts: 8154
  • Saw Gemini live on TV
  • A large LEGO storage facility ... in Michigan
  • Liked: 4903
  • Likes Given: 3328
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. :)
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline georgegassaway

  • Full Member
  • *
  • Posts: 160
    • George's Rockets
  • Liked: 168
  • Likes Given: 57
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

  • Full Member
  • ****
  • Posts: 424
  • Liked: 258
  • Likes Given: 26
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

  • Full Member
  • *
  • Posts: 136
  • Liked: 153
  • Likes Given: 25
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

  • Night Gator
  • Senior Member
  • *****
  • Posts: 31130
  • Cape Canaveral Spaceport
  • Liked: 9388
  • Likes Given: 296
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

  • Member
  • Full Member
  • ****
  • Posts: 740
  • Liked: 104
  • Likes Given: 6
Re: Falcon Heavy Separation Method
« Reply #77 on: 07/11/2017 03:55 PM »
Just spin the rocket and let the boosters go. :-)

Tags: