Falcon Heavy Separation Method

[intrepidpursuit]

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?

[GWH]

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.

[intrepidpursuit]

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.

[GWH]

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.

[intrepidpursuit]

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.

[GWH]

I wouldn't use the renderings to try and analyze this.

[yokem55]

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?

[TomH]

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.

[intrepidpursuit]

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.

[old_sellsword]

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.

[gospacex]

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.

[Jim]

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

[Jim]

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

[Jim]

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.

[StuffOfInterest]

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?

[Mader Levap]

I wouldn't use the renderings to try and analyze this.

We do not have anything better (until we see actual FH photos), so...

[matthewkantar]

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

[Lars-J]

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)

[old_sellsword]

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" (90º and 270º) of the octaweb are different than standard ones on an F9 octaweb, implying connection to the side boosters' octawebs.

[Eerie]

Crazy question: could you help the boosters separate by spinning the rocket along the axis of flight?

[Lars-J]

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)

[Lars-J]

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.

[Jdeshetler]

Click on black box to run GIF.

PR animation by SpaceX.

[octavo]

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

[Eerie]

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.

[TomH]

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)

[Hotblack Desiato]

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).

[jak Kennedy]

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)

[Welsh Dragon]

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.....

[GWH]

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.

[jak Kennedy]

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 

[matthewkantar]

The grid fins not are going to do anything before or during booster separation.

Matthew

[Welsh Dragon]

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

[old_sellsword]

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).

[GWH]

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.

[jak Kennedy]

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.

[GWH]

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.

[old_sellsword]

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.

[Welsh Dragon]

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.

[jak Kennedy]

Great photo of the wind tunnel model and with a wider fairing although probably already discussed. Which forum was this originally posted in?

Thanks

[old_sellsword]

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.

[Jim]

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

[Welsh Dragon]

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).

[GWH]

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.

[Lars-J]

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.

[envy887]

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.

[Jim]

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?

[envy887]

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.

[Jim]

He isnt used at high pressure directly.   It is regulated down

[envy887]

Works fine for second stage separation.

[intrepidpursuit]

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.

[meekGee]

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?

[Jim]

Works fine for second stage separation.

Not the same requirements. 

[guckyfan]

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.

[LouScheffer]

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.

[Semmel]

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!

[Jim]

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

[Lars-J]

  Jim, taking cryptic comments to the next level.

[envy887]

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.

[laszlo]

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.

[dcporter]

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

[intrepidpursuit]

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.

[meekGee]

Jim, taking cryptic comments to the next level.

Not everything that's undecipherable is cryptic.

[envy887]

Jim, taking cryptic comments to the next level.

Not everything that's undecipherable is cryptic.

We've finally driven Jim mad...

[Perchlorate]

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.

[LouScheffer]

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.

[meekGee]

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.

[AncientU]

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.

[JamesH65]

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.

[Norm38]

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.

[Mike_1179]

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.

[Semmel]

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.

[Lar]

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.

[georgegassaway]

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)

[vanoord]

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.

[laszlo]

...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.

[Jim]

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.

[Eerie]

Just spin the rocket and let the boosters go. :-)

[intrepidpursuit]

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.

[Jcc]

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.

[rpapo]

You forget one thing: only three of the nine engines are set up for restart.

[Herb Schaltegger]

So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

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

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

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

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

[Jcc]

So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

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

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

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

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

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

[aero]

So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

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

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

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

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

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

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

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

[Cherokee43v6]

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

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

[intrepidpursuit]

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

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

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

[Semmel]

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

[rakaydos]

Just spin the rocket and let the boosters go. :-)

https://xkcd.com/1244/

[Eerie]

Just spin the rocket and let the boosters go. :-)

https://xkcd.com/1244/

Centrifugal force works everywhere. :-)

[Kaputnik]

So for one, they need some rear attachment system on the Octawebs that will either allow the outer boosters to pivot outwards, or for the rear attachments to actively push the rear of the boosters outwards during separation (but at less of an acceleration rate than the noses are pushed outwards, to produce an outwards yaw rotation rate).  While the forward attachments will actively push the noses outwards pneumatically.

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

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

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

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

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

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

[StuffOfInterest]

Since everyone else is throwing out ideas, might as well add mine in.

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

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

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

[rpapo]

I don't recall off the top of my head if the boostback is single engine or three engine.

Boostback starts with one motor, then adds in two more, then shuts down in reverse order.  Just like the entry burn.

[Mike_1179]

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

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

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

[the_other_Doug]

And what kind of aeroloads will FH be seeing at side booster separation?  Will the trajectory be lofted simply to get out of most of the sensible atmosphere before booster sep?

[envy887]

And what kind of aeroloads will FH be seeing at side booster separation?  Will the trajectory be lofted simply to get out of most of the sensible atmosphere before booster sep?

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

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

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

OneSpeed's FH sim shows a flattish trajectory, 69 km staging, with a dynamic pressure at staging about 1% of MaxQ.
http://forum.nasaspaceflight.com/index.php?topic=42389.msg1742890#msg1742890

[Lar]

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

I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

[old_sellsword]

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

I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

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

[Lars-J]

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

I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

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

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

[Wolfram66]

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

I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

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

[hkultala]

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

I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

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

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

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


So practically:

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


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

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


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

[spacenut]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in. 

[Lars-J]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Yes. And was this in response to a specific point?

[hkultala]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Not 1-2 minutes for launches with center core RTLS.

One minute more flying time for center core means about 100km more distance for the core before the boostback burn can begin, meaning about 100km longer way back home, requiring MUCH more fuel for the boostback burn.

[mme]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Yes. And was this in response to a specific point?

Yes, if the core throttles down after liftoff then the side booster separation is likely earlier than single stick separation. So lower and slower.

[mme]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Not 1-2 minutes for launches with center core RTLS.

One minute more flying time for center core means something like 100km more distance for the core before the boostback burn can begin, meaning about 100km longer way back home, requiring MUCH more fuel for the boostback burn.

Ignoring the SpaceX promotional video, I don't think all 3 cores RTLS is common.

[hkultala]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Yes. And was this in response to a specific point?

Yes, if the core throttles down after liftoff then the side booster separation is likely earlier than single stack separation. So lower and slower.

FH has so much higher T/W that it would have to throttle very deeply to have effective average T/W worse than F9 and stage lower and slower.


And that deep throttling makes absolutely no sense with center core RTLS.

The center core will typically throttle mildly, and the T/W of FH will still be higher than T/W of F9, so that it will stage higher.

[Lars-J]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Yes. And was this in response to a specific point?

Yes, if the core throttles down after liftoff then the side booster separation is likely earlier than single stick separation. So lower and slower.

Yes, but it is still accelerating a much lighter relative mass - FH's larger payload is dwarfed by the mass of the fully loaded upper stage - in the 110t range. 

For RTLS:
- F9, max upper stage + payload mass for F9: ~120t
- FH (boosters RTLS, center ASDS), max upper stage + payload mass: ~150t?


Even at liftoff, an F9 has a T/W ratio of ~1.33... FH will have the same, even if it lifts off with the center core at 50% thrust! So this should show you that it will go faster/higher before booster staging.

[hkultala]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Not 1-2 minutes for launches with center core RTLS.

One minute more flying time for center core means something like 100km more distance for the core before the boostback burn can begin, meaning about 100km longer way back home, requiring MUCH more fuel for the boostback burn.

Ignoring the SpaceX promotional video, I don't think all 3 cores RTLS is common.

3 core RTLS is the cheapest recovery option.

And they will use the cheapest recovery option the mission allows.

Most FH missions will not be launching 16 tonne satellites to GTO, or 14 tonne payloads towards mars, or 40 tonne station parts to LEO. There just are not much this kind of missions.

They will be launching 6-8 tonne satellites to GTO. And maybe also 20-tonne spy satellites to LEO.
The ones F9 cannot do while recovering the first stage.

And 3-core RTLS is enough for these. So 3-core RTLS will be the MOST COMMON recovery method for FH.

[envy887]

I thought after liftoff, the core engines would throttle down to conserve fuel while the side boosters would throttle up to maximum.  Then at separation, the core would throttle up while the side boosters separate and fly away from the core.  The core would continue to burn another 1-2 minutes before second stage kicked in.

Not 1-2 minutes for launches with center core RTLS.

One minute more flying time for center core means something like 100km more distance for the core before the boostback burn can begin, meaning about 100km longer way back home, requiring MUCH more fuel for the boostback burn.

Ignoring the SpaceX promotional video, I don't think all 3 cores RTLS is common.

OneSpeed's sim shows 34 tonnes to LEO which is 14 tonnes to GTO with the center core landing downrange. My calculations using the simulated mass and velocity at staging (205.2 tonnes and 3248 m/s) indicate that for the center core to RTLS it will have to stage 870 m/s earlier, lowering the payload to LEO to about 25.5 tonnes and the payload to GTO to about 9.5 tonnes.

Virtually all GTO payloads are less than 9.5 tonnes, so this seems like a feasible option should SpaceX want to use it. However, it might be much easier on the center booster to use that performance margin for a longer entry retroburn to reduce entry heating.

[mme]

...
Yes, if the core throttles down after liftoff then the side booster separation is likely earlier than single stick separation. So lower and slower.

Yes, but it is still accelerating a much lighter relative mass - FH's larger payload is dwarfed by the mass of the fully loaded upper stage - in the 110t range. 

For RTLS:
- F9, max upper stage + payload mass for F9: ~120t
- FH (boosters RTLS, center ASDS), max upper stage + payload mass: ~150t?


Even at liftoff, an F9 has a T/W ratio of ~1.33... FH will have the same, even if it lifts off with the center core at 50% thrust! So this should show you that it will go faster/higher before booster staging.

...

3 core RTLS is the cheapest recovery option.

And they will use the cheapest recovery option the mission allows.

Most FH missions will not be launching 16 tonne satellites to GTO, or 14 tonne payloads towards mars, or 40 tonne station parts to LEO. There just are not much this kind of missions.

They will be launching 6-8 tonne satellites to GTO. And maybe also 20-tonne spy satellites to LEO.
The ones F9 cannot do while recovering the first stage.

And 3-core RTLS is enough for these. So 3-core RTLS will be the MOST COMMON recovery method for FH.

These are all excellent points and I can't wait to see this thing fly.

[Jcc]

Of course to state the obvious, there are no signs at present of a third landing pad at LZ-1.

If the next mission is STP-2 NET Apr 30, will we see a third pad built for that one?

[old_sellsword]

Of course to state the obvious, there are no signs at present of a third landing pad at LZ-1.

If the next mission is STP-2 NET Apr 30, will we see a third pad built for that one?

They cancelled plans for a third pad and expanded the Dragon area instead. So three-core RTLS is no longer an option.

I really wish we could read the text in their graphic to better understand the elements. Has anyone seen an original for this floating around?

Larger version, plus a site plan attached.

[Lars-J]

Yep. They cold certainly add a 3rd landing pad in the future - but unlike hkultala, I see the FH demo configuration (boosters RTLS and core ASDS) as being the one that will fly the vast majority of FH's payloads.

[Lar]

I expect the proportion of all 3 RTLS to be far lower[1] than booster RTLS and center ASDS. Maybe instead of higher/lower it would be more accurate to speak of faster/slower velocity, as I do think that we'll see more lofted trajectories some times.

Since I expect pretty serious throttledown of the center as soon as a little mass is reduced I think I'm going to stick with my view that the boosters  will stage lower and slower than a single stick, most of the time.... If they had upsized S2, this would be pretty obviously true but even without I think it still is.

1 - possibly zero unless they build another landing pad or land two on one pad.

[hamerad]

I expect the proportion of all 3 RTLS to be far lower[1] than booster RTLS and center ASDS. Maybe instead of higher/lower it would be more accurate to speak of faster/slower velocity, as I do think that we'll see more lofted trajectories some times.

Since I expect pretty serious throttledown of the center as soon as a little mass is reduced I think I'm going to stick with my view that the boosters  will stage lower and slower than a single stick, most of the time.... If they had upsized S2, this would be pretty obviously true but even without I think it still is.

1 - possibly zero unless they build another landing pad or land two on one pad.

I would agree except for the option of having an ASDS just offshore. Just how close would the asds have to be to muddy the waters and make it an ASDS/RTLS

[Robotbeat]

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

I may be confused but I believe that since the FH is a 2.5 stage rocket the boosters will stage lower, not higher than an F9 S1. The center core will stage higher though, I think...

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

Unproven.

[Semmel]

It is pretty obvious that both options are technically possible. Faster and slower staging of the side boosters. Which ever occurs more often is like reading tea leaves and not worth the effort. The question you should be asking in the context of this thread is, does SpaceX want to be able to stage low or are they cool with always staging high? Because that has an impact on the separation method, which is the topic of this thread.

[Kaputnik]

Maybe they will want to look after thise centre cores, since they are specialised and thus rarer, and ASDS landing allows more prop for entry and landing burns, resulting in less damage to the stage.

[Pete]

The Falcon9 Heavy faces some very interesting challenges.
*it needs to liftoff without blowing the launch pad to high heaven. 27 Merlins in one place at one time is serious firepower.
*it has a less aerodynamically forgiving profile. It wants to have higher t/w ratio. The combination of these will make max-q very very interesting.
*did I mention how interesting the superimposed and impinging transonic/low supersonic shockwaves around three parallel cylinders will be?
*A tall narrow cylinder is naturally very strong only in its length. Having differential twr between the side boosters and the core imposes bending moments in exactly the direction that these cylinders are not strong.
*and lastly, the question that this thread is actually about: When you finally Do drop the side boosters, how do you ensure the separation is clean, does not induce transient loads that shatter everything, and ensures no re-contact between anything.

Fortunately, we have Elon Musk's statements on this matter to put our concerns to rest:
"I hope it makes it far enough away from the pad that it does not cause pad damage, I would consider even that a win"
and
"very exciting, major pucker factor, really"

[Jim]

1,  it needs to liftoff without blowing the launch pad to high heaven. 27 Merlins in one place at one time is serious firepower.
2.  it has a less aerodynamically forgiving profile. It wants to have higher t/w ratio. The combination of these will make max-q very very interesting.
3.  did I mention how interesting the superimposed and impinging transonic/low supersonic shockwaves around three parallel cylinders will be?
4.  A tall narrow cylinder is naturally very strong only in its length. Having differential twr between the side boosters and the core imposes bending moments in exactly the direction that these cylinders are not strong.

not really.  All this has been done many times before

1. Previous vehicles that used the pad had higher thrust. 

2 It can throttle and will, hence this will not be an issue. Core is going to throttle down soon after liftoff.   Max q will be similar to other vehicle.

3.  Not a big deal, see Titan III/IV, Delta IV Heavy, STS, etc

4.  There are none.  The attach points are below and above the stage/tanks.  The upper attach points put loads into each other and not the middle vehicle.

[nacnud]

I would agree except for the option of having an ASDS just offshore. Just how close would the asds have to be to muddy the waters and make it an ASDS/RTLS

For the biggest cost savings just land on the ASDS while it is docked...

[the_other_Doug]

As I understand it, unless you run all three cores at full thrust and run through them all at the same time (which is not how SpaceX has said they will launch them), the FH center core is going to be higher and faster than any single-stick F9 core has been before recovery was attempted.  Regardless of how the throttle the center core down and when they separate the side cores.

You would have to leave an awful lot of gas in the core stage's tank to get that back to an RTLS, I would think.  And if you run all the cores at full thrust, due to the very high T/W you would get, you would have to leave an awful lot of gas in *all* of the tanks to get all three cores to RTLS.

I appreciate the concept that recovering all cores RTLS would be economical, but I'm concerned that, if you leave enough gas in the tanks of the stage(s) to accomplish this, you end up with very little greater performance than the Block 5 F9.  In which case, why pay extra for an FH?

Also, I seem to recall SpaceX (both Musk himself, and other SpaceX officials) stating rather certainly -- on a number of occasions -- that on FH, the side boosters will always RTLS, and the core stage will always be recovered (when not expendable) on a droneship.  Seeing that this has always been the stated operational plan, and that there have been no signs of building a third landing pad to accommodate a change to a three-core RTLS plan, I guess I'm not seeing the argument that three-core RTLS is going to be the obvious way they will go, here...

[envy887]

As I understand it, unless you run all three cores at full thrust and run through them all at the same time (which is not how SpaceX has said they will launch them), the FH center core is going to be higher and faster than any single-stick F9 core has been before recovery was attempted.  Regardless of how the throttle the center core down and when they separate the side cores.

You would have to leave an awful lot of gas in the core stage's tank to get that back to an RTLS, I would think.  And if you run all the cores at full thrust, due to the very high T/W you would get, you would have to leave an awful lot of gas in *all* of the tanks to get all three cores to RTLS.

I appreciate the concept that recovering all cores RTLS would be economical, but I'm concerned that, if you leave enough gas in the tanks of the stage(s) to accomplish this, you end up with very little greater performance than the Block 5 F9.  In which case, why pay extra for an FH?

Also, I seem to recall SpaceX (both Musk himself, and other SpaceX officials) stating rather certainly -- on a number of occasions -- that on FH, the side boosters will always RTLS, and the core stage will always be recovered (when not expendable) on a droneship.  Seeing that this has always been the stated operational plan, and that there have been no signs of building a third landing pad to accommodate a change to a three-core RTLS plan, I guess I'm not seeing the argument that three-core RTLS is going to be the obvious way they will go, here...

FH 3x RTLS can undoubtedly launch more than F9 with ASDS landing, and probably more than F9 Block 5 full expendable. If OneSpeed's sim is even close to correct, no current GTO payloads will require a downrange landing for performance reasons.

The economic reason for RTLS is to save wear on the booster, rather than consideration of the cost of sending out the ASDS to catch it. But there is likely a easier trajectory with a partial boostback and shortened downrange landing that allows even less entry heating than a F9 RTLS entry. In the best case, the core stage would probably boostback to completely null downrange velocity and simply fall straight down on the ASDS.

[hkultala]

As I understand it, unless you run all three cores at full thrust and run through them all at the same time (which is not how SpaceX has said they will launch them), the FH center core is going to be higher and faster than any single-stick F9 core has been before recovery was attempted.  Regardless of how the throttle the center core down and when they separate the side cores.

You would have to leave an awful lot of gas in the core stage's tank to get that back to an RTLS, I would think.  And if you run all the cores at full thrust, due to the very high T/W you would get, you would have to leave an awful lot of gas in *all* of the tanks to get all three cores to RTLS.

If not throttling at all.
Due to the very high T/W the very high velocity would also be reached quickly, before the craft has gone much further than F9 goes. And because of leaving slightly more fuel to the tanks for the return trip, the burning time is actually slightly shorter. This means it might not be at all further away when the staging happens, there might be only higher velocity to nullify for boostback, but not longer distance to fly back.

And higher vertical velocity also means more time for the return trip, meaning less horizontal velocity needed for the return trip.

I appreciate the concept that recovering all cores RTLS would be economical, but I'm concerned that, if you leave enough gas in the tanks of the stage(s) to accomplish this, you end up with very little greater performance than the Block 5 F9.  In which case, why pay extra for an FH?

Block 5 falcon 9 on which mode?
Expandable?

Compared to F9 expendable FH RTLS is definitely cheaper, and MAY have better payload

Compared to F9 drone ship - FH RTLS definitely has better payload.


Some calculations (based on 6-tonne payload)

Three F9 cores can give about 2.8 km/s more delta-v to the second stage+payload than one F9 core.
And if three F9 cores lift the second stage to same velocity than expendable F9, they each have about 60 tonnes of fuel left,  which give the 1st stage cores 3.4 km/s delta-v for boostback and landing.

Though these calculations don't take into account the smaller gravity losses of FH, which gives extra edge to FH.

So, if RTLS requires less than 3.4 km/s delta-v, FH RTLS capacity is for sure greater than F9  expendable capacity.

Also, I seem to recall SpaceX (both Musk himself, and other SpaceX officials) stating rather certainly -- on a number of occasions -- that on FH, the side boosters will always RTLS, and the core stage will always be recovered (when not expendable) on a droneship.

People seem to recall many things that actually have not been said. Words like "initially" and "always" get mixed up.

[Hobbes-22]

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

Thust of the SRBs is about 1% of nominal thrust on separation. IIRC the SRBs are separated when stack acceleration falls below a set value.

[lrk]

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

Thust of the SRBs is about 1% of nominal thrust on separation. IIRC the SRBs are separated when stack acceleration falls below a set value.

Separation was actually commanded when the measured booster chamber pressure fell <50psi, but yeah, same idea. 

[nacnud]

From what I can see it looks a bit like the Soyuz booster attachment points. The thrust from the boosters keeps them attached to the middle core which is running at a lower thrust level. The booster are lifting the centre core from the bottom. Once the boosters thrust levels drop then they can fall away.

Obviously more going on than that, hopefully someone will provide more clarity.

[llanitedave]

From what I can see it looks a bit like the Soyuz booster attachment points. The thrust from the boosters keeps them attached to the middle core which is running at a lower thrust level. The booster are lifting the centre core from the bottom. Once the boosters thrust levels drop then they can fall away.

Obviously more going on than that, hopefully someone will provide more clarity.

Does that mean that in the case of engine out on a side booster that the core will have to throttle down further to keep from surpassing the booster and leaving it behind?

[envy887]

From what I can see it looks a bit like the Soyuz booster attachment points. The thrust from the boosters keeps them attached to the middle core which is running at a lower thrust level. The booster are lifting the centre core from the bottom. Once the boosters thrust levels drop then they can fall away.

Obviously more going on than that, hopefully someone will provide more clarity.

Does that mean that in the case of engine out on a side booster that the core will have to throttle down further to keep from surpassing the booster and leaving it behind?

The core will be throttled down by 2 or 3 engine's worth of thrust, and it's pushing 120+ tonnes of upper stage and payload.

[aero]

From what I can see it looks a bit like the Soyuz booster attachment points. The thrust from the boosters keeps them attached to the middle core which is running at a lower thrust level. The booster are lifting the centre core from the bottom. Once the boosters thrust levels drop then they can fall away.

Obviously more going on than that, hopefully someone will provide more clarity.

Does that mean that in the case of engine out on a side booster that the core will have to throttle down further to keep from surpassing the booster and leaving it behind?

No. Remember all of the extra mass on top of the core. In order for the core to surpass the booster, it would need to shoulder all of that mass, less what help the other booster provides. Someone other than me could calculate how many engines the failing booster could loose before being surpassed by the rest of the stack but it is certainly more than one engine and as always, it depends on the time when the failures occur, at lift-off or just before ... what? BECO? (booster engine cut off).

It also depends on the gimbol authority of the surviving booster and the core engines. The booster's plus core thrust vector must pass through the center of mass of the stack. That is easy if both boosters are thrusting equally but if thrust from one drops off then boosters and core need to gimbol engines to counter the unequal thrust magnitude and maintain a vector sum that passes through the center of mass of the stack. (Note that I am using "stack" as the descriptor for the complete FH less the burned fuel-oops- propellant.)

edit, add oops.

[llanitedave]

From what I can see it looks a bit like the Soyuz booster attachment points. The thrust from the boosters keeps them attached to the middle core which is running at a lower thrust level. The booster are lifting the centre core from the bottom. Once the boosters thrust levels drop then they can fall away.

Obviously more going on than that, hopefully someone will provide more clarity.

Does that mean that in the case of engine out on a side booster that the core will have to throttle down further to keep from surpassing the booster and leaving it behind?

No. Remember all of the extra mass on top of the core. In order for the core to surpass the booster, it would need to shoulder all of that mass, less what help the other booster provides. Someone other than me could calculate how many engines the failing booster could loose before being surpassed by the rest of the stack but it is certainly more than one engine and as always, it depends on the time when the failures occur, at lift-off or just before ... what? BECO? (booster engine cut off).

It also depends on the gimbol authority of the surviving booster and the core engines. The booster's plus core thrust vector must pass through the center of mass of the stack. That is easy if both boosters are thrusting equally but if thrust from one drops off then boosters and core need to gimbol engines to counter the unequal thrust magnitude and maintain a vector sum that passes through the center of mass of the stack. (Note that I am using "stack" as the descriptor for the complete FH less the burned fuel.)

Great answer, thanks!

[speedevil]

It also depends on the gimbol authority of the surviving booster and the core engines. The booster's plus core thrust vector must pass through the center of mass of the stack. That is easy if both boosters are thrusting equally but if thrust from one drops off then boosters and core need to gimbol engines to counter the unequal thrust magnitude and maintain a vector sum that passes through the center of mass of the stack. (Note that I am using "stack" as the descriptor for the complete FH less the burned fuel-oops- propellant.)

And for some failures, the non-failed engines on that booster can throttle up/down/gimbal appropriately to have no net change in thrust vector for that booster.
If the '92%' mentioned as thrust level for the FH launch is peak thrust, then it's probable that over most of the flight, for most failures, this is the case, without throttling any engine beyond 102% or so. And for most, not beyond 100%.

[Hog]

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

There used to be a call just prior to SRB sep that was "PC Less than 50"  That meant that there was less than(or equal to) 50psi of chamber pressure inside the SRM, and the SRB was within a suitable thrust profile for SRB separation. This occurred at approx. T+119 seconds.
The "PC less than 50" call at 55:13 video time(2:08 MET) in this STS-1 video.


 Then shortly after the PAO would announce that the Orbiters guidance is correcting for any trajectory errors induced during SRB sep.
To be able to mentally correlate chamber pressure to actual SRM thrust, during STS-35, at the T+ 7.828 mark the RSRM was thrusting with 3,320,780 pounds of thrust, reaching its mission maximum Chamber Pressure of 909.854psi.
At T+19.9682 seconds the booster reached its mission maximum thrust level of 3,369,170 pounds while the measure Chamber Pressure was 887.526psi.

So during STS-35 the "Less Than (or equal to) Chamber Pressure of 50psi" occurred in between these 2 sets of datapoints:(See pic#1)
 MET/T+ 119.28 seconds with the Chamber Pressure of 52.9018psi while producing 869,761 pounds of thrust
 MET/T+ 119.618 seconds with the Chamber Pressure of 46.9326psi while producing 846,514 pounds of thrust


To add detail to some earlier questions.
SRB apogee occurs 75 seconds after SRB sep
"The two SRBs provide 71.4 percent of the thrust at lift- off and during first-stage ascent. Seventy- five seconds after SRB separation, SRB apogee occurs at an altitude of approximately 220,000 feet, or 35 nautical miles (41 statute miles). SRB impact occurs in the ocean approximately 122 nautical miles (141 statute miles) downrange."


SRB sep. motors
"Eight booster separation motors (four in the nose frustum and four in the aft skirt) of each SRB thrust for 1.02 seconds at SRB separation from the external tank. Each solid rocket separation motor is 31.1 inches long and 12.8 inches in diameter."

Left and Right SRB "Less than 50psi" co-ordination
"For STS-135, the left and right SRBs reached the Less than 50psi point within 0.16 seconds of each other, better than the predicted 0.3 seconds that was predicted for this pair of boosters."  Chris Gebhardt July 8th,2012 https://www.nasaspaceflight.com/2012/07/final-flight-superb-performance-sts-135s-srbs/

SRB Separation
"SRB separation is initiated when the three solid rocket motor chamber pressure transducers are processed in the redundancy management middle value select and the head- end chamber pressure of both SRBs is less than or equal to 50 psi. A backup cue is the time elapsed from booster ignition.
The separation sequence is initiated, commanding the thrust vector control actuators to the null position and putting the main propulsion system into a second-stage configuration (0.8 second from sequence initialization), which ensures the thrust of each SRB is less than 100,000 pounds. Orbiter yaw attitude is held for four seconds, and SRB thrust drops to less than 60,000 pounds.

The SRBs separate from the external tank within 30 milliseconds of the ordnance firing command.

The forward attachment point consists of a ball (SRB) and socket (ET) held together by one bolt. The bolt contains one NSD pressure cartridge at each end. The forward attachment point also carries the range safety system cross-strap wiring connecting each SRB RSS and the ET RSS with each other.

The aft attachment points consist of three separate struts: upper, diagonal and lower. Each strut contains one bolt with an NSD pressure cartridge at each end. The upper strut also carries the umbilical interface between its SRB and the external tank and on to the orbiter.

There are four booster separation motors on each end of each SRB. The BSMs separate the SRBs from the external tank. The solid rocket motors in each cluster of four are ignited by firing redundant NSD pressure cartridges into redundant confined detonating fuse manifolds.

The separation commands issued from the orbiter by the SRB separation sequence initiate the redundant NSD pressure cartridge in each bolt and ignite the BSMs to effect a clean separation."

Information content from the NSTS Shuttle Reference Manual (1988) As such, would be info derived from the Redesigned Solid Rocket Motor(RSRM) program, which used the 360º attach rings(were 270º of encirclement) and the new 3 O-ring seal in the Field Joints.
https://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html

Original SRB design STS-1 through STS-7
High-Performance Solid Rocket Motor (HP-SRM) (STS-8-STS-51L) Empty Mass: 145,750 lbs
Redesigned Solid Rocket Motor (RSRM)(STS-26R_RTF-STS-135) double tang3 O-ring field joint,360º attach rings instead of prior 270º, units redesign add reinforcement brackets and fittings in the aft ring of the skirt.

Filament Wound Case(FWC) vs. 1/2" thick steel cases
"In Aerofax Datagraph on the shuttle, Dennis Jenkins states that a FWC-SRM weighted a massive 28,000 lbs less than a steel SRM, however I have also heard quoted that this only translated to an increase in the orbiter's payload of 5000-6000 lbs."

Additional FWC Info
"Notes: Design work began May 1982 as part of long range performance improvement plans for the Shuttle. It would have been used in place of the HP-SRM for special missions requiring increased performance. In place of conventional steel casings, graphite/epoxy filament wound casings would be used, resulting in a mass reduction of 25,000~ lb of the motor’s dry weight, increasing shuttle payload by about 4,600~ pounds. The full scale FWC qualification motor (QM-5) was assembled and ready for firing when Challenger exploded, and the first flight motors were stacked at Vandenberg AFB, for the July 1986 flight of STS-62A."

4 segment SRB weights:
Empty: 193,000 pounds
 Propellant: 1,107,000 pounds
 Gross: 1,300,000 pounds


Details of side booster release of Space Explorations "Falcon Heavy."
 1. Federal Aviation Administration, Office of Commercial Space Transportation. p. 2-3. Archived from the original on 2013-12-07.
"The center core engines are throttled down after liftoff and up to two engines may be shut down as the vehicle approaches maximum acceleration. After the side boosters drop off, the center core engines throttle back up to full thrust. The center engine in each side core continues to burn for a few seconds after separation to control the trajectory of the booster."

Pic #2 Filament Wound Case SRB ready for test fire