Author Topic: Falcon Heavy Separation Method  (Read 24855 times)

Offline intrepidpursuit

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Falcon Heavy Separation Method
« on: 06/30/2017 07:49 PM »
As we are getting close to FH debut, do we know anything about how the boosters will separate from the central core? The boosters already have N2 thrusters at the top, but not at the bottom. Will the N2 thrusters be enough?

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

Any thoughts or sources on how the separation will occur?

Offline GWH

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

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

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

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

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

Offline intrepidpursuit

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Re: Falcon Heavy Separation Method
« Reply #2 on: 07/03/2017 04:42 AM »
Moving this over from the L2 thread but in an edited format:

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

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

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

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

Is it totally crazy to separate a booster using only rocket power, and to pivot on an attachment point while under power? It meets the simplicity requirement SpaceX seems to prefer, but they are as limited by physics as everyone else. It seems like the stress put on that bottom attachment joint would be incredibly high and it would have to pivot as well.

Offline GWH

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

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

Offline intrepidpursuit

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

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

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

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

Offline GWH

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

Offline yokem55

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Re: Falcon Heavy Separation Method
« Reply #6 on: 07/03/2017 06:40 AM »


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

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

Offline TomH

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Re: Falcon Heavy Separation Method
« Reply #7 on: 07/03/2017 06:58 AM »
Didn't the shuttle SRB's have some residual thurst at separation that sent them to higher trajectories?

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

Offline intrepidpursuit

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Re: Falcon Heavy Separation Method
« Reply #8 on: 07/03/2017 08:29 AM »
I wouldn't use the renderings to try and analyze this.

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

Offline old_sellsword

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Re: Falcon Heavy Separation Method
« Reply #9 on: 07/03/2017 01:05 PM »
I wouldn't use the renderings to try and analyze this.

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

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

Offline gospacex

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

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

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #11 on: 07/03/2017 02:46 PM »
Didn't the shuttle SRB's have some residual thurst at separation that sent them to higher trajectories?

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

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

Offline Jim

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Re: Falcon Heavy Separation Method
« Reply #12 on: 07/03/2017 02:47 PM »

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

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

Offline Jim

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


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

Online StuffOfInterest

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Re: Falcon Heavy Separation Method
« Reply #14 on: 07/03/2017 04:19 PM »
How do the Delta IV Heavy side boosters detach currently?  That seems to be the closest layout to what Falcon Heavy will use.

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

Offline Mader Levap

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

Offline matthewkantar

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

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

Matthew

Online Lars-J

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

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

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

Offline old_sellsword

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

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

Matthew

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

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

Online Eerie

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Re: Falcon Heavy Separation Method
« Reply #19 on: 07/03/2017 06:56 PM »
Crazy question: could you help the boosters separate by spinning the rocket along the axis of flight?

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