Author Topic: Falcon Heavy Separation Method  (Read 31595 times)

Offline nacnud

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Re: Falcon Heavy Separation Method
« Reply #120 on: 11/08/2017 12:12 PM »
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... :D
« Last Edit: 11/08/2017 12:12 PM by nacnud »

Offline the_other_Doug

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Re: Falcon Heavy Separation Method
« Reply #121 on: 11/08/2017 05:14 PM »
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...
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Online envy887

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Re: Falcon Heavy Separation Method
« Reply #122 on: 11/08/2017 10:35 PM »
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.

Online hkultala

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Re: Falcon Heavy Separation Method
« Reply #123 on: 11/09/2017 09:17 AM »
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.

Quote
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.

Quote

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.
« Last Edit: 11/09/2017 09:46 AM by hkultala »

Offline Hobbes-22

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Re: Falcon Heavy Separation Method
« Reply #124 on: 11/09/2017 10:45 AM »


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.

Offline lrk

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Re: Falcon Heavy Separation Method
« Reply #125 on: 11/09/2017 12:14 PM »


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. 

Offline Demidrol

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Re: Falcon Heavy Separation Method
« Reply #126 on: 12/21/2017 10:37 PM »
How is this separation node at the hold-down lug supposed to work?

Offline nacnud

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Re: Falcon Heavy Separation Method
« Reply #127 on: 12/21/2017 11:23 PM »
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.
« Last Edit: 12/21/2017 11:26 PM by nacnud »

Offline llanitedave

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Re: Falcon Heavy Separation Method
« Reply #128 on: 12/22/2017 01:17 AM »
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?
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Online envy887

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Re: Falcon Heavy Separation Method
« Reply #129 on: 12/22/2017 01:33 AM »
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.

Offline aero

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Re: Falcon Heavy Separation Method
« Reply #130 on: 12/22/2017 01:43 AM »
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.
« Last Edit: 12/22/2017 02:02 AM by aero »
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Offline llanitedave

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Re: Falcon Heavy Separation Method
« Reply #131 on: 12/22/2017 01:49 AM »
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!
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Offline Demidrol

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Re: Falcon Heavy Separation Method
« Reply #132 on: 12/22/2017 12:27 PM »
The booster are lifting the centre core from the bottom. Once the boosters thrust levels drop then they can fall away.
Are there any locks or latches in these nodes? Or booster can separate free after bottom pushers did their job?

Online speedevil

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Re: Falcon Heavy Separation Method
« Reply #133 on: 12/22/2017 02:34 PM »
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%.

Offline Hog

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Re: Falcon Heavy Separation Method
« Reply #134 on: 12/23/2017 11:05 PM »
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 





Paul

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