Author Topic: Falcon 9 3 engine reentry/landing burn advantage.  (Read 11743 times)

Offline IanThePineapple

Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #20 on: 02/16/2018 11:40 PM »
1-3-1 seems ok for missions that need that last extra percent of performance.  But something to be avoided if non needed. 

It adds more events to manage and fault points.

Evident with the FH center core :)

Offline bulkmail

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #21 on: 02/17/2018 07:40 AM »
If increasing the number of engines used in any of the landing phases 1-3-1, 1-3-3, 3-1-3, etc. is beneficial, then what's the reason to use only 3? Why no 5 or 9?

Offline tyrred

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #22 on: 02/17/2018 08:10 AM »
Thrust to weight ratio of a nearly empty booster means more engines than 3 causes too much stress on the vehicle. Oh, and more shots of tea/teb needed to restart each engine. Looking for a solution to a non-existent problem.

Offline rpapo

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #23 on: 02/17/2018 11:02 AM »
If increasing the number of engines used in any of the landing phases 1-3-1, 1-3-3, 3-1-3, etc. is beneficial, then what's the reason to use only 3? Why no 5 or 9?
Using more than three engines ups the acceleration of nearly empty stage so high that things can break.  And it requires extremely precise timing for relatively little gain.  Somebody here did an analysis of the fuel cost for each case, and there was little to gain from using more than three engines.

The more engines you have to restart, the more likely things can go wrong.  As happened with the FH core.

And finally, the reason for starting with one engine before starting the other two is to give the stage control authority just in case the other two engines start unevenly, with the resultant unbalanced thrust.  The already started center engine can gimbal to balance things and retain control.
An Apollo fanboy . . . fifty years ago.

Offline deruch

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #24 on: 02/17/2018 07:32 PM »
Also current thinking is that, in their current incarnation, they are hardware limited to 3 engines because those 3 are the only ones capable of in-air relighting.  All others are lit from ground supplied TEA-TEB and only those 3 have the plumbing to be air lit currently. 
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Offline niwax

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #25 on: 02/17/2018 11:57 PM »
Before they have worked out how to throttle down further, three engines might be the maximum practicable number anyways. Several posts here have estimated TWR for three-engined landings at minimum throttle at around 3.7, any more and you'd ping right back into space if you overstep the tiny margin of error. And only turning on additional engines engines for a second or two during deceleration doesn't seem worth carrying the extra equipment and might not be possible with the needed precision.

Online wannamoonbase

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #26 on: 02/18/2018 01:04 AM »
Before they have worked out how to throttle down further, three engines might be the maximum practicable number anyways. Several posts here have estimated TWR for three-engined landings at minimum throttle at around 3.7, any more and you'd ping right back into space if you overstep the tiny margin of error. And only turning on additional engines engines for a second or two during deceleration doesn't seem worth carrying the extra equipment and might not be possible with the needed precision.

In mission profiles where there is enough fuel margin to run a single engine landing burn I'd expect that's what they'll use.  Why add the complication and engine starts if not needed.

Block 5 should be a different animal than what we have gotten use too in the last couple years.
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Online Lar

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #27 on: 02/18/2018 02:17 AM »
apparently, some are saying, the 1-3-1 will be always used, as it simplifies things to always use the same pattern
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"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline acsawdey

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #28 on: 02/18/2018 03:16 AM »
apparently, some are saying, the 1-3-1 will be always used, as it simplifies things to always use the same pattern

They seem to have gotten most of the bugs out because the FH side boosters both did it successfully -- you can see visually just how hard they are decelerating during the 3-engine phase in the videos shot from LC-37.

Offline LouScheffer

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #29 on: 02/18/2018 04:35 AM »
The 3 engine landing as done on this mission does not seem to save all that much.  The single engine ignites at 7:48 and shuts off at 8:05, for 17 engine-seconds.  The two side engines run for 4.5 seconds each, for 9 engine-seconds, or 26 engine-seconds total.  The single engine landings run for 30 seconds.  An engine takes about 270 kg/sec at full throttle, so 4 engine-sec gives savings of 1000 kg.  This would translate to less than 100 kg of extra payload, on the single stick version.

Alternatively, for the same mass, saving 4 engine-seconds on landing gives less than 1/2 second of additional boost, or less than 25 m/s.   

This hardly seems worthwhile given the additional complexity. and failure modes.  I suspect they will try a more aggressive profile later, starting the 3 engine part earlier and extending it closer to landing.

Online Lar

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #30 on: 02/18/2018 04:56 AM »
I think the savings is not for the payload but rather for use with a better entry burn, which I think is higher leverage. (yes, it's all kind of the same tradespace)
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #31 on: 02/18/2018 05:00 AM »
"An engine takes about 270 kg/sec at full throttle"

This is where your comparison breaks down. They throttle the engine quite deep when landing. It's certainly not full power! With 3 engines, we can assume even less about the throttle levels are...

A quick look suggests minimum throttle levels of at least 70%, and perhaps even down to 40%, but that may only be for the vacuum variant.
« Last Edit: 02/18/2018 05:02 AM by Nerva »

Offline Semmel

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #32 on: 02/18/2018 10:13 AM »
"An engine takes about 270 kg/sec at full throttle"

This is where your comparison breaks down. They throttle the engine quite deep when landing. It's certainly not full power! With 3 engines, we can assume even less about the throttle levels are...

A quick look suggests minimum throttle levels of at least 70%, and perhaps even down to 40%, but that may only be for the vacuum variant.

As far as I know, they dont throttle as much as you think. Early analysis after the first few landing (attempts) indicated that they run the engines at about 90% to 95% during landing. They dont have much head room to increase thrust. This is done to reduce the fuel consumption obviously. This statement is from memory because I was quite surprised my self when I learned it. I dont have an update for the 1-3-1 landing profile though.

Offline Adriano

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #33 on: 02/19/2018 01:31 AM »
When you throttle down an engine you reduce the chamber pressure and the performance of the engine deteriorates sharply. More fuel efficient to use less engines at full trust than throttling down engines.

Offline cppetrie

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #34 on: 02/19/2018 01:46 AM »
When you throttle down an engine you reduce the chamber pressure and the performance of the engine deteriorates sharply. More fuel efficient to use less engines at full trust than throttling down engines.
I could see the hypersonic retropropulsive environment increasing the chances for problems when the chamber pressure is lower as well. Just because it can throttle down on the test stand doesn’t mean it can do so in the booster return environment. I could be wrong though. I am far from being a rocket scientist, and this is actual rocket science.

Online Lars-J

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #35 on: 02/19/2018 07:24 AM »
"An engine takes about 270 kg/sec at full throttle"

This is where your comparison breaks down. They throttle the engine quite deep when landing. It's certainly not full power! With 3 engines, we can assume even less about the throttle levels are...

A quick look suggests minimum throttle levels of at least 70%, and perhaps even down to 40%, but that may only be for the vacuum variant.

The regular M1D can also throttle down to 40%.

Offline Demidrol

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #36 on: 03/16/2018 12:48 PM »
It's been the standard landing burn, for both RTLS and ASDS landings, for a little while now.  Nothing new about it.
On what missions except FH Demo did they use the 3 engines landing burn for RTLS?
« Last Edit: 03/16/2018 12:51 PM by Demidrol »

Online Lars-J

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #37 on: 03/16/2018 07:15 PM »
It's been the standard landing burn, for both RTLS and ASDS landings, for a little while now.  Nothing new about it.
On what missions except FH Demo did they use the 3 engines landing burn for RTLS?

I think the FH side boosters were the first use of 1-3-1 landing burns for RTLS.

Offline Craig_VG

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #38 on: 03/16/2018 07:31 PM »
Here are 2 screen caps from the SpaceX video during landing.  It's a bit hard to see in video because the camera has trouble tracking.  But if you watch at 0.25x speed and start at T+07:49, you'll clearly see that there is a very short period of 3-engine burn in the middle there.  This also tracks with the fact that the landing burn is short.  The length of which differs from single engine landing burns which last ~30 seconds.

Here's a wider shot I took at the launch for some perspective, see one booster has three going and the other just one.

And another with both boosters with three engines running (and some TEA-TEB being burned off!)

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