Author Topic: Falcon 9 3 engine reentry/landing burn advantage.  (Read 6023 times)

Online speedevil

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I can't find a thread in which this might have been discussed.

From the very simplest assumptions, if the landing burn is 30s and goes to 10s on three engines, you may get 200m/s less gravity loss. This is approximately two tons of fuel at landing, and backing this out to before SECO, I come up with around perhaps 40m/s extra delta-v on the first stage.

(there are many approximations in this).

Has anyone done a more fully worked approximation for how many kilos of extra payload a 3 engine reentry and landing burn may get without worsening the current reusability penalty?

Offline nacnud

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #1 on: 02/01/2018 07:33 PM »
You'll be falling through more atmosphere too, getting more braking effect and lower (pre landing burn) terminal velocity.

Online speedevil

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #2 on: 02/01/2018 07:34 PM »
You'll be falling through more atmosphere too, getting more braking effect and lower (pre landing burn) terminal velocity.

Quite - I have no clue how to approximate that though.

Offline whitelancer64

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Online Adriano

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #4 on: 02/01/2018 08:51 PM »
People speculate that using three engines to land used less fuel than using only one. I do not think that is correct. To stop the rocket you need a certain impulse: trust times length of burn. Three engine will burn per second three times the fuel needed by a single engine, but the burn will last only a third of the time to produce the same impulse, so no fuel savings. I wonder if the savings comes from the fuel left in the pipes from fuel tank to each engine. To use that fuel, you need to fire multiple engines after the tank is empty...any better explanation?

Offline rpapo

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #5 on: 02/01/2018 08:52 PM »
People speculate that using three engines to land used less fuel than using only one. I do not think that is correct. To stop the rocket you need a certain impulse: trust times length of burn. Three engine will burn per second three times the fuel needed by a single engine, but the burn will last only a third of the time to produce the same impulse, so no fuel savings. I wonder if the savings comes from the fuel left in the pipes from fuel tank to each engine. To use that fuel, you need to fire multiple engines after the tank is empty...any better explanation?
You're forgetting gravity losses.

[Mod] This message and the previous one really belong one some other thread.
Edit/Lar: *this* is the "some other thread", I think... (posts were moved)
« Last Edit: 02/01/2018 09:39 PM by Lar »
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Offline mark_m

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #6 on: 02/01/2018 09:54 PM »
People speculate that using three engines to land used less fuel than using only one. I do not think that is correct. To stop the rocket you need a certain impulse: trust times length of burn. Three engine will burn per second three times the fuel needed by a single engine, but the burn will last only a third of the time to produce the same impulse, so no fuel savings. I wonder if the savings comes from the fuel left in the pipes from fuel tank to each engine. To use that fuel, you need to fire multiple engines after the tank is empty...any better explanation?
You're forgetting gravity losses.
I would also imagine that air resistance would contribute. Not sure if it's at terminal velocity before the engines fire, but either way the longer it's going faster the more of the work that friction is doing for you.

Online Kabloona

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #7 on: 02/01/2018 10:21 PM »
There is also this earlier thread about "pushing the limits of the hoverslam landing:"

https://forum.nasaspaceflight.com/index.php?topic=39267.0

The first post in the thread has an estimate of the propellant quantities used in different landing burn scenarios, including the 3-engine.
« Last Edit: 02/01/2018 10:23 PM by Kabloona »

Online Adriano

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #8 on: 02/04/2018 03:59 PM »
Thank you! Perfectly clear. Is there somewhere an analysis of the New Glenn. With its stubby wings it should be able to glide, saving fuel for retro and reentry bursts. Much harder probably to achieve the landing precision of the grid fins... I guess it would be quite complex to fit wings to the Falcon 9 because it would drastically change thermal and mechanical stresses.

Online Lar

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #9 on: 02/04/2018 06:02 PM »
Thank you! Perfectly clear. Is there somewhere an analysis of the New Glenn. With its stubby wings it should be able to glide, saving fuel for retro and reentry bursts. Much harder probably to achieve the landing precision of the grid fins... I guess it would be quite complex to fit wings to the Falcon 9 because it would drastically change thermal and mechanical stresses.
Try asking in a New Glenn, or at least a Blue Origin, thread.
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Online Adriano

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #10 on: 02/04/2018 08:33 PM »
Ok. Thank you!

Offline John Alan

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #11 on: 02/07/2018 03:35 AM »
Just frame by framed the corrected L&R video released and was dumbfounded...

The RTLS Boosters both used 3 engine landing burns...  :o

Center engine light at 7:48
Other two light about 7:50
They go out just as landing gear deploy
Touchdown just after 8:04... (spaceX clock times used on all)

A short 14 to 16 seconds end to end...  ???

Was that a first?... RTLS three engine Super Slam on land...
And in stereo no less... Balls... :o

https://www.youtube.com/watch?v=bCc16uozHVE?t=29m32s

On edit... redid link address
« Last Edit: 02/07/2018 03:33 PM by John Alan »

Online the_other_Doug

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #12 on: 02/07/2018 04:20 AM »
They used a now-familiar 1-3-1 landing burn.  Center engine starts, then the flanking engines start.  Flanking engines shut down, center engine continues to fire.  Center engine shuts down right at touchdown.

It's been the standard landing burn, for both RTLS and ASDS landings, for a little while now.  Nothing new about it.
-Doug  (With my shield, not yet upon it)

Offline deruch

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #13 on: 02/07/2018 04:44 AM »
The RTLS Boosters both used 3 engine landing burns...  :o

Center engine light at 7:48
Other two light about 7:50
They go out just as landing gear deploy
Touchdown just after 8:04... (spaceX clock times used on all)
It's a single engine burn, look at this from another angle at 8:15 in the video:

It's not.  NGCHunter just happened to be at the perfect angle to make it so you couldn't really tell in that video.  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.
« Last Edit: 02/07/2018 05:38 AM by deruch »
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Online catdlr

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #14 on: 02/07/2018 04:48 AM »
The RTLS Boosters both used 3 engine landing burns...  :o

Center engine light at 7:48
Other two light about 7:50
They go out just as landing gear deploy
Touchdown just after 8:04... (spaceX clock times used on all)
It's a single engine burn, look at this from another angle at 8:15 in the video:

It's not.  NGCHunter just happened to be at the perfect angle to make it so you couldn't really tell in that video.  Here are 2 screen caps from the SpaceX video during landing.  It's a bit hard to see because the camera has trouble tracking.  But if you start at T+ 7:50 and watch at 0.25x speed you'll clearly see that there is a very short period of 3-engine burn in there.  This also tracks with the fact that the landing burn is so short.  The length of which differs from previous, single engine landing burns.

Stand corrected, glad there was that split second coverage of it.  Thanks deruch.
Tony De La Rosa

Offline John Alan

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #15 on: 02/07/2018 05:15 AM »
https://forum.nasaspaceflight.com/index.php?topic=44376.msg1784875#msg1784875

This youtube next door really drives home... it was shorter then the normal 30 secondish... RTLS landing burn...
Zuma was almost 30sec... same with CRS-13... (I re-reviewed both earlier tonight)
I've seen so many landings that I (and some on audio in that video) thought they lit up lower then nominal for RTLS...

Thanks to @deruch for confirming this...  :)

Seems like they redo the re-entry burn 1-3-1 but cut the outer engines and unlatch the legs at some height and/or speed, that works for them... 
And now we actually got to see it...  8)
« Last Edit: 02/07/2018 05:17 AM by John Alan »

Online wannamoonbase

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #16 on: 02/07/2018 02:18 PM »
It's going to be an interesting story on why they didn't have enough ignition fuel on board the center core.

The 1-3-1 makes sense but seems like it's a lot more risk and events to manage.  But at this point I defer to SpaceX's world class expertise in this field.
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Online the_other_Doug

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #17 on: 02/07/2018 03:56 PM »
During the post-launch presser, Musk did say that one concern in landing two stages simultaneously is that one stage may lock onto the radar return from the other, confusing its altitude state.

Maybe the 1-3-1 landing burn was selected because it minimizes the amount of time the stages are in their landing burn phases?  Thus cutting down on the time during which radar problems could crop up by more than two-thirds?

Just a thought... ;)
-Doug  (With my shield, not yet upon it)

Offline fthomassy

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #18 on: 02/16/2018 10:12 PM »
There was a debate in the FH Demo discussion about the landing burn phase.  Lars-J put together some nice graphics to justify the notion that the target path was an overshoot at the start of the landing burn. While I disagree his evidence is definitive, I agree that it looks more over than under.

Below he put together the most concise summary of the entry/landing sequence I've seen yet. Have a read, I'll insert my but (pun intended) and a question after the quote.

Follow the link back to the original thread for full context ...
Yep. And this long exposure I found from the first F9 landing does seem to support such an idea... Look at the entry burn and landing burn... not a straight line, the grid fins have steered the stage to aim near the landing pad.

(Be aware of the extreme wide angle in the photo, the landing burn in this photo is close to vertical, compare with buildings in the low right corner)

Based on this, I think it is safe to assume the following:
 - the boostback and entry burns place the IIP far short of the landing pad
 - after the entry burn, the grid fins is steer to aim to the landing pad
 - when the landing burn begins, the IIP is either close to the landing point or just overshooting it

But why "or just overshooting it"? And I'm not arguing evidence they did or did not. Thinking about this I have a rationale for why overshoot is preferred.

I'll assume there is no safety concern so considerations are only performance. The entire argument for me hinges on the conditions at landing burn ignition. Seems that you'll be at the minimum terminal velocity achievable by grid-fin control and that will be true no matter what point is targeted (over or under). The vertical height at ignition is all about the time needed to control the final descent. I think it is clear that it is most efficient to neutralize the vast majority of horizontal speed quickly. Likely by the end of the three engine phase. This means you will control to be more overhead the target so that the remaining descent is vertical control and horizontal fine tuning. The more time you spend fighting horizontal speed under one engine the more fuel/energy you waste.

So the initial trajectory for maximum efficiency of a 1-3-1 burn would be an overshoot where horizontal speed is close to zero by the end of the three engine burn. Depending on how well the grid-fin can guide lift, the most efficient overshoot could be significant.

Edit for clarity
« Last Edit: 02/16/2018 11:30 PM by fthomassy »
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Online wannamoonbase

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Re: Falcon 9 3 engine reentry/landing burn advantage.
« Reply #19 on: 02/16/2018 11:37 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.
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