Author Topic: SpaceX outlines CRS-3 landing legs plan toward first stage recovery ambitions  (Read 28774 times)

Offline Coastal Ron

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I would a tried to add flotation bags to the stage for recovery and inspection...

That would be a waste of time, weight, and money.  They aren't going to use them for return to launch site, there's no reason to try to develop a new complex system that's only going to be used for a few tests.
They already have a source onboard for pressurization adding a few bags is no big deal for an outfit like SpaceX...

I would think the empty 1st stage RP-1 tank (and even the LOX) would provide all the floatation they need, assuming it stays intact.
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Offline cambrianera

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I would a tried to add flotation bags to the stage for recovery and inspection...

That would be a waste of time, weight, and money.  They aren't going to use them for return to launch site, there's no reason to try to develop a new complex system that's only going to be used for a few tests.
They already have a source onboard for pressurization adding a few bags is no big deal for an outfit like SpaceX...

I would think the empty 1st stage RP-1 tank (and even the LOX) would provide all the floatation they need, assuming it stays intact.

Yep, an intact stage will float like a cork.
http://forum.nasaspaceflight.com/index.php?topic=34065.msg1165692#msg1165692
meadows.st did a wonderful job!
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Offline MTom

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Nice article, but I wasn't sure what I was looking at in the last picture -- is that their test area in New Mexico, or an artist's conception of what it will look like, or something else entirely?

This is Spaceport America.

http://spaceportamerica.com/

Offline Rocket Science

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I would a tried to add flotation bags to the stage for recovery and inspection...

That would be a waste of time, weight, and money.  They aren't going to use them for return to launch site, there's no reason to try to develop a new complex system that's only going to be used for a few tests.
They already have a source onboard for pressurization adding a few bags is no big deal for an outfit like SpaceX...

I would think the empty 1st stage RP-1 tank (and even the LOX) would provide all the floatation they need, assuming it stays intact.

Yep, an intact stage will float like a cork.
http://forum.nasaspaceflight.com/index.php?topic=34065.msg1165692#msg1165692
meadows.st did a wonderful job!
I agree "if" it stays intact...
"The laws of physics are unforgiving"
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Offline Jim

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They already have a source onboard for pressurization adding a few bags is no big deal for an outfit like SpaceX...

Yes, it would be a big deal. 
1.  It is no a good ideal to use the onboard pressurization
2.  It would complicate the stage layout
3.  It is not a simple addition
4.  Flippant posts like this belittle the actual work needed.  Much like stating that Tesla can add dual wheels to their cars.
« Last Edit: 03/01/2014 06:47 PM by Jim »

Offline Lar

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PS I've changed the headline from SpaceX outline to SpaceX outlines, as I've got to get my head around SpaceX being singular - I used to struggle with that when referencing NASA.

It is a UK/US thing.

Corps and other entities in the UK are considered plural so one would say "REL are about to embark on an ambitious development programme for their intercooler at their research centre in Basingstoke." 

Corps and other entities in the US are considered singular so one would say "SpaceX is about to embark on an ambitious development program for their ECLSS at their research center in Hawthorne, CA"

Personally, since you're a Brit, I think you should write the way that you find natural, don't speak USese just for us seppos...
"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

Offline MP99

Nice article, but I thought that the legs were supposed to stabilise rotation of the stage, and avoid prop centrifuging in the stage. I had assumed this would be  *before* ignition of the engines for landing.

cheers, Martin

Offline Rocket Science

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They already have a source onboard for pressurization adding a few bags is no big deal for an outfit like SpaceX...

Yes, it would be a big deal. 
1.  It is no a good ideal to use the onboard pressurization
2.  It would complicate the stage layout
3.  It is not a simple addition
4.  Flippant posts like this belittle the actual work needed.  Much like stating that Tesla can add dual wheels to their cars.
My statement Jim is about their engineering capability... Not being easily done to the stage...

BTW: I can't afford a Tesla...
"The laws of physics are unforgiving"
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Offline rpapo

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Nice article, but I thought that the legs were supposed to stabilise rotation of the stage, and avoid prop centrifuging in the stage. I had assumed this would be  *before* ignition of the engines for landing.
But then Tom Mueller revealed to us last week that the real reason for the engine malfunction was debris from the slosh baffles.  We now also "know" that they were doing slosh-baffle testing back in November in MacGregor.

There's an old computer engineering expression for this: GIGO = Garbage In, Garbage Out.  Not that we had entire garbage as input, but we had incomplete and sometimes wrong input for all our conjectures.

FWIW, I still think that if the aerodynamics are stable (and I don't know they are), then deploying the legs very early on would be best.  Lower terminal velocity means less fuel required for landing.  But the shape the folded legs will have is actually quite nice for a descent tail-first through the atmosphere.  And the faster descent will help with the final landing adjustments when there is a cross-wind to deal with.
An Apollo fanboy . . . fifty years ago.

Offline Hauerg

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Also: Stretching the legs in the final seconds of the flight will make for some cool videos in those days when the F9RS1 (?) will actually land on land.

Offline aero

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Has anyone measured the area of the stowed legs? I have estimates for the deployed frontal area but not for the area while they are stowed. I need to increase the reference area of my simulated F 9 to account drag of the legs during launch as well as recovery.
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Offline meadows.st

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Has anyone measured the area of the stowed legs? I have estimates for the deployed frontal area but not for the area while they are stowed. I need to increase the reference area of my simulated F 9 to account drag of the legs during launch as well as recovery.

What level of accuracy are you trying to achieve? 

I have taken my estimates from the existing photos and I believe the thickness at the thickest point is between 50 and 70 cm.  The angle from which the photos (from both JimNTexas and TomNTex) were taken made it impossible to determine this exactly but I think the range is reasonable.  I have also read on other threads that the diameter of the rocket in shipping would increase the overall effective shipping diameter about 2' larger than the "leg free" shipping  diameter and assuming the body would be shipped with the lowest point being between the legs, my assumed max depth seems reasonable.  The thinnest (at the edges) looks more like 30 to 40 cm. 

Looking at the assumptions from another perspective, we know the legs are about 8m long, the pistons are about 20m long and the surface area of the legs is about 12m^2 and the average diam of the pneumatic cylinders is 20cm.  From what we know about the total mass (~2000kg) of the four sets, a rough density would be 75kg/m^3.  Assuming they are using the lightest carbon fibre (density ~ 1750kg/m^3, the system is about 1/23 solid carbon fibre (this is a loose Order of Magnitude approach I know).  Therefore, the wall thickness, assuming otherwise hollow and all other components are a higher density than carbon fibre, the wall thickness would be <3cm thick if you assume the 70cm max thickness and 40cm min thickness of the legs.  I am not a carbon fibre expert but I believe an inch thick carbon fibre structure would be sufficiently strong.

I have created a roughly to scale oversimplified mock up with the max depth approx 50cm (see picture) at the thickest point and ~30 cm at the thinnest point to depict what I am assuming for the cross sectional area of a tail-first "ZZTop" F9 v1.1.  EDIT: added superimposed version of F9v1.1 for side by side comparison for scale.

Edited: less cluttered picture of the engines superimposed on another version of the legs end view.
Edit: Overlay photo from this link from @cambrianera http://forum.nasaspaceflight.com/index.php?topic=34065.msg1163959#msg1163959
« Last Edit: 03/04/2014 01:37 AM by meadows.st »
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Offline AJW

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Do we have an expected duration for the landing burn?  With the legs extending 10 seconds into this burn, I would expect that the divert towards the LZ won't begin until the legs are fully deployed and locked, and this might give us a distance between the LZ and the abort target.

Online Comga

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Nice article, but I thought that the legs were supposed to stabilise rotation of the stage, and avoid prop centrifuging in the stage. I had assumed this would be  *before* ignition of the engines for landing.

cheers, Martin
"When you assume you make...."
The statement from Musk was that the presence of legs mitigated the spin.  He didn't say "deployed legs".  It has been conjectured that the stowed legs also work against the spin.  It appears that SpaceX is going to open them as late as possible, so they have deemed it not necessary to deploy them to resist the spin.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Offline AncientU

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Nice article, but I thought that the legs were supposed to stabilise rotation of the stage, and avoid prop centrifuging in the stage. I had assumed this would be  *before* ignition of the engines for landing.
But then Tom Mueller revealed to us last week that the real reason for the engine malfunction was debris from the slosh baffles.  We now also "know" that they were doing slosh-baffle testing back in November in MacGregor.

There's an old computer engineering expression for this: GIGO = Garbage In, Garbage Out.  Not that we had entire garbage as input, but we had incomplete and sometimes wrong input for all our conjectures.

FWIW, I still think that if the aerodynamics are stable (and I don't know they are), then deploying the legs very early on would be best.  Lower terminal velocity means less fuel required for landing.  But the shape the folded legs will have is actually quite nice for a descent tail-first through the atmosphere.  And the faster descent will help with the final landing adjustments when there is a cross-wind to deal with.
One disadvantage of deploying the legs early (I thought it would happen above the atmosphere after the boost-back burn) is that your terminal velocity is so low that you have an extremely short final burn.  There is no shortage of propulsive power to stop the descent, so higher velocity actually provides more inertial mass and widens the burn window.  The trade-off is a bit of fuel vs. higher margin for a successful landing.
« Last Edit: 03/02/2014 12:31 AM by AncientU »
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Online Comga

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Nice article, but I thought that the legs were supposed to stabilise rotation of the stage, and avoid prop centrifuging in the stage. I had assumed this would be  *before* ignition of the engines for landing.
But then Tom Mueller revealed to us last week that the real reason for the engine malfunction was debris from the slosh baffles.  We now also "know" that they were doing slosh-baffle testing back in November in MacGregor.

There's an old computer engineering expression for this: GIGO = Garbage In, Garbage Out.  Not that we had entire garbage as input, but we had incomplete and sometimes wrong input for all our conjectures.

FWIW, I still think that if the aerodynamics are stable (and I don't know they are), then deploying the legs very early on would be best.  Lower terminal velocity means less fuel required for landing.  But the shape the folded legs will have is actually quite nice for a descent tail-first through the atmosphere.  And the faster descent will help with the final landing adjustments when there is a cross-wind to deal with.
One disadvantage of deploying the legs early (I thought it would happen above the atmosphere after the boost-back burn) is that your terminal velocity is so low that you have an extremely short final burn.  There is no shortage of propulsive power to stop the descent, so higher velocity actually provides more inertial mass and widens the burn window.  The trade-off is a bit of fuel vs. higher margin for a successful landing.
This makes no sense to me.
Can you put this in equations or a numerical simulation?

The "magic" of the Falcon 9 is that it can be throttled to 8% of liftoff thrust by using one of nine engines and throttling mildly. SpaceX will still bring back a stage with only 3-5% of the liftoff mass.   "Hoverslam" is the only way out of this shy of pushing the throttle limit down to below 50%.

My understanding from the information here (and L2!) is that SpaceX will leave extra fuel in the first stage of the SpX-3 rocket to reduce the terminal acceleration and give themselves more time to tune the landing burn for zero velocity at zero altitude. If they want to minimize the fuel needed they will try that on subsequent flights.
What kind of wastrels would dump a perfectly good booster in the ocean after just one use?

Online rsnellenberger

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Nice article, but I wasn't sure what I was looking at in the last picture -- is that their test area in New Mexico, or an artist's conception of what it will look like, or something else entirely?

This is Spaceport America.

http://spaceportamerica.com/
Ah... Thanks!

Offline guckyfan

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My understanding from the information here (and L2!) is that SpaceX will leave extra fuel in the first stage of the SpX-3 rocket to reduce the terminal acceleration and give themselves more time to tune the landing burn for zero velocity at zero altitude. If they want to minimize the fuel needed they will try that on subsequent flights.

I have missed that. Where do you get this understanding from? It does seem to make sense except they can get such data from the Falcon 9R test vehicle.

Offline Paul Howard

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Any chance they will get video of the legs deploying?

Offline Silmfeanor

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Picture posted in the update thread, i'll post it here aswell. First view of a vertical stage with legs...

The source is SpaceX Facebook

Quote
Falcon 9 and Dragon conducted a successful static fire test in advance of next weeks CRS-3 launch to station! In this final major preflight test, Falcon 9s 9 first-stage engines were ignited for 2 seconds while the vehicle was held down to the pad.

 Launch is targeted for Sunday March 16 at 4:41 am EDT.

They look really small. Good to keep CoG in mind..
« Last Edit: 03/09/2014 09:15 PM by Silmfeanor »

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