Author Topic: CRS-3 Falcon 9 first stage to sport legs and attempt soft splashdown  (Read 213373 times)

Offline Chris Bergin

Better give this a standalone thread.....

Short article based on comments provided to a L2 member at the event with Tom Mueller last night. Note the questions about the legs and such were after his speech and he allowed for his comments to be used.

http://www.nasaspaceflight.com/2014/02/crs-3-falcon-9-first-stage-sport-legs-attempt-soft-splashdown/

Offline Lars_J

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Nice scoop! :)

Offline butters

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So the slosh baffles inside the RP-1 tank failed mechanically due to centrifugation and the center engine ingested metal debris. The question is whether they can reduce the roll rate enough to avoid this happening (possibly aided by deployed legs) or if they need to reinforce the slosh baffles. I suppose CRS-3 is their opportunity to see if the current slosh baffles can handle it.

Offline Elmar Moelzer

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Very interesting information, Chris! Thanks for the article! I am looking forward to the soft landing on water. It is not entirely clear from the article, whether the "boost back" will bring the stage almost all the way back to the launch site, or whether it will land much further down range.
Either way, I hope that SpaceX will have cameras on site.

Offline rpapo

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Well, if they don't do a boost-back, and the flight profile is more or less normal, then the "landing" location should be past the Bahamas, judging by that video those British people vacationing in the Bahamas took of the SES-8 launch.  In that video, it looked like the stage separation was occurring over them.
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Offline Rocket Science

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Great update Chris! This article makes me think “buddy can you spare a barge”... ;)
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Offline MTom

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The question is whether they can reduce the roll rate enough to avoid this happening (possibly aided by deployed legs) ...

See the effect of the legs by ice skating (video form 1:25)

Offline cartman

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That's very good news! When do we expect the first photos of the rocket in KSC? That is, how many days before launch do we usually get the first pics?

Offline Chris Bergin

Thanks chaps, but that one was all down to some luck we had a L2 member at an event with 100 people there! I just wrote it up.


See the effect of the legs by ice skating (video form 1:25)
 

Nice synergy there as Elon described the array deploy on the misbehaving Dragon with the same analogy!
http://www.nasaspaceflight.com/2013/03/resilient-crs-2-dragon-pursuit-iss-sunday-berthing/

Quote
Although the initial plan was to deploy Dragon’s solar arrays once two Quads were available, SpaceX decided to unfurl the arrays with only one Quad working. This proved to be helpful for the unstable Dragon, with SpaceX CEO Elon Musk describing it as not unlike an ice skater stretching out her arms during a spin maneuver to slow the rotation.

Offline rcoppola

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So I'm assuming the legs were / are being attached at the Cape? Or were / are they being attached in TX post stage firing and shipped to the Cape pre-attached? Curious, as we have bandied this question about quite a bit.
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Offline boinc

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@admin
hmm  :o, common sense tells at least me that the increase of the moment of inertia and thus reduced rotation rate via extended legs has a negligible effect in the lower atmosphere. The legs only weight 2 metric tons or so and extend no so far out. we are surely talking about aerodynamic effects here.
« Last Edit: 02/20/2014 05:59 PM by boinc »

Offline Lars_J

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@admin
hmm  :o, common sense tells at least me that the increase of the moment of inertia and thus reduced rotation rate via extended legs has a negligible effect in the lower atmosphere. The legs only weight 2 metric tons or so and extend no so far out. we are surely talking about aerodynamic effects here.

You might want to recalibrate your common sense ;) and apply physics. Remember that the stage is going to be very light - almost completely empty. In addition to the "moment of inertia" effect, remember that the extended legs will also slow rotation through air friction.
« Last Edit: 02/20/2014 06:08 PM by Lars_J »

Offline rpapo

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You might want to recalibrate your common sense ;) and apply physics. Remember that the stage is going to be very light - almost completely empty. In addition to the "moment of inertia" effect, remember that the extended legs will also slow rotation through air friction.
Unless the legs aren't completely symmetrical.  Then they can actually increase the spin.  Time will tell.
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Offline TrueBlueWitt

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@admin
hmm  :o, common sense tells at least me that the increase of the moment of inertia and thus reduced rotation rate via extended legs has a negligible effect in the lower atmosphere. The legs only weight 2 metric tons or so and extend no so far out. we are surely talking about aerodynamic effects here.

You might want to recalibrate your common sense ;) and apply physics. Remember that the stage is going to be very light - almost completely empty. In addition to the "moment of inertia" effect, remember that the extended legs will also slow rotation through air friction.

The real key here is  to keep the total induced torque low enough for the cold gas thrusters to keep Rotational velocity at ZERO. 

It's a force(torque) balance in the end.

While the added inertia keeps the rotational velocity lower, so you may not have issues with losing thrust, there is however no reduction in rotational energy. 

To make this work the rotational velocity(energy) still has to be very near ZERO for a survivable landing. 


Drag on the legs as the stage tries to rotate will provide a moment in the opposite direction which will reduce the amount of thruster force need to counter the imparted spin. Turbulence from the legs may also play a roll in reducing any aerodynamic forces that are creating the roll torque.
« Last Edit: 02/20/2014 06:24 PM by TrueBlueWitt »

Offline guckyfan

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Better give this a standalone thread.....

Short article based on comments provided to a L2 member at the event with Tom Mueller last night. Note the questions about the legs and such were after his speech and he allowed for his comments to be used.

http://www.nasaspaceflight.com/2014/02/crs-3-falcon-9-first-stage-sport-legs-attempt-soft-splashdown/

Short but juicy, great info. Thanks.

Offline hrissan

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My thorough explanation of why skater arm reference has nothing to do with roll of Falcon 9 has disappeared from the site. Will repeat it quickly and without numbers (I did them, so will just say what follows).

The fins on the rocket prevent roll because of wing lifting force. When the rocket does not roll, the air slides parallel to fin, hence no force. But as soon as the rocket starts to rotate, the air hits the fin with slight angle, generating lift which tries to cancel roll. This lift is very large even for small fins.

If the fins are a bit assymetric (and they always are), there is a small rate of roll when the lift forces of all fins cancel each other. It depends on the rocket speed.

Falcon 9 legs (when folded) have large area looking from the side, they are rather good fins. When I plugged the numbers, the forces cancelling roll at terminal velocity were large (like pair of Dracos mounted for roll control) at the roll rate of around 6 rpm, which is modest and not enough to centrifuge fuel.

When legs are deployed, they just become slightly better fins (because their lift is generated farther from the center, hence more torque).

Stretching skater's arms does work like stretching Dragon's solar arrays, but not like deploying Falcon 9 legs. :)

Offline john smith 19

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The real key here is  to keep the total induced torque low enough for the cold gas thrusters to keep Rotational velocity at ZERO. 

It's a force(torque) balance in the end.

While the added inertia keeps the rotational velocity lower, so you may not have issues with losing thrust, there is however no reduction in rotational energy. 

To make this work the rotational velocity(energy) still has to be very near ZERO for a survivable landing. 


Drag on the legs as the stage tries to rotate will provide a moment in the opposite direction which will reduce the amount of thruster force need to counter the imparted spin. Turbulence from the legs may also play a roll in reducing any aerodynamic forces that are creating the roll torque.
True.

Like all LV's F9 1.1 is designed to be strong in some directions and not others. Anyone wanting to know how it would survive a spinning landing might like to glue an empty  soda can to a piece of rod and put the rod in a drill chuck. Get it up to speed and then slam it into the ground.

The can should crumple quite spectacularly.   :(

I think the fact it was the slosh baffles breaking up and getting into the engine was quite interesting.

I wonder how many "armchair" engineers predicted that one?  :(

BTW IIRC either the later Saturn V's or Saturn !'s flew with Teflon  baffles for lighter weight. They were not entirely rigid and by flexing absorbed more slosh than rigid metal designs. I'm surprised they did not become SOP for all LV's.  :(

Unfortunately most of the papers that cover spinning tanks of fluid seems to be written by the US Army for the physics of artillery shells delivering nerve gas and operate at the 10s of 1000s of RPM while more then 95% full.

Not very useful in this context.  :(


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Offline PahTo

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Well, if they don't do a boost-back, and the flight profile is more or less normal, then the "landing" location should be past the Bahamas, judging by that video those British people vacationing in the Bahamas took of the SES-8 launch.  In that video, it looked like the stage separation was occurring over them.

Boost back would be very nice--as this is a NASA mission and thus will have NASA TV coverage (and if boost back occurs to an area proximate to the coast, NASA will likely pick it up).  If boost back doesn't happen, the trajectory won't be near the Bahamas--that was a GTO launch inclined 28 degrees.  This is an ISS run, so will be inclined 51.6 degrees, and likely fly N and not S.

Clearly this mission has margin for legs to a relatively highly inclined orbit--a light Dragon or a good sign for future manned Dragon?

Thanks NSF member and Chris--nice scoop!   Here's hoping for success all the way around!

Offline Elmar Moelzer

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Well the article clearly says that there will be a "boost back". But a boost back could still cause the stage to land anywhere downrange between the point of stage separation (or even beyond that) and the launch site.
I hope they bring it back close the launch site. So we can get a good view of it.
« Last Edit: 02/20/2014 07:55 PM by Elmar Moelzer »

Offline Chris Bergin

Well the article clearly says that there will be a "boost back". But a boost back could still cause the stage to land anywhere downrange between the point of stage separation (or even beyond that) and the launch site.
I hope they bring it back close the launch site. So we can get a good view of it.

I better be careful to note that part is my explanation as to what has been attempted with the likes of CASSIOPE. If they don't have to boost back, then they may not.

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