Author Topic: Landing legs design : SpaceX vs Blue Origins  (Read 15488 times)

Offline mheney

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #20 on: 05/10/2016 04:57 PM »
I was surprised to see flashing lights on the inside of the landing legs.  Anyone have any idea of why they're there?

The only reason I can think of is for barge operations - "Be careful walking over here or you'll hit your head on an engine bell..."  But that's what yellow "Caution" tape is for ...  And SpaceX wouldn't have added the weight and complexity to the legs without a real good reason for them. 


Offline sewebster

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #21 on: 05/10/2016 09:50 PM »
I was surprised to see flashing lights on the inside of the landing legs.  Anyone have any idea of why they're there?

The only reason I can think of is for barge operations - "Be careful walking over here or you'll hit your head on an engine bell..."  But that's what yellow "Caution" tape is for ...  And SpaceX wouldn't have added the weight and complexity to the legs without a real good reason for them.

Conjecture in other threads is that they may provide a status signal in terms of stage safety (pressure released, TEA, TEB purged, etc.) so that workers had a visual indication of whether they could approach. But you'd think they could get that from telemetry...

Maybe the real good reason is that they are cool? Might be an LED that does not weigh much at all... I guess there is a battery though, maybe uses main battery...?

Online Lemurion

Re: Landing legs design : SpaceX vs Blue Origins
« Reply #22 on: 05/11/2016 11:57 AM »
The two boosters operate in different flight regimes, and under different mass and performance constraints, of course the legs are different. I personally think Blue's design is probably better for their needs and SpaceX's design is better for theirs.

Blue can accept a proportionally more massive design with less of a performance hit. Blue can also accept a slower deployment rate because New Shepard's landings are far less aggressive than Falcon's. Add in the relatively short and wide design of the New Shepard propulsion unit, and you get a very different set of requirements.

Neither company is trying to solve exactly the same problem, naturally their solutions differ.

Online Hobbes-22

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #23 on: 05/11/2016 02:00 PM »
Has anyone calculated the tipover radius/height for NS vs F9?  I would bet that NS is has a larger tipover radius than F9 per unit height, which would make it more stable (assuming similar CG height/height, which I think is hard to know)...

Does anyone have these numbers?
A post on another site which I can't seem to find at the moment estimated using mass and CG numbers for Falcon 9 that it would take a 23 degree tilt to move the center of mass past the end of the legs.

You mean this post?
http://space.stackexchange.com/questions/8771/how-stable-would-a-falcon-9-first-stage-be-after-it-has-landed-on-a-barge

Offline Alastor

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #24 on: 05/11/2016 03:56 PM »
Has anyone calculated the tipover radius/height for NS vs F9?  I would bet that NS is has a larger tipover radius than F9 per unit height, which would make it more stable (assuming similar CG height/height, which I think is hard to know)...

Does anyone have these numbers?

I believe Musk mentioned 11 in the post-CRS9 conference.
My calculation in L2 was a little higher than that, but not by much.

Offline gospacex

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #25 on: 05/11/2016 04:16 PM »
In that kind of sea state, a previously-flown rocket tipping over would be the least of their concerns.


All seaworthy vessels should be able to withstand ~30 degrees list, and more. Meaning, listing by 30 degrees is rough seas for a large vessel, but it's not a disaster.

For those of you land creatures, the sea is a harsh mistress. It may seem calm and serene at times, but sailors know better than trust it. For any work at sea, you must prepare for far worse conditions than seem reasonable at a first glance. And sailors do.

Offline SoulWager

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #26 on: 05/11/2016 04:35 PM »
Has anyone calculated the tipover radius/height for NS vs F9?  I would bet that NS is has a larger tipover radius than F9 per unit height, which would make it more stable (assuming similar CG height/height, which I think is hard to know)...

Does anyone have these numbers?

I believe Musk mentioned 11 in the post-CRS9 conference.
My calculation in L2 was a little higher than that, but not by much.
I think musk was talking about actually landing on that incline, which, if the stage is descending vertically, would mean the rocket would tip to ~double that from the momentum caused by the legs touching down asymmetrically.

Offline mvpel

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #27 on: 05/12/2016 02:59 AM »
Yes, he was, SoulWager. Josh Brost said about the same thing earlier in the day.
"Ugly programs are like ugly suspension bridges: they're much more liable to collapse than pretty ones, because the way humans (especially engineer-humans) perceive beauty is intimately related to our ability to process and understand complexity. A language that makes it hard to write elegant code makes it hard to write good code." - Eric S. Raymond

Offline envy887

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #28 on: 05/12/2016 02:56 PM »
I think musk was talking about actually landing on that incline, which, if the stage is descending vertically, would mean the rocket would tip to ~double that from the momentum caused by the legs touching down asymmetrically.

Tipover is a fairly complex problem to model, but unless the wind is blowing it won't get to twice the angle. The stage is nearly weightless at touchdown (because the thrust is almost perfectly canceling gravity), so there's little angular momentum imparted by one leg striking first.

It's theoretically possible to land on any incline up to the tip-over angle of the stage as long as it can accurately determine the height of the stage off the surface during landing.

Offline Kabloona

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #29 on: 05/12/2016 03:12 PM »
Has anyone calculated the tipover radius/height for NS vs F9?  I would bet that NS is has a larger tipover radius than F9 per unit height, which would make it more stable (assuming similar CG height/height, which I think is hard to know)...

Does anyone have these numbers?

I believe Musk mentioned 11 in the post-CRS9 conference.
My calculation in L2 was a little higher than that, but not by much.
I think musk was talking about actually landing on that incline, which, if the stage is descending vertically, would mean the rocket would tip to ~double that from the momentum caused by the legs touching down asymmetrically.

Twice 11 degrees is 22 degrees. Which agrees quite nicely with this stability analysis showing that the CG passes over the leg tip at 23 degrees:

http://space.stackexchange.com/questions/8771/how-stable-would-a-falcon-9-first-stage-be-after-it-has-landed-on-a-barge

His CG calculations resulting in the 23 degree tipover angle seem correct. So there must be a dynamic component in the 11 degree limit that Musk mentioned. Otherwise, at a static angle of 11 degrees, the stage should be only halfway to tipping.
« Last Edit: 05/12/2016 03:17 PM by Kabloona »

Offline SoulWager

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #30 on: 05/12/2016 03:18 PM »
I think musk was talking about actually landing on that incline, which, if the stage is descending vertically, would mean the rocket would tip to ~double that from the momentum caused by the legs touching down asymmetrically.

Tipover is a fairly complex problem to model, but unless the wind is blowing it won't get to twice the angle. The stage is nearly weightless at touchdown (because the thrust is almost perfectly canceling gravity), so there's little angular momentum imparted by one leg striking first.

It's theoretically possible to land on any incline up to the tip-over angle of the stage as long as it can accurately determine the height of the stage off the surface during landing.
To land close to the tip over angle you couldn't descend vertically, you'd have to touch down with the rocket already at that angle.  If you land vertically on an incline, you have say two feet touch down while the other two are still several feet in the air. It doesn't matter how heavy the stage is, you still get angular acceleration until the other two legs touch down. In fact, an empty stage will have a higher angular acceleration due to the low center of mass.

Maybe this will help you visualise it: Balance a pen or something on an incline, almost to the point where it tips over. Now push the pen until it's vertical then let go.

Edit: No, it's not exactly double, there's some small damping that I have no hope of calculating, and sin2x does not equal 2sinx. That's why there's a '~' in front of the 'double' on my previous post.
« Last Edit: 05/12/2016 03:49 PM by SoulWager »

Offline envy887

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #31 on: 05/12/2016 03:54 PM »
Your model is missing the live, variable, vectorable Merlin under the stage. Angular acceleration = (weight - thrust) x offset distance / inertial moment (plus a bunch of second-order factors like damping, wind, and air drag). Since thrust is variable, angular acceleration can be controlled. In fact, you can see the Falcon pitching and yawing to vertical as it approaches landing in the CRS-8 video. You can also see transient shutdown thrust as the stage bounces.

Musk's 11 degrees probably includes safety factors for various issues including wind and some things they just don't know exactly yet. This is all new stuff.

Offline SoulWager

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #32 on: 05/12/2016 04:52 PM »
Your model is missing the live, variable, vectorable Merlin under the stage. Angular acceleration = (weight - thrust) x offset distance / inertial moment (plus a bunch of second-order factors like damping, wind, and air drag). Since thrust is variable, angular acceleration can be controlled. In fact, you can see the Falcon pitching and yawing to vertical as it approaches landing in the CRS-8 video. You can also see transient shutdown thrust as the stage bounces.

Musk's 11 degrees probably includes safety factors for various issues including wind and some things they just don't know exactly yet. This is all new stuff.
Thrust vectoring after touchdown? Seems dubious. The only way it makes any difference is if you're sliding the already touched down legs across the deck, and if you're starting from a vertical touchdown your only options are increasing angular velocity just like gravity would, or increasing the total kinetic energy of the rocket by trying to arrest the angular velocity. To get a benefit from thrust vectoring after touchdown you'd have to plan the landing burn knowing what angle and direction of incline you're going to be landing on, and touch down with some translational velocity and a non-vertical attitude.  Even then, there's no point to doing so after touchdown, if you were going to land like that you'd do the maneuvering purely in the inverted pendulum regime before touchdown.

All the information I've seen says the falcon doesn't get real time telemetry as to the pitch and yaw of the landing platform. It's just targeting a vertical touchdown at X coordinates, with radar for altitude on final approach.

Offline Kabloona

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #33 on: 05/12/2016 05:16 PM »
Quote
Thrust vectoring after touchdown? Seems dubious.

He's talking about residual angular momentum induced by vectoring on final approach *before* touchdown. Although the guidance algorithm does seem to damp it out quite well. On the CRS-8 landing when the stage did its final "righting" maneuver to go vertical just before touchdown, it turned into a sideways "sliding" translation, but the angular rate seemed to zero out.
« Last Edit: 05/12/2016 05:53 PM by Kabloona »

Offline envy887

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #34 on: 05/12/2016 05:59 PM »
The stage doesn't need to know the pitch of the deck, or even if one leg is touching. From it's point of view, it hasn't reached zero altitude until ALL the legs are down. So it will still be thrusting after one legs touches on a pitched deck.

That thrust will directly counter and reduce the angular acceleration due to inertia & gravity as the stage falls flat on four legs. It will touch flat with some angular velocity and the thrust near zero. As it pendulums over, the leg that hit first will try to lift up, but now it's fighting gravity with no thrust to help it (depending how fast the transients die).

The thrust immediately before the 2nd through 4th legs touch is canceling out both vertical and angular velocity... effectively, it's being damped by the Merlin.

Vectoring isn't really relevant after touchdown, although it could be used to control translational thrust induced by the angled offset from the weight vector. Probably not necessary on small angles, and you would want the stage to know the deck pitch (or at least leg contact force).

Offline Jim

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #35 on: 05/12/2016 06:07 PM »
The stage doesn't need to know the pitch of the deck, or even if one leg is touching. From it's point of view, it hasn't reached zero altitude until ALL the legs are down.

We don't that.  They could have sensors that detect weight on the legs

Offline dorkmo

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #36 on: 05/12/2016 06:10 PM »
The stage doesn't need to know the pitch of the deck, or even if one leg is touching. From it's point of view, it hasn't reached zero altitude until ALL the legs are down.

We don't that.  They could have sensors that detect weight on the legs

or pressure of pnematic system of each leg?

Offline envy887

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #37 on: 05/12/2016 06:47 PM »
The stage doesn't need to know the pitch of the deck, or even if one leg is touching. From it's point of view, it hasn't reached zero altitude until ALL the legs are down.

We don't that.  They could have sensors that detect weight on the legs

Let me clarify. The altimeter won't read zero until all the legs are on or very near the deck, assuming it is high accuracy (fractions of a meter) and reads altitude either true vertical or coaxial to the stage.

Offline Jim

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #38 on: 05/12/2016 06:50 PM »
Let me clarify. The altimeter won't read zero until all the legs are on or very near the deck, assuming it is high accuracy (fractions of a meter) and reads altitude either true vertical or coaxial to the stage.

There is a radar altimeter on the vehicle.  We don't know what they have as zero altitude in relation to the vehicle.

Offline Kabloona

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #39 on: 05/12/2016 06:58 PM »
It would be interesting to know what their "trigger" is for thrust termination. I doubt it's leg sensors, because that's 4 sensors that all have to work, so 4 potential failure modes. More likely it's the radar altimeter or maybe the IMU sensing the "jerk" of touchdown.

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