Author Topic: Landing legs design : SpaceX vs Blue Origins  (Read 12206 times)

Online b0objunior

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Landing legs design : SpaceX vs Blue Origins
« on: 01/24/2016 04:00 AM »
Hello,

Ok, I want to know what is the feedback from you on the design differences of the landing legs from SpaceX and Blue Origins. Why would they be so different and which is better in your opinion?

Thanks.

Offline JBF

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #1 on: 01/24/2016 04:54 AM »
Hello,

Ok, I want to know what is the feedback from you on the design differences of the landing legs from SpaceX and Blue Origins. Why would they be so different and which is better in your opinion?

Thanks.

Based on what I've seen, it all comes down to size.  The falcon 9 is a much taller rocket and it needs a wider stance to be stable.  With a wider stance and a heavier rocket the legs must be more substantial.  The weight penalty to make them as articulate as New Shepard's legs would be high.
"In principle, rocket engines are simple, but that’s the last place rocket engines are ever simple." Jeff Bezos

Offline cambrianera

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #2 on: 01/24/2016 12:24 PM »
Layout of SpaceX landing gear is simple, but execution has been complicate:
-Telescopic cylinder, with a swing movement of over 140°, many seals and locking rings.
-"A" frame, big, complex and heavy, but effectively carrying no load.

Personally I prefer the general layout of New Shepard landing gear, at the cost of more joints you get simpler struts, with reliable deployment and steady open position.
That said I don't know much of it other than the general layout, therefore no way to say better or worse.
Oh to be young again. . .

Offline funkyjive

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #3 on: 01/24/2016 02:46 PM »
New Shepard also does a preflight check by extending, locking, then retracting the gear giving some extra confidence prior to a flight commit. The f9 legs are one way.

Offline kevinof

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Landing legs design : SpaceX vs Blue Origins
« Reply #4 on: 01/24/2016 04:05 PM »
Remember that blue Shepard is about the size of one falcon 9 leg. It's all to do with weight and forces. I've no doubt that if BO ever build a reusable orbital stage it's leg design will be more like SpaceX.
« Last Edit: 01/24/2016 05:56 PM by kevinof »

Online John Alan

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #5 on: 01/24/2016 05:17 PM »
It's like comparing a car tire... a semi truck tire... and a 747 tire...
Each is designed to the need at hand...

Both leg designs have their pluses and minuses... from an engineering standpoint... IMHO
I think SpaceX worked off their 'in use' rocket structure and weigh savings as key items...
It was an "add on" in other words... but it fit the need at hand...

Both may have design flaws still...
Only using them and time passing will find out...
IMHO...

Online meekGee

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #6 on: 01/24/2016 06:56 PM »
Layout of SpaceX landing gear is simple, but execution has been complicate:
-Telescopic cylinder, with a swing movement of over 140°, many seals and locking rings.
-"A" frame, big, complex and heavy, but effectively carrying no load.

Personally I prefer the general layout of New Shepard landing gear, at the cost of more joints you get simpler struts, with reliable deployment and steady open position.
That said I don't know much of it other than the general layout, therefore no way to say better or worse.

SpaceX's design is motivated by mass.   The main compression strut, when extended, is a cylinder, which is the lightest possible compression beam.    I also initially disliked the multiple slip joints in each leg, but I came to understand where they're coming from.
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Online Robotbeat

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #7 on: 01/24/2016 07:19 PM »
SpaceX will no doubt eventually use a different leg design for their rockets (post-Falcon). I doubt Blue Origin's is perfect, either. And it is a suborbital craft, so yeah, very different story.
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Offline Beittil

Re: Landing legs design : SpaceX vs Blue Origins
« Reply #8 on: 01/26/2016 08:12 AM »
It would seem to me that the BO landing system provides a lot less stability once they would be 'forced' to start barge landings as well on future rockets. The wide base of SpaceX's landing gear combined with the very low center of mass is what would keep the rocket from tipping over on a barge after landing, especially with the barge rolling around in de waves. BO's current system on a larger rocket would probably mean a lot of tipped over rockets before a crew can get aboard to secure it.

Offline sanman

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #9 on: 01/27/2016 02:23 AM »
It would seem to me that the BO landing system provides a lot less stability once they would be 'forced' to start barge landings as well on future rockets. The wide base of SpaceX's landing gear combined with the very low center of mass is what would keep the rocket from tipping over on a barge after landing, especially with the barge rolling around in de waves. BO's current system on a larger rocket would probably mean a lot of tipped over rockets before a crew can get aboard to secure it.

Surely Blue won't keep those same legs for larger rockets. Their orbital vehicle will probably have lighter and longer legs - but maybe they'll have their own original look that's different from F9R's legs. Hoping there'll be some more one-upsmanship.

Offline guckyfan

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #10 on: 01/27/2016 06:36 AM »
Surely Blue won't keep those same legs for larger rockets. Their orbital vehicle will probably have lighter and longer legs - but maybe they'll have their own original look that's different from F9R's legs. Hoping there'll be some more one-upsmanship.

A Blue rocket won't be as long and slender as Falcon 9. The legs can be very different than the wide spreading legs of Falcon.

Offline Elmar Moelzer

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #11 on: 01/27/2016 11:45 AM »
A Blue rocket won't be as long and slender as Falcon 9. The legs can be very different than the wide spreading legs of Falcon.
Would that mean that they wont do road transport?

Offline rpapo

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #12 on: 01/27/2016 12:15 PM »
A Blue rocket won't be as long and slender as Falcon 9. The legs can be very different than the wide spreading legs of Falcon.
Would that mean that they wont do road transport?
Does it matter?  I thought they intended to build the factory a couple of miles away.
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Offline guckyfan

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #13 on: 01/27/2016 12:43 PM »
A Blue rocket won't be as long and slender as Falcon 9. The legs can be very different than the wide spreading legs of Falcon.
Would that mean that they wont do road transport?
Does it matter?  I thought they intended to build the factory a couple of miles away.

This. Most, initially all launches, would be from the production site. Plus they are reusable, which means less transport and there is a harbour nearby, so quite easily transportable anywhere.

Edit. The first launch vehicle is supposed to be one engine. It might even be road transportable. The next generation larger vehicle won't.
« Last Edit: 01/27/2016 12:44 PM by guckyfan »

Offline sanman

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #14 on: 01/27/2016 01:24 PM »
So was the decision to locate factory and launch site together done with the road transport constraint particularly in mind?

Offline leaflion

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #15 on: 01/27/2016 02:39 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?
« Last Edit: 01/27/2016 02:40 PM by leaflion »

Offline mvpel

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #16 on: 01/29/2016 12:09 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.
"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 rpapo

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #17 on: 01/29/2016 01:01 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.
Don't forget the dynamics of the problem, though: on the barge there will be a sinusoidal side-to-side rocking motion, which motion will impart a momentum to the entire rocket, and the legs have to not only absorb that energy, but do so quickly enough that the momentum doesn't result in overshooting that critical tilt angle.

In other words, if the barge tilted all the way between +23 and -23 degrees, even slowly, I don't think the rocket would remain upright for long.

Also consider another worst case: the rocket is tilted directly in line with one of its legs.  In that case, the leg in that direction may take a load far higher than any of the other three, the most extreme being right before tipping, when that leg is taking nearly 100% of the tilted rocket's weight.  How much can one of these legs take before they give?
« Last Edit: 01/29/2016 01:50 PM by rpapo »
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Offline alang

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #18 on: 01/29/2016 02:08 PM »
Does the Spacex approach lend itself, with modification, more to using the legs as an airbrake at partial extension than the blue origin approach

Offline mvpel

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #19 on: 01/30/2016 09:42 PM »
In other words, if the barge tilted all the way between +23 and -23 degrees, even slowly, I don't think the rocket would remain upright for long.

Also consider another worst case: the rocket is tilted directly in line with one of its legs.  In that case, the leg in that direction may take a load far higher than any of the other three, the most extreme being right before tipping, when that leg is taking nearly 100% of the tilted rocket's weight.  How much can one of these legs take before they give?

Nobody seems to grasp that the ASDS is the size of a football field, and for it to be tilted at 23 degrees bow to stern the bow would be about 120 feet higher than the stern, and port to starbord it would be 40 feet higher. In that kind of sea state, a previously-flown rocket tipping over would be the least of their concerns.

"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 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...?

Offline 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.

Offline 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

Online 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

Online 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 »

Online 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 »

Online 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

Online 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?

Online 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.

Online envy887

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #40 on: 05/12/2016 07:06 PM »
Sure, it's probably offset. Still, whatever altitude it reads with four feet on the deck will be less than the altitude it reads with only one foot down.

Online envy887

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #41 on: 05/12/2016 07:14 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.

I believe transient throttle response of the Merlin is proprietary information to SpaceX, but it's possible that the shutdown has to be commanded before the legs hit since there will be some lag as thrust decreases.

IMO shutdown time is likely calculated realtime from a control model using inputs from all control sensors (altimeter, gyros, and accelerometers).

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #42 on: 05/12/2016 07:18 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.

I believe transient throttle response of the Merlin is proprietary information to SpaceX, but it's possible that the shutdown has to be commanded before the legs hit since there will be some lag as thrust decreases.

IMO shutdown time is likely calculated realtime from a control model using inputs from all control sensors (altimeter, gyros, and accelerometers).

Yes, on second thought that must be the case.

Online John Alan

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #43 on: 05/12/2016 07:51 PM »
It's likely a carefully crafted sequence developed thru many simulations then just played back and run thru...
There is likely code to adjust some things in real time... but mostly just to adjust the timeline playing out...

Example... compares timeline expected radar altitude to actual reading...
Jumps back or forward in timeline to make equal and continues to monitor...

Example... I'm falling faster then I should be at this point on timeline...
Applies small plus offset to thrust commands in timeline to attempt to compensate...

Point is with a good timeline sequence laid out and then played out... it works...
Just has enough wiggle room built in to adjust for the actual deck height before it gets there...

The above is very simplified and just my opinion...  ;)
« Last Edit: 05/12/2016 10:51 PM by John Alan »

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #44 on: 05/12/2016 07:54 PM »
I'm sure the Blue Origin legs are more efficient, but the New Shepard stage is much squatter and so can get by with the much narrower stance.

Falcon 9's legs are wide because they need the wider stance for the long orbital first stage.

However, F9's long legs are also potentially useful as large aerobrakes, a feature SpaceX doesn't really use much yet but has hinted they may use in the future.
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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #45 on: 05/15/2016 05:17 AM »
... but the New Shepard stage is much squatter and so can get by with the much narrower stance.

You DO realize that the New Shepard has a wider stance than the Falcon9?
Stance is a measure of the ratio of base to height, which is almost 40% greater for the New Shepard than for the Falcon 9.

Offline docmordrid

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #46 on: 05/15/2016 06:06 AM »
But what are the ratios between c/g height and base width?
DM

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Re: Landing legs design : SpaceX vs Blue Origins
« Reply #47 on: 05/16/2016 12:52 PM »
CG height to base width is indeed the ratio you want. Wind loading (horizontal cross sectional area, height, horizontal drag coefficient, and max operating wind velocity) also need to be considered.


And is New Shepard ever going to land on a non-level surface, like a pitching barge in the ocean?

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