Author Topic: BFR landing cradle discussion and updates  (Read 39500 times)

Offline envy887

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Re: BFR landing cradle discussion and updates
« Reply #40 on: 01/26/2017 03:01 PM »
Nah, you're still not getting it. This has to do with total impulse, not that the thrusters are "too small." I promise the cold gas thrusters are pretty dang powerful on the F9 booster.

Same thing, for various definitions of "small". Sure you can build a huge N2 thruster, but the prop it needs will mass 3x a SuperDraco and 4x a methalox thruster. Put the other way, for the same mass budget a N2 thruster will either be 3 to 4x underpowered or run out of propellant 3 to 4x sooner.

Online Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #41 on: 01/26/2017 03:21 PM »
You've got it! It has nothing to do with the thruster being "too small," but the tanks being too heavy. I've made you think about it so now you're changing the definition of "small" so it's the opposite of what you originally intended. My work here is done. :)
« Last Edit: 01/26/2017 03:24 PM by Robotbeat »
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Offline envy887

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Re: BFR landing cradle discussion and updates
« Reply #42 on: 01/26/2017 03:39 PM »
Or, they could make the tanks the right mass and feed the thrusters long enough, but the thrusters would be underpowered. You can pick two of the three (mass, impulse, thrust), but that's it. :D

Online Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #43 on: 01/26/2017 03:55 PM »
Or, they could make the tanks the right mass and feed the thrusters long enough, but the thrusters would be underpowered. You can pick two of the three (mass, impulse, thrust), but that's it. :D
As I said, my work here is done.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline Lars-J

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Re: BFR landing cradle discussion and updates
« Reply #44 on: 01/26/2017 04:43 PM »
it is obvious that moving the cradle is easier then moving the rocket. in final moments any wind gust can ruin everything. and cradle is only need to hold 300 tons. for example some bridge cranes could move 750 ton things. after catching the booster it could be positioned/transported (if you have enough long rails you could catch it 300-400m off the pad)  to launch hold downs.

No, it is not obvious. And the rocket is already moving! :) Adding a 2nd moving piece is just adding a bunch of complexity and failure modes.

The rocket will not launch during adverse weather conditions, similar constraints for the landing. So there will be no landing during heavy winds. Powerful thrusters at the base and top of the rocket should be sufficient for lateral accuracy.
« Last Edit: 01/26/2017 04:44 PM by Lars-J »

Online Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #45 on: 01/26/2017 05:15 PM »
Parts of the cradle will need to move simply to absorb the energy of impact. Just like a docking port or landing legs.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #46 on: 01/26/2017 05:20 PM »
We know that... To your method, the main engines can be cold thrusters too.  Granted. Semantically.

Meanwhile, in practical land, cold gas thrusters have very low ISP, and high dry tank mass.  Lose-lose. 

They win on simplicity, when the penalty is small.

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Offline Lars-J

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Re: BFR landing cradle discussion and updates
« Reply #47 on: 01/26/2017 05:30 PM »
Parts of the cradle will need to move simply to absorb the energy of impact. Just like a docking port or landing legs.

Obviously. But dampening motion is not the same has having an actively moving landing cradle.

Online Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #48 on: 01/26/2017 06:45 PM »
Parts of the cradle will need to move simply to absorb the energy of impact. Just like a docking port or landing legs.

Obviously. But dampening motion is not the same has having an actively moving landing cradle.
But the mechanism will have to extend and retract anyway, and being actively guided means you can better absorb the energy without exceeding your limits. It also could help position the booster after capture to mate up to the launch hold-downs (or launch mount posts, in case hold-downs aren't used) prior to refilling.

Again, the NDS docking system is actively positioned, too, and I think you might as well actively guide the cradle since it already needs everything else (pistons with significant travel) anyway.

Again, I think it's helpful to think of the cradle as a giant docking port.
« Last Edit: 01/26/2017 06:48 PM by Robotbeat »
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Offline matthewkantar

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Re: BFR landing cradle discussion and updates
« Reply #49 on: 01/26/2017 07:46 PM »
The equipment on the ground can be as heavy as it needs to be. The idea is all about shedding weight from the booster. Adding X tons to the docking device is worth it if it takes X pounds off of the booster. Step one is to calculate how much error there will be, step two is design the catcher to account for it.

Some sort of sensor needs to be able to precisely locate the base of the rocket while flames and smoke are present, then the catcher has to twist and tip to catch the hold downs, then drop to absorb the impact while straightening out out any tilt.

Hydraulics can be arbitrarily large and can be extremely agile. The booster will do its best until it touches the cradle and then surrender its fate to the equipment on the ground. I think this is doable.

Matthew

Online Doesitfloat

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Re: BFR landing cradle discussion and updates
« Reply #50 on: 01/26/2017 08:17 PM »
I'm on the boat that believes the only thing that moves on the landing platform is the do-dad that locks it down after it lands. The booster will have the precision to find it's landing pad from space; the center of the landing pad from the start of the landing burn and the ability to adjust to the center and land gently from 2 meters up.
Without those abilities just put legs on it.

Online Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #51 on: 01/26/2017 08:21 PM »
The only way to gently land is with some kind of springiness. On F9, that's the legs and the cylinders of the legs. You're not going to have two hard surfaces (one stiffly connected to the rocket and the other to the ground) collide without some kind of damage. Just like docking in space, you're going to need something to absorb that energy. Basic physics.
« Last Edit: 01/26/2017 08:22 PM by Robotbeat »
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Offline Jim

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Re: BFR landing cradle discussion and updates
« Reply #52 on: 01/26/2017 10:16 PM »
The booster will have the precision to find it's landing pad from space; the center of the landing pad from the start of the landing burn and the ability to adjust to the center and land gently from 2 meters up.


not possible.  It may find it, but hitting is a different matter.  Show me an airplane on autoland hitting the exact same spot.
« Last Edit: 01/26/2017 10:17 PM by Jim »

Offline Space Ghost 1962

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Re: BFR landing cradle discussion and updates
« Reply #53 on: 01/26/2017 10:30 PM »
The booster will have the precision to find it's landing pad from space; the center of the landing pad from the start of the landing burn and the ability to adjust to the center and land gently from 2 meters up.


not possible.  It may find it, but hitting is a different matter.  Show me an airplane on autoland hitting the exact same spot.

Interesting. I've actually witnessed attempts at this. The scatter was about a yard with a modified MLS with doppler. In best conditions. A far smaller craft than a F9 first stage.

Offline Lars-J

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Re: BFR landing cradle discussion and updates
« Reply #54 on: 01/26/2017 10:47 PM »
The only way to gently land is with some kind of springiness. On F9, that's the legs and the cylinders of the legs. You're not going to have two hard surfaces (one stiffly connected to the rocket and the other to the ground) collide without some kind of damage. Just like docking in space, you're going to need something to absorb that energy. Basic physics.

Nobody disputes this, so I'm not sure why you keep writing it. But it could be as simple as heavy duty springs or something with a crush zone.

Online Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #55 on: 01/26/2017 10:52 PM »
No one disputes this? The guy above mine just said "the one thing that moves is the do-dad that locks it down..." Also: Crush needs to be replaced, and in the F9 is placed in cylinders in the legs. Either way, not amenable to turnarounds of just hours without removing the rocket from the pad. Springs also no good by themselves, must be damped, i.e. By cylinders.

You'll end up with cylinders of some sort whether passive or active. Since you need positioning of the rocket and since you could improve performance by actively catching the rocket, I think you might as well make the cylinders actively actuate.
« Last Edit: 01/26/2017 10:59 PM by Robotbeat »
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Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #56 on: 01/27/2017 12:02 AM »
Hey - opinions vary.

I don't think hitting the release mechanism (mm accuracy) is feasible, and even if it were, it's a waste of resources on the rocekt.

I don't think an active catcher for such a large body makes sense either.

So I have a passive or half-active "final guidance" rails that will bring the rocket in from ~1-2 m accuracy to final capture.

This will remain open for debate till we see what SpaceX has in mind, and even then, it'll be open to debate as to how they should have done it.

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Offline Space Ghost 1962

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Re: BFR landing cradle discussion and updates
« Reply #57 on: 01/27/2017 12:14 AM »
Hey - opinions vary.

I don't think hitting the release mechanism (mm accuracy) is feasible, and even if it were, it's a waste of resources on the rocekt.

I don't think an active catcher for such a large body makes sense either.

So I have a passive or half-active "final guidance" rails that will bring the rocket in from ~1-2 m accuracy to final capture.

This will remain open for debate till we see what SpaceX has in mind, and even then, it'll be open to debate as to how they should have done it.


Agreed.

There is so much presumption here about landing systems and approaching BFR level RTLS.

First off, Falcon 9 is neither built like BFR nor scales like BFR. So it is unlikely to actually land like BFR.

What could you do to bring it close enough? Precision landing like BFR. You'd want, as Jim pointed out above, reproducible within feet/inches, which would be hard if not impossible.

That you could do with the current design, forget about the mechanism for the moment. Because neither is the vehicle suitable structurally nor is the landing platform (it moves!).

So what would be structurally and operationally suitable? Well, the vehicle accepts landing loads from where the legs attach, as well as being stablized with. So remove the legs from the LV and have a mechanism from the barge that performs the same functions when the LV has completed its precision approach.

Then you can move forward to BFR landings. Don't get hung up on the need to use the exact same mechanism. That would be suboptimal.

« Last Edit: 01/27/2017 12:14 AM by Space Ghost 1962 »

Offline dglow

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Re: BFR landing cradle discussion and updates
« Reply #58 on: 01/27/2017 12:18 AM »
What could you do to bring it close enough? Precision landing like BFR. You'd want, as Jim pointed out above, reproducible within feet/inches, which would be hard if not impossible.

That you could do with the current design, forget about the mechanism for the moment. Because neither is the vehicle suitable structurally nor is the landing platform (it moves!).

So what would be structurally and operationally suitable? Well, the vehicle accepts landing loads from where the legs attach, as well as being stablized with. So remove the legs from the LV and have a mechanism from the barge that performs the same functions when the LV has completed its precision approach.

Appreciate the gradual approach, and learning with F9 first. But do so on land; don't bother with the barge.

Offline Space Ghost 1962

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Re: BFR landing cradle discussion and updates
« Reply #59 on: 01/27/2017 12:56 AM »
What could you do to bring it close enough? Precision landing like BFR. You'd want, as Jim pointed out above, reproducible within feet/inches, which would be hard if not impossible.

That you could do with the current design, forget about the mechanism for the moment. Because neither is the vehicle suitable structurally nor is the landing platform (it moves!).

So what would be structurally and operationally suitable? Well, the vehicle accepts landing loads from where the legs attach, as well as being stablized with. So remove the legs from the LV and have a mechanism from the barge that performs the same functions when the LV has completed its precision approach.

Appreciate the gradual approach, and learning with F9 first. But do so on land; don't bother with the barge.

I agree it would be easier. Thus the "it moves!" reference. However, now you have other considerations too.

First, they'll want to have an operational tempo with RTLS to become commonplace to build a launch skill set and expectations also for BFR - you might not want to interfere with that.

Barge recoveries always will be chancy and slow by their nature. Also, since the barge recoveries are downrange, you get an immediate benefit for the mass reduction of missing the legs. Plus, high performance FH core recoveries will doubtlessly push to the max any attempts to recover a usable stage, so it will remain experimental for some time to come, if even tried at all.

Another advantage of NOT modifying the LV is that you could continue to push reuse either way until you decided which to use at the last moment for a given mission.

Finally, I think SX would rather have a "boom" on a barge than on land.

But again, you're right land is much easier.

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