Author Topic: BFR landing cradle discussion and updates  (Read 45626 times)

Offline matthewkantar

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Re: BFR landing cradle discussion and updates
« Reply #20 on: 01/25/2017 08:58 PM »
Reminds me of this:


Matthew

Offline Lar

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Re: BFR landing cradle discussion and updates
« Reply #21 on: 01/25/2017 08:59 PM »
Drawn on a paper napkin (Stone Age MS Paint) during my lunch break. 

Pure Speculation.

The Cradle Ring and 8 hydro-shafts would need to be massive and blast proof.

I assume you used 4 lobes for easy mirroring in paint but the real one would have 3 (or multiples) of everything to  match the fins/alignment tabs, right?
"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 vanoord

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Re: BFR landing cradle discussion and updates
« Reply #22 on: 01/25/2017 09:04 PM »
Imagine that there are guide slideways, and the final cone of acceptance has a half angle of about 15 degrees.

Assume the slam is set for 1g.

If the rocket was able to land within 1 m of ground zero, it would only engage the slides about 4 m above ground, at a vertical speed of 9 m/s, and this will move it at a horizontal speed of about 2 m/s.

Not so crazy.

Aye.

Landing into something which acts as a funnel / guide, allowing a tolerance of say +/- 5m, is about the only way this is going to work.

Suspect it may have to be relatively tall, to deal with the lean of the stage if it's guided sideways to the maximum possible, but that could be designed out of accuracy were improved.

Matched thrusters top and bottom may enable some sort of horizontal translation in the final few metres.

Essentially, guiding the stage down to somewhere that it can rest until the launch clamps are re-engaged (and the legs retract) is the critical bit, ie dealing with any potential horizontal displacement - presumably caused by wind and targeting errors.

The fix is probably as much in software as it is in physical design - the experience with F9 will be vital.

Offline Req

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Re: BFR landing cradle discussion and updates
« Reply #23 on: 01/25/2017 09:20 PM »
I picture something like a very tall king's crown with 3 triangles/points.  Possibly wider diameter at the top than the bottom.  Some kind of low friction material(and/or lubrication) along the leading edges and the fins.  Maybe the whole thing is 3 interlocking pieces that retract vertically/horizontally or even swing up and down like the TEL.

Edit:  Attached an mspaint image.  I'm afraid you'll have to use your imagination a bit, but this is a very very rough sketch of the idea shown directly from the side.  The red line indicates a seam between two of the segments, and you're seeing around half each of 2 of the 3 segments(third segment in the background omitted).  From the top it would just look like a circle.  In this example the diameter does not increase towards the top, but I wouldn't be surprised if that was a desirable trait.  Maybe not necessary with so much room to be off-nominal at the top already.  I'm picturing fairly thin material here - maybe an inch or two(or less) as viewed from the top, just enough to handle the landing environment and structural loads.  It likely wouldn't be solid/sheeted but it's easier to think of it and depict it that way.

I don't go into the mechanism to securely clamp it down once it's there, but I imagine you'd want that to be separate so you can retract these guides once it's secure.  Ideally these guides would get the rocket directly where it needs to be to use the same hold-downs that it uses for launch with little or no dancing cradle/mount equipment.

Edit again - This idea has a fairly slammy landing in mind.  If they plan on bringing it down nice and slow, then something like this is probably the completely wrong idea.
« Last Edit: 01/26/2017 05:12 AM by Req »

Offline Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #24 on: 01/25/2017 09:44 PM »
You can imagine the launch cradle like a giant docking port. Put a huge Stewart platform on it (like the NDS/iLIDS docking port) to help catch the stage.

This Stewart platform can handle a 10 ton payload:


 The ITS booster is 275 tons dry, so you're talking 30x greater.

Attached is another 10 ton Stewart platform. The amount of play needed in absolute terms is probably similar, but not in proportional terms. So you could probably gang up like 30 actuators each 6x stronger around the perimeter instead of the usual 6.

I also attached a picture of the NDS docking port which is based on the 6 degrees of freedom Stewart platform.
http://www.dynamic-concepts.com/projects/nds.html
« Last Edit: 01/25/2017 09:53 PM by Robotbeat »
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Offline matthewkantar

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Re: BFR landing cradle discussion and updates
« Reply #25 on: 01/25/2017 10:08 PM »
One thing to remember that will be helpful, the landing engine are 19 feet or so away from the tips of the bosses shown in the IAC presentation. This should make exhaust impingement more manageable.

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #26 on: 01/26/2017 02:19 AM »
I'll give it a try too.

A couple of things about the sketch:

- The size of the "Flippers" shown is arbitrary.  It depends on what is the 3-sigma(?) confidence in getting the booster within a certain deviation of the zero position.  I drew them way too large probably.

- The flippers are shown on the rocket since it's conceptually easier to understand, but they can (and probably would) be put on the cradle (and so the "cone" will be on the rocket.)

- The flippers are set at a wider angle than the receiving cone.  So during the mating, the rocket gets centered, and the flippers get compressed inward, gradually.

- The flipper suspension is a spring-shock, so it both centers the vehicle and dissipates lateral motion energy.  It's important not to transmit sideways shock, since the rocket is very weak in that direction.

- The flipper's anchor points straddle the CG of the empty stage, so they move it laterally with minimal tilting.

- Final capture happens after the rocket centers at the bottom of the cone, by a different mechanism at the bottom of the rocket, not shown.

« Last Edit: 01/26/2017 02:41 AM by meekGee »
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Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #27 on: 01/26/2017 04:05 AM »
And this is the "shocks on the ground" version.

The difference in angles is the same, to guarantee that first contact is made at the height of the C.G., and then the flippers center the stage and cushion the sideways forces.

The choice is not clear, and depends on the ability to control the clocking of the stage, even after the first "flipper" makes contact.  If clocking is guaranteed, then both sides of the mechanism can be only 3 planar members.

If clocking is not guaranteed, then the "cone" needs to really be a cone.  In that case, maybe it's easier to put the flippers on the rocket and the cone on the ground, like in the first picture.

In the picture below, the "hole" is larger, since it has to fit rocket+fins.   In the picture above, it had only to fit rocket plus retracted (flush to the body) flippers.
« Last Edit: 01/26/2017 04:22 AM by meekGee »
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Online drzerg

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Re: BFR landing cradle discussion and updates
« Reply #28 on: 01/26/2017 06:38 AM »
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.

Online Semmel

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Re: BFR landing cradle discussion and updates
« Reply #29 on: 01/26/2017 07:13 AM »
And this is the "shocks on the ground" version.

The difference in angles is the same, to guarantee that first contact is made at the height of the C.G., and then the flippers center the stage and cushion the sideways forces.

The choice is not clear, and depends on the ability to control the clocking of the stage, even after the first "flipper" makes contact.  If clocking is guaranteed, then both sides of the mechanism can be only 3 planar members.

If clocking is not guaranteed, then the "cone" needs to really be a cone.  In that case, maybe it's easier to put the flippers on the rocket and the cone on the ground, like in the first picture.

In the picture below, the "hole" is larger, since it has to fit rocket+fins.   In the picture above, it had only to fit rocket plus retracted (flush to the body) flippers.

I like this version better, meekGee. You can make the cone V-shaped that aligns the rockets rotation first and then hits the cone part where it slides into the middle.

It would certainly help if the rocket would be able to hover. Not for hovering as such, but for canceling the gravity of earth. The flight profile would be like breaking to 10m/s or so just before the cradle and then slide down with constant velocity. That might increase the burn time by 1 to 2 seconds but would limit the force when touching the cradle and gives more time to align the rocket.

Online JamesH65

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Re: BFR landing cradle discussion and updates
« Reply #30 on: 01/26/2017 11:06 AM »
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.

Been pondering how much of an effect the wind can have. The ITS weighs quite a lot (cannot find figures, but 100t?), so therefor has a lot of momentum. It will take quite a gust to cause excessive movement.

Online envy887

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Re: BFR landing cradle discussion and updates
« Reply #31 on: 01/26/2017 01:40 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.

Been pondering how much of an effect the wind can have. The ITS weighs quite a lot (cannot find figures, but 100t?), so therefor has a lot of momentum. It will take quite a gust to cause excessive movement.

Booster mass is 275 tonnes per the IAC presentation.

A calculation assuming a Cd of 1.0 and a wind speed of 10 m/s gives a wind force of 58 kN on the 12m x 80m booster. Handy (but simplistic) calculator here: http://www.engineeringtoolbox.com/wind-load-d_1775.html

If the terminal control thrusters produce 100 kN and have an adequately responsive control scheme they should have little trouble countering wind gusts in moderate weather.

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #32 on: 01/26/2017 01:56 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.

Been pondering how much of an effect the wind can have. The ITS weighs quite a lot (cannot find figures, but 100t?), so therefor has a lot of momentum. It will take quite a gust to cause excessive movement.

Booster mass is 275 tonnes per the IAC presentation.

A calculation assuming a Cd of 1.0 and a wind speed of 10 m/s gives a wind force of 58 kN on the 12m x 80m booster. Handy (but simplistic) calculator here: http://www.engineeringtoolbox.com/wind-load-d_1775.html

If the terminal control thrusters produce 100 kN and have an adequately responsive control scheme they should have little trouble countering wind gusts in moderate weather.
That's no cold gas thruster...  that's a superdraco...
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Online envy887

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Re: BFR landing cradle discussion and updates
« Reply #33 on: 01/26/2017 02:11 PM »
That's no cold gas thruster...  that's a superdraco...

AIUI Musk stated at IAC that the ITS booster will use 10 tonne gaseous methalox thrusters for terminal landing guidance. Grid fins are useless for terminal guidance and cold gas thrusters are far too small.

Offline Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #34 on: 01/26/2017 02:13 PM »
You can build cold gas thrusters as big as you want. They're not "too small." They're too heavy for the total impulse needed.
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Offline Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #35 on: 01/26/2017 02:15 PM »
By the way, 10 ton methane thrusters is the same order of magnitude as the typical (8 ton) superdraco.
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Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #36 on: 01/26/2017 02:19 PM »
That's no cold gas thruster...  that's a superdraco...

AIUI Musk stated at IAC that the ITS booster will use 10 tonne gaseous methalox thrusters for terminal landing guidance. Grid fins are useless for terminal guidance and cold gas thrusters are far too small.
I meant "SD-sized" - for a sense of scale...

Yes, if it's designed for rapid turn around, working with methane makes more sense of course.

By gasous I take it that it takes gas into the combustion chamber.  A big blow torch.  Does anyone know if the that's correct?
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Online envy887

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Re: BFR landing cradle discussion and updates
« Reply #37 on: 01/26/2017 02:26 PM »
You can build cold gas thrusters as big as you want. They're not "too small." They're too heavy for the total impulse needed.

For a reasonable mass budget to meet the ITS booster thrust requirements they are far too small.

Offline Robotbeat

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Re: BFR landing cradle discussion and updates
« Reply #38 on: 01/26/2017 02:28 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.
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Online envy887

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Re: BFR landing cradle discussion and updates
« Reply #39 on: 01/26/2017 02:31 PM »
That's no cold gas thruster...  that's a superdraco...

AIUI Musk stated at IAC that the ITS booster will use 10 tonne gaseous methalox thrusters for terminal landing guidance. Grid fins are useless for terminal guidance and cold gas thrusters are far too small.
I meant "SD-sized" - for a sense of scale...

Yes, if it's designed for rapid turn around, working with methane makes more sense of course.

By gasous I take it that it takes gas into the combustion chamber.  A big blow torch.  Does anyone know if the that's correct?

Yes. Mostly likely pressure fed, either from high-pressure bottles of O2 and CH4 gas, or possibly directly from the autogenous pressurant system for the main landing tanks.

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