Author Topic: BFR landing cradle discussion and updates  (Read 23100 times)

Offline OneSpeed

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Re: BFR landing cradle discussion and updates
« Reply #220 on: 02/26/2017 10:21 AM »
... As a vehicle scales up, the leg need to scale up in mass and strength much faster. A booster scaled 2x in all dimensions will mass ~ 8X, forcing the legs to become a much more significant part of the vehicle. Or to put it another way: Landing legs scale poorly with size increases.

The nominal strength of a structure is independent of the structure size when geometrically similar structures are considered. Any deviation from this property is called the size effect, and is only encountered when the relative sizes differ by orders of magnitude. For a booster scaled 2x, all of its components will mass 8x, and their strengths will scale equally. Landing legs scale no differently than any other part of the rocket.

Offline cartman

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Re: BFR landing cradle discussion and updates
« Reply #221 on: 02/26/2017 11:07 AM »
My opinion is that what we saw at IAC was Elon's idea of how things would look like when they have achieved their goals for ITS. So this is their vision, not how ITS will look like in its first years.
My guess is that the first generation may be with legs, until they manage to never miss their target by more than a meter or so. Then they remove the legs and land onto a pure landing cradle. Once they manage to do that reliably and in general fly without incidents for hundreds of flights then they will begin launching from the landing cradle. Experience will help refine all the designs. At the beginning they will have large fuel reserves and will only fly in very low wind conditions and as they get more comfortable they will lower the reserves and loosen the weather constraints. A tall wind barrier could help here.

Offline livingjw

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Re: BFR landing cradle discussion and updates
« Reply #222 on: 02/26/2017 01:14 PM »

Yes. That is indeed the primary reason for a leg-less design. As a vehicle scales up, the leg need to scale up in mass and strength much faster. A booster scaled 2x in all dimensions will mass ~ 8X, forcing the legs to become a much more significant part of the vehicle. Or to put it another way: Landing legs scale poorly with size increases.

Using a landing cradle is a way of putting *most* of that leg mass on the ground instead... The landing cradle handles the dampening of the final motion.

Combining the landing cradle with the launch mount is simply intended to increase flight frequency, nothing more.

Landing legs are probably in the neighborhood of 1% of the landing weight, independent of size.

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #223 on: 02/26/2017 03:09 PM »
OK accepting that no legs equals less fuel even with slower precision landing.

My point that they will not want a separate pure landing cradle to facilitate fast turn around stands.
Indeed.
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Offline dglow

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Re: BFR landing cradle discussion and updates
« Reply #224 on: 02/26/2017 04:44 PM »
OK accepting that no legs equals less fuel even with slower precision landing.

My point that they will not want a separate pure landing cradle to facilitate fast turn around stands.

Unless:

i.  There is more than one booster available for a given launch pad
ii.  Flight rate improves by rotating the boosters

Offline Patchouli

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Re: BFR landing cradle discussion and updates
« Reply #225 on: 02/26/2017 05:13 PM »
One option could be they make the cradle transportable similar to the shuttle mobile launch platform and have several of them.

That way landing occurs away from the service tower etc and if a mishap does occur only the cradle and booster are damaged.
« Last Edit: 02/26/2017 05:15 PM by Patchouli »

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #226 on: 02/26/2017 09:10 PM »
One option could be they make the cradle transportable similar to the shuttle mobile launch platform and have several of them.

That way landing occurs away from the service tower etc and if a mishap does occur only the cradle and booster are damaged.

True.  But now you've added a crawler transporter, which is a rare beast and much more complex then "just a concrete pad", a system for moving BFR from the transporter to the pad, and extra processing time.

It's possible, just like landing on a dedicated pad is possible, but it's not a no-brainer - it's a trade-off.

I can't imagine SpaceX didn't cover all three options before deciding on "back-to-the-pad".

I'm not even going to say they'll 100% stick with their choice. They're nothing if not agile.  But to say that "back-to-the-pad" is clearly dumb because they obviously didn't factor in risk to the pad is just nonsense - that was the whole point of the argument upthread.

And now, as Lar hinted - back to the landing cradle itself.
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Offline dglow

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Re: BFR landing cradle discussion and updates
« Reply #227 on: 02/26/2017 09:15 PM »
But now you've added a crawler transporter, which is a rare beast and much more complex then "just a concrete pad", a system for moving BFR from the transporter to the pad, and extra processing time.

They'll need a transporter regardless of where it lands. Extra processing time, yes, that's the tradeoff.

Offline RobLynn

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Re: BFR landing cradle discussion and updates
« Reply #228 on: 02/26/2017 10:12 PM »
Landing legs are probably in the neighborhood of 1% of the landing weight, independent of size.

Current Falcon 9 landing legs are "less than a model S" ie less than 2000kg.  assuming clsoe to the upper limit then that is closer to 5-10% of landed weight than 1%.  Of course this is pretty close to a worst case design given wide base for very tall thin stage and aero drag incorporated.

I believe helicopters are about 2% and airliners around 5-8% of MTOW. 

Landing leg (and aircraft undercarriage) mass is dependant on landed mass and length to be bridged.  Bigger longer nozzles mean longer legs, and larger diameter stages also add greater length to be bridged by extra thrust structure or longer legs or adding bending stiffness to the tank to make it more square-bottomed.

1% sounds very optimistic for a 12m rocket like ITS.
I'm a "glass is twice as big as it needs to be" kinda guy

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #229 on: 02/26/2017 10:30 PM »
But now you've added a crawler transporter, which is a rare beast and much more complex then "just a concrete pad", a system for moving BFR from the transporter to the pad, and extra processing time.

They'll need a transporter regardless of where it lands. Extra processing time, yes, that's the tradeoff.

For the BFR?  As part of the launch cycle?  Why would they? 
Or are you referring to transport from the assembly site?  This may be true, unless they assemble it on a different pad and fly it in. (Such a staging pad also solves the whole "rocket rotation" issue.)
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Offline RobLynn

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Re: BFR landing cradle discussion and updates
« Reply #230 on: 02/28/2017 09:31 AM »
A high speed cable driven motion simulator:

Perhaps this has potential for adaption and scaling up into an actively positioned landing cradle?  It is nice in that it keeps the cradle light with most of the positioning control hardware far from the rocket exhaust, and has potential to accommodate a lot of inaccuracy in speed and height to reduce the landing guidance demands.

With arrestor hooks on the BFR or ITS vehicle it could potentially even absorb some of the falling velocity (or taken to extremes with 3 huge towers all of the landing fuel could be eliminated.
I'm a "glass is twice as big as it needs to be" kinda guy

Offline Hobbes-22

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Re: BFR landing cradle discussion and updates
« Reply #231 on: 02/28/2017 12:03 PM »
One option could be they make the cradle transportable similar to the shuttle mobile launch platform and have several of them.

That way landing occurs away from the service tower etc and if a mishap does occur only the cradle and booster are damaged.

True.  But now you've added a crawler transporter, which is a rare beast and much more complex then "just a concrete pad", a system for moving BFR from the transporter to the pad, and extra processing time.


Transporters are far less rare than they used to be. You can rent (or buy) self-propelled modular transporters that can easily scale (by combining modules) to the size needed to transport a BFR.

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #232 on: 02/28/2017 02:20 PM »
One option could be they make the cradle transportable similar to the shuttle mobile launch platform and have several of them.

That way landing occurs away from the service tower etc and if a mishap does occur only the cradle and booster are damaged.

True.  But now you've added a crawler transporter, which is a rare beast and much more complex then "just a concrete pad", a system for moving BFR from the transporter to the pad, and extra processing time.


Transporters are far less rare than they used to be. You can rent (or buy) self-propelled modular transporters that can easily scale (by combining modules) to the size needed to transport a BFR.
He was talking about transporters into which the rocket lands.
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Offline Hobbes-22

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Re: BFR landing cradle discussion and updates
« Reply #233 on: 02/28/2017 02:25 PM »
One option could be they make the cradle transportable similar to the shuttle mobile launch platform and have several of them.

That way landing occurs away from the service tower etc and if a mishap does occur only the cradle and booster are damaged.

True.  But now you've added a crawler transporter, which is a rare beast and much more complex then "just a concrete pad", a system for moving BFR from the transporter to the pad, and extra processing time.


Transporters are far less rare than they used to be. You can rent (or buy) self-propelled modular transporters that can easily scale (by combining modules) to the size needed to transport a BFR.
He was talking about transporters into which the rocket lands.

Like the MLP used for the Shuttle, you'd split the fireproof ironmongery from the transporter.

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #234 on: 02/28/2017 02:31 PM »
One option could be they make the cradle transportable similar to the shuttle mobile launch platform and have several of them.

That way landing occurs away from the service tower etc and if a mishap does occur only the cradle and booster are damaged.

True.  But now you've added a crawler transporter, which is a rare beast and much more complex then "just a concrete pad", a system for moving BFR from the transporter to the pad, and extra processing time.


Transporters are far less rare than they used to be. You can rent (or buy) self-propelled modular transporters that can easily scale (by combining modules) to the size needed to transport a BFR.
He was talking about transporters into which the rocket lands.

Like the MLP used for the Shuttle, you'd split the fireproof ironmongery from the transporter.
OK, so the rare thing becomes the movable cradle/flame-duct.

The transporter meanwhile has to handle not just static carrying, but the forces and moments of capture.

And still you have a transfer operation to the pad itself.

Like I said above - all these variants are feasible, but none of them is "simply simpler", and they are all pretty obvious, and so were not selected for the current plan.
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Offline dglow

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Re: BFR landing cradle discussion and updates
« Reply #235 on: 02/28/2017 07:47 PM »
Simply simpler than what? The ITS video showed us but a snapshot.

meekGee, you contend upthread that a secondary (non-launch pad) landing cradle would necessitate the creation of a booster transporter. 

I assert they will need a transporter regardless. How could they get by without one? SpaceX would need to:

1. Assemble the booster vertically
2. On a site from which the booster can launch
3. Fly it to a site for test-firing
4. Fly it to the primary/departure launch pad
5. Never have more than one booster per primary launch pad

Now we entertain the thought exercise whereby the locations of 1 and 2 can be consolidated with 3, or 3 can be consolidated with 4, or 1 through 4 are consolidated. But it starts to seem a bit unlikely, doesn't it? And we haven't even gotten to 5.

Unless the launch tower unleashes a factory of nano-assemblers and ITS springs into existence on the launch pad itself, the booster will need to be moved and transported outside of its own power.

I love the idea of a rocket remaining vertical throughout its lifetime, but don't see it happening for ITS.

Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #236 on: 02/28/2017 08:15 PM »
Simply simpler than what? The ITS video showed us but a snapshot.

meekGee, you contend upthread that a secondary (non-launch pad) landing cradle would necessitate the creation of a booster transporter. 

I assert they will need a transporter regardless. How could they get by without one? SpaceX would need to:

1. Assemble the booster vertically
2. On a site from which the booster can launch
3. Fly it to a site for test-firing
4. Fly it to the primary/departure launch pad
5. Never have more than one booster per primary launch pad

Now we entertain the thought exercise whereby the locations of 1 and 2 can be consolidated with 3, or 3 can be consolidated with 4, or 1 through 4 are consolidated. But it starts to seem a bit unlikely, doesn't it? And we haven't even gotten to 5.

Unless the launch tower unleashes a factory of nano-assemblers and ITS springs into existence on the launch pad itself, the booster will need to be moved and transported outside of its own power.

I love the idea of a rocket remaining vertical throughout its lifetime, but don't see it happening for ITS.

Context, please.

What the vid showed was a booster that lands in a cradle co-located with the launch pad.

A bunch of people jumped up crying "that's insane, the risk, why don't they land somewhere else", which boils down to two alternative:

(1) Land on stationary cradles, and have a "simple" transporter take the rocket (vertical?  horizontal?) back to the pad - thus requiring two transfer operations.

(2) Land on a movable cradle-transporter, drive back to the pad (vertical?  horizontal?), and have one transfer operation.

I was showing that both alternate systems add complexity and time, but don't save a whole lot of risk.

You're arguing they need a transporter anyway, but:

- First, even if they do, it's not a cradle-transporter anymore (so strike option 2)
- Second, for MRO and such, time is not a problem like it is within an hours-turn-around operation, so having a transporter doesn't mean they want to use it every launch cycle.
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Offline RobLynn

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Re: BFR landing cradle discussion and updates
« Reply #237 on: 03/22/2017 05:23 AM »
Automatic bullseye dartboard (moving board around using servo controlled winches), includes visual processing of dart trajectory.

Just need to scale it up a little :)
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Offline meekGee

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Re: BFR landing cradle discussion and updates
« Reply #238 on: 03/22/2017 07:01 AM »
Automatic bullseye dartboard (moving board around using servo controlled winches), includes visual processing of dart trajectory.

Just need to scale it up a little :)
And clearly it can catch three cores in quick succession too.

Just as long as they don't mislabel the cores and cause it to miss on purpose.
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Online Norm38

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Re: BFR landing cradle discussion and updates
« Reply #239 on: 03/23/2017 02:52 AM »
The shuttle transporter had to carry the entire STS.  Orbiter, main tank, payload, and full solid rocket boosters.

This transporter would have to carry an empty booster stage.  Quite a bit less mass to move.

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