Author Topic: New Glenn landed booster: Why 6 legs? Hexapod Locomotion?  (Read 6010 times)

Offline TrevorMonty

I do wonder what payloads they plan to fly that demand downrange recovery. At estimated 50t expendable should still be good for 30t at RTLS or 40t downrange.

Offline leaflion

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Re: New Glenn landed booster: Why 6 legs? Hexapod Locomotion?
« Reply #21 on: 10/19/2016 03:34 AM »
Taking a look at the existing spaces & load bearing structures in a rocket design and then going with 6 legs is plausible. Walking rockets, not so much.

Rapid reuse is not required to reduce launch costs. Cheap reuse is. If a stage needs to spend x days or even weeks in transfer that should not be that expensive in itself, you "just" need more hardware in rotation.
That said a leg design that can be retracted at will goes a long way to make recovery and refurbish operations simpler and faster.

What I find more interesting is the small footprint of the deployed legs (~1.8x tank diameter) compared to the F9 (~5.6x ?), even smaller(!) than NS (~2x). Blue must be very certain that they can land the stage perpendicular, with very small tipping moments. 
Given the offshore recovery plans downrange that makes me think less barge and more Sea Launch style platform. (Land on a platform, transfer on a ship with platform cranes, unload at launch site, repeat.)

How are you calculating landing footprint?
I'm guessing you are circumscribing a circle around the shape of the legs to get the footprint.  A better method is to inscribe a circle within the shape of the legs.  That will be your minimum tipover radius.  This the primary reason nobody has tried 3 legs, it requires really long legs to get a bigger tipover radius.  6 legs will get you a significantly larger tipover radius for the leg length than a 4 legged design would.  That said, 6 legged New Glenn still has a smaller tipover radius than a 4 legged F9

Some math: R=L cos (180/n)
where R=tipover radius, L=leg length, n=# of legs.

As n->infinity, R=L

With one leg not deployed, R=L cos (360/n)

Decide if you require fault tolerance, then optimize for weight.  That's how many legs you want.
« Last Edit: 10/19/2016 03:35 AM by leaflion »

Offline Steven Pietrobon

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Re: New Glenn landed booster: Why 6 legs? Hexapod Locomotion?
« Reply #22 on: 10/19/2016 04:48 AM »
Some math: R=L cos (180/n)
where R=tipover radius, L=leg length, n=# of legs.

Some calculations below.
n   R/L
3   0.5
4   0.707
5   0.809
6   0.866
7   0.901
8   0.924

My personal preference is five legs. That's the minimum number where you can have one leg fail and still be standing. Here's the probably of falling given probably of a leg failure is p = 1% = 0.01. With five legs there are five combinations of single leg and five combinations of dual leg failures that are survivable. Compared to four legs, we can reduce the failure rate from about 1 in 20 to about 1 in 2020. Six legs only goes down to 1 in 3250.

n  Pf  p=0.01
3  1-(1-p)^3 = 2.97%
4  1-(1-p)^4 = 4.94%
5  1-(1-p)^5 - 5p(1-p)^4 - 5p^2(1-p)^3 = 0.0495%
6  1-(1-p)^6 - 6p(1-p)^5 - 12p^2(1-p)^4 = 0.0308%
« Last Edit: 10/19/2016 04:50 AM by Steven Pietrobon »
Akin's Laws of Spacecraft Design #1:  Engineering is done with numbers.  Analysis without numbers is only an opinion.

Offline Darkseraph

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Re: New Glenn landed booster: Why 6 legs? Hexapod Locomotion?
« Reply #23 on: 10/24/2016 08:38 PM »
Perhaps since it is landing on a barge out in the sea, that is not the most stable platform, having more points of contact has some advantage when landing. Or more legs allows shorter lighter structures; the span on this booster seems much less than Falcon 9.
"For a successful technology, reality must take precedence over public relations, for Nature cannot be fooled." R.P.Feynman

Offline Dante80

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Re: New Glenn landed booster: Why 6 legs? Hexapod Locomotion?
« Reply #24 on: 10/26/2016 06:40 AM »
Not only is New Glenn destined for barging - we've heard nothing about RTLS, but the hints we have from Blue are for landing much, much further from the launch pad than even the most energetic F9 GTO mission. I don't remember right now where this came from exactly (I think it was the DPL patent Blue did), but it is I think pretty credible.
« Last Edit: 10/26/2016 06:41 AM by Dante80 »

Offline Chasm

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Re: New Glenn landed booster: Why 6 legs? Hexapod Locomotion?
« Reply #25 on: 10/26/2016 07:53 AM »
We got a landing 750 nautical miles downrange from a launch site Drainage Analysis Technical Memorandum [PDF]. Here is the relevant part:

Quote from: Drainage Analysis Technical Memorandum, page 1-4
After arrival at the Integration Facility, the 1st Stage and 2nd Stage, and a possible 3rd Stage, would then be mated together and integrated onto the Transporter Erector system. Following integration of the booster stages, the SV (or PA) would be attached, and then the entire system would undergo a readiness test. The OLV would then be transported from the Integration Facility to the Launch Pad and erected for launch. After a successful launch the first stage would return to the Earth for recovery in the Atlantic Ocean approximately 750 nautical miles downrange in the Atlantic Ocean, east of and well off the Carolina coast, and any payload or capsule would land under parachute at a yet to be determined land site in Texas.


More recently Jeff Bezos said during the 2016 Pathfinder Awards that landing will be on a ship. (Link to that moment)

Drawing a 750nm circle around the launch site "well of the Carolina Coast" can be as close as ~100nm short of Bermuda. (Or further south as close as ~200nm short of Puerto Rico.)
Should be interesting to see if they try something with that in the future.

Offline ZachF

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Re: New Glenn landed booster: Why 6 legs? Hexapod Locomotion?
« Reply #26 on: 11/25/2016 01:54 AM »
Some math: R=L cos (180/n)
where R=tipover radius, L=leg length, n=# of legs.

Some calculations below.
n   R/L
3   0.5
4   0.707
5   0.809
6   0.866
7   0.901
8   0.924

My personal preference is five legs. That's the minimum number where you can have one leg fail and still be standing. Here's the probably of falling given probably of a leg failure is p = 1% = 0.01. With five legs there are five combinations of single leg and five combinations of dual leg failures that are survivable. Compared to four legs, we can reduce the failure rate from about 1 in 20 to about 1 in 2020. Six legs only goes down to 1 in 3250.

n  Pf  p=0.01
3  1-(1-p)^3 = 2.97%
4  1-(1-p)^4 = 4.94%
5  1-(1-p)^5 - 5p(1-p)^4 - 5p^2(1-p)^3 = 0.0495%
6  1-(1-p)^6 - 6p(1-p)^5 - 12p^2(1-p)^4 = 0.0308%

They are probably using 6 legs because the it allows you to put the leg hardware in between the 6 outer rocket engines. 5 legs would need 5-6 engines (5 engine outer ring) and the rocket nozzle area ratio is pretty poor for such designs compared to 7 engines like NG.

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