Rocket landing legs

[Kaos]

What kind of landing legs do you think is best for rockets that are meant to land rocket powered on legs? The more legs, the more weigh, but if one breaks, you might still stand with more legs. Perhaps legs of different strength can also help?

Both SpaceX/Falcon 9 and Blue Origin/New Shepard use four legs and I am sure they have thought about this topic. But four is the biggest number where you do not stand stable if one leg is missing. But on the other hand it is not sure that you stand stable enough with more legs when one is broken. Perhaps that will be no problem anymore, when the technique is mature enough?

Or is there a feasible way to help the landed rocket from the ground? I would guess it is technical unfeasible to do so, but are not sure: Could tethers connected from the ground to the rocket in the landing process enhance stability?

My personal guess is that 6 legs would be the right number of legs. What do you think?

[kerbiloid]

Three legs and a landing onto a heavy cargo elevator bringing it down, into the dungeon.

Edited January 18, 2016 by kerbiloid

[Nibb31]

Three legs is the minimum for stability. The Apollo LM designers considered both 3 and 4 leg configurations, but the 4 legged version offer more tolerance to landing on a slope. It also provided an easier arrangement regarding RCS thrusters. In a 3 legged arrangement with 4 RCS quads, one of the legs would have been too close to a thruster. I don't think they ever considered more legs because weight was the main concern.

The same is true for rockets. I think four legs is the optimal arrangement, but I think that a pantograph system like the New Shepard vehicle or the LT-05 landing strut in KSP might offer better strength.

I also think that VTVL first stages should be designed from scratch for landing. The Falcon 9 booster is a conventional booster with landing equipment bolted on as an afterthought. In the future, VTVL booster designs should probably be shorter and wider with an integrated landing gear, like the old SASSTO or ROMBUS designs.

[kerbiloid]

If use 3 RCS, they match with 3 legs very well.

Anyway, a computer uses them, not a pilot - so no need in 4-symmetry. Efficiency would be a little worse due to non-90° thrust angle, but 3 mechanical parts and additinal monopropellant tank volume is better than 4 mechanical parts.

Slopes are actual on a Moon surface. If a reusable rocket lands on a slope it probably already becomes not reusable.

[Nibb31]

3 minutes ago, kerbiloid said:

If use 3 RCS, they match with 3 legs very well.

Anyway, a computer uses them, not a pilot - so no need in 4-symmetry. Efficiency would be a little worse due to non-90° thrust angle, but 3 mechanical parts and additinal monopropellant tank volume is better than 4 mechanical parts.

Slopes are actual on a Moon surface. If a reusable rocket lands on a slope it probably already becomes not reusable.

I was talking about the Apollo LM, so Moon landing only. 3 RCS quads would have made manual control more complicated and lacked redundancy and efficiency. It would have been a bad idea.

Edited January 18, 2016 by Nibb31

[magnemoe]

4 legs give an significant larger footprint than 3 legs so an 3 leg lander would had to extend the legs further out to compensate and this would add weight and complexity. 
3 legs are often used on probes who are smaller and have lower center of gravity.
5-6 legs give redundancy if leg fail but not much larger footprint, common in KSP as max leg size is pretty small. 

[Frozen_Heart]

Four legs works best IMO. Easier to lay out, more stable than three, and keeps the weight low.

I agree that the Falcon 9 isn't really the best design to land, but they chose that width so they could still take it by road which is massively cheaper.

[Kaos]

17 minutes ago, Frozen_Heart said:

Four legs works best IMO. Easier to lay out, more stable than three, and keeps the weight low.

I agree that the Falcon 9 isn't really the best design to land, but they chose that width so they could still take it by road which is massively cheaper.

If we had a sufficient reliable landable main stage, we could just fly it to the location where we need it, so transportation on the road is no necessity on long sight. At least if we need a small number of relocations in relation to the number of launches.

[Kaos]

3 hours ago, Nibb31 said:

Three legs is the minimum for stability. The Apollo LM designers considered both 3 and 4 leg configurations, but the 4 legged version offer more tolerance to landing on a slope. It also provided an easier arrangement regarding RCS thrusters. In a 3 legged arrangement with 4 RCS quads, one of the legs would have been too close to a thruster. I don't think they ever considered more legs because weight was the main concern.

The same is true for rockets. I think four legs is the optimal arrangement, but I think that a pantograph system like the New Shepard vehicle or the LT-05 landing strut in KSP might offer better strength.

I also think that VTVL first stages should be designed from scratch for landing. The Falcon 9 booster is a conventional booster with landing equipment bolted on as an afterthought. In the future, VTVL booster designs should probably be shorter and wider with an integrated landing gear, like the old SASSTO or ROMBUS designs.

I did not know that reason for 4 legs, interesting information, thank you.

But for New Shepard: I thought it had four legs. Also all photos I have found look like four legs. Do you have a source for that?

[AngelLestat]

I am not sure.. the leg system of falcon9 is good enough, they find a good solution to the deployment issue without affect the aerodynamic much.  It has its weakness but it needs to be also light.

Pictures from falcon9 legs:

https://upload.wikimedia.org/wikipedia/commons/9/91/CRS-6_first_stage_booster_landing_attempt.jpg

http://i.stack.imgur.com/qPirl.jpg

But maybe there is a way to design a landing system that can deploy without using rigid structures, which can expand the leg radius.
This can be achieve it with tensairity beam structures.

 

 

So using cables instead rigid structures, you can achieve an structure that can be deploy using extra hellium from the tanks, which transform into a rigid structure, without the deployment and weight  limits of rigid legs.
You can read more about tensairity beams on internet, there are videos.

Edited January 18, 2016 by AngelLestat
english errors

[Nibb31]

3 hours ago, Kaos said:

I did not know that reason for 4 legs, interesting information, thank you.

But for New Shepard: I thought it had four legs. Also all photos I have found look like four legs. Do you have a source for that?

I didn't say it didn't have 4 legs, I said the legs had a pantograph structure, which looks stronger and more capable of handling a vertical load.

[fredinno]

7 hours ago, kerbiloid said:

If use 3 RCS, they match with 3 legs very well.

Anyway, a computer uses them, not a pilot - so no need in 4-symmetry. Efficiency would be a little worse due to non-90° thrust angle, but 3 mechanical parts and additinal monopropellant tank volume is better than 4 mechanical parts.

Slopes are actual on a Moon surface. If a reusable rocket lands on a slope it probably already becomes not reusable.

But Falcon 9 first stages are long, and need extra stability, hence 4 legs.

5 hours ago, Kaos said:

If we had a sufficient reliable landable main stage, we could just fly it to the location where we need it, so transportation on the road is no necessity on long sight. At least if we need a small number of relocations in relation to the number of launches.

Atlas V does it, but air transport would limit the diameter of the Falcon 9 not much higher then by road, which is why Delta IV uses a barge.

[Albert VDS]

9 hours ago, Nibb31 said:

The Falcon 9 booster is a conventional booster with landing equipment bolted on as an afterthought. In the future, VTVL booster designs should probably be shorter and wider with an integrated landing gear, like the old SASSTO or ROMBUS designs.

The F9 was designed to become a reusable rocket, so the landing legs are hardly an afterthought.

SASSTO and ROMBOS both have relativly flimsy landing legs compared to the F9.
Besides, the F9 legs widens the base while keeping most of the mass at the bottom and center of the stage.

[KerBlammo]

Four legs good.

[Kartoffelkuchen]

43 minutes ago, Albert VDS said:

The F9 was designed to become a reusable rocket, so the landing legs are hardly an afterthought.

SASSTO and ROMBOS both have relativly flimsy landing legs compared to the F9.
Besides, the F9 legs widens the base while keeping most of the mass at the bottom and center of the stage.

Not directly. As far as I know and heard, during development of F9 elon musk said that the rocket will be reusable too, which surprised the engineers quite a bit I guess.

But yeah, all following versions were designed with reusability in mind.

Correct me if I'm wrong, but I am pretty sure it was like this.

[Albert VDS]

31 minutes ago, Kartoffelkuchen said:

Not directly. As far as I know and heard, during development of F9 elon musk said that the rocket will be reusable too, which surprised the engineers quite a bit I guess.

But yeah, all following versions were designed with reusability in mind.

Correct me if I'm wrong, but I am pretty sure it was like this.

Reusiblity of the Falcon 9 was mentioned way before it flew: http://www.nasaspaceflight.com/2009/01/musk-ambition-spacex-aim-for-fully-reusable-falcon-9/
Everything on the first Falcon 9 was designed to be reusable, although it didn't have any capabilities to be reused.
IIRC the surprised engineer part was when they started designing the Falcon 9.

A good reason to not have integrated landing legs is so a disposable Falcon 9 has more delta-V. Having 2 different tanks with integrated legs and no legs would defeat the whole cost saving aspect of mass production.
 

[Kaos]

3 hours ago, Nibb31 said:

I didn't say it didn't have 4 legs, I said the legs had a pantograph structure, which looks stronger and more capable of handling a vertical load.

Thank you for clarification.

[fredinno]

4 hours ago, Albert VDS said:

Reusiblity of the Falcon 9 was mentioned way before it flew: http://www.nasaspaceflight.com/2009/01/musk-ambition-spacex-aim-for-fully-reusable-falcon-9/
Everything on the first Falcon 9 was designed to be reusable, although it didn't have any capabilities to be reused.
IIRC the surprised engineer part was when they started designing the Falcon 9.

A good reason to not have integrated landing legs is so a disposable Falcon 9 has more delta-V. Having 2 different tanks with integrated legs and no legs would defeat the whole cost saving aspect of mass production.
 

Does that part about expendable F9s even matter? Considering they want ALL their rockets to be reusable, and just upgraded their rockets 2 times to do so... 

[Albert VDS]

6 hours ago, fredinno said:

Does that part about expendable F9s even matter? Considering they want ALL their rockets to be reusable, and just upgraded their rockets 2 times to do so... 

There could be flights where they need all the delta-V they can get and landing is not even possible.

[Frozen_Heart]

4 hours ago, Albert VDS said:

There could be flights where they need all the delta-V they can get and landing is not even possible.

The Falcon Heavy is designed to fill those roles.

[Albert VDS]

48 minutes ago, Frozen_Heart said:

The Falcon Heavy is designed to fill those roles.

As long as the Falcon 9's 1st stages aren't reused, and thus the price isn't going down, then a non-reusable Falcon 9 launch is good enough for heavy payloads which require the extra delta-V.

Who's going to pay for a Falcon Heavy if it's easily launched on a non-reusable Falcon 9?

Edited January 19, 2016 by Albert VDS

[KSK]

I thought that Falcon 9 was intended to be reused but the original plan was for parachute recovery? That didn't work (they did try a couple of times IIRC) so they went for powered landing + legs as their next plan. Pure speculation here but when they redesigned the thrust assembly (to the current Octaweb configuration) for version 1.1 of Falcon 9, that would seem like a good time to have designed in the landing gear as well (and all the load bearing issues that implies), or at least the ability to attach landing gear at a later date.

[Mitchz95]

2 hours ago, Albert VDS said:

As long as the Falcon 9's 1st stages aren't reused, and thus the price isn't going down, then a non-reusable Falcon 9 launch is good enough for heavy payloads which require the extra delta-V.

Who's going to pay for a Falcon Heavy if it's easily launched on a non-reusable Falcon 9?

If SpaceX can recover all three cores, a Heavy might be the cheaper option.

[Albert VDS]

10 minutes ago, Mitchz95 said:

If SpaceX can recover all three cores, a Heavy might be the cheaper option.

Sure, but that's why I said "As long as the Falcon 9's 1st stages aren't reused".

[Mitchz95]

1 hour ago, Albert VDS said:

Sure, but that's why I said "As long as the Falcon 9's 1st stages aren't reused".

Missed that part.