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The rocket would keep same descent system as now (no parachute). But instead of landing on feet, it would have long grid fins on its top that would be catched by an elevated cable net.

The previous spaceX attempts showed that the landing position is very precise, but that it's hard to land slowly and stable enough to avoid falling on its side or break a leg. This solution objective would solve this issue, plus reduce the rocket weight.

The global idea is to transfer as much as possible the landing system (and its weight) from the rocket to the ground.

Elevated net for rocket first stage landing

  • No parachutes, keep the current advantages of controlled descend targeting a precise and optimal position in the sea.
  • No more heavy legs.
  • Cannot fall after landing or break a leg anymore.
  • The long top grid fins would help stabilize the descent, and slow down a little the rocket (opposite of the current bottom legs that makes it harder to control).
  • The landing impact force would be absorbed by the elasticity of the fin (and the whole plaform on water) on several meters ; much longer than the current brutal impact on feet. So the rocket could land with a higher speed, more fuel saving.

So, surely there must be flaws and issues with this idea... waiting for your opinion and ideas!

EDIT: Top-view of the net below. Metal structure would be 50m wide (same as the current landing platform), the net hole 15m of diameter, and top legs 15m length, so it cannot fall through the hole nor be in contact with the solid structure.

enter image description here

About some very interesting issues reported in the replies:

  • Rocket precision to enter the net hole: the hole would be 15m of diameter, and the target precision of spaceX is 10m, so it should be already OK.

  • Net would burn/melt from the rocket fire: as seen in the point above, flames should not touch the cables - and cables should be quite resistant to them anyway.

  • Top-Legs resistance (15m instead of the actual 7m):

    • I think a lot of the weight of the current legs come from the huge hydrolic cylinders required to open the legs against the air friction (not the case for that in top-legs).
    • Also they need to be very strong to support the instant landing impact, that would be smoothed by the net elasticity.
    • We can also imagine some "legs" a bit different: if the structure is high enough, it could be some hooks that catch the net, not horizontall by at 45° vertically, and so take all the force mostly in tension and not compression, more like cables and so very resistant for a very low weight.
    • (at this point I made an error of calling them "grid fins", they would just be top-legs, automatically doing the same stabilizing job as the grid fin hanks to their lenght)
  • Stage structure resistance. The stage is not conceived to support top-legs, but the rocket weights 325t at lift-off, so I suppose 20t on a smoother landing should be OK without too much reinforcement

  • Movement of the barge that would make the top of the net very unstable. Good point. Bug the barge is very big (50*90m) so quite stable, and we could add very heavy and deep keel (not sure of the word) to stabilize it more, as it doesn't need to move fast at this time (it may already be the case).

  • The net would break if rocket would come too fast. Absolutely, but the rocket would still slow down to almost 0 like now. It would just have a little more flexibilty, could come slightly faster or less straight.

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Dry weight: ~25 Tons, each 1% remaining fuel is another 4 Tons which is not a show stopper by itself, but making the rocket pass almost completely through a small hole may a bigger burden on navigation - it's certainly a smaller aperture in phase space than landing (which i this case has 10 instead of the usual six dimensions since tilting is important. But it's worth considering further! A potential advantage is that your net could conceivably absorb some variable amount of down-velocity - it doesn't have to reach zero at zero. – uhoh Mar 5 at 6:25
    
The "hole" in the middle of the net could be quite large (as large as the grid fins could be long), and I had the impression that all the past attempts were already really precise for this point. And on the other side, a lot of fuel would be saved by removing the legs weight and the perfectly steady landing at almost 0 speed. (but of course i did not calculation and I wouldn't be able to ^^) – Gaddy Mar 5 at 6:36
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I think they are aiming at developing systems which can land on unprepared terrain, even on the Moon and Mars. Even if it isn't useful for landing F9 1st stage, I think they use it as a test vehicle. – LocalFluff Mar 5 at 7:32
    
That's true. But I might still be best to save as much cost as possible for "common" earth rockets. – Gaddy Mar 5 at 7:59
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This is very similar to this question: space.stackexchange.com/questions/7866/… – Hobbes Mar 5 at 10:03

Edit 2: added a paragraph about what happens after the landing (see at the bottom)

So I've got a pile of arguments against this scheme, but this is the big one:

You've made the legs twice as long (the real ones are about 7 m), which means they're at least twice as heavy. Probably more, since the load will be applied further out so the legs need to be stronger. So far from saving weight, you've just added several tons to the empty weight of the stage.

Other arguments:

The legs are at the bottom of the stage, conveniently located where the stage already has to be strongest to withstand the loads from the engines. The bottom hinges are attached to the Octaweb engine frame. The top hinge has a compression load, which is easy enough to engineer for (a band of thicker skin is enough). Due to the leg geometry, the load is spread between the bottom and top hinges.

When you move the legs to the top, you have to strengthen the stage at the top, where you don't already have a strong structure inside. When the legs are straight out, the top hinges won't take any load so all of it has to be absorbed by the lower hinge. The lower hinge will be pulling outwards with a force of several tons, so you need to strengthen the skin here.

The top of the stage is made to support weight in the vertical direction, not to support large horizontal loads.

So...

  1. you don't save any weight, because you still need legs that can support the stage, you've just moved them from the bottom to the top.
  2. you need to strengthen the top of the stage, which adds weight.

You still have the failure mode of 'a leg breaks after landing' or 'a leg fails to lock into position', because you're using a very similar mechanism.

The hydraulic cylinders are still there: this time they're not pushing against the wind, they're preventing the wind from snapping the legs back violently. Longer legs = greater aerodynamic forces, so again more weight to prevent stuff from breaking.

A net is inherently less resistant to the rocket's flames than a slab of asphalt on top of a slab of steel. The net also creates point loads where a cable meets the landing leg. As the rocket settles in the net, it will tend to move around. The leg will slide over the cables, which will abrade the legs (steel cable is a decent saw blade).

The barge moves in the water, rocking on the waves. Any movement is amplified by the height of the poles that hold up the net: a few degrees of tilt ends up putting the poles several m out of place. This makes the hole a moving target that's much harder to hit than the barge itself.

The hoverslam maneuver aims to get the rocket's vertical speed to 0 at the moment of touchdown. Catching the rocket while it still has some vertical speed increases the loads on the rocket. So you have to make the legs stronger and heavier again.

When the stage uses its main engine to maneuver, the engine gimbals, spewing a ten-meter column of flame in a large off-vertical angle.

It's possible to build a platform that's not affected by wave motion. You can add a large underwater structure that provides the buoyancy. Offshore drilling rigs are sometimes built like this. This would make the barge far less mobile, and 10x heavier (=10x more expensive) than it is now.

To get flexibility in the landing, it'd be easier to replace the current leg design with one that doesn't lock, and adds some shock absorption to the hydraulic system. This requires more complex, heavier hydraulics though.

And another thing: what happens after the landing? In the current design, the crew (presumably) comes aboard and welds some clamps over the legs to secure the rocket for the return voyage. Simple.
In your scenario, the rocket's dangling from a net. It'll have to be secured, and pretty quick, or it'll bang into something and be destroyed. You can't lower it vertically (no legs at the bottom), so you'll have to devise a complex mechanism to set it down horizontally on a cradle.

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Thanks for the answer. Are you sure the stage that support a weight of 325t at lift-off would really need to be strengthen to support a smoother catch by by its top at 20t ? And about the barge movement, that's a issue sure. But the movement should be limited with a very long and heavy keel (not sure of the word). No need to be suited for speed travel etc, only purpose to maintain balance. – Gaddy Mar 5 at 14:36
    
The net would be cable so can resist a bit to the flames, and they shouln't be touched by flames anyway, there is a 15m hole in the center. Legs could break but the impact would be much smoother (thanks to the net's elasticity) than on solid ground, so less chance of breaking. And the objective would still be to get "about 0" speed, it would just not be forced to be exactly that, there would be more flexibility than now – Gaddy Mar 5 at 14:36
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It's actually quite a substantial wave that can suddenly tilts a ~170 x 300 foot barge by a few degrees. If such a wave hit while the rocket was just about to touch down - it could have bad effects as well. I think this one really needs to be simulated - the tradeoffs may not be so clear with real numbers. – uhoh Mar 5 at 15:09
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Good point about the top-legs weight. Although I think a lot of the weight of the current legs come from the huge hydrolic cylinders required to open the legs against the air friction (not the case for that in top-legs). Also they need to be very strong to support the instant landing impact, that would be smoothed by the net elasticity. Finally, the top-legs would need to be mostly resistant in tension, not compression, and that's far easier to achieve (a cable is very resistant in tension, much more difficult to get the same resistant in compression) – Gaddy Mar 5 at 23:47
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I've added some thoughts on the hydraulics, and what happens after the landing. – Hobbes Mar 6 at 19:27

Issue #1: the rocket body is really thin (like a soda can) and the smallest force in the direction the net would apply would probably dent it.

Issue #2: there was never a problem with stability which would need solving with some extra contraptions. It was:

  • not enough hydraulic fluids for steering
  • stuck valve and algorithms not accounting for the latency it created
  • leg not locked
  • not sure yet (maybe coming too fast because not enough fuel margins?)

The first 3 just needed fixing the specific problem and the 4th attempt was a test of upper limits anyway.

Issue #3: the net and surrounding structures would have to withstand rocket exhausts of 1 or even 3 (supposedly in the 4th attempt) engines.

Issue #4: SpaceX is training for Mars and you won't have such benefits there.

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These are the problems so far - not enough data to say they are the only problems! – uhoh Mar 5 at 6:28
    
About issue #1: the hole could be quite large (as large as the grid fins could be long), I think the current precision would be already enough for it. I'll edit the post about it About issue #2: with the net, the same first 3 issues would not have broken the rocket (theoretically of course). The idea would be to save the rocket even if the landing is not perfect/steady of if there are some various problems – Gaddy Mar 5 at 6:43
    
The larger the hole, the greater the speeds and forces applied by it in case it is "used", and the more important the #1 is. This has been discussed too many times already - it even become a FAQ – jkavalik Mar 5 at 6:47
    
Very good points, sorry i had not seen this faq. However it's not really the same suggestion (keeping the legs, just adding something to prevent it from falling) so most of their answers do not really apply here. But they have good point about the forces (my fin grids might just break if they are not too strong and so too heavy), and the objective of landing on other planets – Gaddy Mar 5 at 6:53
    
About issue #3: I don't think it would be that much of a issue, there would not be direct contact with the exhausts, less than for the lift-off structure. – Gaddy Mar 5 at 6:55

You're using way too high speeds to make it feasible, similar to a water landing (you could say, why not "dive" into the water). At the speeds a rocket touches down, if not heavily controlled, hitting a net is equal to hitting the ground/water surface.

So the landing legs (which anchor to the net) need to withstand the full force, AND dissipate all kinetic energy, without creating a high momentum.

So what makes this "easier" than a ground landing according to you? The net needs to have a very high yielding strength to withstand those forces. But not only that: either the net needs to be really flexible, or the landing anchors simply need to withstand the same forces as a ground landing.

A high yield strength, highly ductile material (low Young's modulus) that can be created of large enough size simply does not exist yet. Sure, theoretical materials such as graphene might prove useful in the future, but if you allow that, why not make the whole rocket from that material?

Now there is also an actual drawback to using a net compared to a landing on land such as the Soyuz do. (This is similar to why water landings are so much harder to both air and spacecraft). Land is well defined structure, you know its shape and it's (probably) nicely flat. A net (and sea) are not that way, they deform (waves) and are nearly never flat. This effect is, in a net, exaggerated by the workings of netted structures: you never land perfectly, so one part of the rocket will touch the net first, this creates a wave which will actually create an upward force at other parts that are landing.

Try simulating the last part.

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The speed would still be very slow, almost 0 - just more flexibility than now, it would stand a little higher speed at landing thanks to the elasticity. It would still be a heavily controlled descent. What makes it easier than on solid groudn is that there woudl be some elasticity to absorb the impact forces on a longer time, impact smoothed. Just a little flexibility is already far better than solid ground with instant impact. Solid cables should be ok, with the distance there will be a bit of elasticity with a little deformation of the cable and the structure itself – Gaddy Mar 5 at 14:41
    
Your point about water/land is true but spacex already tries to land on sea to avoid spending a great lot of fuel to get back to land. About the "wave" created when one side land first, it's true but the effect is worse on solid ground, as it's instant impact instead of being smoothed by the elasticity. I mean that if you land on solid ground not pefectly straight... all the instant impact force goes on 1 leg, very hard to resist. On the net this initial touch will not be that important, as it's smooth on a longer time – Gaddy Mar 5 at 14:42
    
Steel cable and springs exist, and come in an incredibly large array of sizes. [1], [2], [3]. Any suspension bridge builder would love this project! – uhoh Mar 5 at 15:21
    
[4] – uhoh Mar 5 at 15:34
    
@uhoh: Yet stell is actually really stiff (high young's modulus) compared to it's maximum strength - so you won't really gain a "soft landing", using springs isn't a "magical solution" - you are still limited in the material choice. (Aluminum and Titanium are both way more easy to bend than a spring). – paul23 Mar 6 at 21:46

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