Good work! I still think that they should have some grippers on the barge, or a net, ready to spring and grasp the rocket on landing, instead of carrying flimsy legs all the way to space and back.
The current legs aren't flimsy, and they seem way less complicated than a gripper system would have to be. How would a gripper system handle off-centered landings? It would have to catch the falling rocket very quickly because there's not much fuel left in the tank to hover. Your proposed system would add a lot of complexity, all for the small decrease in launch mass.
I was all set to post something about how the Grasshopper used a different engine, but apparently both the Grasshopper and the F9 first stage use Merlin 1D engines.
Wikipedia says that these produce 700kN at sea level and can be throttled down to 70%, which means at minimum power one engine could hover about 50 tonnes.
I can't find out what Grasshopper massed. Does anyone know?
I'm still surprised there's no active system on the barge to secure the rocket. Even now they seem to be able to stick the landing with some reliability, it's always nice to have belt and suspenders.
I'm thinking a system with a pillar at each corner of the barge, pairs of cables between opposite pillars, and puller cables to open out the main cables into an iris-type aperture which can be rapidly closed on the rocket after it lands to keep it upright. Do that at 2-3 heights above the barge and it'll cradle the rocket even in heavy seas.
Now you've created a collision risk with all those pillars and cables sticking up. The rocket actually comes in from the side, because it initially aims next to the barge so that an engine failure doesn't cause it to crash into the barge. You'd have to change that approach, and your margin for error would become much smaller.
All that to solve a problem that doesn't seem to actually exist. A scheme like this might have saved the Jason-3 landing, where the rocket fell over due to a leg that didn't lock, but none of the other barge landing failures would have been helped by this.
I think there's a fundamental failure to understand the scale of this hardware, too. This is a massive machine (50 meters tall or so) built to weigh as little as possible, and therefore to only be strong where it's needed. It has approximately zero ability to withstand lateral forces from things like cables pulling it upright.
The center of mass of the rocket is _very_ low. According to Elon, the CRS-8 core was very stable (it needed even less stabilization than they thought it did).
They may add some means to secure it if it needs to travel through rough seas on the way home, but if the conditions are calm enough to land, they're calm enough for the stage to be stable. Watch the CRS-8 landing and look at how much the barge is rolling while the stage is parked on it...
I remember reading somewhere that the actual shell of the rocket is surprisingly thin and very fragile against horizontal forces.
Think of it like a very long can of soda with a ton of lead at the bottom. Any kind of attempt to keep it upright only at the top will most likely end up damaging the shell.
That's why their method of securing it after it lands is to weld the feet, the bottom of this thing is pretty much the only part that's solid, the rest is flimsy as hell in every direction but one.
Permanent decrease in launch mass is big money, whereas complications on the barge do not really matter. Like all good engineering problems, it only looks problematic at first sight and I am sure that SpaceX could rise up to the challenge. Use a big funnel mesh?
However, I accept the comment that perhaps they are really practising for landings on Mars, where fair number of probes have overturned on landing.
I'm not sure you realize just how fragile the stage is. The side walls are made of _very_ thin aluminum. Any landing support mechanism must put the impact force on the 'octaweb' structure that supports the engines. That's the advantage of the legs. They can be engineered to distribute the force exactly where they want it.
A significant problem with any sort of "catching" mechanism is that the load on the stage is far less predictable.
> Your proposed system would add a lot of complexity, all for the small decrease in launch mass.
While I agree with your overall sentiment, I imagine that people building rockets make complexity tradeoffs to save tiny amounts of weight all the time.
The rocket can land anywhere in a area of a couple hundred square meters. How would you design the "grippers" to work anywhere on the barge? How would you "grab" the stage without punching a hole in it?
The landing legs aren't all that heavy, are on the first stage (where the mass penalty isn't as bad), and can be engineered to distribute the force through the structure of the stage that can handle it.
