There are several reasons why landing legs make more sense:
– Any flat chunk of cement is a landing spot. That means more places to land in case of contingencies. For yesterday's mission, SpaceX had one primary and four alternate landing zones.[1]
– I doubt industrial robots can withstand rocket exhaust. As helicopter footage shows, the landing pad got lit-up pretty good.[2] Remember, the first stage is over 40 meters tall. Those are some massive flames.
Landing legs are heavy, because they need to absorb a big shock. They also need actuators. "The best part is no part"; if there is a way not to drag this extra weight from the sea level up to the stratosphere and back, it counts as an improvement.
For the SuperHeavy booster that will launch Starship, Elon wants to take the legs away and instead have it land on a platform intended to catch the rocket on its grid fins near the top.
This requires a very precise landing. But gets rid of a very heavy piece, and makes it easy to build a shock absorber. The downside is that you'll need to replace platforms any time one of these misses, hence the need for a very precise landing.
(The landing already has almost no margin of error given that they land with a suicide burn.)
It's true that SpaceX's previous landings have used legs, but that's either been Falcon boosters (much smaller/lighter than Starship boosters, AKA "Superheavies"), or for Starship's upper-stage (again, much smaller/lighter than Superheavy boosters).
The catching system looks crazy, but it might actually be easier than landing a Superheavy on legs. The ground is a larger target than the catching arms; but its maneuverability and shock-absorption are famously low.
I am sure they did the math but at first I thought that landing legs would be unnecessary if you have a system that 'catches' the rocket on the ground. Since the rocket can hover, it would be possible to grab the rocket with a structure/claw.
The weight of the legs is probably quite low so this might have been the easiest solution.
Landing mechanism is cool too. The lander has deformable metallic sponge balls on the legs, hits the soil at its "heel" and tip forward[1].
Official reasons given is it allows reduction of structural margins, physical interferences and tipover risks on inclined surfaces, which seem to make sense. Maybe it's just me that it looks a bit like someone had to have a spacecraft that lands like the Space Shuttle with engines pointing back.
They were originally trying to land on pad, but they couldn't get legs light enough and strong enough not to bend while landing, there were also problems with maintaining structure around those legs. It was simpler to have two hardpoints and legs be more massive but on the ground.
That's a gymnastics phrase for landing on your feet (with no bounce) during a dismount and is a pretty appropriate analogy for what they're trying to do with the Falcon 9 first stage.
To carry the analogy further, the second landing attempt could have been a "one hop" landing if they'd had a bigger drone ship. The first landing (which needed a bit more propellant to keep the stage upright) would have succeeded as a "one step" dismount - if the stage could actually take a step with it's legs.
To be fair they attempted a soft water landing three launches ago with CASSIOPE, though without the legs. They are basically attempting this for a second time, and the addition of the legs will probably improve their chances (with CASSIOPE, the first stage began spinning around the vertical axis shortly before reaching the water, causing an engine cutoff. The legs are expected to improve the aerodynamics of the returning first stage to prevent this from happening again.)
Yes, the first stage has landing legs. They're planning to deploy the landing legs over water and attempt to hover the booster just before it hits the surface.
This question might be a bit naive, but could someone explain to me if the SpaceX illustration (https://www.nasa.gov/sites/default/files/styles/full_width/p...) is feasible at all? From the (many, oh, so many) hours of KSP I've played landing legs seem to be the way to go with an uneven and uncertain and especially slightly sloped landing surface and not a flat rocket underside. Or is there any explanation how Space Ships are supposed to land? Thanks Thanks
I'm pretty sure the landing legs will be fine if they can pull off the controlled burn and keep the rocket spin under control. The last time they tried this (without legs) the falcon 9 spun too fast in the atmosphere damaging the in tank baffles so that the fuel centrifuged causing the engines to go out on the second burn.
They landed a lot of falcon 9 first stages without legs on the water, vertically, to test the control system. And a number of attempted barge landings with legs failed, before the first successful landing. Now it's almost a routine thing.
I have an idea for how to improve the landing of the rocket stage: instead of fold-out landing-legs, why not fold-out wings? Or, a combination of both - legs with wing-like features - they function as wings until close to touch-down and then feather to support the rocket laterally upon landing?
The point is that the ability to land is much harder to develop than the ability to stand up after landing. Improving the leg locking mechanisms is relatively trivial compared to developing the ability to turn a booster around in space, fly it back to earth and land on a floating platform.
If I was involved in SpaceX then I would be proud to have helped solve such a tricky problem, and optimistic about fixing the far less difficult leg locking problem
I think the former is avoided because attitude control thrusters don't tend to be very powerful, so you're adding another variable you need to zero out right as you land, and because sensors can't necessarily be rotated and still provide useful data.
Allowing the lander to take a bit of a hit is kind of what IM-1 ended up doing :P but jokes aside, the legs add a bit of tolerance for this, and it has been done on Mars (wrapping rovers in inflatable balloons to absorb the impact), but I guess the logic with the Moon is that you need to do a controlled propulsive descent anyway (and with heavy/large payloads you have no other options), so might as well focus on doing it properly. With Mars there's an atmosphere to help, reducing how controlled the descent needed to be for lighter things.
You can see that sometimes in the landing footage, the legs are pretty flexible, and they get some help from RCS thrusters at the top of the booster. Plus, as soon as the engine has purged any excess lox/fuel, a robot slides underneath and tries to latch onto several points, which probably helps by bringing the center of mass down.
There's footage online of some of the early landings when the robot had not been implemented, where boosters slid across the barge in part due to landing at a bad angle.
That's for the booster. The goal is to skip landing legs for the booster, as it'll only operate around Earth, so it's "easy" to build infrastructure instead.
Starship (the 2nd stage) will always do that flip & land on legs, as it also needs to do it on Mars.
– Any flat chunk of cement is a landing spot. That means more places to land in case of contingencies. For yesterday's mission, SpaceX had one primary and four alternate landing zones.[1]
– I doubt industrial robots can withstand rocket exhaust. As helicopter footage shows, the landing pad got lit-up pretty good.[2] Remember, the first stage is over 40 meters tall. Those are some massive flames.
…and most importantly:
– Landing legs work on other planets.
1. Map: http://www.americaspace.com/wp-content/uploads/2015/12/LZ1.j... (from http://www.americaspace.com/?p=89910)
2. https://www.youtube.com/watch?v=ZCBE8ocOkAQ
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