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SpaceX's Latest Launch Successful, But Ends With a "Hard Landing" (theverge.com)
Eloking writes with this news from The Verge: SpaceX successfully launched its Falcon 9 rocket into space this afternoon, but — as expected — failed to land the vehicle on a drone ship at sea afterward. CEO Elon Musk said the rocket 'landed hard' on the drone ship. The mission requirements made a successful landing unlikely. This was SpaceX's fourth attempt to land the Falcon 9 post-launch on an autonomous drone ship floating in the ocean. All of the previous sea landings failed too, though the third attempt came very close. The company had low hopes of a successful landing from the start of this mission, since the rocket had to send a heavy satellite into a high orbit. That requires a lot of fuel for the launch itself, so there wasn't much fuel left for the rocket's return to Earth and powered landing.
The rocket is not designed to handle stress from all angles. Flopping into a net would entail coming down hard in a direction that it is not designed to handle stress. The primary advantage of a vertical landing is that most of the stress remains vertical just like when the rocket is being launched. Building it to handle other directions would require much more mass. They'll get this to work eventually, and this was a very difficult run anyways because the orbital profile required the rocket coming down from higher up, at a higher velocity and with less fuel to work with. Please be patient.
It's all about the fuel. On launches that leave enough spare fuel, they actually return the rocket all the way back to a landing pad at the launch site in Florida. They successfully landed the rocket once in that manner. But on launches that require more fuel (to put a heavier payload into a faster orbit), there isn't enough fuel leftover for the burn that would send the rocket back towards the launch site. As a result, they are limited to a relatively ballistic trajectory from the launch site, which means landing somewhere out to sea. The landing destination is actually pretty precise (the drone ship is trying its best to stay stationary, not move to meet the rocket), it's just that it's the only place they have enough fuel to get to.
The first stage of the rocket never reaches orbit: it's still going really really fast, but not orbital velocity. So after the second stage separates, left alone, the first stage would start falling down again downrange and crash into the ocean.
Normally, after separation, the first stage flips itself over and then does a boostback burn to kill the forward momentum, and give it enough momentium backwards to line up its trajectory back towards the launch site. Then later it does a deceleration burn to slow itself down to keep the atmosphere from ripping it apart. And then finally, it does a landing burn for the last segment to slow it to a stop.
On some missions, they don't have the fuel to do that full boostback burn, so they kill some of the forward momentum, but that's it.
The challenge here for SpaceX is that a single engine can not throttle down enough to hover the empty booster. That's why they call it "hoverslam", if the engine stayed on the rocket would bounce back up. There would be a lot more room for error correction if that was the case.
Bruce Perens.