SpaceX Successfully Lands Its Rocket On A Floating Drone Ship For The First Time (theverge.com)

An anonymous reader quotes a report from The Verge: SpaceX has finally landed its Falcon 9 rocket on a drone ship at sea, after launching the vehicle into space this afternoon. It's the first time the company has been able to pull off an ocean landing, after four previous attempts ended in failure. This is the second time SpaceX has successfully landed one of its rockets post-launch; the first time was in December, when the company's Falcon 9 rocket touched down at a ground-based landing site in Cape Canaveral, Florida, after putting a satellite into space. Now that SpaceX has demonstrated it can do both types of landings, the company can potentially recover and reuse even more rockets in the future. And that could mean much greater cost savings for SpaceX.

Comments

That came in at a pretty steep angle

[drew_kime]

Anyone know why they would come in at an angle and straighten up at the last moment? Is it actually easier to control that way, or is it to protect the landing pad in case of a list-second abort?

Re:That came in at a pretty steep angle

[fgodfrey]

There are two reasons that I've seen.

Because the rocket is almost out of fuel, even burning only one engine at minimum throttle, the thrust to weight ratio is more than one (ie, the rocket would fly, not land). So, they can't hover, they have to hit the ship and shut the engine off at the exact moment that the velocity is zero (or very close to it). So, to help with that problem, they come in at an angle which helps consume at least some of the thrust in a direction that isn't upward.

The second reason is, as you say, to protect the landing platform. If they run out of fuel (or the engine fails or....), the stage just drops into the ocean rather than crashing into the barge at a very high speed. That said, based on their last several failed landing attempts, that barge can take quite a hit and stay in one piece....

More physics than economics, and it checks out

[goodmanj]

SpaceX, and the people in this thread, are comparing the vehicle cost to fuel cost, which is kinda cheating. It's not the cost of the fuel that matters, it's the cost of building the vehicle larger to hold that fuel -- and the fuel needed to launch that fuel -- that matters. So let's do the math!

Most data taken from http://spaceflight101.com/spac...
Basic info:
Stage 1: 23 tonnes structure, 400 tonnes fuel
Stage 2: 4 tonnes structure, 93 tonnes fuel
Payload: 13 tonnes

When launching, the first stage burns all 9 engines at full thrust for two and a half minutes. The re-entry burn and landing happen on a single engine, and from eyeballing the videos (including this one that shows the re-entry burn) appear to take about 30 seconds total. Assuming all burns are near full thrust (which is the best way to do it), that means the landing burn takes about (1/9) * (0:30 / 2:30) = 2% of the first stage fuel. Let's double that to 4% to provide a generous safety margin: that works out to about 400 * 0.04 = 16 tons more fuel.

This fuel is carried up to the moment that the second stage separates, so it subtracts from the mass of the second stage. Second stage plus payload weighs 110 tonnes: without the landing fuel, you could have scaled that up to 126 tonnes, a 15% increase.

So, landing the first stage reduces the payload SpaceX can launch, and thus the money they earn, by about 15%. In exchange, they recover about 75% of the cost of the launch hardware. So it's worth doing, even after you subtract off the cost of recovery and refurbishing. Maybe not the game-changer Elon Musk wants it to be, but it's a win.