Elon Explains

… some of the background for last nights successful Falcon 9 landing

 A lot about how things work in space is counter-intuitive, as all of our intuition is gained from daily experiences where the air is thick, gravity doesn’t seem to change and movement is relatively slow. We do see lots of movies about space, but, unless you’re watching an IMAX documentary, they vary from slightly wrong, like The Martian (good movie!), to mostly absurdly wrong, like Red Planet (don’t watch this, it will hurt your brain), which also doesn’t help intuition.

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In the case of the Falcon 9 rocket, the boost stage is able to accelerate a payload mass of 125 metric tons to 8000 km/h and land on an ocean platform or to 5000 km/h and land back at the launch site. The second one is lower because the rocket is moving super fast away from the launch site, so it has to do a screetching U-turn with nitrogen attitude thrusters, then fire the engines to create a reversed ballistic arc, then reorient again for atmospheric entry and have the engines pointed in the right direction for the landing burn. Since the propellant is liquid, it wants to centrifuge out during these maneuvers, so there has to be a system of baffles and internal holding tanks to keep it in place. It also needs three axis control surfaces that don’t melt easily and work well from hypersonic through subsonic speeds.

For a sea platform landing, the Falcon 9 figure of merit is therefore roughly 300 gigajoules (GJ) of kinetic energy and for a return to launch site landing, the number is about 120 GJ. These are fairly sizable by terrestrial standards. To put it into perspective, the city of San Francisco uses about 1 GJ per second of electricity, so the Falcon 9 booster transfers enough energy to power a city of almost a million people for five minutes. 

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Read more about the physics and technology involved in first stage Return To Launch site, plus a short history of SpaceX’s efforts in this regard at:

http://www.spacex.com/news/2015/12/21/background-tonights-launch

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