Friday, June 01, 2012

Catching a Dragon

As promised last time, here's the update for events since last Friday's launch attempt.

The problem with the center engine in the Falcon 9 first stage cluster was fairly simple: a faulty check valve that needed to be replaced. But, the check valve couldn't be replaced until the vehicle had been made fully safe, which meant offloading all of the fuel and oxidizer from both the first and second stages. That's a lesson we learned fairly early on in the Space Age. On October 24, 1960, Marshal Mitrofan Nedelin ordered his technicians to fiddle around with a fully-fueled ICBM prototype. It was the last order he ever gave.

So, we just don't do that anymore. It pushed the launch back from Friday to Tuesday. But that's way better than having the launch pushed back to "whenever we finish rebuilding the launch pad, and get a whole bunch of new technicians trained."

In any case, Tuesday morning came around. The same sequence of events happened: they loaded fuel and LOX, and performed all of the countdown checks to verify that the ship was ready to fly. It came down to the final count, when they lit off all nine engines. As always they held the missile down to the pad until all nine engines reported ready for flight. And this time, they were.

(Embedded videos courtesy of SpaceX, originals and more available here.)

The nice thing about autonomous inertially-guided vehicles is that they don't need to see where they're going. So it doesn't matter what time of the day or night you fly. The down side to that is, you don't see much of anything interesting from the external camera views. The Falcon 9 vehicle performed flawlessly, in pre-dawn darkness, carrying the Dragon spacecraft to precisely the right orbit for its rendezvous with the International Space Station on Flight Day 4. Once the Dragon was in orbit, and had extended its solar panels, one big hurdle had been cleared.

Now, the waiting. As I've said earlier, space travel is a lot like baseball in that the participants spend most of their time standing around waiting for something interesting to happen. In this case, the waiting was necessary in order to synchronize the orbits of the Dragon with the Space Station. Now, if fuel is no constraint, and you don't care how much of it you use, the simple way to synchronize orbits is to figure out the angular distance in-plane between you and your target, figure out how much time it would take for the target to get back around to where you are, and then kick yourself up into an orbit that chews up exactly that much extra time. The problem is, no one ever has that much fuel to spare. So what we do is to put ourselves into a slightly lower orbit, which eventually puts us into a position to kick ourselves up into proximity with our target. It's a question of fuel or time, and time is something you don't have to pay extra Delta-V for.

This brings us to Thursday, the first "fly-under", where the Space Station crew tested their ability to issue commands to the Dragon spacecraft. These tests went very well, and the two spacecraft spent the next day or so in proximity operations. Dragon was in what we call an "osulating" orbit relative to the Space Station: relative to the Space Station, it looked like it was moving in an ellipse, sometimes going out in front, sometimes above, sometimes behind, and sometimes below. This demonstrated its ability to maneuver safely, and to stay well away when commanded.

Then, on Friday, the really tricky part: autonomous approach. I had the day off, so I tuned in to the CNN live feed online to watch this part. It was gripping TV in the same sense that a golf tournament is gripping TV: it wasn't. But seeing people do something completely new in real time is instructive, because there were a few glitches with the approach. It turns out that the LIDAR gear that Dragon uses to automatically find the right docking port was being confused by spurious reflections off of the Japanese module. They had to back off until they figured out how to deal with this problem. The solution they settled on was to reduce the LIDAR's field of view, so that it wouldn't be able to "see" the offending module, and could focus on the bits it needed to see. That seemed to fix things, and Dragon was able to move from its 70-meter hold position to a 30-meter hold position. At that point, Dragon was exactly where it needed to be, relative to the berthing port and robot arm. So, they issued the final command, which took Dragon to a 10-meter hold position. This put it in position for Astronaut Don Pettit to reach out with the robot arm, and grab a Dragon by the tail, so to speak.

And there was much rejoicing.

On Saturday morning, they opened the hatch, and started to unload the cargo. Over the next week, they filled up the Dragon with trash, experiments, and return equipment to be sent back to Earth. Then, on Thursday, the Dragon spacecraft undocked from the Space Station. Shortly after that, it fired its thrusters to re-enter the atmosphere, and then splashed down in the Pacific Ocean. This video was captured from the chase plane, a U.S. Navy P-3 Orion. At the beginning, you can see the Dragon falling beneath its two drogue parachutes. The drogues don't do a whole lot to slow the ship down, but they do stabilize it as it descends. A few minutes later, the main chutes deploy, slowing the vehicle for splashdown.

Two up, two down. Not bad for the new guys on the block.

I'll go into more detail next time, but it's hard to over-state the importance of what's just happened. Suffice it to say that the real Space Age has just begun.

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