How SpaceX Tamed a Finicky, Orbiting Dragon

On Sunday morning, a SpaceX Dragon capsule delivered over a half-ton of supplies and scientific hardware to astronauts on the International Space Station. The successful berthing – it was grappled by the station’s Canadarm – came just 19 hours and 22 minutes after launch. That’s the good news. The bad news is there were plenty of problems during the flight. It seems that SpaceX’s continued success isn’t without its trials.

The last time a Dragon launched to the ISS, it almost didn’t make it. One of the Falcon 9’s nine engines shut down early, decreasing the rocket’s thrust and putting the capsule into too low an orbit. Some orbital maneuvers got the Dragon aligned with the ISS and salvaged the mission, but a secondary payload was lost after the launch failure. The mission was mostly a success.

Friday’s Falcon 9 (it launched on March 1) flew without a hitch. Whatever hurt the last rocket didn’t resurface this time, and the Dragon easily achieved its desired orbit. But once it got there, it ran into trouble.

The Dragon gets its power from the Sun via deployable solar panels. They’re supposed to deploy 10 minutes after the capsule reaches orbit, but on Friday they didn’t. The capsule does have a backup system of batteries to keep things running and give engineers time to sort out any solar panel issues, but they can only provide between 12 and 14 hours of power. That put a pretty severe time limit on how long SpaceX had to fix the problem.

But it wasn’t just the solar panels that were acting up. The problem was rooted in the capsule’s thrusters, the series of small rocket nozzles that expel gas. The thrusters are vital to a mission. In space, even the tiny bursts of propellant from these rockets give the capsule a big enough kick to change its orbit. Without thrusters, there was no way for the Dragon to meet the ISS.

Thrusters need two things to work: fuel and an oxidizer to light it. The tanks that hold the oxidizer weren’t pressurized properly so they couldn’t deliver any oxidizer to the thrusters. It’s not entirely clear what happened. SpaceX’s founder Elon Musk said in a press conference, held while all the orbital drama was unfolding, that the pipes that carry oxidizer hadn’t frozen but the substance itself might have. That wasn't totally bad news; frozen oxidizer is a simpler problem to fix. But there was also a chance some debris got into the pipes and blocked the flow of oxidizer or that a valve got stuck somewhere, which would be more difficult to solve.

Here’s how the problems compounded: the Dragon needs two of its four thruster pods up and running to do a series of maneuvers before the solar panels can deploy. It only had one.

SpaceX’s engineers knew that the longer the solar panels stayed folded up in orbit the colder the hardware was going to get, so they improvised. They sent an override command to the capsule, forcing the solar panels to deploy in spite of non-functional thrusters. It worked. About an hour and 40 minutes after launch,the panels were deployed and working.

In the time it took to deploy the solar panels, one of the thruster oxidizer tanks began to pressurize. It didn’t come back online immediately, but five hours after launch a second thruster pod was finally operational. The others eventually followed. By mid-afternoon on the East Coast, all four thruster pods were online and the capsule had regained control over its attitude. SpaceX ran a series of tests to ensure the spacecraft could safely approach the space station. It did, berthing a day after originally planned.

It was an impressively fast recovery from what looked for a minute like a fatal failure. It appears there’s still an underlying engineering problem to fix in the spacecraft, but the company has time to work out the kinks. This is the second of at least 12 ISS resupply missions the company is launching for NASA, all leading up to the day when it will start ferrying astronauts up to the station.

The Dragon is still up there, docked to the station where it will stay for about three weeks. The crew will load it with over a ton and a half of equipment, supplies, and trash before it separates and falls back through the atmosphere to splash down in the the Pacific a few hundred miles off the coast of Los Angeles.