An anomaly with the thrusters aboard its Dragon spacecraft caused SpaceX to miss a scheduled burn to adjust the capsule orbit on Friday, causing SpaceX to delay by at least 24 hours Saturday’s planned rendezvous with the International Space Station.
SpaceX engineers have been working full-throttle to resolve the thruster issue, which followed a picture-perfect launch atop the Falcon 9 rocket. SpaceX founder and CEO Elon Musk and NASA officials said they are confident Musk’s engineers will resolve the problem quickly and could have Dragon on track for a rendezvous Sunday.
Engineers at SpaceX mission control in southern California saw the first signs of trouble shortly after Dragon reached orbit.
“It was a little frightening there,” Musk told reporters in a post-flight press conference. “We noticed after separation that only one of the four thruster quads was ready to engage. We saw that the oxidizer pressure in three of the four tanks was low.”
Dragon has four thruster quads, used to maneuver the spacecraft in orbit. They are essential to the docking procedure and three of them are required by NASA to be working flawlessly in order to make the final approach to the International Space Station. Two of the “quads” are comprised of four small Draco thrusters while the other two actually have five thrusters apiece. The Dracos are initially used to adjust Dragon’s orbit to begin its rendezvous with the station. Once Dragon has used them to achieve general proximity with the ISS, they are used to make the small adjustments needed for docking.
An initial investigation by SpaceX points to a possible blockage in a line that leads from a helium tank to an oxidizer tank that provides the oxygen needed for combustion. Inert helium is used to pressurize the four oxidizer tanks. There are four corresponding kerosene fuel tanks for the thrusters as well.
Musk called the problem, “a glitch of some kind, and not a serious thing.” He says the best guess is a piece of “frozen oxidizer” got stuck in the line. The oxidizer is liquid oxygen, and Musk says he believes it eventually warmed up enough that the liquid oxygen could resume flowing.
“We’ve been able to free that up by cycling the valves,” he said, referring to the control valves between the helium and oxidizer tanks.
As the day wore on, Musk said the four oxidizer tanks and the thruster quads were operating nominally. A successful co-elliptical burn happened late in the afternoon to raise Dragon from an altitude of 200 kilometers to something between 250 and 300 kilometers. This procedure was essential, Musk said, because Dragon’s orbit would decay within days at the lower altitude, forcing an unwanted re-entry.
With only one of the thruster quads ready to go shortly after launch, there was a delay in deploying Dragon’s power-generating solar arrays because engineers want two functioning thrusters so they can maneuver the capsule prior to the deployment. Engineers later decided to proceed with deployment with a single thruster after temperatures continued dropping on the array’s actuators. Deploying the arrays provides Dragon with power beyond the on-board batteries.
Because SpaceX missed the initial burn required to begin the rendezvous with the space station, Dragon will miss Saturday morning’s planned docking with the ISS. Once engineers have confirmed all four thruster quads are working properly, they’ll begin the planned burns and maneuvers needed to approach the station. NASA officials said those maneuvers will allow them to see if the thrusters are working properly and provide the confidence needed to proceed with docking.
In order to operate inside of a safe ellipsoid around the ISS, Dragon must have at least three working thruster quads. Once inside the ellipsoid, there are several contingency plans that allow for aborting the approach. The final safety zone around the station is known by the acronym KOS — the Keep Out Sphere, an area 200 meters around the space station. The final decision to enter KOS will be made only if everything is functioning properly.
NASA officials said the next likely chance for docking will be Sunday and there are several windows of opportunity during the next week, but both NASA and SpaceX said they must wait until they know Dragon is working as planned. There is no rush, everyone said, and Musk said that, in a worst-case scenario, SpaceX “would keep [Dragon] up there for at least a month” to troubleshoot the problem. The only blackout days are on and around March 15, when six astronauts aboard the station are slated to come home aboard a Russian Soyuz capsule.
This isn’t the first time SpaceX has had problems during flight. During the demonstration flight to the space station in May, a stuck nitrogen purge valve led to an automatic engine shutdown that aborted the launch of the Falcon 9 rocket.
Thruster Problem Forces SpaceX to Reschedule ISS Docking
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Thruster Problem Forces SpaceX to Reschedule ISS Docking
