Propellant Requirements Force Razor-thin Window for SpaceX Launch

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Update: Launch was aborted in the final seconds of the countdown due to a high chamber pressure reading on Engine No. 5 of the Falcon 9 rocket’s first stage. Next launch opportunity is May 22 at 3:44 a.m. EDT.

WASHINGTON — Space Exploration Technologies Corp. (SpaceX) has no margin for error in its single instantaneous launch opportunity May 19. Liftoff must occur at precisely 4:55 a.m., or else the company will have to wait three days for the next attempt to loft what is expected to be the first privately operated spacecraft to deliver cargo to the international space station (ISS).

If the Hawthorne, Calif., company has to reset its countdown for the predawn launch, as it did on the previous two Falcon 9 launches, it will miss its chance to get its Dragon capsule to ISS with enough propellant to complete several days of on-orbit maneuvers the craft must perform before it is allowed to berth with the station, Alan Lindenmoyer, manager of NASA’s Commercial Crew and Cargo Program, said May 18 during a preflight briefing.

 “Because of the number of objectives on this flight, that requires a lot of propellant,” Lindenmoyer said at the briefing, which was held at Kennedy Space Center in Florida. “Therefore, they need to have very precise timing on the launch.

“They need as much propellant margin as they can possibly get.”

Falcon 9 and Dragon are slated to lift off from Launch Complex 40 at Cape Canaveral Air Force Station, Fla, at 4:55 a.m. EDT. If SpaceX is forced to scrub the launch attempt, it will have to wait until May 22 for the next opportunity.

On top of the fuel needed to complete these maneuvers, Lindenmoyer said, SpaceX is packing enough propellant on Dragon to allow for one mulligan.

“They would like to protect for an abort case so that they could even come back and try again,” he said.

SpaceX President Gwynne Shotwell told reporters she thinks Falcon 9 has better than even odds of lifting off May 19, weather permitting.

“We have not hit a T-0 yet on our first attempt,” Shotwell said at the May 18 briefing. Nevertheless, she said, “I’m going to give myself better than a 50-50 shot of lifting off tomorrow.”

If SpaceX does not launch on May 19, it could try again on May 22, May 23, May 25, May 26 or May 29, Shotwell said.

If it cannot launch by May 29, it will run afoul of the so-called beta cutout period, when, due to the angle of the sun, Dragon’s intended orbit would cease to take the spacecraft into the Earth’s shadow, pushing temperatures into potentially unsafe ranges, Shotwell said.

“Things get very hot,” she said.

Once Dragon finally reaches orbit, it will begin checking off flight objectives which, if completed successfully, will result in the company delivering to ISS a cargo shipment that includes food, water and student experiments. If Dragon can then fly back to Earth and be recovered by SpaceX, the company will be clear to begin regular cargo runs to ISS later this year under a $1.6 billion contract it won in 2008.