HOUSTON — It took two attempts, but SpaceX carried out a successful hot-fire test of the Falcon 9 first stage on April 30, as part of a dress rehearsal for the countdown to the prospective first U.S. commercial resupply mission to the International Space Station.

The test on Space Launch Complex 4 — the Hawthorne, Calif.-based company’s launch pad at Cape Canaveral — concluded at 4:15 p.m. EDT.

“SUCCESS. 2 second burn!” the company announced by Twitter.

SpaceX engineers planned a full review of the hot-fire data.

The first attempt on April 30 was interrupted by a computer glitch with 47 sec. remaining in the countdown. After the successful second test, the day ended with the planned 18-day Falcon 9/Dragon test mission tentatively set for liftoff on May 7 at 9:39 a.m. EDT. However, a repeat of the countdown interruption on launch day would result in a scrub, since the Falcon 9/Dragon launch window is instantaneous (1 sec.).

The company is reviewing the data from the hot fire as well as the countdown rehearsal difficulties.

“So far things look good,” says Kristin Brost Grantham, SpaceX spokeswoman. “Engineers will now review data as we continue preparations for the upcoming launch.”

After tracing the difficulty to a “bad limit” that prompted a flight computer abort, SpaceX reset the clock at the 14-min. mark and resumed the countdown at 4:01 p.m. EDT. The second terminal count unfolded without interruption, and the Falcon 9’s nine Merlin first-stage kerosene/liquid oxygen engines erupted on schedule for 2 sec., generating a combined 3.8 million lb. of thrust.

Plans to launch the mission on April 30 were postponed last week to provide SpaceX engineers with more time for hardware-in-the-loop software control testing.

The mission plan combines what were previously the company’s Phase 2 and 3 COTS orbital test objectives into a single flight.

During the Phase 2 portion, the company will attempt to demonstrate the unpiloted Dragon supply capsule’s rendezvous capabilities, and during Phase 3, the final approach, grapple and berthing mission elements.

If the scheduling holds, the Falcon 9 will boost the Dragon capsule and a noncritical station cargo of just more than 1,000 lb. into orbit on a rendezvous trajectory. On May 9, the supply ship will maneuver to a position 1.5 mi. below the station for a test of the station crew’s close-proximity communications and commanding capabilities. If the rendezvous phase of the flight goes well, the Dragon will move away from the station and return on May 10 for the berthing.

NASA astronaut Don Pettit and Andre Kuipers of the European Space Agency will control the station’s Canadarm 2 as Dragon closes within reach of the 58-ft.-long robot arm.

Once securely in the arm’s grasp, Pettit will turn over primary control of Canadarm 2 to Kuipers, who will maneuver Dragon to its berthing port on the station’s U.S. segment Harmony module. The reusable freighter would be released on May 25 for re-entry and recovery in the Pacific Ocean. Station crew access to the freighter is scheduled for May 11.