The Falcon 1 rocket stands atop the launch pad built on Omelek Island in the Central Pacific. Credit: Thom Rogers/SpaceX

An initial launch try for the partially reusable rocket was scrubbed November 26 because a ground liquid oxygen tank's vent was accidentally left open. Significant quantities of the super cold oxidizer escaped before workers could correct the problem. The engines on both stages of the rocket use liquid oxygen and kerosene fuel.

SpaceX has built its pad on Omelek, part of the Marshall Island chain in the Kwajalein Atoll of the Central Pacific. The site permits flying rockets on trajectories to reach either polar or equatorial orbits -- an advantage not practical from the U.S. mainland.

But getting replacement liquid oxygen and helium supplies for additional launch attempts has taken time. And a U.S. military missile defense test from Kwajalein has the Army-operated Range reserved, pushing Falcon's next try to December 19 (U.S. time).

A C-17 cargo aircraft was charted by SpaceX to transport two liquid oxygen containers to Hawaii for re-filling. Another tank was ordered in Hawaii. All three are aboard a barge slated to arrive in Kwajalein around December 14.

SpaceX CEO Elon Musk says his team expects to have enough liquid oxygen available to fuel the Falcon 1 four or five times, offering plenty of margin for delays.

In a remarkably open and candid assessment of the first launch attempt, Musk explains in a statement the unusual set of circumstances that led to the liquid oxygen loss. It all began with postponing the launch from October to November for additional testing of the Merlin first stage main engine.

"Many have asked how we could run out of LOX on a remote tropical island on the last launch attempt. The team tried hard to avoid it, but several issues conspired to create the problem:

• "The additional month of Merlin testing resulted in additional LOX boil-off on island. Even though it is stored in vacuum jacketed containers, LOX at -300F degrees does not like being on a tropical island at 85F.
  
  
• "The SpaceX LOX plant on island broke down a few weeks prior to launch, which meant we could not top up.
  
  
• "We ordered replacement LOX from Hawaii, but the container quality was poor, so only 20% of what we ordered actually arrived.
  
  
• "Ground winds were unusually high on launch day, which amplifies the boil-off rate significantly, since the Falcon's first stage LOX tank is uninsulated.
  
  
• "All of the above would not have mattered if our final storage tank did not have a small, manual vent valve incorrectly in the open position. Somewhat agonizingly, we were only a few percent away from being full. We just needed a little sip from the last tank.
  
  
• "After a while, we were able to close the vent and fill the vehicle's LOX tanks. However, we use LOX to chill our onboard helium and the absence of ground LOX to do so resulted in the helium heating up and venting back to storage. In the end, we did not have enough LOX to stay filled on the rocket and chill and pressurize the helium."

Meanwhile, SpaceX has resolved a separate problem that occurred during the November 26 countdown. The rocket's first stage main engine computer experienced a reboot. The glitch, which was unexplained at the time, would have been reason enough to scrub the launch that day even without the liquid oxygen troubles.

Technicians have since determined a ground-power problem triggered the reboot.

"The reason it cropped up at Kwajalein was that the higher load on the longer umbilical (three times longer than in prior tests) coupled with high temperatures in Kwajalein resulted in increased resistance in the ground umbilical. This was just enough to lower the voltage below minimums and cause an engine computer reset when drawing maximum power," Musk said in his launch update message.

The maximum power test was repeated with Falcon running on its internal batteries -- like it will be during the launch -- instead of the ground-provided power source. No problems were encountered, SpaceX said.

The launch team will prevent the problem during the next countdown by "slightly increasing voltage on the ground umbilical," Musk said.

Pending the missile defense test being completed as scheduled this month, SpaceX could get three consecutive days for launch attempts on December 19, 20 and 21 (U.S. time). The Range closes December 22 for the holidays.

The inaugural payload for the Falcon 1 rocket is the Air Force Academy's cadet-built FalconSat 2 space plasma probe. It will be delivered into a 250 by 310 mile orbit inclined 39 degrees to the equator during the launch.