UPDATE: The static test fire was successful on the second attempt at about 4:15 pm ET.

SpaceX will conduct a static fire engine test at 3:00 pm ET today in preparation for its scheduled test launch to the International Space Station (ISS) next week. The company’s webcast of the engine test begins at 2:30 pm.

It is a 2-second test of the nine Merlin engines on SpaceX’s Falcon 9 rocket and is part of a complete dress rehearsal for the next Falcon 9 launch scheduled for May 7.

If all goes according to plan, SpaceX will launch the Falcon 9 with a Dragon spacecraft at 9:38 am ET on May 7 as the second test launch in NASA’s Commercial Orbital Transportation Services (COTS) program. The COTS program is funding two companies, SpaceX and Orbital Sciences Corp., to develop launch vehicles and spacecraft that can deliver cargo to the ISS.

Three SpaceX test launches were planned as part of the COTS program. The first was successfully conducted on December 8, 2010 when the Falcon 9 delivered a Dragon spacecraft to orbit and was then deorbited and recovered at sea. SpaceX convinced NASA to let it combine the objectives of the last two test launches into this flight, which NASA calls C2+ — the second COTS test flight plus the additional objectives that had been planned for the third. Under the type of agreement NASA and SpaceX have — a Space Act Agreement — the company is paid only when it successfully completes milestones. However, NASA cannot set requirements and has more limited insight and oversight of the company’s activities than with a traditional procurement method such as a fixed price or cost plus contract.

A lot is riding on this launch and the COTS program overall. NASA has no way of its own to send cargo — or people — to the ISS now that the space shuttle program has ended. Russia, Europe and Japan, all of whom are partners with NASA in the ISS program, have cargo spacecraft that deliver supplies to the ISS crews, but the volume of cargo required to support six crewmembers 365 days a year exceeds their capacity. When the ISS program was planned, it assumed that the space shuttle would be available throughout the ISS’s utilization period. President George W. Bush’s decision to terminate the space shuttle program as soon as ISS construction was completed changed those plans and the COTS — or “commercial cargo” — program emerged.

SpaceX is planning to evolve its Dragon spacecraft to carry people as well as cargo as part of NASA’s commercial crew program to take astronauts to and from the ISS. Today, only Russia can launch and return ISS crews.

Demonstrating commercial cargo is the first step. NASA and SpaceX are cautioning everyone to keep their expectations in check for this flight, stressing that it is a test flight. In fact, this flight has been postponed several times, mostly recently from April 30, as SpaceX strives to ensure it goes well. Today’s test is another step in that process.