The SpaceX Falcon 9 test rocket launch is pictured at Complex 40 at the Cape Canaveral Air Force Station in Cape Canaveral.

Cape Canaveral - SpaceX on Tuesday readied a fresh bid to become the first private firm to launch a craft to the space station after fixing an engine problem that grounded its earlier attempt.

The launch of the Dragon space capsule atop the Falcon 9 rocket is scheduled for 3:44am (0744 GMT) from Cape Canaveral, Florida, on a mission to carry supplies to the International Space Station, an orbiting research lab.

“The Falcon 9 has been fully fuelled and is awaiting launch. Weather in Florida is warm and clear,” Nasa said on the Twitter micro-blogging site. Officials said there was an 80 percent chance of favourable weather.

The test flight - which should include a fly-by and berthing with the station in the coming days - aims to show that private industry can restore US access to the ISS after Nasa retired its space shuttle fleet last year.

No humans are travelling aboard the Dragon, but six astronauts are already at the $100-billion space lab to help the capsule latch on, to unload supplies and then restock the capsule with cargo to take back to Earth.

The Saturday launch attempt was scrubbed at the last second when computers detected high pressure in the central engine of the Falcon 9.

SpaceX engineers discovered the root cause was a faulty check valve.

“The failed valve was replaced on Saturday and after thorough analysis the vehicle has been cleared for launch,” SpaceX spokeswoman Kirstin Brost Grantham said in a statement.

If for some reason Tuesday's launch window cannot be met, another opportunity opens up on Friday. SpaceX had mentioned Wednesday as a potential launch day but later decided Friday would be a better alternative.

California-based SpaceX is the first of several US competitors to try sending its own spacecraft to the ISS with the goal of restoring US access to space for human travelers by 2015.

The company successfully test-launched its Falcon 9 rocket in June 2010, then made history with its Dragon launch in December 2010, becoming the first commercial outfit to send a spacecraft into orbit and back.

Its reusable Dragon capsule has been built to carry both cargo and up to seven crew.

Until now, only the space agencies of Russia, Japan and Europe have been able to send supply ships to the ISS.

The three-decade US shuttle program, which ferried astronauts and cargo to the research outpost, ended for good in 2011, leaving Russia as the sole taxi to the ISS until private industry comes up with a replacement.

The US space agency has given SpaceX about $390 million so far of the total $680 million that the company has spent on cargo development. SpaceX has a $1.6 billion contract with Nasa for future supply missions.

Orbital Sciences Corporation is working on its own supply ship, with a preliminary launch scheduled for later this year, and has a $1.9 billion contract with Nasa.

SpaceX also gets funding from Nasa for a separate effort to develop a commercial crew vehicle for carrying astronauts to space, along with competitors Blue Origin, Boeing and Sierra Nevada.

In a few years' time, SpaceX says it will be able to undercut the hefty price Nasa pays Russia for US astronauts to get a seat aboard the Soyuz space capsule - around $63 million a ticket.

But in the meantime, SpaceX and Nasa have both cautioned that the attempt is risky.

“Demonstration launches are conducted to determine potential issues so that they might be addressed and - by their very nature - carry a significant risk,” SpaceX said in a statement on Monday.

“If any aspect of the mission is not successful, SpaceX will learn from the experience and try again.”

If the launch goes as planned on Tuesday, Dragon would orbit the Earth on May 23 as it travels toward the ISS.

On May 24, the spacecraft's sensors and flight systems are to undergo a series of tests to see if the craft is ready to berth with the space station, including a complicated fly-under at a distance of about 1.5 miles (2.5 kilometres).

If Nasa gives the green light, the Dragon will then approach the ISS on May 25 in an attempt to berth with the station.

The astronauts on board the ISS will maneuver the station's robotic arm to help capture the capsule and attach it to the orbiting research outpost.

The hatch of the Dragon is set to open on May 26 for unloading and restocking. On May 31, the Dragon is to detach from the station and make a safe landing in the Pacific Ocean off the coast of southern California. - Sapa-AFP