A row has broken out within Nasa over the future of the rocket system needed to replace the Space Shuttle fleet which will retire in 2010.

Nasa administrators are pushing the Ares rocket programme as a replacement, but this will not be ready until 2014 at the earliest.

However, Nasa engineering staff working in their own time have developed a different plan which they say could be ready in two years and save the agency $35bn in development and fabrication costs.

Rather than develop an entirely new rocket the engineers say that their Direct v2.0 system would extend the existing rocketry used to launch the Space Shuttle and enable larger payloads to be carried into orbit for less.

"Nasa states, with 65 per cent confidence, that the Ares-I will be operational in March 2015 to carry the Orion spacecraft with a crew for the first time," the team in a briefing document (PDF).

"Current budget constraints and high technology requirements continue to push out this schedule with recent estimates now placing this first flight into 2016.

"By utilising existing production hardware and launch infrastructure in this way, Direct v2.0 is able to retarget all of the 'long lead time' items needed and remove them from the critical path to the first flights.

"This enables the Jupiter-120 to fly much sooner - within 54 months of a green light."

But administrators have been scornful of the plan, saying that the Ares is the best way to lift payloads into orbit.

"It's not feasible," Steve Cook, head of the Ares office, said in a recent interview with Associated Press after reviewing the plans.

"We said that it doesn't work and moved on. We're down to the nuts and bolts ... on this [Ares] rocket. This is not a napkin drawing."

Nevertheless the engineers are asking for a full review of their project before it is dismissed.