One of Endeavour's three main engines was replaced inside the Vehicle Assembly Building as testing raised concerns about the powerplant. Photo: NASA

A potentially catastrophic engine problem encountered during the last space shuttle launch could force NASA officials to postpone Monday's planned liftoff of Endeavour.

On this 14th anniversary of the space shuttle Challenger accident in which seven astronauts were lost, space agency officials acknowledged post-flight inspections have revealed turbine blades cut a groove into a metal seal inside a main engine fuel turbopump.

The so-called tip seal is considered crucial to the engine's operation as it directs super-hot gases through the fast-spinning turbine blades. The turbine shaft spins at 37,000 rpm.

NASA says some contact between the blades and the nickel-plated seals has occurred previously, but the gouging during shuttle Discovery's launch last month was deeper and more severe.

If the contact had been worse on Discovery, it could have caused a failure of the engine and possibly even destruction of the shuttle, sources said.

With that in mind, NASA says it must fully understand the problem and the ramifications before clearing Endeavour for liftoff at 1747 GMT (12:47 p.m. EST) Monday on an 11-day radar mapping mission of Earth.

Boroscope inspections performed at Kennedy Space Center this week raised the concerns with one of the engines that powered Discovery into space on December 19 to service the Hubble Space Telescope.

"During routine inspections, we discovered what appeared to be a bit more rubbing or scoring on this tip seal that we usually see," NASA test director Doug Lyons told reporters this morning.

NASA says it does not yet know why the incident occurred on Discovery, but data from the launch indicated the pump's overall performance was not harmed.

Each turbopump contains two tip seals, each made up of six segments. They run the circumference of the pump housing, about 18-thousandths-of-an-inch away from each of the two turbine blade wheels.

"The function of the tip seal is to direct hot gas as it passes through the turbine into the turbine blade. This prevents gas from passing past the tips of the turbine blades, thus the name tip seal."

The suspect hardware has been shipped from KSC to the engine manufacturer Rocketdyne in Canoga Park, California, for further analysis.

To physically inspect the seal, technicians must first disassemble the turbopump. That work is beginning and more in-depth inspections and material tests were expected to start midday on Friday.

NASA would like the tests will be completed by Saturday afternoon, allowing managers to debate the issue during a regularly scheduled meeting.

Endeavour is lowered into position for mating to the external fuel tank. The three main engines are clearly visible. Photo: NASA

How the problem will impact Endeavour's launch date is not yet known.

"It is too early to say now. We got to go and see what really is going on there," Lyons said. "At this point, what we are doing is planning on proceeding with the countdown timeline."

Countdown clocks are scheduled to begin ticking at 2230 GMT (5:30 p.m. EST) Friday.

NASA has two attempts to get Endeavour off the ground, on Monday and Tuesday, before having to wait until around February 9 because other rocket launches planned from nearby Cape Canaveral Air Station.

The U.S. Air Force-controlled Range that provides required tracking, communications and safety services to all shuttle and rocket flights from Florida's East Coast needs 48 hours between launches to reconfigure a vast network of systems.

Once in space, Endeavour's six astronauts representing the U.S., Germany and Japan will use a sophisticated radar system to generate the highest resolution 3-D surface elevation map of Earth.