NASA announced April 17 it has pulled a spacewalk dedicated to heat shield repair techniques from the schedule of its next space shuttle flight to make way for additional inspections to be conducted using the orbiter’s robotic arm.

The launch of NASA’s STS-121 shuttle mission to the international space station aboard the shuttle Discovery currently is scheduled to take place no earlier than July 1.

Mission planners decided to eliminate the third of three planned spacewalks to allow time for in-depth scans of the orbiter’s heat shield, NASA officials said, adding that the extravehicular activity (EVA) is not being abandoned altogether.

“The crew and the team are going to continue training for the third one in the event that the consumables would support an extra day for the mission, which is likely,” NASA spokesperson Kylie Clem said of the decision, which was first reported by CBS News.

The third planned EVA for STS-121 spacewalkers Michael Fossum and Piers Sellers called for the astronauts to test a series of shuttle tile and reinforced carbon-carbon (RCC) repair techniques.

The repair methods, which use a black putty-like material to fill in RCC cracks and a dabbed-on gray material to coat broken tiles, were first tested during an hour-long procedure during an STS-114 spacewalk last summer. That flight, also aboard Discovery, was NASA’s first shuttle mission following the 2003 loss of the STS-107 astronauts aboard Columbia, which sustained heat shield damage during its launch. That accident led to the tile and RCC repair tests during the STS-114 mission. STS-121 will be the second post-Columbia test flight.

Clem said the revised schedule now calls for the mission’s six-astronaut crew, commanded by shuttle veteran Steven Lindsey, to use a 15-meter boom extension on Discovery’s 15-meter-long robotic arm to conduct extra heat shield scans before and after Discover undocks from the station. Inspections were already planned for the second day of the 13-day mission, NASA said.

Discovery’s mission could be extended an extra day to allow the heat shield repair spacewalk to take place, but that will depend on the amount of consumables available to power the orbiter’s fuel cells, Clem added.

Sellers and Fossum will continue to train for that EVA along with the two others, first of which is aimed at testing the stability of the boom extension with an astronaut standing on the end. The test, which NASA station managers have said will be conducted over the orbiter’s payload bay, is designed to determine whether the boom can be used as a repair platform if needed.

The second spacewalk is dedicated to space station work, with Fossum and Sellers repairing a cable system that transfers power, data and video to and from the orbital laboratory’s mobile transporter railcar. The railcar is used to move major components along the length of the station.

Meanwhile, shuttle engineers at NASA’s Kennedy Space Center spaceport in Florida are replacing one of Discovery’s three main engines in preparation for its STS-121 launch.

Shuttle engine checks at NASA’s Stennis Space Center in Mississippi discovered a crack in a solder joint associated with the engine’s controller, prompting orbiter managers to swap Discovery’s lower left engine because of the possibility that it has the same problem, said Jessica Rye, a spokeswoman at NASA’s Kennedy Space Center.

Engineers are expected to have the replacement engine installed by April 22, Rye said.

“The end result is no impact to the shuttle’s rollover date,” Rye said .

Discovery currently is scheduled to roll from its hangar-like Orbiter Processing Facility May 12 to the Vehicle Assembly Building, where it will be mated with solid-rocket boosters and an external tank.

The mission’s launch window stretches from July 1 to July 19.