New technical issues are emerging for NASA’s James Webb Space Telescope (JWST) that have eaten up much of the management reserve on the program’s crucial instrument suite as it prepares for its scheduled October 2018 launch on an Ariane 5 ECA rocket.

For more than a decade NASA’s most expensive science mission suffered cost growth and schedule delays due to poor management and inadequate budgets.

But until recently, technical progress on the enormous space observatory appeared sound.

Since the project was rebaselined in 2011, however, NASA has been grappling with delayed delivery of two primary instruments, looking at ways to reduce mass on the spacecraft bus and addressing technical problems with key subsystems, one of which required the addition of a third round of lengthy cryo-vacuum testing to the Integrated Science Instrument Module (ISIM).

Combined, these problems have cost 18 of 26 months of schedule reserve on the ISIM, the heart of the telescope that houses JWST’s four instruments, which are designed to detect light from distant stars and galaxies, and planets orbiting other stars.

One of the late instruments is the Near-infrared Spectrograph (NIRSpec), a 200-kg (440-lb.) spectrometer designed to observe up to 100 celestial bodies simultaneously at various spectral resolutions that is being supplied by the European Space Agency (ESA) and built by Astrium GmBH of Ottobrun, Germany.

According to ESA, NIRSpec was found in July 2011 to have three cracks in the part that holds the optics components for the instrument. A failure review board was held in January 2012, and ESA had to reassemble the instrument using a flight spare optical bench.

ESA’s rework of NIRSpec will not be completed in time for ISIM’s first cryo-vacuum test, which gets under way in August. But the agency says NIRSpec is now reassembled and has passed the first cryo performance and vibration tests, with a final cryo test now under way. Shipment to NASA is planned for mid-September, more than one year late but in time to incorporate NIRSpec into ISIM’s third round of cryo-vacuum tests in 2015.

NASA is also awaiting delivery of a second instrument that was delayed nearly a year, the Near-infrared Camera (NIRCam) built by Lockheed Martin Advanced Technology of Palo Alto, Calif.

It is expected to arrive later this month after NASA determined the need to electrically ground a mirror on the instrument, a hardware change that required the contractor to attach a small wire to the edge of the mirror, with the other end attached to the metal of the support mechanism holding it, says Lockheed Martin spokesman Buddy Nelson.

Another challenge JWST must address is reducing mass on the spacecraft bus, the mission’s least-mature subsystem and one that can still accommodate design changes to account for the mass of its electrical wiring harnesses, which turned out to be more than expected. Mass constraints have been allocated for each subsystem, including the spacecraft, which is limited to 1,754 kg. As of September 2012, however, its estimated mass was 1,960 kg, a 12% overage.

NASA spokesman J.D. Harrington says design changes were insignificant, and included the use of lighter-weight components and finding low-mass means for meeting spacecraft requirements. “We also received additional mass margin from the launch provider,” he said, referring to Arianespace, which manages Ariane 5 launches. “All of this is completely normal as the observatory design matures.”

Conceived in the late 1990s as a follow-on to the Hubble Space Telescope, JWST was projected to cost just $1 billion. By 2011, however, the program had been plagued by almost a decade of cost overruns and schedule delays, prompting NASA to rebaseline the program with a revised cost estimate of $8.8 billion, a new launch date of October 2018, and a healthy amount of schedule margin to maintain both.