WASHINGTON — Manufacturing difficulties plagued major elements of the James Webb Space Telescope this year, forcing prime contractor Northrop Grumman Aerospace Systems to rebuild key structural elements declared unfit for flight while continuing to grapple with a persistently problematic cryogenic compressor needed to keep JWST’s infrared sensors cold.

Despite the latest setbacks, launch remains on track for October 2018, Eric Smith, JWST’s deputy program manager, told the National Research Council’s Committee on Astronomy and Astrophysics at a Nov. 4 meeting in Irvine, California.

The structural elements that had to be rebuilt were the pair of roughly 10-meter-long Unitized Pallet Structures that unfold in orbit to provide a frame for the ultrathin sun shield that shades JWST’s infrared instruments. During launch, the carbon-composite Unitized Pallet Structures protect the sun shield from vibration damage.

The first set of pallet structures, produced in the spring, did not meet NASA’s strength specifications because of moisture contamination traced to their mold at Northrop’s Space Park campus in Redondo Beach, California, Smith said in a Nov. 6 interview. The company was forced to fabricate new structures, burning two months’ worth of JWST’s reserve funding in the process and leaving just over 11 months of funded reserves as a cushion against a launch slip.

One replacement pallet structure was finished “within the past couple of weeks,” and the second is under construction, Smith said in the interview. The structures, slated to be integrated with the Northrop-built JWST spacecraft bus in 2016 or 2017, would be among the last hardware installed on the observatory before it is delivered to Kourou, French Guiana, to be integrated with its Ariane 5 launch vehicle.

Also threatening to guzzle reserve funding is the compressor for the cryogenic cooler that will keep JWST’s European-built Mid-Infrared Instrument chilled to its operational temperature of about minus 255 degrees Celsius. Northrop is building the helium-filled cryocooler under a separate 2007 contract from NASA’s Jet Propulsion Laboratory in Pasadena, California.

The cryocooler comprises a warm half and a cold half mounted about 20 meters apart on the finished JWST. The heat-generating compressor and its associated electronics are part of the warm half, which has suffered from manufacturing difficulties that have driven up NASA’s costs on Northrop Grumman’s cryocooler contract by 120 percent since 2012 and nearly tripled the project’s headcount to 110 workers, according to U.S. Government Accountability Office reports released in January and April.

A senior member of the JWST science team who is familiar with the compressor’s design said Northrop has built test units that perform to NASA’s specifications, proving the design is essentially sound. However, parts of the demonstration compressors are connected by bolts rather than the brazed seals required for spaceflight, and Northrop has not been able to get a brazed unit to perform to specification, the source said.

Northrop Grumman thinks it has a solution, according to Smith, and the compressor is now expected to be finished and shipped to JPL around April 2015 for about a year of integrated testing with duplicates of the Mid-Infrared Instrument and the cold half of the cryocooler. The compressor was supposed to be ready to ship this year, and the delay means nearly all the schedule padding in Northrop’s cryocooler contract is gone. Further delays in shipping to Pasadena, or any problems uncovered in tests there, will burn even more of JWST’s 11 remaining months of reserve funding, Smith said.

The compressor creates heat by compressing helium gas that then expands and cools as it works its way to the cold half of the cryocooler, which is known as the cold head assembly. The cold head assembly now appears to be working as intended after NASA swapped some leaky valves with replacements from manufacturer ValveTech of Phelps, New York. A flight-ready cold head will be installed on the Integrated Science Instrument Module that houses JWST’s four main science instruments in time for a thermal vacuum test scheduled for late July or early August 2015 at the Goddard Space Flight Center in Greenbelt, Maryland. Two previous thermal vacuum tests of the integrated science module used an engineering unit as a stand-in for the cold head.

Once Goddard finishes the thermal vacuum tests, JWST’s science module will be sent to NASA’s Johnson Space Center in Houston for integration with the observatory’s Optical Telescope Assembly in early 2016. The combined units will go into thermal vacuum testing at Houston the same year and, assuming no anomalies are found, be shipped to Redondo Beach for integration with the spacecraft bus Northrop is building.

NASA’s overseers in Congress pay close attention to the JWST program, which had to be overhauled in 2010 when it became clear the telescope would not adhere to the $5 billion budget and 2014 launch date NASA committed to in 2008, when development on the long-planned observatory began in earnest.

The pressure Congress puts on NASA, NASA passes on to Northrop, which is building key JWST elements and assembling the observatory under a prime contract awarded in 2002 and now worth roughly $3.5 billion.

“NASA is unhappy with performance and expects everyone on the team to do better,” Smith told the National Research Council Nov. 4. Smith gave Northrop credit for being “very proactive” on the prime contract, but said NASA is “going to have to watch” the cryocooler contract.

Northrop Grumman Chief Executive Wes Bush got a similar message recently from NASA Administrator Charles Bolden, who summoned Bush from Northrop headquarters in Falls Church, Virginia, for what Smith called a pair of “come to Charlie” meetings.

Northrop Grumman representatives Christina Thompson and Randy Belote, who declined multiple requests to provide comment for this story, also declined to say when Bush last met with Bolden.

JWST, an infrared successor to the Hubble Space Telescope, is by far the largest science mission now in development at NASA. Expected to cost some $8.8 billion to build, launch and operate for a five-year primary mission at the gravitationally stable Lagrange point some 1.5 million kilometers from Earth, JWST consumed more than $650 million in funding in 2014, when development costs were expected to peak.