NASA has been concerned with JWST's mass since its inception in 1999, due to the telescope's size and the payload-carrying capacity of available launch vehicles. Mass constraints were allocated for each subsystem, including the spacecraft, limited to 1,754 kg. But as of September 2012, its estimated mass was 1,960 kg, a 12% overage.
NASA spokesman J.D. Harrington says design changes were insignificant, and included finding lighter-weight components and low-mass means for meeting spacecraft requirements.
“We also received additional mass margin from the launch provider,” he said, refering to Arianespace, which manages Ariane 5 launches. “All of this is completely normal as the observatory design matures.”
NASA says despite these concerns, JWST remains on cost and schedule under the rebaselined plan, asserting the project has overcome several challenges in the past 18 months alone: Mitigating an increase in the amount of heat expected on the instruments, accelerating optics work that added a month of funded reserve to the schedule, and completing 18 segments of the primary mirror—the project's highest technology risk—six weeks ahead of schedule.
“We are managing to the 2013 budget, and in the replan there is a plan for 2014 and each year after, so we're on track,” said Jonathan Gardner, deputy senior project scientist for JWST, at a NASA Advisory Council astrophysics subcommittee meeting July 17. Gardner warned, however, that any changes to the rebaselined plan—including a proposed $74-million funding cut to JWST in the House Appropriations Committee's version of NASA's 2014 budget—would give the program an out. “The project is committed to the budget and schedule, the profile that's in the replan, and if we don't have that profile then we're no longer committed to maintaining the same budget and schedule.”
Under the rebaselined plan, NASA has excluded the ISIM—one of JWST's four primary components that will eventually be integrated with the optical telescope, mirrors and spacecraft bus—from the so-called 'critical path' of milestones that could delay JWST's launch. Although the ISIM has burned through 75% of its schedule margin and added a third round of thermal vacuum tests, “since the ISIM is not on the critical path, there is no change in the launch date,” Harrington says.
Once ISIM test and integration is complete, JWST will have 14 months of funded schedule reserve before it launches in 2018. Only half of that time is allotted for the final three of five complex integration and test efforts, during which problems are commonly found and schedules tend to slip.
“For now, when they run into problems they can shift things around to accommodate late deliveries and other issues,” says a congressional source familiar with the project. “Once you get farther along in integration and test, you lose that flexibility, so any delays could delay the launch.”
Tap the icon in the digital edition of AW&ST for a timeline of JWST integration and test efforts, or go to AviationWeek.com/jwst