Webb Telescope lost time to shutdown; latest technical setbacks seen as minor
Despite recent technical progress, 's James Webb Space Telescope (JWST) is headed into its peak year of spending with less schedule margin and fewer funding reserves than anticipated for the $8 billion flagship astronomy mission as it advances toward a planned 2018 launch.
Less than three months into the current fiscal year,says the JWST program has lost one of 14 months from its development-schedule margin due to the October government shutdown, which interrupted the first of three thermal vacuum tests and bumped the spacecraft's critical design review (CDR) to January from December.
In addition, NASA says the cryo-cooler system designed to regulate Webb's MIRI mid-infrared instrument is running well behind schedule due to multiple technical issues. “The cryocooler has been a struggle for a long time in terms of technical performance and cost,” says NASA's JWST deputy program manager, Eric Smith. “We have a new cost plan but they are close to running out of any schedule reserve going into fiscal 2014.”
Smith told the NASA Advisory Council this month that multiple technical issues have delayed completion of MIRI's so-called cold-head assembly, including valve leaks and a more recent issue involving welding of key components.
“There's just been a number of technical issues that have [trimmed] a lot of reserves to get this done,” he said, adding that only two months of schedule margin remain for the cryocooler system, ahead of more comprehensive thermal vacuum testing slated for next year.
Smith touts progress on new microshutters that had to be redesigned for Webb's NIRSpec camera due to a manufacturing flaw, but says the improvements are yielding mixed results: While an initial set of new-generation shutters has held up in environmental and acoustic testing, a second batch revealed the need for additional work.
Moreover, a new technical issue arose: Four pins intended to hold the spacecraft in place during launch will need to be redesigned to give the observatory a gentler nudge away from the Ariane 5 launch vehicle's upper stage once the payload fairing has been jettisoned post-launch.
“We call them launch release mechanisms, but their primary job is restraint,” says Scott Willoughby, the JWST program manager at. “Those four devices are being redesigned to minimize the output shock when they separate. It's a non-explosive device, so it is not a charge, but even when the coil unwinds, there's stored energy. That's in process, but it's not on the critical path of the telescope.”
Willoughby says testing of the main Kapton membranes that form the five layers of JWST's massive sunshield is going well (see page 48), though some of the subsystems that work to keep the telescope's infrared sensors and composite optical bench at a steady temperature below 40K need more work. Of particular concern are “ancillary thermal management subsystem products” near the base of the telescope's mirrors that are called the frill and bib, Willoughby says.
“These are panels that prevent heat from the sunshield from getting back and pinging back on the spacecraft and making the spacecraft too hot,” Smith says. “They are having to be redesigned, and so that's a new issue that is going to hang out for us past the spacecraft CDR. So their CDR will occur later in the year.”
Going forward, Smith says, thermal issues will plague the JWST through final integration. The supercold temperatures on the infrared detectors and high pointing stability in the optical bus afforded by steady temperatures will allow astronomers to study red-shifted light that originated more than 13.5 billion years ago, when the first stars and galaxies were emerging from the opaque post-Big Bang environment dubbed the “cosmic dark ages.”
“These five layers of sunshield have to be aligned properly so an intermediate layer doesn't either stick out beyond the layer below it, and therefore get heated by the sun [so] you see a hot outline of a sunshade or, more subtly, if the edges of the sunshade aren't perfectly flat, if they droop down, they can impede the heat-flow out,” Smith says. “Then the sunshield can get hotter, and the one closest to the telescope is hotter, and that raises the infrared background the observatory has to generate.”
Earlier concerns with the spacecraft's mass margin have been alleviated, both by the maturity of the JWST's design as well as performance improvements associated with the European Ariane 5 heavy-lift rocket that is expected to loft JWST to orbit in October 2018, according to Smith. While the program was reset after an outside panel in 2010 found it 14 months behind schedule and $1.5 billion over budget, some elements were accelerated because they were going well. Those schedule gains remain in place, says Willoughby, listing the spacecraft bus as an example.
Although once the most tardy of the telescope's four main elements—which include the telescope, instrument module and sunshield—the bus was accelerated to a CDR this month from May 2014. Despite the one-month slip in the spacecraft CDR to January, Willoughby says work is going well, “even with the challenges of operating a spacecraft in continuous sunlight at the L2 Lagrangian point.
“Because we always have Sun, we do need to deal with the thermal environment on the bus,” he continues. “Having this large sunshield such that it maintains the cold side cold, it reflects more heat back into the bus, so it makes the bus hotter than [it] normally would [be]. So we have other items in there that help with that, like deployable radiator shades.”
Despite good-news stories such as the spacecraft bus, other issues remain. Decaying infrared detectors discovered in 2010 still need to be replaced on some of the JWST's instruments ahead of thermal vacuum testing next year. Smith says detectors on NIRSpec have been swapped out for new ones. However, the Canadian-built Fine Guidance Sensor (FGS) that has been integrated onto the spacecraft will need to be de-integrated so the detectors can be replaced.
On the program side, Smith says contract renegotiations with Northrop Grumman have taken longer than expected since the mission was rebaselined in 2011 but they should be finalized this month. Willoughby says the remaining contract issues have not slowed development because there is agreement on the main points. However, if Congress does not appropriate NASA's combined $1.3 billion request for astrophysics in 2014, including $658 million for the JWST, other astrophysics projects could face a roughly 10% funding cut in order to keep the flagship astronomy program on track.
“I don't think the appropriate thing to do in astrophysics is to give everybody the same peanut-butter spread; we need to prioritize,” says Paul Hertz, director of NASA's astrophysics division. “The JWST project is the highest priority in all of astrophysics. I would certainly advocate that we fully fund it and that any reductions in the total budget come out of other areas of astrophysics.”
Hertz says lawmakers in the U.S. House of Representatives have proposed funding the JWST in 2014 at roughly $74 million less than requested. The Senate, he says, has thus far upheld the president's budget proposal.
“It's important to keep in mind that this is the peak year of funding requirements for JWST,” Hertz says, noting that NASA is operating under a continuing resolution (CR) that funds the JWST at the 2013 level of just $607 million. “If you do calculations under a CR, where everything is flat, you still have to find money elsewhere so that you can spend more on JWST in 2014 than in 2013 if you want to stay on plan for an October 2018 launch.”