Thursday, October 5, 2017
JWST launch slips to early 2019
NASA ordered the launch delay to keep ground teams from rushing to ready JWST for its previous launch date in October 2018. Assembly of the spacecraft at Northrop Grumman’s facility in Redondo Beach, California, ran behind schedule the last few months, consuming most of the remaining slack in the schedule leading up to the launch opportunity next October.
“During the summer, we started to analyze what’s the work we have yet to do, and what’s the pace at which we’re accomplishing the current work, which is the testing of the telescope down at the Johnson Space Center, and the integration and eventual testing of the spacecraft bus element, the combination of the sunshield and the spacecraft, and then the whole observatory,” said Eric Smith, JWST program director at NASA Headquarters. “It’s about this time that Northrop was encountering difficulties with the integration, not in the sense that they couldn’t do the work, but that it was taking longer than they had planned.”
Most of the delays encountered during spacecraft assembly have been small, Smith said, taking just a few days to resolve. But the difficulties added up to consume most of the three-and-a-half months of schedule reserve JWST had early this year.
“We’re uncovering the normal issues as you go through the integration of something this big and complicated, but there are no showstoppers that are making us scratch our heads, and ask, “How do we do this?” Smith said. “It’s just a matter of there are so many things to do, that they’re all taking a bit longer.
Managers also decided to take more time testing the fully-assembled observatory next year, ensuring engineers can carefully verify JWST’s readiness for launch.
“It’s the first time for doing this activity,” Smith said in an interview with Spaceflight Now. “No one’s ever built one of these before, so I guess it shouldn’t be too surprising, but once we did that analysis, we realized it should be prudent to move the launch date out so that we have time to do all the activities very carefully.”
The schedule slip will not add to the observatory’s cost or impact the telescope’s scientific output, officials said.
JWST is NASA’s next great observatory, promising new discoveries about the earliest galaxies in the universe, the formation of stars, and the environments of planets orbiting nearby stars.
The telescope’s primary mirror is made of 18 hexagonal gold-coated beryllium segments that will fold up for launch, then unfurl once in space. JWST also carries a five-layer sunshield to keep the observatory’s scientific instruments cold, giving the telescope visibility in infrared light.
JWST is set to launch on top of a European Ariane 5 rocket from Kourou, French Guiana. The Ariane 5 launch is a major contribution to the mission by the European Space Agency.
The JWST program escaped cancellation in 2011 as delays mounted and costs skyrocketed. NASA officials re-planned the program’s budget and schedule at that time, committing to launching the mission by October 2018 at a cost of $8.84 billion to the U.S. space agency.
Adding the contributions of European and Canadian partners, including the Ariane 5 launcher, pushes the mission’s total cost to around $10 billion.
ESA and Arianespace, the Ariane 5’s operator, planned to formally set a target launch date for JWST this month in a customary schedule tag-up 12 months before liftoff.
We’re sort of seeing (the assembly delays) right about the time we’re having to do this schedule assessment, and that’s when we realized you can’t go into detailed testing of the spacecraft element and the observatory with no reserves, so you need to build yourself some adequate reserve,” Smith said. “That’s what we’ve done.”
Webb’s new launch window opens in March 2019 and extends three months.
The delay opens up Arianespace’s October 2018 launch slot for BepiColombo, an interplanetary mission to Mercury jointly managed by ESA and the Japan Aerospace Exploration Agency. Engineers in Europe are in the final stages of testing BepiColombo, which includes two Mercury orbiters developed by ESA and JAXA, plus a transfer stage propelled by ion thrusters.
BepiColombo’s launch period opens Oct. 5, 2018, and extends until Nov. 28. The mission must launch during that window to begin a lengthy interplanetary journey that will use a flyby of Earth, two gravity assist swingbys of Venus, and six flybys of Mercury to set up for orbit insertion around the solar system’s innermost planet in December 2025.
The Mercury mission remains on track for its October 2018 launch window, wrote Johannes Benkhoff, ESA’s BepiColombo project scientist, in an email last week.
JWST comes in two major sections: the spacecraft and the telescope.
Smith said JWST’s sunshield, which will deploy to the size of a tennis court after launch, has been attached to the spacecraft by Northrop Grumman. But the sunshield’s installation took longer than planned.
Workers have re-welded transducers into the spacecraft’s propulsion system to replace units damaged during testing.
“If you were fortunate enough to go see it at Northrop Grumman in the big clean room there, you would see it looks fully assembled as far as the spacecraft element goes,” Smith told Spaceflight Now. “The transducers were repaired. New ones were put in, so that fix is behind us now, and they’ve been tested. We’re all good there.”
Engineers plan to conduct the first deployment test of the JWST spacecraft this month to evaluate how it will transform from launch configuration into an operational layout after liftoff.
Depending on how you count, Smith said JWST will have more than 300 deployments after it separates from from the upper stage of the Ariane 5 launcher. Counting steps in a similar way, the Curiosity Mars rover had around 70 deployments, according to Smith.
Smith said there were many small issues confronting the assembly team at Northrop Grumman, but no showstoppers.
“If you were to look at the list of things that would cause you to use reserve for the spacecraft element, you’d see there are many things, a small one a day or two here and there,” he said. “Installation of the membrane release devices, for instance. There are more than 100 of them. That was scheduled for a certain period, and it took longer.”
While Northrop Grumman puts together the spacecraft in California, the telescope is wrapping up a three-month stint inside a super-cold cryogenic thermal vacuum chamber at NASA’s Johnson Space Center in Houston.
Technicians placed the telescope, a structure that includes the mirrors and science instruments, inside the vacuum chamber in June. The test chamber was chilled to 37 Kelvin (minus 393 degrees Fahrenheit) and air pumped out to create a vacuum, mimicking the environment JWST will experience at its operating post at the L2 Lagrange point nearly a million miles (1.5 million kilometers) from Earth.
The test chamber in Houston is the largest such test facility in the world, and it will be the last time the telescope sees the extreme temperatures it will encounter in space. There is no chamber large enough for a thermal vacuum test of the fully-assembled observatory.
Smith said a preliminary look at the outcome of the cryogenic test looks good.
“The initial results from the thermal vacuum test are all very positive,” Smith said. “They showed some images that the optics team had taken of the mirror, and when you look at it with the detailed instruments that we have down there, you can see the gravity sag. It’s sitting in a 1g gravity field right now, so that puts a little deformation in the mirrors on the order of microns. But it was exactly as their models had predicted, so they were delighted by that picture, and the instruments have performed well.”
The telescope passed acoustic and vibration tests earlier this year at NASA’s Goddard Space Flight Center in Maryland. Those tests subjected the telescope to the shaking and sound it will experience during launch.
Ultra-sensitive micro-shutters inside JWST’s NIRSpec instrument, which proved troublesome earlier in the mission’s development, also worked as expected during the cryogenic test, a result Smith said was “very important.”
Engineers used mechanical actuators during the cryogenic test to align each of the 18 primary mirror segments to function as a single monolithic unit. The focusing actuators can also adjust the alignment after the launch of JWST, the largest space telescope ever built.
Methodical checks of JWST’s optics after vibration testing, and in cryogenic conditions, were aimed at avoiding a repeat of the mirror deformity that plagued the Hubble Space Telescope after its launch in 1990. Space shuttle astronauts had to add corrective vision aids to the orbiting observatory in 1993 to fix the problem.
No such astronauts visits are planned to JWST, which is not designed to be serviced in space. The first assessment of test data suggests no such deformity exists on JWST.
The three-month cryo test continued as the Houston area was flooded by drenching rains from Hurricane Harvey.
“They kept testing all through Harvey,” Smith said. “I think, in the end, it might have added two days fo the 93-day test. They did a wonderful job down there.”
The telescope is scheduled to come out of the cryogenic vacuum chamber Oct. 22, Smith said, and test crews will need about one month to prepare it for shipment to Northrop Grumman’s factory in Redondo Beach.
“Engineers will perform the warming gradually … to ensure the safety of the telescope, its science instruments, and the supporting equipment,” said Randy Kimble, an integration and test project scientist for the Webb Telescope. “Once the chamber and its contents are warmed to near room temperature, engineers will begin to pump gaseous nitrogen into [the chamber] until it is once again at one atmosphere of pressure (at sea level) and no longer a vacuum.”
“Depending on how long that hardware removal takes, some time in December you could expect to see it showing up in Southern California,” Smith said.
Workers will mate the telescope and spacecraft in Redondo Beach for another full round of combined vibration and acoustic tests next year. NASA says JWST will perhaps be the most tested vehicle ever sent into space after undergoing systematic tests at the instrument, spacecraft and telescope, and observatory levels.
The spacecraft will travel by boat from Southern California through the Panama Canal to the Ariane 5 launch base in French Guiana around the end of 2018 for fueling and final launch preparations.