Results from the JWST Testing Assessment Team
Review
Erin Elliott
18 November 2010
Context
• Testing Assessment Team (TAT) was tasked by NASA HQ to
review JWST’s integration and test plans.
• Prompted by a request for review from Senator Mikulski.
• Members:
John Casani, Chair, Jet Propulsion Laboratory
Alan Dressler, Carnegie Observatories
Matt Mountain, Space Telescope Science Institute
Jerry Nelson, University of California–Lick
Observatory
Jim Oschmann, Ball Aerospace
Al Sherman, Allan Sherman, LLC
Georg Siebes, Jet Propulsion Laboratory
Erick Young, USRA
Bill Irace, Jet Propulsion Laboratory
Milt Heflin, NASA Johnson Space Center
Jeff Kegley, NASA Marshall Space Flight Center
Mike Ryschkewitsch, NASA
• Review focused on:
– ISIM testing at GSFC
– OTIS testing at JSC (OTIS = OTE + ISIM)
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JWST Architecture
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Overview of JWST I&T
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ISIM Testing Overview
• ISIM testing uses an OTE
simulator (OSIM)
• Each cryo cycle takes 20 weeks
(7 wks of cooldown & warmup)
• Thermal tests include
– Measurement of total load
ISIM puts on cryocooler
– Measurement of ISIM
radiators and straps
• Optical tests include
– SI to ISIM pupil shear and
focus
– SI wavefront errors
– Data needed for ISIM to
AOS alignment
– Plate scale measurements
– WFSC calibrations
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OTIS Testing
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OTIS Testing
• Single cryo cycle planned, taking
170 days.
• Cost is about $1,000,000 per day!
• Thermal tests include:
– OTE temperature
– Core isolation performance
– ISIM electronics compartment
isolation performance
– ISIM & line loads
Optical tests include:
• Primary / secondary alignment to the AOS
• Primary / secondary actuator ranges
• ISIM to AOS alignment
• Final plate scales
• Primary mirror ROC, conic, astigmatism, tilt
• Closed-loop WFSC demonstration
• Rogue, truant path checks
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Cryo Position
Metrology
Primary Mirror LF
WFE & Stability
SM
PG
PG
AOS
PG
PG
PM
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TAT Conclusions
The TAT Team concluded that the ISIM testing at GSFC can be
reduced from 14 months to 10 months and believes the OTIS
testing at JSC can be reduced from 167 days to 90 days with
acceptable reduction in pre-launch predictability of performance
and without predictable loss of science capability. This reduction
could shorten the critical path, avoiding significant cost growth.
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TAT Recs – Science & Testing Priorities
• Prioritize key system requirements, and design and implement the
test program using those priorities.
• Conclude OTIS testing in 90 days. Continued work is only justified by
substantial risk to Level 1 science requirements.
• Prioritize functional tests over performance tests. Complete functional
tests early in the test sequence.
• There is a range of test temperatures in which all instruments but
MIRI can be verified. MIRI verification should not drive a second OTIS
cycle nor prolong the first test cycle.
– Test plan should be driven by NIRCam, FGS, and NIRSpec.
• Test all detectors at temperatures up to at least 60 K.
– NIRSpec data wasn’t taken at the time.
• Make an effort to obtain new NIRSpec flight detectors with improved
dark-current performance.
– Observatory could operate over wider range of on-orbit temps.
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TAT Recs – Organization and Decision Making
• Establish an I&T Lead responsible for defining and documenting a
clear I&T approach.
• Adopt overall I&T plan that maximizes efficiencies over the entire
test program. Goal is to minimize risk of cost and schedule growth
during OTIS testing.
– High risk of needing a second cryo cycle at OTIS.
• Develop decision criteria and contingency plans for OTIS testing.
• Employ a command-and-decision structure during OTIS testing
similar to the one used for the Hubble Servicing Missions.
• Assess implications of proposed SE reorg. Make sure GSFC has the
necessary staff to support it.
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TAT Recs – Optical Testing
• Priorities at JSC should be:
– Mechanism tests
– Photogrammetric optical alignment
– AOS to ISIM alignment
– Verification of integrated FSM/ FGS functionality
– COC testing of the primary mirror
• Establish clear pass/fail criteria. Put cross-checks and
extrapolations to on-orbit performance as optional tests.
• Eliminate cryo frill and rogue-path tests and reduce the scope of
the pass-and-a-half testing.
• Eliminate CPT testing at the ISIM and OTIS levels. Do at SI level.
• Reduce WFSC demonstrations, concentrating on functional testing
and polarity checks.
• Increase concurrent optical and thermal testing during ISIM testing.
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TAT Recs – Thermal Testing
• Combine the two thermal balance points in the OTIS testing to a single
test.
– Add a core test, updated to the current flight design.
– Add thermal testing to the Pathfinder testing.
• Eliminate the first ISIM cryo test.
– Add a cryo test with OSIM and the NIRCam ETU (off the critical
path).
– Measure SI displacements during proof-load testing.
• Postpone installation of OTIS GSE until after chamber commissioning.
• Shorten thermal model turnaround times to a few hours.
– Prevents costly delays during OTIS testing.
• Develop tiered criteria that permit progressive assessments based on
information as it evolves, and reduce test time where possible.
• Increase parallel optical and thermal testing during OTIS testing.
• Apply available radiator margin to reduce stabilization times.
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How do we build and test larger missions?
• Interesting higher-level discussion during TAT proceedings.
• “Test as you fly” is a well-established NASA rule.
– Waivers required when testing doesn’t meet the requirement.
• JWST already in the regime where that is not possible.
– OTIS testing takes place with a sunshield and spacecraft
simulator.
• Electrical connections, etc., made after OTIS testing won’t be
performance-tested at cryo.
– A return flat isn’t available for a full-aperture end-to-end optical
test.
• Three small flats are used instead.
– The JSC chamber doesn’t recreate the flight thermal
environment.
• Clearly can’t build larger chambers and test mirrors as the collecting
areas increase.
• How do we build systems in pieces and guarantee that the system
will work correctly after assembly?
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How do we ensure thermal performance in
JWST and larger missions?
• Two schools of thought on thermal testing:
1. Test in a very flight-like environment.
–
Thermal models used during design stage only.
2. Accept testing in a non-flight-like environment and use thermal models to predict on-orbit
performance.
•
•
•
•
JWST uses approach # 2.
–
Facility large enough to create a flight-like environment doesn’t exist.
–
JWST is passively cooled. Error in thermal design could equal lost mission, not just a
shorter mission lifetime.
–
Accuracy and turnaround times for the thermal models is a constant struggle.
–
Thermal balance tests are very time-consuming and $.
Several NASA missions launched recently are running warmer than predicted. These are
actively cooled missions, so leads to shorter mission lifetimes.
There is a need to review the missions as a group to look for common issues.
But also: How do we make systems more robust w.r.t. operating in varying thermal
environments?
–
Part of the answer is to pursue detectors with improved dark current vs. T
performance.
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