44th International Conference on Environmental Systems ICES-2014-304 13-17 July 2014, Tucson, Arizona

James Webb Cryogenic Thermal Pathfinder Test

Wes Ousley1 and William Burt2 Genesis Engineering Solutions, Lanham, MD, 20706

and

Angelique Davis3 Edge Space Systems, Glenelg, MD 21737

The James Webb Space Telescope (JWST), scheduled for launch in 2018, includes a 6.5m 18- segment primary mirror passively cooled below 55 degrees Kelvin. The observatory cannot be optically or thermally tested as a system at flight temperatures due to its large sunshield, so the telescope portion along with its instrument complement will be tested as a single unit. This test will be very late in the program, and on the program schedule critical path, so a set of cryogenic tests with some flight-like hardware will be performed earlier to help mitigate this schedule risk. The JWST Pathfinder will be used in these optical and thermal tests, and includes a test-only telescope structure segment, two flight-like primary mirror segments, a flight-spare secondary mirror, and associated electronics.

This paper describes the JWST Thermal Pathfinder (TPF) cryogenic test, and the analysis used to design the test configuration. Scheduled for early 2016, this test will characterize telescope thermal performance during transient conditions, and will demonstrate a cool-down procedure that meets hardware gradient requirements. Items will be added to the Pathfinder structure to more closely match telescope thermal characteristics, including flight-like insulation and blanks to simulate the mirrors not present on the Pathfinder. To meet mass targets, the blanks will be much lighter than flight mirrors. Therefore, a heater system is being designed to slow their cool- down to allow a better simulation of the structure/mirror system cool-down during the test. Other heater systems are being designed to make up for the mass of instruments and other structures that are not part of the Pathfinder structure.

Additional hardware will be installed in the chamber to help prepare for the flight telescope test by providing a thermal environment similar to that test, including a simulator for the warm spacecraft, support structures, a sunshield simulator, and test cables. A large room-temperature electronics box simulator will also be tested for the effects of the gas-assisted cool-down on its sophisticated thermal isolation system.

Nomenclature BSF = Backplane Support Fixture EM = Engineering Model GSFC = NASA Goddard Space Flight Center IEC = Instrument Electronics Compartment ISIM = Integrated Science Instrument Module JSC = NASA Johnson Space Center JWST = James Webb Space Telescope K = Kelvin (unit of Temperature) MLI = multi-layer insulation

1 Senior Thermal Engineer, 4501 Boston Way, Lanham, MD, 20706. 2 Cryogenic Thermal Systems Engineer, 4501 Boston Way, Lanham, MD, 20706. 3 Thermal Engineer, P.O Box 310, Glenelg, MD 21737. NASA = National Aeronautics and Space Administration OTE = Optical Telescope Element OTIS = OTE and ISIM Assembly PF = JWST PathFinder PMSA = Primary Mirror Segment Assembly SAO = Smithsonian Astrophysical Observatory SINDA = Systems Improved Numerical Differential Analyzer SLI = single-layer insulation SMA = Secondary Mirror Assembly SMSS = Secondary Mirror Support Structure SVTS = Space Vehicle Thermal Simulator TMM = Thermal Math Model TPF = Thermal PathFinder TSS = Thermal Synthesizer System

I. Introduction The James Webb Space Telescope, scheduled for launch in late 2018, is designed to observe the early universe as it emerged from the dark ages that followed the Big Bang. Featuring a cryogenic 18-segment, 6 meter primary mirror and near- and mid-infrared cameras and spectrometers, JWST provides a unique capability to study the evolution of galaxies, the history of the Milky Way, and the origin and formation of planetary systems.

As an international collaboration among the space agencies of the US, Europe, and Canada, JWST is to be launched by an launch vehicle from Kourou, French Guiana. After a six-month journey, JWST will enter orbit around the Earth-Sun L2 Lagrange point, about 1.5 million km from the Earth in the anti-Sun direction. In this orbit, the telescope and instruments are shadowed from Earth and Sun inputs by the large deployable sunshield, allowing passive cooling to cryogenic temperatures (see figure 1). Due to the size and 1-g distortion of the sunshield and the cold temperature of the optical system, the deployed JWST cannot be thermal-balance tested, or optically tested as a system, in any facilities which currently exist. Instead, sub-assemblies will be optically and thermally test-verified, and analysis will be used to show compliance with system requirements, including margins1.

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Figure 1. James Webb Space Telescope (JWST)

The JWST optical system will be cryogenically tested to verify optical performance at predicted temperatures. The Integrated Science Instrument Module (ISIM) was assembled and is undergoing environmental testing at NASA/Goddard Space Flight Center (GSFC), and the Optical Telescope Element (OTE) structure is being built up at Aerospace Systems. The mirrors will be installed on the OTE structure at GSFC to complete OTE assembly, and then the ISIM is integrated with the OTE. This combined assembly is called the OTIS (see figure 2). After transport to the NASA Johnson Space Center (JSC) Chamber A thermal vacuum facility, the OTIS will undergo end-to-end optical and thermal tests at flight temperature levels in mid-2017, with the JWST launch schedule depending on timely test completion. To reduce this schedule risk, a series of three tests will be executed in Chamber A prior to OTIS, using “pathfinder” hardware in 2015-2016.

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OTIS = OTE + ISIM

Secondary Mirror Assembly (SMA)

Primary Mirror Segment Assemblies (PMSA) Optical Telescope (18 Total) Element (OTE)

Aft Optics Subsystem (AOS)

Primary Mirror Backplane Support Structure (PMBSS) Integrated Science Instrument Module (ISIM)

Figure 2. OTIS

In general, the “pathfinder” tests will check out optical test hardware, chamber systems, analyses, team communications, and procedures2. Test personnel will demonstrate test processes, enable improvements in OTIS test efficiency and accuracy, and foster team-building and training in the test environment without significant risk to flight schedules or hardware. The first two Pathfinder tests will emphasize optical metrology and thermal distortion measurements starting with a bare center section of the telescope backplane structure. An engineering-model secondary mirror assembly (SMA) and its support structure is installed, along with two flight-like primary mirror assemblies (PMSAs), to provide an effective optical test configuration (see figure 3). The Thermal Pathfinder test originated from a recognition that the open and bare center backplane in the optical and thermal distortion tests is so different from flight as to provide little thermally useful pre-OTIS level information, and a realistic thermal configuration would obstruct photogrammetry targets. Further was a recognition of an opportunity to evaluate critical telescope backplane heat flows, which are never tested for prior to OTIS. Finally, it was recognized that the complicated thermal aspects of the OTIS test require high fidelity thermal pre-testing to characterize the test equipment and the facility, and to train personnel. This resulted in a separation of the Pathfinder optical and thermal testing events.

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. Secondary Mirror

. Primary Mirrors

. Aft Optical System Mass Mockup

. Pathfinder Structure

Figure 3. Pathfinder optical test configuration

II. Thermal Pathfinder Test Objectives The TPF test will focus on generating thermal data for an OTIS-like thermal system. This includes demonstrating an OTIS-like cool-down procedure, providing data on structure temperature gradients and heat flows, and verifying OTIS test thermal environment simulators. A primary objective of the TPF test is to demonstrate that the schedule-critical OTIS cool-down timeline will meet critical mirror gradient constraints. Full thermal instrumentation on flight mirrors is not practical due to mass, thermal distortion, and data system considerations, but the Pathfinder flight-spare mirror assemblies will have thermal sensors on each sensitive component. Showing gradient compliance during an OTIS-like cool-down will increase confidence in the safety of the OTIS test plan. At this point in time, the OTIS cool-down is planned to last about 30 days, but this plan will be thoroughly scrubbed and analyzed before the TPF test plan is finalized. Gradients and heat flows along the telescope backplane are critical elements in thermal distortion and stray light analyses. The backplane thermal blankets and coatings have inherent performance and workmanship uncertainty, strongly affecting heating parasitics. This test represents the only opportunity prior to OTIS to evaluate these characteristics, and the TPF thermal sensor suite will allow a thorough examination. Sensitivity of the backplane heat balance to temperature changes in the space vehicle simulator, structure interfaces, and ISIM Electronics Compartment will also be evaluated. Thermal effects of motor operations will be realistically simulated for the two primary mirrors, including electronics heating, development of gradients, and structure settling time. One phase of the OTIS cool-down will be gas-assisted using helum, likely in the higher rarefied pressure regimes currently lacking an accurate analytical framework. The OTIS-like cool-down in this test will provide key gas-cooling data for OTIS timeline prediction, including effects of gas pressure and shroud temperature changes, gas removal effects on temperature stability, and performance of multi-layer insulation systems in the gas environment. The TPF test will have an IEC simulator in place to evaluate the effects of the reduced-effectiveness thermal shields

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on nearby structures, and verify that flight survival heaters will be adequate to protect the IEC during OTIS gas cooling. The TPF test will help prepare the test staff and analytical tools for the OTIS test. Since the TPF cool-down will be a dress rehearsal for OTIS, shift personnel and thermal analysts will be able to practice the real-time use of thermal model in a realistic manner. Projections of transient trends and effects of adjustments to heaters and shroud temperatures will be used as input into event and personnel schedules. Comparisons to pre-test predictions and sensitivity study results will enable model upgrades in real-time, improving model performance during later test phases. For example, effects of adjustments of gas pressure and shroud temperature during cool-down will be compared to model predictions. Consequently, model adjustments could be made and follow-up tests could be designed to isolate parameters that need more definition. Another primary objective of the TPF test is the cryogenic demonstration of the Space Vehicle Thermal Simulator (SVTS), which will provide a flight-similar thermal environment for the telescope during the OTIS test. The SVTS also accommodates the interface with the (non-flight) cryocooler used to cool down the MIRI instrument. The cooling envelope of this cryocooler in the OTIS configuration will be demonstrated in the TPF test. Other test objectives include demonstration of systems for data monitoring and display, communications protocols, and contamination control monitoring.

III. Test Configuration The JSC Chamber A, used to test Apollo hardware fifty years ago, has undergone a serious makeover to prepare for the JWST Pathfinder and OTIS test program3. The heritage chamber liquid nitrogen (LN2) shroud volume is 36m tall and 20m in diameter and is gravity fed to reach about 80Kelvin. A helium-cooled shroud with 20m height and 14m diameter has been installed inside the heritage shroud in preparation for the JWST test program. Helium shroud temps can be controlled as low as 15K using the new cryogenic helium refrigeration system. During the OTIS test, the Space Vehicle Thermal Simulator (SVTS) will provide thermal simulators of the JWST sunshield, spacecraft, and harness interfaces (figure 4). The interior “hub” area will be heated to provide a flight-similar and controllable thermal environment in that area, and harness interfaces will be controlled at flight- like temperatures. The TPF test will provide a thorough check out of this system, and effects of hub and harness temperature changes on the Pathfinder test article will be evaluated.

Secondary Mirror Assembly

SVTS with sunshield simulator

Backplane with blankets IEC with and coatings nearby Mini BSF structure

Figure 4. Thermal Pathfinder Configuration

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A simulator for the room-temperature Instrument Electronics Compartment (IEC, figure 4) with a flight-like Conformal Shield thermal isolation system will be included. Nearby structure segments will also be simulated to evaluate the effects of the increased heat flow from IEC caused by gas conduction during the cool-down. To provide an OTIS-like cool-down, the Pathfinder structure will be augmented to provide a thermal system similar to OTIS, as much as practical. Structure thermal insulation will be installed as on OTIS, with flight-like configuration and attachment. One of the key objectives in the flight thermal design is to keep mirrors and structures cold to enhance science, so the back side of the backplane is enclosed in thermal insulation. Many areas are covered with black Kapton single-layer insulation (SLI) to facilitate radiation to space, while other areas have multi-layer insulation (MLI) to reduce heat flow into the backplane volume from warmer areas. Figure 5 shows the configuration of the Thermal Pathfinder backplane insulation.

Single-Layer Insulation

Multi-Layer Insulation

Figure 5 Thermal Pathfinder Backside Coatings

The 18 primary mirrors cover most of the area on the “front” side of the fully deployed OTIS backplane, with 12 of the mirrors on the center section. The Pathfinder has only two flight-similar mirrors, so mirror blanks were designed for TPF to simulate the thermal effects of the other 10 center section mirrors (Figure 6). The mirror blanks will have realistic coatings, but have to be much lighter to reduce the stresses on the non-flight Pathfinder structure. Heaters added to the low-mass mirror blanks will enable them to cool down on an OTIS-like timeline. Backplane structures will also be heated during cooldown, since the Pathfinder structure lacks the mass of some significant OTIS elements.

(10) Mirror Blanks

(2) Flight-Like Mirror Assemblies

Figure 6 Pathfinder Mirror Configuration

Some hardware items considered for Pathfinder upgrades were shown by analysis to be insignificant contributors to cool-down or backplane thermal balance. Mirror electronics boxes were shown to be unnecessary due to the short 7 International Conference on Environmental Systems

duty cycle of mirror operations. The mirror-to-structure conduction coupling through the support flexures is a very small number, and its removal affected backplane gradients by less than 0.1K. Backplane harness effects proved to be somewhat more significant. Since there are only two flight-like mirror assemblies on the Pathfinder, the backplane harness would not be well represented unless a flight-like harness is fabricated and installed. Copper wire harnesses can have considerable effect on cryogenic systems, since they bridge between the “warm” and the “cold” areas, and the copper conductance is much higher in the telescope temperature range than at ambient temperatures. However, analysis showed that removing the harnesses had relatively little effect on flight backplane temperature gradients. Figure 7 shows that the bulk temperature of the backplane increased by an average of 2K, although heat conducted by the harnesses raised small areas of the structure by up to 5K. Further, the science-critical warm end of the backplane increased by only about 1K, which would not justify the fabrication and installation of a flight-like harness on the Pathfinder.

Figure 7. Flight Model Temperature Difference With and Without Backplane Harness

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IV. Instrumentation The Thermal Pathfinder test article is heavily instrumented, as are the thermal simulators, optical test equipment, structures, and chamber shroud. All of the temperature sensors are commercial silicon thermal diodes (Lakeshore DT-670, 4 wire sensors calibrated from room temperature to 20Kelvin), although several different styles and packages are used. The complete suite of sensors can be read every second, using commercial readouts with custom software and interfaced into the JWST Eclipse data handling system developed for the JSC test program. A computer menu driven Thermal Telemetry System developed by Exelis provides the sensor data management. The expected accuracy on the test article sensors is about 0.3K per sensor when all error sources are combined. There are approximately 480 thermal sensors currently planned for the test article, with final sensor placement reviews underway. Given the number, placement, and reliability of these sensors, it was determined that redundant sensors were not necessary, except for those areas requiring heater control. Sensor locations were chosen to enable the Thermal Pathfinder thermal objectives as listed above, with particular emphasis on:  Mirror assembly transient gradients, to confirm that the OTIS cool-down will meet these relatively tight temperature constraints  Composite structure gradients and inferred heat flows, by instrumenting every large beam in the backplane to provide a comprehensive thermal map for this distributed mix of thermal arteries  Confirmation of predicted heat pulse behavior of mirror actuators and drive electronics, and transient effects on nearby structures  Control of mirror blanks and mini-BSF interfaces, to produce OTIS-like boundary temperatures on the flight-like mirror assemblies and backplane  Validation of steady state Thermal Pathfinder modeling performace accuracy  Replication of the sensor locations on flight hardware, to assess locations prior to OTIS  Checking effectiveness of key thermal blankets on backplane and IEC during the gas-assisted transient cool-down, and during an OTIS-like steady state thermal balance

In addition to the temperature sensors, the TPF will utilize 15 GSFC-developed radiometers. These are based on a Winston cone design and are described in reference 4. The radiometers will be used to measure heat flows from within the chamber shroud (including shroud gaps, penetrations, and harnesses), and to examine critical test article MLI closeouts, which could have workmanship-dependent performance uncertainty that is difficult to quantify by other means.

V. Thermal Pathfinder Modeling The Thermal Pathfinder thermal model is a conglomerate of models attained from Smithsonian Astrophysics Observatory (SAO), Goddard Space Flight Center (GSFC), and Exelis, along with models of the hardware items specifically designed for the test. SAO provided the model of the Pathfinder structure with two primary mirror assemblies, Secondary Mirror Assembly (SMA), Secondary Mirror Support Structure (SMSS), and the Mini Backplane Support Frame (BSF) as discussed in reference 2. GSFC provided thermal models of the Primary Mirror Blanks, the thermal coatings enclosing the rest of the Pathfinder structure, and the IEC Engineering Model (EM). Exelis provided models of the SVTS including the hub area, sunshield, and spacecraft simulators, plus other items such as the Deployable Tower Assembly and the cryocooler line isolation system. The Thermal Pathfinder model utilizes Thermal Desktop (version 5.5) and SINDA Fluint. The Thermal Desktop software models the geometry of the Pathfinder with appropriate coatings and calculates radiation couplings. Node data, conductor data, array data, register data, and interior radiation couplings were provided for each component via SINDA include files and not calculated by Thermal Desktop. All models that were provided to the thermal team were modeled in this format, with the exception of the IEC Engineering Model which was originally provided in the TSS/ SINDAG format. Conversion of this model to Thermal Desktop/ SINDA Fluint was performed and checked. As noted in Section II, Thermal Pathfinder Test Objectives, the test model will be used to predict transient profiles, gradients, and heat flows. During the test this 15,000 node model will be used to project trends and predict the effects of adjustments to shroud temperature, helium pressure, SVTS temperatures, etc. This real-time analysis will also be useful to prepare for similar activities planned for the OTIS test. Since the current Thermal Desktop models have their origins from other modeling tools that don’t support submodels, only the MAIN submodel is used. The current model has been created to run trade studies to determine the necessity of including details of OTIS, like harnessing, in the Thermal Pathfinder test. For this purpose, the test chamber was modeled as a one node cylinder; future analysis will add more detail from the correlated JSC chamber model. The Instrument Electronics 9 International Conference on Environmental Systems

Compartment (IEC) Engineering Unit model represents the IEC in the Thermal Pathfinder analytical effort, and will be used in IEC hardware configuration trades.

VI. Conclusion The JWST Thermal Pathfinder test philosophy and test objectives have been defined. This test will be used to practice for the JWST flight telescope optical and thermal test, which is on the critical schedule path towards observatory launch. To evaluate cool-down procedures and backplane heat flows, flight-like thermal coatings will be added to the existing Pathfinder structure, and telescope test timelines will be previewed. Thermal sensors will be located to show hardware safety, gradients, and heat flows. Key thermal simulators and gas-cooling effects will also be checked out. Thermal Pathfinder test thermal models were developed and used to perform analysis used in configuration trade studies, including design of mirror blanks, structure heaters, and thermal sensor locations.

Acknowledgments The authors would like to thank Walt Ancarrow of WCA Engineering for his thermal analyses of the flight backplane conduction sensitivities. Thanks are also due to Sang Park of SAO, Randy Franck of Ball Aerospace, Jesse Huguet of Exelis, and Chris May of Maze Engineering for generously sharing their models and knowledge thereof to assist with the test analysis effort. Finally, thanks are due to Keith Parrish and Shaun Thomson of NASA, Al Sherman of Genesis Engineering Solutions, and John Pohner of Northrop Grumman for their vision that resulted in the development of the Thermal Pathfinder test concept.

References

1Cleveland, P., Parrish, K., “Thermal System Verification and Model Validation for NASA’s Cryogenic Passively Cooled James Webb Space Telescope (JWST)”, International Conference on Environmental Systems, SAE International, Warrendale, PA, 2005 2Park, S., Ousley, W., Cohen, L., Parrish, K., Burt, W., “James Webb Space Telescope Pathfinder First Cryogenic Test Thermal Analysis”, International Conference on Environmental Systems, AIAA, Washington, DC, 2012 3Homan, J., et. al., “Creating the Deep Space Environment for Testing the James Webb Space Telescope at NASA Johnson Space Center’s Chamber A”, International Conference on Environmental Systems, AIAA, Washington, DC, 2013 4DiPirro, M., Hait, T., Tuttle, J., and Canavan, E., “A Low Cost, Low Temperature Radiometer for Thermal Measurements”, Astronomical and Space Optical Systems, Proceedings of SPIE, Vol. 7439, 74391A-1, 2009

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