Keith Parrish

Passive thermal control of the NGST

Preliminary studies of passively cooling the NGST utilizing a lightweight deployable subshield are described. The NGST mission concept of a passively-cooled large-aperture optical telescope is unique from any other mission flown to date. We show that achieving operational temperatures of less than 50 K appears feasible by passive cooling alone through a combination of (i) operating the observatory far from the Earth so that the Sun becomes the only significant source of environmental heating, (ii) selecting an observatory configuration that isolates all significant heat dissipation from the cold telescope, and (iii) employing a high performance sunshield to attenuate the incident solar radiation. The observatory configuration consists of the sunshield with cold telescope and instrument elements on the anti-sun side, and warm spacecraft avionics and propulsion elements on the sun-side of the sunshield. A sunshield thermal configuration trade study, preliminary telescope thermal analyses, and a mechanical concept for a lightweight deployable sunshield are presented. Also discussed are the remaining issues to be addressed.

Science promise and conceptual mission design for SAFIR: the single aperture far-infrared observatory

We report on completion of the SAFIR Vision Mission study, as organized by the NASA Science Mission Directorate. This study resulted in a focused baseline design for this large aperture space observatory that capitalizes on architectures being actively developed for JWST and other missions. Special opportunities for achieving thermal performance of this <10 K telescope are reviewed, as well as efforts to understand capabilities and needs for focal plane instrument and I and T on this large (10 m-class) telescope.

Thermal system verification and model validation for NASA's cryogenic passively cooled james webb space telescope (JWST)

A thorough and unique thermal verification and model validation plan has been developed for NASA s James Webb Space Telescope. The JWST observatory consists of a large deployed aperture optical telescope passively cooled to below 50 Kelvin along with a suite of several instruments passively and actively cooled to below 37 Kelvin and 7 Kelvin, respectively. Passive cooling to these extremely low temperatures is made feasible by the use of a large deployed high efficiency sunshield and an orbit location at the L2 Lagrange point. Another enabling feature is the scale or size of the observatory that allows for large radiator sizes that are compatible with the expected power dissipation of the instruments and large format Mercury Cadmium Telluride (HgCdTe) detector arrays. This passive cooling concept is simple, reliable, and mission enabling when compared to the alternatives of mechanical coolers and stored cryogens. However, these same large scale observatory features, which make passive cooling viable, also prevent the typical flight configuration fully-deployed thermal balance test that is the keystone to most space missions thermal verification plan. JWST is simply too large in its deployed configuration to be properly thermal balance tested in the facilities that currently exist. This reality, when combined with a mission thermal concept with little to no flight heritage, has necessitated the need for a unique and alternative approach to thermal system verification and model validation. This paper describes the thermal verification and model validation plan that has been developed for JWST. The plan relies on judicious use of cryogenic and thermal design margin, a completely independent thermal modeling cross check utilizing different analysis teams and software packages, and finally, a comprehensive set of thermal tests that occur at different levels of JWST assembly. After a brief description of the JWST mission and thermal architecture, a detailed description of the three aspects of the thermal verification and model validation plan is presented.

The Role of Integrated Modeling in the Design and Verification of the James Webb Space Telescope

The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2011. System-level verification of critical optical performance requirements will rely on integrated modeling to a considerable degree. In turn, requirements for accuracy of the models are significant. The size of the lightweight observatory structure, coupled with the need to test at cryogenic temperatures, effectively precludes validation of the models and verification of optical performance with a single test in 1-g. Rather, a complex series of steps are planned by which the components of the end-to-end models are validated at various levels of subassembly, and the ultimate verification of optical performance is by analysis using the assembled models. This paper describes the critical optical performance requirements driving the integrated modeling activity, shows how the error budget is used to allocate and track contributions to total performance, and presents examples of integrated modeling methods and results that support the preliminary observatory design. Finally, the concepts for model validation and the role of integrated modeling in the ultimate verification of observatory are described.

Thermal-Structural Analysis of Sunshield Membranes

Future large infrared space telescopes, such as the James Webb Space Telescope (JWST), will require deployable sunshields to provide passive cooling for optics and instruments. Deployable sunshield structures for such applications typically consist of multiple thin-film membrane layers supported by deployable booms. The mechanical design of the sunshield must accommodate thermal strains due to layer-to-layer temperature differences as well as potentially large in-plane temperature gradients within individual film layers. This paper describes a thermal-structural analysis for predicting the stress state in a thin-film membrane subject to both mechanical thermal loads that could aid in the mechanical design of future sunshield structures. First the temperature field predicted by a thermal analysis is mapped to a structural finite element model, and then the structural response is predicted using a nonlinear static analysis. The structural model uses membrane elements in conjunction with a tension field material model to predict the response of the thin-film membrane layer. The tension field material model accounts for no-compression behavior associated with wrinkling and slackness. This approach was used to study the problem of a single membrane layer from the NASA reference concept for the JWST sunshield. Results from the analysis show that the membrane can experience a loss of tensile preload due to the presence of an in-plane temperature gradient representative of the cold-side layer temperature distribution predicted for the reference concept JWST.

Systems engineering on the James Webb Space Telescope

The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2014. System-level verification of critical performance requirements will rely on integrated observatory models that predict the wavefront error accurately enough to verify that allocated top-level wavefront error of 150 nm root-mean-squared (rms) through to the wave-front sensor focal plane is met. This paper describes the systems engineering approach used on the JWST through the detailed design phase.

On-Orbit Thermal Performance and Model Correlation of the Fast Auroral Snapshot Explorer

The Fast Auroral SnapshoT explorer (FAST) spacecraft, the second of NASAs Small Explorer (SMEX) series of scientific satellites, was launched on August 21, 1996 by a Pegasus XL launch vehicle. Due to slightly higher than expected temperatures during early orbit operations, an extensive thermal model correlation effort was undertaken to understand and characterize FASTs thermal performance in order to properly orient the spacecrafts attitude during its mission. FASTs thermal design and the on-orbit thermal model correlation and resolution are described. Finally, the correlated models predictions are compared with nine months of flight data.

The use of high thermal conductivity composites in the satellite bus structure of the wide field infrared explorer

The Wide-field Infrared Explorer (WIRE) spacecraft, the fifth of NASA’s Small Explorer (SMEX) series of scientific satellites, is scheduled to be launched in mid 1998 by a Pegasus XL launch vehicle. The WIRE mission will conduct a wide field infrared survey in search for galaxies with an unusually high star formation rate. Once detected, these interesting galaxies can be studied more closely with ground based observatories or with on-orbit telescopes. To maximize the mass allocated to the spacecraft’s single infrared telescope, the spacecraft will utilize a lightweight, fully bonded fiber reinforced composite structure. The use of composite materials presents unique challenges to the design of the spacecraft’s relatively simple passive thermal control system and necessitates the use of Amoco’s high thermal conductivity K1100X fibers in specific areas of the structure. This paper describes the spacecraft’s passive thermal control system and focuses on the engineering design, analyses and tests performed to...

James Webb Space Telescope Pathfinder First Cryogenic Test Thermal Analysis

The JWST observatory, scheduled for launch in 2018, has a large optical telescope passively cooled to below 50K. Due to the size of its large sunshield in relation to existing test facilities, JWST cannot be optically or thermally tested as an observatory system at flight temperatures. As a result, the telescope portion along with its instrument complement will be tested as a single unit very late in the program, and on the program schedule critical path. To mitigate schedule risks, a set of cryogenic tests with non-flight and flight-spare hardware will be performed earlier. These tests will demonstrate the optical testing capabilities of the facility, characterize telescope thermal performance, and allow project personnel to learn valuable testing lessons to reduce program risks. This paper describes a risk reduction thermal analysis of the “Pathfinder” cryogenic test program, focusing on the first test in the series. The process of developing the independent thermal analysis model for the JWST ‘pathfinder’ program is described, as well as analysis model guidelines developed specifically for the JWST program. The thermal math model consists of the “Pathfinder” telescope structure, two flight-spare primary mirror assemblies, a flight-spare secondary mirror, and a 20K cryogenic chamber shroud simulation. The rigorous model validation process is discussed, as is the thermal analysis performed to verify that test requirements could be met. Results of this analysis include thermal stability for multiple optical measurements, cooldown time to cryogenic temperatures enhanced by gaseous-helium free-molecular heat transfer, and sensitivity studies of shroud emissivity, shroud temperature profile, and properties of the cooling gas.

An update on the role of systems modeling in the design and verification of the James Webb Space Telescope

The James Web Space Telescope (JWST) is a large, infrared-optimized space telescope scheduled for launch in 2014. The imaging performance of the telescope will be diffraction limited at 2μm, defined as having a Strehl ratio >0.8. System-level verification of critical performance requirements will rely on integrated observatory models that predict the wavefront error accurately enough to verify that allocated top-level wavefront error of 150 nm root-mean-squared (rms) through to the wave-front sensor focal plane is met. Furthermore, responses in several key disciplines are strongly crosscoupled. The size of the lightweight observatory structure, coupled with the need to test at cryogenic temperatures, effectively precludes validation of the models and verification of optical performance with a single test in 1-g. Rather, a complex series of incremental tests and measurements are used to anchor components of the end-to-end models at various levels of subassembly, with the ultimate verification of optical performance is by analysis using the assembled models. The assembled models themselves are complex and require the insight of technical experts to assess their ability to meet their objectives. This paper describes the modeling approach used on the JWST through the detailed design phase.