Wes Ousley

Hubble Space Telescope Thermal Blanket Repair: Design and Implementation

Substantial damage to the outer layer of Hubble Space Telescope (HST) thermal blankets was observed during the February 1997 servicing mission. After six years in LEO, many areas of the aluminized Teflon(R) outer blanket layer had significant cracks, and some material was peeled away to expose inner layers to solar flux. After the mission, the failure mechanism was determined, and repair materials and priorities were selected for follow-on missions. This paper focuses on the thermal, mechanical, and EVA design requirements for the blanket repair, the creative solutions developed for these unique problems, hardware development, and testing.

Augmented Method to Improve Thermal Data for the Figure Drift Thermal Distortion Predictions of the JWST OTIS Cryogenic Vacuum Test

The JWST Optical Telescope Element (OTE) assembly is the largest optically stable infrared-optimized telescope currently being manufactured and assembled, and is scheduled for launch in 2018. The JWST OTE, including the 18 segment primary mirror, secondary mirror, and the Aft Optics Subsystem (AOS) are designed to be passively cooled and operate near 45K. These optical elements are supported by a complex composite backplane structure. As a part of the structural distortion model validation efforts, a series of tests are planned during the cryogenic vacuum test of the fully integrated flight hardware at NASA JSC Chamber A. The successful ends to the thermal-distortion phases are heavily dependent on the accurate temperature knowledge of the OTE structural members. However, the current temperature sensor allocations during the cryo-vac test may not have sufficient fidelity to provide accurate knowledge of the temperature distributions within the composite structure. A method based on an inverse distance relationship among the sensors and thermal model nodes was developed to improve the thermal data provided for the nanometer scale WaveFront Error (WFE) predictions. The Linear Distance Weighted Interpolation (LDWI) method was developed to augment the thermal model predictions based on the sparse sensor information. This paper will encompass the development of the LDWI method using the test data from the earlier ‘pathfinder’ cryo-vac tests, and the results of the notional and as tested WFE predictions from the structural finite element model cases to characterize the accuracies of this LDWI method.

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.

The submillimeter-wave astronomy satellite: On-orbit thermal performance and design retrospective

A large telescope aperture, stringent thermal stability and temperature range requirements, and a passivelycooled 150°K module presented major challenges in thermal design and hardware fabrication of this Small Explorer satellite. This paper reviews briefly the thermal design of the SWAS science instrument, and examines the first three months of on-orbit thermal history. Measured temperatures for both the science payload and the spacecraft module and solar arrays are compared with those predicted by the correlated analytical model. Similarities and differences are interpreted in terms of the major uncertainties remaining after thermal-balance testing, especially those of MLI performance and telescope aperture properties. Review of the thermal model adequacy and thermal design verification are included to suggest improvements in the thermal design process for future missions.