Shaun Thomson

Environmental Exposure Conditions for Teflon® Fluorinated Ethylene Propylene on the Hubble Space Telescope

The outer layer of Teflon® fluorinated ethylene propylene (FEP) multi-layer insulation (MLI) on the Hubble Space Telescope (HST) was observed to be significantly cracked at the time of the Second HST Servicing Mission (SM2), 6.8 years after HST was launched into low Earth orbit (LEO). Comparatively minor embrittlement and cracking were also observed in the FEP materials retrieved from solar-facing surfaces on the HST at the time of the First Servicing Mission (3.6 years exposure). After SM2, a failure review board was convened to address the problem of degradation of MLI on the HST. In order for this board to determine possible degradation mechanisms, it was necessary to consider all environmental constituents to which the FEP MLI surfaces were exposed. Based on measurements and various models, the environmental exposure conditions for the FEP surfaces on the HST were estimated, including: the number and temperature ranges of thermal cycles; equivalent sun hours; fluence and absorbed radiation dose of x-rays, trapped protons and electrons and plasma electrons and protons; and atomic oxygen (AO) fluence. This paper presents the environmental exposure conditions for FEP on the HST, briefly describing the possible roles of the environmental factors in the observed FEP embrittlement and providing references to the published works which describe in detail testing and analysis related to FEP degradation on the HST.

Incorporation of molecular adsorbers into future Hubble Space Telescope instruments

The Hubble Space Telescope (HST) has been designed to accommodate changeout and/or repair of many of the primary instruments and subsystem components, in an effort to prolong the useful life of this orbiting observatory. In order to achieve the science goals established for this observatory, many HST instruments must operate in regimes that are greatly influenced by the presence of on-orbit propagated contaminants. To insure that the required performance of each instrument is not compromised due to these contaminant effects, great efforts have been made to minimize the level of on-orbit contamination. These efforts include careful material selection, performing extensive pre-flight vacuum bakeouts of parts and assemblies, assuring instrument assembly is carried out in strict cleanroom environments, performing precision cleaning of various parts, and most recently, the incorporation of a relatively new technology -- molecular adsorbers -- into the basic design of future replacement instruments. Molecular adsorbers were included as part of the wide field/planetary camera 2 (WFPC-2) instrument, which was integrated into the HST during the servicing mission 1 (SM1) in 1993. It is generally recognized that these adsorbers aided in the reductio of on-orbit contamination levels for the WFPC-2 instrument. This technology is now being implemented as part of the basic design for several new instruments being readied for the servicing mission 2 (SM2), scheduled for early 1997. An overview of the concept, design, applications, and to-date testing and predicted benefits associated with the molecular adsorbers within these new HST instruments are presented and discussed in this paper.

The use of molecular adsorbers for spacecraft contamination control

In recent years, the technologies associated with contamination control in space environments have grown increasingly more sophisticated, due to the ever expanding need for improving and enhancing optical and thermal control systems for spacecraft. The presence of contaminants in optical and thermal control systems can cause serious degradation of performance and/or impact the lifetime of a spacecraft. It has been a goal of the global contamination community to develop new and more effective means for controlling contamination for spacecraft. This paper describes an innovative method for controlling molecular contaminants in space environments, via the utilization of Molecular Adsorbers. It has been found that the incorporation of appropriate molecular adsorbing materials within spacecraft volumes will decrease the overall contamination level within the cavity, thereby decreasing the potential for contaminants to migrate to more critical areas. In addition, it has been found that the placement of a Molecu...

Thermal and contamination control of the mid-infrared instrument for JWST

The Mid-Infrared Instrument (MIRI) is the coldest and longest wavelength (5-28 micron) science instrument on-board the James Webb Space Telescope observatory and provides imaging, coronography and high and low resolution spectroscopy. The MIRI thermal design is driven by a requirement to cool the detectors to a temperature below 7.1 Kelvin. The MIRI Optics Module (OM) is accommodated within the JWST Integrated Science Instrument Module (ISIM) which is passively cooled to between 32 and 40 K. Thermal isolation between the OM and the ISIM is therefore required, with active cooling of the OM provided by a dedicated cryostat, the MIRI Dewar. Heat transfer to the Dewar must be minimised to achieve the five year mission life with an acceptable system mass. Stringent cleanliness levels are necessary in order to maintain the optical throughput and the performance of thermal control surfaces. The ISIM (and MIRI OM) is launched warm, therefore care must be taken during the on-orbit cooldown phase, when outgassing of water and other contaminants is anticipated from composite structures within the ISIM. Given the strong link between surface temperature and contamination levels, it is essential that the MIRI thermal and contamination control philosophies are developed concurrently.

Stray light performance for the James Webb Space Telescope

The James Webb Space Telescope (JWST) is a large cryogenic telescope observing over a spectral range from 0.6 μm to 29 μm. A large sun shield blocks sunlight and provides thermal isolation for the optics. Analyses characterizing the stray light reaching the instrument focal planes from the galactic sky, zodiacal background, bright objects near the line-of-sight, and earth and moon shine are presented along with the self-generated thermal infrared background from Observatory structures. The latter requires thermal analysis to characterize the Observatory temperatures. Dependencies on the surface properties of BRDF and emittance are discussed for the underlying materials and the effects of contamination

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.

TRMM project contamination control using molecular adsorbers

The Tropical Rainfall Measuring Mission (TRMM) is a spacecraft under development by the National Aeronautics and Space Administration (NASA) and the National Space Development Agency of Japan (NASDA) and is scheduled for launch in August 1997. The spacecraft design includes the use of numerous optical instruments and the thermal control surfaces. In addition to the inherent contamination sensitivities of the optical and thermal systems, TRMM has had the added challenge of designing systems to function at a relatively low altitude (350 km), with solar exposure. Under these conditions, high atomic oxygen densities and potentially high levels of backscattered contamination (self‐contamination), as well as UV photopolymerization effects, all pose major threats to sensitive TRMM elements. In considering the various contamination control paths to follow, the TRMM project management has opted for pursuing a relatively new, but very promising technology for the TRMM spacecraft in order to lower the on‐orbit conta...

Applying Contamination Modeling to Spacecraft Conventional Propulsion System Designs and Operations

Molecular and particulate contaminants generated from the operations of a propulsion system can impinge on spacecraft critical surfaces. Plume depositions or clouds can hinder the spacecraft and instruments from per- forming normal operations. The interconnection between the functions of spacecraft contamination modeling and propulsion system implementation is presented. An innovative contamination engineering approach is addressed during a spacecraft mission, which includes concept design, manufacturing, integration and test, launch, and on-orbit operations. A summary of the implementation on several successful missions is also presented. NE potential source of concern facing the instruments of or- biting spacecraft is the effect of molecular contaminant inter- action with sensitive thermal control and optics surfaces. Typically, the sources of these on-orbit contaminants can be categorized into e ve general areas: 1 ) material outgassing (water, hydrocarbons, sil- icones) from materials of construction; 2 ) spacecraft and multiple- layer insulation venting; 3 ) e uid leakage from pressurized vessels (e.g., cryogen tanks ), dumps, and lubricant loss; 4 )exhaust material generated through thruster e rings; and 5 ) extravehicular activity. 1 Once released, contaminants can propagate to the receiving sur- faces through direct line-of-sight transport (direct e ux), ree ections with spacecraft surfaces, and scattering through self-scattering or with the local ambient atmosphere (return e ux). The efe ciency of these transport mechanisms is a complicated function of spacecraft geometry, mission /e ight operations, and environmental effects. In the past the purpose of computer modeling was concentrated in the assessment of contamination damage during the late design phase, integration and test, and on-orbit operation. The impact of modeling on the mission was limited to minor design changes (such as vent locations ), verie cation (for meeting contamination require- ments), and on-orbit operation (such as operational constraints im- posed to avoid contamination ). Becauseofincreasedsensitivityofspacecraftcomponentstocon- tamination effects, contamination engineering has begun to play a more notable role in overall spacecraft development. Early involve- mentrepresents themosteffectivedirectionoffuture contamination modeling efforts. By ine uencing the early design, cost savings can be very signie cant because many inefe cient contamination avoid- ance remediesestablishedlate in thedesign cycle canbeeliminated. In recent years improved contamination modeling techniques have been used extensively by contamination sensitive projects to improve spacecraft and instrument performance during the early design stage. One good example is the detailed modeling effort for the Tropical Rainfall Measuring Mission (TRMM). Contami- nation modeling efforts for this mission resulted in several design changesespeciallyinthepropulsionsystem.Thepaperdescribesthe

Atomic oxygen erosion of a graphite coating on a TQCM onboard the Return Flux Experiment (REFLEX)

A TQCM coated with graphite was flown aboard a Spartan carrier in January 1996. During a flight of about 46 hours at an altitude of 305 km, the graphite reacted with the atomic oxygen (AO) in the environment and was eroded away. The 15-MHz TQCMs frequency dropped from 6800 to 4000 Hz in about 15 hours of exposure and was shown to be a strong function of the TQCMs orientation to the ram direction. The erosion rates for four different ram angels was measured and found to be both consistent and repeatable. The average graphite volume loss for the 61 degree and -62 degree ram angles was calculated to be about 2 X E-08 cm3/hr and for the 18 degrees and 19 degrees angles to be about 8.5 X E-08 cm3/hr, which is slightly less than previous flight data. The erosion data was then correlated with AO density numbers for the particular times and positions of the spacecraft in orbit. From this analysis, an equation was derived that shoed the carbon volume loss as a function of both atomic oxygen density and ram angle. For example, 1.59 E-07 cm3/hr would be the calculated carbon volume loss for a ram angle of 0- degrees and an AO fluence of 3.52 E+17 atoms/hr. The results of this data and analysis may lead to the development of a sensor capable of monitoring the AO fluence on a spacecraft.

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.