A venting equation commonly used to describe the transient evolution of pressure within a volume containing an outgassing or offgassing source and a restrictive vent conductance under conditions of molecular flow has been solved analytically. Solutions are found for sources of finite thickness characterized by classical diffusion-limited behavior (proportional to inverse square root of time), as well as responses for thick material sources often observed in testing that are characterized by a more general form of power-law decay, up to inverse time behavior associated with surface desorption. Solutions involve evaluating integrals where both numerators and denominators of the integrands diverge with time, making wide-ranging transient solutions difficult to directly compute numerically. Usually, one can avoid evaluating these integrals by assuming quasistatic conditions at long time scales. A novel approach is used in this work to analytically produce solutions by generating bespoke mathematical functions, some of which solve integrals that have apparently had no previous analytical solution.
A general, lumped-parameter, control volume equation is developed to describe the rapid loss of pressure associated with a launch vehicle fairing jettison event in its initial phase. Beginning with a general mass conservation statement, a sonic constraint is applied to the expanding gap between receding fairing halves to produce a statement for transient density. This is related to fairing pressure by assuming the expansion may be described by a polytropic process. A generic, singlestep, lateral fairing half separation case is created and explored for illustrative purposes.
Maintaining molecular cleanliness during the JWST’s Optical Telescope/Instrument Module (OTIS) Cryogenic Thermal Vacuum (TV) test campaign was critical to the success of its optical mission on orbit. In the thermal vacuum tests leading up to the final cryogenic test to validate the OTIS flight hardware, NASA Johnson Space Center’s (JSC’s) TV Chamber A was fully characterized for molecular contamination. It was found to contain common volatile condensable materials (VCM), including hydrocarbons, plasticizers, and silicones, all of which absorb in JWST’s infrared wavelength region. Due to the risks involved, cleaning molecular contamination from the OTIS mirrors was not an option and heating the Primary Mirror (PM) segments would have also been a risky and expensive endeavor. As a result, a monitoring process was developed and implemented during four different Pathfinder or risk reduction tests that were scheduled to occur prior to the flight hardware test. The goal was to quantify and assess the risk of molecular contamination depositing on the PM resulting from relatively warm chamber shrouds “leading” colder PM mirrors during warmup, by a margin of 10-50 Kelvin (K). This was accomplished using Cryogenic Quartz Crystal Microbalances (CQCMs), held at temperatures slightly cooler than the segments to signal the onset of contamination events. Per the JWST Contamination Control Plan (CCP), the total Primary Mirror molecular allocation requirement was 50 angstroms. In all tests, the results showed an average accumulated molecular contamination of <10 angstroms.
This paper summarizes a recent numerical analysis of water vapor and volatile condensible material deposition on the James Webb Space Telescope from the initial orbit insertion up to 180 days post launch. The analysis utilized 17 distinct geometry files capturing observatory configuration changes during the deployment. Surface temperature was set from a time-dependent thermal analysis solution. A vapor pressure model was used to calculate the net water ice adsorption. Molecular contamination included a contribution from UV photopolymerization. The analysis predicted levels of ice and molecular accumulation were found to be within the allowable limits specified by the observatory contamination control plan.
The James Webb Space Telescope (JWST) has a primary mirror, made of 18 segments, and a secondary mirror (SM) that are used to direct the light of desired targets. After launch, the secondary mirror assembly (SMA) is stowed for approximately 10 days and is subject to molecular contamination outgassing from the cavity of the secondary mirror support structure (SMSS) in-board hinge (IBH) which contains cables, motors, resolvers, and coatings. The main concern during this period before SMA deployment is the accumulation of ice due to the lack of a heater on the SMA. The temperature differentials between the IBH surfaces and SMA could cause redistribution of water vapor contamination. To address this concern, single layer insulation (SLI) was reconfigured to direct the vent path of IBH outgassing sources away from the SM. Two separate thermal vacuum (TVAC) tests were performed to quantify this contamination: a Z307 ASTM E 1559 materials test of the radiator paint used on the motor of the IBH and a separate test on the hinge motor from the primary mirror backplane assembly (PMBA) qualification engineering test unit (ETU). The PMBA ETU hinge was similar in design to the IBH. These tests approximately followed the predicted SMA predeployment thermal environment. To quantify source rates in case of a leak in the new SLI enclosure or baffle, the motor and resolver sides were separated, and quartz crystal microbalances (QCM) were used to measure the deposition of water. The SLI redesign and implementation and outgassing measurements to understand leak effects from the IBH were essential to mitigate the deposition of contamination on the SMA.
KEYWORDS: Contamination control, Particles, James Webb Space Telescope, Optical telescopes, Space telescopes, Aerospace engineering, Atmospheric particles, Fermium, Frequency modulation, Nitrogen
At the conclusion of cryogenic vacuum testing of the James Webb Space Telescope Optical Telescope Element Integrated Science Instrument Module (JWST-OTIS) in NASA Johnson Space Center’s (JSCs) thermal vacuum (TV) Chamber A, contamination control (CC) engineers are postulating that chamber particulate material stirred up by the repressurization process may be kept from falling into the Integrated Science Instrument Module (ISIM) interior to some degree by activating instrument purge flows over some initial period before opening the chamber valves. This manuscript describes development of a series of models designed to describe this process. The models are strung together in tandem with a fictitious set of conditions to estimate overpressure evolution from which net outflow velocity behavior may be obtained. Creeping flow assumptions are then used to determine the maximum particle size that may be kept suspended above the ISIM aperture, keeping smaller particles from settling within the instrument module.
KEYWORDS: James Webb Space Telescope, Space operations, Contamination, Contamination control, Inspection, Observatories, Manufacturing, Foam, Acoustics, Picture Archiving and Communication System
This paper will continue from Part 1 of JWST contamination control implementation. In addition to optics, instruments, and thermal vacuum testing, JWST also requires contamination control for a spacecraft that must be vented carefully in order to maintain solar array and thermal radiator thermal properties; a tennis court-sized sunshield made with 1-2 mil Kapton™ layers that must be manufactured and maintained clean; an observatory that must be integrated, stowed and transported to South America; and a rocket that typically launches commercial payloads without contamination sensitivity. An overview of plans developed to implement contamination control for the JWST spacecraft, sunshield, observatory and launch vehicle will be presented.
The derivation of contamination control (CC) requirements for the JWST Optical Telescope Element (OTE) was presented at the SPIE conference in 20081. Since then, much work has been done to allocate contamination at each phase of Integration and Test (IandT) and to plan for achieving the allocations. Because JWST is such a large and complicated observatory, plans for meeting the requirements are many and varied. There are primary mirror segments that must be cleaned early and maintained clean; there are four science instruments that each have tight contamination requirements but cannot be cleaned after they are integrated onto the Integrated Science Instrument Module (ISIM) structure; there is the composite ISIM structure that is fragile and must be minimally handled; there are numerous cryo-vacuum tests that must be controlled and monitored in order to minimize molecular contamination during return to ambient; … and more. An overview of plans developed to implement contamination control for JWST optics, instruments, and thermal vacuum testing for JWST will be presented.
KEYWORDS: Space operations, Contamination control, Space telescopes, James Webb Space Telescope, Ruthenium, Gases, Standards development, Telescopes, Satellites, Contamination
As a spacecraft undergoes ascent in a launch vehicle, its ambient pressure environment transitions from one
atmosphere to high vacuum in a matter of a few minutes. Venting of internal cavities is necessary to prevent the
buildup of pressure differentials across cavity walls. These pressure differentials are often restricted to low levels to
prevent violation of container integrity.
Such vents usually consist of fixed orifices, ducts, or combinations of both. Duct conductance behavior is
fundamentally different from that for orifices in pressure driven flows governing the launch vehicle ascent
depressurization environment. Duct conductance is governed by the average pressure across its length, while orifice
conductance is dictated by a pressure ratio. Hence, one cannot define a valid “equivalent orifice” for a given duct
across a range of pressure levels.
The purpose of this paper is to develop expressions for these two types of vent elements in the limit of small
pressure differentials, explore conditions for their validity, and to compare features regarding ascent
depressurization performance.
KEYWORDS: Data modeling, Temperature metrology, Solids, Crystals, James Webb Space Telescope, Molecules, Sensors, Instrument modeling, Data conversion, Contamination control
In the harsh vacuum environment of deep space, surfaces shielded from the Sun may easily develop temperatures
low enough to condense water vapor for extended periods of time. The condensed vapor will subsequently desorb at
rates consistent with its temperature-sensitive equilibrium vapor pressure, and under certain circumstances it is
important to predict this release rate. A review of available scientific literature to confirm model predictions
indicated no such measurements had been reported below 131 K. Contamination control personnel at NASA
Goddard Space Flight Center recognized the possibility they readily possessed the means to collect such
measurements at lower temperatures with an existing apparatus commonly used for making outgassing observations.
This paper will describe how the ASTM E-1559 “MOLEKIT” apparatus was used without modification to measure
water vapor sublimation down to 120 K and compare this data to existing equilibrium vapor pressure models. In
addition, an in-depth analysis of theoretical formulations for vapor pressure gives insight into the physical basis
underlying characteristics associated with high-fidelity models.
KEYWORDS: Crystals, Cryogenics, Quartz, Contamination, James Webb Space Telescope, Systems modeling, Annealing, Measurement devices, Temperature metrology, Liquids
Sublimation rates and energies are critical for modeling the transport of water ice in cryogenic systems.
A quick test was devised using an ASTM E-1559 [1] device that measures deposition with quartz crystal
microbalances (QCMs). Credible results were obtained at temperatures as low as 120K and compared
well with published data above 130K. Deposition and the following sublimation were performed with
the QCMs held at constant temperature to alleviate variability due to ice morphology.
As a spacecraft undergoes ascent in a launch vehicle, its pressure environment transitions from one atmosphere to high
vacuum in a matter of minutes. Venting of internal cavities is necessary to prevent the buildup of pressure differentials
across cavity walls. Opposing the need to vent these volumes freely into space are thermal, optical, and electrostatic
requirements for limiting or prohibiting the intrusion of unwanted energy into the same cavities. Bus vent design
evolution is discussed for the Solar Dynamics Observatory. Design changes were influenced by a number of factors and
concerns, such as contamination control, electrostatic discharge, changes in bus material, and driving fairing ascent
pressure for a launch vehicle that was just entering service as this satellite project had gotten underway.
The proposed paper will describe development of a time-dependent, one-dimensional counterflow diffusion model meant to help study the issue of helium exposure of the IRU (Inertial Reference Unit) used in the Earth Observing-1 (EO-1) spacecraft. The IRU features sensitive quartz crystal hemispherical resonating gyros (HRG's). Although the IRU enclosure was purged with high-purity liquid boil-off nitrogen, the HRG's were still quite susceptible to contamination generated by exposure of the enclosure to levels of helium above atmospheric background levels. This helium would be preferentially collected by the HRG's, changing their mass, and hence the driving voltage required for operations. The paper will discuss a comparison of theoretical results with test data for an IRU enclosure, and how contaminant gases can enter vent holes despite the presence of a purge. These observations are then used to describe a possible improvement for purge effectiveness.
The Wide Field Camera 3 (WFC3), a scientific instrument being prepared for the next Hubble Space Telescope (HST) servicing mission, has two detectors providing wavelength sensitivity from 0.2 to 1.7 microns. In a departure from previous HST detector enclosure designs that required extensive bakeouts, the WFC3 enclosures are vented to space. Several contamination analyses were performed to assist in the design and validation of the vent tube, detector and enclosure bakeout requirements, and instrument operational constraints. The benefits derived from the vented enclosure configuration are discussed, and the analysis techniques and implementation are presented.
Contaminants degrade the thermal properties of spacecraft systems. This paper describes a method of quantified visual inspection, which assumes a minimum visible particle size that can be easily evaluated for any observer. It also assumes a log-normal particle size distribution, but not the slope of it. By counting the particles visible in a convenient unit area, the fractional area coverage can be calculated. The method can be applied to real systems during ground inspection. This paper discusses methods for assessing the radiant cooler sunshields of the Imager and Sounder instruments on the Geostationary Operational Environmental Satellite (GOES) spacecraft. Then the method for converting the percent area coverage to the amount of solar power, which is scattered, to the cooler patch is explained. The essence of this conversion is based on careful consideration of the paths of specularly reflected and scattered solar light within the sunshield enclosure. A three-dimensional Bidirectional Reflectance Distribution Function (BRDF) model of the radiant cooler sunshield was constructed in order to estimate the illumination of a contaminated spot on a sunshield wall based on relative sun orientation, direct illumination, and specularly-reflected illumination from other walls. Radiant energy reflected from the contaminated spot to the radiant cooler was estimated using a BRDF model based on data reported by Young in previous studies.
This paper describes a series of analyses undertaken to estimate the operational neutral direct flux contamination environment within the transition Region and Coronal Explorer telescope. These analyses included details concerning vent effectiveness. TQCM measurement response during instrument and observatory-level thermal-vacuum testing, and indirect location of unidentified sources. In particular, this paper describes analytical and computational model development and the level with which model results corroborated with a and supplemented data collected during instrument and observatory-level thermal vacuum testing. One issue successfully dealt with a recognized problem associated with TRASYS calculations of internal geometries through viewfactor normalization. Confidence in model result affected telescope design, with removal of a previously unrecognized source from the primary mirror base, and redesign and replacement of the telescope ascent vents.
The fraction of neutral molecules transmitted through a restrictive passage in molecular flow can be highly dependent on the shape of the reservoir from which the gas originates. A particular reservoir shape was investigated to determine its effect on the angular distribution of molecules entering two types of simple restrictive passages. It was determined that a reservoir consisting largely of two-dimensional shells imparted a bimodal distribution to the reservoir gas, resulting in lemniscate flux distributions for the gas entering a baffle network. A Monte Carlo code was developed and compared to analytical transmission probabilities calculated by Clausing. After establishing the validity of the code, the effects of lemniscate and Lambertian flux distributions for molecular flow were simulated for two-dimensional channels and right- angle bends for various length-to-height ratios. It was determined that the shape of the entrance distribution can play an important role in the calculation of transmission probabilities.
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