2. System Description

2.2 Mechanical/ Thermal

2.2.1 Material Selection
2.2.2 Design Requirements
2.2.3 Instrument-Shuttle Structural Interfaces
2.2.4 Description of WUPPE Structures
2.2.4.1 Detector
2.2.4.2 ZOD
2.2.4.3 Spectrometer
2.2.4.4 Apertures and Filters
2.2.4.5 Primary Bulkhead (and WUPPE Coordinates)
2.2.4.6 Hardpoint and Handling Ring
2.2.4.7 Articulating Drive Ring
2.2.4.8 Aperture Door
2.2.4.9 Sunshade
2.2.5 Thermal Control
2.2.6 Thermo-Electric Cooler

2.2.1 Material Selection

The first requirement in the selection of any and all materials for WUPPE is that they are acceptable by NASA. NASA's general criteria for acceptance are that the material be non-flammable, non-corrosive, non-toxic, non-reactive with adjacent material and have a TML (Total Mass Loss) of 0.1% or less. In addition, the CVCM (Collected Volatile Condensable Materials) must be 0.10% or less.

Selection of materials involves many factors in varying degrees. Materials have to meet design criteria of weight, strength, stiffness, thermal expansion, thermal conductivity, and corrosion with other materials. Some materials in contact with other materials create electrolytic action which causes hydrogen embrittlement and material cracking. Once the materials meet the above criteria there are other considerations, such as cost, availability of required shapes and sizes, ease of fabrication, method of finishing (paint, anodize, etc.), and then finally there is the question of who has the equipment and experience to fabricate a quality product from these selected materials. Take for instance the case of deciding between using stainless steel and aluminum. Stainless steel is stronger, stiffer, heavier, more costly, more difficult to machine than aluminum. Therefore stainless steel was used sparingly where its strength was needed and its mass could be tolerated. Aluminum's limitations of strength and stiffness were handled by designing larger cross sections for stiffness and strength where required. If WUPPE's primary material were stainless steel instead of aluminum, it would have a mass of over 4000 pounds, instead of 800 lbs., it would have taken about twelve times longer to fabricate it, and it would have been 20 times more costly. Also, 85% of the fabrication could not have been handled by University shops.

WUPPE's materials by percentage of mass are approximately the following:

85% Aluminum alloy 6061T651
7% Aluminum alloy 2219T8
3% Invar (a thermally stable nickel alloy)
1% Noryl (a glass-filled plastic electrical/ thermal insulator)
2% Titanium 6Al4V
1% Hexcell (a composite aluminum material).

All materials and hardware required certificates of compliance to either MIL (Military) specs, NASA specs, NAS (National Aeronautical Standard) specs, SAL specs, or some recognized specifying agency. All fasteners were NDT (Non-Destructive Test) tested by dye penetrant and/or X-rayed for cracks. All structure-critical and safety-critical parts were X-rayed. All structural elements underwent fracture analysis. The WUPPE structure was Finite-Element Modelled (FEM) for stiffness, since NASA imposed a stiffness requirement of more than 24 Hz but less than 35 Hz. Structures of > 35 Hz tend to be too heavy with superfluous mass.

2.2.2 Design Requirements

NASA requirements are based on available space and orbiter lift capacity. Astro was assigned a maximum mass and size which in turn was allotted to each experimenter in accordance with their science proposals.

The requirements are negotiable between NASA and the experimenters and were reviewed as designs progressed. Each experiment team negotiates with NASA as to who supplies what, the Instrument Interface Agreement (IIA) is the result of the negotiation. WUPPE was originally allotted a space of 1x1x4 meters at about 1000 lbs., and NASA was to provide a thermal canister. About halfway into the development phase the experimenters were made responsible for their own thermal environment. WUPPE was already committed to an aluminum structure therefore causing some design adjustments.

In developing a flight instrument there is normally a development/ design phase, an engineering-model phase, and a flight-model phase. For WUPPE our engineering model is our flight model, due to cost and manpower constraints.

In order to help NASA design the cruciform, several areas required early development, such as location of electrical interface, instrument mounting interface, and aperture door layout. At SAL early development of the spectrometer and articulating drives were required.

The following is a list of instrument design requirements and needs:

Operation in vacuum, thermal dissipation, electrical continuity through entire instrument (to dissipate static charge), structural stiffness to 24 Hz, light weight (less than about 35 Hz), strength through optical-path structure to accept a 1x10-5 Torr vacuum (for vacuum calibration with the collimator), size of 91 cm x 91 x m x 3.8 m (revised size after reviews, actual size is 81.3 cm x 83.8 cm x 3.8 m), mass of 900 lbs. (actual mass is 820 lbs.), worst-case G-load of 15 G in z direction minimum of 1/16 inch radius for all exposed edges, no single-point failures (all structure was designed with a minimum of three point failures). All mechanisms are sealed at one atmosphere (surfaces sliding and rubbing in the absence of oxygen weld together, even dissimilar materials).

2.2.3 Instrument-Shuttle Structural Interfaces

This is a general description of how WUPPE fits into the shuttle. WUPPE, HUT, and UIT are mounted to a common structure called cruciform. The cruciform is an aluminum structure with a horizontal blade of about 80" x 80", an intersecting vertical blade of 40" x 80" called keel blade, and a back blade which stabilizes the horizontal to vertical blade. When viewed from the front, the cruciform looks like the University-of-Texas symbol (letter T superimposed on the letter U).

The cruciform is attached to the IPS (Instrument Pointing System) through the back blade. The IPS computer is housed in the igloo.

The IPS is mounted to one of two cradles assigned to Astro. A cradle is a U-shaped structure, about 3 m long and 4 m wide. Each cradle is locked into place in the shuttle payload bay with launch locks in 6 places: two at the bottom of the U, and two at each side of the top edge.

During the mission, there is no provision for any space walks, i.e. in-flight maintenance and repair of the instruments.

2.2.4 Description of WUPPE Structures

Closest to the cruciform-back blade are two rectangular aluminum radiators of 20" x 36" covered with a silver-colored thermal control tape. The radiators run along the telescope tube on two sides, they are the -y and -z radiators. The next geometric shape is an aluminum tube of 22" in diameter and 56" long, the telescope tube, which is covered with MLI (Multi-Layer Insulation). The MLI consists of alternate layers of kapton and scrim (scrim is a netting material) and topped with white Beta cloth. All exposed parts of WUPPE are covered with MLI. The next geometrical shape is a rectangular box, the sunshade, which is 32" x 56" on a side and is covered with silver thermal-control tape. The sunshade is made of Hexcel. This material has a honeycomb core of about 1/2" thickness sandwiched between two aluminum sheets of 0.010" thickness. The inside of the sunshade box is baffled by an exposed honeycomb that is painted a matt black. Between the tube and the sunshade is a transitory piece of MLI in the shape of a trapezoid.

The radiators support 90% of the electronics and dissipate the heat generated by that electronics. The -z radiator houses the power switcher, TEC, latch driver, heat pipe, and two HVPS mounted in thermal contact. The -y radiator accommodates the DEP and the backup DEP.

Underneath the MLI on the tube is the wiring harness supported in 3 cable trays which run the length of the tube. Directly adjacent to the cable trays are the 3 Invar metering tubes. The metering tubes have virtually no thermal expansion/ contraction. They are solidly attached to the articulating drive ring at one end and loosely mounted to the primary bulkhead at the other end through teflon brushings. At the primary bulkhead each Invar tube is mounted to a LVDT (Linear Variable Differential Transformer) which can measure displacements to 1 Ám. Also installed on the telescope tube are a large number of strip heaters to maintain uniform thermal environment.

The WUPPE instrument or "flight hardware" is attached to the cruciform at three points by a kinematic mount. The kinematic mount constrains the instrument to a precise location relative to the cruciform while allowing some thermal movement. This is accomplished by allowing zero degrees of freedom on one bracket, 1 degree of freedom on the second bracket, and 2 on the third bracket. All brackets are in the same plane parallel to the horizontal blade of the cruciform (see also Figures in the IIA).

The following describes the materials and finishes of further WUPPE structural elements. There are basically two types of finishes chosen for electrical or optical properties. Chromate conversion coating is a finish that is electrically conductive and is a protection against corrosion; it can be greenish or brown in color but is generally light yellow to gold, and is known commercially as Iridite or Alodine. Anodizing is an electrically induced oxidation process which creates a coating that can be dyed to most colors, but is not electrically conductive. Therefore all anodized parts have to be externally grounded to have static and electrical continuity. The SPD box is made mostly of aluminum and Noryl, the aluminum is chromate conversion coated, the Noryl has no finish. The spectrometer box is made of aluminum and anodized in matt black. The ZOD box is mostly made of aluminum, plus Noryl, and some Iridited aluminum and raw Noryl. The spectrometer interface housing is made of matt, black, anodized aluminum, as are the filter and aperture wheel. The primary bulkhead is also fabricated in aluminum, with a matt, black anodized finish. On the same plane as the bulkhead reside the xyz T-bracket and the z T-bracket. The brackets are made of titanium and have no finish. The mirror centering tube is made of aluminum and is anodized in matt black. The primary mirror is made of Schott Zerodur glass and coated with aluminum and MgF2. The mirror is contained in an aluminum spider that has a matt, black, anodized finish. Also attached to the bulkhead is the telescope tube. The hardpoint ring and Cruciform I/F are both fabricated in aluminum with a matt, black, anodized finish, as are the articulating drive ring and the drive housings. Internally 17" in -x direction from the drive ring resides the secondary mirror, which is made of the same materials as the primary mirror. In +x direction is the IMC made of black anodized aluminium. The cantilevered spider vanes hold the secondary mirror in place. They are made of Invar and painted with Z306. On the front end of the telescope tube sits the aperture door jamb, which is made of aluminum and has a black, matt anodized finish. Mounted to the door jamb are the aperture door, door mechanisms, door latch, and 12 sunshade support arms, all made of aluminum coated with black, matt anodize. On the inside of the aperture door sits the calibration test lamp. The structure continues with the previously described sunshade.

The entire optical path from the primary mirror to the end of the sunshade is baffled. Inside of the telescope tube are annular rings about 2" apart. The primary-mirror centering-tube extends about 15" beyond the primary mirror and has baffling rings. The baffle on the secondary mirror is 6" long. The baffles are made of aluminum and have a dead black anodize finish called Martin Optical Black. Further baffling is provided by open-cell Hexcel on the secondary spider vanes, the inside aperture door surface, and the inside of the sunshade.

2.2.4.1 Detector

The electrical and optical functions of the detector have been described elsewhere. The items in the Reticon detector box are: Image intensifier, Reticon, TEC, Heat-pipe junction-plate, printed circuit boards, manifold, insulation, and a nitrogen purge. The design had to obey several constraints. The Image tube and Reticon interface had to be electrically isolated. The Reticon and TEC had to float to allow for thermal expansion and yet maintain the optical contact between the Reticon fiberoptic bundle and the image intensifier. Electrical isolation was accomplished by using Noryl. The optical contact between the Reticon and the Image tube was done by using contour-fitting flat springs. This allowed for a side-ways float. The float between the Reticon and the TEC was accomplished by soldering silver chevrons (an accordion-like spring) to the cold side of the TEC. The cold from the TEC is delivered to the back of the Reticon through the chevrons to a silver block in thermal contact to the back of the Reticon. The hot side of the TEC is soldered to a copper plate, which is in turn thermally contacted to the heat-pipe junction-plate and the manifold. The heat of the TEC travels the length of the heat pipe to the -z radiator. For ground operations, when the heat pipe does not operate, the manifold is chilled by a Vortex tube which uses compressed air to create cold air. There is a continuous nitrogen purge for ground operations only that keeps the inside of the detector box dry.

2.2.4.2 ZOD

The ZOD is very similar to the detector. A few less restrictions applied. The parts that constitute the ZOD are: the pick-off mirror-snout, image intensifier, CCD, and printed circuit boards. The assemblies had to provide a vacuum seal with the spectrometer, and be electrically isolated. As the box is not actively cooled, no floating, purging, and insulation were necessary.

2.2.4.3 Spectrometer

The spectrometer looks like a smashed box of eight sides, or, with some imagination, it looks like a horse's head without ears. Along the optical path the parts and most openings are as follows: In the horse's head analogy the forward wall or where head would connect to neck is an opening large enough to pass on-axis and off-axis light. It is also used to evacuate the spectrometer. At a short distance toward the nose is a titanium cell which holds the Wollaston prism. From here the light passes to the "nose" opening where the camera mirror is installed. The reflected light passes adjacent to the "nose bone" to the "eyes" area, again at the forward wall, where the grating is mounted. The grating reflects the light to the detector in the "mouth" area and the zero order is picked off and bent into the "jaw" area for the ZOD. The spectrometer was fabricated from a solid aluminum block, 5" thick, 9" wide, and 15" long. The precise angles and bores for the camera mirror, the detector, and the ZOD were machined into the block before it was hollowed out.

The grating is a separate assembly, and its position and angle could be adjusted at assembly. That is was also true of the Wollaston prism; its position was arrived at empirically. The side walls of the spectrometer are vacuum sealed to the spectrometer, and are contoured to provide lateral support to the spectrometer case. One side of the case has provisions for the instrument positive nitrogen purge. The forward wall seals to the Filter-Aperture Plate which in turn is sealed to the Interface Plate.

2.2.4.4 Apertures and Filters

The Filter-Aperture Plate is actually the mounting platform for the Filter-Aperture wheel spindles. It resembles a shallow pie dish. The side that seals to the spectrometer has three holes roughly in a straight line across the face. The center hole is the optical path hole, the other two are for the Filter-Aperture mechanisms. The open side of the pie dish is sealed to the Interface plate. The Interface plate resembles a large, deep pie dish with no bottom. The filter and aperture wheels and their encoders protrude into the space provided by the interface plate.

The Filter Wheel and encoder assembly consists of 32 parts: one spindle, 2 bearings, wheel hub, wheel, retainer ring, clamping ring, 16 filter holders, 4 magnets, encoder cup, LED support, photodiode support, and two circuit boards. The filter holders are made of titanium to match the filters' thermal coefficients. Each filter is held in place with flight approved RTV (Room Temperature Vulcanizing silicone adhesive), and can be adjusted individually. The encoder cup has holes drilled in in binary code, to be sensed by the LED/Photodiode for position. The position detenting is done with magnets. In moving from position to position the stepper-motor logic knows how many steps are required to bring the wheel close to the desired position. Then the motor stops and the wheel is free wheeling and can be picked up by the closest magnet. The detenting system has four stationary magnets 22.5° apart and four equally spaced dynamic magnets 90° apart on the wheel. The wheel magnet is picked up sequentially, 1, 2, 3, and 4, of the stationary magnets. Thus as the first wheel magnet leaves the #4 magnet, the second wheel magnet can start its sequence, etc..

The Aperture Wheel and encoder assembly is essentially the same as the Filter-Wheel assembly. Significant differences are the accuracy of the detenting (▒ 0.001"), number of aperture positions (30 plus a corner cube and a Rochon), and the aperture disk.

In detenting there are still 16 positions, but on the aperture disk there are two aperture holes on the same radial line, one on axis and one off axis. The aperture disk was made by Dow Corning from a ceramic disk (metal would deflect the optical path) in which the required holes were etched very much like making a circuit board.

The Filter and Aperture wheels overlap at the telescope optical axis.

2.2.4.5 Primary Bulkhead (and WUPPE Coordinates)

Next piece on our tour is the Primary Bulkhead. It is a large aluminum block of sizes 28" x 28" x 4". The geometric center of the Primary Bulkhead is also the geometric center of the WUPPE coordinate system, which was selected to parallel the Shuttle's coordinates. The Shuttle's x axis is nose to tail, y axis is wing to wing, and z axis is top to bottom. WUPPE's coordinates are -x from the center of the bulkhead to the cruciform back blade,+x from center bulkhead to sunshade, +y from center bulkhead to cruciform vertical blade, -y is bulkhead to -y radiator, +z is bulkhead to horizontal blade on cruciform, and -z from bulkhead to -z radiator. The Primary Bulkhead supports and is the index point for most critical parts: the spectrometer assembly on its -x face, the primary mirror and the telescope tube on its +x face, XYZ T-brackets on its +y edge +z position, and Z T-bracket and -y radiator support on its -y edge +z position, -y and -z radiator support on its -y and -z edges, optical cube (a cube with reflecting surfaces used for instrument alignment) on its -z edge -y position, -z radiator support and electric on its -z and +y edges, three mirror preload assemblies (see below) equally spaced on about a 6" radius, and the mirror centering tube and spider. The Primary Bulkhead is webbed and contoured beyond description, its finished weight is 48 lbs., down from a starting weight of 320 lbs. before hollowing out.

The Primary Mirror mount to the bulkhead is accomplished by a centering tube which is mounted to the bulkhead. The mirror is placed over the centering tube. A 3-legged spider is mounted on the +x end of the centering tube. The spider has 3 Lexan pads which duplicate the mirror's curvature and at the same position as the preload assemblies. The assemblies are preloaded by springs, they force the mirror forward in +x to contact the Lexan pads. There is a total of 750 lbs. preload on the 50 lbs. primary mirror, this preload is enough to overcome launch and landing loads. The mirror also has an anti-rotation dowel. Protruding beyond the spider is a baffle tube which has baffles on the inside and outside.

The telescope tubes are stacked and bolted, and sealed to the Primary Bulkhead with 21 high-strength bolts and are dowel pinned in three places for rotational integrity at each joint. The two tubes are each 28" long and 0.20" thick, except for the internal flanges (1.75") and the internal stiffeners (0.40"), which are thicker to gain stiffness. The tubes were made by rolling up aluminum plates and then welding the seam. Each welding pass was X-rayed for homogeneity before the next pass. The plates were 2" thick and weighed 450 lbs. before machining and 48 lbs. after finishing.

2.2.4.6 Hardpoint and Handling Ring

The next part along the optical axis is the Hardpoint and Handling Ring. This is a heavy, sectioned ring to which a structurally strong bracket is mounted to which in turn the YZ T-bracket is attached. In order that the alignment of WUPPE to the cruciform and therefore to coalignment to the other instrument be possible, the T-brackets had to be perfectly aligned to each other and the optical axis. The T-bracket surfaces had to be on the same plane. The line established by a center point on the xyz bracket and the center point on the yz bracket must be exactly parallel with the optical axis and be perpendicular with the line from the xyz bracket to the z bracket. These center points and the surfaces were established with a theodolite and correctively machined accordingly. The handling and support points are almost exactly opposite from the T-bracket mounting-points but on the bottom (-z) side of the instrument.

2.2.4.7 Articulating Drive Ring

The Articulating Drive Ring is sealed and bolted to the Hardpoint Ring. Externally this ring supports various power supplies, the articulating drives for the secondary mirror, and the hard mount for the Invar tubes. Internally it supports the secondary-mirror vanes, the IMC, and the mirror itself. The secondary mirror is housed in a cell. The back side of the cell is the wobble plate for the IMC. At the center of this plate and the stationary IMC plate is a single ball bearing. The actuation of the IMC is controlled by two PZTs which push a spring-loaded plunger in the pitch and yaw directions. The PZTs are capable of movement in the 0.1 Ám range. The IMC stationary plate is attached to the secondary spider, which looks similar to a tripod. The mirror spider is not the traditionally looking spider, because the mirror is cantilevered by 17". The vanes which make up the tripod spider are made of Invar and have open-cell Hexcel for baffling of the spider itself. The base side of the tripod is connected through a bellows-seal arrangement to the articulating drives in three places. One end of the bellows is attached to each of the tripod vanes, and the other end is a spring-loaded ball and socket which is on a translator stage within the drive housing. The translator rides on two curved rods which describe a radius of the cosine of the distance from the drive. The translator captures a cam between two follower bearings. The action of the translator and cam are analogous to a bicycle clamp brake and a twisted wheel, where the translator is the brake and the cam is the twisted wheel. The brake follows the action of the twisted wheel as the translator does the cam. The action of the translator is parallel to the x axis or optical axis and is capable of a translation of just over one half inch. The cam is driven by an in-line reduction device called a harmonic drive, in turn driven by a stepper motor. Each step of the motor results in a 10 Ám motion at the tripod-ball joint. By selectively actuating the drive we can aim or focus the secondary mirror.

As you have no doubt noticed there is a fair amount of mechanism in each drive and all drives require lubrication. The lubrication is a synthetic lubricant by Brayco that meets the NASA requirements. All of the ball bearings were ordered and lubricated as specials. The ball bearings' phenolic-ball separators were vacuum impregnated with Brayco oil and the inner and outer ball- bearing races were grease plated. This process insures that there is adequate but not excessive lubrication. The action of any mechanism is bound to create contaminants and therefore it is now apparent why the mechanisms were sealed separately from the rest of the instrument.

2.2.4.8 Aperture Door

The next in line is the Aperture Door Jamb which is host to the twelve sunshade support arms, latch mechanism, two witness mirrors, the Aperture Door, and its two mechanisms.

Figure 2.2.4.8-1 :WUPPE Aperture Door.

Figure 2.2.4.8-2a : WUPPE Aperture Door Mechanism.

Figure 2.2.4.8-2b : WUPPE Aperture Door Mechanism.

A photograph of the WUPPE Aperture Door is shown in Figure 2.2.4.8-1. Some details of the door mechanism are displayed in Figures 2.2.4.8-2 a and b.

Two sunshade support arms double as door axle supports, a third support arm doubles as the latch-mechanism housing. The aperture door provides a contamination enclosure for the optics on the ground, during launch/reentry, and during on-orbit water dumps or RCS (Reaction Control System) firings. It is locked in the closed position by redundant latches and driven open or closed by redundant, independently- controlled mechanisms (Fig 2.2.4.8-2b). The telescope tube is vented on launch and reentry through one-way valves and filters in the door, with a flow rate guaranteeing less than 0.1 atm differential pressure. Since the telescope door has to open in orbit or else no observations can be obtained, its design and operation are critical.

The latch mechanism consists of a simple solenoid-actuated rocker which bears on a roller mounted to the outboard end of the door. The solenoids are only used to move the cam for releasing the door. Each solenoid piston is attached to a stainless steel belt which is wrapped around the cam shaft so that the action of either of the solenoids has the same rotation. The redundancy in this mechanism is the second solenoid. The latch is sprung shut when power is off, thus holding the closed door against its seal against differential pressure and launch/ landing loads. When power is applied to either solenoid, the rocker is forced back and held back as long as power remains on. The latch solenoids are powered directly from the heater-power bus. One of the solenoids is to be activated as the first step before power is applied to the heaters since the solenoid draws a large amount of power (about 200 Watts) while moving, and then settles to a low power (4-Watt holding-state). The "latch unlocked" state is indicated by a microswitch which is read by DI and by the DEP. The door should not be driven open with the latch locked, as this will cause motor stall and possible mechanism jamming. The latch is then to be released as the last step of final deactivation. If the latch should fail in the unlocked position, the door will withstand launch/reentry loads in the unconstrained position (closed is best), although the mechanism will likely be jammed.

Each drive mechanism is a stepper motor which drives a spur-gear cluster and worm/worm-wheel gear mounted at the pivot of the door. The mechanisms are sealed in one atmosphere of dry air. The primary mechanism is attached to the door jamb structure and drives a torque tube at the door pivot; the backup mechanism is attached to the door and drives the opposite end of the torque tube. The stepper motors are operated at 1 rev/ second; the gear reduction on the primary mechanism is 150 : 1 and on the backup mechanism is 250 : 1. This results in a quarter turn of the door in 30 and 50 seconds, respectively. The door operation depends on the fact that the

Figure 2.2.4.8-3 : Door Latch Mechanism (conceptual drawing).

worm-gear mechanisms cannot be driven backwards, so that the non-operative mechanism effectively transmits the torque generated by the operative motor to the other side of the hinge. Either motor can drive the door in any direction from any position; both mechanisms could be operated simultaneously (not a normal operation ): driven in the same direction the door would move faster with added torque; driven in opposite directions the door would move at the difference rate between the primary and backup mechanisms. A positive mechanical stop prevents the door from opening beyond a safe position in case of door-position sensor or software failure. Microswitchs indicating the open and closed positions automatically shut off motor pulses in the respective directions. The primary mechanism is operated by the DEP (see section 3.2 ) and the backup mechanism is operated by a hardwired controller (section 3.1).

The 3 Bright Object Sensors are mounted on the sunshade support arms adjacent to the door jamb (see Fig. 2.2.4.8-2a).

The Aperture Door is essentially a frame to which a piece of Hexcel is fastened, the inside of which is open cell baffle. When the door is open for observing, the open cell presents itself as a baffle to prevent stray reflections. Installed on the inside (-x) of the door is an optical bench with a test lamp and its optics. A design requirement in this area was to keep the fragile lamp and optics contained in such a way as to prevent a safety hazard for the telescope. Also installed on the door is a series of umbrella valved. Half of the valves release air into the instrument, and half release air from the instrument. Obviously for ascending and descending the air pressure within the instrument needs to be balanced with the surrounding air. In conjunction with these valves is a filter which restricts particles from passing into the instrument.

2.2.4.9 Sunshade

The last element of the instrument is the sunshade. The sunshade is a large, hollow box, measuring 32" x 33" x 56". The length of 56" was selected so that all of the UV instruments have the same total length and will not reflect stray light into an adjacent instrument. The sunshade is a six-sided box with openings at the -x end where it is attached to the support arms and at the +x end for observing. The sunshade is made of Hexcel. A cross section of the sunshade would be thermal control tape, external skin, core sandwiched with the internal skin and another core.

The core is made up of perforated foil sheets of aluminum. A foil sheet was placed flat, then parallel lines of adhesive were put down on the sheet. The separation of the lines are determined by the cell size. Then a second foil sheet was put down on the first and more adhesive lines were put on the second sheet, but these lines start halfway between the previous lines. The foil sheets and adhesive lines were laid down alternately until a huge plate 3 ft. thick and 8 x 8 ft long was achieved. This huge plate was put under pressure and heat until cured, and then cut across the grain to obtain the core. The core block was then grabbed by its 3 ft. edges and pulled apart until it showed the hexcel or honeycomb shape. Adhesive applied to the front and back edges of the core binds sheets of aluminum to complete the sandwich. Perforation allows for outgassing of the cells. The obtained structure is light weight, yet extremely strong.

Some other bits of information not previously covered are the following: NASA has rigid requirements for welding of aluminum. Thus all parts with the exception of the telescope tubes were made from solid blocks. All fasteners inside and outside of the instrument are safety-wired or epoxied to the instrument. Each piece of MLI has a grounding strap which grounds them piece by piece and finally to the instrument.

2.2.5 Thermal Control

Thermal control is provided by passive radiators, insulation, and stand-alone thermostatted heaters (see Fig 2.3-5 ). The heaters are powered straight from the unregulated 28 V heater bus; the heaters are on independently of the electronics power. There is a primary and redundant bank of heaters selected by DO on the WAC display (section 3.1). Each bank of heaters has four independently thermostatted heater "islands", the telescope/optics structure, the DEP-electronics radiator, the electronic-interface radiator, and the TEC (Thermo-Electric Cooler ) radiator for the detector. The power consumed by each of these islands when its thermostat is on is shown in Table 2.3-1.Typical temperature histories for the temperature readouts on the WAC page will be available after Astro-1.

When WUPPE was first developed, NASA was to provide a thermal canister, i.e. a thermally insulating container around the cruciform. After the mission was moved from GSFC to MSFC, the canister was dropped and each instrument made responsible for their own thermal environments. MSFC provided a cruciform-model study, which provides some flight predicitions for the shuttle environment. A detailed model of the cruciform itself is not available, however, its thermal behavior will be studied on orbit using the TGSS (Thermal Gradient Sensor System) display pages.

Thermal predictions for WUPPE are based on the Flight Predictions and Thermal Math Model Documentation for WUPPE. Model predictions for 5 baseline flight cases are included in this document. The orientation of WUPPE relative to the cargo bay and space determined the boundary conditions. The two exmtreme thermal environments are "cold", with the payload bay not illuminated by the sun and UIT towards the bay, and "hot" with the sun at 45 degrees to the payload-bay normal and either WUPPE to bay (worst hot condition for radiators, i.e. DEPs) or UIT to basy (worst hot condition for tube). The cold case is expected to be experienced during the majority of the operational mission; the hot case will be seen for at most one hour at a time. All the above cases are modeled for a depoyed IPS. The thermal environment is unknow during the non-operational parts of the mission and cold during the stowed checkout phase.

The spectrometer and telescope structure is maintained at 15-25° C by the heaters, and has a very long thermal time constant, about 20 hours. This island will therefore always behave like the cold case, with heaters on for 5 hours and off for 3-5 hours. The DEP and electronic I/F radiators are maintained above -35 degrees, and heaters go off above -15 degrees; these islands have time constants of about 3 hours and the heaters are on typically 5 hours and off 2 hours. The electronics islands will therefore not heat up into the hot case, where heaters are off entirely and temperatures rise at 5 °/hr, unless more than an hour is spent in the hot case. The detector TEC hot side is coupled by heat pipe to its own radiator which is thermostatted to 15-25 degrees. This narrow range is required to accommodate the DEP-controlled TEC stabilization algorithm. The time constant for the TEC island is about one hour, and the duty cyle of the heaters is about 80 % in the cold case and 10 % in the hot case. For ground testing, the TEC hot side is cooled directly by a chilled-air TEC-purge.

2.2.6 Thermo-Electric Cooler

The spectrometer Reticon detector is cooled to approximately T(RET) = -50° C by a 4-stage thermoelectric cooler. The cold side of the cooler is attached through a flexible silver strap to the back of the Reticon chip, where the temperature is monitored by two thermistors. Control of the TEC is entirely through the DEP, which reads the Reticon thermistors and controls the voltage V(TEC) to the TEC module using a programmable power supply. The hot side of the TEC is attached to a copper heat sink and a heat pipe which runs to a radiator on the -Z side of the instrument. The temperature of the TEC radiator T(RAD) is controlled to 15-25° by standalone thermostats in order to keep the hot side of the TEC within range of the DEP control algorithm. The Radiator temperature and TEC voltage are reported on the WCO display. Possible states of the TEC voltage and Reticon temperature are as follows:

1) DEP control algorithm not on: V(TEC) = 0 V, and T(RET) = T(SPEC).

2) DEP algorithm on, cool-down phase. V(TEC) = 10 V, and T(RET) (t) given by

ÆT(t) = T(░) + (ÆT(0) - ÆT(░)) exp (-t / Æt (on))

where

ÆT = T(RAD) - T(RET)

ÆT(0) = approx. 75° C

Æt (on) = approx. 780 seconds (13 min.).

As a result, T(RET) will initially drop at 0.1° / sec, and will reach within 10o of its asymptotic value in 27 minutes.

3) DEP algothrithm on, fine control. V(TEC) controlled by DEP between 7 and 11 V, based on ÆT(ERROR) = T(RET) - T(SETPOINT), an input parameter. This phase is entered automatically when ÆT(ERROR) becomes less than 5°. The V(TEC) is then cycled around V(SETPOINT) (initially 9 V) based on the sign of ÆT(ERROR):

IF ÆT(ERROR)>0.1 or <-0.1° C

THEN V(TEC) = V(SETPOINT) ▒ 1 Volt

ELSE V(TEC) = V(SETPOINT)

ENDIF

When not at setpoint, ÆT(ERROR) will converge to 0 at about 0.006°/ sec; also, V(SETPOINT) is adjusted at 0.003 V/sec in the appropriate direction in order to follow temperature drifts in T(RAD).

4) DEP algorithm off, warm-up: V(TEC) = 0 V, and T(RET) (t) given by

ÆT / ÆT(0) = exp ( -t / Æt (off) )

where

ÆT = T(SPEC) - T(RET),

Æt (off) = approx 100 seconds.