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OPM:
Design
These pages
contain information on the various aspects
of the OPM's design, including the mechanical
and electrical design, the primary science
instruments, and the environmental monitors.
Also to be found here is a discussion of
design constraints for the Mir mission.
Mechanical Design:
The OPM mechanicals were designed to make
a lightweight, effective,
and mechanically uncomplicated payload which
required no more resources than absolutely
necessary. The OPM was designed to fit through
all of the hatchways and tunnels in the
Shuttle, SPACEHAB, and Mir, including the
Shuttle docking adapter and the Mir's airlock,
plus all of the expected sizes of hatchways
and tunnels aboard the ISS (according to
the data that was available at the time
of the design work). The OPM was constructed
with a variety of mounting and hard points,
and a versatile mounting base that allowed
it to be mounted to a variety of different
spacecraft exterior accommodations by way
of a simple interface plate. The OPM had
its own shipping rack; i a standard
size double rack which was compatible with
both Shuttle and ISS ISPR mountings. The
OPM's dimensions were:
- 82.87 cm in length (left to right, not
including EVA handrails)
- 68.26 cm in depth (at the longest measurement)
- 52.07 cm in height (to tallest point,
at top of instrument enclosure)
- Mass: 115 kg
Placement and layout of the various OPM
components was chosen for maximum science
return while maintaining mechanical simplicity.
The samples were mounted on a rotating carousel,
which rotates to first expose the samples
and then bring them into the corresponding
instruments for measurement. Half of the
carousel's service contained samples; the
other half was blank. The placement of the
instruments in an enclosure which overhangs
half of the carousel allowed for easy alignment
of the samples for measurement, and the
enclosure created a protected stowage area
for the samples when the OPM was not operating
(the carousel simply rotated so that the
blank half is exposed). The instrument arrangement
also allowed for placement of some parts
of the Vacuum Ultraviolet (VUV) instrument
underneath the samples, which was necessary
due to the nature of the instrument. This
arrangement of the instruments slightly
limited the experiment's exposure view angle,
but placement for a total 180-degree view
by the samples would have required a much
more complex mechanical transport to move
the samples inside the OPM's body for measurements.
The body itself contained the OPM's power
supplies, the carousel drive, and the DACS
electronics.
The carousel was driven by a stepper motor,
which could position the carousel to within
an angular position accuracy of 5.4 arc
minutes (0.084 degrees, or .0015 radians).
A rotation of 180 degrees (½ turn)
took approximately 1 minute, 5 seconds,
at the drive's maximum rate. The carousel
contained a number of sensors (radiometers
and calorimeters) on the rotating portion.
These were electrically connected by way
of a flexible cable; no slip rings or commutators
were used. The carousel did not have a hard
stop; the flexible cable allowed for a rotation
range of slightly less than 1-1/2 turn,
which was enforced by the software. One special
feature of the carousel was a calibration
sample for the Total Integrated Scatter
(TIS) instrument; this was mounted on the
blank half of the carousel and had a trap
door which covered the sample. The door opened
only when the sample was inside the TIS instrument.
The OPM was designed for flight aboard a
manned spacecraft, and the design accomplished
maximum efficiency in a complete package
while posing minimal burdens on the flight
crew, and avoiding hazards to the crew.
The requirements for flight aboard a manned
spacecraft, which OPM met, can be summarized
as:
- Minimize size and mass.
- Be capable of surviving Shuttle launch
and landing shocks and vibrations under
anticipated nominal and off-nominal conditions.
- Physical compatibility with Shuttle,
Mir, and SPACEHAB interfaces and accommodations,
such as hatchways and payload racks.
- Minimize use of spacecraft resources
and crew labor.
- Avoid posing hazards to the spacecraft
and crew.
Most of the OPM's structure was composed of
6061-T6 Aluminum, which was selected for surviving
extended exposure to the space environment.
For thermal control, the OPM made extensive
use of advanced thermal control coatings;
this allowed the OPM to operate outside the
spacecraft, in the space environment, and
maintain reasonable internal and touch temperatures
without requiring a cold plate. The instrument
cover enclosure served as a controlled radiative
surface for dissipating instrument heat during
measurement sequences without reaching an
excessive temperature, and without creating
a heat sink during the OPM's low-power passive
exposure periods. The mounting base and interface
plate mechanism was also designed to minimize
heat transfer between the OPM and the spacecraft
to which it was mounted. Extensive thermal
analysis was performed to verify these
properties.
The OPM contained a number of design features
to facilitate handling during EVA. The two
EVA handrails were gold anodized for easy
recognition by the crew, and met the NASA
and RSA requirements for reach and accessibility
by the gloved hand, as well as touch temperature
constraints. The handrails were hard points
and, when the OPM was mounted, could be used
as tether points by the crew. The sample
cover was anchored with EVA-qualified pip
pins, and its handle could be used as a third
handle for the OPM itself when the cover
was in place (although the sample cover handle
could not be used as a tether point). The OPM
also had a fixture for attachment to the
Mir crane, and this fixture could be changed
to accommodate other transport mechanisms
(such as the Space Station Remote Manipulator
System) as needed. The electrical connectors
were twist-to-lock types which were designed
for use with the gloved hand, and the OPM's
base plate contained attachment points for
tie down of the electrical cable while the
OPM was being translated to and from its
operating location. The base plate also
contained a stowage for the sample cover.
Stowage or restraint was provided for all
electrical connector caps and covers, so
that there were no components dangling when
the OPM was in place.
In flight and in operation, the OPM was designed to present no hazards to the
spacecraft crew. In order to avoid even
the remote possibility that a spilling or
broken sample might pose any hazard to the
flight crew, the OPM employed four layers
of protection:
- The carousel is rotated to the samples-stowed
position at any time when the experiment
is accessible to the crew. In this position,
all of the material samples are concealed
underneath the instrument area, the cover
of which fits very closely to the top
surface of the carousel.
- A plastic cover is taped to the exposed
half of the carousel, which completely
seals the carousel and interior areas
of the OPM. This is removed prior to deployment.
- A sample cover plate is placed over
the carousel area and, together with the
instrument enclosure, completely covers
the sample carousel and all of the sensors
and instruments. This sample cover is
removed as the final step in the deployment
EVA, and replaced as the first step in
the retrieval EVA. (The sample cover also
protects the environmental sensors mounted
on the carousel deck from damage.)
- The entire OPM payload is wrapped in
a heavy beta cloth protective cover when
stowed inside the spacecraft.
One other safety consideration is the use
of the lasers in the Total Integrated Scatter
(TIS) instrument. To avoid presenting a vision
hazard, the TIS was designed so that with the
instrument enclosure cover in place, all laser
light is "trapped" and there is
no possibility of any scatter or reflection
of the beam reaching a crew member's eyes.
Electrical Design:
The OPM's electricals consisted
mainly of a power supply, active thermal
control system, and the DACS computer. The
power supply converted the 28V DC power from
the spacecraft to the various voltages needed
by the OPM electronics.
The active thermal control system consisted
of two 50-watt case heaters, under the control
of the DACS. Each heater had its own temperature
sensor and operated independently.
The DACS is the brains of the OPM. Adapted
from a commercial off-the-shelf spacecraft
processor system from Southwest Research,
it consisted of two printed-circuit boards:
a processor board and an I/O interface board.
The processor board contained a space-hardened
version of the commercial 80186 processor
which has also been the basis for many personal
computers. The processor board was equipped
with 512 kilobytes of random access memory
(RAM), and 6 megabytes EEPROM to store the
OPM control software and for archival memory
for the data collected by the OPM. All program
and data information was stored in this memory;
the OPM did not contain any mass storage
device such as a disk drive.
OPM Software:
In operation, the DACS controled the OPM
instruments, collected and stored data, and
communicated with the Mir MIPS laptop via
its RS-422 interfaces. The software initiated
the measurement sequences, controled the
operation of each instrument and the carousel,
and stored the data collected in the EEPROM.
Data from the environmental monitoring instruments
was collected continuously between measurement
sequences, and stored in the EEPROM along
with the measurement data.
Pre-loaded on the MIPS laptop was software
which enabled it to communicate with the
OPM through the laptop's RS-232 port. This
software used a modified version of the
common X-Modem protocol to retrieve data
from the OPM and send commands to it. The
normal procedure, executed once a week by
an astronaut, transfered all of the data
collected since the last data transfer.
The data was transferred directly from the
EEPROM to the laptop. The OPM software kept
track of which data had already been collected,
and only transfered new data. (previously
transferred data could be re-retrieved via
a special procedure if need be.) As a backup
data source, the OPM retained collected data
in the EEPROM as long as possible; its capacity
(as designed) was sufficient to retain all
of the data for a mission of up to 20 months.
Special procedures could be executed on the
laptop to alter OPM operational parameters.
Many aspects of the OPM's operation could
be changed, including the scheduling of
the measurements and operational parameters
of the individual instruments. The OPM software
also contained an end-of-mission deactivation
procedure that stowed the samples and prepares
the payload for retrieval.
Instruments Design:
Space environment effects on the OPM samples
were measured using three instruments: a
Vacuum Ultraviolet spectrometer (VUV), a
spectro-reflectometer, and a total integrated
scatterometer. Each row of samples on the
carousel corresponded to one of these instruments.
The outer row was the VUV experiment, the
next row was the solar absorption and emittance
study, and the third row was the scatter
experiment. The innermost row of samples
on the carousel consisted of passive samples
which were not measured in flight.
The Vacuum Ultraviolet spectrometer (VUV)
measured the ability of materials samples
to reflect and transmit light in the near-
to middle-ultraviolet portion of the electromagnetic
spectrum (approximately 1216-2500 Å
wavelength range). A special arc-discharge
lamp, known as a deuterium lamp (similar
in principle to a mercury vapor streetlight)
produced the ultraviolet light, and a set
of filters selected the specific wavelength
to be measured. The transmittance and reflectance
data from the VUV helped to fill a general
void of optical properties data for materials
used in the UV range (MgF, LiF, CaF, etc.).
The data is useful to experiment designers
which must expose these materials to the
space environment.
The spectro-reflectometer is a big word
for an instrument that measures the reflectance
of materials samples at wavelengths ranging
from the middle-infrared, through the near
infrared and visible light wavelengths,
to the near ultraviolet. The range of wavelengths
measured are in the range of 250-2500 nm.
(Visible light spans approximately 400 to
800 nm.) To cover this large range, the
reflectometer used two light sources: a
tungsten halogen lamp (similar to the halogen
lamps used in some residential light fixtures)
and a deuterium lamp are combined; the deuterium
lamp served the ultraviolet and visible
blue and violet portion while the tungsten
lamp covered the remainder of the visible
light and all of the infrared portion. The
reflectometer measured the total hemispheric
reflectance, i.e., all of the radiation
reflected in any direction by the sample
being measured; it did this using an integrating
sphere which captured all of the reflected
energy. The wavelength to be measured was
selected using a mechanism known as a prism
monochromater which splits the light source
using a prism, and then employs a moving
mirror in front of a slit to screen out
all but the desired wavelength. Many surfaces
can be tested for degradation by the spectro-reflectometer,
including heat-reflecting paints and coatings
intended for use both on the ISS and for
commercial use.
The Total Integrated Scatter (TIS) instrument
measured how much light is scattered (in
other words, reflected away at an angle
other than the angle of direct reflection).
It performed this measurement at two specific
wavelengths, 532 nm (visible green) and
1064 nm (near infrared), using a pair of
lasers as the light sources. The scattered
light was collected for measurement by way
of a Coblentz sphere, which is a hemispherical
mirror. The TIS scatter measurement is an
indication of how rough the surface of a
material is, which in turn indicates to
what extent the material's surface has been
damaged by the space environment. Smooth
mirrored surfaces, such as those used in
solar collectors and radiators, and heat-resistant
coatings are measured by this instrument.
Also measured and of particular interest
are plastics which are intended to be used
as overcoatings and electrical insulators.
One such material is Kapton, widely used
in spacecraft but known to be damaged by
atomic oxygen (AO) in low Earth orbit;
some of the TIS samples consisted of AO-resistant
materials which have been developed as substitutes
for Kapton for possible use on the ISS.
Design Constraints:
The OPM was originally designed to fly on
the European Space Agency's EURECA satellite.
The EURECA was an unmanned, free-flying
payload that was launched in the Space Shuttle,
released to fly free for a one-year mission,
and then returned to Earth by another Shuttle
to be refurbished and launched again. However,
the EURECA program was cancelled in 1994
after one flight, and it became necessary
to find another spacecraft on which to fly
the OPM.
At this time, an opportunity arose to fly
as part of NASA's International Space Station
Phase I program. This was a joint American-Russian
program to, among other things, fly U.S.
payloads aboard the Russian Mir space station
in order to gain experience for subsequent
operations aboard ISS. Among its advantages
were very long duration, a stable platform,
and crew presence to fix problems if necessary.
However, some changes were necessary to
the OPM design in order to fly aboard Mir,
and some additional steps had to be taken
to qualify to fly the OPM on Mir.
The most significant impact on the design
of the OPM were the Mir hatch and passageway
constraints. As originally planned for the
EURECA mission, the OPM would be pre-integrated
with the EURECA satellite prior to the mission,
and since the EURECA would be loaded directly
into the Shuttle's payload bay, it would
not ever be necessary to pass the OPM through
any spacecraft interior. After the switch
it became apparent that the OPM would have
to be deployed through either the Mir or
the Shuttle airlocks, which meant the payload
had to pass through tunnels and hatchways
in the interior of the respective spacecraft.
Originally, the main (lower) portion of
the OPM's case had been designed as a simple
rectangle. However, in order to pass through
a circular hatchway, the lower front panel
was beveled inwards; this reduced the diameter
required for passage. Another change was
in the method of mounting the payload to
the spacecraft; in the original plan, the
OPM would have been integrated with the
EURECA before flight, so there was no reason
to provide any in-flight attach and detach
capability. However, on Mir, installation
would be done by astronauts/cosmonauts during
an extravehicular activity (EVA), and they
had to be able to mount and dismount the
OPM while in EVA gloves and suits and lacking
any leverage. In addition, the mounting
means itself was different; on EURECA the
OPM would have been attached to a satellite
truss, while the Mir used a ball-and-socket
mechanism. An interface plate was fabricated
which adapted the OPM to a device called
variously the "Mir interface adaptor"
or the "solar array latch" (the
mounting site had originally been used to
transport a solar array), supplied by RSC
Energia. The interface adaptor had a three-position
handle which the EVA crew members used to
lock and unlock the interface adaptor from
the mounting base on the Mir docking module,
and this constituted the complete mechanical
attachment between the OPM and Mir.
It was also necessary to provide for handling
of the OPM by EVA astronauts. Towards this
end two large handles were added, one at
each end of the case; these handrails were
sized and qualified to be handled by EVA
astronauts, allowing for factors such as
the limited grip ability of the gloves (it
is difficult to close one's hand in a pressure
suit because the pressure tends to hold
the glove fingers straight) and the clearance
required between the handle and the side
of the case in order for the gloved fingers
to fit in the space. The electrical cable
connector was changed to one capable of
being connected and disconnected in the
EVA operation. In addition, a ball fixture
was added to the right side for capture
by the Mir cargo crane, so that the OPM
could be moved that way rather than by hand,
saving significant amounts of time during
the EVA.
Finally, changes were necessary to allow
for the differences in EURECA's and Mir's
data interfaces and downlink capability.
EURECA provided a near-continuous, real-time
data downlink via the TDRSS satellite system
and was operated mainly from the ground
except during the deploy and retrieval.
However, Mir at the time of the OPM mission
did not have a real-time downlink capability
available for payload use; rather, downlink
was performed using a file transfer process
executed by a crew member. The OPM's data
I/O software modules were changed to conform
to this interface. In addition, a small
software module was needed on the computer
(a laptop) to which the data was transferred,
and this was written; in the process, the
OPM engineers took the opportunity to provide
additional user interface capabilities on
the laptop so that the crew could send commands
to the OPM and execute off-nominal procedures
if need be. The electrical/data cable was
changed to a design using two parts. Part
of the cable was outside of the spacecraft
and part was inside; the two halves met
at a bulkhead connector which was already
built in to the side of the docking module.
The interior cable connected to a power
outlet to provide power to the OPM, and
provided primary and backup data ports for
the laptop. In addition, a pair of signal
lights were added to the cable to give the
crew a visual indication of the OPM's general
operating state without having to connect
the laptop. Data retrieval and transfer
to the ground were executed once per week.
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