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Phase 3D thermal vacuum test unofficial status report



The Phase 3D spacecraft arrived at Orbital Sciences Corporation's 
facility in Germantown, Maryland on October 13. After functional
checkout and installation of thermocouples in OSC's clean room,
the satellite was moved to the 11 by 16 foot vacuum chamber on
Wednesday afternoon. The satellite was lifted by a chain hoist
and a triangular lifting fixture which lifts the satellite on
three of its six corners. A six sided aluminum framework was
installed around the spacecraft to hold 30 quartz halogen heat
lamps. (A photo of this arrangement can be seen at:
http://www.magicnet.net/~phase3d/dailyphotos/dp981022.html).

Fifteen teflon coaxial cables were cleaned with alcohol and 
installed inside the chamber to connect the spacecraft's RF
modules to a bulkhead feedthrough plate leading to the outside
of the chamber. Additional cables were connected to supply
DC power and telemetry and command connections between the
satellite and the ground support equipment located outside 
of the chamber. A quick trip to a local camera store procured 
two haze filters which were placed over the SCOPE camera lenses
to protect them from any volatile crud that might be deposited
on them during the vacuum test. Another trip was made to Radio 
Shack to obtain a piezoelectric buzzer element which was 
connected to yet another coax cable feedthrough, this provides 
a nice little microphone which allows vibrations on the 
spacecraft to be recorded on an oscilloscope outside of the 
chamber. Most of the transmitters were terminated with dummy 
loads outside of the test chamber, but the 2 meter and 70 cm 
transmitters are feeding antennas on the roof of the facility. 
The beacons from these transmitters should be audible to local
hams who live within VHF propagation distance of Germantown.
Finally the spacecraft was carefully leveled to insure proper
operation of the heat pipes and was secured in place with 
steel aircraft safety wire to prevent it from moving during 
the test.

After another quick functional test was run to verify that the 
spacecraft was properly connected to its ground support cables 
and a brief delay while OSC technicians replaced a leaky
SMA coax feedthrough on the bulkhead plate, the chamber door
was closed at 11:45 pm on Thursday night and pumpdown began
1/2 hour later. At 2 am the chamber pressure was at 20 millitorr
(0.020 mmHg) and the twin diffusion pumps were turned on. By
4:30 am the chamber pressure was at 3 x 10^-5 torr and Phase 3D
experienced for the first time the vacuum environment in which 
it will spend its working lifetime. At 7:45 am the 70 cm 
transmitter was powered up in vacuum for the first time and
P3D was on the air. Later today liquid nitrogen will be flowed
through a network of pipes on the inner wall of the vacuum
chamber to cool the chamber walls to an estimated temperature 
of 100 degrees Kelvin (-173 deg C). This will freeze out any 
remaining air molecules in the chamber and simulate the cold 
blackness of space.

In school you no doubt learned about the three methods of heat 
transfer, Conduction, Convection, and Radiation. Conduction 
occurs when heat is transferred between two bodies in physical 
contact. Soldering is an example of conductive heat transfer. 
Convection occurs when a fluid flows around a heated object 
and carries heat away from that object. The fan that blows air 
through your computer or ham transceiver is an example of forced 
convective cooling. Without a fan, the natural tendency of warm 
air to rise (at least in a one-G environment) causes a natural 
heat flow through the air vents of your equipment and results 
in natural convective cooling. The third method of heat transfer 
involves electromagnetic radiation, usually in the infrared or 
visible part of the electromagnetic spectrum. The heat that we 
feel in front of a roaring fireplace and the warmth of the sun 
are two examples of radiative heat transfer.

A satellite floating weightless in space is not in physical 
contact with anything, so there will be no conductive heat 
transfer outside of the satellite. Heat generating components 
on Phase 3D such as RF power transistors are carefully mounted
on aluminum plates so that their heat can be conducted to
the heat pipes. The heat pipes carry waste heat by convection
of ammonia inside the pipe to the cold side of the spacecraft,
where it is radiated into deep space. Since there is no air in 
space, convective heat transfer is also impossible (outside of
the ammonia heat pipes). A circuit board or module that depends 
on natural convection of air to stay at a safe operating
temperature may not be cooled properly once the air is removed.
The purpose of a thermal vacuum test is to expose the spacecraft
and its electronics to a vacuum environment while still on the
ground, to verify that all of the modules and electronic 
components will maintain a safe operating temperature in the 
absence of natural air convection. This will allow any problems 
that might be uncovered to be fixed before launch. In this test,
Phase 3D will be allowed to lose heat by radiation to the cold
walls of the test chamber until it reaches a minimum temperature
of -20 degrees C. At that point the quartz halogen lamps will be
turned on to radiate heat onto P3D's exterior panels until it
heats up to a maximum of 40 degrees C. It is hoped to complete
at least 5 heating and cooling cycles before we vacate the test
chamber at the end of the month.

Orbital Sciences Corporation has committed significant resources 
to support this thermal vacuum test, including 24 hour support 
by two technicians to keep the vacuum chamber in good working
order. They deserve a big "Thank You" from all supporters of the
Phase 3D project.

Dan Schultz N8FGV

---------------------------------------------------------------
The above writing represents my personal observations as a
volunteer present during the P3D thermal vacuum test and should
not be considered an official status report from Amsat.
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