The paramount consideration is, however, that of smoothly blending the
mechanical design technology needs of the spaceframe with all of the
other technologies that comprise the complete object, a successful
satellite fulfilling its mission. While this presentation will deal
mostly with the Mechanical Engineering aspects of the Phase 3D
spacecraft, the observer must keep in mind that none of this is done
out of the context of all of the technologies involved in the whole
mission.
The Phase 3D spaceframe is principally fabricated of thin-gauge sheet
aluminum. Its formation, to rather unusually close tolerances for
sheet metal structures, caused more than passing concerns by all who
were involved in the effort. Typically these tolerances are in the
range of 0.2mm (0.008in.). The secret to this type of construction is
to place all of the load stresses into the shear plane of the sheet
metal, where it is notably strong for its weight. An example of this
are the six Divider Panels, one on each corner of the spaceframe.
Three of these will be anchored to the launch vehicle which will get
Phase 3D into space. Thus, during launch, these three points will be
quite heavily loaded in all motions. The only machined parts in the
spaceframe are the six Corner Posts at the outer ends of the Divider
Panels. These must be robust enough to carry all of these launch
thrust loads into the spaceframe, translating all of those forces into
the plane of the 0.8mm thick sheet metal Divider Panels as sheer
forces. It is difficult to completely convey these concepts in words
and pictures, but those seeing the spaceframe in-person will be able to
more readily grasp the concepts employed in the design.
Working with panels of this type is not all that easy when it comes to
mounting massive single elements, such as an 11.5kg battery assembly.
As such mountings are done to the flat surface of a Divider Panel, the
panel will flex and bend at the slightest of influences, such as
vibration. Such a mounting really needs to have its natural resonance
frequency greater than 100 Hz, but without any modifications, such a
panel resonance is less than 20 Hz. We found that by using
light-weight aluminum stiffeners, formed into the shape of a hat, that
we could bring the panel resonance up to 38 Hz. By placing a thin
aluminum panel across the tops of the several stringers, we formed an
aluminum sandwich, much like a honeycomb panel, and the resonance of
the battery mass went up to 128 Hz. All of this added structure was
done on the assembled spaceframe (not an easy way to so such a task)
and joined with both rivets and epoxy bonding agents. Co-bonding
structures like this is a very messy process.
Mention was made, above, of panel resonance frequencies. We did such
tests in a manner that is one of the "flagstones" of AMSAT practice.
The method, learned and 'borrowed' from our German compatriots, employs
a high fidelity amplifier (a 45 year old Ultra Linear Williamson
amplifier) and a speaker held inside of the spacecraft propellant bay.
By driving the amplifier with a sine wave signal generator, the
frequency was swept over the range of interest. On thin panels a light
object, like a screw washer, will dance with panel resonance. Massive
objects, such as the simulated battery, will not respond as well and
required another method. This time we used an old ceramic microphone
element attached to the 'battery' and to observe the output signal on
an oscilloscope. Resonance frequencies are readily observable with
such simple instrumentation. This is AMSAT.
Construction of two of these SBS units has been completed in Utah,
mainly at Weber State University. Each of the $10k machined flange
rings, or Frames, on each end of the SBS cylinder were done in Florida,
obviously on a very large lathe. These were made from the highest
quality of ring-forged aircraft aluminum. The Frames were mounted to a
purpose-built precision steel table, holding flatness and roundness
values to less than 0.05mm (0.002in.), far better than ESA requires.
The 4mm thick sheet aluminum rolled shell sections are attached to the
Frames with a total of 976 special fasteners. A total of 2600
additional rivet fasteners are also used in each assembly.
Tests have been conducted at Weber State University to verify the
operation of the mechanism to separate the spacecraft from the SBS, and
the launch vehicle. For this purpose, a 500kg Mass Mockup Unit (MMU)
was constructed to take the place of the P3D satellite. The Separation
Nut units were operated using Nitrogen gas, rather than the
pyrotechnically generated gas sources that will be used in flight. In
this manner, we could reuse the Nuts over a number of tests. After a
little travail at the start of the tests, all went very well over quite
a number of repeated tests, with concurrence of the three Nut
separations being within 3ms (0.003 second) of each other. Witnessing
the exiting of a 500kg object from the SBS was impressive, to say the
least.
In the case of the Phase 3D spacecraft, the internal ring-shaped heat
pipes remove heat from one part of the spacecraft and re-distribute the
energy to other parts where it is ultimately transported through the
sides of the spacecraft and radiated to space - the ultimate "heat
sink". What is felt to be a unique feature of this heat pipe system,
as employed on Phase 3D, is that none of the pipes come in direct
contact with space-facing panels. Instead they depend upon indirect
re-radiation of the heat from internal equipment mounting panels to
side panels that are deliberately allowed to become cold. All along,
however, the electronic equipment modules maintain their desired
temperatures because of the thermal influence of the heat pipe system,
regardless of whether those modules are mounted on the solar heated
side, or on the space-cold backside of the spacecraft. These design
concepts put to practice the concepts of energy conservation,
eliminating any need for active thermal control or supplemental
heating.
The earlier Phase 3A, B and C satellites employed several multi-layer
thermal insulation blankets to assist these spacecraft through the
thermal rigors of spaceflight. Quite simply, such blankets are a
first-class nuisance to fabricate, as the required assembly technology
is very exacting. In the case of Phase 3D, the side panels of the
spacecraft will be painted to provide the necessary radiative heat
rejection. The top and bottom panels will be mostly solar energy
absorbing metallic finishes of several different types, depending upon
the location and desired temperatures of that section of the
spacecraft. In general, the thermal design calls for the mean
spacecraft temperatures to be between -5° and +20°C for the
expected range of sun angles (() from -80° to +80°.
Extensive computer thermal analyses of the Phase 3D spacecraft have
given us a very comfortable confidence that this design will provide
the desired results, without the use of the kind of thermal blankets
used on most other satellites. These thermal analytic computations
were accomplished on a home computer to produce a series of temperature
performance curves as functions of ( angle.
While solar panels are satisfactory as a sole source of power, some
form of energy storage must also be provided. This is accomplished
with a battery. Energy storage is necessary, not only to power the
spacecraft during times that the sun is eclipsed by the earth, but also
to operate the arc-jet thruster. It's power requirements exceed the
capability of the solar arrays, even under the best of conditions.
Actually the Phase 3D satellite will carry two batteries, a "main" and
an "auxiliary". This is to provide redundancy in case of failure of
the main battery. The Phase 3D design team evaluated several sources
and types of batteries. A final decision was made to select a more or
less conventional nickel-cadmium battery, albeit with a new plate
design, as proposed by a German firm. Another contender was from a US
firm which proposed the use of an assembly of Nickel-Metal Hydride
cells for the main battery and a more conventional Nickel-Hydrogen
stack for the auxiliary. As in the case of the solar cells, cost was
an important factor in reaching this decision.
The Main Battery is composed of 20 cells of 40 Ahr capacity, for a
22-28 VDC supply. These rectangular cells are from a terrestrial
application, but have been very well characterized for space service.
They are contained in three subassemblies, two of seven cells and one
of six cells. The seven cell assembly is 11.5kg mass and the
respective assemblies will be mounted to reinforced Divider Panels, as
previously discussed. The selection of the location of the lighter six
cell subassembly will give us an option to use in achieving spacecraft
balance.
For the Auxiliary Battery, a relatively new 10 Ahr cylindrical cell has
been selected. A total of 40 of these cells will be mounted in two
20-cell parallel strings. The entire group is divided into four, ten
cell subassemblies, mounted to the remaining three Divider Panels. As
one panel will need to mount two of these subassemblies, we are again
provided with a tool for spacecraft balancing.
Spaceframe and Launch Adaptor
The Phase 3D electronic equipment must be housed in something. That
something is the spaceframe. In the case of Phase 3D, those designing
and building the structure were continuously learning new techniques
for such fabrications and getting quite an education in the process.
Along the way a number of quite good lessons on light-weight aircraft
structural construction methods were learned. While the end product is
not as light as some would have liked (about 60kg), it has already
demonstrated itself to be very strong. The spaceframes for OSCARs 10
and 13 weighed only 7kg.SBS
Since Phase 3D will be a secondary payload on an
Ariane launch vehicle, it must conform to
whatever space the European Space Agency (ESA) can make available. ESA
has the conical 1194V Adaptor which interfaces between the (2624mm
((103.3inches) diameter bolt circle on the Ariane upper stage to a
(1194mm clamp-band used for payloads. Although hollow, the conical
adaptor does not provide sufficient space to house Phase 3D, or any
other reasonably sized payload. Accordingly, ESA offered the amateur
satellite community the opportunity to launch aboard the new Ariane 5
vehicle if we would provide a cylindrical "spacer" that could be
mounted between the (2624mm diameter bolt circles on the bottom and the
conical section on the top. Phase 3D could then ride to orbit inside
this cylinder while a prime payload satellite would attach to the
conical adaptor with the (1194mm clamp-band. Thus, they require that
our cylindrical section must be able to support the launch loads of
this prime payload fellow passenger. This means that this AMSAT
provided (2624mm diameter Specific Bearing Structure (SBS) must be able
to withstand the load forces imposed by a 4.7 T (metric ton, 10,350
lb.) satellite load. In order to assure ourselves that our design is
capable of handling such a load, extensive structural Finite Element
Analysis (FEA) computer work has been performed (on the same home
computer used to accomplish the thermal analysis).Thermal Control Subsystem
Three axis stabilization satellite operation planned for Phase 3D
drives the thermal design of the spacecraft. Most of the commercial
communication satellites spin in order to keep any side from getting
too hot and the opposite side too cold. Once in its desired orbit and
orientation, Phase 3D will not spin, but will be oriented in
three-dimensional space, with the antennas continually facing Earth.
This continual attitude adjustment, with one side facing the sun,
causes some interesting thermal design problems. The Phase 3D solution
to overcome most of these problems is through the use of the four heat
pipes. A heat pipe is a thermal linkage of very high conductivity
consisting of a closed, evacuated tubular chamber with walls lined with
a wick and partially filled with a pure fluid. The fluid used in Phase
3D is anhydrous ammonia. The fluid is vaporized at the hot end. The
vapors then move through the hollow core of the tube, and condense at
the cold end; from which the resulting liquid is returned through the
wick to the hot end by capillary action. By this process, heat is
transported from the hot to the cold end. Heat pipes typically offer
heat transport characteristics that are many times greater than the
heat transfer capability of the best heat conducting materials, while
maintaining an essentially uniform temperature. The process requires
no power and operates to its maximum performance in a zero-gravity
environment.More Power Needed
One of the design features intended to make Phase 3D more accessible to
smaller ground stations will be the use of higher power transmitters.
This increased power carries with it another set of problems. First,
the power must be generated. This means more solar array area. To
achieve this, Phase 3D will employ four deployable solar panels in
addition to the two mounted on the spaceframe. This will be the first
use of deployable panels on an amateur satellite. Of course,
deployable panels means mechanisms to initiate the unfolding plus
appropriate hinges and latches to achieve the desired final
configuration. This type of hinge is able to swing both ways but
always return to the desired center position. One of the German
members of the Phase 3D Design Team first suggested the use of this
type of hinge, and actually obtained one at a hardware store to
demonstrate the utility of the principle on the model of the former
Phase 3D "Falcon" spacecraft design. As there is not the luxury of a
lot of excess space around the current Phase 3D spacecraft when
installed in the SBS, this hinge design had to go through several
gestations in order to achieve the desired device in a compact manner.
This effort included finding a spring wire able to withstand the
metallurgical and thermal rigors of anticipated operation at
temperatures as low as -100°C.
Power System
Since satellites must get their primary power from a the sun, the only
practical means of obtaining power is the use of semiconductor solar
panels. Generating the power needed to support the transmitters aboard
Phase 3D requires large solar panels. The design which evolved calls
for a total solar panel area of 4.46 m2 (48 ft2) and BSFR Silicon solar
cells of 14.3% efficiency. This array will produce about 620 Watts of
power at the beginning-of-life (BOL) and at optimum sun angle ((=0°). After 10 years in orbit, this power number will still be about 350
Watts at a (=45°). This amount of power is still sufficient to
operate at least two transmitters and the other necessary spacecraft systems.
Like almost anything else, solar arrays deteriorate with age. This is
why their performance after a specified number of years is an important
design consideration. The cells for Phase 3D are being obtained
through a very attractive agreement with DASA, the German Space Agency.
These cells are of US manufacture and were surplus inventory from a
prior satellite program. While other sources and configurations of
solar cells were considered, it was concluded that this one represents
the best trade-off between performance and cost. Solar cells, and
their assembled panels, represent one of the single highest cost items
which go into building a spacecraft.Summary
Providing the 'containment' for all of the systems and experiments for
the Phase 3D spacecraft has provided an interesting engineering
experience for quite a number of us on the team. We are confident that
this effort will translate into a very long lived mission for us to all
enjoy in the communications afforded by Phase 3D.
Last updated: Feb 15, 1996
by Ralf Zimmermann, DL1FDT