Introduction

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.

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°.

Power System

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.