The 1970’s were exciting years in the developing US space program. NASA was developing means to explore both near and far space, and Excelco/Newbrook (ENI) was deeply involved in creating the necessary hardware.
One of the premier projects in the early 1970’s was to produce instrumentation cases for the atmospheric probes for NASA’s Pioneer Venus Multi-Probe mission. The client was Lockheed Missiles and Space Corp., under contract to NASA. ENI’s long relationship with Lockheed, producing rocket motor cases, deep-sea pressure vessels and other specialized hardware, proved that the Venus probe work was a natural fit for us.
This was a build-to-print project to make three 20-inch spherical diameter cases and one 36-inch spherical diameter case. We were tasked with turning the raw materials into complex shaped parts with literally zero deviations from print.
The instrumentation cases housed the sensors, instrumentation, communication equipment and power sources, protecting them during launch, the transit to Venus and for a sufficient time during the entry through the Venusian atmosphere to gather and transmit scientific data. They were never expected to survive the impact with the planet’s surface.
The components of these cases were all machined from solid titanium alloy forgings. At the time, working with titanium was still a specialty art form. Welding was not allowed, even for the repair of small machining errors. If mistakes were made during planning or production, we would have to start all over, and there was little or no time for recovery.
The Journey (Approach)
The smaller cases were hemispheres with bolting flanges. Each hemisphere had several openings with external reinforcements and pads for mounting sensors that significantly complicated the machining process.
The large case was made up of one full hemisphere, an instrumentation ring and a partial spherical dome, all with bolting flanges. These parts all had the complicating reinforced openings and pads. The dimensions were very precisely controlled to maintain the overall spherical shape and the contours of the reinforcement around each of the openings through the vessel wall.
Flawless engineering and manufacturing , as well as careful planning of each machining step, would be absolutely necessary if we were going to get it right.
Making these parts required multi-axis machining, something that couldn’t be done by ordinary XYZ axis machines. At the time, computer-controlled machining was in its infancy and not available for this project, so we had to use the machines and methods available to us to make these parts perfectly.
All of the multi-axis machining, therefore, was performed using pneumatic tracing equipment.
The solution to producing precise final surfaces was to adapt existing tools and equipment to provide all the cutter movements that were required.
The final machining setup was essentially a three axis pantograph with a pneumatically driven probe which followed the contours of a full scale model of the finished part, driving a machine spindle to reproduce the contours on the final part. The model and rough machined part were set up on mechanically linked turntables to provide matched rotary movements while the probe drove movements in the vertical and horizontal planes. Precise matching of the probe and cutting tools maintained the required tolerances.
The Use of Aluminum
Full scale models were made of each finished section from aluminum. The models were made by first machining all of the features that could be produced by conventional turning, such as the spherical contours and integral flanges. All of the reinforcements and raised pads, including the radii blending the surfaces, were machined as separate parts and inset into precisely located milled recesses in the turned sections. Precise handwork took care of the contours that couldn’t be machined.
To ensure that the tolerances would be met in the finished parts, the allowable deviations in the models were limited to half that of the finished part tolerances. Fabricating each precise model was, in and of itself, a significant project.
To maintain the history of this remarkable project, one of these aluminum models still exists today.
The individual sections were rough machined using conventional machining practices and then finished by tracing off the models. Each section was hand finished to remove all of the machining marks and bring all the surfaces into tolerance.
It is difficult to overstate the painstaking nature of this process.
The Pioneer Venus mission was launched in August, 1978, and successfully met all of its objectives. The probes were not expected to survive the descent through the Venusian atmosphere, but one of the smaller probes continued to transmit data for a full 45 minutes after reaching the surface.
The techniques and innovations were considered cutting edge at the time. The project still serves as a source of pride for all involved — most manufacturers will never build anything that winds up on Venus.