Figures 2.2.01 and 2.2.02: Goembel Instrument's dual turbo-pumped vacuum chamber and the assembly of our unique electron gun

With our equipment, experience, and track record, those who want flight-quality charged particle spectrometers should now consider contracting Goembel Instruments.

Figure 2.2.03: High performance electron gun

While equipping the laboratory and SCM test apparatus, I designed innovative components for the SCM. One such part was the SCM channel electron multiplier (CEM). No off-the-shelf part was available that would meet my demanding criteria for the SCM so I worked with Dr. Sjuts (of Dr. Sjuts Optotechnik GmbH, Germany). His CEMs are renowned for their extraordinary performance and have an enviable flight heritage.

Figure 2.2.04: Custom designed, low profile, rugged CEM for the SCM

Each of the parts I designed for the SCM needed to be lightweight, but also sturdy enough to withstand the vibration of launch, thermal shock while in flight, etc.

Figures 2.2.05 and 2.2.06: SCM part being machined and Examples of internal parts for the SCM

As challenging as it was to make many of the mechanical parts for the SCM, none were as challenging as the electronics. The building of the SCM electronics (by subcontract to Goembel Instruments) took the most time and effort to produce and was responsible for almost all of the delays and re-designs for the project.

'Space flight qualifiable' electronics are difficult to make. The SCM presented some special problems. The SCM needed high voltage (kilovolts) electronics to power the CEMs. It is difficult and expensive to make high voltage electronics for space flight. The SCM also required very sensitive electronics able to detect individual electrons in order to determine spacecraft charge. Furthermore, the funds budgeted for electronics were small compared to most flight instrument projects. In fact, one 'name brand' aerospace electronics firm said that the amount of money I was awarded for the entire SCM project would not pay for them to even design and build one flight-quality board (the SCM ended up having three boards). Another firm considered getting involved in the project, but said that the funds I had budgeted for complete, flight-quality electronics would be enough for them to work on concepts only (no hardware build of any kind) Flight quality boards were entirely out of the question.

After finding a subcontractor to produce the SCM electronics we encountered problems. In the 6-months time they had estimated it would take to complete the work they had only gotten a small fraction of the way toward completion. They didn't appear to be committed to the project. When it became clear work on the electronics was not going well, we conducted another search for an electronics subcontractor and replaced the original subcontractor with Payload Systems, Inc. The Goembel Instruments/Payload Systems association has been an excellent one. Payload Systems was very committed to the project and has produced superb electronics for the SCM at very low cost.

The following example shows how Goembel Instruments worked with others to overcome difficult electronics design challenges.

The SCM CEMs need to be supplied with approximately 2,000 volts power to detect electrons. A 2,000 volt power supply for space flight is a pretty expensive and exotic piece of hardware. Some estimates I received were in the $20,000 - $30,000 range per unit, which was far beyond the amount budgeted for all of the SCM's electrical components.

Payload Systems suggested that we use an off-the-shelf DC-DC converter that was capable of providing up to 5,000 volts and was also small (a one inch cube), lightweight, and energy efficient. It would meet the needs of the SCM. However, it was not a part built for space flight. Staff at Payload Systems (the SCM team included Jim Littlefield, Joanne Vinning, John Merk, and Julianne Zimmerman) and I did some investigating and found that it was not uncommon to fly commercial off-the-shelf electronics components such as the miniature DC-DC converters we were interested in. More investigation revealed that there was nothing inherently 'space flight intolerant' about the component. It was likely that such devices could withstand significant radiation without ill effect. They could certainly withstand the vibration of launch. The only modification required was the replacement of the original leads and the vacuum potting of the flight units (the original leads and potting might prove 'outgassy' in a vacuum). It turned out that finding replacement low-outgassing high-voltage leads was difficult. Ultimately, W.L. Gore & Associates donated 4 yards of Teflon insulated high voltage wire to the SCM project and EMCO High Voltage Corporation agreed to sell Goembel Instruments unpotted units. Numerous other times Goembel Instruments worked together with others to find innovative ways to meet the budget and produce high quality electronics. I am especially grateful for the help the Air Force Research Laboratory (Hanscomb AFB) provided during the development of the SCM.

Figures 2.2.07 and 2.2.08: Unpotted 5kV DC-DC converter and Vacuum potting process at Goembel Instruments

Shortly into Phase II an image of the completed SCM was emerging.

Although a two-part instrument was considered (a sensor head and electronics box) I decided a single unit instrument would be simpler. Payload Systems and I decided on a three printed circuit board (PCB) design. One PCB would contain the high voltage and electron detection electronics, another would contain the low voltage analog electronics and the final board would contain the microprocessor.

Figure 2.2.09: Three board function of the SCM electronics.

It wasn't long before the layout and three-dimensional shape of the populated printed circuit boards could be determined. At that time I started designing the chassis for the completed SCM.

Figure 2.2.10: Early view of SCM flight instrument design

Again, advice from the Air Force Research Laboratory and others was very helpful in determining what sort of interface to include, what sort of temperature and vibration tolerances to allow for, etc. Goembel Instruments and Payload Systems even planned a meeting with some staff of the Air Force Research Laboratory as an informal 'design review' before the SCM electronics design was solidified. I am grateful to the Air Force Research Laboratory for their advice throughout the production of the flight prototype SCM.

Dorothy Gordon, originally of Elf Electronics (her own company) designed the logic circuitry for the SCM. She is expert at designing FPGA-based systems for space flight electronics and worked with Goembel Instruments through thick and thin to get the SCM built. She had even contributed to work on proposals back before there had been even one cent in funding for the SCM. She designed a very energy efficient, high performance, inexpensive logic circuit for the SCM and has always done very rapid, innovative, inexpensive prototyping for Goembel Instruments.

Figure 2.2.11: Input/output scheme for the SCM

After the electronics were fully designed we were ready to design the complete flight-prototype. A number of mock-ups were built to find the best way to house the electronics.

Figure 2.2.12: Plastic 'see-through' mock-up of SCM electronics housing

Ultimately, the design of the SCM came about through careful iteration. The electronics, hemispherical analyzer and chassis were all designed to fit together as a compact, rugged unit. Other constraints such as time and budget played a role as well.

Figures 2.2.13, 2.2.14, 2.2.15: Views of the SCM during assembly, and Complete (right)

I am pleased that I have not only reached, but have surpassed, my original design goals and have produced a truly extraordinary instrument to monitor spacecraft charge.

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