Evolution of the instrument design

The first entry in my notebooks dealing with actual hardware design is dated 27 March 1956.8 On that date, I began breadboarding several transistor circuits that I had found in Electronics magazine.9 Although those circuits served as a starting point, they required far more electrical power than we could afford. I used a binary scaler (variously referred to as a binary counting circuit, flip-flop, or bistable multivibrator) as my learning tool.10 As for the transistors themselves, at first I used several early germanium types that had been identified in the Electronics magazine articles. They gave mixed results, with their high leakage currents making the necessary extreme power reduction problematical. A sample of a new type of surface barrier germanium transistor (Philco type SB-100) arrived at the laboratory on 4 May 1956. It was the first readily available production transistor that had the low leakage current, stability, and uniformity that I needed. I immediately began testing those transistors in my circuits and continued to use them until more desirable silicon transistors became available later that fall.

The Vanguard engineers at NRL were busily developing various electronic circuits and testing components for the satellite program, and they and the experimenters freely exchanged information on our respective efforts.

Instrument development went into high gear in early May 1956. From that date forward, my notebooks are full of descriptions of preliminary, intermediate, and final designs; of meetings attended; and of records of telephone calls to coordinate with the NRL engineers, program managers, IGY officials, and other experimenters. They also record literally hundreds of calls to collect information about suitable components and equipment, including everything from transistors to resistors, capacitors, time standards, recording and playback heads, recording tape, gears, bearings, switches, batteries, circuit board materials, encapsulating materials, and environmental test chambers.11 Those contacts continued throughout the entire duration of the project.

My notebook entry on 9 May contains a rough sketch of the complete block diagram for our cosmic ray instrument. By 10 days later, it had taken the form shown in Figure 5.1.12 By the time Explorer III was launched in 1958, only three major changes were made to that design: a change from a drum to tape as the data storage medium, the addition of a continuously transmitting channel, and a change in the encoding scheme.

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CHAPTER 5 • THE VANGUARD COSMIC RAY INSTRUMENT

FIGURE 5.1 Block diagram from the author’s laboratory notebook of the University of Iowa satellite cosmic ray instrument as it existed in May 1956 during its development as a part of the Vanguard program.

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May 1956 A meeting of the Working Group on Internal Instrumentation at the Naval Research Laboratory from 31 May to 1 June presented our first opportunity to report on our progress. Homer Newell began that meeting by stating, “We [collec­tively] are entering the brass tacks phase.” He announced that the most likely date for the first launch would be the fall of 1957 and listed the specific national objectives that had been established for the Vanguard program. They were (1) to put an object in orbit around the Earth, (2) to prove that it was in orbit, and (3) to conduct at least one scientific experiment using its internal instrumentation.

In terms of the physical arrangement of the planned Vanguard satellite, He stated that “[the] party line so far is 21.5 lb., 20-inch sphere. Line of retreat—no payload— third stage bottle only—18" dia. x 50" lg.” Following his general introduction, the by-then-active experimenters outlined their individual plans and the status of their developmental efforts.

Our status report included the block diagram shown in Figure 5.1, a full expla­nation of its operation, and a listing of expected characteristics. They included an expected instrument weight of 2.66 pounds (exclusive of the transmitting and receiv­ing equipment and their batteries), sizes and volumes of modules, and a total power requirement of 80.9 milliwatts.13

July 1956 Another pivotal technical working session was held at NRL on 30 and 31 July 1956.14 As far as our Iowa instrument was concerned, the most significant progress included a first attempt to detail the overall physical arrangement, good progress in designing the data recorder and electronic circuits, and investigation of sources for components and fabrication materials.15

Although the initial evaluation of GM counters embraced a wide variety of types, Van Allen’s familiarity with the devices in general, and, in particular, with the halogen- quenched counters that Herbert Friedman had developed at NRL, soon narrowed our focus. Halogen-quenched counters were being produced on a routine basis by the Anton Laboratory in Brooklyn, New York, and Van’s longtime association with the laboratory’s founder, Nicholas Anton, paved the way for a wonderfully effective association. Anton and his chief engineer, Herbert Kalisman, were extraordinarily helpful throughout those early years, when they produced numerous special versions for our evaluation, often within only a few days.

The choice of halogen-quenched counters for the Iowa instruments turned out to be fortuitous. They operated in orbit without degradation for the satellites’ entire lifetimes, in spite of the unexpected extremely high counting rates resulting from repeated incursions into the Earth’s trapped radiation. In retrospect, had we used the more conventional alcohol-quenched counters, they would almost certainly have failed before the end of the satellites’ operating lifetimes because of the high radiation intensities that they encountered.

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Well before the July meeting, I was becoming convinced that a major change would have to be made in the recorder design. The storage medium in the initial Mark I design was a cylindrical drum surfaced with ferric oxide. The recording and playback heads were to be supported above that surface by a very small gap. As the drum rotated, it would move axially, producing a continuous 18 inch long data track as a spiral around the surface of the drum.

I abandoned that drum approach in late June after realizing how rapidly the pulse packing density decreased with increasing head-to-recording-surface spacing. That spacing would have to be as small as 0.5 mil (0.0005 inch), and problems of drum concentricity would be large.16

Using tape instead of the drum permitted the recording medium to ride in direct contact with the heads. The tape version was identified as the Mark II recorder, and by the time of the July meeting, our instrument shop had produced a very rough first unit that I was able to show to the attendees.

The Vanguard technical discussions at the July 1956 meeting included details of the launcher, satellite structure, temperature control, some of the circuit development efforts at NRL, telemetering and radio commanding, and environmental testing. We saw a mockup of the by-then-envisioned satellite structure. Its exterior shell was a 20 inch diameter sphere consisting of two aluminum hemispheres joined at their equator. It was stated that the shell would have a 40 micron coating of silicon monoxide for temperature control.

Internally, the shell contained a small cylindrical chamber at its bottom to house a spring mechanism for separating the satellite from the final rocket stage. A larger cylinder for the scientific instrument was mounted on top of the separation mechanism, supported on its sides by a cantilever structure fabricated from welded aluminum tubing. The model was shown with a 3.5 inch diameter instrument cylinder, and the meeting discussions focused on that size. It was stated, however, that the instrument cylinder could be as much as 6.5 inches in diameter, and it was on that basis that we proceeded with our 6 inch configuration.

One-quarter wavelength antenna rods were mounted on the exterior of the shell’s equator. They were to be folded for launch and snapped into place on tapered me­chanical sockets following satellite separation.

October 1956 The Working Group on Internal Instrumentation held its third meet­ing at NRL on 9 October 1956. By that time, NRL was well along in designing the two different models of the satellite. The first was to contain the 3.5 inch diameter version of the scientific instrument package, and the other was for the 6 inch version.

Throughout the program, Van Allen and I conversed frequently, in his office, the laboratory, the hallways, or over lunch, to review progress and to exchange ideas about the instrument development. It was at just such a discussion on 22 September that we

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agreed to increase from a single to two channels of telemetry, one for continuously transmitting the raw counter rate and the other for transmitting the tape recorder data readout upon ground command.

At that time we addressed the question of possible effects of cosmic radiation on the transistors. Van Allen had become concerned that cosmic ray interactions within the body of the transistor chips might either trigger false results or, in extreme cases, damage the devices. After some back-of-the-envelope calculations, he concluded that reasonably expected cosmic ray rates should introduce less than one interaction in a one cubic millimeter pellet during an entire orbit—an acceptable error rate, even if all of those interactions should result in false counts. He also concluded that the chance of damage to the chips would be remote.

On a different subject, Van Allen mentioned that Wayne Graves, an engineer at the Collins Radio Company in Cedar Rapids, was interested in working on the satellite. He soon made the necessary arrangements with his friend Arthur Collins, and Wayne worked closely with me on the instrument development and testing from that October until June 1957. A very capable engineer, he helped tremendously in the design and testing efforts.

By the time of the October meeting, our work on the satellite instrument had progressed substantially. Major work on electronic circuitry had been completed, and many electronic and mechanical components suitable for flight had been chosen. Silicon transistors from the Texas Instruments Company had entered the picture. Their new 2Nxxx series was coming into early production. I had received early samples and found that their temperature and electrical properties were far superior to the germanium units that we had been using.

Van Allen and I had initially expected that we would contract with a commercial firm to complete the design and fabricate the tape recorders. My telephone discus­sions and visits to several prospective manufacturers proved disappointing, however, and we decided to build them in-house. Our instrument makers had completed the first Mark II recorder, shown in Figure 5.2, and I was subjecting it to extensive testing.

By that time, I had evaluated and ordered the first of a number of new environmental testing facilities. It was a temperature chamber, capable of testing our components and modules at both high and low temperature extremes.

The October 1956 meeting focused on detailed satellite design. A new satellite weight allocation listed 2.00 pounds for the shell, 1.10 pounds for the internal support­ing structure, and 2.50 pounds for our internal experiment packages, including their thermal-mechanical control switches, but not including the telemetering components. The spring device to separate the satellite from the final rocket stage was projected to weigh 1.00 pound. The Minitrack telemetering system, consisting of the antennas, transmitter, and batteries, was estimated at 6.07 pounds. One pound was set aside

CHAPTER 5 • THE VANGUARD COSMIC RAY INSTRUMENT FIGURE 5.2 The Mark II tape recorder as it appeared in October 1956. The centrifugal governor is in the short vertical cylinder on the left of the upper flat plate. The tape-advancing ratchet is visible above the governor, and the Mylar tape is clearly visible in the center foreground, where it wraps around two idler rollers and the recording and playback heads.

for wiring and miscellaneous items. That made a total projected satellite weight of 13.67 pounds.

December 1956 As 1956 was ending, a meeting at NRL of the Vanguard Science Program Committee reviewed the status of the satellite development and worked out additional technical details. By then, we had progressed from general system de­sign to very specific engineering details—the meeting discussions concerned satellite structure, internal temperature control, instruments, the environmental testing pro­gram, and orbit details. The results of internal NRL design work on circuits, batteries, telemetering, and ground receiving station recording received considerable attention.

With respect to our Iowa instrument, by December, the most substantial accom­plishments included extensive thermal testing with our new temperature chamber, finalization of the data-encoding scheme, and more changes to the data recorder.17

My greatest problem with the recorder had been in controlling the speed of the tape during playback. It was necessary to control the speed of the tape to produce

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a reasonably constant data rate for transmission. To initiate playback, a ratchet was released, permitting a spring to rewind the tape onto the supply reel. A normal spiral – wound spring provides a torque that varies considerably as it winds and unwinds. Attempts were made to find a spring formed in an S shape that would provide a more nearly constant tension (a so-called Negator spring), but I was unable to locate a suitable source.

The Mark II version had employed a mechanical governor having centrifugally actuated brake shoes in frictional contact with a stationary drum. That approach could not be made to work smoothly in such a small configuration. For the Mark III version, I used an eddy-current speed controller, where a retarding torque was produced by a silver disk rotating at high speed in a strong magnetic field. Since the retarding torque in such an arrangement varies as the square of the rotational speed, it provided a rough but acceptable speed control. The result was a 6.5 second playback time for dumping the entire tape content, with a speed variation of less than a factor of two during the playback. That speed variation, although certainly not desirable, was compensated for in the ground data processing.

I had been using ordinary consumer-grade Mylar-based recording tape but was concerned about its durability in the space environment. My greatest fear was that the recorder might get warm enough for the Mylar to stretch. I finally located a metal recording tape that had the desired ruggedness and dimensional stability. The UNIVAC I computer that had been introduced in 1951 by the Univac Division of Remington Rand in Philadelphia employed a 0.5 inch wide by 0.001 inch thick phosphor bronze recording tape with an electroplated nickel-cobalt recording surface. Rand donated a twenty-five foot length of that tape. I arranged with the tape’s original manufacturer, the Somers Brass Company in Waterbury, Connecticut, to slit this length of tape to the desired narrow 5/32 inch width. A 55 inch long piece of that tape was incorporated in each Mark III and IV recorder.

The tape-advancing ratchet was also redesigned. The Mark II mechanism had been unbalanced. I was concerned about the effect of vibration, acceleration, and spin on that device and designed a more completely balanced version for the Mark III recorder.

I had had great difficulty in finding very small but sufficiently high performance recording and playback heads. Throughout the summer of 1956, I obtained specifi­cations and samples from every supplier I could locate. In early October, I obtained new samples from the Dynamu Division of Maico Corporation, a maker of consumer- grade reel-to-reel tape recorders. Finally, I had heads that were small enough to fit in the recorder but which still had good enough high-frequency performance to produce the desired data packing density on the tape. The recording head had a gap width of only 0.00015 inch, quite remarkable for that time.

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Other changes in the Mark III version were relatively minor, but they illustrate the extreme care taken to assure high reliability. The three metal idler rollers were replaced by ones made of Teflon, which has a slippery surface. Thus, if a bearing were to freeze, the tape could still slip across the idlers and permit the recorder to operate. Finally, a pair of cam-operated mechanical limit stops was added to augment the previously included electrical limit switches. Then, if there should be an electronic failure, the mechanical stops would stall the tape to assure that it could not be pulled off the tape reels.

It was announced at the December meeting that NRL would deliver a first aluminum prototype satellite shell on 30 January 1957 for our use in test fitting the cosmic ray instrument package and for initial system tests. The first two magnesium models were due on 1 May. One of those was earmarked for our system testing at the State University of Iowa (SUI), while the second (the true prototype model) was to be delivered back to NRL with our instrument package installed for the tests that they were to conduct. Three flight models of the satellite shells and instrument supporting structures were due to us at Iowa City on 15 June.

One important action taken at the December meeting was the naming of spe­cific NRL individuals to work with each of us. The team for our Iowa instrument consisted of Leopold Winkler as chairman, Robert (Bob) C. Baumann for mechan­ical structures, Milton Schach for internal temperature control, Roger Easton for the radio frequency components, and Whitney Mathews for the telemetry system. The group’s initial charge was to review our complete system, prepare a break­down of the relative NRL and SUI responsibilities, and review the SUI instrument budget.

Our transmitters and receivers were also being built by NRL. A first transmitter was promised for 30 January. Their first receiver was due on 15 February. The second transmitter, able to switch between two power levels to accommodate our two-channel instrument design, was due on 1 March. Four flight units of both the transmitters and receivers were scheduled for delivery to us on 1 May.

We scheduled a first vibration test of a prototype data recorder for 1 February. One completely assembled cosmic ray instrument package was due in Washington on 1 May 1957 for environmental testing at the design levels, and three flight models were promised for 15 June.