Additional notes on the data recorder

Since the data recorder developed for Vanguard and launched on Explorer III played such an important role in the radiation belt discovery, a few more words about the device are appropriate.

Variations on the Iowa design Variations of my recorder design were developed by two other groups. Starting in late 1956, Gerhardt Groetzinger and his group at RIAS in Baltimore, Maryland, designed a Vanguard instrument package with an ionization chamber for measuring the flux of primary cosmic ray nuclei with atomic number greater than eight. He, too, needed to obtain broad orbital coverage. In May 1957, I sent him a full set of drawings and an actual working model of my recorder (unit 2 of the Mark III design), as well as my electronic circuit designs. He used that material as the basis for a version for his instrument.

Unfortunately, Dr. Groetzinger died before his instrument could be launched. That ion chamber, with P. Schwed of RIAS and Martin A. Pomerantz of the Franklin Institute serving as investigators, was eventually launched aboard Explorer 7 on 13 October 1959. Explorer 7 employed a solar-powered, continuously radiating, rel­atively high power 20 MHz transmitter that could be received by radio amateurs via conventional receivers throughout the world. Thus, adequate coverage by direct radio transmission was expected, and their tape recorder was not needed in that satellite.

For many years, I enjoyed a wonderfully productive relationship with Verner (Vern) E. Suomi at the University of Wisconsin. He first contacted me in mid-1956 about the possibility of using my recorder and electronic circuit designs, and I immediately provided a full set of drawings and other details. His “Proposal on Radiation Balance of the Earth From a Satellite,” which included a variation of my recorder design,

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was submitted in the early fall as a candidate for flight in the Vanguard program. He received preliminary endorsement from the IGY committee in early December 1956. On 25 February 1957, they received formal approval and funding for their proposal, and a month later, they reported that fabrication had begun on their tape recorder – sequencer. Their variation on the design increased the recorder tape speed to two steps per second instead of one and added an elaborate set of sequencing switches that cycled between five sensors and two 10-stage scaler circuits. Those switches were actuated by the ratchet drive in the tape recorder.

An attempt was made to place the full University of Wisconsin instrument into orbit on 22 June 1959 on Vanguard Satellite Launch Vehicle (SLV)-6, but that launch attempt failed. A modified version of the instrument was finally orbited aboard Ex­plorer 7 on 13 October 1959. Since that satellite benefited from global reception of the higher-powered 20 MHz transmissions, as mentioned before, the data recording feature was not needed and dropped. However, the stepping mechanism and general layout were used for operating the programming switches.

Other early satellite data recorder designs A somewhat similar unit was developed in the early 1960s by the Raymond Engineering Company in Middletown, Connecti­cut, under contract from JPL. It had many similarities with the SUI recorder—it also operated entirely in the digital domain, with the tape advancing one physical step only when an input pulse was applied. The Raymond device, however, being produced later than the Iowa design, was able to use a superior configuration. Although its tape was also held in tension by a spring, the spring applied a torque to the tape take-up spool rather than the supply spool, and the tape was pulled forward by the spring when it was incrementally released by an escapement mechanism similar to that found in mechanical clocks. The Negator spring used in their design provided a nearly constant torque. An electric motor drove the tape in the other direction at a nearly constant speed to read out the data upon command.

The tape in the Raymond device moved 0.005 inch for each step, the same as in the SUI recorder. With a much longer tape, it had a total capacity of 100,000 bits versus 8000 in mine. That made it substantially larger, 2.25 by 4 by 5 inches, and heavier. Its power consumption was comparable to that of the SUI recorder. Developed to permit individual instruments aboard future multi-instrument spacecraft to have their own data storage, its use was overtaken by two developments. First, the rocket launching capacity increased rapidly after the initial space launches, so that extreme weight minimization was less important. Second, demands for data storage and transmission increased dramatically as the instruments became more discriminating, and the Raymond design did not meet that need.

Only a breadboard model of the Raymond recorder was built for testing, and it was never used in flight.30

OPENING SPACE RESEARCH

A completely different type of space tape recorder was launched into orbit on 17 February 1959 as a part of the Vanguard II instrument package. That recorder, de­signed by a group under William Stroud’s leadership at the U. S. Army Signal Corps Engineering Laboratories at Fort Monmouth, New Jersey, was designed to record cloud cover distribution over the daylight portion of the satellite orbit. It was a minia­turized version of more conventional analog recorders, with a weight of a pound or so. It used a 75 foot length of tape in an endless loop that did not require rewind­ing and had recording and playback times of 50 minutes and 1 minute, respectively. The recorder operated properly in orbit. Interpretation of the data was not possible, however, because of the unanticipated complex rotation of the satellite.

As mentioned earlier, the Soviets included a much larger data recorder in Sputnik 3, launched on 15 May 1958. However, that recorder failed before the launch. Political considerations caused them to launch without the delay that would have been required to fix the problem. Thus, they were deprived of the continuous data coverage that they wanted.

Explorer III tape recorder summary The Explorer III tape recorder set new stan­dards in terms of size, weight, required electrical power, and ruggedness. Some of the recorder’s most notable features were as follows:

• The final model was only 2.5 inches in diameter, less than 3 inches long, and weighed just over eight ounces.

• Its average power consumption was only 0.035 watts. That low power require­ment was a major factor in permitting the full Explorer III instrument to operate in orbit for 44 days.

• It was fully digital in its operation, making it relatively insensitive to temperature, battery voltage, and other variables. The fact that the tape was motionless most of the time contributed to the low power demand.

• All moving mechanical components except for the playback solenoid were dy­namically balanced, making it largely insensitive to vibration and various trans­lational and rotational motions of the satellite.

• All mechanical components were physically constrained so that high acceleration and shock in any direction could not damage the recorder.

• Ball bearings held friction to a minimum. Nineteen of them supported all rotating components and served as cam followers. The smallest ball bearing was only 5/32 (0.156) inch in outside diameter.

The tape recorder required more than 2000 hours of precision tooling in the Physics Department instrument shop over a one and a half year period. Edward (Ed) Freund worked with me as the principal instrument maker.31 He was helped at times by shop manager Joseph George Sentinella and fellow instrument makers Robert (Bob)

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Markee and Robert (Bob) Russell. For their work on the data recorder, the four were recognized in 1959 with Certificates of Recognition from the American Society of Tool Engineers.

I kept a spare recorder in my possession from the time of my departure from Iowa City in 1960 until early 1968. That year, I offered it to the Smithsonian Institution’s National Air and Space Museum, and arrangements were made with Frederick C. Durant III to transfer it to their care. In due course, it was cleaned and restored to fully operating condition by Ed Freund. In September 1968, I carried it to Washington and handed it to Fred. The official transfer to the museum was by a letter from Van Allen.32

The unit was identical to the ones launched in Explorer II (launch failure) and Explorer III, except that its relatively short-lived magnesium coating was replaced with gold plating to make it more immune to long-term oxidation.

Although the device included a number of unique and remarkable features, the Uni­versity of Iowa business office decided not to apply for patents. They concluded that there would not be enough demand for similar devices to make patenting economically attractive.