Measures for Progress: A History of the National Bureau of Standards

THE RADIO PROXIMITY FUZE 391
A variety of principles were available for obtaining proximity detona-
tion against a target, including photoelectric and reaction oscillator types
wider British investigation, a beating oscillator arrangement proposed by
the Department of Terrestrial Magnetism, a pressure type based on the radio
altimeter, and one on acoustic principles. The most promising for the non-
rotating fuze, however, proved to be that utilizing the Doppler effect of
reflected radio waves. Hinman and Diamond devised a diode detector ar-
rangement that acted when the amplitude of the reflected signals exceeded
a predetermined value, and with that the section began its experiments.76
Tests of the first series of crude box models using the radio principle
were made between January and April 1941. Despite the fact that only a
third of the cumbersome models functioned properly, they proved that a
radio proximity fuze was practicable. Turning it into an operational service
item was to take almost 2 more years.
Much effort was expended in the early months on the electronic cir-
cuits activating the fuze and then on its mechanical switches and safety
mechanisms, since a serviceable fuze had to be so safe that anyone could
handle and even abuse it without danger.77 While the circuits and mecha-
iiisms proved out on the early models tested at low altitudes, dropping the box
fuzes from 10,000 feet and higher produced dismaying results. The higher
velocities in the drop set up vibrations that the radio tubes and other com-
ponents could not withstand.
In the next series of models, instead of the original shock mounting, all
components were made so stiff and rugged and mounted so rigidly that they
were capable of resisting the severest mechanical vibrations. Circuit ele-
ments were either immersed in wax or fixed to a frame and given a heavy
protective wax coating. As for the electronic tubes, small hearing aid tubes
offered the best solution to difficulties with the large and structurally weak
commercial tubes that had been used. Raytheon, Hygrade Sylvania, Gen-
eral Electric and others, already at work on this problem for the shell fuze
project, subsequently produced small, high quality, exceedingly rugged tubes
for both the shell and bomb fuzes.
A new method of arming the fuze, to improve its exploding time,
was introduced in February 1942. It consisted of a special type of circuit
that provided a delay in the charging time for producing the current that
set off the detonator. Another improvement eliminated use of the bomb
body as the antenna of the radio fuze, by building two bars into the fuze
itself, providing an antenna separate from the projectile.78 In May 1942, at
Hinman, p. 9; RP1723, "Radio proximity fuze design" (Hinman and Brunetti, 1946).
W. B. McLean and J. Rabinow were the chief designers of the switches and safety
mechanisms (NBS War Research, p. 20).
78 Hinman, pp. 11—13.