Model rocket manufacturer Estes Industries sold a radio-based locating device called Transroc that fit inside the main body of a model rocket or model airplane.

Transroc Cover
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From the manual:

Section B. Modes of Operation

The TRANSROC has three basic modes of operation one of which, TM (telemetering), can be further broken down and expanded into several subdivisions. This versatility offers the ESTES rocketeer many hours of educational enjoyment and a wide field of new horizons in model rocketry.

B.1 Rocket-Finder Mode

The most basic mode of operation for the TRANSROC is as a rocket finder. The basic kit is supplied complete with all components needed for operatiom in this mode. The only accessory needed for rocket finding is a good receiver.

In the rocket-finder mode, the transmitter is pulsed on approximately once per second for approximately one-fifth second and is off during the remainder of the cycle. This provides a beacon signal similar to the ones used in space capsules and survival equipment for locating astronauts and fliers after they lower by parachute into the ocean. This "cw" (continuous wave) type signal provides excellent penetration in the presence of severe interference. The rocket-finder signal from the TRANSROC mixes with any other signals which are being received to produce an easily distinguishable audio beat note in the receiver's speaker.

Depending upon the level of interference present and the sensitivity of the receiver used, it is possible to receive the TRANSROC in the rocket-finder mode at distances of up to five miles or more during flight and up to a few hundred yards after the rocket has landed. Field tests have shown battery life up to 24 hours or more.

B.2 Telemetering

The telemetering modes are similar to the rocket-finding mode in that the transmitter is turned on and off in sequence to convey information. The cycle is much shorter for TM, however. The time during which the transmitter is off during each cycle is controller by the appropriate transducer such as a photocell, thermistor, etc.

Current consumption is only one-third greater in the telemetering modes than for rocket finding, but useful battery life is shortened more than this. The shorter usable battery life is mostly due to the fact that a much stronger signal is needed for receiving good TM than is needed for rocket finding.

B.3 Microphone Transmitter

The microphone transmitter mode is significantly different than the rocket-finder or telemetering modes in that insteawd of the transmitter output being turned on and off intermittently, its amplitude (strength) is varied in proportion to the sound striking the microphone. It thus transmits in the same manner as an AM (amplitude modulated) radio station or walkie-talkie. This signal can be tape recorded as it comes from the receiver. The tape recording thus obtained will include your voice during countdown, engine burning noises, and a variety of sounds caused by the wind rushing by, the ejection charge, and the rubbing between the rocket body and the parachute shroud lines, etc., after ejection.

Battery life is shortest in the microphone mode due to higher current drain and a higher minimum usable battery voltage. A new battery is adequate for six to ten flights if recovered and turned off after a few minutes each time. THe battery can then be used for several TM flights followed by several rocket-finder flights.


The following article appeared in the May 1972 issue of Popular Electronics:



Recently we had the opportunity to build and test the "Transroc" model aircraft and rocket transmitter put out by Estes Industries. Since it was designed to be used in conjunction with a conventional Citizens Band receiver, we were surprised at the compact size of the six-transistor, crystal-controlled transmitter.

The Transroc is no simple little toy meant to be used by aspiring young scientists. In fact, the transmitter did things that surprised even old model rocketry veterans like us. When properly set up, the Transroc can provide any one of three different modes of operation: first, it is a "beeper" which transmits one beep per second on the CB band - a signal used to locate a downed model aircraft or rocket. Second, it can be used for telemetry by employing some form of signal-to-resistance sensor (photocell, etc.). Finally, used with a microphone, it becomes an under-100-mW AM CB phone rig.

Circuit Design. To give you an idea of the sophisticated circuit design used in the Transroc, the schematic diagram shows a portion of the modulator - in this configuration, a beeper. For the analysis, start at the collector of Q2 and assume that this transistor is cut off. In this state, the diode gate consisting of D3 and D4 is switched on by the current through D2, R3 and R4. The junction of D3 and D4 is supplied with charging current through R5. This current builds up a voltage across C1 and is also supplied to transistor switch Q1 as one of its inputs. The other input to Q1 is via voltage divider R1/R2.


When the anode voltage of Q1 reaches the level of its gate voltage, Q1 conducts and C1 begins to discharge through the transistor, the base-emitter junction of Q2, and current-limiting resistor R6. The flow of current turns on Q2, and the voltage drop at the transistor's collector cuts off the D3/D4 gate. This stops the charging current to C1. After a time period approximately one-half as long as was required to charge C1, Q1 automatically resets to the nonconducting state cutting off Q2 and allowing the process to repeat as long as power is applied.

The Transroc was tested (in the beeper mode) in a working model aircraft using a conventional CB receiver with a loop antenna. There was no trouble locating the downed aircraft almost a mile away. We listened to similar units operating in the telemetry mode and were surprised at the excellent signals.

Price of the Transroc is $21.95 assembled or $14.95 in kit form.

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Last updated September 30, 2016