The Jetex engine

In both the S.I.G. interview and the FAB interview, Derek Meddings makes reference to 'Jetex motors'. Since I was unaware of this technology before encountering the previously mentioned references, I turned to the Net for enlightenment and found the following. Wikipedia mentions that "The Jetex motor is a type of solid-fuel rocket motor produced for use as a powerplant for model aircraft."

Jetex 50 motor in original packaging
Jetex 50 motor in original packaging

It goes on to say that "[It was] originally developed in 1947, by Wilmot, Mansour & Company Ltd. of Southampton, it was first demonstrated to the modelling press in early 1948, and was available to the public in June 1948, when Aeromodeller featured Jetex power on its front cover. The first motors were the Jetex 100 and 200, with the more powerful Jetex 350 following in November 1948. The most popular motor, the Jetex 50, was introduced in May 1949, along with kits for a model plane and model car using Jetex power.

Jetex motors are powered by a solid pellet of guanidine nitrate, which burns to release a variety of gases in copious volume, leaving no solid residue or ash. Thrust developed is fairly modest, suitable for horizontally launched flying models rather than vertically launched rockets. The exhaust gas is not excessively hot, which confers a safety advantage. Fuel and wick to ignite the pellets was manufactured by Imperial Chemical Industries (ICI). The engine casing was made of an aluminium alloy and was reusable, new fuel pellets and ignition wick being a consumable that could be bought and used in the engine."

Since the motors release 'a variety of gases in copious volume', developed a 'fairly modest' thrust and the exhaust gas 'is not excessively hot', they seemed eminently suited for Meddings' purpose of disturbing the Fuller's Earth his model sets were sprinkled with. I would say that Meddings used the Jetex 50 motor for his purposes: it is small and was the most popular model which probably means it was widely available.

Another source of information both Wikipedia and Google pointed me to was the website which is where I found this cutaway drawing and explanation of a Jetex 200 motor.

cutaway drawing
Move the pointer over the numbers in the drawing to access the legend
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Safety clip spring
Threaded hole for attachment bolt
Spring plate
Wire clip
Solid fuel charge
Gauze disc
Combustion chamber
Igniter wick
Jet orifice
Back plate
Asbestos washer
End cap
cutaway drawing of a Jetex 200 from the instruction sheet, Wilmot, Mansour and Co. Ltd., 1948
The burning of the solid fuel charge (5), once ignited by the wick (8), generates a large quantity of gas in the combustion chamber (7). This is forced out of the jet orifice (9) at great speed, producing (by Newton's third law of motion) a reaction which drives the motor in the opposite direction. The springs (1) are intended to hold the end cap (12) tightly seated on the asbestos washer (11) so as to seal the combustion chamber (7). They are seated on the spring plate (3) and act through the clips (4) to provide a safety release mechanism in the event of the orifice becoming clogged. The gauze disc (6) serves to hold the coiled wick tightly against the fuel charge, while the back plate (10) protects the washer.