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It’s fifty five golden years since Jet-X first appeared (1947). Roy Lever has been an ardent fan for all of that time and has been the designer and manufacturer of the engines and the new-type fuel for the last twelve years. So here is his very original story on how it all started…..allegedly:

As a small boy, keen on anything that flew, I was somewhat amazed to read adverts in “Aeromodeller” magazine for “THE WORLDS SMALLEST JET ENGINE”. Designed and developed by the Wilmot Mansour Co. to fly the new sleek scale jet aeroplane kits, also made by Wilmot Mansour, as well as by other manufacturers of the day, scale models of this sort needed something more realistic than a prop stuck on the front. Hence the need for a realistic miniature jet engine was born in or around 1947. OR WAS IT? ……. After many years of discussions with people and companies “in the know”, a word here a sentence there, many many intriguing stories told in the UK, USA and by enthusiasts in Germany etc., I have put together this story.

From all this information I think the idea started late in the summer of 1939 when war was on the horizon and the Wilmot brothers in association with Joseph Mansour were busy making a living by inventing and selling very successfully wonderful model aeroplanes that really did fly well. They were sold through a company they had formed in 1931, The INTERNATIONAL MODEL AIRCRAFT CO. using the brand name FROG (Flies Right Off the Ground).

Now there is no doubt that they had the best and the fastest-flying rubber-powered aeroplane (the aluminium Interceptor, selling at ten shillings and six pence, achieved a distance of 300 ft at a height of 70ft and a scale speed of 236 mph) .Therefore it is not surprising that thoughts of what to do next led to Charles Wilmot and Joe Mansour deciding to put wings on a halfpenny rocket to make it go even faster. The results were not too exciting at first; the flights were extremely erratic, whilst the flame from the rocket presented a persistent threat of dangerous fire every time it crashed. How then to get thrust without flame? This is not an easy task by any means. Then “Eureka” – Joe Mansour had the idea of a compressed air-powered jet. This unfortunately didn’t seem to work too well (owing to Boyles Law, no doubt), so it seems the idea for a pressurised gas jet with no burning flame (Reaction Motor) was born. Experiments with all kinds of gas pressure systems followed in the next few weeks, some with unknown and rather dangerous explosive tendencies which demonstrated a precedent that still holds good today: that is that all chemical pyrotechnics are potentially dangerous and, unfortunately, in some respects not by any means an exact science. Warnings to the avid experimenter here – I am well aware of this rule from forty years of handling explosives for films, TV and model events and I have the scars to prove it.

So it was with this dangerous and erratic set of results that the chemical gas jet engine was evolved using gas-producing chemicals and a catalyst reactant, first as prototype made in cardboard (at least it did not set the tail plane on fire like the rocket did), then later in a thin Bakelite tube . Both brothers Wilmot and Joe Mansour knew exactly what they had to achieve because of their unique skills and experience in aerodynamics. But they did not have the know how of a pyrotechnical chemist. However they did know a man who might: one Captain E. Grattan Thomson, who had links to the armament industry and access through his friend Group Captain C.H. Keith to specialised armament units working for the defence of the UK . Even with this kind of help and advice it still took till midsummer 1940 to make a prototype working engine (Bakelite and a removable cardboard tube) with a fully workable gas generating powder rammed up the tube and ignited with a fuse. Some of the flights were so successful that the men from the Ministry heard about it and took a sudden interest in the experimental project, promptly producing a senior pyrochemist from the War Office, who saw the potential not in toy aeroplanes but in weapons of war.

The ability to be able to fly fast and silently with an explosive payload into hostile territory was of great interest and had never been used before.

At this point everything disappeared to some far-flung secret location. Over six months elapsed before reports appeared of a “six foot silent jet powered orange coloured plane” seen flying at the army range near Larkhill, launched from a steel ramp and powered by the mother of the “JET-X” engine. The fuel was a cordite-based concoction, as opposed to true Jet-X fuel which was nitrated guanidine. This “drone” weighed six pounds and the motor weight was two pounds with a maximum thrust of just under three pounds. Apparently it was so successful as a target and as a secret weapon that hundreds of thousands were built by the International Model Aircraft Co. This however is another story for another day – it might still be on the secret list (SSSSsssssssssh). 

Now to the nitty gritty of the latest engines and how to look after them. First the all- new 50 Z. This motor is a big improvement over the rest of the 50 series and boasts a much thicker and metal-reinforced sealing washer (NOT suitable for any other engine) and the latest-technology rocket nozzle, which tests have shown can increase the effect of thrust up to 25% depending on the model. The 50Z spec. is weight 5g length 44mm dia.18mm., takes up to four of the new Z-type (hole in centre) Jet-x pellets for approx. 25 sec. run (av. thrust 1.15 oz) but can be used with as little as two for test flying. Can be bought as motor only for �7.99 or in a nice new presentation box 50th. anniversary set with 36 pellets, 2yds fuse, five washers. and a gold-anodised 50Z motor. With full instructions the cost is �24. All spares available.

Next the all- new Jet-x 100Z ……. This new size motor has been in development for over a year and is the most powerful produced so far. Average thrust is 2oz and the motor spec. is weight 19g. length 63mm dia. 24mm, internal dia. 18.5 mm. Will take up to four pellets but can be test flown on two or three. Motor only costs �12.99 or the 50th anniversary set with 24 pellets, 2yds fuse and gold anodised Jet-x motor, all in a useful display box with full instructions costs �27.99. All available from your local model shop.

Now how do these motors differ from the run-of-the-mill Jet- x? First they are made from a lightweight high temperature silicone alloy, then deep anodised and hardened which helps to protect from corrosion and is much easier to clean. The new and vital thrust washer has been doubled in thickness and is now metal-reinforced.. The new rocket- type nozzle directs the gas thrust in a tight stream for much longer behind the model (2ft ) thus giving less pressure loss in the earlier part of the jet stream. This results in smoother flights for the scale modeller in particular. The fuel has changed also: it now has a hole in the middle to accommodate the fuse and has much improved smoother burn. A few important things to remember: (1) always spend a little time bedding down the sealing washer so it doesn’t leak: (2) it is vital to pull the fuse wire out when the motor has started but only with tweezers or pliers if you don’t want a burnt finger and thumb like us old jet fanatics (or, if you want to go a bit more upmarket, why not treat yourself to a Jet-X igniter pen for �24.99 – the 50th anniversary edition is gold plated!): (3) try not to let the fuel get damp; if it does, put it on top of a central heating radiator overnight: (4) keep your motor clean at all times – it will help to put a smear of oil inside before loading.

By the way, I almost forgot: Bob Davis has produced a new jet model for the 50Z called the Jetsetter. It’s 18″ span, 15″ long and the weight is only 32g. The kit is contest-grade balsa and costs �9.99 Also a nice little hand drill the correct size for the jet nozzle at only 99p, a must for your tool kit. Sams have produced a wonderful scale kit of the famous Komet i.e. the ME 163, one of the first rocket powered gliders from WW2 and it costs �13.99. Both of these models fly really well. So no excuses, now is the time to put away the half scale B17 and become a Jet-X pilot – it’ fun, it’s back to basics. All .the above can be purchased from Megamodels.

By Roy Lever
Good preparation is the only way to succeed with Jet-X.


This is very important and can mean the difference between a good flight and a frustration fizz. First make sure that the engine is spotless especially the jet hole, the body and particularly the thrust groove (Pic. 1). Now slide the fireproof sealing washer into place as in Pic. 2, then rotate and press the body hard on to the washer until a bright ring appears (Pic. 3). Look at this picture carefully: they tell the whole story of good preparation.

Now load three pellets into the motor, taking care to have the hole in the pellets facing upwards The hole can be enlarged by using the Jet-X drill (Pic. 4) and drill down to the next pellet. This gives an initial fast burn of the first pellet. Then insert about two inches of fusewick, as shown and feed through the jet hole. Close the engine making sure that the fuse is not disturbed, slide into the spring clip and you are ready to go. (Pic. 5)


Using a convenient source, light the fuse, let it burn and disappear inside the motor. Wait until a hissing sound is heard. Wet finger and thumb, take hold of the fuse at the end and carefully withdraw it
(Pic. 6). Do not snatch or it will leave the fuse inside and this will block the hole. If you are not an old Jet-X enthusiast, use a pair of pliers, again removing gently – don’t snatch it. (Pic. 7) Wait a few seconds until you see reasonable volume of gas and thrust, launch at at least 10-15 mph. This assists the venturi on both the 100 and 50Z size to build up turbulence on the motor. Another method of lighting is available as shown in Pic. 8, in the form of John Emmett’s Jet-X igniter. This costs �24.99 and avoids the use of fusewick. Care must be taken with this item, since it gets very hot but it does ensure that no fusewick blocks the jet hole. In the case of the 100 size the present model of Jet-X igniter is not satisfactory, owing to the length of the jet nozzle.


Burn times will always vary dependent on ambient temperature and humidity levels. Humidity in particular plays a very important role. Should it be high the pellets must be kept in a warm, dry place to get the best performance. Contest pellets have approximately in the 50 motor 21/2oz maximum thrust. This occurs approximately nine seconds into the flight. If the model is at around 50 feet when this occurs, it will soar to double that height. Maximum burn time for the 50 is 23 seconds. If four pellets are inserted, maximum thrust is the same but running time is approximately 29 seconds. The optimum number of pellets for the 100 size is three. Although it flies well on four pellets there is a greater risk that the thrust will exceed eight ounces. On three pellets, the maximum thrust is six ounces and the average is three ounces. Burn time is 22 seconds with three pellets. See chart below.

Jet-X MotorWeightWeight inc. 3 PelletsAv. ThrustMax ThrustAv. Burn Time
508 g10 g1.2 oz1.9 oz20 sec.
50Z10 g12 g1.6 oz2.5 oz20 sec.
100Z20 g24 g2.8 oz6.1 oz20 sec.
100ZMk.II18 g22 g2.8 oz6.1 oz20 sec.



By far the best material to clean the motor is paraffin. The thrust groove must be spotlessly clean in order to function correctly. Remove the thrust washer and immerse the whole engine in paraffin then clean with an old toothbrush and a thin screwdriver round the groove. It is a good idea to take a jam jar with paraffin to your flying field, so that you can immerse the whole engine after flying. This prevents corrosion in the case of the non-anodised engines and will lengthen their lives. Don’t forget to re-tighten the spring every time you change the thrust washer. The JET hole can be cleaned with a 1mm drill for the 50Z and a 1.5mm drill for the 100Z – DO NOT DRILL THE HOLE ANY LARGER.

***NOTE: Some of the 100Z (Gold) engines have the wrong size hole, please check it, it should measure 1.5mm NOT 1mm. Either return direct to Megamodels or drill out to correct size only.

In order to protect your model from the heat of the engine thin aluminium is the best, since it reflects as well as protects. Stick the aluminium onto the airframe in the vicinity of the jet hole. Be aware that on the 100 size the actual jetstream can still be hot three inches behind the motor. In the case of the 50 this reduces to 11/2 inches. Never, never paint the motor with anything, since it runs at temperatures which no paint – even heatproof – can stand. A good tip is to keep all your Jet-X in a small biscuit tin – this keeps it dry and stops it from decaying.


A number of kits are now available for Jet-X, both chuck gliders and scale – contact KeilKraft, Sams Models or Powermax for further details. Plans are also available from Keith Harris and David Boddington (see especially his new Vampire plan for the 100Z – I lost mine first flight).

Megamodels, Lancaster House, Bentinck Street, Farnworth, Bolton, Lancashire, BL4 7EP
Telephone: 01204 792921 Fax: 01204 792922 E-mail: [email protected]