Jetex/Rapier powered models

Many people of my age have fond memories of playing with Jetex motors when they were kids - I had several back in the late 60ís and early 70ís, and got some good flights using them. The motors are back on the market again now, though the fuel currently available is not as good as it used to be, having a different chemical formulation.

The motors are available in different sizes, but all consist of a metal body, with removable nozzle cap, into which fuel pellets are loaded. A fuse is placed in contact with the surface of the fuel and passed out through the small nozzle hole in the end cap. The end cap is held in place against a washer by a spring, so that in the event of pressure build-up getting too high, the cap can move and let the gas out at the side (better than an explosion). If all works well, you light the fuse (which has a wire core), the sparks enter the motor, the motor starts hissing, you pull out the fuse, and the burning combustion products jet out of the nozzle to give thrust. The current fuel can be tricky to light - I think the main problem is water absorption, so drying it before a flying session is a good idea. Some people use their microwave oven for this. The current fuel also spits and crackles a lot, and seems to give variable amounts of power during a flight - the old stuff used to just give a continuous hiss, and a very smooth power delivery.

A relative newcomer to the market is the Rapier range of motors, manufactured in the Czech Republic. These differ in being one-use only items, basically a cardboard tube filled with propellant and a ceramic nozzle at one end with a hole in it. They currently come in three sizes, the L1, L2 and L3. The L2 comes in three versions, the standard L2, the L2LT (long time) and L2HP (high power). There used to be two different bigger motors, called L3 and L4 which disappeared for a couple of years, but now new style L3 motors are available again (2005).

The fuses are included when you buy a box of motors, and unlike the Jetex fuse these have no wire core. Thus when you drop them in the motor hole and light them, there is nothing left once they have burnt. This is a definite advantage. The Rapiers almost always light first time, but in the rare occasions they do not, a small twist drill or jewellers screwdriver inserted in the hole and twiddled briefly to disturb the surface of the propellant will do the trick.

Choosing the correct model size for each motor depends on various factors, such as size, span, weight and the drag of a particular airframe. To further complicate matters, the thrust of the motors varies from batch to batch, although this is always given on the box, in mN (milliNewtons), together with a burn time. This varies from 16 to 26 seconds, depending on the batch, so quite a difference.

The thrust of an L1 motor can range from about 50 mN to 80 mN, with the higher powered units generally having a shorter run time.

L2 motors range from about 100 mN up to 140 mN, L2LT motors are lower powered (about 85 mN) but burn for up to 26 seconds and L2HP motors go from approximately 220 all the way up to a rather staggering 270 mN. The L2HP is marketed as a direct replacement for the Jetex 50 motor. In my opinion, however, many of the old kit designs for Jetex 50 power (such as the Keil Kraft scale range) do not actually need this much thrust, especially if built light.

The latest L3 motors have a substantial rating of 300 mN with a 20 second burn time.

The variation in thrust from batch to batch can lead to problems, in that a model designed to fly with a 110 mN L2 motor may well be underpowered with a 80 mN L2, but uncontrollably overpowered with a 140 mN L2. The best solution is to buy stocks of motors with different thrust levels as they appear, so you have a good range to choose from and experiment with.

To help you size your models, here are the power units that work best in some of my collection of Rapier powered models (all pictured elsewhere on this site). Weights quoted are with a loaded motor installed.

Saab J29, 12.5 inch span, weight 18.5 grams:

This works with any Rapier L1 between 50 and 75 mN. I tried it with an L1 rated at 80 mN but it it went into a series of uncontrollable loops - this was just a bit too much power.

Boeing 727, 13 inch span, 34 grams:

This needs a 115 mN rated L2 motor to get any altitude. With a 100 mN motor it does not get above head height.

Keil Kraft Cub, 20 inch span, weight about 20 grams:

The plan calls for a Jetex 50, but mine flew very well with a 105 mN rated L2 motor. Andy Blackwell flies his successfully with L2 LT motors rated at 85 mN

Douglas Skystreak, 11 inch span (but 16 inch length), weight 36 grams:

Goes fine with a 110 mN rated L2 motor.

Douglas Skyray, 11 inch span, 26 grams:

A 100 to 115 mN rated L2 motor will work fine with this model, which is small, but draggy.

Martin XB-51, 18.5 inch span, length 27 inches, weight 64 g:

Slightly overpowered with 2 x 100 mN rated L2 motors. Goes better with 2 x 80 mN rated L1 motors (which also saves on all up weight).

Skyleada Mystere 4, 15 inch span, weight 33 g:

Initially flown with a 100 mN rated L2, which had ample power, then tried with an L2LT rated at just 85 mN. Surprisingly this gave very satisfactory cruising flights, probably helped by the low wing loading of this aircraft.

Above: A fine collection of Rapier powered models seen at the 2000 Old Warden Vintage weekend. Top left is a Fairey Delta FD.1 for Rapier L2 by Pete Smart and bottom right is his Avro 707 scaled from the old Keil Kraft kit by about 85% to suit the smaller Rapier L1. Bottom left is a Fiat G80, scaled down slightly by Richard Crossley from the little known Keil Kraft kit to suit the Rapier L2.

Pete Smart built this large Vulcan and was flying it with Rapier L4 power at Old Warden - very impressive in the air.

Good First Scale Models:

I suppose the sensible approach would be to say build a quick and simple profile model first to get a bit of experience with trimming and using the motors, but what the heck, letís go straight for a proper scale job.

My top choice for a really good scale jet kit that flies well would have to be the Aerographics Bell XS-1.

Here they are the prototypes photographed together at the 2000 Peterborough Flying Aces meeting - the Orange one is by Richard Crossley (who designed it) and the white one was built by myself.

I had many excellent flights (30 or 40 I guess) with mine before the wing broke off just one time too often, and I have seen several people make their first successful jet flights with this kit. As long as you are comfortable covering a streamlined stringered fuselage, it is not difficult to build, and the instructions are comprehensive. It flies fine with almost any Rapier L2 unit, and it will even cope with an L2HP.

Here are two shots of Richard flying his Bell XS-1 at Old Warden. Above, the motor is lit, the thrust is building up and the launch is imminent. Below the model can be seen climbing away leaving its smoke trail.

Below is another photo of my white example, highlighting the attractive lines of the Bell X-1.

Another variation on the Bell X-1 theme is this X-1A, built by Richard Crossley. The nose is longer, and a new canopy is fitted. The model weighs 30 g and a Rapier L2 provides plenty of power. Even with a longer nose, some nose weight was still needed.

The model has proved to be an excellent flier, as can be seen in the lower photograph.

A more recent Aerographics kit that has proved an outstanding flyer is the Focke Wulf Flitzer. This goes very well on Rapier L2 power, and is very stable. My only reservation would be the slightly trickier build than the XS-1 due to having to line up the tail booms accurately. Here is a picture of Shris Strachan's example:

One of the all time classic Jetex scale designs was the Keil Kraft Mig 15, which was an excellent stable performer. David Boddington published a reduced size version (13 inch span) in Model Flyer magazine, March 2001 for Rapier L2 power.

Unfortunately the Keil Kraft range are no longer produced, but Easy Built kits still make a Mig 15, which I suspect may be derived from the Keil Kraft one. Could be worth considering, and should work OK with a higher rated Rapier L2.

The Aerographics Me 163 is a bit more challenging, but with careful trimming can be made to fly very well. It is also very strong, and bounces well! You are going to have less problems with an L2 in it than if you use the Jetex 50 recommended on the plan. One tip if you decide to make this kit, balance the model so the centre of gravity is further forwards than shown on the plan. It should be somewhere near the pilots bulkhead. When test giding it is vital that the glide be steep, with not a trace of float. Otherwise your Komet will loop the loop when you try a powered flight. You might also want to reduce the amount of reflex built into the wing (where it curves up at the trailing edge). The amount shown in the kit has proved to be a bit too much.

The Aerographics Lightning is another good flyer. This goes really well with a Rapier L2HP motor - very spectacular!

If you do not mind working from plans, you could do a lot worse than try my Saab J29 "Tunnan", which has proved to be a very stable and consistent flier. I am now building my third example! Span is 12.5", all up weight 18.5g, and it is powered by a Rapier L1. Click here for more details and a free plan download.

As added inspiration, here are a couple of shots of Peter Smart's first Folland Gnat model for L1 power.

The model flew with no dihedral, but was a bit twitchy laterally, so occasionally dropped a wing and dived in. Weight about 14 g unloaded and span 9". Gorgeous, eh?

Top 10 tips for Rapier/Jetex models

(with grateful thanks to Richard Crossley for his considerable input)


1. Mount the motor well forward, with the centre of the motor as least as far forwards as the centre of gravity of the model. Best results are often obtained with the motor in front of the C of G, making the model nose heavy while the power is on, but more floaty in the glide when the fuel is exhausted.

2. If using Rapiers, you need to make a paper tube to mount it in. You need a bit of clearance because the motors expand when used, so roll 2 layers of masking tape around the motor you are using as a former before wrapping the notepaper round it. I use notepaper and PVA glue to make my tubes, wrapping the paper round three times. If you only apply the glue after the first time round, you should be able to slide it off the motor OK afterwards. I cut a hard balsa disc to the inside diameter of the tube, and glue it just inside the end of the finished item. The motor will tend to fall out as you launch if you do not add a small wire spring clip to stop it sliding out of the back of the tube - this is easily made and added to the outside of the tube.

3. Before you add the nose block to your model, hollow out a space, and drill an access hole from underneath. You usually need to add plenty of noseweight later on, and it is useful to be able to hide it inside the model rather than sticking it onto the outside of the nose, which looks horrid!

4. The model is extremely likely to crash during the trimming process , as all testing has to be carried out at full power, and the most likely crash is a banking dive, with a wing hitting the ground. It is worth having a look at your wing/fuselage joins with a view to making these as strong as possible. Cross pieces may be useful in transmitting crash loads through the fuselage to the other wing, so saving damage to fuselage formers.

5. Line the motor trough with aluminium foil, but something thicker than the normal cooking variety - one easy method is to buy a roll of adhesive backed aluminium foil tape from your local car accessory shop - Halfords in the UK sells it as an aid to car repair. If you apply the foil to the trough in sections, start at the back and work towards the motor, so you do not get any steps facing the jet blast (stops the foil being lifted off).

This view of the Bell X1 shows how the motor trough has been completely lined with strips of self adhesive aluminium foil. The green end of the Rapier motor can just be seen poking out of the end of its mounting tube.

Trimming and Flying:

6. Aim for a dead straight glide, rather steeper than you would consider normal for a conventional free flight model. The model will probably turn one way or the other as the speed builds up under power, but letís keep it straight for as long as possible. A floaty glide is not a good idea, as the model will probably loop!

7. Add a small acetate (or similar material) trim tab to the trailing edge of one of the wings, near the tip, preferably on the wing on the outside of any turn. This can be bent up slightly to straighten the flight if necessary (less effective incidence on the wing, also more drag, both of which will tend to straighten things up.

8. Dethermaliser fuse makes a useful ingition source for lighting the motor fuse - slow burning, and no flames to accidentally set fire to your model!

9. Wait for the power to build up before launching the model, and give it a good shove in a horizontal direction - not a good idea to let it just drop delicately from your hand!

10. Wipe down the aluminium motor trough lining after each flight to stop layers of crud building up - if you donít, it will start to bake on, add weight, and be hard to remove (especially with Rapiers).

Above all, have fun - these models are very exciting to fly, and always draw a crowd. If you do pile a model in, the crashes are usually pretty spectacular too, and this will also be appreciated by watching spectators!

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