Roger Simmonds, owner of Jetex.org
Mounting Rapiers in Modelsin Jetex
Generally, our 'one shot' Rapier motors are inserted into paper/cardboard tubes attached to our models.
These mounts are simple and effective, and can be commercially available, or 'own rolled'. In either case they have a wire retaining clip at one end and a balsa plug the other . These can be refined to resemble like 'weapons pods' beneath a profile model and really look quite authentic:
Above: a motor mount is ready to be glued to my profile Corsair II. Note the two short lengths of cocktail sticks to key the mount in the fuselage and ensure (as far as possible) that the mount doesn't come off in a hard arrival. I've also rounded the nose and applied a couple of coats of grey enamel paint.
Above: the latest Corsair II with finished mount and motor in place. Note the foil protection and the thin metal thrust tab behind the motor Rapier exhaust is HOT!.
Rapier motors are these days quite reliable, much better than they were a couple of years ago, when duff motors were not uncommon. Nevertheless, burn-throughs, blow outs, whatever you like to call them, can still happen.
Below is my MiG-19 for L-1. It's a fine flyer and I'm very fond of it:
But this is what happened on its first flight:
OK, not too serious, and profile models are much more robust than a proper scale model, but even so it is upsetting.
More serious is what happened to my Hawker Hunter a few years ago:
Rapiers are pretty good these days, but one would hesitate to put one of the older motors we all have at the bottom of our flight boxes (or given to us by a generous rocketeer who has (alas) 'moved on') into our latest superscale creation.
As can be seen in the case of the MiG-19 and Hunter, card/paper motor mounts give little protection and allow the hot exhaust to quickly attack a precious model. Built up models are, as can be seen with the Hunter, more easily damaged.
So, is there a better way? Time for some thought ...
I have in my possession some genuine Jetex asbestos paper from many years ago:
Above :Jetex asbestos paper and a commercially available mounting tube for an L-2. The L-2 shown is used, and though scorched, it did not, I repeat not, burn through in fliight.
I wondered if a motor mount fashioned from asbestos would be more robust. I glued light 'Bank' paper to the asbestos with PVA glue, and wrapped it around the cardboard mount:
Above: rolled tube and motor.
Above: Asbestos/card motor mounts (or tubes) cut to size after drying overnight.. There are two thicknesses of asbestos paper next to the cardboard. Note the balsa plug and wire clip.
I was quite pleased with these and the asbestos only added a gram or so.
I then mounted one of the new mounts in my test rig:
Note I drilled a small hole in the Rapier L-2about an inch below the nozzle to ensure a 'blow-out'.
Now for the crucial experiment. I ignited the motor:
Above: we have ignition! The motor(which went first go) is giving a healthy 13 grams (a little under 1/2 oz) of thrust. Looking good ....
The flame then reached the drilled hole:
A case of, "Houston, we have a problem!
Above: Through the smoke you can see that the thrust dropped immediately as the flame came through the motor case. After a further couple of seconds, the blow torch-like exhaust seared through the motor mount. Not quite what I was hoping for ...
I let everything cool down:
Above: Here is the motor and mount at the end of the experiment. Glad I hadn't tried it in a model!
I was disappointed the asbestos burned through so quickly, but this is an extreme case, and in real life a Rapier a 'burn-through' is never this bad.
So the results are not clear. I'm inclined to believe asbestos can offer your precious model some protection from a duff motor. Asbestos, which these days has a bad reputation, is probaby not now the best material to use. Thin metal sheet I'm not sure is the answer - perhaps there is modern plastic out there which is better. Nomex, perhaps?
I'm open to suggestions!
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