Roger Simmonds

Roger Simmonds, owner of Jetex.org

Mounting Rapiers in Models

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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: 

Corsair II 1 

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.

CorsairII Mount 

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:

MiG 19 1

But this is what happened on its first flight:

MiG 19 2

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:

Hunter damage

 

 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: 

exp 1 

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:

exp 2 

exp3

Above:  rolled tube and motor.

exp4 

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: 

exp5

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:

exp5A

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:

exp6

 

A case of, "Houston, we have a problem!

exp5B  

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:

The end

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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Comments

  • Roger Simmonds
    Roger Simmonds Sunday, 05 July 2015

    We had an interesting response from Rob McConaghy in the USA, who writes:

    "I read of your experiments with asbestos sleeves with interest. I worked designing high temperature heat exchangers for years and spent a lot of time finding suitable ceramics for various purposes.
    These are usually very expensive, but that was not an issue for us. Usually it took a lot of experimentation to find suitable materials- it’s hard to tell from a catalogue how smooth, or brittle, or machinable a material might be, or bendable or formable.

    One of the best materials I found for machining furnace parts was this stuff:
    http://www.zircarceramics.com/pages/rigidmaterials/specs/zal15.htm

    I have lots left over from these projects, and would be willing to try to machine a few cylinders for you. It’s good to 1600 C and has incredible resistance to thermal shock. It’s density is only .24g/cc, about a tenth that of asbestos. A cylinder 10mm ID x 15mm OD x 35 mm long (my guess at Rapier L1 sleeve dimensions) would weigh about 0.8 g. It’s a little brittle but may be OK. I think I could make them. I responded to this:
    " I would be very pleased if you could fashion some tubes for L-1 and L-2 motors, and I can send you the specs (in mm?) if you are agreeable.
    The ceramic tubes, which you say might be fragile; these can be strengthened with Nomex tape or laminated paper. Again, some experimentation will be called for."
    This brought a very generous response from Rob:

    "Great. I would like to try it- success not guaranteed. An overwrap of paper is a good idea. Yesterday I also ran across sodium silicate, or water glass, which sounds like an interesting adhesive/sealant. Take a look on Wiki. Possibly even a candidate for wrapping “fire proof” tubes, and you may be able to get some at the corner chemists. BTW, I think your asbestos paper has a lot of organic material in it.
    Yes, please send the tube dimensions, mm is fine. Would a 15mm OD be OK? I don’t want the wall too thin. Also, I’m not sure what to shoot for as an ID. Close sliding fit? But maybe the OD tolerance of the Rapiers is not that closely controlled. And, if there is a burn-through, it might be good if the pressurized hot gases had a place to go. It would be easy to create six or so narrow longitudinal grooves on the ID for pressure relief. Lots of questions and things to explore!"

    So, watch this space - we may open a topic on the forum about this.

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