Ken's "Jet"

by Ken Sollars

My desire to build a small jet engine goes back a long way. Frank Whittle had success with Rovers' co-operation and thoughts turned to gas turbines, but miniaturisation with combustion chambers and heat withstanding materials, was a long way off Doodle Bugs came over and soon pulse jets were produced for models, but twenty-two inches seemed about the shortest length of resonant tube possible. My attempts with shorter eight inches or so produced some peculiar whining noises but would not run.

By the 1990's a clever engineer named Kurt Schreckling, was solving the miniature turbine problems. In 2001 I bought his book entitled “Gas Turbines for Model Aircraft", Issue No.29, November 1998. This started me thinking how I could build one with improvements, the compressor shown was made of wood! So the first part I made was one of dural, and aluminium welded. His turbine was made of a disc of 2.5mm. sheet 306 stainless steel with seventeen twisted blades. So I made one from incoloy 800 bar, machined from solid with nineteen blades.

I made the combustion chamber from stainless steel thicker than Kurt used but as I did not at this stage contemplate flying it, I was not concerned about weight. About this time I learned of the Gas Turbine Builders' Association and joined. I was rapidly gleaning information and realizing the pitfalls and probable mistakes I was making. I totally changed the combustion chamber fitting six combustor tubes from the rear end. As I was getting near the testing stage I realized the most important thing was to know the RPM because of the possibility of a runaway and the centrifugal forces involved, so the next thing was to build a very accurate rev. counter capable of reading to at least 100,000 RPM.

This seemed a bit of a joke as the first successful test achieved about 25,000 RPM. on butane gas. After weeks of modifications including increasing the number of combustor sticks to twelve, and dozens of needle size changes, with a propane/butane mix and the bottle turned upside down, feeding liquid gas, suddenly the note changed to a roar and it rapidly accelerated to 40,000 RPM, and would have gone much higher. However the compressor began to grow and would have completely broken up but for the fact of the close tolerance stopping it. Further mods made to the combustion chamber - I seemed to be going backwards - so a new chamber to a tried and tested design had to be made and a new compressor.

Finally running on a parafin petrol mix, I took my incoloy turbine wheel up to 90,000 RPM on about three runs but each time it was growing and binding, and had to be machined on the tips of the blades. At this stage I was lucky enough to acquire a cast inconel wheel which machined and balanced now runs to over 100,000 RPM, and gives about 10 lbs thrust on AVTUR jet fuel from Fairoaks Airport.

The bearings I am using are standard ball races (16 x 8 x 5) manufacturers rating to 43,000 RPM! with attention to oil supply, air cooling, and balancing are proving very satisfactory.

I would like to thank all those that have helped me achieve an ambition, in particular Bill Haylor of Copthorne, for Tig welding, Frank Bone for materials and Ced Tracey for jet needles, to name a few.

What's next?

Still working on a built-in electric start, then possibly an ECU.

Finally, has anybody got a drawing for a suitable Delta aircraft?