Has anyone tried the smoke pellets used for testing flues in gas installations? The PH type is a low temperature gas producing pellet with a formulation that comprises guanidine nitrate and potassium perchlorate. They seem to produce plenty of gas at about the right rate. The 13g pellets claim 18m^3 over 30 seconds. 13g is a bit big for a Jetex motor but something custom is not hard to make and smaller pellets are available. The AX type seems too slow burning. Here are the pellets:
many thanks for this and the link. As to whether these are a viable alternative to a genuine Jetex pellet - there is only one way to find out, which is to try one. A pellet could be cut down, I suppose, to fit a Jetex 50C, or would one fit - say - a PAA loader?
The original Jetex formulation was very carefully developed and I somehow doubt these are as meticulously produced.
I doubt the manufacturers would be interested in any proposal to change their production line.
Sorry to be somewhat negative, but whilst genuine Jetex pellets are readily available on eBay, I would be disinclined to put these in a motor!
What we do lack is a viable (modern) source of fuse - any ideas?
I have some on order. Tempted to try them in my old PAA loader although I think the 5gm pellets may be close to 50 size.
As for ignition, I have experimented with electrical ignition with reasonable success using Sebel fuel, although the most successful version was a faff to use since it involved a resitance wire (nichrome) making good contact with part of the case internally, with the case connected to one terminal and then passing a small insulated wire (standard enamelled copper) through the nozzle to meet the resistance wire internally to close the circuit. Only advantage seems to be that I don't need fuse and the enamelled wire will fit through the small nozzle. When it works ignition is obvious but it was a fiddle to get decent contact in the first place.
Where I want to go with the smoke pellets (which will be made to a very consistent spec for the usual product certification rule reasons) is a custom motor with a larger nozzle if possible. The 13g pellets are getting towards an inch in diameter (don't have any exact measurements yet). As they appear to leave a bit of solid residue after burning a tiny nozzle might not be a happy experience so these may be right for bigger motors/models. Electrical ignition will be easy at larger sizes but, of course, we shall see when I get a chance to try them. The basic Jetex concept of a spring-closed motor body was brilliant in its simplicity and safety and if these things work there may be a whole new development direction to take. I'll let you know how it goes.
I like your the electrical ignition system. As to your experiments with the smoke pellets, note these are not designed to be fired in a semi-sealed metal body, and the pressure will affect the rate of combustion. My advice as a Ph.D physical chemist is not (repeat, not) to do this unless you have access to a fume cupboard (the formulation contains ammonium chloride and the fumes are very nasty).
So please stand well back and wear safety glasses!
The amount of ammonium chloride is low (10mg/m^3 in the smoke emitted), also I have no intention of using these indoors although that is exactly how plumbers use them for testing flues and internal flows. They claim low toxicity but I would not think deliberate inhalation was a particularly smart move.
I am well aware of accelerated combustion under higher pressures (basic thermodynamics in a gas producing exothermic reaction) and so would never burn these in an unfused closed container. The Jetex system was very clever in that it makes the entire end cap into a safety valve retained by a light (for the area concerned) spring and significantly limits pressure. I am extremely wary of stability of combustion (being an ageing Aero-Engineer still with a full complement of limbs and fingers). I'll let you know how it goes.
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