I had an interesting communication from John Emmet, who is looking to make an igniter for TSP motors and old (and sometimes hard to start) Rapiers:
Above: John's well regarded and still-available. Jetex igniter.
John writes
"1. It looks like the electric igniter problems that people are having are down to low thermal energy rather than low temperature. Unfortunately, there are conflicting requirements here, as you need good thermal conductivity down the probe in order to get the heat into the fuel. Unfortunately high electrically conductivity is needed for the batteries sake, and that comes with poor thermal conductivity. Copper has a thermal conductivity 20 times that of (say) Kanthal heater wire, that is why the old coaxial igniters had a copper core, but with 3 volt batteries you need at least 5 x that resistance.
2. The good news comes from the below longitudinal sections of a TSP L-1 and a Rapier;
Here you can see that both have a 1.4mm diameter nozzle, which gives us twice the area of any suitable Jetex probe. Also any TSP/Rapier probe only needs about 7mm depth reach, as apposed to 1mm diameter with a 20mm reach needed for a Jetex 50C.
3. A most professional touch in the nozzle of the TSP is the narrow layer of (probably) black powder as an ignition booster. You can see its effect in the start up sparks, as well as the thrust peak.
I do wonder if an electric igniter is really essential if the fuses turn out to have no problems associated with them? Of course any open flame close to tissue/dope is bound to be dodgy, so I will be going ahead anyway."
I replied:
" many thanks for this, it's good stuff. You have a good point about temperature vs. thermal conductivity : A 30 SWG Nichrome element gives a healthy orange/yellow glow in a Cord Zap with two quality AA batteries, but (in my experience) hopeless at igniting the Rapier propellant. 28 SWG is a good compromise. 26 SWG is better but (a) you need more batteries, (b) the Rapier nozzle is so narrow it's difficult to get the heating element down the 'ole.The Vintage Model Company) have a 2 mm plus nozzle diameter so 26 or even 24 SWG Nichrome would work with these.
That TSP motors have a black powder as an ignition booster. This reflects Piotr's experience with motors for 'ballistic' model rockets. I'm not sure they are such a good idea for our sorts of models though. Troughs will need to give your Hawker Hunter adequate protection. When I first ignited a TSP CE motor the initial squib-like exhaust made me jump! Scary!"
Inspired by john, and blow the expense, I then cut a CE TSP motor in half:
Note the fuse fits the nozzle 'only just'.
Note the different colour (pink) of the CE propellant from the earlier motors black). Note also the black powder ignition booster.
John then asked, "Is an electric igniter really essential if the fuses turn out to have no problems associated with them? Of course any open flame close to tissue/dope is bound to be dodgy, so I will be going ahead anyway."
I answered,
"I didn't find poking the CE green fuse in the nozzle and getting a good contact that easy, So yes, we do need one"!
I look forward to what John, a very clever engineer, comes up with.