TOPIC: Lighting Rapiers on the Flying Field

I did some experiments this morning with David's igniter and a number of TSP L-2 motors. I too, could not light a motor with David's igniter 'as sent'. In a way, this was pleasing, because it meant I could track down any problems and not just say, 'Well it works for me' . There was, indeed a puzzle here, as it should work!

I eventually got David's 'Lugeresque' igniter working, and ignited both TSP L-2 (150 mN thrust) and TSP L-2 (300 mN) motors on my thrust rig. I tested the latter 'as manufactured (1.4 mm nozzle diameter) and after being bored out to 1.7 mm. I was pleased their performance was 'nominal' and well in line what Terry reported last autumn.

These are the modifications I made to David's igniter:

1. David had fitted a 30 SWG (0.32 mm diameter) Nichrome element. Especially when heated, this tended to buckle when pushed in the orifice and contact with propellant was not good. Where the element was in contact with the propellant, the thin wire didn't retain its heat, but rapidly cooled.
I therefore changed the element to 28 SWG (0.42 mm diameter) which retained its shape even when glowing, provided (importantly) it was not too hot.

2. I adjusted the 'off load' voltage' to around 4V, this gave a nice glow,and the element was robust enough to be thrust through the motor oifice and make good contact with abraded propellant. Motors ignited within 2 seconds or so.

3. In my experience the Nichrome elements need quite frequent replacement, so to make this more convenient I fitted the modified end of a 'Cord'Cord Zap. This worked well:



All this was quite encouraging: bore out the motor, 4V with a 28 SWG element (made thin enough to go up the spout!) and ignition was as expected.
I will send David some 28 SWG Nichrome wire for him to play with ( I trust he had a selection of jewellers screwdrivers to bore out the nozzles and abrade the propellant) and a few fuses. These also ignited the motors, which, so far, appear to store well.


One final thought: with all those volts (and thus amps) available, 26SWG Nichrome wire may be better than 28SWG.

So there you have it - problem solved. What's next?

Just for the record I'm using 0.5mm nichrome in my, "Eddy Grundy Special" igniter.

Roger adds:

Ah, this explains its robustness - 26 SWG Nichrome wire may indeed be better than 28 SWG - worth a try in David's igniter gun!

Hi, I mentioned in my email to Roger that when making the original version, I had accidentally ordered 30 SWG nichrome, rather than 28SWG - and yes, the AWG is rather thinner. Thanks for the offer but I do now have some 28SWG wire - I have tried this with no success, but will give it another go, running hotter than before. I might even get some 26SWG in to try.

aha- quick furtle in the 'bits' box, and I do indeed have some 0.45mm 26SWG nichrome - will give that a go ...

Hi David,

this must be very frustrating for you!:

I have now tried 26 SWG (0.48 mm) Nichrome. I turned up the (unloaded) voltage to 4.5 volts and got a nice healthy glow which ignited a motor pretty well instantly.
So, something else for you to try!

Though the new TSP L-2 motors light quite easily with fuses, the TSP L-1 motors where the nozzle diameter is such that the fuse is a (very) tight fit and the propellant is packed in up to the end plug, were trickier, we found. The nozzle can be reamed or drilled out with a screwdriver or drill bit of 1.2mm (not more) but this did result in a drop of thrust, and my Jetex Wren only cruised around Old Warden. True, for a nice long time, but I missed seeing it high up.

So these definitely benefitted from being ignited electrically.
When I was selling modified Cord Zaps I didn't stress enough that good (read pricier then yer average) AA batteries were needed like Duracell 'Ultra'. Inferior brands scarcely heat up the 28 SWG element to skin burning temperatures (I know, I've tried it!)

David Lane, whose 'Lugeresque' 6- NiMH AA igniter did such sterling work at Peterboro Flying Aces and September's Old Warden, thinks Duracell batteries inadequate to the current an element draws, citing their high internal resistance, and recommended I investigated rhe use of. Rechargeable NiMH batteries, Which I did:



Above: my rather crude (though effective) four cell igniter. With these Duracell batteries it needed to be 'rested' after continuous use. This, says David, is due to polarisation (which NiMH cells suffer from far less).



Above: the same igniter with four 2800 mAH rechargeable cells.

In summary, comparing batteries in 2, 3 and 4 cell igniters, I get the following results:



The voltage drop under load reflects the internal resistance of the batteries and the current drawn. The NiMH batteries obviously are delivering more amps, but the difference between the two types of battery are not as great as I expected. David says that perhaps I bought the wrong NiMH batteries, in that these are more suited to toys and torches. The ones we need for our igniters are those sold by hobby outlets for electric cars and boats. I will repeat the experiments when these arrive and are charged up.

Meanwhile, David is working oh a new version of his igniter, perhaps with fewer cells that will fit in the handle.

So: watch this space