One of my hobbies is building and launch solid fuel rockets. They can be launched using fuse or an igniter of some sort. I just prefer a bare nichrome wire no larger than .0130". The wire is in a circuit and is touching the solid fuel at the rocket engine nozzle for ignition. I usually use my Ram 1500 auto battery and have ample power. But I don't want to be tied to my truck if I get a good field I cannot drive into. Batteries are bulky and unreliable unless you've got a motorcycle battery. And then you have maintenance, etc. I don't launch very often. My brother back in about 1970 helped me by putting together a circuit with a surplus army hand crank generator maybe 4" by 3" in size used by the signal corps and a capacitor (large) with a diode to indicate when the capacitor was fully charged. Actually, the igniter (nichrome) actually exploded more than glowed, but it worked. I gave all that stuff away to my nephews when I entered college. Like most hobbies I do this when I have nothing else I am involved with at any one time. I have a small generator that puts out 15 volts at a maximum of 1.5 Amps. So lets say about 12 to be comfortable, but could get to almost 15 volts if I really move. What do I need to complete the circuit including capacitor, diode, resistor(?) for me to "charge" the capacitor and maintain a 5 to 10 second red hot wire when I close my launch circuit. For me a nichrome wire at .0130" diameter 3" long between 400 and 600 degrees F. I thought this problem would be easier than it turned out. Even large capacitors just don't have the joules available to work. Voltage is very important as available energy is equal to 1/2*C*V*V. Even so my brother put together a unit from old used parts (capacitor rating unknown, but about 5" long and about 2" diameter) that worked. This is not an easy problem and I don't want to go into a trial and error method. That would be expensive. Any help would be appreciated. My background is not in science, but I have some knowledge, just not enough.

 

What is the resistance of the nichrome wire and how much current does it take to ignite the rocket?

 

Resistance from tables online (my multimeter is rudimentary) it is about 1.1 ohms for 3" of .013" diameter. And on Wikipedia under nichrome I found the amps needed as 1.75 or so. No voltage given for that current.

 

PDF reference, here

A small 12 V SLA should do it...

 

Simple power calcs says you only need about 2.5 watts at the igniter. Your generator produces much more wattage than that, but its voltage/current ratio is not right for a nichrome wire. So, you could use a Buck power converter at the base of your launcher to convert the 12-15 volts (low current) from your generator to 2-3 volts at high current to light your igniter.

A buck power converter changes high voltage, low current into low voltage, high current. In this power range they are also cheap and simple.

You would not need huge capacitors. The energy produced by just cranking the generator would light the igniter.

How does this sound to you?

 

www.gwiz-partners.com/igniters.pdf

A small 12 V SLA should do it...

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Trying again

Better reference

 

.......... I just prefer a bare nichrome wire no larger than .0130".

The OP wants to use nichrome wire instead of the more expensive higher voltage igniters. This is the original way model rocketeers lit their motors, other than fuses. The desired power source were Ni-cads. Ni-cads have very low internal resistance, making them perfect for nichrome igniters.

This is old school and I like old school.

 

The OP wants to use nichrome wire instead of the more expensive higher voltage igniters. This is the original way model rocketeers lit their motors, other than fuses. The desired power source were Ni-cads. Ni-cads have very low internal resistance, making them perfect for nichrome igniters.

This is old school and I like old school.

So how about charging some NiCds with the generator he has? You could charge them in series and discharge them in series-parallel.

 

A doubler or tripler circuit circuit should work for you if you have pulsed DC (AC) from the generator.

You might then be able to store enough energy in your final capacitor for the wire to flash.

 

Yes, your suggestion is simple and one I had not anticipated. Just crank the rocket off the pad!! I will look into the Buck converter and see what I may need. Thanks.

 

Thanks Crutschow! Excellent idea. No need to test the battery before going out and could be a very light setup. Only concern is the time to charge even though some charge would remain from last use. Also, this system would allow for me to be in control of power source and not the passive battery way.

 

Actually, I went to the bench and tried to set off an igniter directly off the generator and I got it to red hot on a .0085" wire!! I was working with a little capacitor (super cap 1F at 5.5v), but nothing. I had a broken patch lead before so I tried the capacitor again with a good circuit and still nothing. So now I have something to work with. I just have to see what 15' of lead wire will do since I don't want to be too close to the launch pad.

 

FWIW

3" of .008 glows red at 6 volts and 1.5 amps.

A 50,000 cap charged to 25 volts will heat wire red, but nothing I would count on for ignition.

I'll try some higher voltage caps when I get back.

It seems for reliable ignition, a heavier wire, stretched thin at the ignition point would keep the energy in the fuel.

That would cut the energy required by 1/2 or 1/4.

 

Twist the middle of your 3-inch section of nichrome around a thin (22 gauge) solid wire. Slide it off of the 22 gauge wire to get a lamp-filiment. Then Use some superglue to put some broken match-head sulfur in the wound lamp-filiment. Make sure each coil of nichrome is physically away from adjacent turns but still close. The superglue will help keep it that way. 4 or 5 turns is enough. Make sure that your form' (22-gauge wire is small enough to allow the wrapped nichrome to fit in the solid fuel rocket engine).

Use a small piece os parachoot wadding to hold the ignitor in place.

Two AA batteries will be enough to get the nichrome warm enough to set off a solid-fuel rocket. The wound filiment will heat especially hot like a lamp filiment.

Make 10 to 20 at the same time so you have a stash of them on launch day.

 

PDF reference, here

A small 12 V SLA should do it...

Agree...
I made my own igniter, I've used 12V SLA batts down to 1.4Ah - they have plenty of power to light igniters....

 

This post brings back memories of the launch of a 6 ft V2 shaped rocket powered by 10 sky rockets & catapult. I made igniters from 1 in strips of Ni Cr wite, 16 ga jumpers & 2 circles of 14 ga wire, all in parallel sucesspully fired from a 6 V car battery. Was suposed to have been tethered, one line broke and rocket made multiple circular passes over , or near, a crowd at a college fair-- I was allowed to graduate, 1950.
Couple of years later designed, and used, synchronized blasters for seismograph oil exp. co. Caps fired on 1/10 sec line on chart. Used 300 V dry cells on capacitive discharge via SCR.