# Contrail Hybrid ignition

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#### monkey_bolt

##### Active Member
Hi all,

Not sure if this is the correct place for this thread but here goes....
I've seen on the Contrail hybrid page that they use a Pyrodex pellet and a resistor to ignite there motors. I can't find any info into what type or value of resistor they use. Does anyone use this method of ignition.

I currently use PIC and a Davey fire, although this looks like a cheaper alternative.

Many thanks

Mike

#### ttabbal

##### Well-Known Member
The resistor requires 24V and a lot of amps to heat it up enough to light things. I've heard of people using diodes that are a little cheaper and ignite with 12V systems in common use.

I just dip my own ignitors and ematches. Cheap, reliable.

#### rsynoski

##### Well-Known Member
I don't have one of the resistor igniters handy, but I believe that they are 100 ohm. They do require 24 volts. My launch system uses a pair of small SLA batteries, and it works just fine.

Contrail is testing a new 12v igniter, I've tried a couple and they work well. I believe that Contrail's intent is to discontinue the resistor type igniter.

#### monkey_bolt

##### Active Member
Many thanks for the replys, unfortuantly i'm limited to a 12v launch gear so I guess this method is out. I would love to make my own ignitors, but the kits available are not allowed in the UK.

Cheers

Mike

#### davel

##### Well-Known Member
I don't have one of the resistor igniters handy, but I believe that they are 100 ohm. They do require 24 volts. My launch system uses a pair of small SLA batteries, and it works just fine.

Contrail is testing a new 12v igniter, I've tried a couple and they work well. I believe that Contrail's intent is to discontinue the resistor type igniter.
P = E^2/R. So we have 576/100 or 5.76 watts.

To get that same power with a 12 volt system would require 144/5.76 or a 25 ohm resistor. Don't know why it wouldn't work......

#### bobkrech

##### Well-Known Member
I'm guessing a 100 ohm resistor would be a bit high. Typical igniters have resistances from ~1 ohms to ~3 ohms.

You can tell the resistor code from the color band code. 100 ohms is brown-black-brown. 10 ohms is brown-black-black. As I look at the photos in the Contrails manual, it looks like brown-black-black which is 10 ohms which is more reasonable than 100 ohms, but I can't tell if it's a 1/4 or 1/2 watt resistor. Hears a neat online resistor calculator.

http://www.csgnetwork.com/resistcolcalc.html

A conventional igniter with ~1.6 ohms resistance operated at 12 volts develops P = (12^2)/1.6 = 90 watts and draws I = 12/1.6 = 7.5 amps.

A 10 ohm resistor igniter operated at 24 volts develops P = (24^2)/10 = 57.6 watts and draws I = 24/10 = 2.4 amps.

Bob

##### Well-Known Member
A conventional igniter with ~1.6 ohms resistance operated at 12 volts develops P = (12^2)/1.6 = 90 watts and draws I = 12/1.6 = 7.5 amps.

A 10 ohm resistor igniter operated at 24 volts develops P = (24^2)/10 = 57.6 watts and draws I = 24/10 = 2.4 amps.

Bob
Is there any technical reason that same job can't be done with 12V and a different resistor?

I emailed Contrail about that once but never did get an answer. I'd try one of their motors but I've got enough gse to drag out to the pad without adding another battery.

#### Wingarcher

##### Well-Known Member
TRF Supporter
In my hands on, practical experience, a 1/4 watt 10ohm resistor shorted across a 7Ah 12V gel-cell will burst into flame pretty convincingly after about 2 seconds. You have to get the right sort of resistor- the "flame proof" are right out.

A 1/8 watt resistor of the same value will go a lot faster, but doesn't produce as much flame as it's about 1/2 the size.

The 1/4 watt size will ignite pyrodex pellets if it's taped carefully to the side but there is a slight delay.

I'd use two regular igniters- dipped wirewrap sort or an igniter and ematch. Why waste nitrous with a failed ignition?

N

#### bobkrech

##### Well-Known Member
Is there any technical reason that same job can't be done with 12V and a different resistor?

I emailed Contrail about that once but never did get an answer. I'd try one of their motors but I've got enough gse to drag out to the pad without adding another battery.
I don't know what type of resistor is being used: wire wound, metal film, metal oxide, carbon or carbon film, but there's no good technical reason why you couldn't use a 1-2 ohm 1/8-1/2 watt resistor on 12 volts to ignite the Pyrodex(R).

In my hands on, practical experience, a 1/4 watt 10 ohm resistor shorted across a 7Ah 12V gel-cell will burst into flame pretty convincingly after about 2 seconds. You have to get the right sort of resistor- the "flame proof" are right out.

A 1/8 watt resistor of the same value will go a lot faster, but doesn't produce as much flame as it's about 1/2 the size.

The 1/4 watt size will ignite pyrodex pellets if it's taped carefully to the side but there is a slight delay.

I'd use two regular igniters-dipped wirewrap sort or an igniter and ematch. Why waste nitrous with a failed ignition?

N
Contrails has chosen the resistor/Pyrodex(R) ignition route to avoid BATFE issues, however I don't consider a 10 ohm resistor that takes ~2 seconds to burn a suitable e-match replacement. E-matches actuate within a few milliseconds of current application, and an uninhibited Pyrodex(R) pellet will burn for 30-35 milliseconds, and a bore inhibited Pyrodex pellet will burn for 60-70 milliseconds, and is the preferred embodiment IMO.

Pyrodex ignites at ~600F. IMO you want your Pyrodex(R) pellet heater to reach >> than 600F in < 0.1 seconds. For this you need power!

A 1/4 watt resistor might have 0.04 grams of mass. If we estimate the heat capacity is ~1 J/gram, the heat capacity of the resistor is ~25 C/J assuming no losses and a close estimate of the real heat capacity. A heating rate of 14 watts from a 12 volt battery would raise the temperature ~350 C/s (~630F/s) (the minimum ignition temperature of Pyrodex(R)), or to ~700C (~1300 F) in two seconds which is where I would expect a resistor to flame. With the recommended 24 volt battery, these times are reduced by at least a factor of 4.

If the more standard 1.6 ohm resistance valve was chosen at 12 volts, the heating rate is 90 watts, and the times to temperature would be reduced by more than a factor of 6. The time to obtain pyrodex ignition would be ~0.15 seconds and the resistor flame time would be ~0.3 seconds. This latter resistor value is preferred.

Not all resistor types are readily available or cheap (both are necessary) in 1-2 ohm values, but 100 resistors will cost less than $10. Chose wisely and you have a nice cheap ignition system. Bob #### rsynoski ##### Well-Known Member In my hands on, practical experience, a 1/4 watt 10ohm resistor shorted across a 7Ah 12V gel-cell will burst into flame pretty convincingly after about 2 seconds. You have to get the right sort of resistor- the "flame proof" are right out. A 1/8 watt resistor of the same value will go a lot faster, but doesn't produce as much flame as it's about 1/2 the size. The 1/4 watt size will ignite pyrodex pellets if it's taped carefully to the side but there is a slight delay. I'd use two regular igniters- dipped wirewrap sort or an igniter and ematch. Why waste nitrous with a failed ignition? N The only time I've seen a Contrail fail to ignite is when a dipped igniter was used instead of the resistor or an e-match. The extra heat of the igniter burned through the fill tube too early, resulting in a dump of nitrous instead of ignition. I have had 100% success rate with the resistor or an e-match #### FROB ##### Well-Known Member <snip>... Not all resistor types are readily available or cheap (both are necessary) in 1-2 ohm values, but 100 resistors will cost less than$10. Chose wisely and you have a nice cheap ignition system.
Bob
Thats exactly why i picked up a pack of 100 ea. of 1 ohm and 2.4 ohm resistors (they had nothing in between) at a local surplus store last summer for \$1.99 ea. Haven't have a chance to try my contrail motor yet tho- hoping to first spring launch.

#### init 6

##### Well-Known Member
unfortuantly i'm limited to a 12v launch gear
Just as an aside, why are you limited to 12v? What launch gear are you using? Your own or someone elses?

#### bobkrech

##### Well-Known Member
The only time I've seen a Contrail fail to ignite is when a dipped igniter was used instead of the resistor or an e-match. The extra heat of the igniter burned through the fill tube too early, resulting in a dump of nitrous instead of ignition. I have had 100% success rate with the resistor or an e-match
Robert

AFAIK, the heat from relatively long burning Pyrodex(R) pellet combustion, and not the short burning igniter, is supposed to melt the fill tube and light the Contrails motor. It sounds like the ignition failure you witnessed with the dipped igniter was probably due to improper placement of the igniter with respect to the fill tube, rather than the type of igniter used.

Bob

#### rsynoski

##### Well-Known Member
Robert

AFAIK, the heat from relatively long burning Pyrodex(R) pellet combustion, and not the short burning igniter, is supposed to melt the fill tube and light the Contrails motor. It sounds like the ignition failure you witnessed with the dipped igniter was probably due to improper placement of the igniter with respect to the fill tube, rather than the type of igniter used.

Bob
Igniter positioning could have been a factor. Since it can be a little tricky getting the pellet, igniter and fill tube in the correct orientation I would have to believe that using a lower energy igniter would give you a wider margin of error.