Igniters

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Ejection Charge Igniters

Rockets that use electronic altimeters to deploy the parachute need igniters to ignite the black powder ejection charge. The igniters used for this purpose need to be relatively low current so that the electronics can readily fire them. Furthermore, the igniters themselves need to be as reliable as possible to ensure they will function properly.

Daveyfire Inc. makes a broad line of low current igniters. A few specific types have been adopted for use by the high power rocketry community. These are the N28F, N28B and N28BR. Their properties are listed in the table below. More information can be found here at the Daveyfire web site.

The photo below shows four different types of igniters. I believe three of them are Daveyfire units. The ones with black, orange and yellow wires. The one with red/blue wires is an Oxral igniter and has properties similar to the Daveyfire units.

Four different types of ejection charge igniters.

Click on this photo to see a closer view of these igniters.

Black is a Daveyfire N28F.

Orange is a HiRMI-Standard. (Probably an N28F)

Yellow is a HiRMI-Sensitive. (Probably an N28B)

Red/Blue is an Oxral igniter.

Model	Resistance (Ohms)	Max No Fire Current (Amps)	Min All Fire Current (Amps)	Ave. Firing Time	Pyrogen (grams)	Wire Color
N28F	1.6 +/- 0.3	0.40	1.00	40ms at 2 amp		black
N28B	1.6 +/- 0.3	0.20	0.37	2ms at 1 amp	0.16	white
N28BR	1.6 +/- 0.3	0.20	0.37	2ms at 1 amp	0.48	orange
HiRMI Std	~ 2	0.40	1.00			orange
HiRMI Sen	~ 2	0.10	0.40			yellow
Oxral	~ 2	0.20	0.50			red/blue

The HiRMI (High Reliability Electric Match Igniter) is branded and sold by Blacksky. Consequently I don't know for sure if they are manufactured by Daveyfire, but they certainly look like they might be.

The N28B and N28BR are identical units except that the N28BR has three times as much pyrogen on the match head. They can be purchased from Public Missiles.

The Oxral igniters seem to be basically equivalent to the Daveyfire units except that the leads are separate wires (one red and one blue). The separate wires becomes a problem when you install more than one igniter in the rocket and run all the leads together to the altimeter connector. You have to make sure to pair up the correct red wire with the correct blue wire! The Daveyfire wires are a form of "zip cord" so they automatically stay paired together. Oxral igniters are available from ALL HOBBIES.

NOTE: The Daveyfire N28F igniters (and I suspect the HiRMI standard igniters) can be used for outboard motor ignition or second stage motor ignition but ONLY after they have been dipped into additional pyrogen. (For example the FireStar pyrogen.) The N28F igniter from Daveyfire has a longer burn time and will therefore have a better chance of igniting the second layer of pyrogen. All of the other igniters in the table burn so fast that they should NOT be used for motor ignition. They burn with a quick "snap" where as the N28F burns with a little more of a "sizzle". For ejection charges the N28B type are usually a better choice since they require less current from the altimeter electronics.

My personal favorites for ejection charge igniters are the HiRMI-Sensitive units with the yellow wires. They are very low current. I buy them from ACME Rocketry whenever I attend the launches at Brothers Oregon or at Black Rock Nevada.

HiRMI-Sensitive igniter head with protective tubing.

This is an HiRMI-sensitive igniter with some red protective plastic tubing over the match head. This is how they are shipped.

View of the match head once the protective tubing has been removed.

HiRMI-Sensitive igniter with tubing placed over the solder joints.

I trim the tubing and then re-install it over the electrical solder joints to prevent the contacts from touching something they shouldn't. The match head is fully exposed for placement inside a black powder ejection charge.

Safety and Reliability Considerations

I ALWAYS use more than one igniter for my ejection charges. Although these igniters are very reliable, I do not want to trust the safe return of the rocket to that one single very small piece of wire in the match head. (The wire is supposed to get hot and transfer enough heat to the pyrogen to ignite it. However, if the wire opens before it gets hot enough, then the igniter will not ignite.) On large rockets with two altimeters I use two igniters per altimeter for a total of four igniters placed in two independent ejection charges. Click here to view igniter and ejection charge preparation for Wildfire.