The 60-second rule for misfires

I need to fact-check something for my blog. The blog itself is part showing what I'm up to for general rocketry interest, but also part sharing what I've learned and passing it off to other new rocketeers.

Now, I've read a lot and learned a lot since I started, last summer, and I'm careful not to present information that I'm not pretty sure is correct. If it's something to do with craftsmanship, and it's not something I'm sure of or have experienced myself, I'll say "I think that..." or "I have read that..." But for something which is more technical or safety-oriented, I want to make sure I have the facts straight. I'd like to know for my own enlightenment, and also, I don't want to give bad information so that any newer rocketeers who read my blog get the idea that they can't trust it.

That being said, I'm writing something on launching a rocket for the first time, and I want to address the issue of having a misfire and waiting 60 seconds before approaching the pad.

Here's what I've written:

If nothing happens after you've held the launch button down for a few seconds, release the button and remove the safety key. You must wait 60 seconds before approaching the pad, to prevent being right on top of the rocket if it suddenly ignites!

This is particularly true of composite motors, which we're not discussing here, but it's also true of black powder motors. Now, black powder doesn't really smoulder. It's lit, or it's not. But one reason for a misfire is that the igniter might not be touching the propellant. I believe the reason for waiting 60 seconds is that the igniter wire may still be hot, and if it suddenly comes in contact with the propellant, you could have an unexpected ignition, so you want to let that wire cool down.

First question: My comment about black powder smouldering - is that true?

Second: Is the reason for waiting (with BP motors) to allow the wire to cool down, as I've hypothesized?

Third: Though I haven't written about composite motors yet - because I haven't used them yet, and I prefer to write from my own experience - is the statement "This is particularly true of composite motors..." factually correct?

Thanks for your help!

I have seen composite motors light as late as 15-20 seconds after the button was pressed. Sometimes they chuff continuously, but other times they don't. This isn't common, however.

I have no clue about BP motors; I've never seen one ignite late. The igniter wire is really tiny and will burn out completely and then cool down nearly instantaneously. Perhaps it's because in the smaller sizes you can have either composite or BP and you can't be sure which is which.

CarVac said:

I have seen composite motors light as late as 15-20 seconds after the button was pressed. Sometimes they chuff continuously, but other times they don't. This isn't common, however.

I have no clue about BP motors; I've never seen one ignite late. The igniter wire is really tiny and will burn out completely and then cool down nearly instantaneously. Perhaps it's because in the smaller sizes you can have either composite or BP and you can't be sure which is which.

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I've always had the feeling that if you get a misfire with a BP motor, you'll be fine. However, all the sources I've read on low power model rocketry - including your basic Estes instructions in rocket kits and motor packages - state to wait 60 seconds. I'm guessing at the reason why in this post. I suppose the reason itself isn't crucial to the post, but I'd like to know if there's a real reason, or if it's simply because too much caution is not a bad thing, whereas too little caution can be dangerous.

lcorinth said:

I've always had the feeling that if you get a misfire with a BP motor, you'll be fine. However, all the sources I've read on low power model rocketry - including your basic Estes instructions in rocket kits and motor packages - state to wait 60 seconds. I'm guessing at the reason why in this post. I suppose the reason itself isn't crucial to the post, but I'd like to know if there's a real reason, or if it's simply because too much caution is not a bad thing, whereas too little caution can be dangerous.

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Daniel:
your pretty close; with BP motors if the igniter didn't make good contact or was shorted just sort of the propellant as the wire cools it can bend into contact and Whoosh! We've had Late BP ignitions very rarely but they can and do happen. The 60second rule is in place to ensure everyone's safety.
As with all forms of Rocketry Safety must ALWAYS be First and foremost.

If a BP motor doesn't fire when the fire button is pushed and you had continuity before the attempt, the electricity is going somewhere and doing something. Here are some of the situations I can think of:
1) A short outside of the nozzle
2) A short inside the nozzle
3) Proper function of the igniter or starter but no ignition of the BP fuel.

Two out or these three can lead to a misfire some seconds after the fire button is released.
1) On an Estes igniter/starter, the paper that is supposed to hold the wires apart could conceal the short and actually heat to the point of smoldering or flame. This could eventually ignite the fuel but should be visible and diagnosed well within the 60 second safety cushion.
2) Using the Estes plastic plugs or some balled up wadding can hold enough heat inside the nozzle that the pyrogen or other material could "fire" after the power has been removed. Waiting the 60 seconds does allow everything to cool down.
3) Not likely to fire once the electricity is off but why chance it?

If the short is at the clips, either touching each other of both touching the blast deflector or some other metal, there is very little possibility of a misfire. Still, if you don't know why the BP didn't fire, what's the harm in waiting 60 seconds. We can do hundreds of launches in a 6-hour day and have many misfires due to any of the reasons listed above. Sometimes it's annoying if there is a line of people waiting to use that particular launch rod. But safety is our first concern and a little more time has never made anyone miss a flight opportunity.

While I was typing this up, a couple of other answers were posted. I don't think any of are contradicting the others.

All good advice and with good reason. I believe black powder burns at about 2,500 F and composite fuels are often above 4,000 F. The "F" is for fried!

I once saw a fellow have a misfire of a J composite motor (IIRC). The motor chuffed and then appeared dead on the pad. The flyer went to his rocket and checked the connections. As he returned to the flight line and was maybe twenty-five feet away from the pad, his rocket suddenly came to life. The entire event only took about 45 seconds start to finish and only the laundry man knows the full story. :y:

Composite motors do take longer to light than BP, which is why they will tell you to ignite your composites before your BP if you are clustering with both types of motors.

Overall, though, I think the idea is similar to "Never look down the barrel of a gun and pull the trigger, even if you just ensured that it is unloaded." It's that one-in-a-million freak accident that blinds someone or even kills someone that will bring the BATFE down on us and illegalize the hobby.

KennB said:

If a BP motor doesn't fire when the fire button is pushed and you had continuity before the attempt, the electricity is going somewhere and doing something. Here are some of the situations I can think of:
1) A short outside of the nozzle
2) A short inside the nozzle
3) Proper function of the igniter or starter but no ignition of the BP fuel.

Two out or these three can lead to a misfire some seconds after the fire button is released.
1) On an Estes igniter/starter, the paper that is supposed to hold the wires apart could conceal the short and actually heat to the point of smoldering or flame. This could eventually ignite the fuel but should be visible and diagnosed well within the 60 second safety cushion.
2) Using the Estes plastic plugs or some balled up wadding can hold enough heat inside the nozzle that the pyrogen or other material could "fire" after the power has been removed. Waiting the 60 seconds does allow everything to cool down.
3) Not likely to fire once the electricity is off but why chance it?

Click to expand...

Good - sounds like my explanation was not nonsense. Thanks, everybody!

Pretty good discussion here. I've often seen composite motors take 5-10 seconds to light up; rarely 10-20 seconds. There are various ways to mis-ignite them (improper ignitor position, not enough pyrogen, etc.) and if not properly lit they can take quite a while to come up to pressure, sometimes with very little visible smoke. This can happen even if they don't chuff at all.

With black powder motors the delays are typically a lot less - there's less pyrogen on the ignitor and it takes a lot less time for the motor to come up to pressure once lit. In tens of thousands of BP launches I've seen I can't remember a true hang-fire of more than a few seconds.

Another thing that has to be accounted for is launch controller issues. One common syndrome with mass produced controllers is having weak contact or weak battery (common) or a jammed-stuck (rarely) short on the ignition contacts. In the former case the motor may not light until you've been mashing the button for a while (and kids do that really well!). In the latter case the motor could light at anytime *until you take the safety key out*.

The 60-second rule with the "take the safety key out" clause is there to give lawyer-resistant margin for all of the above scenarios. I believe that rule has been around since before composite motors were very common. If you want to be in 6-sigma territory for composites it might even be good to make it 120 seconds.

Thanks for the input, guys. The post is up.