Aerotech composite grain too narrow for ignitor

[RocketSquirrel]

So i bought a bunch of old composite motors from someone. The bag includes a copperhead ignitor, so pretty old. I'm trying to get the motor ready but the grain slit is too small for the ignitor to slide down. Its barely narrow enough for the copper end to slide down.

What would you do? Am i missing something? I have 3 F25-6W that all have this problem. I tried to get a picture of it for reference

 

[markg]

So i bought a bunch of old composite motors from someone. The bag includes a copperhead ignitor, so pretty old. I'm trying to get the motor ready but the grain slit is too small for the ignitor to slide down. Its barely narrow enough for the copper end to slide down.

What would you do? Am i missing something? I have 3 F25-6W that all have this problem. I tried to get a picture of it for reference

Old grains swell up and close the gap for the igniter. I've got the same issue with some older G64s. I successfully flew one a couple of weeks ago by opening up the slit with a long small drill bit. After I got the slot opened up I also sliced out the edges of the slot from the top grain to expose 'fresh' propellant, then use a dipped igniter to get it going. This was on a reload, SU motors are going to be a bit more of a challenge.

cheers - mark

 

[Tractionengines]

Since you're new.. Do NOT use the drill bit in a power drill, or even battery powered drill.

Just wrap some masking tape at the shank end, and turn with your fingers to open the core back up to almost nozzle diameter.

Your big issue is figuring out how deep to go. If you drill into the delay grain, it will go off early. Or if too deep could "cato". ( Motor failure )

 

[Art Upton]

Also the older motors have 'extended" delay times. I found a 6 second turned into 9-10 seconds and ruined the nose of a rocket. This was in 2003 using a motor with a 1988 date stamp. I still have a few 1988 dated motors in my old box.

 

[BrendanH69]

+1 on using a small drill bit by hand to gradually open up the hole to accept the size of the dipped ignitor. I've recently had to do this with composite E20's with batch dates in only in Feb2018.

My experience/tips:

• Before you start, use a narrow item, e.g. blunt toothpick, as a depth gauge to work out how deep the grain is and the start of the delay. Use this to mark your drill bits to avoid hand drilling into the delay.
• Start small and work up in drill bit sizes until you find a sweet spot for the ignitor head you are using. If you are taking a lot out, then reconsider your choice of ignitor.
• I found the the initial throat end was the narrowest part of the grain, so pause after drilling there and try your ignitor before going in full depth.
• Save/store or dispose of your propellant shavings safely.

 

[prfesser]

Sorry, but those motors may be unusable.

APCP motors are designed so that the cross-sectional area of the core is significantly greater than the area of the nozzle throat. Usually at least twice the nozzle throat area, though there is some wiggle room depending on a number of factors. (In this case the core is a slot so area is depth times width, of course.)

The core has closed up quite a lot from what I see at the nozzle throat. It's not just a matter of opening the core enough to get the starter in place. It's a matter of opening up the core such that the area for the entire length of the core is pretty close to what it was originally.

In a reload this isn't too difficult because you can hold the propellant grain in your hand and sand or saw away at it. In a single-use motor that work has to be done through the nozzle throat...without increasing the throat diameter....and you can't really see the progress....

It's true that some APCP motors operate properly even though the core area and nozzle throat area are about the same. But that's the exception and not the rule.

If you insist on using the motors I would test by "fixing" one, then either static-fire or use it in a "beater" rocket that you don't mind losing. Possibly I'm being overly cautious.

 

[cwbullet]

Not all rockets need a delay. Build a saucer to fly them so that you do not need the delay.

 

[86mustang408w]

I bought some First Fire Jr's and used them.

 

[kjhambrick]

I bought some First Fire Jr's and used them.

How did the motors burn and how did your rocket fly ?

Thanks !

-- kjh

 

[86mustang408w]

Great. And it took a little bit to get the Fire First Jr in the F23-4FJ. I bought those motor over 10 years ago with many others and they are fine. I talked to a few guys and they said not to even bother using the copperhead igniters. My other pack of F42-4T came with the orange igniters, the new FF Jr's I got from eRockets in 3 in a pack. They were for spares but I used them for primaries.

Both motors(F23-4FJ on SBB and then Leviathan)ignited and took off right away. I just pushed the igniter as far up as it would go, marked it with my fingers and pulled it out to check the depth outside visually, and then put it back it and wrapped the rubber band around the nozzle 3 times. They worked well, it all did.


Back in July I used a F42-4T for my Leviathan, I left the igniter down from the top probably an 1/8", and it took a while to ignite. Some motors are better if you kind of give the tip of the igniter a "fishhook" and put it in the nozzle. That's hard though with single use F motors.

 

[burkefj]

In my experience the copperhead will probably not light it even if you get it all the way in, you will probably spend more on igniters than the motor is worth if and when you get it to light and most likely thrust will be reduced and delay/ejection might not work or have the right time. I've wasted time and money on these sorts of things before and won't waste any more.