Aft ignition of composite motors vs forward ignition

Has any tried aft ignition in apcp and had any successful results?

843SouthernPride said:

Has any tried aft ignition in apcp and had any successful results?

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As in placing the igniter close to the nozzle?

This risks igniting the base grain only, which doesn't raise the pressure and thus burns for a long time, which can melt the case.

Why do you ask?

No expert here, but it also wouldn't light the delay grain until WAYYYY too late.

They say insert the igniter all the way to the top for a reason. If it was possible in any other way they would tell you.

The way composite motors are designed, they need to be ignited at the top of the propellant grain because that's where the delay grain is and the motor needs to be lit at the top of the grain simply so the motor burns properly. If you disregard the instructions and try lighting the motor at the bottom you're probably going to watch it chuff and puff, wasting precious APCP, until it finally lights, taking your rocket on an underpowered flight. Trust me, light the motor at the TOP of the grain, or, really whereever the instructions say. Some motors (i.e. endburners) have to be lit at the bottom of the grain.

Well bc I tend to place my igniter near the nozzle on my sorbitol motors. Have heard of anyone trying it on composite

No. All commercial composite manufacturers tell you to place the igniter at the forward end of the motor either in contact with or very near the smoke/delay grain. Placing an igniter in the aft of the motor just ahead of the nozzle will lead to excessive ignition delay and chuffing and will totally mess up the timing of a delay grain. It's not good in APCP motors, and should be worse for sugar propellant which burns slower than APCP.

Bob

There was a Video on Youtube of some Yahoo/Moron lighting his Composite Motor with Cannon Fuse. It Chuffed, went on a short Lawn Dart Flight, and then, the Ejection Charge finally fired while it was laying on the ground all crumpled up.:lol:

Yes. Every single E6 reload I've ever flown has been lit from the aft end.

kj

Haha, yeah chuffing is not a good thing. I've always put the igniter all the way to the delay/smoke grain, but was just curious if anyone has tried different.

On aerotech motors I usually put the igniter in all the way then pull out about 1/2" had better luck then all the way in. CTI all the way in

I saw one flight where the igniter fell to the bottom of the motor when the launch button was pushed. The motor lit and burned at partial pressure for 10 seconds before the motor came up to full pressure and the rocket lifted off. Amazingly the flight was nominal with ejection at apogee. We had a video of the flight and saw the igniter drop on the video. The rocket was a beautiful Mercury Atlas on a J.

On Cesaroni's FAQ on their website, they tell of a person who use their own igniter in their reload. The motor never pressurized and burned for about two minutes on the launch pad. When he inspected his rocket, the casing was a melted pile on the ground. I know that's not the same as aft ignition, but if your motor doesn't pressurize (due to aft ignition) then you run the risk of totally destroying your casing.

Apogee D3's as well.

kjohnson said:

Yes. Every single E6 reload I've ever flown has been lit from the aft end.

kj

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Because they have no core. When there is a core, the flame is trying to exit the nozzle and at the same time it is igniting the propellant in the core. If you light the aft end of a core burning motor the flame path will constantly be seeking new propellant, burning from top down is path of least resistance. It would be like trying to swim up a waterfall.

Good anology Jim .

g zilla said:

On aerotech motors I usually put the igniter in all the way then pull out about 1/2" had better luck then all the way in. CTI all the way in

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Some of the Aerotech reloads have a spacer above the propellant and below the delay element. In those motors, the igniter shouldn't be inserted all the way to the delay element. The instructions for some of the motors have you place a piece of tape over the end of the slot in the propellant to stop the igniter.

-- Roger

The rule of thumb is this:

Motor has cored propellant: Igniter goes at the top (not above) of the topmost propellant grain (e.g., space shuttle RSRM)

Motor has no core: Igniter goes to the base of the propellant grain (but really, that's the only place you can place it; e.g., Estes BP motor)

There maybe exceptions, but this is the general rule.

Greg

On my extreme l/d motors, I place the igniter halfway up the core. There were certain Ellis motors that required it to be 2/3 up the core. So not always do you light bates apcp motors from the very top. Depends on the design.

Stopping one grain short of the top was a method of controlling erosion I read in some commercial or military motor. To make it work reliably though, one would have to dump sufficient energy into the core to create an instant on, I would think. Otherwise there is a lack of control of which portion(s) of the core are ignited during pressurization.

Gerald

THIS [video=youtube;JMivWSdCiEQ]https://www.youtube.com/watch?v=JMivWSdCiEQ&list=UUr1Jlv9yNGg9on9FakA5A9g[/video]
Is what happens

Binder Design said:

On my extreme l/d motors, I place the igniter halfway up the core. There were certain Ellis motors that required it to be 2/3 up the core. So not always do you light bates apcp motors from the very top. Depends on the design.

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That's what I heard Mike, that it might lead to a less of a starting jolt for flights with electronic deployment? I've also heard of the 1/3rd from the top on the high L/D motors. For short stuff with motor ejection, I'd say the best route is recommended by the motor maker.:wink: Kurt