Aerotech 38mm reload delay kits. How are they different?

Aerotech 14 second 38mm reload delay kits. They come in 7 different flavors. CRDK38-01 to CRDK38-07. Different delay kits for different propellent types.
(I'm looking at the list on Sirius Rocketry).
https://www.siriusrocketry.biz/ishop/aerotech-reload-delay-kits-rdk-71/crdk-kits-for-38mm-motors-186/

What's the difference between them? I mean, they're all 14 second delays, right? They all fit in the same 38mm forward closure. When is 14 seconds no longer 14 seconds.

I have a couple CRDK38-01 kits I bought for a White Lighting motor I had messed up the delay charge on. I want to fly a blackjack motor this weekend, but it came with a 10 second delay and I need 14 seconds. It looks like I need a CRDK38-03 14 second delay kit for blackjack motors.

Why do different propellent types need different 14 sec delays?

How would a white lightning 14 sec delay work on a black jack motor?

Quick Answer: Each propellant burns at a different temperature and pressure. While the propellant is burning the delay burns at a different rate than at burnout and the rate is different for each propellant. That's why there are so many delays. IIRC the 14 sec white lightning delay on a black jack would be somewhat under 14 seconds, say 10 s to 12 s, but don't bet your rocket on my recollection.

Different propellants (formulas) burn at different pressures and rates. They will consume the delay element at different rates. So, a 14 second delay element for your BJ motor will be different then a 14 second delay for a Redline motor. The delays come in various lengths and maybe even different delay formulas. During the motors burn phase a portion of the delay element is "used up". The remaining portion has to be the right length (and maybe formula) to give the correct delayed time. The spacer portion of the delay train just fills the void of the shorter then "full length" delay elements.

Tony

All Aerotech delay grains burn at 1 sec per 1/32nd inch. It's the propellant that burns differently.

jimzcatz said:

All Aerotech delay grains burn at 1 sec per 1/32nd inch. It's the propellant that burns differently.

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Jim , the 1/32 per second is only after the motor is done burning . The delay time starts right at motor burnout , not ignition.

The delay time, yes. But the length of the delay grain factors both motor burn time and desired delay time. The 1/32 inch starts at ignition.

If you have a onsite motor vendor they may stock kits, I have bought before here onsite

I found a chart once I believe RAH that's gone

To the OP. I recently did what you did and screwed up a delay on a motor. If you just over drilled the delay element, you don't need the CRDK - Complete Reload Delay Kit. That includes everything for the delay assembly. You just need the RDK - which is just the delay grain and a spacer. The RDK costs more, but it comes with 3 delay grains, so you'd have a couple of spares for that type of motor. You can do it however you want, I just wanted to make sure you realize there is a difference between the CRDK and the RDK. I found it a little confusing at first.

Swissyhawk said:

To the OP. I recently did what you did and screwed up a delay on a motor. If you just over drilled the delay element, you don't need the CRDK - Complete Reload Delay Kit. That includes everything for the delay assembly. You just need the RDK - which is just the delay grain and a spacer. The RDK costs more, but it comes with 3 delay grains, so you'd have a couple of spares for that type of motor. You can do it however you want, I just wanted to make sure you realize there is a difference between the CRDK and the RDK. I found it a little confusing at first.

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Thanks for the info Swissyhawk, and for all the other replies. No definite answer. I'll go for it. It will be an interesting experiment. I'll be using a jolly logic chute release so if I deploy at a bit if speed, it wont be to bad.

I'm guessing using a 14 sec "White Lightning" delay in a Black Jack motor will give you about a 10 second delay.

A real 14 second Black Jack delay is .844" long. A 14 second White Lightning delay is .719" long .844 - .719 = .125 .125/ .03125 = 4 14 sec - 4 sec = 10 sec

again..a guess

Good luck with your flight. Let us know how it goes.

Tony

This is a guide I found years ago on the rockstock.org website that I have found to be useful:

https://web.archive.org/web/20050131091348/https://www.rocstock.org/wizards/rms-delays_2938.pdf

FWIW

You can find the complete delay length (RDK's) cross reference here: https://www.aerotech-rocketry.com/resources.aspx?id=2

I tried to attach/ link the PDF did not work.

We'll never know. To windy today. I started with a LOC Graduator on a G76 with a chute release set to 300 feet to test the winds. 2000 ft altitude. It landed a mile away. That was it for me today. The correct 14 second delays for FJ motors will be delivered by my next launch.

jimzcatz said:

The delay time, yes. But the length of the delay grain factors both motor burn time and desired delay time. The 1/32 inch starts at ignition.

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You're claiming that a delay will burn at the same speed during the propellant burn, no matter what the propellant/motor pressure/motor temp is?


(To be clear, I believe you are wrong)

At least in the HDK delays I think that's true for all but the 3/4" Slow. I believe it's because although the burns vary in length, they develop approximately similar case pressure.

So rate A when the thunder's coming out, Rate B when it's just delaying the inevitable. Hence why they vary only by length instead of any other variables.

I'll have to go math the lengths on my HDK sheet, now I'm curious.

Tony

DavidMcCann said:

You're claiming that a delay will burn at the same speed during the propellant burn, no matter what the propellant/motor pressure/motor temp is?


(To be clear, I believe you are wrong)

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You're right, Dave!

Jim, the delays burn faster than .03125"/sec during the motor burn (heat, pressure are higher); different propellants, chamber pressures affect the burn rate during propellant consumption. After the motor burns out, then the delays burn at the slower and more predictable rate of ~1/32" per second.

Why then does the drilled end of the delay face the propellant?

Buckeye said:

Why then does the drilled end of the delay face the propellant?

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Because if it faces the ejection charge, black powder can trickle through the hole and fill the hole in the delay grain. When ignited that is the wrong place to have your BP adding pressure. You want the black powder to ignite in the charge well, not in the delay grain well.


Steve Shannon

Steve Shannon said:

Because if it faces the ejection charge, black powder can trickle through the hole and fill the hole in the delay grain. When ignited that is the wrong place to have your BP adding pressure. You want the black powder to ignite in the charge well, not in the delay grain well.
I hope that explanation makes sense.

Steve Shannon

Click to expand...

Steve Shannon

Steve Shannon said:

Because if it faces the ejection charge, black powder can trickle through the hole and fill the hole in the delay grain. When ignited that is the wrong place to have your BP adding pressure. You want the black powder to ignite in the charge well, not in the delay grain well.


Steve Shannon

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The drilled end using the 1/32" rule is now in the combustion chamber, which some are claiming invalidates the 1/32" rule. :confused2:

Buckeye said:

The drilled end using the 1/32" rule is now in the combustion chamber, which some are claiming invalidates the 1/32" rule. :confused2:

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No, you're misunderstanding. When the motor is at pressure, the delay grain burns at one rate. As soon as the propellant has burned, the rate changes to 1/32 inch per second. That's all people are saying. The length of time the delay grain burns in the same regardless of which end is drilled.
The total EFFECTIVE length of the delay grain is from the bottom of the hole you drill to the other face of the grain. The grain material around the drilled hole doesn't affect the burn time.
So, for a seven second delay, you need a total length that includes the length that burns at the faster rate during the motor's burn time, plus 7/32 inch for the seven second delay.
If you aren't lighting an ejection charge it doesn't matter which way the drilled hole faces.
In a smaller CTI motor you don't have a choice. The only side of the delay grain you can get to is the aft end.


Steve Shannon

Buckeye said:

Why then does the drilled end of the delay face the propellant?

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It doesn't in all applications......when shorting single use motors, you must drill from the front.

I used to do it all the time back when I flew 'double" packs of Econo-jets! Dump out the BP....drill delay,,,,replace bp and tape over a cover. Never failed.....unless you drilled less than a 4 second delay.

Buckeye has a point. If I drill 4 seconds off a delay grain using the tool, I'm drilling the same amount no matter which propellent is being used. If the delay grain burns at different rates based on the propellent, and the drilled portion is facing the propellent, then the delay grains for each propellent must be different sizes if we're to believe the 1/32 per second rule.

blackwing94 said:

Buckeye has a point. If I drill 4 seconds off a delay grain using the tool, I'm drilling the same amount no matter which propellent is being used. If the delay grain burns at different rates based on the propellent, and the drilled portion is facing the propellent, then the delay grains for each propellent must be different sizes if we're to believe the 1/32 per second rule.

Click to expand...

I agree; for different pressures and burn times of propellants, different lengths of delay grains must be used, but the direction the drilled hole in the delay grain is facing doesn't affect the delay time.



Steve Shannon

The black jack delays arrived. Now that I have 2 kinds in house....... let's measure.

Both are 14 second delays.

The delay element for White Lightning (and black max) propellants CRDK38-1 is:
93/128 inches

The 14 second delay element for Black Jack propellant CRDK38-3 is:
100/128 inches

The black jack delay is about 7/128 or 1.75/32 longer than the delay for white lightning. That's almost a second.

Does the J main burn last a bit longer than the W, perchance?