Wildman Rocketry Giveaway!

I am trying to make sense out the Aerotech ejection delays. I need a delay of 12.6 seconds for a G64 motor. I plan on ordering an HDK-22 delay, which is the 0.75 inch delay element. Based on the specs for the 4, 7 and 10 second delay elements, the HDK-22 will give a 16.6 second delay. So my plan is to shave off 4 seconds, or maybe 3 seconds so that it ejects after apogee.

I have used the rule of thumb of 1/32nd of an inch per second when making delay adjustments in the past. However, after looking at the various delays in the HDK table it seems that 1/32nd inch would be too much for the G64. My computations show that the G64 delay burns at a rate of about 0.018 inches per second. I also looked at the F40, which burns at a rate of 0.044 inches per second, which is almost 2.5 times the G64 rate. Has anybody else looked at the delay burn rates for the various motors? Do my numbers make sense?

I understand that the delay element burns quite a bit faster when the motor is under pressure, but why would there be a difference in burn rate after the motor has burned out?

Dave

2. ## Re: Aerotech Delay Adjustment

Originally Posted by DaveHein
I am trying to make sense out the Aerotech ejection delays. I need a delay of 12.6 seconds for a G64 motor. I plan on ordering an HDK-22 delay, which is the 0.75 inch delay element. Based on the specs for the 4, 7 and 10 second delay elements, the HDK-22 will give a 16.6 second delay. So my plan is to shave off 4 seconds, or maybe 3 seconds so that it ejects after apogee.

I have used the rule of thumb of 1/32nd of an inch per second when making delay adjustments in the past. However, after looking at the various delays in the HDK table it seems that 1/32nd inch would be too much for the G64. My computations show that the G64 delay burns at a rate of about 0.018 inches per second. I also looked at the F40, which burns at a rate of 0.044 inches per second, which is almost 2.5 times the G64 rate. Has anybody else looked at the delay burn rates for the various motors? Do my numbers make sense?

I understand that the delay element burns quite a bit faster when the motor is under pressure, but why would there be a difference in burn rate after the motor has burned out?

Dave
The HDK-22 will make a G64-14.

The delay grain will burn ~0.446" during the motor burn and ~0.021" per second after the burn according to a linear regression analysis of the HDK data sheet. http://www.aerotech-rocketry.com/cus.../hdk_x-ref.pdf

See the attached plot.

Bob

3. ## Re: Aerotech Delay Adjustment

Bob,

I did a linear fit as well. However, I only used three data points, and I used the NAR certified data instead of the generic values of 4, 7 and 10. The NAR delays for the G64 are 4.04, 8.39 and 11.08. Fitting a line to this give the equation 55.8X - 25.3. This yields the following delays.

Length Delay
------ ------
0.500 2.6
0.521 4.3
0.562 6.1
0.593 7.8
0.657 11.4
0.750 16.6

The plot of this data is shown below. Is it better to use the values of 4, 7 and 10 instead of the measured NAR values? I would think the measured values would be more accurate.

Dave

4. ## Re: Aerotech Delay Adjustment

Dave

You and I are both over analyzing the problem. For a number of reasons but primarily cost, the inability of some engine manufacturers to produce reproducible delay columns is one of the biggest headaches in rocketry. The burn rates appear to vary batch to batch, year to year and perhaps by phase of the moon.

This is reflected by the large variance allowed by NFPA 1125 for delay timing. Minimum variability is +/- 1.5 second or +/- 20&#37; of the labeled delay which ever is greater but not exceeding +/- 3 seconds. The NAR data simply represents the 10 delays that were submitted for testing, and may not be representative of the one of the hundreds of thousands of delays you could get, so I can't tell you which data set to use.

In practical terms a labeled -4 can be anywhere between 2.5 to 5.5 seconds; a labeled -7 can be anywhere between 5.5 to 8.5 seconds; a labeled -10 can be anywhere between 8 to 12 seconds, so a nominal -14 can range from 11.2 to 16.8 seconds.

Some of the variance can be attributed to differences in atmospheric pressure, but I'll speculate that a good bit is due to variations in the chemical composition of the delay column due to particle settling before the binder cures.

If you really need more precision, use a timer.

Bob

5. ## Re: Aerotech Delay Adjustment

Bob,

Thanks for the explanation. I understand that the delay can be quite variable. The fact that more than half of the delay element is consumed during the motor burn, and the remaining portion burns at one-tenth the rate must be a real QA nightmare for motor manufacturers.

I plan on using the G64 in a minimum diamter 29mm rocket that should go almost a mile up. The rocket is light enough at 10 ounces, and the 14" parachute is small enough that it should be able to withstand ejection a few seconds after apogee.

Based on your numbers I should shave 0.038" off the HDK-22 delay to eject at apogee. My numbers say that I should shave off 0.072". I think I'll go with your number, which will bias it toward ejecting after apogee.

Thanks,
Dave

6. ## Re: Aerotech Delay Adjustment

Dave

I think you will need a 12 -13 second delay so your numbers are within range.

You should drill, not shave, the delay. The drilled side faces the propellant grain. See the instructions at http://www.aerotech-rocketry.com/cus...y_mod_inst.pdf however I'm not convinced that the depths quoted apply for the hobby delays.

Bob

7. ## Re: Aerotech Delay Adjustment

http://www.aerotech-rocketry.com/cus..._rdk_instr.pdf

RDK seems to be for the High Power motor casings only.

RDK does not appear to me to be suitable for the 29mm Model Rocket casing G64 being discussed here.

Am I missing something?

8. ## Re: Aerotech Delay Adjustment

A more current info sheet for hobby line HDK delays.

http://www.aerotech-rocketry.com/cus.../hdk_x-ref.pdf

Bob

9. ## Re: Aerotech Delay Adjustment

http://www.aerotech-rocketry.com/cus..._hdk_instr.pdf

Hobby Delay Kit (HDK) is designed for the 29 mm Model Rocket casing.

10. ## Re: Aerotech Delay Adjustment

OK, I get it.

HDK kit for a longer burning motor is longer than the longest G64 delay and so it does not use the spacer. Result, longer delay burn providing longer than 10 seconds delay after propellant burnout.

Nevermind...

11. ## Re: Aerotech Delay Adjustment

I can't believe that no one pointed out that the resulting motor will not be certified Sorry, couldn't resist.

Join Date
18th January 2009
Posts
2,105

I thought that as long as the motor mfg gives you the process to modify the delay, then it is blessed and the delays could be modified and the motor keep it's cert. This is using the AT delay and modifying it to match AT's direction (more or less) and so it should be blessed.

-Aaron

13. ## Re: Aerotech Delay Adjustment

But you're modifying a component that was never tested in the product. I'm actually not saying you are wrong as much as just wanting to stir the pot to see what the prevailing thought was. I'm a little jealous I hadn't thought of this myself. And the cert issue wouldn't be a problem at MDRA

Join Date
18th January 2009
Posts
2,105

Its no different than the CTI DAT or the LOKI version of the DAT. Aerotech has blessed modifying the delays by drilling. Once they blessed it and gave directions, it is kosher as far as motor certifications go. This means that you can swap delays (need a 6 sec and only have a 10 sec) and then change them to suit your needs. If they sell a delay kit that would make the delay in the motor a 18 second delay, you could use that and then drill it down to the exact time you need. I think there are rules about how much total you can drill down and so I don't think you can make a 18 second delay into a 4 second delay, but you should be able to make it into a 14 second delay. (numbers are purely made up, I don't know the delay kits available for the hobby line)

I think that the variations in delay times that Bob pointed out are very valid. I use electronics whenever possible. Whats the point of drilling a delay when it can be off by +/- 3 seconds anyway?

-Aaron

15. ## Re: Aerotech Delay Adjustment

Originally Posted by rstaff3
I can't believe that no one pointed out that the resulting motor will not be certified
I'm under the impression that the motor is still certified independent of the delay adjustment. Clearly, Aerotech cannot "certify" the resulting delay since they don't give precise equations for determing the delay.

There was a discussion on the EFC a while ago, and it was concluded that the motor did not need to be recertified as long as changes were not made in the propellant area. The delay and ejection charge were not considered to be part of the propellant area.

BTW Bob, I did mean "drill out" when I said "shave off". However, the difference is significant. When I first tried ajusting the delay on an Aerotech motor I actually did shave off a layer of the delay element. I used a flat-blade screwdriver to get the shaved off area nice and even. I figured that this would be equivalent to using a delay element of the same length.

Instead of reducing a six-second delay to four seconds I ended up with a one-second delay. I had never seen a high speed chute ejection up close before. Apparently, I had compromised the seal of the delay element against the side wall by scraping the extra material off. I have since used a drill bit with good results.

Dave

16. ## Re: Aerotech Delay Adjustment

Delay and ejection are most certainly a part of the motor for cert purposes. IIRC the only mods allowed are purely external, such as gluing on a thrust ring. Otherwise plugging motors yourself could be legit. Am I wrong?

Also, the resulting mod will only be certified if you follow the AT instructions vs. an alternate method. My main issue is the longer delay is not listed by AT as applicable to the G64 motor.

BTW, what is EFC?

It looks like this trick may be applicable to the LMS motors. I always needed a delay over a -10 for those G powered 'mach-buster style' rockets.

Join Date
6th July 2006
Posts
2,023

For non-hobbyline motors this is a quick and useful reference.

http://www.rocstock.org/wizards/rms-delays_2938.pdf

18. ## Re: Aerotech Delay Adjustment

Dick

You are correct. A motor with the HDK-22 delay would not be a certified motor in my opinion under anyones present certification rules because the delay element has not be submitted for certification with the G64.

CTI Pro38 motors were the first to be certified with adjustable delays. These certifications were performed by CAR. After that AT announced that shortening delays by drilling was acceptable, and AMW then started manufacturing very long delay that begged for shortening.

Everyone knows that you can drill out a delay column to shorten the delay however user adjustable delay motors have been a PITA because there is currently no 1125 standard for certifying them. NAR S&T is currently writing a policy for testing adjustable delay motors which involves determining the maximum delay, and requiring the manufacturer to specify the minimum delay. We will then shorten the delay in several steps to determine the linearity of the shortening and work from there to insure that a single delay colum falls within the +/- 1.5 second minimum, +/- 20&#37;, and +/- 3 second maximum variation ranges as specified by 1125.

The AT Electronic Forward Closure is a rather cleverly mounted g-switch initiated electronic timer by Perfectflite that powers a glow plug to initiate a BP ejection charge with Warp-9 engines. http://www.aerotech-rocketry.com/cus...structions.pdf IMO the biggest drawback is the price tag of nearly \$200 and the lack of a readily accessible external S/A switch. An altimeter and a hot wire BP igniter will perform the same function at equal or lower cost but it needs to be integrated into the air frame.

Bob

19. ## Re: Aerotech Delay Adjustment

Egads, I knew of the EFC product but for some reason my brain slipped a bit. That sheds a different light to Dave's last comment. My bad. But the EFC is a whole other issue since it is completely external.

20. Senior Member
Join Date
26th February 2009
Posts
450

To inject a random data point into the mix:
I've shortened an HDK kit (I forget which one) for the G64. The delay was much longer than I would've expected following the 1/32" per second guideline.

To get the most accurate ejection possible, electronics are a must. The EFC is cool but seems expensive too me. For that price, I'd rather use an full altimeter.

I particularly like that an altimeter/accelerometer will generally do a good job even if something goes terribly wrong with the flight.

Last month, I had a delay burn-through cause a CATO an AMW I271BB flight about 250' off the pad. The altimeter (PerfectFlite MAWD) still put out the laundry with time to spare.

Jim

21. ## Re: Aerotech Delay Adjustment

Jim

Absolutely correct about a big difference in burn time. A regression analysis of the AT data shows that the correct burn rate for the delay after motor burnout is only 0.021" per second and not the 0.03125 that most folks use as the rule of thumb.

If you took of 1/8" hoping to shorten the delay by 4 seconds, you would have actually shorten it by 6 seconds resulting in an early deployment.

Bob

22. ## Re: Aerotech Delay Adjustment

I still don't understand why a G64 has a burn rate of 0.021" per second (or 0.018" per second based on measurements) and the F40 has a burn rate of 0.044" per second. I've looked at some of the other 29mm reloads and they are close to 1/32" per second, but the G64 and F40 are different for some reason.

I know that the burn rate is highly dependent on pressure, but the pressure should be 1 Atmoshere after motor burnout. Anybody have a theory why the burn rate is different?

Dave

23. ## Re: Aerotech Delay Adjustment

OK, after thinking about this for a while I have a theory why the burn rates are different after motor burn-out. The answer is that the burn rates are <b>not</b> different after burn-out!

My guess is that the perceived difference in burn rate is actually due a difference in the amount of the delay element consumed while the motor is burning. Assuming that the burn rate is 1/32" per second, then a G64-4 would burn 0.407" of the delay element during the motor burn phase, and then 4*0.031 = 0.124" after motor burnout. The 7-second delay would be 0.376" during the motor burn and 0.217" after burnout. The 10-second delay would be 0.347" during motor burn and 0.310" after burnout.

This is only a theory, but it's one possible explanation.

Dave

24. ## Re: Aerotech Delay Adjustment

Originally Posted by DaveHein
OK, after thinking about this for a while I have a theory why the burn rates are different after motor burn-out. The answer is that the burn rates are <b>not</b> different after burn-out!

My guess is that the perceived difference in burn rate is actually due a difference in the amount of the delay element consumed while the motor is burning.

Dave
That is correct.

25. ## Re: Aerotech Delay Adjustment

Originally Posted by DaveHein
OK, after thinking about this for a while I have a theory why the burn rates are different after motor burn-out. The answer is that the burn rates are <b>not</b> different after burn-out!

My guess is that the perceived difference in burn rate is actually due a difference in the amount of the delay element consumed while the motor is burning. Assuming that the burn rate is 1/32" per second, then a G64-4 would burn 0.407" of the delay element during the motor burn phase, and then 4*0.031 = 0.124" after motor burnout. The 7-second delay would be 0.376" during the motor burn and 0.217" after burnout. The 10-second delay would be 0.347" during motor burn and 0.310" after burnout.

This is only a theory, but it's one possible explanation.

Dave
Dave

If you take a look at how I plotted the G64 delay data in post #2 of this thread, you will see that I fit the manufacturer's delay data to a linear equation of the form

y = m*x + b where m = 47.615 seconds per inch and b = -21.267 seconds (y-intercept at x = 0)

The linear delay column burn rate is 1/m=1/47.615=0.021" per second.

The x-intercept (y=0) is the delay column length consumed during the motor burn. For the G64 it is

x = -b/m = 21.267 seconds / 47.615 second per inch = 0.446"

You can make similar plots for other motors such as the F40-4, -7, -10 or the any other motor listed in http://www.aerotech-rocketry.com/cus.../hdk_x-ref.pdf to determine that motor types m and b values.

You will see the linear delay column burn rate 1/m doesn't change (much, it's round off), but each motor type has a different -b/m value which corresponds to the length of the delay column consumed during that motor's burn.

Bob

26. ## Re: Aerotech Delay Adjustment

Bob,

I completely understand your analysis. However, your analysis uses a model that assumes that "b" is a constant that is independent of the starting length of the delay element. This produces a value of "m" that is slightly different for each motor. In the case of the F40 and G64 the value of "m" is dramatically different.

I initially thought that the value of "m" in your model was the burn rate after motor burnout. However, I now realize that "m" is a <b>perceived</b> burn rate that results from "b" depending on the starting length of the delay element.

In my model, the equation should be as follows:

y = m*x - b(x0)

"m" would be 1/32" per second, or whatever the real number is. "m" is the real burn rate after motor burnrate. "m" can be determined in a lab by lighting a delay element and measuring how long it takes to burn at 1 Atm of pressure.

"b(x0)" is a function of the initial element length, "x0". This function is different for each motor. The initial length of the delay element slightly changes the volume of the combustion chamber, which must have a slight effect on the pressure. It also changes the gap distance between the leading edge of the delay element and the insulator and propellant.

The constant-b model is easier to use in practice. I was just trying to understand why "m" was different for each motor. My theory may be way off, but I haven't heard any other explanations.

Dave

27. ## Re: Aerotech Delay Adjustment

BTW, I did fit lines to the delays for most of the 29mm reloads. I used the results published on the NAR site, and I got the following results:

E16 0.035"/s
E23 0.034"/s
F40 0.044"/s
F52 0.023"/s
G33 0.023"/s
G64 0.018"/s

Dave

28. ## Re: Aerotech Delay Adjustment

Originally Posted by DaveHein
"m" would be 1/32" per second, or whatever the real number is. "m" is the real burn rate after motor burnrate. "m" can be determined in a lab by lighting a delay element and measuring how long it takes to burn at 1 Atm of pressure.
What makes you think that the delay burns at atmospheric pressure in the motor? Every one of these motors has a nozzle. While the delay does not produce enough gas to create choked flow at the nozzle, the nozzle still restricts the flow and the chamber pressure will be greater than atmospheric.

Because of the small burning surface the pressure rise will be small but it will not be zero and it will vary with throat diameter.

29. ## Re: Aerotech Delay Adjustment

If I am not mistaken in quoting Gary Rosenfield, the burn rate of the delay during the motor burn is about 3 times of what it is after burnout, due to the pressure dependence of the burn rate.

Juerg

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