Adjusting Aerotech RMS delays

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ghp3

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Hello:

I purchased a few Aerotech reload kits for my 24/40 and 29-40/120 cases that have longer delays than it turns out I need (e.g. F52-11, F39-9). I also have reloads that do have the delays that I need. It is fairly obvious that the delay grains for the 9 and 11 second delays are simply longer grains than for the 5 second and 8 second delays.

So here is my question. Can I cut down the longer delays to equal then length of the shorter delays, make or reuse a delay spacer ring, then use them with the rest of the reload?

If this is dangerous or illegal, or a safety code violation, simply tell me (no flames or lectures, please.) Maybe I'll eventually use of the longer delays, but if cutting them down is an option, I'd like to pursue it.

Thanks!

George
ghp3
Tripoli Pittsburgh
Pittsburgh Space Command
 

bobkrech

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George

You can only fly certified commercial motors at NAR and Tripoli sanctioned launches. It is not legal to altered certified commercial motors under their safety codes. Aerotech does not have an approved certified delay time reduction modification procedure so it is not allowed and is an illegal modification. Using a modified engine at a sanctioned nullifies the launch insurance. Not good. That's the rules.

Pro38 delays can be shortened by drilling out the center of the delay charge with a Pro38 delay adjustment rule. This is an manufacturer approved and certified procedure, and therefore is allowed under NAR and Tripol rules.

https://www.pro38.com/Pro38Instns.PDF

The length of the Aerotech delay and delay spacer is a fixed number: longer delays have shorter spacers. You can not simply cut down the delay because it will not seal and the motor will CATO.

https://www.aerotech-rocketry.com/c...ions/RDK_Instructions/29mm_rmsp_rdk_instr.pdf

The bottom line is that drilling out the Aerotech delay column is not dangerous but it is not approved by the manufacturer so therefore it is not allowed at sanctioned launches.


Bob Krech
 

Thrasher

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I've had great success with adjusting the delays on the 29mm/40-120 motors. Do not cut the delay. Dig a small hole with a screwdriver into the forward end of the delay. (The end with the black powder ejection). Go 1/32 for each second you want to reduce it by. The black powder will go into the hole and eject sooner without any motor CATO problems.

I fly them at organized TRA launches and never had a problem. You are not adjusting the motor, but the delay. Most of my motors have no delay because I use altimeters and my own home-made dual stage recovery ejection charges. If I can do this with 100% success, a little hole in a delay element is no big deal. I'm not saying Aerotech approves, and I don't care what NAR says. I'm saying it can be done and done with ease.

Proper ejection times is better than Aerotech's "bonus delays" and the zipper it causes.

Good Luck.
 

bobkrech

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I fly them at organized TRA launches and never had a problem. You are not adjusting the motor, but the delay.
Trasher

You are correct on how you can alter the delay (but the hole should face the propellant, not the BP), however that's not the point. The Tripoli Safety Code specifically prohibits the alteration of reloadable rocket motor components that are not specified by the manufacturer or in the reloading kit instructions.

4. Motors
I. Use only certified commercially made rocket motors.
II. Do not dismantle, reload, or alter a disposable or expendable high power rocket motor, not alter the components of a reloadable high power rocket motor or use the contents of a reloadable rocket motor reloading kit for a purpose other than that specified by the manufacture in the rocket motor or reloading kit instructions.

Ref. https://www.tripoli.org/documents/safety_code.shtml

Aerotech has not approved this modification, nor is it in the instructions for the reload kit.

Maybe you don't care about anything NAR has to say, but you should care about what Tripoli says since you are a Tripoli member flying at sanctioned launchs. So while you are correct in saying that it can be done, and it is easy to do, it's a violation of the Tripoli safety code, and shouldn't be done at a sanctioned launch.

Bob Krech
 

utahrc

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(but the hole should face the propellant, not the BP)
You sure about that? Andy Woernr, great rocket guru of San Diego, advised me to face the hole towards the BP and I've flown a handful of rockets this way with the delays operating as expected.
 

DPatell

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does this mean that 29-40/120's need to include ejection charges? no where in the instructions does it say you can exclude the black powder...

hmm..I know it says it for 29 HPR and up, I don't know about the 18mm, 24mm and 29-40/120...altimeters are getting pretty small...
 

Stones

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Here is a thread on "adjusting" delays. The drilled out area of the delay should face the ejection charge. It may work both ways but, I'd personally stick with it facing the BP.
 

gerbs4me

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I didn't think this was legal, can you buy just the delays? that way you could get the delay you want
 

n3tjm

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It does not matter which way the hole faces in the delay... but what will happen if the hole is towards the ejection is the delay grain will fire the charge, and then the very hot gasses will shoot up into your rocket for the remainer of the delay and toast the motor tube of the rocket. That was the problem with Aerotech disposable motors. D21's really did a number on my Alien Space Probe.

I once was told that Aerotech is looking into making a tool for delay modification.
 

bobkrech

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There is no difference in the delay times based on hole orientation, but I can think of three reasons why you want the drilled hole to face the propellant grain.

First, drilling increases the surface area and removes any oxidization layer from the delay column. This make it easier to light, and since the delay grain generates tracking smoke, you should also get more tracking smoke when the hole faces the propellant grain.

Second as Doug and others have said, the probability of damage from post BP ignition after-burning appears to be lessened in this orientation.

Thrid, that's the way Cesaroni does it in the Pro38 design.

Bob Krech
 

Stones

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Originally posted by bobkrech
...
I can think of three reasons why you want the drilled hole to face the propellant grain.
...
Three good reasons indeed Bob. Betwixt you and Doug, I'm sold now on the direction of the delay. :eek:
 

als57

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Originally posted by utahrc
You sure about that? Andy Woernr, great rocket guru of San Diego, advised me to face the hole towards the BP and I've flown a handful of rockets this way with the delays operating as expected.
Hole towards the propellant please.

Think about this. Upon ignition delay column lights inculding the tip of hole drilled in column. Column is now effectevly shorter.

With hole facing BP ; your making the assumption that the delay column burns flat across that surface. In the real world that probably isn't happening.

BTW why do you thing CTI's DAT makes a hole in the delay that faces the propellent?

If you make a hole in your delay ; use about a 1/8" drill bit and make sure you clean out the hole.

Al
 

Larry

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I've always put the hole towards the BP, because I was told that was how it suppose to be done, but with all I've read here I think I'll try it the other way. I've never had a problem with the way I'm doing it, but I'd like to see if there is a difference. Is it agreed that you don't want to shorten the delay to less than four seconds?

Larry
 

Stymye

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I don't think it's super accurate when you drill the delay so If you have a very short window that the ejection needs to fire, you may be greatly increasing the chances of a prang.

for example if you have a 10 sec delay and you need a 3 sec
the odds of getting exactly 3 sec are slimmer than going from say 10sec to 7sec... because you have more breathing room if it's not exact.
 

hokkyokusei

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Originally posted by als57

BTW why do you thing CTI's DAT makes a hole in the delay that faces the propellent?
So you don't have to remove the ejection charge to make the hole?
 

FredA

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I would (and have) drilled the forward end.
Why.....because you don't have any possibility of changing the burn profile.

An end-burner (standard delay) is different from a core-burner. If you drill the propellant side, you are making a core. The increased surface area will change the burn profile. How much is up for argument.

If you drill the top, then the you have not changed the burn profile until the delay burns into the drilled portion. After that you don't care about the delay burn profile since the ejection charge has been lit.

The down-side of this is that you MIGHT have some delay still burning after the ejection charge fires. This MIGHT cause some extra burning material to come though the hole in the closure....but I doubt it. Also, you need to make sure the BP packs down into the hole you drilled.

The up-side is that if you are careful, you can drill the delay through the hole in the fwd closure after the motor is built. No need to dissasemble the motor to tune the delay.

But, if you need a non-standard delay, I still recommend that you just choose the right delay element from the get go.

Now - having said this, I need to make it clear that this is not something you can do a sanctioned launches. Modifying commercial motors is not allowed according to the rules. Using a non-standard delay is also out of bounds.

Do this at your own risk.....
Fly legal Fly High Have fun!
 

als57

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Originally posted by freda

An end-burner (standard delay) is different from a core-burner. If you drill the propellant side, you are making a core. The increased surface area will change the burn profile. How much is up for argument.

Do this at your own risk.....
Fly legal Fly High Have fun!
Considering the +-10% the total output of the motor is allowed to vary by design and while remaining in spec ; I would say its insignifigant.

Its too bad they don't offer the delays in 2 second intervals. That would eliminate the need.


Al
 

FredA

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I'm not quite sure how motor variation comes to play with delay variations, unless you are saying that there is no use in tuning the delay cause the motor is going to vary.....
If so, I dissagree - since you don't know which way the motor is going to vary (strong or weak) you should try to nail the delay.


Aerotech DOES make a whole range of delay's.

They just don't characterize/certify them all for each motor....just three or four per motor.

Look at the link I posted to this thread earlier.
 

bobkrech

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Al

NFPA 1125 specifies the allowable tolerances for consumer rocket motor performance, and these are the parameters that NAR and Tripoi measure and certify in their motor testing.

Model Rocket Motors

Total Impulse: absolute maximum variation +/- 20% of certified total impulse value; lot maximum standard deviation +/- 6.67%

Average Thrust: absolute maximum variation +/- 1 N or +/- 20% whichever is greater

Delay Time: maximum variation +/- 1 second or +/-20 % whichever is greater but not to exceed +/- 3 seconds

High Power Rocket Motors

Total Impulse: absolute maximum variation +/- 10% of certified total impulse value; lot maximum standard deviation +/- 6.67%

Average Thrust: absolute maximum variation +/- 10 N or +/- 20% whichever is greater

Delay Time: maximum variation +/- 1 second or +/-20 % whichever is greater but not to exceed +/- 3 seconds

The motor burn time and the ejection delay time variation specifications are largely decoupled.

In practice for most high power motors, the allowable variations in total impulse and average thrust would result in a nominal burn time within +/- 1 second of the certified burn time under the worst conditions. The minimum allowed ejection delay variation is +/- 1 second for delays of 5 seconds or shorter, and +/- 20% for delays between 5 and 15 seconds, and +/3 seconds for delays longer than 15 seconds (which don't currently exist).

In practical terms, when you consider the overall boost and coast vertical flight to apogee, I think the allowable motor variations yield a difference of only +/- 50 ft to +/-100 ft altitude if you normalize the flight to the ground level density altitude. While this is probably not a conventional normalization, it is very relevant for aerodynamic deployment stress which is proportional to air density and and to velocity squared.

Near apogee, gravity determines the velocity of the rocket. The variation in velocity is 32 ft/second/second near apogee, so if you're ejection charge fires within 3 seconds of apogee, the rocket is moving at less than 100 ft/second. The air density determines the maximum deployment shock load. A good HPR should have a recovery system that is rugged enough to withstand this the forces of deployment at this velocity, and most will, so if the delay burn is within +/- 3 seconds of apogee, your rocket will recover without separating.

The stock delay choices selected by Aerotech for each motor maintain these margins. You can and should buy the correct ones and not modify the ones you have if they have the wrong delay. It's not that is unsafe, it's just against the rules.

A comment for Fred.

The burn rate of a propellant is a function of the chamber pressure. The fuel grain(s) geometry and the nozzle throat area determine the chamber pressure. The delay column is also propellant, but its burning surface area, drilled or drilled, is so small compared to the fuel grain(s) that its recession rate is independent of its surface area and is determined only by the chamber pressure temporal profile. As I stated earlier, the end that you drill from won't make a significant difference in the delay time, but if you drill from the propellant side, the greater burning area should give a denser tracking smoke.

I believe that the forward ejection of burning delay column particles is primarily an issue with short delays when residual fuel grain combustion is still providing enough mass to pressurize the motor casing. If the pressure in the casing is still above ambient, buring delay charge could be spitting out on the shock cord for several seconds after the BP has gone off.

Bob Krech
 

FredA

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Bob,

Not arguing with you but.....

I agree there are several variable all with tollerences that may cancel or add. All contribute to your error at getting the correct delay to nail apogee.

Why start with a delay element that is out of band?
If I need a 12 second delay, how would you choose between a 10 or 14? Why? Sure the 10 might be 11 or the 14 may be 12.6...neither is the 12 I seek.... Then again, the 12 might 11 or 13....but it's nominal value is where I need it. Why HOPE for the right delay? At least try to minimize the errors.

As for the delay burning the same speed regardless of drilling....all I can say is I doubt your argument. Burn rate is a function of pressure, I agree. But if there is more surface burning, the material is consumed faster. If you admit there will be more smoke, then you must be consuming material faster....hence a shortened delay. Maybe it is in the noise....then again, maybe not.
 

als57

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Originally posted by freda
I'm not quite sure how motor variation comes to play with delay variations, unless you are saying that there is no use in tuning the delay cause the motor is going to vary.....
If so, I dissagree - since you don't know which way the motor is going to vary (strong or weak) you should try to nail the delay.


Aerotech DOES make a whole range of delay's.

They just don't characterize/certify them all for each motor....just three or four per motor.

Look at the link I posted to this thread earlier.
No actually the previous post idicated making a hole in the delay was going to change the thrust curve. My point was that it was insignifigant compared to the normal manufacturing tolerances in the output of the motor.

Al
 

FredA

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Agree on that - unless you totally screw it up and the motor CATO's - that will result in a different thrust curve ;-)

Yep - the delay generates insignificant thrust regardles of how fast it is consumed .
 
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