Delays

[MarkABrown]

Has anyone ever tried to modify the delay grain on an Aerotech Reload and if so, how would one go about it?

The reason I ask is because the shortage of motors is forcing me to purchase motors with longer delays than I want. What I'd like to do is modify the delay grain to produce a shorter delay.

 

[Austin]

Actually, there are two solutions to your problem. You can purchase delays seperately for your reloads, which is my recommended method.

Also, you can modify your reload by drilling a 1/8" hole down the center. For every 1/32 of an inch in depth, you reduce the delay by approximately 1 second. Some people use this method to "tune" their delays for perfect deployment. Note that the drilled side must be installed FACING the motor grain.

 

[MarkABrown]

Thanks Carl. That's the kind of answer that I was looking for. I agree that the preferred method would be to purchase the appropriate replacement delay however, I suspect that, due to the Aerotech fire, the replacement delay grains are going to be in short supply also.

 

[PGerringer]

Last weekend we went to Whitakers with some looooooooong delays. Mark and I had 7 and 10 second delays which were way too long for the rockets we were flying. Well, we did the "approved" method for altering the delays. And it didn't work.

The first launch was with a 10 second delay that Mark drilled to spec. Burnout - 1 one thousand - 2 one thousand - ... - 10 thousand - pop - crash. It did not affect the delay a bit. I did make sure that the drilled hole was facing the grain.

The first launch was with a 10 second delay that Mark drilled to spec but then drilled some more to try to affect the delay. He then put the hole facing the grain and off he went. Burnout - 1 one thousand - 2 one thousand - ... - 10 thousand - pop - crash. Again, it did not affect the delay.

So, does anyone have any ideas what might have happened?

I've been thinking about it and I wonder... I admit I don't know that much about delays. I know that it starts burning at ignition and finishes burning after burnout + delay. With that in mind wouldn't it make sense that when the delay started burning it would start at the tip and just cause there is a hole it still has to burn all the way through the delay? But, what if you faced the hole towards the black powder? It would make sense that some of the black powder would get in the hole and when the thinner delay burns through, the flame hits the black powder in the hole and then the chain reaction starts. - pop -

Course, I could be wrong.

 

[kgholloway]

IMHO which way the drilled grain faces should not have an effect. So long as the total length of the column, between the bottom of the drilled hole and the opposite face of the delay grain, is constant the total time should be the same.

I'd just feel a lot better using undrilled grains since if something were to go wrong my NAR insurance would cover me. It would not if I drilled the delay since this would be considered a modification to the motor.

One variable that you did not mention is the diameter of the hole. Could this have had an effect?

Ken Holloway, NAR #78336, L-1

 

[PGerringer]

The diameter was 1/8" as everyone suggests. We had done this before at a previous launch and was able to take a 7 sec delay and make it 5 sec. I wonder if being a 10 sec delay has any affect. I'm sure Mark will chime in when he gets back from holiday.

 

[MarkABrown]

The first one that was drilled used a 1/8" bit to create a 5/32" deep hole in a 10 second delay. That should have reduced the delay to 5 seconds. When that didn't work, I tried another using a 3/16" bit and drilled to about 3/16" deep. That should have been at least a 4 second delay but, again, we got a 10 second delay. Luckily, my Diablo was lighter than Phil's Mirage so I had a little more breathing room with a 10 second delay than Phil. The only thing I can think of is that these reloads looked as if they were pretty old. The delay grains looked to have some oxidation on them. I still have two of the drilled 7 second reloads and Phil has one of the 10 seconds left. Next time, I'll at least try to face the hole towards the black powder since that may bring the powder closer to the burning propellant.

 

[kgholloway]

Mark,

Mark, that remark about oxidation rang a bell. One of the routine steps that I take when assembeling a reload is to scrape the surface of the delay column. This was a suggestion from an AeroTech representative to one of our section's members at an LDRS launch a couple of years ago.

It seems that if the surface of the delay column facing the reload's grains is oxidized the column doesn't burn at the normal rate. The delay takes longer to ignite and consequently the delay time increases. You don't have to scrape the delay column enough to shorten the delay. Just "rough up" the surface slightly so that there's not a smooth oxide layer facing inward.

Lightly scraping the last reload grain is also supposed to help the ignition process and the AeroTech rep recommended it for clusters.

Respectfully,

Ken Holloway, NAR #78336, L-1

 

[Stones]

Looks like it's been almost 2 years since this thread was used so, let me bring 'er back up.
I recently picked up a couple of reloads for a decent price that have 10 sec. delays that I'd like to "trim". Seems to me that I looked at a reload with a 5 sec delay and it appeared shorter. Would it be feasible to just trim a thin section from the entire delay grain to acheive a shorter delay time? Just thinkin out loud here.

 

[Ryan S.]

I dont think so....it you did that the delay may not seal....I know this method works, the Pro 38s use it.

 

[Skyhigh]

I talked to Ross at Magnum and he said that it does make a difference on which end the drilled delay is facing. He suggested the drilled end should face forward and not toward the grain. Reason being is if the drilled end is facing the grain, the whole grain from the outer edged ignites and burns normally toward the forward end. If the drilled end of the delay is facing forward and the black powder is in the drilled hole then when the delay gets to the powder it would ignite sooner!

The more I think of it this way it makes sense.

 

[Ryan S.]

that does make sense

However, if you put the drilled out portion towards the grain it will burn ahead of all the rest of the delay (imagine a triangle burn) and hit the ejecction charge first. I have seen both ways work fine

it is true that outer edges of the grain burn normally but there is the drilled out portion burning ahead of it

 

[Stones]

The drilled area being away from the propellent grain , or more precisely, filled with BP, strikes me as the optimal method. I'll post results when the time comes to try.

 

[Rocketjunkie]

Originally posted by Stones
The drilled area being away from the propellent grain , or more precisely, filled with BP, strikes me as the optimal method. I'll post results when the time comes to try.

Either way works. If the hole is toward the propellant grain, it must be large enough to ignite all the way to the bottom immediately. With the hole toward the front and filled with BP, ignition is assured as soon as the burning front reaches the bottom of the hole. The disadvantage is the afterburn of the remaining delay material. This may cause damage to the rocket particularly in minimum diameter designs (least likely to need a cut down delay though).

 

[Stones]

Originally posted by Rocketjunkie
... The disadvantage is the afterburn of the remaining delay material. ...

Ah, now there's the rub. Obviously, the remaining delay element around the sides of the drilled hole will continue to burn after the BP is ignited. (Drilled hole facing forward in casing.) This is what made me think of trimming the ENTIRE delay element by 1/32" for each second that needs to be removed. Looking at a 7 sec. delay vs. a 10 sec. delay, the difference is made up by the use of a delay "spacer", which is included in the reload kit. I have measured this difference and found it to be .100"+/- .001".
Now, I'm just thinking out loud here, again, and I'm not condoning altering delays but...
Could one remove x amount of delay as a full "sliver", remove the "sliver" of element from it's liner, and then use the empty liner as the delay "spacer? Might be a tad more intensive but, would remedy the afterburn issue.

 

[n3tjm]

I have ben trimming delays for years... ever since I first started using reloadables. I find my perferd method of trimming is to put the hole on the side of the ejection. This is how the Disposable motors work, and it works great with the reloadables to. Sure, there is the afterburner affect, but so do the disposable motors. The amount of time it takes the remaining delay to burn out does not heat the ejection cavity enough to melt it...

The touch hole effect on the forward end also increases the probability of ejection. I had a J500 flight where I did not trim the 20 second delay (The rocket relied on an altimeter). The delay burned through, but the charge never ignited. Instead it fell into the motor, and was probabily consumed by the smoldering components inside the freshly fired case, or the extremely hot graphite nozzle.

So from now on... I drill a small touch hole to give the powder more contact with the delay element.

 

[Larry]

I have cut some delays using a delay cutter, and always put the hole towards the ejection powder. With the cutter the hole is always dead center. its always worked well.............Larry

 

[Stones]

Originally posted by Larry
I have cut some delays using a delay cutter ...
Larry

Larry, any chance you could post a pic of this "delay cutter"?

 

[Larry]

Stones,
This is the Delay cutter I use. The separate ring is to place the delay grain in so it drills right in the center. The wingnut has a mark as does the body of the cutter. Each turn of the wingnut = !-sec. My instructions say to always put the cut end towards the ejection charge.
Larry

Oh, keep grease from motor assembly off the delay grain, as it changes the burn rate.

 

[Stones]

Tnx for posting the pic Larry. I can work from that.