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Adjusting Aerotech delay grains?

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TWRackers

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Hopefully I can get a quick answer here, I need to know for tomorrow (July 4th).

  1. Can Aerotech delay grains be shortened by drilling a shallow hole, as I've seen done with other manufacturers' delays?
  2. If the answer to #1 is yes, how many millimeters, 64ths of an inch, or whatever of hole depth are required for each second to be taken off the delay?
  3. Would the drilled side of the delay grain face the propellant or the ejection charge?
Thanks in advance to anyone with the answers.
 

madmax

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1. Yes.

2. The instructions are on AT's site but going from memory it is a 3/16" drill bit and each 1/32" you drill in takes one second off the delay time.

3. Face the drilled end toward the ejection charge.

Hope that helps!
 

mikeyd

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cjl

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Yes - face the drilled end towards the propellant. Other than that, madmax is correct
 

Chrisn

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I dont understand why it matters which direction the drilled end is facing. Care to explain? or has Aerotech just decided on any direction to avoid the question coming up by consumers?
 

cjl

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If the drilled end is facing up, among other things, the black powder can fall through the hole in the forward closure and end up in the hole in the delay grain, rather than in the ejection well where it belongs. There are also some other problems, IIRC, although I can't remember any specifics.
 

TWRackers

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Thank you, I had forgotten that Aerotech had the information on their web site, but I've been away from my computer since right after I posted my original message. I'm putting a hardcopy of the Aerotech instructions in my altitude charts binder as we speak. ;)
 

Chrisn

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There is a hole in the ejection well, the black powder already falls down it and onto the delay?
 

JoeG

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Hope your launch went well today.

Aerotech says to put the drilled portion toward the propellant. Not sure why other than to stop a lot of questioning. I would suggest if you are worried about it do it the way they say.

My understanding is that drilling the delay effectively reduces the thickness of it. No matter which way you put it in it is going to be the same thickness. It is also going to start burning at the same time no matter which way it is inserted.

Taking three seconds off a delay will give you a hole big enough to hold only a few grains (not weight) of black powder so I don't see this making much difference since all the powder goes off at once anyway. I always tap and shake my motors to make sure the powder does get down to the delay element.

I have done it both ways lots of times with no noticeable difference. Someone else already said that the only way to trim a single use delay is to drill it from the powder side, which is true, and I have done this and seen it done successfully many times.

I hope you got the delay you needed and had a great time.
 

gizza2

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Do all RMS+ delays employ the same formulation? And are there any significant chamber pressure effects on burn rate?

If so and if not I'm just going to start buying the physically longest delays for 29/38 and 54mm hardware and modify them by drilling and subtracting motor burn time.
 

TWRackers

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Hope your launch went well today.
Probably would have, except the igniter that came with the I245G reload just popped instead of fully igniting, so no ignition. Since this was part of a HPR demo at an Independence Day event, there was not time to recycle and try again. So I put the motor back into the magazine to save it for our club launch in two weeks.

Aerotech says to put the drilled portion toward the propellant. Not sure why other than to stop a lot of questioning. I would suggest if you are worried about it do it the way they say.

My understanding is that drilling the delay effectively reduces the thickness of it. No matter which way you put it in it is going to be the same thickness. It is also going to start burning at the same time no matter which way it is inserted.

Taking three seconds off a delay will give you a hole big enough to hold only a few grains (not weight) of black powder so I don't see this making much difference since all the powder goes off at once anyway. I always tap and shake my motors to make sure the powder does get down to the delay element.

I have done it both ways lots of times with no noticeable difference. Someone else already said that the only way to trim a single use delay is to drill it from the powder side, which is true, and I have done this and seen it done successfully many times.

I hope you got the delay you needed and had a great time.
There is a BIG difference between facing the hole one way or the other: the burning surface. If you face the hole towards the propellant, you're going to get a roughly spherical flame front as the delay grain burns, and when it burns through you'll get a fairly large hole towards the BP. If you face the hole towards the BP charge, you get the normal flat flame front until it reaches the hole from the back side, then you will get a 3/16" diameter burned-through spot which could behave very differently from nominal in lighting the BP.

It helps to know what the correct (or recommended, anyway) method is in order to reason out why it should be that way.
 

JoeG

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There is a BIG difference between facing the hole one way or the other: the burning surface. If you face the hole towards the propellant, you're going to get a roughly spherical flame front as the delay grain burns, and when it burns through you'll get a fairly large hole towards the BP. If you face the hole towards the BP charge, you get the normal flat flame front until it reaches the hole from the back side, then you will get a 3/16" diameter burned-through spot which could behave very differently from nominal in lighting the BP.
Sounds good in theory. My experience has been different. I have achieved the same results from installing the delay either way. We were trimming delays on single use motors before there were any reloads with accuracy thanks to one of the dealers around here who shall remain nameless.

As far as whether a 3/16" hole is big enough to set off the charge I believe it is since we used to use a 1/8" drill to trim the SU's.

It helps to know what the correct (or recommended, anyway) method is in order to reason out why it should be that way.
Your reasoning makes sense to me and i would suggest you continue following the manufacturers suggestions.

My reasoning is that with the depression toward the BP there is a concentration of powder exactly where the delay will burn through. I have never had part of an ejection charge go off. It's been either all or nothing.

Sorry you didn't get to fly. One thing. Make sure you mark the motor that you changed the delay on clearly. Big letters. You don't want to shove it in a rocket that needs a -M and only have a -3. That makes for a bad day.
Don't ask me how I figured this out. ;)
 

TWRackers

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Sorry you didn't get to fly. One thing. Make sure you mark the motor that you changed the delay on clearly. Big letters. You don't want to shove it in a rocket that needs a -M and only have a -3. That makes for a bad day.
Don't ask me how I figured this out. ;)
No danger of that happening, my Endeavour is the only rocket I have that is an appropriate size for an I-impulse motor at our field. My Explorer is too small, and my L3 bird is waaaaaaay too big.
 
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bobkrech

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The reason why you want to have the delay drilled from the motor end and not the BP end is to minimize after burning of the delay grain.

Delay grains are designed to be end burners. In theory the delay grain face burns uniformly with time and thins out evenly. Break through ideally occurs at the same time across the entire delay grain and ignites the BP.

Since the delay grain is simply a slow burning propellant, it has a pressure dependent burn rate. If a drilled delay grain starts burning from the bottom, the bore gets wider rapidly with time when the motor is burning at high pressure and by burnout has transitioned to an end burner with a planar burning surface so when you burn through and ignite the BP, in theory there is no delay grain left to burn out.

If you drill down from the top of the delay grain, the time delay is unchanged, however since the delay grain is an end burner, when the hot gases break through and ignites the BP, you still have to burn out the remaining cored delay grain. Since the burn rate is slow, approximately 1/32 of an inch per second, the grain will generate hot gases for several seconds, cooking your shock chord. Over several flights this could burn through the shock chord.

For example if you shorten the delay 3 seconds by drilling a 3/32" deep hole, you have 3/32" of unburned core burning delay grain left at breakthrough and BP ignition. The remaining cored delay grain will continue to burn for 3 seconds after deployment spewing hot particle into the airframe towards your recovery gear.

Bob
 

JoeG

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The reason why you want to have the delay drilled from the motor end and not the BP end is to minimize after burning of the delay grain.

Delay grains are designed to be end burners. In theory the delay grain face burns uniformly with time and thins out evenly. Break through ideally occurs at the same time across the entire delay grain and ignites the BP.

Since the delay grain is simply a slow burning propellant, it has a pressure dependent burn rate. If a drilled delay grain starts burning from the bottom, the bore gets wider rapidly with time when the motor is burning at high pressure and by burnout has transitioned to an end burner with a planar burning surface so when you burn through and ignite the BP, in theory there is no delay grain left to burn out.

If you drill down from the top of the delay grain, the time delay is unchanged, however since the delay grain is an end burner, when the hot gases break through and ignites the BP, you still have to burn out the remaining cored delay grain. Since the burn rate is slow, approximately 1/32 of an inch per second, the grain will generate hot gases for several seconds, cooking your shock chord. Over several flights this could burn through the shock chord.

For example if you shorten the delay 3 seconds by drilling a 3/32" deep hole, you have 3/32" of unburned core burning delay grain left at breakthrough and BP ignition. The remaining cored delay grain will continue to burn for 3 seconds after deployment spewing hot particle into the airframe towards your recovery gear.

Bob
Bob,

Has this been observed or is it strictly theoretical?

It does make sense and does give the best reason so far for me to drill the propellant side of the grain.

Thanks,
 

bobkrech

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Bob,

Has this been observed or is it strictly theoretical?

It does make sense and does give the best reason so far for me to drill the propellant side of the grain.

Thanks,
Joe

I don't know if anyone has actually measured it in a delay grain, but many solid rocket motors employ internal grain geometries, some of which are extremely complicated to manufacture, to change the thrust as a function of time for a given mission.

Estes BP motors work this way. Take a look at Figure 4 in this report. http://www.esteseducator.com/Pdf_files/TN-2 REPORT.pdf

Also compare the thrust curves for the Estes B6 and C6, and D12 and E9.

The initial thrust spikes are virtually the same as is the tail thrust plateau for each of the motor pairs. The only difference is the length of the propellant column. http://www.esteseducator.com/Pdf_files/Estes Time-Thrust Curves PG 31.pdf The initial spike is due to core burning and the end plateau is do to the transition to end burning.



The AT I59WN-P (P/N 09059P) shown above work the same way and is AeroTech's first Boost-Sustain offering and uses a coreburning White Lightning™ propellant boost grain coupled with an endburning Warp-9 propellant sustain grain. The I59WN fits RMS 38/480 hardware and delivers a NAR-certified total impulse of 486 N-sec over a burn time of 7.99 seconds. The motor delivers a peak "boost" thrust of 173 newtons followed by a long sustain burn tapering from about 60 to 35 newtons.

AED Electronics GDP program can model the phenomenon. http://www.aedelectronics.nl/gdp/gdp.htm

Bob
 
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