Captain Low-N-Slow
I think I finally have the delay grain drilling thing figured out. But what about the drill bit diameter? The Aerotech instructions say to use a 3/16" bit, but I've seen on the forum 3/32" is recommended. Do you use the same diameter bit for all size reloads (24mm, 29mm, 38mm, etc.)?
Super Moderator
I believe it should be 3/32" for 29mm and smaller and 3/16" for 38mm and up..And 1/32" per second you want to trim the delay by..Though, honestly, the only delays I have ever trimmed have been CTI with their handy DAT(Delay Adjustment Tool)
Chute Happens!!
NAR 86940 L2
TRA 12270 L3 09-01-12
KF4GUL
TeleTubby Fan, Unofficial King of Namby-Pamby Land
Super Moderator
http://www.aerotech-rocketry.com/cus...y_mod_inst.pdf are the official instructions.Originally Posted by qquake2k
The delay grain diameters are all about 5/8" diameter, so the AT specified 3/16" drill bit is fine and what I would use. Be aware that it's really important to have a collar on the drill bit so that you don't go deeper than the 1/32" per second depth or you will find that your delay is way too short.
Bob
Senior Member
You can make one using a 10/32 screw. I got this one from Bob at Aerocon. I think he called it a peach cutter or some thing.
Tony
hybrid user
The Pro DAT drill is 1/4" and I have used it with Aerotech's Delay Drill Adapter on HPR delays and Hobbyline delays. By the way, never shorten delays to less than 4 seconds. I know, I tried.
Karl Baumheckel
TRA 11594 L3
Senior Member
Peach pitter. BTW, here's the pic that used to be up on Bob's site. This gives the directions on how to make one.You can make one using a 10/32 screw. I got this one from Bob at Aerocon. I think he called it a peach cutter or some thing.
And here are mine:
Doug
.
Senior Member
Doug, I was close on the name. Thanks for posting the instructions..I have not seem them posted for a while.
Tony
Captain Low-N-Slow
I have the Cesaroni ProDAT delay tool, but can't find the Aerotech DDA (delay drill adapter anywhere). Where did you get yours?
Captain Low-N-Slow
Is a 5/32" drill bit too small for the Aerotech 38mm delay grains? I have this really cool #0 Jacobs chuck that would make a perfect stop collar, but it will only take up to 5/32" bits.
Oblivious to time
I would like to know the answer to that also. Please let us know.
Captain Low-N-Slow
I just did a search for the Aerotech part number (T062705-1) and found it at a couple of places. It's $25 though, I didn't realize it was so expensive. I only paid $20 for the Cesaroni delay adjustment tool, so I don't think the Aerotech adapter is worth the price. If the more complex ProDAT is only $20, then I'd think the DDA should only be around $10, but that's just me. For that much money, I'll just use a drill bit.
http://www.wildmanrocketry.com/defau...pid2=103590187
http://www.balsamachining.com/hpm.htm
Member
From my experiences, I would caution against modifying an Aerotech delay with drill bits. I tried to use the 1/32" depth per second rule, without any luck. I was using an H123-W, and tried to shorten the delay from 10 down to 6 seconds. The motor ejection charge went off right as the motor burned out (A zero second delay). I can post the video of the flight if anyone is interested. I can not imagine that I was that far off with the drilling, so if the Aerotech DDA really works, then it would be well worth the money.
Senior Member
Which side of the delay are you drilling?
Legs
Member
Both ends of the delay grain are identical (or at least I thought)
Senior Member
When you drill them, where do you put the drilled side? Toward the ejection charge or toward the motor grains?
Legs
Captain Low-N-Slow
According to Aerotech, you put the hole towards the propellant (#4):
http://www.aerotech-rocketry.com/cus...y_mod_inst.pdf
Member
I put the drilled end toward the propellant, so I'm not sure what went wrong
Captain Low-N-Slow
Could it have been a bad o-ring?
Super Moderator
In theory you should drill the side of the delay grain that faces the propellant to minimize the afterburning of the delay grain after the ejection charge goes off.
Why you ask?
AT delay grains are about 0.6" in diameter and are greater than 0.3" long and can be as long as ~0.8 inches. Grains recess perpendicular to the burning surface so if you drill out the delay end facing the propellant, the drill bore widens as the delay grain burns and eventually you have a flat burning surface just like a shorter delay element.
Recession during the propellant burn time is faster than after propellant burnout. The actual recession depend on the propellant burn time and the chamber pressure. For 29 mm motors the recession during the motor burn is between 0.15" and 0.35". If you use a 3/16" drill bit to shorten the delay, the burning surface will be planar after the propellant has burned about 3/16"! Just remember that you should never shorten you delay to less than the S delay time of any motor. If you do, you risk a failure of the delay grain before the motor burns out.
Conversely if you drill out the delay facing the BP, when the delay grain burn up to the bottom of the drill bore, it will ignite the BP and the delay grain will continue to burn radially outward for several seconds after the BP has gone off. This afterburning can cook your recovery gear.
Bob
Senior Member
I would not invest in any aerotech drilling setup at the current time as their high power line is going to be changing. I saw the new system at LDRS and it involves a new forward closure, preassembled delays and a prodat style drilling tool. Apparently, it will be rolled out with the new system priced as low as possible and will be used in all high power reloads. It looked pretty slick.
more rockets then cents
Captain Low-N-Slow
Will the new system be compatible with existing hardware?
Captain Low-N-Slow
Does anybody know if you can use a 5/32" bit?
Super Moderator
You can, but you could also take a larger drill bit and grind down the end to make it 5/32" at the rear end.
The main concern as you go smaller is if there is any delay time in the propagation of the flame front down the narrow bore, the delay will be longer than planned.
Bob
U.S. Manned Space Program
Hmmmm. Interesting.
This has spawned some questions (if you know the answer):
- Was any time table announced, like later this year, next year?
- Do the design changes address any issues, such as improvement in delay time accuracy or shorten motor assembly time?
- Will the current RMS propellants work in the new hardware?
- Is any part of the new hardware compatible with the old hardware?
Greg
Captain Low-N-Slow
I emailed Aerotech, and got the following response:
"It will work...no more than a 32nd per second in depth...if you place the hole forward, just make sure the Black powder gets into the drilled hole before filling up the powder well........Karl"
So apparently a 5/32" bit will work. But his advice about placing the drilled hole forward surprised me. I emailed back, for confirmation, and got this response:
"It is a subject of debate..they both work..but I have always instructed persons in the field to do it that way and it has always worked, now if you were using a CTI delay unit you have no choice but to drill and install on the propellant side....your druthers....Karl"
It kind of makes sense, because as the delay burns upward, as soon as it hits the black powder in the drilled hole, it would set off the charge. I suppose you could put a little dog barf in the ejection charge well to keep the black powder from moving around, especially if the rocket happens to arch over after apogee.
Addicted to APCP
There better be a huge discount as I have at least 30 forward closures from 29 to 98 mm. I have 4 98 mm closures alone.
Senior Member
I ran across a recommendation at LDRS that I had never heard before. It was that for drilling the delays on 54mm Cesaroni motors for short delay times the hole should be given a slight cone shape to ensure that the delay is accurate. The idea being that the burn rate in deep holes can be slower then expected.
more rockets then cents
Super Moderator
Flame front propagation down a deep, narrow hole may not be as fast as expected because there is no convective flow to carry the hot gases forward. It's the same reason that you put your igniter at the head end of the engine because you want prompt motor ignition.
IMO you want a bore diameter to be at least approximately the same diameter as the bore depth. CTI uses a 1/4" diameter drill for a maximim delay shortening of -9 seconds which is ~1/4" deep.
Bob
Member
Hi,
The new pre-assembled delays look like the aluminum delay element in the 38mm adaptor system that AT currently sells, but it is made of black phenolic (?) with the delay element glued in. The delay element looks to be "long" in time and then you just drill out the delay to the desired time you want with AT's drill delay tool.
I mentioned to Gary that it seems to be a better system than the current one as it looks to have less parts for less possibility of user error in assembly (foreward blow-by).
AT's products are well engineered (I have NEVER had a RMS motor failure) but their user friendliness SUCKS! (Sorry Gary) Every time they come out with new loads or some advance in their system, you have to buy more hardware and the old hardware becomes obsolete! It gets expensive.
Pardon the mis-spellings... Daniel
Senior Member
We used to cut the delay time by using a pocket knife to make a cone shaped depression in the delay grain. The shape was not something scientific it is what you get with a knife. Just estimate how deep by eye.
The delay times are not very acurate anyway, +-20% IIRC is the standard they must meet for cert, a 10 second delay may be an 8 or a 12.
Mark
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