Questions about Aerotech Delay Drilling Tool, Delay Kits

I've been spending nearly all weekend trying to figure out a few things. I want to buy some mid power APCP motors to use this spring, and I've been looking at Aerotech reloads as well as single use ones.

I have a few rockets I want to buy motors for, a few PSII kits and a 24mm scratch build. But there are so many motor choices out there, I've spent a large portion of time the past two days running sim after sim, trying to figure out which motor will do what with the rocket, and more importantly, what delay time I need. It's taken me a lot of time to comb through the motors available online, check thrust curves on Thrustcurve.org, run sims, and keep track of what I've already simmed.

So, I know that with SU motors, you get the delay grain the motor comes with. But then during my search, I found out about the replaceable delay kits and delay drilling tool. I've used a Cesaroni drilling tool (once), but thought the Aerotech tool was only for high power motors. Then realized maybe I was wrong about that, and should come here to ask.

So, one motor I really want to try is the 24mm F12 FJ motor. Maximum delay I can find for that motor is 5 seconds. My sim tells me I need about 7-8 seconds. There is one replacement delay kit for the F12-5, the HDK-13. Jumping from 5 seconds to 13 (if I understand how this works right) seems like a big leap to me.

If I get a long replacement delay, can I then drill it back down to the length I need? For that matter, if I buy any Aerotech reload, even 24-29 mm, can I simply buy the longest delay available, and worry about simming it later, then drill the delay back down a few seconds if I need to?

If I could spend less time simming each and every rocket and motor combination before shopping, that would be great. Especially because some of these rockets aren't finished yet, so I'm having to guestimate how much more weight glue and paint will add.

Thanks.

I think you should take another look at the listing...the bit where it says that it is for the F12-5. it is (when used with an F12 reload) a 5 second delay. the hdk-13 just means that it is the 13th delay in the series not 13 seconds.
Rex

Rex R said:

I think you should take another look at the listing...the bit where it says that it is for the F12-5. it is (when used with an F12 reload) a 5 second delay. the hdk-13 just means that it is the 13th delay in the series not 13 seconds.
Rex

Click to expand...

I can't find where it says that. It says that it's for the F15-5J motor, but the motor comes with a 5-second delay already. Are you supposed to stack one on top of the other?

Aerotech Hobby Delay Kit (HDK) for Hobby Line Motors. HDK-13 Reload Delay Kit 3-Pack to make the following 24mm reload kits: (delay kits contain all necessary parts and spacers)

F12-5J

just what it says, a 5 second delay for the F12 reload. to get a 13 second delay your motor case would need to be a 1/4" longer, there just isn't room in the case for a longer delay.
these kits are useful if, lets say you have an F12-3 and want a longer delay, or if for some reason you don't have a delay grain(lost, kit was lacking one or more delay grains).
Rex

Rex R said:

Aerotech Hobby Delay Kit (HDK) for Hobby Line Motors. HDK-13 Reload Delay Kit 3-Pack to make the following 24mm reload kits: (delay kits contain all necessary parts and spacers)

F12-5J

just what it says, a 5 second delay for the F12 reload. to get a 13 second delay your motor case would need to be a 1/4" longer, there just isn't room in the case for a longer delay.
these kits are useful if, lets say you have an F12-3 and want a longer delay, or if for some reason you don't have a delay grain(lost, kit was lacking one or more delay grains).
Rex

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OK, I see what you're saying, but I'm kind of baffled by that, because the F12-5FJ already comes with a 5 second delay. And the listing for anther delay kit, the HDK-08, is listed as a replacement for:

D9-4W, F39-6T

Two different times.

I'm not an expert at this stuff, but why replace a 5-second delay with a 5-second delay? My understanding was that the HDK was to change the delay of a motor, not replace one it already has.

Here, the HDK-13 is referred to as a "slow" delay, but I don't know how many seconds "slow" is.

If you have a 3 second delay then the HDK-13 will get you 5 seconds if that is what you need instead of 3. 5 second delay is the longest possible for that motor. I guess it will not work for your particular rocket. I need to get some new delays for some of my motors due to the age and the fact that some propellants and delays do not "get along" with each other if not packaged separately.

AT delays 101
with composite motors the delay starts burning (in theory) when the propellant lights(the stated delay time is measured from motor burn out). the burn rate before burn out depends on the temperature and pressure inside the case which is dependant on propellant type (and nozzle diameter)and the surface area of the burning propellant, so different reloads will net you a different delay using the exact same delay grain. the delays come in different lengths and formulations (which you don't need to memorize, the motor makers have done this for you). all you need to do is get the reload kit that has the closest delay time(a bit longer works as you can adjust down).
as for why you might need a delay kit, well...on occasion a reload kit might be missing one(had that happen twice), or it might get lost/misplaced. not to mention that one might buy a reload kit with the wrong delay for the rocket they wish to fly.
Rex

You might need to replaced a damaged delay assembly or one contaminated with grease. You also might need to go from a F12-3 to an F12-5 delay.

Well, the F12 issue is a bummer. I was hoping I could stretch the delay to 7 or 8 seconds. Well, perhaps once I've finished the build, it will be heavy enough to use a 5-second delay (I got the weight I was simming with by weighing all the parts, then adding a few grams of "guess weight").

Still, it sounds as if I am in luck when it comes to drilling out delays that are a little long. For some reason, I got it into my head that you could only drill out AT delays on certain high power motors. Not sure why I thought that.

I guess for now I'll stick with just getting motors for rockets I'm sure of. I'm just trying to stock up bit by bit during the winter so I have a variety of motors to choose from when spring comes.

Check the AT delay chart for the correct delay part number. https://www.aerotech-rocketry.com/c...ary/Catalogs_Flyers_Data_Sheets/hdk_x-ref.pdf

Bob

Daniel, I don't do the computer simulations and thus I'm talking beyond my pay grade. I see that the F12 is a low impulse, long burning motor and comes with either a 2 or 5 second delay. https://www.nar.org/SandT/pdf/Aerotech/F12.pdf

I am familiar with shortening delay grains by drilling but haven't a clue as how one could lengthen a delay without using a different, slower burning chemical formula for the delay grain. What size rocket (weight, exterior dimensions) were you planning to fly this in? I think you would need a very light weight, small or minimum diameter rocket for a delay longer than 5 seconds. I would follow the link that Bob Krech recommended and note that the HDK-13 grain is noted as "slow" and I assume this means that it will yield a 13 second delay which could be shortened down to the 7-8 second range by careful drilling. Verify anything I say.

Ted

Pour a little water into the nozzle before you stick the igniter in. Guaranteed to create a very long delay.

tmacklin said:

Daniel, I don't do the computer simulations and thus I'm talking beyond my pay grade. I see that the F12 is a low impulse, long burning motor and comes with either a 2 or 5 second delay. https://www.nar.org/SandT/pdf/Aerotech/F12.pdf

I am familiar with shortening delay grains by drilling but haven't a clue as how one could lengthen a delay without using a different, slower burning chemical formula for the delay grain. What size rocket (weight, exterior dimensions) were you planning to fly this in? I think you would need a very light weight, small or minimum diameter rocket for a delay longer than 5 seconds. I would follow the link that Bob Krech recommended and note that the HDK-13 grain is noted as "slow" and I assume this means that it will yield a 13 second delay which could be shortened down to the 7-8 second range by careful drilling. Verify anything I say.

Ted

Click to expand...

I just looked at this again and have come to the conclusion that I should have waited until the drugs wore off. The HDK-numbers are merely a list of delay grains that vary in length and propellant type, most being "standard" and others being "slow". You're on your own...carry on.

Try a F35W-11 ... nice long delay for an AT 24mm motor.

Ever since i started buying RMS from Wildman have simplified and only buy them wirh the longest delay.

BONUS : almost never have to account for or install delay spacer into the assembly liner..

Kenny

tmacklin said:

I just looked at this again and have come to the conclusion that I should have waited until the drugs wore off. The HDK-numbers are merely a list of delay grains that vary in length and propellant type, most being "standard" and others being "slow". You're on your own...carry on.

Click to expand...

This is the rocket I'm looking to use it in:



Having simmed it again, it looks like it will be fine. The optimal delay for that motor should be 6.46 seconds, but there are no red flags from OpenRocket about deployment at high velocity. I'd like to coax a little more time to get the rocket to apogee before deployment, but the motor should be fine. I plan on trying others as well, but I like the longer burn time on this motor.

Your simulation seems to be predicting more altitude than I would expect. I ran a quick and dirty sim on ThrustCurve https://www.thrustcurve.org Using a body diameter of 2.60in and a empty weight of 211g, they predict an altitude of 994feet (303meters) and an optimum delay of 4.4seconds. They also warn of that "speed at end of launch guide too slow (40ft/s)", but that is for a 3foot rod/rail.

Here is a little more background regarding the delays. The chart that Bob K. referenced, says an and HDK-13 is the delay used in the F12-5J. It is already the longest physical delay at .625inch length and it's delay formula is the SLOW composition. There is simply no way to make a longer delay for the F12. The reason for this is the delay burns faster while under the higher pressure during thrust, and the F12 is a longish burning motor at 2.9seconds.

I hope that helps and good luck

Graduator said:

Your simulation seems to be predicting more altitude than I would expect. I ran a quick and dirty sim on ThrustCurve https://www.thrustcurve.org Using a body diameter of 2.60in and a empty weight of 211g, they predict an altitude of 994feet (303meters) and an optimum delay of 4.4seconds. They also warn of that "speed at end of launch guide too slow (40ft/s)", but that is for a 3foot rod/rail.

Here is a little more background regarding the delays. The chart that Bob K. referenced, says an and HDK-13 is the delay used in the F12-5J. It is already the longest physical delay at .625inch length and it's delay formula is the SLOW composition. There is simply no way to make a longer delay for the F12. The reason for this is the delay burns faster while under the higher pressure during thrust, and the F12 is a longish burning motor at 2.9seconds.

I hope that helps and good luck

Click to expand...

It does help. I didn't know that about the pressure affecting the delay burn time.

As for the launch velocity, I'll be launching it off a longer rod - or perhaps a 1500mm MakerBeam rail - I haven't decided yet (the rocket is still in construction).

I think the delay will be OK. I'll redo the sim once the rocket is built and painted, and I plan to fly it with an altimeter to see what the actual altitude ends up being.

Thanks for the help.