CTI delay element drilling hole size

[billdz]

With AT motors, I was advised that drilling into a delay element with a 1/4" bit reduces the delay by 1 second for each 1/32" of depth of the drill hole. Is the same true for CTI, or is there a different formula? Today I flew my first CTI and it appears that the hole was not deep enough, ejection occurred later than expected.

Thanks for any info on this,
Bill

 

[mikec]

I presume this means you didn't use the official CTI ProDAT adjustment tool?

I actually don't know what the CTI tool does, I've always used one and it's always worked.

 

[jimzcatz]

Close enough to be considered the same.

 

[billdz]

@mike - Correct, I do not have the official tool, perhaps I should have bought it. Never had a problem with AT motors just drilling 1/32" deep for each second of delay shortening.
@Jim - If CTI is also 1/32" per second, any thoughts on why my G50 with a 15 second delay did not go off after 6 seconds with a 9/32" hole drilled into the delay element?

 

[tim cubbedge]

@mike - Correct, I do not have the official tool, perhaps I should have bought it. Never had a problem with AT motors just drilling 1/32" deep for each second of delay shortening.
@Jim - If CTI is also 1/32" per second, any thoughts on why my G50 with a 15 second delay did not go off after 6 seconds with a 9/32" hole drilled into the delay element?

How long after motor burnout do you think it actually took?

 

[mikec]

any thoughts on why my G50 with a 15 second delay did not go off after 6 seconds with a 9/32" hole drilled into the delay element?

How late was it? Do you have data from the flight or was this just estimated? How did you determine how to set the delay? Did you have an accurate weight for the rocket?

 

[billdz]

It was very late, no ejection until the rocket was near the ground. I did weigh the rocket, and the Open Rocket sim said I needed a 6 second delay. So I drilled a 9/32" deep hole, to take 9 seconds off the stock 15 second delay.

 

[dhbarr]

NOT trying to critique, only asking questions: post the sim?

My immediate hunch would be higher drag and/or more crossbreeze than the sim had leading to more post-burnout decel, lower apogee, overlong delay.

 

[terryg]

I have found that when greatly shortening long delays, that accuracy is improved by coning the drilled hole with an xacto knife.

 

[billdz]

Here's the sim.

 

[Buckeye]

Here's the sim.

7 m/s off the rod? This launch was grossly underpowered.

 

[blackjack2564]

I have found that when greatly shortening long delays, that accuracy is improved by coning the drilled hole with an xacto knife.

This^ .............don't forget when setting delays, the TMT standard is + or - 20 %
Meaning set for 10 seconds could be 8 or 12. That's a 4 second spread.

I just use my pocket knife.

 

[billdz]

Could someone with the official CTI ProDAT adjustment tool please advise the diameter of the drilling bit and the length of the bit for a 9 second reduction in the delay?

 

[tim cubbedge]

Could someone with the official CTI ProDAT adjustment tool please advise the diameter of the drilling bit and the length of the bit for a 9 second reduction in the delay?

I have 2 ProDats and measured each one twice.
Dia = 0.249"
9 second delay = 0.359"

 

[billdz]

Thanks Tim. Well, 9/32 equals .281 so my hole was quite short, guess that explains the problem. Your 0.359" is almost 12/32.

 

[joe_grain]

I have 2 ProDats and measured each one twice.
Dia = 0.249"
9 second delay = 0.359"

Could you please provide measurements for other delay times as well? I would like to do a 5 second delay, but cannot get my hands on an official ProDAT.

Thanks in advance,
Joe

 

[Steve Shannon]

It’s really important to understand that HPR delay grains are inaccurate in the first place. Alan Whitmore wrote an article for the Tripoli Report in 2018 (I think) where he documented how inaccurate they are.
They are pressure sensitive, meaning that for different motors delays burn at different rates before burnout. At different altitudes they burn at slightly different rates after burnout.
They are also sensitive to contamination. A little oil or grease on the surface of a delay can really mung things up.
Bottom line is that if you want accuracy use electronics.

 

[bill_s]

Thanks Tim. Well, 9/32 equals .281 so my hole was quite short, guess that explains the problem. Your 0.359" is almost 12/32.

A tool length measurement needs to be the difference between contact and drill depth. I recall the delay grain surface is maybe 1/32" below the plastic (just drilled one yesterday but it's gone now). Also don't count the first 1-2 32nds of the drill tip, as it just mashes down the material. Try drilling a one second reduction with a bare bit and you'll see how much you have to drill before it removes much anything.

I don't know how it was measured but that could explain some difference. Also the rocket may have needed a shorter delay than that too, depending if you clocked it or just basing this on the flight appearance.

 

[tim cubbedge]

Could you please provide measurements for other delay times as well? I would like to do a 5 second delay, but cannot get my hands on an official ProDAT.

Thanks in advance,
Joe

If you don’t have the tool, with a 1/4” drill bit , remove 1/32” for every second you want the delay shortened. It’s an approximation but should be close.

 

[boatgeek]

Our TARC team found (through trial and error) that if you don't clean the drill shavings out of the hole, the delay gets longer. Their procedure now is to drill to depth, clear the hole, then spin a couple more times to clear out all the crud.

 

[Nytrunner]

I always take an xacto knife and turn the hole into a gentle cone. Nothing terrible has happened so far