I just drilled the delay wrong...

[Spitfire222]

I just made a pretty significant and stupid mistake, and I could use some advice on how to fix it. To date, I've only ever used single-use motors. However, I would like to venture into reloads, so I purchased a CTI 3-grain case a while back, and I bought an F59-12 reload for it. I bought the Pro-DAT 38mm tool, plus the 29mm spacer to drill the delay. OpenRocket sims showed that a 7 second delay would work, so I set about drilling that out. Well, being my first time doing this, I was so focused on doing it correctly that I got to work having completely forgotten to change the tool from the default -9 setting to the -5 setting, and ended up removing a whole 9 seconds from the delay. I'm now left with an F59-3. I obviously can no longer use this motor in it's intended rocket, and no rocket currently in my fleet will handle such a short delay. Did I just waste this motor? Is there anything I can do to add delay again so I can actually use it? Thank you for any advice.

(Just to reiterate, I know that the tool setting is for seconds of removed delay. That's not the problem, the problem is I just forgot to even set it to what I needed.)

 

[jmasterj]

If it were an Aerotech motor you could buy a new delay element, but I'm not sure if there's anywhere to do that with CTI.

 

[Worsaer]

You could fly with an altimeter for electronic deployment and remove the delay charge. No need to waste the motor.

 

[Sandy H.]

You could fly with an altimeter for electronic deployment and remove the delay charge. No need to waste the motor.

Is there a chance that the residual motor pressure could cause a separation but not full deploy when the delay burned through? I've only flown CTI motors a few times, but just figured I'd ask. Maybe a bit of epoxy where the charge used to be would be extra insurance?

Sandy.

 

[rocketgeek101]

It can't hurt to enquire with your dealer about getting a new delay element, but I think the answer is most likely to be a no. I think your only realistic options for flying this motor are to build a new rocket that needs a 3 second delay, or remove the BP charge from the motor and use electronic deployment. You could view this as an excuse to learn electronic deployment if you haven't already. Or maybe with some luck someone at your club has one of these motors and would be willing to swap your delay element for theirs if they have a rocket that could work with yours.

By the way, the F59 is one of my favourite motors!

 

[ThirstyBarbarian]

Time to build a saucer, pyramid, or spool type rocket!

There’s no way to add back the delay, so I think your best option is to ask your vendor if there is any way to buy a replacement delay element.

Also, it’s possible someone has a spare from a motor that has a different kind of problem. In fact, I probably have one, but it’s not something I can legally mail to anyone, otherwise I’d send it to you. If you live near the SF Bay Area, send me a PM.

If sourcing a replacement doesn’t work out, you could use electronic deployment, which is not very common for rockets the size you are probably talking about. You would remove the paper disk on top of the delay element, dump out the black powder, pack the charge well with dog barf, and then tape it closed.

And if electronics is not an option, then I’m serious about building some high drag designs, like a saucer, pyramid, or spool. Those are fun.

Good luck.

 

[Steve Shannon]

Fill the drilled hole with epoxy and drill it again to the correct depth.

 

[scschulte]

Fill the drilled hole with epoxy and drill it again to the correct depth.

An interesting issue and I'm having this too! (reduced a 15 second delay on a CTI Pro29 159 G118-15A 3 grain Blue Streak to 8 seconds of delay but now realize 10 seconds would be better).

OK but if we just put epoxy in the hole and redrill it- isn't there a layer of epoxy below the now shorter "drilled-out" section? When the motor burns out, that epoxy will prevent any burning there so won't the delay charge just start from the end (and a little from the sides of the drilled-out section) but basically have a delay of 12 seconds? Instead of epoxy, would it not be better to "scrape-off" some delay charge from around the top (area around the drilled hole); stuff that into the hole, compress it with a small dowel or pencil; then redrill to the correct time in this case -5 seconds instead of -9 seconds -- to give a 7 second delay? Or would that "compressed" delay charge, no matter how hard it was 'tamped' just burn-through rapidly because there would not be any binder there? (Could one add something to act as a binder?).

Be great to hear some comments / feedback here! I'm certain this issue is relevant to lots of us!!

Steve Schulte
Sunday 17 July 2022

 

[John Kemker]

An interesting issue and I'm having this too! (reduced a 15 second delay on a CTI Pro29 159 G118-15A 3 grain Blue Streak to 8 seconds of delay but now realize 10 seconds would be better).

OK but if we just put epoxy in the hole and redrill it- isn't there a layer of epoxy below the now shorter "drilled-out" section? When the motor burns out, that epoxy will prevent any burning there so won't the delay charge just start from the end (and a little from the sides of the drilled-out section) but basically have a delay of 12 seconds? Instead of epoxy, would it not be better to "scrape-off" some delay charge from around the top (area around the drilled hole); stuff that into the hole, compress it with a small dowel or pencil; then redrill to the correct time in this case -5 seconds instead of -9 seconds -- to give a 7 second delay? Or would that "compressed" delay charge, no matter how hard it was 'tamped' just burn-through rapidly because there would not be any binder there? (Could one add something to act as a binder?).

Be great to hear some comments / feedback here! I'm certain this issue is relevant to lots of us!!

Steve Schulte
Sunday 17 July 2022

You're forgetting the annular ring of delay grain around the epoxy. Face the epoxy plug towards the propellant grains.

 

[Frederocket]

Fill the drilled hole with epoxy and drill it again to the correct depth.

Steve,
Doing as you say, probably will not work, reason being: The web of the delay will now have to burn down to the bottom level of the remaining epoxy plug depth before the hoped for delay time begins. At best, the actual delay time probably would be the three seconds pre-drilled plus the length of time added by the remaining epoxy plug, after re-drilling.

The best course of action is to purchase a new delay from CTI or a vender. Other options would be; "build a rocket that requires a three second delay, or remove BP and plug the touch hole with epoxy and using the motor in a electronic recovery configured rocket".

IMO; "once a delay is drilled, it is what it is, unless you decide you need to further shorten the delay time".

 

[Steve Shannon]

You're forgetting the annular ring of delay grain around the epoxy. Face the epoxy plug towards the propellant grains.

Thanks for being the one who understood! The delay grain will burn alongside and around the epoxy plug. It doesn’t just burn within the hole.
It would probably be a good idea to chase the previously drilled and epoxied hole to the next larger diameter to get to fresh delay material. It’s the same as plugging the drilled hole and drilling a new hole alongside it.
If a new delay assembly is available that would be the best solution, but there’s no reason this couldn’t work.

 

[sharkbait]

An interesting issue and I'm having this too! (reduced a 15 second delay on a CTI Pro29 159 G118-15A 3 grain Blue Streak to 8 seconds of delay but now realize 10 seconds would be better).

OK but if we just put epoxy in the hole and redrill it- isn't there a layer of epoxy below the now shorter "drilled-out" section? When the motor burns out, that epoxy will prevent any burning there so won't the delay charge just start from the end (and a little from the sides of the drilled-out section) but basically have a delay of 12 seconds? Instead of epoxy, would it not be better to "scrape-off" some delay charge from around the top (area around the drilled hole); stuff that into the hole, compress it with a small dowel or pencil; then redrill to the correct time in this case -5 seconds instead of -9 seconds -- to give a 7 second delay? Or would that "compressed" delay charge, no matter how hard it was 'tamped' just burn-through rapidly because there would not be any binder there? (Could one add something to act as a binder?).

Be great to hear some comments / feedback here! I'm certain this issue is relevant to lots of us!!

Steve Schulte
Sunday 17 July 2022

If you know of any EX research motor makers in your club, they most likely can tell you how to fill the hole with smoke/delay mix and then you can re-drill it without issue. If you have access to the restricted research threads, post it there and I can tell you how to do it, did this once myself and a couple times for fellow flyers.

 

[Handeman]

I doubt any EX guys have delay element mixes that burn at the same rate as the factory delays. Just as propellants burn at different rates depending on mix and pressure, so does the delay element. Check the number and delay times in different motors for all the different AT delay grains.

I think @Steve Shannon solution is the best available, short of replacing the delay grain. When the motor fires, the delay element will start burning sideways in the drilled hole and down around the epoxy plug. I would consider the biggest risk is if the epoxy liquefies and burns away faster than the delay element, which it could depending on type of epoxy. If that does happen, you'll have a short delay, although not as short as the original drill time.

 

[Steve Shannon]

I doubt any EX guys have delay element mixes that burn at the same rate as the factory delays. Just as propellants burn at different rates depending on mix and pressure, so does the delay element. Check the number and delay times in different motors for all the different AT delay grains.

I think @Steve Shannon solution is the best available, short of replacing the delay grain. When the motor fires, the delay element will start burning sideways in the drilled hole and down around the epoxy plug. I would consider the biggest risk is if the epoxy liquefies and burns away faster than the delay element, which it could depending on type of epoxy. If that does happen, you'll have a short delay, although not as short as the original drill time.

I agree; it would be difficult and unnecessary to match the burn rate. Because epoxy is a thermosetting resin, it shouldn’t liquify. I have used lots of heat to try and remove epoxy. It softens and chars, but I’ve never seen it liquify. And because it has no oxidizer, it won’t burn as fast as the delay material. As the delay grain burns around the epoxy plug it should expel it through the nozzle.

 

[scschulte]

You're forgetting the annular ring of delay grain around the epoxy. Face the epoxy plug towards the propellant grains.

Sorry I am not following you… Can you attach a drawing or photo— or maybe just explain again in more detail for me. What I see is a 9 second “hole” that gets filled with epoxy (now it’s a 12 second delay) but then gets drilled down 5 seconds (so a 7 second delay)- but isn’t there epoxy just below the drilled “5 second hole” that will prevent the shorter delay? Or am I really missing something here… Thanks! Steve 17 July 2022

 

[scschulte]

I agree; it would be difficult and unnecessary to match the burn rate. Because epoxy is a thermosetting resin, it shouldn’t liquify. I have used lots of heat to try and remove epoxy. It softens and chars, but I’ve never seen it liquify. And because it has no oxidizer, it won’t burn as fast as the delay material. As the delay grain burns around the epoxy plug it should expel it through the nozzle.

OK now I’m seeing that yes, the epoxy will “block” the burning of the delay charge in the downward direction, but the sides will burn and “go-around” the epoxy plug.

JB Weld is another type of epoxy that seems to have even a higher temperature resistance than ZAP or others. For me the 2 seconds might not be enough to make a change - but for someone wanting to add 4 or more seconds of delay time to an already drilled delay charge, I’d think the epoxy plug and re-drilling should be something to try without too much risk— the delay could actually be longer than drilled however, as the burning is going around the epoxy plug.

If anyone out there has actually done this with an epoxy plug, be great to hear the results.

Thanks! Steve 17 July 2022

 

[Frederocket]

Thanks for being the one who understood! The delay grain will burn alongside and around the epoxy plug. It doesn’t just burn within the hole.
It would probably be a good idea to chase the previously drilled and epoxied hole to the next larger diameter to get to fresh delay material. It’s the same as plugging the drilled hole and drilling a new hole alongside it.
If a new delay assembly is available that would be the best solution, but there’s no reason this couldn’t work.

With a delay grain that is drilled, the delay burn starts at the bottom of the drilled hole immediately at ignition. If the pre-drilled hole is plugged, at any depth, the timed burn will not start until the delay propellant burns past the plug. That being the case, there is no way the desired burn rate will equal the desired drilled indication. Am I missing something, as I thought I understood how propellant/delay grains burn???

EDIT:
I guess I should of read closer. As you indicate, drilling the hole larger would start the delay burn at the desired depth, resulting in the desired delay.

Further edit: I was confused. What I wrote in red is not true. What is true; "the total delay time would equal the time of the remaining length of the epoxy plug, plus the distance from the end of the plug and the remaining delay grain". Hope that makes sense.

 

[Frederocket]

I thought earlier someone mentioned, (don't see the post anymore), you can replace or flip the delay grain and re-drill from the un-drilled end of the delay after epoxy filling the mis-drilled hole. I wasn't sure if that was possible and could not remember, so, I just unwrapped a CTI G250 VMAX. You can't remove the delay grain from the loads FWD closure, as the delay grain is glued into the disposable FWD closure.

 

[jmasterj]

I thought earlier someone mentioned, (don't see the post anymore), you can replace or flip the delay grain and re-drill from the un-drilled end of the delay after epoxy filling the mis-drilled hole. I wasn't sure if that was possible and could not remember, so, I just unwrapped a CTI G250 VMAX. You can't remove the delay grain from the loads FWD closure, as the delay grain is glued into the disposable FWD closure.

You could with an AT delay, but obviously not with a CTI delay.

 

[Frederocket]

You could with an AT delay, but obviously not with a CTI delay.

Yes, AT at one time, sell packaged replacement delays, may still. In addition years ago, (some may remember), AT sold some motors without delays and sold or gave you the delays separately, but that didn't last very long.

 

[crossfire]

I would just purchase another reload and use delay forward end drilled correctly and save misdrilled reload for altimeter based ejection.

 

[Spitfire222]

Thank you all for the input. I'm going to try and source a replacement delay element, otherwise I will shelf this motor for another time and take this is as a lesson learned to be more careful drilling delays next time.

 

[fyrwrxz]

Vince, pm me. I may have a solution for you but it's a ton of research on my end. All I can say is Jerone was very good to me before his accident. I MAY have a cure in the "Felix Bag" ( google Felix the Cat) Where do you live? I'm in S.Cali. Worse comes to worse, I may be able to send a homemade repair kit with exact instructions. I don't have my screen name for nothing. Include the motor designation again in your pm. I'll prolly forget.

 

[sharkbait]

The de

I doubt any EX guys have delay element mixes that burn at the same rate as the factory delays. Just as propellants burn at different rates depending on mix and pressure, so does the delay element. Check the number and delay times in different motors for all the different AT delay grains.

I think @Steve Shannon solution is the best available, short of replacing the delay grain. When the motor fires, the delay element will start burning sideways in the drilled hole and down around the epoxy plug. I would consider the biggest risk is if the epoxy liquefies and burns away faster than the delay element, which it could depending on type of epoxy. If that does happen, you'll have a short delay, although not as short as the original drill time.

lay

I doubt any EX guys have delay element mixes that burn at the same rate as the factory delays. Just as propellants burn at different rates depending on mix and pressure, so does the delay element. Check the number and delay times in different motors for all the different AT delay grains.

I think @Steve Shannon solution is the best available, short of replacing the delay grain. When the motor fires, the delay element will start burning sideways in the drilled hole and down around the epoxy plug. I would consider the biggest risk is if the epoxy liquefies and burns away faster than the delay element, which it could depending on type of epoxy. If that does happen, you'll have a short delay, although not as short as the original drill time.

The delay’s are end burning slug grains, the AT & CTI delays burn at approximately 1/8”/sec under pressure and approximately 1/32”/sec ambient. I use the word approximate because there is up to a 20% deviation with a 3 second max allowable deviation within the NFPA 1125 which is what the certified motor committee follows. After numerous tests, this is also the same burn rates that I have achieved with my delay mix. Delay mixes are very simple.
There are different delay times given for the same delay grain, with for example a 1/2” long delay grain because of burn times of different motors, a 2 second burn motor would result in a 8 second delay, the same 1/2” delay grain in a 3 second burn motor would result in a 4 second delay ect ect. There are not a bunch of different burn time delay mixes being used by the commercial manufacturers.
I would think that you also run a sizable risk of debonding or tearing away an epoxy plug while trying to drill it out with a delay cutting tool, and I wouldn’t be real keen on spitting a slug of epoxy out of my nozzle either, especially on a tiny motor like an F59

 

[sharkbait]

The de

I doubt any EX guys have delay element mixes that burn at the same rate as the factory delays. Just as propellants burn at different rates depending on mix and pressure, so does the delay element. Check the number and delay times in different motors for all the different AT delay grains.

I think @Steve Shannon solution is the best available, short of replacing the delay grain. When the motor fires, the delay element will start burning sideways in the drilled hole and down around the epoxy plug. I would consider the biggest risk is if the epoxy liquefies and burns away faster than the delay element, which it could depending on type of epoxy. If that does happen, you'll have a short delay, although not as short as the original drill time.

lay

I doubt any EX guys have delay element mixes that burn at the same rate as the factory delays. Just as propellants burn at different rates depending on mix and pressure, so does the delay element. Check the number and delay times in different motors for all the different AT delay grains.

I think @Steve Shannon solution is the best available, short of replacing the delay grain. When the motor fires, the delay element will start burning sideways in the drilled hole and down around the epoxy plug. I would consider the biggest risk is if the epoxy liquefies and burns away faster than the delay element, which it could depending on type of epoxy. If that does happen, you'll have a short delay, although not as short as the original drill time.

The delay’s are end burning slug grains, the AT & CTI delays burn at approximately 1/8”/sec under pressure and approximately 1/32”/sec ambient. I use the word approximate because there is up to a 20% deviation with a 3 second max allowable deviation within the NFPA 1125 which is what the certified motor committee follows. After numerous tests, this is also the same burn rates that I have achieved with my delay mix.
There are different delay times with for example a 1/2” long delay grain because of burn times of different motors, a 2 second burn motor would result in a 8 second delay, the same 1/2” delay grain in a 1 second burn motor would result in

 

[jimzcatz]

Steve,
Doing as you say, probably will not work, reason being: The web of the delay will now have to burn down to the bottom level of the remaining epoxy plug depth before the hoped for delay time begins. At best, the actual delay time probably would be the three seconds pre-drilled plus the length of time added by the remaining epoxy plug, after re-drilling.

The best course of action is to purchase a new delay from CTI or a vender. Other options would be; "build a rocket that requires a three second delay, or remove BP and plug the touch hole with epoxy and using the motor in a electronic recovery configured rocket".

IMO; "once a delay is drilled, it is what it is, unless you decide you need to further shorten the delay time".

Unfortunately no composite motor will hold together with a 3 second delay, it will cato on ignition. There is not enough delay material to hold the pressure of the motor burning. I was always told never go below 4 seconds, and 5 just to be safe. I agree with Steve, that method should work. To be safe and sure, get another delay or use an altimeter removing the BP.

 

[Rocketjunkie]

Unfortunately no composite motor will hold together with a 3 second delay, it will cato on ignition.

Not necessarily true. With my 18 mm motors, I started with a 1/2" long delay grain. In a Bates grain motor, burning for 1 sec. I got a 10 sec delay. In a D grain motor, burning for 3-1/2 sec, I got 0, give or take about 2. Not pretty on the - delays but the motor held together and the 0 or greater delays worked fine. No, I could not get enough consistency for reliable booster motors. A 3/4" long delay grain gave me 10 sec on the D grain motors.

 

[Rocketjunkie]

I doubt any EX guys have delay element mixes that burn at the same rate as the factory delays. Just as propellants burn at different rates depending on mix and pressure, so does the delay element. Check the number and delay times in different motors for all the different AT delay grains.

I've been using my 40-40 mix since the late '90s. It has a burn rate very close to 1/32"/sec. at ambient. To determine delay, you need at least 2 motors identical except for delay grain length. Static fire in front of a video camera and determine delay time. Ambient burn rate is difference in length divided by difference in delay time. Note that bubbles in the delay grain will shorten the delay time.

 

[fyrwrxz]

Thank you all for the input. I'm going to try and source a replacement delay element, otherwise I will shelf this motor for another time and take this is as a lesson learned to be more careful drilling delays next time.

Vince, belay that last. Do NOT pm me. Use my email: "[email protected]"
If you are really serious sbout saving that motor, I may have a fix. If not, nvmd, moving on.
Also, where are you?