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  1. #1
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    Can't shorten delay enough

    I have a CTI 1 grain 24mm E75 Vmax. For some reason CTI made the delay on that reload an extremely long 17 seconds. I'm going to be flying it in a Big Daddy and OpenRocket says the optimum delay is 5 seconds so I would need to shorten the delay by 12 seconds. The most I can drill out with the delay adjustment tool is 9 seconds. That is still too much delay and would result in a 60 mph velocity at deployment. So my question is: Is it safe to drill the delay beyond the maximum amount that you can do with the delay adjustment tool? The tool drills 1/32" for each second so if I drill out 12/32" which is 3/8" would that safely give me a 5 second delay?

    Or would it be better to just drill it down to 8 seconds with the delay adjustment tool and hope that I don't get a zipper when the chute deploys at 60 mph.

    NAR Level 3 #96210

  2. #2
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    Personally, I think drilling the delay down to 5 seconds should be fine. I would do it. Use a 1/4" drill bit and carefully measure how deep you want to drill the delay. Use a wrap of masking tape to mark how deep you want to go.

    In the late 90's I flew an LOC Warlock on an H242 at a field with a 1000' waiver. I drilled the delay down to 2-3 seconds and it popped right at apogee.

    Chris Attebery
    TRA 6602 L3
    Personal best: 37,789' 1335mph

    www.ape-rc.com

  3. #3
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    I couldn't find the page on the NAR site (a previous link hit a 404 error) but the Tripoli site has the letter from both organizations. Also, drilling beyond the maximum recommended by the vendor could void any warranty or coverage. For the best results, try to think of a better match for your rocket.
    KENN BLADE
    NAR #80160
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  4. #4
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    Tricky question possibly. If you drill extra you need to make sure that the internal pressure from the motor doesn't just push through the delay grain, by mechanically breaking it where it is thinned. That tells me there is a limit to the minimum you can go. I suspect the variability of the delay goes up (slightly more unpredictable) if you adjust more, although I might be wrong on that (or most things I say ).

    This is how I would approach thinking about it if it were my flight/rocket: Vmax are fast-burning motors, but I suspect the internal pressure in the motor is remarkably similar to the other motors that CTI have for that size case (just a big nozzle to let the reaction mass out quicker). I would make the assumption that all the delay grains are approximately the same in composition, so mechanically similar properties. So, if there is a motor delay in the 24mm series that can be drilled down to five seconds (or below) it should be able to withstand the pressure if you drill the E75 out to a similar required length. I would drill to the calculated depth and fly it.

    If anyone can correct my thinking if it is awry would be great!

    Do CTI have any recommendations? I have found them great to deal with on the odd occasion I have needed to.

    Consider getting some electronics for deployment. It takes all the guesswork out of apogee
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  5. #5
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    Make sure you read Kenn's link. I don't believe there's been an update since then, so the requirement for electronic deployment for VMAX motors should still stand.

  6. #6
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    Quote Originally Posted by Nathan View Post
    I have a CTI 1 grain 24mm E75 Vmax. For some reason CTI made the delay on that reload an extremely long 17 seconds. I'm going to be flying it in a Big Daddy and OpenRocket says the optimum delay is 5 seconds so I would need to shorten the delay by 12 seconds. The most I can drill out with the delay adjustment tool is 9 seconds. That is still too much delay and would result in a 60 mph velocity at deployment. So my question is: Is it safe to drill the delay beyond the maximum amount that you can do with the delay adjustment tool? The tool drills 1/32" for each second so if I drill out 12/32" which is 3/8" would that safely give me a 5 second delay?

    Or would it be better to just drill it down to 8 seconds with the delay adjustment tool and hope that I don't get a zipper when the chute deploys at 60 mph.
    The grain is probably the same physical length as the other motors, but Vmax motors burn for such a short period of time that relatively more delay remains after the propellant has finished burning.
    As has been mentioned, but Iíll say it more directly: Vmax motors are prohibited in motor deployment only rockets at NAR or Tripoli launches.



    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  7. #7
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    I wasn't aware of the restriction, but I don't like fast burn motors anyway. If it were me, I'd choose a different motor. The 1 grain Smoky Sam delay can go down to 4 seconds.

    http://pro38.com/products/pro24/motor.php
    NAR 91107, Level 2

    I really, really hate bugs.

  8. #8
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    Make sure you read Kenn's link. I don't believe there's been an update since then, so the requirement for electronic deployment for VMAX motors should still stand.
    I had forgotten about that Vmax issue. Very important. No people skewers please . Another good reason for electronic deployment!
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  9. #9
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    5 second delay on an E75 in a big daddy?
    Ryan Winslow
    Level 1 5/28/16 3" kitbash Patriot H128W
    Level 2 5/13/17 Polecat 4" BBIII J270W

  10. #10
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    Quote Originally Posted by Flyfalcons View Post
    5 second delay on an E75 in a big daddy?
    It has 4 oz of nose weight. According to OR max altitude would only be about 500 ft. I'm going to be flying it at a small field and this is the first flight so I wanted to keep it low. I have some 2 grain F motors but they will all take it above 1000 ft.

    This will be an NAR launch so I guess I will fly it on a F51 motor instead, and use a JLCR to hopefully keep it out of the trees.
    NAR Level 3 #96210

  11. #11
    Join Date
    22nd September 2017
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    If I were you... I would attempt a electronic trigger deployment with a power source to a FireWire and a ejection charge sized. You can run a calculation for charge size then ground test in increments if you are dead set on the Vmax motor. Aerotech Warp9 has similar issue. Delays were included but instructions were plug it then fly electronic deploy. FireWire inserts into a 9/32" vac tube or a custom charge holder. Please don't forget a magnetic switch. I'm new at this, but the timers can ignite a charge on connection when you least expect it if a safety switch isn't installed. (I had no experience and gaining it is rough some days.) Keep hands out of rocket Airframe tube when installing battery if in doubt.


    The other silly answer is pick a delay trim-able motor with less Newton seconds for a lower apogee if you don't want to risk losing a JCLR. It sounds like the field and delays are limiting your flight performance envelope. But hey I'm new.

  12. #12
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    On the Aerotech i1299N the forward closure was epoxied shut with the delay grain inserted oddly with Superlube on one end to null igniting the delay grain. Then you added the electronics and the deployment charge separately. Not certain how a CTI would work as I've seen the delays on the 29mm H motors as a self contained unit. I've heard filling a burnt forward inert CTI closure with a epoxy for head ignition is a TRA research motor. CTI instructions may vary.

  13. #13
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    I had this exact conversation with Jereon one day. He said never ever below 4 secs but 5 would be preferred. You need the delay material to contain the pressure.
    Jim Scarpine
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  14. #14
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    Jim how would you approach this problem? Would you leave material in a delay and do electronic deploy too for a shorter delay than you could trim? I'd imagine the delay would fire after the electronic deploy charge.

    The only time I messed with this was when delay wasn't long enough and a team decided to remove delay for an inert piece for electronic deploy but we had a CATO from not containing pressure as you said. The epoxy on a CTI closure wasn't the same strength as a delay grain closure.

  15. #15
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    Nathan, I would just ditch the E75 for something you can use the delay on. The rocket is not setup well for electronics.


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    Mark Koelsch
    Tripoli 6155 L3
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  16. #16
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    Is an E31 enough to get the rocket off the ground? That would be my choice- you can go down another 2 seconds, plus it'll likely go higher- WT is one of the more efficient propellants around, and the top white smoke propellant by a landslide (typ. 220s as opposed to 190 (Loki), 180 (AT, Gorilla), or 175 (CTI "slow" white)
    NAR #104043

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  17. #17
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    He said never ever below 4 secs but 5 would be preferred. You need the delay material to contain the pressure.
    That's what I thought (in post #4).

    As a few have suggested, a different motor is a good fix by the sound of it.
    TRA 13430, Level 3

    "Everybody's simulation model is guilty until proven innocent" (Thomas H. Lawrence 1994)

  18. #18
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    Quote Originally Posted by Andrew_ASC View Post
    Jim how would you approach this problem? Would you leave material in a delay and do electronic deploy too for a shorter delay than you could trim? I'd imagine the delay would fire after the electronic deploy charge.

    The only time I messed with this was when delay wasn't long enough and a team decided to remove delay for an inert piece for electronic deploy but we had a CATO from not containing pressure as you said. The epoxy on a CTI closure wasn't the same strength as a delay grain closure.
    Standard procedure is to leave the delay full length and do your electronic deployment however you want. If you don't want anything going "pop" when the delay finally burns through, empty the ejection well of the black powder and fill with dog barf. Tape over the top of it. No problemo.

  19. #19
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    I built this Big Daddy specifically to fly at a low power launch on a small field, with no electronics other than maybe a JLCR. We are starting a new NAR section in the Fredericksburg area called Northern Neck Rocketry Society. It is low power only and this Sunday will be our first launch. There will be no motor vendors so I have to go with what I've got, so I'll fly it on a F51 Blue Streak as long as the winds are low.
    NAR Level 3 #96210

  20. #20
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    Tangental question -- if you drill the delay to something other than factory-specified lengths, does it become and EX motor?

    John S. ---- NAR #96911 ---- TRA #15253 ---- MDRA #067 ---- BARC #028
    L1, 3/15/14: Aerotech Sumo, CTI H133BS
    L2, 6/21/14: Giant Leap Vertical Assault, CTI J240RL
    L3, 3/12/16: MAC Performance Radial Flyer, CTI M1101WH
    Altitude: 13,028', L3 flight; Speed: Mach ???, L3 flight

  21. #21
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    Quote Originally Posted by Bat-mite View Post
    Tangental question -- if you drill the delay to something other than factory-specified lengths, does it become and EX motor?
    If you mean shorter than tested by the certification organization, I believe the answer is no, as long as itís not shorter than the manufacturer recommends as the shortest length, i.e. ďdonít do thisĒ, but thatís the portion of the adjustment that is the very least precise. Because the timing of the delay grain is affected by so many different variables, including ambient pressure, length of motor burn (which has its own set of variables), age of delay grain, storage conditions, etc. weíre probably expecting too much trying to get within a second or two. When it does happen Iím surprised.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  22. #22
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    Quote Originally Posted by Bat-mite View Post
    Tangental question -- if you drill the delay to something other than factory-specified lengths, does it become and EX motor?
    If you overdrill a 5 to a 3, most definitely.

    If you take a motor that's only certed 7,10 and drop a 14 in there, same again.

    If they offer 4,6,8 and you drill a 6 to a 5, almost assuredly not.

  23. #23
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    Quote Originally Posted by dhbarr View Post
    If you overdrill a 5 to a 3, most definitely.

    If you take a motor that's only certed 7,10 and drop a 14 in there, same again.

    If they offer 4,6,8 and you drill a 6 to a 5, almost assuredly not.
    IIRC, there was a time when a modified commercial motor was not even EX. I don't know what the rules are now. I'd be curious to know.

    Edit: Scratch this comment, I found the TRA Research FAQ, which says kit-bashed motors are research motors.
    Last edited by rstaff3; 27th November 2017 at 05:42 PM.
    Dick Stafford
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  24. #24
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    Quote Originally Posted by Steve Shannon View Post
    If you mean shorter than tested by the certification organization, I believe the answer is no, as long as itís not shorter than the manufacturer recommends as the shortest length, i.e. ďdonít do thisĒ, but thatís the portion of the adjustment that is the very least precise. Because the timing of the delay grain is affected by so many different variables, including ambient pressure, length of motor burn (which has its own set of variables), age of delay grain, storage conditions, etc. weíre probably expecting too much trying to get within a second or two. When it does happen Iím surprised.


    Steve Shannon
    You can produce delay grains that can be pretty immune to temperature and moisture cycling over time- of course that becomes more challenging if the manufacturer is striving for effects like dense (tracking) smoke.
    As I've mentioned previously and as Steve touched on, the primary gotcha with drilling delay grains beyond the limitations of the provided recommendations is the modelling of the burn rate.
    Like the propellant, the delay grain might have a non-trivial difference in burn rate between the high pressure conditions of motor operation, and the virtual ambient pressure conditions post propellant burn ie. the rate might not be very linear. In these circumstances, you'll really need the internal ballistic parameters of the composition to accurately model the delay at a given depth.

    TP

  25. #25
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    Quote Originally Posted by rocket_troy View Post
    You can produce delay grains that can be pretty immune to temperature and moisture cycling over time- of course that becomes more challenging if the manufacturer is striving for effects like dense (tracking) smoke.
    As I've mentioned previously and as Steve touched on, the primary gotcha with drilling delay grains beyond the limitations of the provided recommendations is the modelling of the burn rate.
    Like the propellant, the delay grain might have a non-trivial difference in burn rate between the high pressure conditions of motor operation, and the virtual ambient pressure conditions post propellant burn ie. the rate might not be very linear. In these circumstances, you'll really need the internal ballistic parameters of the composition to accurately model the delay at a given depth.

    TP
    I absolutely believe you when you say those problems can be designed out. Iím just reporting what has been seen.
    For one motor that TMT analyzed, the burn rate of the delay grain while the motor was under pressure averaged five times that of the burn rate of the delay grain after the motor has ceased burning. That means that if a motor burns a fifth of a second different (longer or shorter) than the certification value, the delay may be a full second shorter or longer (or more depending on what portion of the burn profile was extended) than the certification value. TMT has also seen motors burn a second longer than the certification value. That could possibly result in the delay being five seconds shorter than specification.
    Motors and delays are just two parts of a complex system that is sensitive to many outside influences that are outside the control of end users. My point is simply that attempts to drill a delay to one second precision are probably unrealistic given all the other variables that may affect the burn rate and length remaining after motor burnout.


    Steve Shannon
    Steve Shannon
    L3CC, TAP, Director, Tripoli Rocketry Association

  26. #26
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    Quote Originally Posted by Steve Shannon View Post
    My point is simply that attempts to drill a delay to one second precision are probably unrealistic given all the other variables that may affect the burn rate and length remaining after motor burnout.


    Steve Shannon
    Agree totally.

    Troy


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