Modifying RMS plus and RMS std. delays

[DeltaVee]

I wanted to get a few reloads for my 24mm rms motor casing in the 4 second range. I recently obtained Aerotech's rms delay drilling tool so I thought I'd get the E18-7 and F24-7 kits and mod them as required (openrocket says some of my rockets needed a 4-5 second delay for these motors) to use in a few cases. So far I've only used the E18's twice trying to drill them down to 4-5 seconds. In both cases the delay seemed to take the full 7 seconds although I didn't actually count very carefully. Lets just say there is "no way" the delays were closer to 4 than to 7 seconds! On the other hand I just used a F37-14a reload that I needed to drill down to 5 seconds. The thing is, the delay tool can only take up to 8 seconds off so I used my ryobi drill set to eyeball an extra second off of the delay. When I flew it in my HV Arcas kit the flight was as perfect as one could observe. I noted that the RMS plus delay not only fit better in the tool (it *says* 18-38 mm for the tool), but they were far less rubbery than the consumer delays and as a result far, far easier to drill, with a hole dead center which can't be said for the hobby delays. I can't for the life of me figure out what I might have done wrong with the E18 delays, as the depth of the hole I drilled, although a bit off-center, seemed correct. Is this a feature or a bug??

This is my first time modding delays so I'd say I need to get some expert advice at this point. The RMS plus stuff seems to be on the mark, while the hobby delay mods don't seem to handle it quite as well. Off-center hole an issue perhaps??

 

[Handeman]

Drilling delays can be tricky. It seems the current idea is that 1/32" = 1 second delay. That wasn't the case when Aerotech first started describing drilling delays. In the beginning, you obtained the length of a 10 sec. delay and a 4 or 7 second delay and divided the length by the seconds to get the depth per second of delay. All the Hobbyline motors seemed to be different and none of them seemed to be really close to 1/32" per second. The delays were different for different motors. A 0.7" delay might give you 10 sec delay in a White Lightning motor, but only 7 seconds in a Blue Thunder motor. Mostly they didn't have cross compatible delays.

I don't know where the 1/32" per second drill depth came from, but unless Aerotech changed all of their delay materials, I think that using that, or a drill tool based on that, is still looking at a +- 20% accuracy at best.

Now, with all that said, I've been using a 1/4" drill and drill stop and the 1/32" per second on Hobbyline motors with pretty good results. At least as good as you get with any motor delay estimate.

As for the off center hole, it shouldn't make any difference. Only the depth should affect the delay. You are putting the drilled part of the delay toward the propellant, right?

 

[DeltaVee]

Drilling delays can be tricky. It seems the current idea is that 1/32" = 1 second delay. That wasn't the case when Aerotech first started describing drilling delays. In the beginning, you obtained the length of a 10 sec. delay and a 4 or 7 second delay and divided the length by the seconds to get the depth per second of delay. All the Hobbyline motors seemed to be different and none of them seemed to be really close to 1/32" per second. The delays were different for different motors. A 0.7" delay might give you 10 sec delay in a White Lightning motor, but only 7 seconds in a Blue Thunder motor. Mostly they didn't have cross compatible delays.

I don't know where the 1/32" per second drill depth came from, but unless Aerotech changed all of their delay materials, I think that using that, or a drill tool based on that, is still looking at a +- 20% accuracy at best.

Now, with all that said, I've been using a 1/4" drill and drill stop and the 1/32" per second on Hobbyline motors with pretty good results. At least as good as you get with any motor delay estimate.

As for the off center hole, it shouldn't make any difference. Only the depth should affect the delay. You are putting the drilled part of the delay toward the propellant, right?

Thanks @Handeman

I did have the hole toward the propellant as far as I can remember (no proof positive after firing of course!). The 1/32" per second burn rate after motor burnout does appear to be the case for the hobby delays... I did some numerical work using the values for a given motor (eg. F24/E18) plotting things up and getting a slope for a line that shows the HDKs all falling on the line and the slope is very close to 32 seconds per inch of delay (or 1/32 inch per second if you prefer) for the hobby delays. Naturally for a given lenght, the remaining delay depends very strongly on the burn length of the propellant when the combustion chamber is pressurized and burning much faster. I think I need to try and get a good handle on drilling the delay. Like I said it was far easier to drill (and the hole was a lot cleaner) for the RMS plus delay than it was for the hobby delay.

 

[dhbarr]

Perhaps this would be useful?

https://docs.google.com/spreadsheet...cVEcUGcFRjfZyOA0cqzaaT_0/pubhtml?gid=90280042

 

[DeltaVee]

Perhaps this would be useful?

https://docs.google.com/spreadsheet...cVEcUGcFRjfZyOA0cqzaaT_0/pubhtml?gid=90280042

Useful for sure...

It does look like (from the motors listed) that it's for the 29/40-120 system and as such is consistent with the delays for the 24/40 system. However to restate things perhaps more simply, I'm starting with E18-7s, drilling 3/32" off the delay, and still get a 6-7 second delay, meaning that if the removal of material had the expected effect, then the original delay length is in the neighborhood of 9-10 seconds rather than 7. I've never seen one of these off by that much... It was as if the hole had no effect at all.

 

[Handeman]

I think @dhbarr's chart illustrates what I said about different materials and rates. The G64 has 2/32" different length delays for a 3 second delay change. The G33 uses the same length delays for both 5 and 7 second delays, obviously different materials/burn rates. The F52 uses delays that are 1/32" different for 8 and 11 second delays, again obviously different materials/burn rates.

With all those differences in Hobbyline delays, you might have had a E18-7 that would actually give you 8+ second delay and when you drilled it you got down near 6 sec. With the inconsistencies for the delay times, you might have actually shortened the delay 2 - 3 seconds, but it just didn't seem like it in flight. Did the motor chuff or was it slow to come up to pressure? That could have also affected the delay times.

I can speculate all day, but the only way I know of to get consistent deployments is with electronics.

 

[sl98]

All I fly is AT and based on my experience the delay times are not accurate with or without drilling. For example, yesterday, I had bonus delay on a D13-7. Charge fired at 9 seconds based on both on Jolly Logic altimeter data and visual observation. I've seen bonus delays at club launches. I have drilled AT delays but without much success. I mentored a TARC team a few years ago that used CTI. CTI delays when drilled seemed dead accurate.






The picture above contains delays from two different packs of F24-7. The delay from "pack A" is white in color and contains a delay space (#1). When you put the delay and spacer in the liner it protrudes slightly (#2). The delay from "pack B" is brown in color and does not contain a delay spacer (#4). The brown delay fits perfectly in the liner and does not protrude (#3). My guess is they changed the delay grain material.

I have to agree with Handeman...the only way to get consistent deployments (with AT) is with electronic deployment.

 

[DeltaVee]

General inconsistency is exactly the response I was fearing. So much for 10% variance.

 

[sl98]

General inconsistency is exactly the response I was fearing.

The only thing that is consistent is the inconsistency.

 

[DeltaVee]

All I fly is AT and based on my experience the delay times are not accurate with or without drilling. For example, yesterday, I had bonus delay on a D13-7. Charge fired at 9 seconds based on both on Jolly Logic altimeter data and visual observation. I've seen bonus delays at club launches. I have drilled AT delays but without much success. I mentored a TARC team a few years ago that used CTI. CTI delays when drilled seemed dead accurate.


View attachment 327377



The picture above contains delays from two different packs of F24-7. The delay from "pack A" is white in color and contains a delay space (#1). When you put the delay and spacer in the liner it protrudes slightly (#2). The delay from "pack B" is brown in color and does not contain a delay spacer (#4). The brown delay fits perfectly in the liner and does not protrude (#3). My guess is they changed the delay grain material.

I have to agree with Handeman...the only way to get consistent deployments (with AT) is with electronic deployment.

I guess if the pack of E18s I have is giving me a bonus delay of +2 seconds then that certainly would account for what I'm seeing. Doesnt matter much at the moment as my 24/40 casing is hanging deep in a wooded area somewhere in Maine right now :-(

 

[DeltaVee]

I think @dhbarr's chart illustrates what I said about different materials and rates. The G64 has 2/32" different length delays for a 3 second delay change. The G33 uses the same length delays for both 5 and 7 second delays, obviously different materials/burn rates. The F52 uses delays that are 1/32" different for 8 and 11 second delays, again obviously different materials/burn rates.

With all those differences in Hobbyline delays, you might have had a E18-7 that would actually give you 8+ second delay and when you drilled it you got down near 6 sec. With the inconsistencies for the delay times, you might have actually shortened the delay 2 - 3 seconds, but it just didn't seem like it in flight. Did the motor chuff or was it slow to come up to pressure? That could have also affected the delay times.

I can speculate all day, but the only way I know of to get consistent deployments is with electronics.

The motor didn't chuff btw but ignition wasn't instantaneous either. Since it was an E18 I assembled it as per instructions with the spacer in the forward end and a piece of tape over the slot... which is where the igniter bottomed out (i.e. NOT touching the delay element). I *suppose* that could have helped to delay the ignition of the delay element. (egad that looks weird in writing) Hard to imagine that it would take a second or two more for the delay to start burning after the propellant... A good experiment might be to put the propellant spacer on the nozzle end of the casing (once my replacement 24/40 comes)

 

[DeltaVee]

btw I came up with the following graphic for F39/E28s and F24/E18's. Noting in the aerotech document https://www.aerotech-rocketry.com/c...ary/Catalogs_Flyers_Data_Sheets/hdk_x-ref.pdf that there are two hdks for each motor, which can establish the slope and intercept of the function describing delay time vs. length... the lines are plotted and markers show the expected location on the lines as per the published data for length vs. delay. HDK 07 to HDK 12 is shown (markers left to right), the y axis is in seconds. Note that for F24/E18 HDK 08 effectively gives a zero length delay. As the F24/E18 F39/E28 have nearly the same thrust duration it would seem that'd be why everything falls on both lines shown.

 

[Lowpuller]

DeltaVee

I really want to understand what you are saying but I can't figure it out. Please explain more.

Thank you,

 

[DeltaVee]

DeltaVee

I really want to understand what you are saying but I can't figure it out. Please explain more.

Thank you,

The post was a bit of stream of consciousness... but basically if you focus on one of the lines, say the upper one which is valid for the 24/40 E28 or F39 reload delays....

Given that the list of delay kits for 24mm reloads, if you review the literature, for an F39 motor, the 9 second delay and 6 second delay of the prepackaged F39 reloads use the HDK 10 and HDK 7. Knowing the full length of each delay and the alleged delay they produce, this defines the equation of the line. So for this line, you can see plotted on it (as a series of asterisks) are the known lengths and the resulting expected delay of each HDK from 4 seconds for HDK 7 to 13 seconds for the HDK 12 standard (NOT the slow buring one) delay. It so happens that the E28 motor has the same burn time so the equation for that is extremely close to being the same as for the F39 and in fact HDK 9 is listed as the 7 second delay for the E28. That is represented on the upper line as the third asterisk from the right hand side as you go up the line... and what you see for that is in fact 7 seconds. So this line is valid for both the F39 and E28 reload.

The lower line represents the same approach, starting with the stated HDKs to define the line for the F24 and E18. In fact the same HDKs included in the F24 and E18 reloads have the same advertised delays, namely HDK 11 and HDK 12 for 4 and 7 seconds respectively. That means that the line definded by these two HDKs works for both reloads as well. The amusing thing is that what the lower plot shows is that HDK 7 (which is NOT on the lower line... that first asterisk is the HDK 8 data point) would mathematically yield a negative delay which is impossible. That means that the delay would be entirely consumed during the burn of the motor. HDK 8 (given the equation which produced the line of delay length vs delay time) gives a delay length of very close to zero which indicates that when the motor burnout occurs for the F24 or E18, the HDK 8 delay is completely consumed. That also tells you immediately that the burn rate of the delay for the F24 or E18 during thrust is (ideally) 13/32" per 2.14 seconds (the hdk 8 delay length per the 2.14 seconds burn time) or about 6/32" per second. So for the F24 or E18, any delay shorter than 13/32" will burn through before the end of thrust.

The delay time vs. delay length functions in fact are:
F39/E28: delay-time = 32 *(delay length) - 7
F24/E18: delay-time = 32 *(delay length) - 13

The slope of the line (32) has the units of seconds per inch (eg. 1/32" per second)

All this is based on aerotech's numbers in the xml document from their website... you have to have faith that you're not getting a bonus. The burn times of the F39/E28 are very nearly the same (as is the F24/E18) as shown by the NAR test data so you can in principle select HDKs to give you a somewhat broader range of delays before resorting to drilling for example. That's all assuming aerotech's data is ostensibly correct...

 

[dhbarr]

Just remember, putting one of the not-as-sold delays ( e.g. to get a g76-13g ) can be considered EX, though drilling is approved.

Never go below 4s.

 

[DeltaVee]

Just remember, putting one of the not-as-sold delays ( e.g. to get a g76-13g ) can be considered EX, though drilling is approved.

Never go below 4s.

The 4 second rule is a good one considering the variability of the delays in the first place. Odd thing is, I would think using the not-as-sold delay would be a better bet than drilling however, Da' rules is da' rules.

 

[dhbarr]

The 4 second rule is a good one considering the variability of the delays in the first place. Odd thing is, I would think using the not-as-sold delay would be a better bet than drilling however, Da' rules is da' rules.

I think it just means no one has asked AT for a ruling

 

[DeltaVee]

Just remember, putting one of the not-as-sold delays ( e.g. to get a g76-13g ) can be considered EX, though drilling is approved.

Never go below 4s.

dhbarr,

Can you point me at any documentation that drilling the standard delays are an approved practice? At our last launch, our LCO wagged his finger at me after a flight when I blurted out that I had shaved a few secs of my F39-9 reload. I searched in vain for a definitive ruling and would love to know that I'm not in violation!

 

[rharshberger]

iirc the 4 second rule was to keep the delay from being blown through while motor was at full pressure due to the delay being drilled to a point it was too thin to hold up to the pressure.

 

[DeltaVee]

iirc the 4 second rule was to keep the delay from being blown through while motor was at full pressure due to the delay being drilled to a point it was too thin to hold up to the pressure.

I'm good with the 4 second rule, but my LCO implied that modification of standard shipped delays (eg. the 9 and 6 sec delays for the F39) is not technically allowed, and that allowed modification only applies to those motors shipped with a single long delay (eg. F51NT-10 and the DMS G80NT-14A). My intention was to turn an F39-9 into an F39-7. Since the package designates the F39 as a 9(6), I was then "sure" that I had at least a "4"!

 

[dhbarr]

https://www.aerotech-rocketry.com/c...tions/RDK_Instructions/rms_delay_mod_inst.pdf

https://www.aerotech-rocketry.com/news.aspx?y=2010#n127

So it's clearly approved by the manufacturer as they have a printed procedure and indeed will sell you the tooling. Note that the tooling is not mandatory, it just makes it easier.

More of AT's new stuff is only coming with 10s+ delays. It wouldn't surprise me to see everything heading that way so e.g. manufacturers and vendors have less inventory to maintain.

HTH,
-dh.

PS: The 2005 example explicitly demonstrates turning an M into an S.

 

[DeltaVee]

https://www.aerotech-rocketry.com/c...tions/RDK_Instructions/rms_delay_mod_inst.pdf

https://www.aerotech-rocketry.com/news.aspx?y=2010#n127

So it's clearly approved by the manufacturer as they have a printed procedure and indeed will sell you the tooling. Note that the tooling is not mandatory, it just makes it easier.

More of AT's new stuff is only coming with 10s+ delays. It wouldn't surprise me to see everything heading that way so e.g. manufacturers and vendors have less inventory to maintain.

HTH,
-dh.

Makes sense... and the discussions here indicate it's a common practice... but does the NAR approve? Our LCO and RSO know the rules well and try to keep us out of trouble!

FWIW, according to my shiny, new, altimeter 3 (I wouldn't have gone to a launch with temps in the teens if my lovely wife hadn't given me this gem for the holidays and was so itching to get it into the air!), I ended up with a 8.3 second delay overall. The rubbery nature of the delay makes is kind of hard to get the depth clean with the level of certainty that I'd want (and I did in fact use the AT tooling... drill could be a touch sharper!).

 

[rharshberger]

Not all LCO's and RSO's have the same opinions even if they are supposed to be based on facts. As a manufacturer approved modification drilling delays is an approved proceedure and afaik NAR nor TRA has said it invalidates the motor cert or has had specific guidance against drilling delays.

 

[DeltaVee]

Not all LCO's and RSO's have the same opinions even if they are supposed to be based on facts. As a manufacturer approved modification drilling delays is an approved proceedure and afaik NAR nor TRA has said it invalidates the motor cert or has had specific guidance against drilling delays.

Seems reasonable to me... "if it's not forbidden, it's allowed" (American System) vs. "if it's not allowed, it's forbidden!" (Prussian System)

 

[Steve Shannon]

iirc the 4 second rule was to keep the delay from being blown through while motor was at full pressure due to the delay being drilled to a point it was too thin to hold up to the pressure.

Not just being able to withstand the pressure, but to avoid burn-through. If a motor burns one second longer than its certified burn time, that reduces the length of the delay. On one motor for which I did the calculation the delay grain burns at about 5/32 inch per second while the propellant is burning. Standard assumed rate after the motor has burned is 1/32 inch per second, so a motor that burns one second longer may have just shortened the delay grain by five seconds. If that delay had been shortened to four seconds, the last fifth of a second the motor could be jetting flame through the top.
Fortunately that doesn’t happen frequently, but it could.

 

[jimzcatz]

I'm not going to read through all these posts. I have over 25 years in this hobby. Adjusting the delay, whether single use or reload DOES NOT MAKE YOUR MOTOR A RESEARCH MOTOR!! Anyone telling you otherwise us just plain wrong. I have gotten the specs from both Gary from Aerotech and Jeroen from CTI. They both support the practice, hence no research. And 1/32nds per sec, don't go below 4 sec delay. Now can we put this issue to rest?

 

[UhClem]

Makes sense... and the discussions here indicate it's a common practice... but does the NAR approve?

Maybe, maybe not.
NFPA 1127 was modified in the 2012 edition to allow for user adjustment of delay times (4.5.3) while NFPA 1122 has no such provision. (1122 prohibits any alteration of model rocket motors - 4.19. ) So by the NFPA codes you can adjust the delay of a high power motor but not a model rocket motor. (NAR policy requires these be followed at all NAR launches.)

But it gets murkier. NAR insurance has been written to only care about the NAR codes. Which have some conflicts with the NFPA codes.

Both the NAR model and high power codes prohibit tampering with motors and have not been updated since the change to NFPA 1127 at 4.5.3. Nothing in the NAR codes mentions altering delay times.

 

[Andrew_ASC]

You can buy E18W-4 rather than mod an E18W-7. I drill delays on H130W-14’s down to 10s with the RMS tool and so far those have been spot on. No offense I think you are complaining about a non issue. I leave the hole drill side facing the fuel grain.

 

[DeltaVee]

Maybe, maybe not.
NFPA 1127 was modified in the 2012 edition to allow for user adjustment of delay times (4.5.3) while NFPA 1122 has no such provision. (1122 prohibits any alteration of model rocket motors - 4.19. ) So by the NFPA codes you can adjust the delay of a high power motor but not a model rocket motor. (NAR policy requires these be followed at all NAR launches.)

But it gets murkier. NAR insurance has been written to only care about the NAR codes. Which have some conflicts with the NFPA codes.

Both the NAR model and high power codes prohibit tampering with motors and have not been updated since the change to NFPA 1127 at 4.5.3. Nothing in the NAR codes mentions altering delay times.

Not sure what to say about this... the short answer for me is: do what my LCO and RSO say. Unless things get clarified I'll simply purchase the AT hobby delay kits... what the heck, it's only money.

 

[Steve Shannon]

Maybe, maybe not.
NFPA 1127 was modified in the 2012 edition to allow for user adjustment of delay times (4.5.3) while NFPA 1122 has no such provision. (1122 prohibits any alteration of model rocket motors - 4.19. ) So by the NFPA codes you can adjust the delay of a high power motor but not a model rocket motor. (NAR policy requires these be followed at all NAR launches.)

But it gets murkier. NAR insurance has been written to only care about the NAR codes. Which have some conflicts with the NFPA codes.

Both the NAR model and high power codes prohibit tampering with motors and have not been updated since the change to NFPA 1127 at 4.5.3. Nothing in the NAR codes mentions altering delay times.

This requirement from the 2018 version of NFPA 1122 would seem to permit manufacturer permitted alterations to model rocket motors:
4.19.3 No person shall alter the components of a reloadable model rocket motor or use the contents of a reloadable rocket motor reloading kit for a purpose other than those specified by the manufacturer's instructions for the reloadable rocket motor or reloading kit.

Also, NFPA 1125 allows for the certification of user adjustable delays for model rocket motors
8.1.8.1 Any user-adjustable delay reductions shall be accomplished using the manufacturer’s instructions and delay adjustment tool, if provided.

That’s good enough for Tripoli, but I can’t speak for NAR. I do know that TARC teams regularly adjust delays.