24/40 E28 and F39 "certified" delay times?

[4regt4]

I'm confused (not the first time...) on the labeling of the delay times for the E28 and F39 RMS motors. For example, the short delay for the F39 reads "F39-6T(3)*" with the asterisk saying NAR certified delay time. So which is it? 6 seconds, or 3 seconds?

Hans.

 

[Rocketclar]

It's 6 seconds. There are three F39 24mm reloads in a package.

 

[4regt4]

It's 6 seconds. There are three F39 24mm reloads in a package.

No, that's not what the (3) means. The next longer delay F39 reads: F39-9T(6)*. There are not 6 in the package.

The (3) or (6) is the "certified" delay, whatever that means. I get that the motors are certified, but what is the actual delay, and why are there 2 delay times listed for each motor?

Hans.

 

[wsume99]

Recommend you ask this question in the AeroTech thread. They regularly reply to questions there.

 

[RocketTree]

There was some extensive discussion on this subject here:

https://www.rocketryforum.com/threads/delay-time-confusion-aerotech-f39-6t-reloads.129487/


Thrustcurve shows the cert data:

https://www.thrustcurve.org/simfiles/5f4294d20002e90000000287/

 

[mbeels]

I was very confused on that as well. I asked Tim at Apogee and eventually came to the conclusion that the NAR certification process measured different delays than Aerotech. Much/some of that difference was due to the different altitudes where the tests were performed. (Air pressure affects the rate that the delay grain burns) The short of it is that there is a lot of variation in delays when using motor eject. They are just not very precise at all.

 

[4regt4]

Thanks for posting the link to that older thread. But after reading it, it's still clear as mud. Seems every other post conflicts with the one above it.

I recently fired an F39-9(6) that I drilled 1/16" to shorten it by 2 seconds hoping for a 7 second delay. What I got was a 9 second delay, verified both by altimeter and a really good video shot by a friend.

So I'm still confused....

Hans.

 

[4regt4]

Oh, and PS: What is also weird is that the motors in question (F39 and E28) are both Blue Thunder propellant. But I can't fathom why that would make a difference in delay times.

Hans.

 

[4regt4]

I was very confused on that as well. I asked Tim at Apogee and eventually came to the conclusion that the NAR certification process measured different delays than Aerotech. Much/some of that difference was due to the different altitudes where the tests were performed. (Air pressure affects the rate that the delay grain burns) The short of it is that there is a lot of variation in delays when using motor eject. They are just not very precise at all.

I read in the other thread linked above about the different altitudes that the delays were measured at (sea level for NAR, 6000ft for Aerotech). But that doesn't hold water, at least to me. If that is true, shouldn't ALL Aerotech motors show a second delay in parenthesis?

Hans.

 

[wsume99]

I read in the other thread linked above about the different altitudes that the delays were measured at (sea level for NAR, 6000ft for Aerotech). But that doesn't hold water, at least to me. If that is true, shouldn't ALL Aerotech motors show a second delay in parenthesis?

Hans.

There's a reason why many flyers avoid motor eject. Delay times are not super reliable based on what I've read. I haven't experienced anything far outside the published delay time +/- 20% but many flyers have reported issues with large variances from published delay times.

My advice... always use a longer shock cord than you think you need if using motor eject. Never know when a late separation might happen.

 

[mbeels]

I read in the other thread linked above about the different altitudes that the delays were measured at (sea level for NAR, 6000ft for Aerotech). But that doesn't hold water, at least to me. If that is true, shouldn't ALL Aerotech motors show a second delay in parenthesis?

Hans.

My understanding (which is also mud) was that it was realized how confusing it was having two different delays on the motors. So they standardized to only present one delay. They only did that for a short period of time.

The duration of the motor burn has a very significant impact on the delay time. The delay grain burns very rapidly during the boost phase of the rocket. So the same delay grain has a different delay time when using in different motors.

 

[wsume99]

The duration of the motor burn has a very significant impact on the delay time. The delay grain burns very rapidly during the boost phase of the rocket. So the same delay grain has a different delay time when using in different motors.

Yes, however the change in delay is accounted for in the published nominal delay time for a given motor. This wouldn't explain variation of delay time for repeated launches with the same motor.

Bottom line, expect the delay time to vary by unknown amounts at random.

 

[4regt4]

There's a reason why many flyers avoid motor eject. Delay times are not super reliable based on what I've read. I haven't experienced anything far outside the published delay time +/- 20% but many flyers have reported issues with large variances from published delay times.

My advice... always use a longer shock cord than you think you need if using motor eject. Never know when a late separation might happen.

While overall good advice , it doesn't address *which* of the 2 numbers I should be referencing .... That was the original question.

Hans.

 

[samb]

Thanks for posting the link to that older thread. But after reading it, it's still clear as mud. Seems every other post conflicts with the one above it.

I recently fired an F39-9(6) that I drilled 1/16" to shorten it by 2 seconds hoping for a 7 second delay. What I got was a 9 second delay, verified both by altimeter and a really good video shot by a friend.

So I'm still confused....

Hans.

I don't think you'll get a better answer than the one DeltaVee got in 2017 (post #29 in the link provided by RocketTree):

"This is the answer I got from Aerotech regarding the parenthetical stuff:

The numbers in parenthesis are from NAR data observed at sea level,
Our numbers are taken at 6,000 feet. As a rule of thumb, delay material
burns faster at lower elevation due to increase pressure, so that is why
the variation is noted for contest purposes. Also delays can vary up to 20%
either way according to NFPA standards and actual physical practice.




Now charts that I've seen give pressure at sea level as 29.92 inches Hg while at 6000 feet it gives 23.99 inches Hg.
That means that pressure at 6000 feet is 80% that of sea level. I imagine that the delay consumption rate during thrust
would be relatively unaffected since the chamber pressure ought to dwarf the value of the atmospheric pressure... that
*could* account then for the 3 second shortage across the board for all delay times I suppose since the chamber presssure
would drop close to atmospheric once the main propellant charge was expended. Still the size of the difference does make
me skeptical somewhat about this explanation... but I guess I would not fully dismiss it. It just seems unlikely since if this
is in fact the cause it would be more commonly known I would think."

My personal experience since about 2008 is Aerotech delays vary. Make harness attachment points strong and mitigate zippers.

 

[4regt4]

I don't think you'll get a better answer than the one DeltaVee got in 2017 (post #29 in the link provided by RocketTree):

"This is the answer I got from Aerotech regarding the parenthetical stuff:

The numbers in parenthesis are from NAR data observed at sea level,
Our numbers are taken at 6,000 feet. As a rule of thumb, delay material
burns faster at lower elevation due to increase pressure, so that is why
the variation is noted for contest purposes. Also delays can vary up to 20%
either way according to NFPA standards and actual physical practice.




Now charts that I've seen give pressure at sea level as 29.92 inches Hg while at 6000 feet it gives 23.99 inches Hg.
That means that pressure at 6000 feet is 80% that of sea level. I imagine that the delay consumption rate during thrust
would be relatively unaffected since the chamber pressure ought to dwarf the value of the atmospheric pressure... that
*could* account then for the 3 second shortage across the board for all delay times I suppose since the chamber presssure
would drop close to atmospheric once the main propellant charge was expended. Still the size of the difference does make
me skeptical somewhat about this explanation... but I guess I would not fully dismiss it. It just seems unlikely since if this
is in fact the cause it would be more commonly known I would think."

My personal experience since about 2008 is Aerotech delays vary. Make harness attachment points strong and mitigate zippers.

Actually, that's still a bit of a non-answer. It explains why there is a discrepancy, but doesn't address *which* number to use when selecting a motor. And why don't White Lightning motors have this apparent altitude problem?

Hans .

 

[4regt4]

My understanding (which is also mud) was that it was realized how confusing it was having two different delays on the motors. So they standardized to only present one delay. They only did that for a short period of time.

It's not for a short period of time .I have some F39s with this year's date code , the label is still the same .

Hans .

 

[4regt4]

OK, so here is the bottom line. I want a 6 second delay. Which motor do I buy? And will the one highlighted in yellow pop the chute at 6 seconds, or 3? And again, if it's an altitude issue, why is it not a concern with White Lightning?

Hans.

 

[samb]

Actually, that's still a bit of a non-answer. It explains why there is a discrepancy, but doesn't address *which* number to use when selecting a motor. And why don't White Lightning motors have this apparent altitude problem?

Hans .

Fair enough. I've used the parenthetical number. I fly at about 750 feet asl. In general, though both are less than optimal, I'd rather deploy on the way down than on the way up. YMMV. Next stop, Aerotech customer service ?

 

[4regt4]

Looking at the F24 White Lightning, the NAR site shows 4 and 7 second delays, which correspond to the packaging from AT. It doesn't appear to be a date issue as to when the certs were done, as the dates for the F24 and F39 are reasonably close to each other.

Strange...

Hans.

 

[4regt4]

Fair enough. I've used the parenthetical number. I fly at about 750 feet asl. In general, though both are less than optimal, I'd rather deploy on the way down than on the way up. YMMV. Next stop, Aerotech customer service ?

Yeah, perhaps I should ask there. I hope they don't repeat the answer about the difference in altitude for the tests, because that should also be an issue for all their other motors, not just these 2 Blue Thunder loads.

Thanks all, it's been constructive.

Hans.

 

[mbeels]

It's not for a short period of time .I have some F39s with this year's date code , the label is still the same .

Hans .

Yes, the labeling hasn't changed. I mean the practice of defining the delay in that confusing way was one done for a short period of time. That motor was certified during that time, and the label has stuck with it ever since, because that was how it was certified. Motors since then have been labeled in a less ambiguous way.

Yeah, perhaps I should ask there. I hope they don't repeat the answer about the difference in altitude for the tests, because that should also be an issue for all their other motors, not just these 2 Blue Thunder loads.

It is an issue for all delays when using motor eject, it is just the physics of how delay grains burn. If you launch at 6000 feet, you'll get an average of 3 seconds longer delay, compared to sea level. If you're in between sea level and 6000 feet, you'll get something in between (on average).

Consider every delay to be +/- 3 seconds. It's all very approximate.

 

[Tractionengines]

ALL delay grains burn differently depending on conditions. Plus there is a LARGE tolerance on them to start with. Just think of it as +/- A LOT.

The (#) Is the NAR Certified value. These were only put on the listing on a few motors. [ But those that are certified thus way need to stay that way unless recertified. ] To compare to any other motor before, or after, these "oddballs" use the manufacturer values. That is what every other certification listing shows.

 

[4regt4]

I think the 800lb gorilla here is if the delay grains currently being shipped are the same as the "cert" grains from 25 years ago ....

If so , I need to use the parenthesis value , if not, the other value. But it's my very limited understanding that you can't significantly deviate from the specs that were submitted for certification. Or maybe not?

Hans .

 

[Steve Shannon]

Those certified times are what the NAR data actually saw in testing. They are right off the certification paperwork that are provided to the manufacturer (which are available on the Aerotech-Rocketry website under “resources“.
The delay times that are marked on motors now, are those that the certification authorities actually saw in testing. The certification organizations are required (NFPA 1125) to test the delays or adjust the tested delay times to sea level. However, if you’re flying from a high elevation to begin with, or your rocket flies to a high altitude, the delay grains will burn at a slower rate, more closely resembling the original Aerotech number.
Lots of things can affect the length of time a delay grain burns. Probably the two biggest factor is the burn time of the motor and the pressure within the motor while it’s burning. In composite motors with a core the delay is burning the entire time the motor burns, but at a rate that’s sometimes five times as fast as after the motor burns out. So, if a motor burns for one second longer, that can shorten the delay time by five seconds! That’s why two delay grains of identical physical length can have greatly different burn times. The age of the delay grain and any dirt on its surface also can cause it to burn differently.
So, which time should you rely on? If I were launching from sea level I’d use the NAR certified time. If I were launching from 6000 feet, I’d use the AT time.
Alan Whitmore compiled a really interesting report about how delay grains vary. It was published in the Tripoli Reports about four years ago. If you’re a Tripoli member it’s available on the Tripoli website.

 

[Andrew Brown]

Which one to buy? Personally, I'd buy the 9 second delay, drill 2 seconds out, and try it with a datalogging altimeter. If you get the nominal 9 minus 2 seconds at that point, no harm done. If you get the parenthesis value of 6 minus 2, you are at 4 s, probably no harm done with a long enough shock cord. We've burned a whole lot of E28-7's for TARC, logged the data, and they have a 7-9 s delay depending on the lot, leaning a little more on the 9 s side though. I'm also at 700 ft above sea level. Note that Aerotech plays a game where one of the delay grains is individually wrapped for both the E28 and the F39 to make USPS shipping easier. We consistently get 1 s shorter delay on the individually wrapped delay grain. The bottom line is that there is variation, and you need to characterize it. If this is for contest flying, you'll see there is a distribution and will want to center the distribution probably just after apogee to avoid the ejection charge from determining the altitude. If you get a longer delay, you can always trim it back. You will also get delay variation based on how the ignitor is fed in and how quickly the motor lights vs the delay lights.

As a side note, here is the NAR cert data on the F39 motor:
http://www.nar.org/SandT/pdf/Aerotech/F39.pdf
The average measured delay for the certification on an F39-6 was 6.35 s. I have no idea what the parenthesis actually means... It is possible that Aerotech certified the motors with a different product offering (3,6 and 9 s delays), they changed the product offering, and argued that the motor is certified by similarity.

 

[Johnly]

The parenthesis value is no longer relevant and I thought that Aerotech updated the instructions and removed it from the instruction document.
The whole parenthesis value was an artifact caused by a delay time discrepancy issue that occurred close to 30 years ago and and has since been resolved.

John Lyngdal
NAR Standards & Testing

 

[4regt4]

The parenthesis value is no longer relevant and I thought that Aerotech updated the instructions and removed it from the instruction document.
The whole parenthesis value was an artifact caused by a delay time discrepancy issue that occurred close to 30 years ago and and has since been resolved.

John Lyngdal
NAR Standards & Testing

Thanks much. But Aerotech is still selling the F39 and E28 with the confusing parenthesis value.

As for being an artifact from the original certification, does this mean that we should ignore the parenthesis value on currently produced product?

Hans,

 

[Johnly]

Thanks much. But Aerotech is still selling the F39 and E28 with the confusing parenthesis value.

As for being an artifact from the original certification, does this mean that we should ignore the parenthesis value on currently produced product?

Hans,

Can you post a picture of the motor instruction so I can read the manufacturing date code?
Yes, ignore the value in the parenthesis as it is no longer correct.

 

[4regt4]

Can you post a picture of the motor instruction so I can read the manufacturing date code?
Yes, ignore the value in the parenthesis as it is no longer correct.

Thanks.

 

[Johnly]

Much Appreciated.

John