Why Engines Codes Are Often Wrong

mh9162013

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Here's an excerpt of an article I wrote for publication. I have recently realized that it won't be published, so I'm posting some portions of that article here. I hope you find it enlightening or at least interesting.


Why Are the A8-3 and A10-3T Estes Engine Codes “Wrong?”

To be clear, the A8-3 and A10-3T Estes engines aren’t the only ones that have NAR test data that are inconsistent with the engine codes. For example:
This phenomenon doesn’t just apply to Estes engines. Take the Quest C6-0. It’s an 18mm black powder engine that should have an average thrust of 6 Newtons. But according to the NAR test data for the Quest C6-0, its average thrust is only 3.95 Newtons. So what’s the explanation for this? There are at least two of them.

The first explanation applies to older engines that were named before the NAR motor labeling conventions were in effect. The Estes A8-3 engine is an example of this, so it’s been allowed to keep its original name/engine code, even though it’s now a “legacy” designation.

The second explanation comes from testing standards and rules from the NAR and the NAFPA 1125. According to Section 10.3 from NAR’s Standards & Testing Committee Motor Testing Manual (Version 1.5), a motor’s average thrust rating needs to be within 10% of the tested average thrust rounded to the nearest whole number.

Chapter 8.1.7 of the NFPA 1125 says that the “[a]verage thrust shall be within 20 percent [or 1 N (0.22 lbf), whichever is greater] of the average thrust that is computed by dividing the mean total impulse measured during propellant burn time by the mean propellant burn time.” Yet this is an incomplete explanation because of how burn time and total impulse are measured.

In Chapter 7.8.3 of NFPA 1125, it says that the engine burn time doesn’t start until the point where measured thrust rises above 5% of its eventual peak value. And the engine burn time is deemed to have stopped when the thrust falls below 5% of the peak thrust value.

Chapter 7.8.5 of NFPA 1125 says that when it comes to measuring total impulse, measurements won’t begin until the thrust of the engine rises to 5% of that motor’s eventual peak thrust. And thrust measurements won’t end until “…the point of last measurable thrust prior to ejection…”

I’m not sure if the above explains any discrepancy between an engine’s label and its NAR testing data, but I imagine it explains a lot. At the very least, it gives us a good look into the process of testing motors and certifying them and why how a motor performs in real life won’t always match what its label says.
 

cls

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Estes motors are derated when not fired at 5000 feet ASL. Where was the motor test jig?
 

Johnly

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Here's an excerpt of an article I wrote for publication. I have recently realized that it won't be published, so I'm posting some portions of that article here. I hope you find it enlightening or at least interesting.


Why Are the A8-3 and A10-3T Estes Engine Codes “Wrong?”

To be clear, the A8-3 and A10-3T Estes engines aren’t the only ones that have NAR test data that are inconsistent with the engine codes. For example:
This phenomenon doesn’t just apply to Estes engines. Take the Quest C6-0. It’s an 18mm black powder engine that should have an average thrust of 6 Newtons. But according to the NAR test data for the Quest C6-0, its average thrust is only 3.95 Newtons. So what’s the explanation for this? There are at least two of them.

The first explanation applies to older engines that were named before the NAR motor labeling conventions were in effect. The Estes A8-3 engine is an example of this, so it’s been allowed to keep its original name/engine code, even though it’s now a “legacy” designation.

The second explanation comes from testing standards and rules from the NAR and the NAFPA 1125. According to Section 10.3 from NAR’s Standards & Testing Committee Motor Testing Manual (Version 1.5), a motor’s average thrust rating needs to be within 10% of the tested average thrust rounded to the nearest whole number.

Chapter 8.1.7 of the NFPA 1125 says that the “[a]verage thrust shall be within 20 percent [or 1 N (0.22 lbf), whichever is greater] of the average thrust that is computed by dividing the mean total impulse measured during propellant burn time by the mean propellant burn time.” Yet this is an incomplete explanation because of how burn time and total impulse are measured.

In Chapter 7.8.3 of NFPA 1125, it says that the engine burn time doesn’t start until the point where measured thrust rises above 5% of its eventual peak value. And the engine burn time is deemed to have stopped when the thrust falls below 5% of the peak thrust value.

Chapter 7.8.5 of NFPA 1125 says that when it comes to measuring total impulse, measurements won’t begin until the thrust of the engine rises to 5% of that motor’s eventual peak thrust. And thrust measurements won’t end until “…the point of last measurable thrust prior to ejection…”

I’m not sure if the above explains any discrepancy between an engine’s label and its NAR testing data, but I imagine it explains a lot. At the very least, it gives us a good look into the process of testing motors and certifying them and why how a motor performs in real life won’t always match what its label says.
You might want to consult the 2022 edition of NFPA 1125 which states:
Chapter 8.1.7(c) of the NFPA 1125 says that the "Labeled average thrust shall be within 20 percent [or 10 N (2.25 lbf), whichever is greater] of the average thrust that is computed by dividing the mean total impulse measured during propellant burn time by the mean propellant burn time.”

The text I proposed and was adopted replaced a circular statement that allowed the labeled average thrust to be any value the manufacturer desired. Is it perfect, no it isn't, but the bounding box was that the text change couldn't impact any currently certified motors.
 

mh9162013

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You might want to consult the 2022 edition of NFPA 1125 which states:
Chapter 8.1.7(c) of the NFPA 1125 says that the "Labeled average thrust shall be within 20 percent [or 10 N (2.25 lbf), whichever is greater] of the average thrust that is computed by dividing the mean total impulse measured during propellant burn time by the mean propellant burn time.”

The text I proposed and was adopted replaced a circular statement that allowed the labeled average thrust to be any value the manufacturer desired. Is it perfect, no it isn't, but the bounding box was that the text change couldn't impact any currently certified motors.
What you quoted with 10 N in the quote: is that the current version of the NFPA 1125? I ask because it doesn't make any sense...or maybe I'm missing something?
 
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Johnly

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10N is in the current 2022 version of NFPA 1125. I need to find a really old version of NFPA 1125 so I can find the text that was replaced with the current version, to show the improvement in the code meaning. NAR S&T certifies motors to be compliant to NFPA 1125 and before Bob Krech wrote the NAR S&T manual we didn't have an operational document to share with the motor manufacturers. Things happened prior to that time like the A10 motor designation, that wouldn't occur today. The "or 10N" clause in the current document was driven by the A10, as changes to the code can not be made to NFPA 1125 that would push a previously compliant motor out of compliance, unless it is for safety reasons.
 

mh9162013

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But 10N is a massive "margin" for rockets to be in compliance.

So if you had a (hypothetical) A10-5 engine, it could have an average thrust of 0 to 20 N and still be within parameters. But that makes no sense.

What am I missing?
 

Johnly

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Keep in mind that 8.1.7(c) of the NFPA 1125 is for the "Labeled average thrust", which is different than the actual motor performance.
NFPA 1125 has several significant issues and the opportunity to resolve them is limited.
 

mh9162013

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Keep in mind that 8.1.7(c) of the NFPA 1125 is for the "Labeled average thrust", which is different than the actual motor performance.
Right, and that's what my article excerpt attempts to address...at least partially.

But from what you're saying, the NFPA might just be a convoluted mess?
 

CoyoteNumber2

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mh9162013, I think you're making a mountain out of a molehill.

I don't know if your article was intended to be an exposé on the matter, but the fact that motor designations do not precisely match their measured values is not an earth-shattering secret.

As far as "how a motor performs in real life" goes - motor testing data are freely available, so it's not like a manufacturer is going to pull a fast one on us. Heck, they usually include a printed thrust curve in/on the package.
 

mh9162013

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the fact that motor designations do not precisely match their measured values is not an earth-shattering secret.
No, it's not a secret. But the reason why might be...or maybe not a secret, but an unknown fact that's interesting to learn more about.

One way to learn more about why values don't match is to look at codes/regulations. But Johnly has brought to everyone's attention that the current version of the NFPA might be...oddly written.

Or maybe he's saying something else. I'm trying to figure out what he's saying.

As far as "how a motor performs in real life" goes - motor testing data are freely available, so it's not like a manufacturer is going to pull a fast one on us. Heck, they usually include a printed thrust curve in/on the package.

It's freely available, but many casual users won't be aware of it. They'll just read the packaging/instructions that came with their engines (if that) and rocket(s) and the Estes website and assume that engine codes are gospel.

I know I did until just a few months ago.
 

shockie

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Keep in mind that 8.1.7(c) of the NFPA 1125 is for the "Labeled average thrust", which is different than the actual motor performance.
NFPA 1125 has several significant issues and the opportunity to resolve them is limited.
Why is the opportunity to resolve them limited? What are these limitations of which you speak? Can you enumerate,elaborate and enlighten us?

Isn't that why the NAR and TRA has multiple influences on the NFPA 1125 technical committee to resolve issues like that?
 

Steve Shannon

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Why is the opportunity to resolve them limited? What are these limitations of which you speak? Can you enumerate,elaborate and enlighten us?

Isn't that why the NAR and TRA has multiple influences on the NFPA 1125 technical committee to resolve issues like that?
Anyone can suggest corrections to NFPA 1122, 1125, or 1127. It certainly doesn’t have to come through the NAR or Tripoli representatives on the technical committee. NAR and TRA each have a single counted vote, but have established sufficient respect that the rest of the technical committee accept their recommendations. In addition there are several others who are rocketry people who are on the committee.
In the last cycle I had a few suggestions that were accepted. The primary limitation is mostly the long revision cycle for policy changes. Errata can be revised more quickly.
 

UhClem

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Why is the opportunity to resolve them limited?
The revision cycle runs about 5 years. NFPA will be accepting input (from anyone) until June 2023. Then you can comment on stuff at a couple of points.

Anyone can do it. I have submitted changes and comments. Sometimes it even makes a difference. For example, I noticed a problem with the high power igniter rules as applied to vented hybrid rocket motors. As in the rules prohibted installing the igniter before reaching the pad which pretty much no one was following. So I suggested an exception.

You need to support your requested changes.

It is possible, barely, to make changes between cycles. I found a typo in 1127 a while back (a difference between the adopted text in a report and the published text) that was fixed.
 

Alan Whitmore

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Keep in mind that 8.1.7(c) of the NFPA 1125 is for the "Labeled average thrust", which is different than the actual motor performance.
NFPA 1125 has several significant issues and the opportunity to resolve them is limited.
I agree.
 

smstachwick

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I did some number crunching a while back and I think that explains the A8-3, but not the A10-3T.
The A8 is actually more like an A3. It was originally the A.8 (as in 0.8) when thrust was expressed in pounds.

The Estes A3 and Estes A10 are more like A2s, but the A10T has more peak thrust.

The C5 is descended from the old Centuri C5 and B8 family. They shared a nozzle design that allowed them to deliver more peak thrust over their stablemates in the B-C range. The C5 restarted production a couple of years ago and is only available as a C5-3. It's marketed as the "Super C5" on the packaging that boasts of its 50% increase in max thrust over standard C6 engines. I think the motor wrap has a "Super C" emblem as well.

Why this was allowed in the first place will likely continue to be the subject of much finger pointing, but the combination of more easily differentiated products, thrust figures useful enough to figure out what to load them in, and readily available product use suggestions is likely what allows it to continue.
 
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