Why Engines Codes Are Often Wrong

[mh9162013]

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:

• Estes C5-3 engines should have an average thrust of 5 Newtons, but NAR test data for the C5-3 engine says it’s only 3.91 Newtons.
• Estes C6 engines (with 0, 3, 5 and 7 second time delays) should have an average thrust of 6 Newtons, but the NAR test data for the C6 engines say it’s only 4.74 Newtons.
• Estes B6 engines (with 0, 2, 4 and 6 second time delays) should have an average thrust of 6 Newtons, but the NAR test data for the B6 engines say it’s only 5.03 Newtons.

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.

 

[lakeroadster]

A lot of good data here... thanks for posting this.

 

[cls]

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

 

[Johnly]

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:

• Estes C5-3 engines should have an average thrust of 5 Newtons, but NAR test data for the C5-3 engine says it’s only 3.91 Newtons.
• Estes C6 engines (with 0, 3, 5 and 7 second time delays) should have an average thrust of 6 Newtons, but the NAR test data for the C6 engines say it’s only 4.74 Newtons.
• Estes B6 engines (with 0, 2, 4 and 6 second time delays) should have an average thrust of 6 Newtons, but the NAR test data for the B6 engines say it’s only 5.03 Newtons.

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]

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?

 

[Johnly]

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]

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]

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]

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]

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]

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.

 

[mh9162013]

IGNORE; comments merged.

 

[shockie]

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?

 

[shockie]

Always attack the messenger and ignore the message.

 

[Steve Shannon]

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]

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]

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.

 

[Ronz Rocketz]

I read somewhere that the A8-3, A10-3T, and C5-3 are old motors so the units were different than Newtons.

 

[mh9162013]

I read somewhere that the A8-3, A10-3T, and C5-3 are old motors so the units were different than Newtons.

I did some number crunching a while back and I think that explains the A8-3, but not the A10-3T.

 

[Ronz Rocketz]

I did some number crunching a while back and I think that explains the A8-3, but not the A10-3T.

I think it was Stine who said when they rereleased the C5-3, they thought about updating the name but decided to keep the old name for historical purposes.

 

[smstachwick]

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.

 

[Lord Rory Gin]

I've pretty much given up on going by Estes engine codes. I know they are rated at 5000+ ft in Colorado which gives them a ~20% better rating than at sea level. I suppose Estes could rename their entire engine lineup but that might be more confusion than it is worth.

Also look at rocket tubes - everything from the BT-5 to BT-80 don't correspond to any measurements in the real world either. If they went and changed those designations it would also foul up the entire model rocket industry for several years. Young kids wouldn't care but anyone in the hobby for more than 5 years would. This would require buy-in from all the manufacturers in the industry. It reminds me of the conversion to the metric system. In Canada we started in 1975-79 but in real life we have been bi-lingual in measurement systems ever since then because our neighbours refuse to do the same!

We just accept the dysfunctional naming systems because they are legacy and have been with us for nearly 70 years...

 

[Steve Shannon]

I've pretty much given up on going by Estes engine codes. I know they are rated at 5000+ ft in Colorado which gives them a ~20% better rating than at sea level. I suppose Estes could rename their entire engine lineup but that might be more confusion than it is worth.

Also look at rocket tubes - everything from the BT-5 to BT-80 don't correspond to any measurements in the real world either. If they went and changed those designations it would also foul up the entire model rocket industry for several years. Young kids wouldn't care but anyone in the hobby for more than 5 years would. This would require buy-in from all the manufacturers in the industry. It reminds me of the conversion to the metric system. In Canada we started in 1975-79 but in real life we have been bi-lingual in measurement systems ever since then because our neighbours refuse to do the same!

We just accept the dysfunctional naming systems because they are legacy and have been with us for nearly 70 years...

No, the manufacturers don’t certify their own products. Certification of Estes motors has been done at NAR S&T at a lab at MIT I believe. If you want to know what to expect, read the certification documents.

 

[Lord Rory Gin]

No, the manufacturers don’t certify their own products. Certification of Estes motors has been done at NAR S&T at a lab at MIT I believe. If you want to know what to expect, read the certification documents.

Yeah, No. I'm not talking about certification which is completely different. I'm talking about Estes own manufacturing rating for the motors and the naming convention from the '60s. An A8 today and an A8 from 1962 - are they the same thrust? I don't know. Was there a certification requirement back when Estes first came to market with their engines and if so was the certification done to specify the engine before it was named by the manufacturer? I do know that Estes has said many many times that their classification was based on their own testing which is done at their facility in Colorado...

 

[CoyoteNumber2]

Maybe they should just call it an A1, that way you'll be pleased when it over-performs.

 

[ghughes1138]

Yeah, No. I'm not talking about certification which is completely different. I'm talking about Estes own manufacturing rating for the motors and the naming convention from the '60s. An A8 today and an A8 from 1962 - are they the same thrust? I don't know. Was there a certification requirement back when Estes first came to market with their engines and if so was the certification done to specify the engine before it was named by the manufacturer? I do know that Estes has said many many times that their classification was based on their own testing which is done at their facility in Colorado...

Prior to 1969 the NAR and manufacturers used English units. From 69 onwards they used metric units. In 62 you would have bought an A.8 (the decimal is important) implying 0.8 lb-wt average thrust (roughly an A3 or A4 in metric).

I have a NAR S&T listing from 1965 so certification has been in effect since at least then. Curiously that list includes both the manufacturer assigned code and the NAR assigned code and they weren't always the same.

At some point in time a policy decision was made to include only the manufacturer code and the manufacturer assigned total impulse in the S&T listing. In the mid-70s members of the MIT RocSoc became interested in the discrepancies and began their own testing and publishing the results. Quite interesting as there were a few engines that not only had incorrect average thrusts, but were not even in their labeled letter class (Ds that were really Cs etc).

gary

 

[Alan15578]

Prior to 1969 the NAR and manufacturers used English units. From 69 onwards they used metric units. In 62 you would have bought an A.8 (the decimal is important) implying 0.8 lb-wt average thrust (roughly an A3 or A4 in metric).

I have a NAR S&T listing from 1965 so certification has been in effect since at least then. Curiously that list includes both the manufacturer assigned code and the NAR assigned code and they weren't always the same.

At some point in time a policy decision was made to include only the manufacturer code and the manufacturer assigned total impulse in the S&T listing. In the mid-70s members of the MIT RocSoc became interested in the discrepancies and began their own testing and publishing the results. Quite interesting as there were a few engines that not only had incorrect average thrusts, but were not even in their labeled letter class (Ds that were really Cs etc).

gary

Please scan and post that 1965 NAR listing.

 

[BEC]

Please scan and post that 1965 NAR listing.

+1 for this request.

I flew a 55-year-old A.8-3 yesterday (date code of 7 5 67) and it took an Alpha III to over twice the altitude that a contemporary A8-3 does. In digging a little deeper I have found that the actual total impulse ranges (units notwithstanding) for 1/2A, A and B changed at the same time that the English to metric units usage changed. An A under the original system could have up to 0.7 lb-sec of total impulse. That value is now 0.56 lb-sec (or 2.5 N-s). I would really like to see S&T data from that general time period!

 

[shockie]

I believe I may have some NAR motor data from the early 60's. These predate the establishment of NAR S& T.
Currently at work. When I get home I'll track the .pdf down and post it here.

The answer to the question are yesterdays Estes motors more powerful than today's?

The answer is yes, because the BP used today is not as powerful as it was back in the 60's.

Estes has used Goex primarily, but at times used Kik from the Czech Republic or CIL from Canada.

Not all BP is created equal if even the same manufacturing processes are used because different charcoal from different trees produce BP with varying levels of Carbon. High content carbon charcoal is the best. I've read that trees with greater cellulose levels make a higher carbon charcoal, which makes sense if you think about it.

 

[shockie]

I read somewhere that the A8-3, A10-3T, and C5-3 are old motors so the units were different than Newtons.

The A8-3 dates back to the A.8-3. there's a long thread over on YORF from years past, where all the old-timers relayed the history between the 2 motors. The A10-x motors are 70's motors, while the original C5-x didnt appear till the 80's I believe. I'm doing this from memory and I'm 67 so take that into consideration.
Yes up until 68-69 Estes used the Imperial or English system of units and then changed to the metric system.