Confused

A teacher brought up a point today that has me stumped.

An Estes A8-3 engine is classified as:
A - thrust range of 1.26-2.50 total impulse
8 - average thrust
3 - delay

Looking at the certification for the engine, everything is fine except for the average thrust. It is not even close to 8, but only 3.18 ( 2.32/0.73 )
from https://www.nar.org/SandT/NARenglist.shtml
https://www.nar.org/SandT/pdf/Estes/A8.pdf

The engine should be classified as A3-3 or A 4-3 according to the certification.

So what do I give as the answer ?

:confused2:

( and no hitch hiker's joke please)

I thought it was:

A8-3

A - Average total power (area under the thrust curve)
8 - Max impulse (peak of the thrust curve)
3 - Delay in seconds

That's why a D12-3 can actually be a better engine for heavy models than a E9-4...because it gives you more 'punch' off the pad...but a E9-4 will burn longer.

FC

The actual tested average thrust is often different from the manufacturer's designation shich is a combination of advertizing and history.

you see, motors hcanged design over the years, but manufacturer's cannot easily change the motor designations since the consumers have 'grown up' with the motor types (A8-3, B6-4, C6-5, etc.) and will balk at buying a motor type they are not familiar with. Also, instructions are printed and list the familiar motor types.

Of course, this has happened in the past when they switched from English to Metric motor designations, but the average thrust was relatively accurate back then. THEN later, the propellent has changed over time and there have been large motor redesigns (The older super thick motor casings held the B4 and the thin casings held the B6, but then the thick casings went away and the B4 was re-engineered for the thin casing with the larger internal diameter of the propellant grain, hence the HUGE nozzle compared to the B6).

The NAR changed their S&T rules not too long ago to prevent this average thrust issue, and motors may actually be redesignated in the future. There was a statement somewhere regarding this effort that is going on now in advance of the next major motor list update for publication (August 2013 ?).

Not sure if this will affect motors like the Estes A8 and the Quest C6.

Minor average thrust differences from tested data and the advertized designation are allowed, especially if there are several differnt motors with essentailly the same average thrust. Look at G motors and understand how confusing it would be to have 4 or 5 different G80 motors with different propellant types. So, they call them G80, G79, G78, G77, G76 etc.

sunward said:

A teacher brought up a point today that has me stumped.

An Estes A8-3 engine is classified as:
A - thrust range of 1.26-2.50 total impulse
8 - average thrust
3 - delay

Looking at the certification for the engine, everything is fine except for the average thrust. It is not even close to 8, but only 3.18 ( 2.32/0.73 )
from https://www.nar.org/SandT/NARenglist.shtml
https://www.nar.org/SandT/pdf/Estes/A8.pdf

The engine should be classified as A3-3 or A 4-3 according to the certification.

So what do I give as the answer ?

:confused2:

( and no hitch hiker's joke please)

Click to expand...

The value for average thrust can be higher than total impulse if the burn time is less than 1 second, so I always figured that motors like the A8 got up to the 8 by fudging the calculation and using "effective" burn time rather than the actual total.

I can't remember where I saw this, but I did see an explanation like that someplace besides my own musings, I think. If you stop measuring the burn time of the A8 after that first peak of thrust is over (around 0.3 sec), you get closer. So total impulse is 2.32 Newton-seconds, and let's call the burn time 0.29 seconds (a little short, but what's a hundredth of a second between friends) and you get 2.32/0.29=8. Voilà. I told you it was fudged.

shreadvector said:

The NAR changed their S&T rules not too long ago to prevent this average thrust issue, and motors may actually be redesignated in the future. There was a statement somewhere regarding this effort that is going on now in advance of the next major motor list update for publication (August 2013 ?).

Not sure if this will affect motors like the Estes A8 and the Quest C6.

Click to expand...

Unfortunately it's not retroactive so currently certified motors get to keep their average thrust even when they are recertified. Any newly certified model rocket motors have to be labeled with an average thrust that is + or - 10% of the tested average thrust.

And a 2x4 isn't 2" x 4", nor a 1" dia pipe actually is 1" in diameter inside or out, nor...

just something we've come to expect, i guess..

And then there's the ultimate example of "World's Finest Chocolate". Bleeech

dr wogz said:

And a 2x4 isn't 2" x 4", nor a 1" dia pipe actually is 1" in diameter inside or out, nor...

just something we've come to expect, i guess..

Click to expand...

I'm not sure, but In the specific case of the A8, it may go all the way back to the metric conversion that happened back in the 60s -- the motor called the A.8 (0.8 lbf-seconds) in the 1967 Estes catalog became the A8 in the 1968 catalog, even though the conversion factor should have made it the A3 or A4. There may have been some fudging of the burn time to make this happen. See https://www.ninfinger.org/rockets/catalogs/estes67/67est44.html and https://www.ninfinger.org/rockets/nostalgia/68estp48.html

Estes can do and say whatever it wants. It's the 400lb gorilla of rocketry. Heck, they could call it a royale with cheese-3 if they wanted to. I'd buy a pack.

mikec said:

I'm not sure, but In the specific case of the A8, it may go all the way back to the metric conversion that happened back in the 60s -- the motor called the A.8 (0.8 lbf-seconds) in the 1967 Estes catalog became the A8 in the 1968 catalog, even though the conversion factor should have made it the A3 or A4. There may have been some fudging of the burn time to make this happen. See https://www.ninfinger.org/rockets/catalogs/estes67/67est44.html and https://www.ninfinger.org/rockets/nostalgia/68estp48.html

Click to expand...

No. As I said, there were thick walled casings and the thick walled A motor was the A5 and the B was a B4. The thin walled motor was the A8 and the B was a B6 and there was finally enough room inside for the C6.

ORIGINAL A8, B6, and C6 motors had somewhat different nozzles and definitely had different Black Powder than available today. They have been re-engineered and have slight tweaks. For instance modern B6 motor have had the same nozzle for a few decades, but the depth of the centerbore is now shallower.

I think you will find that the ORIGINAL A8 was indeed an A8.

shreadvector said:

I think you will find that the ORIGINAL A8 was indeed an A8.

Click to expand...

The table in the 1968 Estes catalog (the first year that the motor was called that AFAIK) says that the A8 has a total impulse of 2.5 N-s and a burn duration of 0.42 seconds, making it an A6. Unfortunately the scan of the thrust curve is too low-res to read in detail. As you say, there was also an A5 with 2.5 N-s and a burn duration of 0.56s, which would be have an average thrust of 4.5 N. I don't know if the motors were reengineered when the metric conversion happened, or just relabeled (I'd have guessed the latter for no good reason.)

As Sully points out upthread, the calculation of average is quite sensitive to what the burn duration is said to be.

JPVegh said:

Estes can do and say whatever it wants. It's the 400lb gorilla of rocketry. Heck, they could call it a royale with cheese-3 if they wanted to. I'd buy a pack.

Click to expand...

Estes is very good about playing by the rules. They sent us a batch of motors after we made the average thrust labeling change but before we notified the manufacturers. They missed the 10% by a tiny amount. S & T would have let them keep the slightly inaccurate average thrust because we (actually it was me) were late in getting the notice out. However, they made an adjustment and sent out a new batch that was within the limits.

The average thrust for an Estes A8 is about 3.5 N, so it is really should be classified as an A4, at best.



Greg

mikec said:

I'm not sure, but In the specific case of the A8, it may go all the way back to the metric conversion that happened back in the 60s -- the motor called the A.8 (0.8 lbf-seconds) in the 1967 Estes catalog became the A8 in the 1968 catalog, even though the conversion factor should have made it the A3 or A4. There may have been some fudging of the burn time to make this happen. See https://www.ninfinger.org/rockets/catalogs/estes67/67est44.html and https://www.ninfinger.org/rockets/nostalgia/68estp48.html

Click to expand...

I'm 99% confident that you are correct. In the early days of hobby rocketry when Estes was the dominant manufacturer, US engineering calculations were performed in US units (pound, feet, seconds) instead of the metric units (kilogram, meters, seconds) we use today.

I believe the change occured in the late 1960's so the old A.8 (lb.) motor would ahve been relabeled as an A-4 (N) motor, but motors in commerce before the metric conversion were allowed to keep the US system labels they were known by to avoid market confusion.

Back in 2007 we formally recodified the NAR S&T motor testing standards to 100% comply with NFPA 1125, and we tightened the NAR motor labeling standards to require the labeled average thrust to be within 10% of the actual certified value. Again motors in commerce were exempt, however if a motor went out of production and was later put back into production, the reintroduced motor has to comply with the new labeling standards.

Bob

bobkrech said:

... 1960's so the old A.8 (lb.) motor would have been relabeled as an A-4 (N) motor, ...

Click to expand...

uhm, no.

8 pounds force = 35.5857729 newtons from online conversions.

bobkrech said:

..... Again motors in commerce were exempt, however if a motor went out of production and was later put back into production, the reintroduced motor has to comply with the new labeling standards....

Click to expand...

Even of the conversion were true, why would they be exempt?

NAR publishes the standards and certifies the engines.

Still confused.

sunward said:

uhm, no.

8 pounds force = 35.5857729 newtons from online conversions.

Click to expand...

.8 pounds force = 3.5585773

Close enough to a A4 for me.

sunward said:

uhm, no.

8 pounds force = 35.5857729 newtons from online conversions.


Even of the conversion were true, why would they be exempt?

NAR publishes the standards and certifies the engines.

Still confused.

Click to expand...

See below.

bandman444 said:

.8 pounds force = 3.5585773

Close enough to a A4 for me.

Click to expand...

And because the prior standards were in US units....and that's how the older certified Estes motors were labeled....so the ~10 preexisting certified motor labels were grandfathered....so it did not adversely effect sales.....of a known entity....that everyone in the hobby purchased....if they were in the hobby before the standard changed.

Bob

Interesting, I didn't know that about pounds-force units in the 60s. And the average is close to 0.8lbf (based on the NAR thrust curve):



https://www.thrustcurve.org/simfilesearch.jsp?id=1251

By the way, the average thrust is normally calculated not over the whole burn time, but discounting the ignition and trail-off time by imposing a threshold of 5% of the maximum thrust. (You will see these start and end times marked on all the ThrustCurve.org graphs and the calculated average is only over this time.)

sunward said:

uhm, no.

8 pounds force = 35.5857729 newtons from online conversions.


Even of the conversion were true, why would they be exempt?

NAR publishes the standards and certifies the engines.

Still confused.

Click to expand...

Until we tightened up the average thrust labeling requirements a couple of years ago, S & T went strictly by NFPA 1125 which was worded in such a way that, for a model rocket motor, a manufacturer could pretty much pick any number and call it the average thrust. The wording for a high power motor clearly says what it's supposed to be so this wasn't a mistake. I've been told that, because NFPA doesn't put "grandfather" clauses in its codes, the only way keep existing mislabeled motors was to make the wording vague. S & T has known about this for years. My predecessor as secretary made several attempts to get the NAR Board to allow us to go outside 1125. When the Quest C6 came out and we began reading reports of bad flights by people who assumed a C6 was a C6 it opened the door for us to make a case to the Board that something needed to be changed. We have always published the correct numbers on the engine data sheets. Unfortunately very few people take the time to read them.

The current A8 thrust curve is *NOT* the same as the older A8 thrust curve from the early 1970's (and I mean metric A8, not english A.8).

The motor changed over time.

THe old English A.8 motor had the super thick casing just like the A5 motor had. I think the English A.8 became the Metric A5. A5 m0tors went away in the 1970's....

https://www.ninfinger.org/rockets/nostalgia/70est092.html

https://www.ninfinger.org/rockets/nostalgia/70est084.html

A5 and B4 motors were metric versions of the older super thich casing original motors. A8, B6 and C6 motors used the newer thin walled casings. The different propellant slug diameters resulted in different thurst/time curves.

Over time the A5 went away and the B4 was re-engineered into the thin walled casing with the larger nozzle. The A8 was also re-engineered. It used to have a tiny nozzle, but it now has a larger nozzle (just like the B4 and 1/2A6 motors).