AeroTech M1939

[lmt56]

Can anyone tell me if the AeroTech M1939 motor is now rated as an N Motor?
From thrustcurve.org.
Cert. designation N1691
Total impulse 10481.5 N/s
2 percent N.
I have been told the M1297 motor was at one time rated as a L motor.
Insight anyone.

 

[UhClem]

What happened to the L1120, now M1297 is a mystery. When that happened I asked some questions and as a result I have serious doubts about the TMT procedures. It appears that (at the time at least) they did not send their load cells out for calibration. Instead they put a test load on it and called that a calibration. Ignoring the problem that the test load was far below the thrust of the L1120. (There being no way to know if the load cell was linear out to that load or not.) None of the other similar Aerotech 75mm reloads had anything close to the same thing happen.

If you go the the TMT web pages you will find that the "Motor Designation" is M1939 but the "TMT Metric Designation is N1691. (I have no idea what if any significance that has.)

Now if the motor certification class controls the safe distance then you need to be 500' back but if it is the certified impulse, then it is 1,000'. Looking at the table in NFPA 1127 I think a good case could be made for 1,000'

It has usually been treated as an M motor.

 

[stickershock23]

its that 20% plus or minus thing...

 

[jimzcatz]

Can we say M2900 turning into an M3500?

 

[JStitz]

Once Gary got the M Blue Thunder loads for those casings everyone kinda looked at the WL M's as boring...

 

[cwbullet]

I have wondered how that happens. Then again, an M has a lot of propellant so a little change in the quality of measurement might make a big difference.

 

[ECayemberg]

If I remember correct, the M1939 has *always* been a baby N, at least since the early-mid 90's. I have a feeling that the original certification either tested (or was simply declared) to be a full M at 10,240 N-s, then was tested to be a 2% N after the designation "M1939" was already in place.

Jstitz; I don't know many people that find the M1939 boring; I'm certainly not one of them! I've seen a lot more 98mm White Lightning burned over the years than 98mm Blue Thunder!

-Eric-

 

[JStitz]

Yea....some folks will take the soft ride.:bangpan: No "M" is boring...now, that baby K-550..that is boring if you have seen a hundred.

Frank broke the monopoly up.

 

[JDcluster]

Yes, I believe this is true. I remember reading it in TMT published certified motor directory.
They haven't published one in ink in quite a few years now.
I think it was 1996 or so.....



JD

If I remember correct, the M1939 has *always* been a baby N, at least since the early-mid 90's. I have a feeling that the original certification either tested (or was simply declared) to be a full M at 10,240 N-s, then was tested to be a 2% N after the designation "M1939" was already in place.

Jstitz; I don't know many people that find the M1939 boring; I'm certainly not one of them! I've seen a lot more 98mm White Lightning burned over the years than 98mm Blue Thunder!

-Eric-

 

[bobkrech]

its that 20% plus or minus thing...

With NAR certifications that does not apply at impulse class boundaries. Deviations over 1% require relabeling or adjustment.

https://www.nar.org/SandT/docs/ST-MotorTestingManual.pdf Section 6.6 Motors Testing Outside their Total Impulse Class

In round numbers a full M or minimum N has about 5000 grams of propellant. A 1% adjustment is 50 grams. That a readily measurable and reducible quantity of propellant.

Bob

 

[UhClem]

With NAR certifications that does not apply at impulse class boundaries. Deviations over 1% require relabeling or adjustment.

It is nice to see that NAR has standards but I was looking at NFPA 1125 and I see no requirements on labeling of motor impulse. While there is a requirement that the imprinted average thrust be within 20% of the tested average thrust, the only requirement on total impulse is that the standard deviation be less than 6.7%.

Since the total impulse is so important to determining the correct minimum safe distance for use, this seems like an odd oversight by NFPA. Perhaps this could be fixed in the next version.

 

[CCotner]

My impression has been that TRA/NAR/CAR have always skated under the loose and unenforced definitions in NFPA documents. If Aerotech wants to call it an M, then it becomes an M. It disgusts me a little bit but I get over it.

 

[mikec]

Since the total impulse is so important to determining the correct minimum safe distance for use...

Safe distance is obviously quantized. It's not like a motor suddenly becomes 1.5x more hazardous going from 10235 N-s to 10245 N-s (for example.) You're always free to launch at more than the specified M distance if you feel it's advisable.

 

[cjl]

My impression has been that TRA/NAR/CAR have always skated under the loose and unenforced definitions in NFPA documents. If Aerotech wants to call it an M, then it becomes an M. It disgusts me a little bit but I get over it.

Why does it disgust you? It's 2% off, which is close enough that some of them probably are M motors in reality, depending on manufacturing tolerances and conditions on the day of flight. There's probably at least that much variance introduced simply due to different ambient pressures at different launch sites, not to mention temperature of the grains before ignition. As Mike said, it isn't like a motor is suddenly substantially more hazardous by going up a percent or two in total impulse. Actually, I'd consider the 1939 a pretty tame motor - it uses a pretty cool burning propellant, and doesn't run at a terribly high pressure either. It's well proven, and has been flown an enormous number of times.

(Actually, running some calculations, the ambient pressure can make a surprisingly large difference - motors with the CTI 98mm XL nozzle have about 25 pounds more thrust at Hartsel, CO than they do at sea level, which is enough to bump the N5800 just barely into the O class, going by certified numbers)

 

[bandman444]

Why does it disgust you? It's 2% off, which is close enough that some of them probably are M motors in reality, depending on manufacturing tolerances and conditions on the day of flight. There's probably at least that much variance introduced simply due to different ambient pressures at different launch sites, not to mention temperature of the grains before ignition. As Mike said, it isn't like a motor is suddenly substantially more hazardous by going up a percent or two in total impulse. Actually, I'd consider the 1939 a pretty tame motor - it uses a pretty cool burning propellant, and doesn't run at a terribly high pressure either. It's well proven, and has been flown an enormous number of times.

(Actually, running some calculations, the ambient pressure can make a surprisingly large difference - motors with the CTI 98mm XL nozzle have about 25 pounds more thrust at Hartsel, CO than they do at sea level, which is enough to bump the N5800 just barely into the O class, going by certified numbers)

That is very interesting Chris. What equations or programs are you running to get those numbers.

I'm just curious.

 

[CCotner]

You can calculate the difference in relative pressure ratios across the nozzle based on the nominal air pressure differences at the respective altitudes, and from that calculate a scalar correction to the thrust curve-and then re-integrate the thrust curve to get the total impulse-all of which can, more or less, be done by hand or on wolfram alpha.

 

[cjl]

You can calculate the difference in relative pressure ratios across the nozzle based on the nominal air pressure differences at the respective altitudes, and from that calculate a scalar correction to the thrust curve-and then re-integrate the thrust curve to get the total impulse-all of which can, more or less, be done by hand or on wolfram alpha.

Actually, it's easier than that - I just assumed that the exhaust velocity would be unchanged (a pretty darn good assumption, since the flow is fully choked in either case), meaning that the momentum thrust would be unchanged. The difference in pressure thrust is simply equal to the exit area of the nozzle multiplied by the difference in ambient pressure between hartsel and sea level (which is about 4.1 pounds per square inch). It can be done on google calculator in about 30 seconds. The one piece of information that can be a bit hard to find is nozzle exit area - I happened to know it for the 98mm xl nozzle off the top of my head, but it isn't in the dimensioned drawings for most motors.

 

[bobkrech]

My impression has been that TRA/NAR/CAR have always skated under the loose and unenforced definitions in NFPA documents. If Aerotech wants to call it an M, then it becomes an M. It disgusts me a little bit but I get over it.

That's interesting. You are not a member of NAR S&T, and AFAIK not a member of TRA TMT or CAR MTC either. I can not speak for TMT or MTC but as I compiled the NAR S&T Motor Testing Manual, I take exception to your comments.

The S&T motor certification procedures are a superset of the NFPA recommendations. We are 100% compliant with NFPA and have several additional S&T requirements, such as the Impulse class edge restrictions and periodic recertification. These additional requirements are consumer protection related requirements imposed by S&T on the manufacturers to check if the NFPA mandated manufacturer lot QA testing is producing motors similar to those certified, and is an independent check to see if the motors you are purchasing are indeed that they are claimed to be.

By convention, the impulse class boundaries are based on the total impulse measured at, or corrected to, sea level pressure. S&T does its certifications at sea level using load cells calibrated with NIST calibrated load cells, so correction factors are not required. Specific impulse increases with altitude, so the situation is reversed from what you think it is. For example, a motor measured as a baby H at altitude will measure and be certified as an almost full G at sea level. Etc.

Bob

 

[JStitz]

So,that brings up where the TMT testing was done.Was the testing done in rotating locations? Do they calibrate the load cells? how? with what?
no attitude here..just a curious george..:cyclops: BTW,how many times has Gary re-certified this load? If I remember ISP made this load back in the early 90's.

 

[stickershock23]

I have bench tested 100's maybe 1000's of motors and to think that you could get them all within 1% of each other is ABSURD.

heck 20 deg temp changes or a few hundred feet altitude can make that kind of change in ANY motor..


Edit: (bob this is NOT directed at you)

 

[CCotner]

That's interesting. You are not a member of NAR S&T, and AFAIK not a member of TRA TMT or CAR MTC either. I can not speak for TMT or MTC but as I compiled the NAR S&T Motor Testing Manual, I take exception to your comments.

The S&T motor certification procedures are a superset of the NFPA recommendations. We are 100% compliant with NFPA and have several additional S&T requirements, such as the Impulse class edge restrictions and periodic recertification. These additional requirements are consumer protection related requirements imposed by S&T on the manufacturers to check if the NFPA mandated manufacturer lot QA testing is producing motors similar to those certified, and is an independent check to see if the motors you are purchasing are indeed that they are claimed to be.

By convention, the impulse class boundaries are based on the total impulse measured at, or corrected to, sea level pressure. S&T does its certifications at sea level using load cells calibrated with NIST calibrated load cells, so correction factors are not required. Specific impulse increases with altitude, so the situation is reversed from what you think it is. For example, a motor measured as a baby H at altitude will measure and be certified as an almost full G at sea level. Etc.

Bob

That is not what I have heard from individuals with less authority than yourself. I stand corrected, and I apologize for misguided comments.