Thrust/Weight Ratio and Flight Forces

CzTeacherMan said:

Test data can be off by as much as 20%, so, especially for a cert flight, I would want an abundance of caution.

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So, then, basically every "successful" Cert flight, ever flown, with a "Baby H", "Baby J", or "Baby M" could actually be "illegitimate", since it might have performed below the minimum required total impulse level, based on your performance range figures . . . ?

Dave F.

Ez2cDave said:

So, then, basically every "successful" Cert flight, ever flown, with a "Baby H", "Baby J", or "Baby M" could actually be "illegitimate", since it might have performed below the minimum required total impulse level, based on your performance range figures . . . ?

Dave F.

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No. Certification flights are based on how the rocket is certified, not on whether the motor performed better than the midpoint of the range.

Of course if you really want to make sure you hit the spirit of the cert instead of the letter, just pick motors that are no way debatable under the impulse threshold like the H120, K500, and O3400

Steve Shannon said:

No. Certification flights are based on how the rocket is certified, not on whether the motor performed better than the midpoint of the range.

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Steve,

"CzTeacherMan" posted that certified motors could vary by as much as 20% in their performance . . . ( I replied to him, initially ).

What I was referring to were motors that "under-perform" . . . LEVEL 1 - A "Baby-H" that only performed as a "G" motor . . . LEVEL 2 - A "Baby-J" that only performed as an "I" . . . LEVEL 3 - A "baby-M" that only performed as an "L".

The result would be that Cert Flights could be flown that do not meet the minimum Total Impulse for the Cert Level.

By the same token, someone Certifying with a "maximum" motor might actually be flying a motor above the Cert Level they are attempting . . . LEVEL 1 - A "Maximum I" that performs as a "J" motor . . . LEVEL 2 - A "Maximum L" that performs as an "M" motor . . . LEVEL 3 - A "Maximum O" that performs as a "Class 3" motor.

QUESTION : How accurate are the published specifications for Certified Motors and what is the Tolerance Range for them ?

Thanks, Steve

Dave F.

I think this is a case of "you can only do so much"

Someone that can successfully fly the M1297, could still shred a rocket on the L2200.

It's a decently structured system, and of course there are extreme end cases at the level limits.

Ez2cDave said:

Steve,

"CzTeacherMan" posted that certified motors could vary by as much as 20% in their performance . . . ( I replied to him, initially ).

What I was referring to were motors that "under-perform" . . . LEVEL 1 - A "Baby-H" that only performed as a "G" motor . . . LEVEL 2 - A "Baby-J" that only performed as an "I" . . . LEVEL 3 - A "baby-M" that only performed as an "L".

The result would be that Cert Flights could be flown that do not meet the minimum Total Impulse for the Cert Level.

By the same token, someone Certifying with a "maximum" motor might actually be flying a motor above the Cert Level they are attempting . . . LEVEL 1 - A "Maximum I" that performs as a "J" motor . . . LEVEL 2 - A "Maximum L" that performs as an "M" motor . . . LEVEL 3 - A "Maximum O" that performs as a "Class 3" motor.

QUESTION : How accurate are the published specifications for Certified Motors and what is the Tolerance Range for them ?

Thanks, Steve

Dave F.

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I understand what you’re referring to; it’s ridiculous to suggest that an off the shelf motor which falls in the bottom portion of the allowed range makes a certifications “illegimate.”
Off the top of my head I don’t know the accuracy of the TMT test stand and load cells. I know that Alan Whitmore is very careful to calibrate them.
As far as the tolerance limits, they are dictated by NFPA 1125. I think Erik already told you what they are. I don’t have a copy on my phone but you can look them up.
https://www.nfpa.org/

Steve Shannon said:

As far as the tolerance limits, they are dictated by NFPA 1125. I think Erik already told you what they are. I don’t have a copy on my phone but you can look them up.
https://www.nfpa.org/

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Steve,

NFPA does not "openly share" information, as you know. They require joining NFPA for access . . .

Dave F.

No they don’t. They require you to register (create a free login) and then agree to their terms and conditions in order to read the individual documents, which is also free. It’s the same amount of work whether you do it or I do it, but I’m traveling right now so it’s better for me if you do it. There are several different tolerances permitted in the testing section but what you should pay attention to is the section that covers lot testing by the manufacturers.

Although I’m an alternate to the technical committee I am not an NFPA member.

Ez2cDave said:

Steve,

NFPA does not "openly share" information, as you know. They require joining NFPA for access . . .

Dave F.

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Here ya go...

CzTeacherMan said:

Here ya go...

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QUESTION : Does that phrase "within 20 percent . . ." mean that it can be 20% below or 20% above ( a 40% spread ), or does it mean a 20% spread ( 10% above or 10% below ) ?

I can only assume that that is only a "maximum permissible range" and does not speak to the actual "quality-control standards" of the various motor manufacturers . . .

Dave F.

Within 20% means +/-20%.
Manufacturers are required to test each production lot also. There are standards in NFPA 1125 that they are required to comply to.
I’m traveling and I don’t have easy access but you should look into it.

Steve Shannon said:

Within 20% means +/-20%.
Manufacturers are required to test each production lot also. There are standards in NFPA 1125 that they are required to comply to.
I’m traveling and I don’t have easy access but you should look into it.

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Steve,

Already done by another person and myself . . . That is where the quoted information came from ( 1125 ).

Dave F.

On a side note, subsection a) in the list is interesting to me. I wouldn't be trusting any standard deviation calculated from only two data points, but the result is considered valid in the definition.

OverTheTop said:

On a side note, subsection a) in the list is interesting to me. I wouldn't be trusting any standard deviation calculated from only two data points, but the result is considered valid in the definition.

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Interesting observation . . . That would tend to suggest the "a" and "c" contradict each other.

Dave F.

Ez2cDave said:

Interesting observation . . . That would tend to suggest the "a" and "c" contradict each other.

Dave F.

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I don't think that's necessarily true. a) is total impulse and c) is average thrust. You could have a higher thrust but shorter burn (or vice versa) and have the same total impulse. I don't have enough motor theory to know *why* that would happen, but it certainly seems possible.

Ez2cDave said:

Steve,

Already done by another person and myself . . . That is where the quoted information came from ( 1125 ).

Dave F.

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No, the quoted information cited the certification standards. There are similar production standards that the manufacturers are required to follow in a different section of the requirements.

boatgeek said:

I don't think that's necessarily true. a) is total impulse and c) is average thrust. You could have a higher thrust but shorter burn (or vice versa) and have the same total impulse. I don't have enough motor theory to know *why* that would happen, but it certainly seems possible.

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That is exactly what happens. If something is done that results in lower average thrust (such as a slightly larger nozzle throat or propellant geometry) the burn time is extended. If something happens that results in higher average thrust, burn time is shorter for a given total impulse.

Okay, I’m back home and able to look at NFPA 1125. There have been some misstatements here.
For high power motors, the imprinted average thrust is allowed to deviate by up to 20%, {requirement 8.2.7(1)c} but that doesn’t mean the “power of the motor” may deviate that much. That’s just how much the average thrust printed on the label can vary. So, an H100 could actually have an average thrust (not total impulse) between 80 Newtons and 120 Newtons. That limits how manufacturers are allowed to label their motors, not the actual amount average thrust may vary between tested samples!
Standard deviation of the total impulse of the test samples may not exceed 6.7% from the mean measured value. The imprinted total impulse must be the mean measured value.

The production testing requirements are different and are covered in 7.8.6. Section (1) requires that for high power motors the total impulse must not vary more than 10% from the established mean total impulse value. That’s the standard manufacturers must follow. It’s 20% for model rocket motors.

djs said:

Remember when you're talking about "high thrust".. there's "high thrust" and "sustained thrust". What usually does rockets in is where the thrust continues over the length of time. That's why a I250 and J250 may sound equivalent, but the J burns for twice as long and can cause much more problems.

Watch the fins on this video here- note that it didn't break loose right at the ground- it's after some sustained force that everything went all kafloopy:

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I don't think this was an adhesive failure, since the kablooie happened right after the text in the video said the rocket was traveling between 700 and 800 mph. That's the trans-sonic speed range when compressibility shock waves are at the strongest level.

gldknght said:

I don't think this was an adhesive failure, since the kablooie happened right after the text in the video said the rocket was traveling between 700 and 800 mph. That's the trans-sonic speed range when compressibility shock waves are at the strongest level.

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Fin flutter failure, the fins looked to snap off at the airframe. Looked like a classic failure, and most likely did happen at or near MaxQ.

OverTheTop said:

On a side note, subsection a) in the list is interesting to me. I wouldn't be trusting any standard deviation calculated from only two data points, but the result is considered valid in the definition.

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Two data points are pretty thin if that is all the information available. But the manufacturer tells you what they think the numbers are so perhaps a more Bayesian approach is in order. If you have a good idea on that front, or any other, NFPA is accepting proposals to change 1125 until 26 June for this revision cycle.

UhClem said:

Two data points are pretty thin if that is all the information available. But the manufacturer tells you what they think the numbers are so perhaps a more Bayesian approach is in order. If you have a good idea on that front, or any other, NFPA is accepting proposals to change 1125 until 26 June for this revision cycle.

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I think even the Bayesians would like some more data points. Yes, some statistical value can be extracted from two data points, but not very much.

It seems to be working well, so if it ain't broke don't fix it...

The NFPA requirement is for two samples but for smaller HPR motors I think we require more. You’re right, a sample size of two doesn’t reveal much.

Adjusted charts attached.
Updated the labels to say Thrust, Newtons, and Pounds. View attachment QCRS RSO Charts.pdf

Steve Shannon said:

Okay, I’m back home and able to look at NFPA 1125. There have been some misstatements here.
For high power motors, the imprinted average thrust is allowed to deviate by up to 20%, {requirement 8.2.7(1)c} but that doesn’t mean the “power of the motor” may deviate that much. That’s just how much the average thrust printed on the label can vary. So, an H100 could actually have an average thrust (not total impulse) between 80 Newtons and 120 Newtons. That limits how manufacturers are allowed to label their motors, not the actual amount average thrust may vary between tested samples!
Standard deviation of the total impulse of the test samples may not exceed 6.7% from the mean measured value. The imprinted total impulse must be the mean measured value.

The production testing requirements are different and are covered in 7.8.6. Section (1) requires that for high power motors the total impulse must not vary more than 10% from the established mean total impulse value. That’s the standard manufacturers must follow. It’s 20% for model rocket motors.

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Let me weigh in on this and give some actual data for a known, certified motor which was tested by TMT back in 2017 to address the question under discussion here (how much variation exists in total and average thrust figures for a popular commercial motor) and another question (how much variation exists in the delay element timing of that motor). I need to point out that this test was not for certification purposes but for "education and training of the new TMT crew".

We purchased 12 Aerotech I161W reload kits. These were purchased from 2 different suppliers and had 3 different day-codes printed on the packaging. The average total impulse for these 12 motors was 455.69 N.s with a standard deviation of 11.00 N.s. Let me point out that 11 N.s is just 2.4% of 455.7. Our results from other manufacturers show variation of similar magnitude, although mostly with smaller sample sizes.

The point I want to make is that all of our commercial manufacturers do an exemplary job of providing affordable motors and reloads with predictable and reproducible performance. The worry that motor performance might vary as much as 20% from published values is not justified by the data.

Alan Whitmore
Chair, Tripoli Motor Testing

Alan Whitmore said:

Let me weigh in on this and give some actual data for a known, certified motor which was tested by TMT back in 2017 to address the question under discussion here (how much variation exists in total and average thrust figures for a popular commercial motor) and another question (how much variation exists in the delay element timing of that motor). I need to point out that this test was not for certification purposes but for "education and training of the new TMT crew".

We purchased 12 Aerotech I161W reload kits. These were purchased from 2 different suppliers and had 3 different day-codes printed on the packaging. The average total impulse for these 12 motors was 455.69 N.s with a standard deviation of 11.00 N.s. Let me point out that 11 N.s is just 2.4% of 455.7. Our results from other manufacturers show variation of similar magnitude, although mostly with smaller sample sizes.

The point I want to make is that all of our commercial manufacturers do an exemplary job of providing affordable motors and reloads with predictable and reproducible performance. The worry that motor performance might vary as much as 20% from published values is not justified by the data.

Alan Whitmore
Chair, Tripoli Motor Testing

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Thank you, Alan!

Alan Whitmore said:

Let me weigh in on this and give some actual data for a known, certified motor which was tested by TMT back in 2017 to address the question under discussion here (how much variation exists in total and average thrust figures for a popular commercial motor) and another question (how much variation exists in the delay element timing of that motor). I need to point out that this test was not for certification purposes but for "education and training of the new TMT crew".

We purchased 12 Aerotech I161W reload kits. These were purchased from 2 different suppliers and had 3 different day-codes printed on the packaging. The average total impulse for these 12 motors was 455.69 N.s with a standard deviation of 11.00 N.s. Let me point out that 11 N.s is just 2.4% of 455.7. Our results from other manufacturers show variation of similar magnitude, although mostly with smaller sample sizes.

The point I want to make is that all of our commercial manufacturers do an exemplary job of providing affordable motors and reloads with predictable and reproducible performance. The worry that motor performance might vary as much as 20% from published values is not justified by the data.

Alan Whitmore
Chair, Tripoli Motor Testing

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Alan,

It is a relief to see how limited the performance deviation actually is . . . Many thanks for taking the time to clear things up !

Dave F.

I never meant to imply that motors WOULD deviate by 20%... I merely made a statement that marginal flight profiles are sometimes a bit more marginal than intended because a motor's thrust COULD deviate by as much as 20%. Thereby being a reasonable reason that an RSO might reject a flight profile given certain conditions.

CzTeacherMan said:

Test data can be off by as much as 20%, so, especially for a cert flight, I would want an abundance of caution.

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CzTeacherMan said:

I never meant to imply that motors WOULD deviate by 20%... I merely made a statement that marginal flight profiles are sometimes a bit more marginal than intended because a motor's thrust COULD deviate by as much as 20%. Thereby being a reasonable reason that an RSO might reject a flight profile given certain conditions.

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CzTeacherMan,

You stated that the TEST DATA could be off by 20% . . . In truth, the TEST DATA is not what is being evaluated here. The Test Data MUST be ACCURATE . . .

NFPA 1125 states that the Average Thrust of Motors is ALLOWED to deviate by, UP TO, 20% and that the Total Impulse can only vary by 6.7%.

The data presented by Alan Whitmore, Chairman of Tripoli Motor Testing, clearly shows that today's manufacturers maintain tight production standards and, thereby provide for consistent perfomance of their motors.

To quote Alan . . .

" We purchased 12 Aerotech I161W reload kits. These were purchased from 2 different suppliers and had 3 different day-codes printed on the packaging. The average total impulse for these 12 motors was 455.69 N.s with a standard deviation of 11.00 N.s. Let me point out that 11 N.s is just 2.4% of 455.7. Our results from other manufacturers show variation of similar magnitude, although mostly with smaller sample sizes.

The point I want to make is that all of our commercial manufacturers do an exemplary job of providing affordable motors and reloads with predictable and reproducible performance. The worry that motor performance might vary as much as 20% from published values is not justified by the data."

END QUOTE :

BTW - Alan Whitmore is my TAP . . .

Special thanks to Steve Shannon & Alan Whitmore !

Dave F.

Ez2cDave said:

CzTeacherMan,

You stated that the TEST DATA could be off by 20% . . . In truth, the TEST DATA is not what is being evaluated here. The Test Data MUST be ACCURATE . . .

NFPA 1125 states that the Average Thrust of Motors is ALLOWED to deviate by, UP TO, 20% and that the Total Impulse can only vary by 6.7%.

The data presented by Alan Whitmore, Chairman of Tripoli Motor Testing, clearly shows that today's manufacturers maintain tight production standards and, thereby provide for consistent perfomance of their motors.

To quote Alan . . .

" We purchased 12 Aerotech I161W reload kits. These were purchased from 2 different suppliers and had 3 different day-codes printed on the packaging. The average total impulse for these 12 motors was 455.69 N.s with a standard deviation of 11.00 N.s. Let me point out that 11 N.s is just 2.4% of 455.7. Our results from other manufacturers show variation of similar magnitude, although mostly with smaller sample sizes.

The point I want to make is that all of our commercial manufacturers do an exemplary job of providing affordable motors and reloads with predictable and reproducible performance. The worry that motor performance might vary as much as 20% from published values is not justified by the data."

END QUOTE :

BTW - Alan Whitmore is my TAP . . .

Special thanks to Steve Shannon & Alan Whitmore !

Dave F.

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Ok, my dude. Ok. You took one thing I said out of context and griped at me for it. Cool. Get on with your bad self. Congrats. I'll be sure to bite my tongue next time I think about speaking. I thought I was pretty clear and then I clarified what I meant, but you go ahead and tell me what I meant. I'm totally cool with that.
Hope your L3 goes well, my dude. Best of luck to ya!