Thrust and Impulse variance in commercial APCP motors

[ghostfather]

I tell beginning rocketeers that the commercial APCP motors we fly from Cesaroni and Aerotech can vary in Thrust and Impulse for any two of the same motors, maybe by about 10%. I had heard that from someone else, or maybe read it somewhere, though I also seem to remember that the certifying authorities require the values measured to be within 20% of the claimed/published values. The message that I try to bring to beginners is why we work with safety margins, like a minimum 5:1 thrust to weight ratio.
Rocketry isn't an exact science, and there is probably more variance between flights because of wind or other factors than motor variance.

I'm curious as to what contributes to any variance, and whether it does vary much between motors. If you take five identical motors from the same manufacturer, with different dates or batches, some brand new, some 3-4 years old, even motors from the same batch, and bench test them, will there be significant variance? 10% seems like a lot, 20% even more unbelievable, though I don't have the luxury or equipment to make the tests myself.

I'm also curious as to what may cause any variance, and whether some propellant formulations are more prone to variance than others. Do slow burners vary more than fast burners, for example.

Just something to talk about after the launch....

 

[Steve Shannon]

NFPA 1125 lists the actual performance tolerances that are allowed when certifying motors.
All manufacturing is subject to imperfection (called tolerance) when measuring or in purity of chemicals. Those tolerances can stack up. The inverse of imperfection is price, so in order to keep costs reasonable a performance tolerance is allowed. However, the fact that it’s allowed doesn’t mean the manufacturers take advantage of it. I suspect most manufacturers stay well within the allowed tolerances.
Other things can also greatly affect performance, such as ambient pressure. Alan Whitmore and I tested delay grains and found that burning at my elevation (a mile above sea level) a delay grain required about 8% more time to burn the same length compared to his elevation, which is nearly sea level.

 

[UhClem]

I suggest you look at the TARC results. A random (or not so random) 10% variation in motor performance would be a big problem there.

 

[Antares JS]

My experience has definitely been that variance between performances of the same motor is much less than 20%. Just because that much variance is allowed per the motor certification rules doesn't mean that much variance actually happens. I would guess five of the same motor would perform the same within a couple percent of each other.

 

[ghostfather]

I'm guessing that the real life variance (or tolerance, from the manufacturer's point of view) in motors is probably a couple percent, but can, at most, approach 10%. What do bench tests for certification show, when they test three identical motors, and average them?

I'm curious in general if older motors lose some of their impulse with age, or perhaps they burn a bit slower. And whether some propellant sorts are more variant than others.

 

[Steve Shannon]

That question needs to have a pinned response it’s asked so frequently. Some motors are sensitive to humidity and can become more difficult to ignite. White lightning is one. But for the most part motors are very stable, especially when left sealed in their original packaging. Many of us have flown many year old motors. Without placing them on a test stand it’s impossible to know of slight changes, but I haven’t noticed any.
If the performance of a motor changes substantially during 10 years on the shelf, manufacturers are required (by NFPA 1125) to mark them with a “use by” date. I don’t know of any motors that are marked that way.

 

[waltr]

If the performance of a motor changes substantially during 10 years on the shelf, manufacturers are required (by NFPA 1125) to mark them with a “use by” date. I don’t know of any motors that are marked that way.

I have a couple of CTI F120 29-1G reloads that state "use within one year of date". Only motors I have seen this on.

 

[G_T]

There are propellant formulations that are not long-term stable. I do not know if any such are used in commercial motors. Those do have to be burned soon after casting. And, there are certainly hygroscopic formulations that can be badly affected by humidity exposure. Those should be kept in a sealed bag, with dessicator, before use.

Ambient temperature also affects burn rate.

If there is evidence of porosity in the grains, then it is likely that the propellant density will not be super consistent. Depending on whether this is being compensated, the total propellant mass in the motor might vary by enough to make a notable difference. Also the more porous the propellant, the faster it will burn. I've seen plenty of commercial propellant that I'd reject and throw on the burn pile. Not naming names, just saying... I expect closer to 1% variation in my own motors. But a commercial company would not be able to charge what they would cost. Too many extra processing steps and extra time during processing. The rate of conversion from chemicals to sold motors would be too slow!

Conventional formulation APCP motors should be quite stable over long periods of time and over a variety of storage conditions. That's a big part of WHY we use APCP.

Gerald

 

[Steve Shannon]

There are propellant formulations that are not long-term stable. I do not know if any such are used in commercial motors. Those do have to be burned soon after casting. And, there are certainly hygroscopic formulations that can be badly affected by humidity exposure. Those should be kept in a sealed bag, with dessicator, before use.

Ambient temperature also affects burn rate.

If there is evidence of porosity in the grains, then it is likely that the propellant density will not be super consistent. Depending on whether this is being compensated, the total propellant mass in the motor might vary by enough to make a notable difference. Also the more porous the propellant, the faster it will burn. I've seen plenty of commercial propellant that I'd reject and throw on the burn pile. Not naming names, just saying... I expect closer to 1% variation in my own motors. But a commercial company would not be able to charge what they would cost. Too many extra processing steps and extra time during processing. The rate of conversion from chemicals to sold motors would be too slow!

Conventional formulation APCP motors should be quite stable over long periods of time and over a variety of storage conditions. That's a big part of WHY we use APCP.

Gerald

I did go look at a few certifications (they’re publicly available on both NAR and Tripoli’s websites) and the variation I see is around 1 to 2% for impulse.

 

[Neutronium95]

I suggest you look at the TARC results. A random (or not so random) 10% variation in motor performance would be a big problem there.

I have heard of top TARC teams buying large numbers of motors, and then weighing them to try to weed out any that might have a tad bit more or less propellant.

 

[Andrew Brown]

A simple question with a complicated series of answers. I'm coming from a TARC perspective, where we did purchase large numbers of motors from single date codes, controlled launches as best we could, measured mass of propellent, measured nozzle diameter (both before and after), accounting for temp variations, etc. The certification data of say, an E28, has 3 sigma variation of sub 1%. However, we saw a bimodal distribution across 20+ launches. One line was within 1%, the second line, 3% away, was also within 1%. No correlation to mass or starting diameter of the nozzle. There was a correlation with ending diameter of the nozzle. This year, we stepped up to the F51, and we had problems all over the place with non-axial thrust during the first 40 ft being the chief problem. However, variation in nozzle erosion was typically the dominant factor in thrust difference. I believe the propellant itself is very well controlled within a single date code, but the nozzle has variation in abrasion. This was clearly accentuated by this fast burning propellant type.

I'm not sure this answers the question of the OP. I think the chemistry of the propellant is well controlled to within a few percent, but there are other elements that are harder to determine.

 

[tim cubbedge]

That question needs to have a pinned response it’s asked so frequently.

Done

 

[Neutronium95]

However, we saw a bimodal distribution across 20+ launches. One line was within 1%, the second line, 3% away, was also within 1%. No correlation to mass or starting diameter of the nozzle. There was a correlation with ending diameter of the nozzle.

That sounds really interesting. Have you considered the air temperature that you were flying in? Motors will burn at different speeds if the propellant has different starting temperatures.

 

[rocket_troy]

Conventional formulation APCP motors should be quite stable over long periods of time and over a variety of storage conditions. That's a big part of WHY we use APCP.

That's a big part of why the military and professional aerospace use it, but it's not necessarily a primary driver for hobbyist usage. As has been mentioned, Aerotech White Lightening (the variant used for the smaller motors) will degrade significantly over time (on the skin) if not protected from atmospheric moisture - that's probably the longest running commercial composite propellant used in the hobby space.
Any propellant containing both AP and Mg (used for bright plumage) will also generally tend to suffer from this issue. Most manufactures will tend to use Al instead so it's generally not an issue in most cases but sometimes when scaling down to the smaller sized motors, you're probably more constrained in what you can use to achieve a certain visual or acoustic effect as it needs to be more "amplified".

TP

 

[rocket_troy]

Other things can also greatly affect performance, such as ambient pressure. Alan Whitmore and I tested delay grains and found that burning at my elevation (a mile above sea level) a delay grain required about 8% more time to burn the same length compared to his elevation, which is nearly sea level.

I'm absolutely gobsmacked reading that.

TP

 

[AeroTech]

This is one of the reasons full K and larger motors must be used with electronic ejection systems. We learned in the early days that pyro delays in large motors burned much longer in high altitude flights than when tested closer to sea level.

 

[AeroTech]

Variation within batches made from the same lots of raw materials is minimal. Variation of batches made from different lots of raw materials over longer periods of time can be substantial. Peak thrust and burn time will usually vary much more than total impulse though.

 

[rharshberger]

This is one of the reasons full K and larger motors must be used with electronic ejection systems. We learned in the early days that pyro delays in large motors burned much longer in high altitude flights than when tested closer to sea level.

I had wondered why 75mm and larger motors had no delay grain or facility for an ejection charge.

Variation within batches made from the same lots of raw materials is minimal. Variation of batches made from different lots of raw materials over longer periods of time can be substantial. Peak thrust and burn time will usually vary much more than total impulse though.

Not surprising, consistency should be a standard of quality control, still I learned two thing today that I didn't know. Thank you AT.

 

[G_T]

Anecdote here...

I was just mixing this weekend, at a friends, for a first test of a propellant idea that is not in the range of normal propellants you would come across. My friend had a test batch of something else ready to pack. It was lower viscosity than I expected with the particular trimodal AP blend he used, but that blend had been working well for him.

Well, I altered my intended trimodal blend based on what I saw with his test batch. When I weighed out the AP I saw the difference. His 200 was 200 + fines, whereas I was used to working with and designing for straight 200 (though I've also had 200 with some included fines before). I've seen this between batches of AP. We get what we can get, then compensate for it!

Change batch of chems, then one might need to reformulate. It's not just the chemistry; the particle morphology matters. If the motor is big enough that the chemical reactions go to completion (ours generally aren't big enough), it is the chemistry which determines what happens. But, it is the morphology of the particles which has a big affect on the rate at which it happens. Hence, the effects of size. Coal dust burns a whole lot faster than lumps of coal, for example. I won't go into it more here but it is a fun topic.

I understand reformulation was needed at least once and probably a couple times for the SRB's propellant during the shuttle program. When a batch of chemicals changed, the formula had to be tweaked to give back the same performance or at least something quite close. Designing initially with multimodal blends gives room to compensate for such variations.

Quite possibly the smaller commercial and hobby manufacturers have to do the same thing. We're not big enough to dictate chemical supply specifications, or reject it if it is out of spec! There aren't many suppliers of some of what we use.

Gerald

 

[Grog6]

When I see a variance in my mixes, it's always bubbles in the mix.

 

[ghostfather]

I did go look at a few certifications (they’re publicly available on both NAR and Tripoli’s websites) and the variation I see is around 1 to 2% for impulse.

Thanks, Steve.
That was exactly the sort of answer I was looking for, backed up with sources as well. I wasn't aware that the certification details were publicly available.

It looks like I may need to nuance my information a bit to beginning rocketeers, as it would seem that motors don't vary as much as 10%, but real life motor variance is probably within 2%, which is what I would expect.

 

[OverTheTop]

I hate to mention this but can we please use variation instead of variance. I think it is what you (several of you) actually intend to mean.

"Variance is a parameter of a distribution (standard deviation squared) that helps us describe the distribution’s shape and the data spread. Variation is a less precise term intended to describe changes in the values of a variable or the spread of data. "

 

[ghostfather]

I hate to mention this but can we please use variation instead of variance. I think it is what you (several of you) actually intend to mean.

"Variance is a parameter of a distribution (standard deviation squared) that helps us describe the distribution’s shape and the data spread. Variation is a less precise term intended to describe changes in the values of a variable or the spread of data. "

Thanks, though I'm not in to statistics, I bow (deeply) to your definition of variance.
I used it in the vernacular.

 

[OverTheTop]

Thanks, though I'm not in to statistics, I bow (deeply) to your definition of variance.
I used it in the vernacular.

Thanks, but please don't bow or do any other silliness

 

[Steve Shannon]

Thanks, Steve.
That was exactly the sort of answer I was looking for, backed up with sources as well. I wasn't aware that the certification details were publicly available.

It looks like I may need to nuance my information a bit to beginning rocketeers, as it would seem that motors don't vary as much as 10%, but real life motor variance is probably within 2%, which is what I would expect.

I posted the wrong link. Please see Alex B's link down below: https://www.rocketryforum.com/threa...in-commercial-apcp-motors.172341/post-2270754

 

[SecondRow]

I hate to mention this but can we please use variation instead of variance. I think it is what you (several of you) actually intend to mean.

"Variance is a parameter of a distribution (standard deviation squared) that helps us describe the distribution’s shape and the data spread. Variation is a less precise term intended to describe changes in the values of a variable or the spread of data. "

A pet peeve of mine that happens at work consistently. The business units use variance incorrectly and it’s trickled down to the analysts who should know better.

 

[G_T]

Air in propellant should be treated as an additional particle type in the mix (air). But how do you adequately control for the percentage of that particle and its morphology? So of course if there are bubbles present the consistency is going to be lower than if they are not present. More bubbles and/or larger bubbles -> faster burn rate at a given pressure if the propellant has a classic burn rate curve. Bubbles -> lower, and probably inconsistent, density. IMHO any such propellant belongs on a burn pile.

 

[Scott_650]

A pet peeve of mine that happens at work consistently. The business units use variance incorrectly and it’s trickled down to the analysts who should know better.

Wow, flashback to the time I spent doing analysis - both of pertinent data and of work processes - folks at every level would mix terms during our “How Goes It” meetings and I’d explain what my charts actually showed. I learned not to grimace or grit my teeth - not because I wanted to but so they’d pay attention to the information (and my analysis) rather than my facial expressions…that skill has served me well since retirement while substitute teaching in elementary school classrooms

 

[jderimig]

 

[AlexBruccoleri]

The most comprehensive listing is here: https://www.nar.org/standards-and-testing-committee/nar-certified-motor-list/

I may not be following this, but that is just NAR certified motors. The full list with TRA/CAR is
https://www.nar.org/SandT/pdf/CombinedMotorsByImpulse.pdf
Cheers, Alex