Thanks, Alex, you're right. That's the link I wanted to post.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
Thanks, Alex, you're right. That's the link I wanted to post.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
The TARC team I mentor opens several motors (CTI, 24mm 2G), weighs grains, and then mixes and matches to get consistent propellant mass. That seems to work well. The max-to-min variation is on the order of 1-2% of total grain mass, including ignition pellet and casting tubes.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.
According to John, the limit for thrust variation is 5%. Actual typical variation I don't know.Peak thrust and burn time will usually vary much more than total impulse though.
I had a J motor delay run a full 7 seconds longer than drilled in Alamosa, CO, last year. They're up around 7K MSL.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.
After burnout the chamber pressure is quite low. Not sure how the delay grain/residual propellant pressurize the chamber though.I just reread this thread, and I noticed something that caught my attention: apcp motors burn longer at elevated altitudes.
I have to wonder WTF? as the burning environment for the propellant is pretty isolated from atmosphere; it's at hundreds of pounds of pressure, separated from the rest of the world by a nozzle.
Any Ideas?
I just spotted this thread. If you check in on www.thrustcurve.org you will find links to lots of the official certification letters. The data in those letters includes standard deviations from the mean for all of the measured parameters, if enough motors were tested to make such a calculation meaningful.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....
@Spacedog49Krell --Attached is variance data I've recorded from Aerotech G40-10 motors. Some of the variances are extremely short lived and can only be detected with high speed flight computers. View attachment 597916
a sample size of two motors is too small to call a single motor an anomaly.@Spacedog49Krell --
My eyeball planimeter says the total Impulse MIGHT be a little higher for the 'Anomoly Motor' ( the red line ) ...
I used to see similar differences in the olden days when I extracted Thrust Curves from 8-bit AltAcc Data.
Have you tried integrating the area under the two curves ?
After adjusting for the 1G offset and Drag-vs-Velocity I'll bet the variation is negligible or at least within spec.
-- kjh
I agree @rharshbergera sample size of two motors is too small to call a single motor an anomaly.
The total-impulse between the two motors is within specifications. This is 16-bit data recorded every 2.5 ms of either slag or a small chunk of unburned propellant passing through the nozzle based on the time into the burn. My question is, "How many of these millisecond events have occurred? Are these events common or rare? Are there other millisecond events that we should be concerned about? Could CATO's be related to a nozzle blockage issue?"@Spacedog49Krell --
My eyeball planimeter says the total Impulse MIGHT be a little higher for the 'Anomoly Motor' ( the red line ) ...
I used to see similar differences in the olden days when I extracted Thrust Curves from 8-bit AltAcc Data.
Have you tried integrating the area under the two curves ?
After adjusting for the 1G offset and Drag-vs-Velocity I'll bet the variation is negligible or at least within spec.
-- kjh
What's different? Base Fill?The most glaring issue is that PROPULSE assumes drag -vs- velocity during the burn phase is the same as it is during the coast phase. It is not.
I am not sure @FredAWhat's different? Base Fill?
First, I agree with everyone else that the Total Impulse of those two motors are close enough to be within specs..The total-impulse between the two motors is within specifications. This is 16-bit data recorded every 2.5 ms of either slag or a small chunk of unburned propellant passing through the nozzle based on the time into the burn. My question is, "How many of these millisecond events have occurred? Are these events common or rare? Are there other millisecond events that we should be concerned about? Could CATO's be related to a nozzle blockage issue?"
Wow, old Blacksky AltAcc 1 data graphs, back when the G40 was 112 N*sec. Around 2000, Scott heard that I was flying 12-bit data flight computers that recorded acceleration every 10ms and altitude every 40ms. A coding structure I still use today, just faster. I had a few conversations with Scott on writing new code, I believe, for the AltAcc 2(?). My day job travel schedule didn't work out for the project.I like your data @Spacedog49Krell
Thanks for sharing it !
I don't know the As to your Qs but there are all kinds of unimaginable chemical and physical events that will combine in unknown ways to make each motor as different as are snowflakes -- they're all different but they're much the same in the important ways ( total impulse and general shape of the thrust -vs- time curve ).
One thing I found useful when flying an accelerometer is to be sure to record the Motor Manufacturer's BATCH ID from the paperwork ... you might see something useful in that info.
One thing I see is that the deceleration after burnout is significant in your data plots. I am not sure of the units ( ??? Gs ??? )
I've got an awk script that I wrote specifically for 8-bit, 16 Hz AltAcc Acceleration Data that more-or-less automates the process of extracting thrust curves from raw accelerometer data and a launch parameter file.
In keeping with the Black Sky naming scheme, the script is called PROPULSE.
I plan to port so it can process Blue Raven Data when I can get around to it.
In a nutshell, the script takes the rocket characteristics and launch site conditions and then back-calculates Force -vs- Time from the raw acceleration data by factoring in the force due to drag during the motor burn phase.
There are some invalid assumptions in PROPULSE.
The most glaring issue is that PROPULSE assumes drag -vs- velocity during the burn phase is the same as it is during the coast phase. It is not.
Another less important thing I never bothered with was the rate of consumption of the delay grain and the effect of the hot smoke from the delay on drag during the coast phase. But I figured it was insignificant with large enough rockets and motors.
The script is very specific to AltAcc Data Sets but porting it for other Accelerometers is absolutely doable,
Be happy to share it if you want it.
-- kjh
p.s. @OverTheTop --
Please stop tickling my lab tech nostalgia bone. I sold my micro beam balance when I moved from the SF Bay Area to San DIego ( but I did keep my K&E Polar Planimeter )
EDIT: these are PROPULSE plots from AltAcc Data for three G40's I flew at Ocotillo, CA in 1997 ...
I agree with you Walt.First, I agree with everyone else that the Total Impulse of those two motors are close enough to be within specs..
If the 'spike' was caused by "a small chunk of unburned propellant passing through the nozzle", my concern is that it may cause a "Vectored" thrust. I and others have seen enough flights where the rocket left the rail straight then at a distance above the rail the rocket made a fairly sharp turn (maybe only a few degrees but sometimes much more).
Also have been plenty of posts asking why the rocket made a sharp turn. This may be the reason.
Very nice @Spacedog49Krell !
That would have been the AltAcc 2C -- 2000 was about the time I had to give up rocketry for a new job in L.A.
I would love to see your base drag data !
Please post it if you can share it ?
Thanks !
-- kjh
No worries @Spacedog49KrellI only have a rough layout of the sensor cluster. There is a limited gap between the 29mm motor mount and the BT-60 body tube for sensors and electronics. This project started three months ago, but was interrupted with grandkids for the summer.
A quick way of integrating and comparing the outcomes is to print the graph twice and cut out each graph and the zero line. If you have accurate scales you just weigh the cutouts. Integration is just the area under the graph, which is proportional to the density of the paper. That's how chemical analysis using spectrometers used to be done before the advent of digital computer interfaces.
Assuming the person has the relevant programming skills, yes. Otherwise they just need a scale and to be able to operate scissors .Very clever and an interesting bit of history!
Though, a trapezoid rule or Simpsons rule written on the data would be the easiest way to integrate.
The most glaring issue is that PROPULSE assumes drag -vs- velocity during the burn phase is the same as it is during the coast phase. It is not.
Another less important thing I never bothered with was the rate of consumption of the delay grain and the effect of the hot smoke from the delay on drag during the coast phase. But I figured it was insignificant with large enough rockets and motors.
Any decent propellant is vacuum processed, in commercial circles ambient mixing is unheard of and air bubbles are a great way to sensitize propellant.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.
Vmax propellant is very storage stable and retains it's mechanical and ballistic properties for years. It uses no migratory burn rate catalysts such as ferrocene derivatives. It handily passed a wide range of tests to qualify it for various commercial applications. The only reason to put a "use before" date on such a product is to satisfy a rule or regulation that requires such. I would not hesitate to light one that is 20 years old.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.
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