A plea to motor manufacturers for consistent and error free motor file collections

TopRamen

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I would actually prefer to see data come from the certification organizations than the manufacturers.

That sounds like a good idea to me too.

I can build a rocket based on a thrust curve, but if that thrust curve is wrong, my design will not fly as I envision it.
 

Buckeye

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Here is a typical NAR certification report as shown in paperwork for the AT 38 mm I218R https://www.nar.org/SandT/pdf/Aerotech/I218.pdf

Page 1 provides the official averaged parameters obtained by S&T during certification testing.

Page 2 provides the statistics obtained during the certification testing.

Page 3 contains the .eng file for the most typical motor obtained during the certification. Simply copy and paste it into your .eng file library.


Remember that your motor may vary from the certification data. Note that for these 9 motors submitted for certification, the total impulse had a standard deviation of 0.3%, the average thrust had a standard deviation of 1.8% and the burn time had a standard deviation of 1.5%. The allowable variation is specified in the motor testing manual. https://www.nar.org/wp-content/uploads/2014/08/ST-MotorTestingManual.pdf

Right. This is all good stuff, and I use the statistics of the motors in my own variation analyses. I mentioned "digital" files, because yet another party (usually Mark) has to actually create and edit them for practical use in the simulators.

I believe the Thrustcurve motor guide averages all motor entries when doing a simulation, as a means of including variation. I say chuck all the extraneous motor files and use only the cert data with standard deviations to show expected range of performance.

AFAIK, RSE files simply add more precise info about mass and CG of the motor, and include a more modern number of significant digits. The thrust vs. time and total mass data is exactly the same as eng.

Lastly, I think this topic boils down to accountability. Mark and John have taken on the responsibility of distributing motor data in useful form (much THANKS!), but they are not the ones accountable for the data itself. It is easy to see how this can be confusing.
 
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JohnCoker

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I say chuck all the extraneous motor files and use only the cert data with standard deviations to show expected range of performance.

In the new implementation, I've dropped the ALT4 and CSV files, keeping only the ENG (RASP) and RSE (RockSim) files. However, there's no good way to choose which among multiple thrust curves have the most correct data. We keep track of the source of the data as well as the date entered, so that should help, but there's no mechanical way of figuring out which one is more "typical".

The good news is the inherent limitations of simulations mean that perfect data isn't really necessary, not to mention the variation between motors means that there is no single "accurate" data file.

As always, if you find inaccuracies in the motor data (dimensions, weight, etc) or bad thrust curves, let us know and we'll fix them.
 

Len B

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As one of the contributors of CTI motors and probably a major source of the errors in naming conventions, I would like to volunteer to help fix the errors.
All of my contributions were made by using data from CAR/ACF. Initially, I was supplied raw cert data from the head of CAR/ACF motor testing. Later, I used the thrust curves as supplied by the same person. At his request, data was submitted as coming from the Cert. Org. I worked with Andre Choquette on this. He has now become the head of CAR/ACF motor testing. He is also a Regional Director for CAR/ACF. Talk about volunteer work!!

If I can assist in cleaning things up in any way, John, Mark, please let me know.

Thanks,
 

markkoelsch

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I am working on a plan. As I said, I want to start with AT motors as I started looking at those already.

Give me a few days to get it down, I will post it, and see about asking for a few volunteers to assist.
 

Peter Olivola

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This! Ever so much, this! Motor variability exceeds the accuracy of all simulation programs. This thread is a request to achieve the unachievable.

In the new implementation, I've dropped the ALT4 and CSV files, keeping only the ENG (RASP) and RSE (RockSim) files. However, there's no good way to choose which among multiple thrust curves have the most correct data. We keep track of the source of the data as well as the date entered, so that should help, but there's no mechanical way of figuring out which one is more "typical".

The good news is the inherent limitations of simulations mean that perfect data isn't really necessary, not to mention the variation between motors means that there is no single "accurate" data file.

As always, if you find inaccuracies in the motor data (dimensions, weight, etc) or bad thrust curves, let us know and we'll fix them.
 

Buckeye

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... but there's no mechanical way of figuring out which one is more "typical"....there is no single "accurate" data file.....

I think there is. Look at the certification data, per your suggestion in post #9. Take my G80 example mentioned above. Thrustcurve contains the NAR Cert Data Sheet and 5 simulator file entries. I plotted the curves here:

Thrustcurve.g80.JPG

RASP file #1 from DeMar agrees exactly with the certification data sheet typical curve. All others do not. RASP Coker is very similar, the others are not even close.

Let's see if the other curves fall within the measured statistics. This table shows the total impulse firing average and 6 sigma standard deviation from the cert data sheet. Also shown are the computed total impulse of the individual thrust curves:

Thrustcurve.impulse.g80.JPG

Cert Data Sheet Typical, RASP1 DeMar, and RASP Coker all fall within 99.7% probability. The others, again, not even close. RASP2 DeMar, RASP3 DeMar, and RSE Coker should be purged from Thrustcurve. What do you think?
 
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Johnly

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Kudos to Mark & John for Thrustcurve, it's a wonderful effort that we should all appreciate.

It's worth considering that the the NFPA 1125 criterion for motor certification allows a large allowance for total impulse variation.
Most manufacturers produce product that only use a small fraction of the permitted variation, but over time those values may shift.

John
 

Len B

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I think there is. Look at the certification data, per your suggestion in post #9. Take my G80 example mentioned above. Thrustcurve contains the NAR Cert Data Sheet and 5 simulator file entries. I plotted the curves here:

View attachment 302186

RASP file #1 from DeMar agrees exactly with the certification data sheet typical curve. All others do not. RASP Coker is very similar, the others are not even close.

Let's see if the other curves fall within the measured statistics. This table shows the total impulse firing average and 6 sigma standard deviation from the cert data sheet. Also shown are the computed total impulse of the individual thrust curves:

View attachment 302187

Cert Data Sheet Typical, RASP1 DeMar, and RASP Coker all fall within 99.7% probability. The others, again, not even close. RASP2 DeMar, RASP3 DeMar, and RSE Coker should be purged from Thrustcurve. What do you think?

Yes, but which is the "old" G80 and which is the "new" G80. Perhaps a bad example since the motor did actually change. If you looked hard enough, you might find that there is thrust curve data to support this. I remember when the new motors came out and there were suggestions that it was different enough that it would have been nice to have a new name. I'm sure that Aerotech wouldn't want to rename an iconic motor like this. The cert. orgs leave that to the manufacturer.
 

Buckeye

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Yes, but which is the "old" G80 and which is the "new" G80. Perhaps a bad example since the motor did actually change. If you looked hard enough, you might find that there is thrust curve data to support this. I remember when the new motors came out and there were suggestions that it was different enough that it would have been nice to have a new name. I'm sure that Aerotech wouldn't want to rename an iconic motor like this. The cert. orgs leave that to the manufacturer.

Dunno. This goes to the plea by the OP for the OEMs to get their data in order and provide only currently available motors. Or at least label their data more carefully.
 
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mikec

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Yes, but which is the "old" G80 and which is the "new" G80.
It's documented in the comments for the RASP files.

AFAIK all of the major differences in motor thrust curves are from production changes in the motor.
 

markkoelsch

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Yes, but which is the "old" G80 and which is the "new" G80. Perhaps a bad example since the motor did actually change. If you looked hard enough, you might find that there is thrust curve data to support this. I remember when the new motors came out and there were suggestions that it was different enough that it would have been nice to have a new name. I'm sure that Aerotech wouldn't want to rename an iconic motor like this. The cert. orgs leave that to the manufacturer.

That is just it- there are two versions of the G80T by Aerotech.

Another example would be the two version of the J350.
 

markkoelsch

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Dunno. This goes to the plea by the OP for the OEMs to get their data in order and provide only currently available motors. Or at least label their data more carefully.

That then precludes the older motors still being out there and people flying them.

A great example of that was seen yesterday at Tripoli Wisconsin launch where a friend flew an old Vulcan H100. Motor worked great, but good luck finding motor file for it.
 

JohnCoker

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Cert Data Sheet Typical, RASP1 DeMar, and RASP Coker all fall within 99.7% probability. The others, again, not even close. RASP2 DeMar, RASP3 DeMar, and RSE Coker should be purged from Thrustcurve. What do you think?
You're right, it looks like two different motors.

FWIW, ThrustCurve.org includes a block that allows comparison of the data to the official (certified) values. For example the RSE file I submitted:

Screen Shot 2016-09-25 at 9.29.39 PM.png

Maybe this is too dense to be easily readable?

Here's how it looks on the new site:

Screen Shot 2016-09-25 at 9.33.42 PM.png

That seems more readable, but may still not be easy enough to understand.
 

rcktnut

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First of all THANKS to those providing the motor files. I can see where there is some confusion with the motor files with some motors, most of them are correct. As mentioned some motors have been changed throughout their production history. Sticking with the G80T there are 2 versions, the old and new. According to the Aerotech motor instructions/matrix have the old G80T at 120 Ns total impulse, and yet the "official, certified, whatever file" had them at 99.7 Ns. That's pretty far off, but the "new" G80T file looks good.

I have several G75J's in my inventory and the motor instruction sheet has total impulse at 155 Ns. The current .rse file has them at 161.4 Ns. and Thrust Curve has them at 135.6 Ns. I was aware of the new G80's but have no idea if the G75J's were changed or not. I see no problems with 5 Ns. or so but 20+ could make a difference on the delay needed.

So there are a few files out there that could use a little attention, but again for most, looking good, THANKS AGAIN.
 

bobkrech

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Between the manufacturer's changes in motor construction and the nomenclature used to describe a motor, it is frequently difficult to determine which motor the thrust curve represents. Designating motor in the format total impulse, thrust class. average thrust the confusion but it would be impossible to incorporate easily in the existing simulation programs.

I think that many folks who use sims do not understand how or why they work, and that fact is the reason why theses questions arise.

1.) A sim is a differential equation problem solver. It is an optimum mathematical solution to the equations of motion but it is limited to the accuracy of the data that can be input to the simulation, and the availability of certain types of data. Data that does not exist can not be used in a sim.

2.) Motor certifications are performed at 25C +/- 5C. (Room Temperature). The quantity of propellant in the motor is accurately measured, so the total impulse of a motor is the only reasonable invariant in the motor. For practical purposes, the total impulse of a motor is a constant, and does not vary with temperature.

3.) The propellant burn rate depends on temperature so the thrust is temperature dependent. Motors operated below room temperature will burn slower and have a lower average thrust than labeled. Conversely motors operated above room temperature will burn faster and will have a higher average thrust than labeled. Regardless the product of the average thrust multiplied by the burn time is for all purposed constant independent of temperature.

4.) For most average flights, you will get an accurate apogee and time to apogee if you describe your rocket accurately, shape and mass. The calculated velocities have more error because of the unaccounted for drag of a faster or slower burning motor, however the differences are often just a few percent.

5.) Sorry folks but for the prices you are willing to pay for hobby rocket motors, software and the time you are willing to invest in collecting better data, you can only expect to get about +/-5% accuracy in a sim for most rockets. If you have a more accurate description of your rocket, or make your own thrust curve to account for changes in motor performance with temperature and actually measure your environmentals, then you can get more accurate results, but don't expect it.
 

Buckeye

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You're right, it looks like two different motors.

OK, so is this enough info to make a change in Thrustcurve? Have two G80 entries (old and new, or whatever they should be called), since they are so dramatically different in performance. Since the Motor Guide will average all 5 thrust profiles when running a simulation, any G80 results will be very wrong in the current set up.
 

JohnCoker

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OK, so is this enough info to make a change in Thrustcurve? Have two G80 entries (old and new, or whatever they should be called), since they are so dramatically different in performance. Since the Motor Guide will average all 5 thrust profiles when running a simulation, any G80 results will be very wrong in the current set up.
I removed two of the old files, keeping only one old on that stated it was from certified data and added an appropriate note.
 

highflyer1968

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Not sure if this belongs here but the thrustcurve app is missing the max thrust for the j315R 54 mm motor
 

ATGM

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For your motor files, please fix their most common errors and inconsistencies I've found in Thrustcurve.org downloads, errors and inconsistencies that really mess up the motor selection presentation within simulation programs, then offer them in a zipped file as detailed below. The common errors and inconsistencies I've found:

Make sure all available motor delays for a given motor in both ENG and RSE file types are included and correct.

If both file types aren't available on Thrustcurve.org, create the missing type using EngEdit 9 for inclusion in your zipped file mentioned below.

In a motor's RSE file, be sure the motor TYPE is specified.

Make sure your company name or the abbreviation for your company name is exactly the same in all files for your motors.

Correct any other errors or missing information within each file.

CSI, please standardize the motor naming convention within your motor files to be the same as the one you use on your web site. This isn't your fault, of course, and has to do with the preferences of the much appreciated individuals who upload CSI motor files to Thrustcurve.org.

Then, with those nice, clean, consistent and error free motor files:

Create a file name dated zip file with both the ENG and RSE files only for the motors you currently offer for sale and make it available for download. Why only the ones you currently offer for sale? So that we can select a motor from a list of motors that are actually available. The owners of OOP motors can just go to Thrustcurve.org and download the motor file(s) for their OOP motor(s); no need to have those OOP motors polluting everyone else's motor choices within the sim.

Please do not offer a zip file containing all of your motors in a merged *.eng file without also providing a merged *.rse file. Why? Because the by far most popular simulation programs can use RSE files which, unlike ENG files, contain the very useful piece of information about motor type (single use/reloadable).

Also, if you don't make certain that all common errors and inconsistencies mentioned above are corrected (but why didn't you?) in your motor files, individual rather than merged *.eng and *.rse files would be preferable so we can correct the individual files ourselves using EngEdit 9.

-----

CSI offers a current zip file with merged ENG and RSE files, but I haven't checked whether they are free of the common errors and inconsistencies listed above and only for motors they currently sell.

Aerotech offers a current zip file with merged ENG, but not RSE files, but I haven't checked whether they are free of the common errors and inconsistencies and only for motors they currently sell.

Estes provides nothing that I can find.

-----

To any individuals who might already have the above products due to their own efforts, please don't offer a download link unless you are willing to make the commitment that your product will be properly updated indefinitely. It's up to the manufacturers to have one of their employees spend an hour or two to do this and, later, much smaller amounts of time maintaining their file as new motors are offered and old motors are discontinued.

The individuals gracious enough to create and upload those files for your motors to Thrustcurve.org have already done most of the work for you. Please do the rest so we all don't have to do the same individually in order to have neat and valid motor selections within our sims.


First off all, a big thanks to Mark who is constantly on top of these files and frequently contacts us directly for any clarification. As he states, the information is entered manually so errors will happen. With that said, AeroTech offers both ENG and RSE zip files on the site. This should help:
https://www.aerotech-rocketry.com/resources.aspx?id=8

I have had a few errors sent to my inbox here and I will get those to Mark as they come in. If any of you find errors let myself or Mark know and we can get it sorted out from there.

Again, thank you Mark for your efforts.
 

Buckeye

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I think that many folks who use sims do not understand how or why they work, and that fact is the reason why theses questions arise.

The questions in this thread are precisely because people do know how the simulators work. The questions pertain to improving the nominal motor simulation files by going after very fixable things like naming convention, missing information, and data errors (see the OP's plea to manufacturers). Natural variation and the physics of motor burn rates are a different subject.
 
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Winston

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The questions in this thread are precisely because people do know how the simulators work. The questions pertain to improving the nominal motor simulation files by going after very fixable things like naming convention, missing information, and data errors (see the OP's plea to manufacturers). Natural variation and the physics of motor burn rates are a different subject.
As the OP, I thank you for that re-clarification. I haven't visited this thread since I started it and shortly thereafter made clearer who it was aimed it, and I've only read your correct comment. I hope this hasn't evolved into a "there's no room for improvement or any utility in his suggestions," but if it has, oh, well.
 
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