Weird Open Rocket Happenings With 13 mm Estes Engines

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Cape Byron

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This is an odd one.

Been using OR 15-03 for some time with good results. Love the software. Now working with 13 mm motors I'm getting some weird results with large CG changes between A3-4T and A10-3T.

The Estes Engine Chart shows the A3-4T having an initial weight of 7.6 gm. The A10-3T has an initial weight of 7.9 gm.

On an A10-3T I'm getting a stability of 1.79 calibre. If I change to an A3-4T I get a stability of 1.04 calibre. A stability reduction on a lighter engine.

Every 13 mm motor design I have shows the same behaviour. Version problem? Operator error? Alcohol induced operator error?

OR File attached.

TIA as always.
 

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kuririn

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I think the weight (launch mass) of the A10 in the rasp engine file is incorrect.
Showing .185 oz.
S/B closer to .30 oz.
Substantial difference.
A3 weight looks correct at .30 oz.
Got out my scale and dug up those motors outta my stash.
A10-3: .30 oz.
A3-4: .28 oz.
Actual weights.
 

BrendanH69

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Now... how do you edit a RASP file?
Check out the engine files in Thrustcurve.org. I believe the OR engine files are sourced from there, so maybe the source file has an error. If not, import the correct file from Thrustcurve. I learned to do it recently for a couple of Aerotech DMS motors not in the standard OR library.
 

Cape Byron

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Check out the engine files in Thrustcurve.org. I believe the OR engine files are sourced from there, so maybe the source file has an error. If not, import the correct file from Thrustcurve. I learned to do it recently for a couple of Aerotech DMS motors not in the standard OR library.
Cool, thanks for that.

I'll look at it tomorrow. Apparently I'm cooking swordfish steaks in a few minutes...
 

Cape Byron

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Hmmm, I still have issues.

I copied the A10 .eng file to the directory that holds my motor curves.

Loaded OR and the rocket file in question.

Deleted the motor configurations from the rocket and saved the .ork

Quit OR.

Reloaded OR and the rocket .ork

Added the A3-4T and A10-3T to the sim.

Poop. Still the same result.

Is the A10.eng file not being used because it already exists in the OR internal library?

BTW, dinner was swordfish steaks, king prawns, baked potatoes, Greek salad, aioli and sliced Meyer lemons...
 
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kuririn

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Burp........
What's the motor weight of the A10 file you loaded?
 

Cape Byron

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This is the line from the .eng file from John Coker.

'A10T 13 45 3-100 0.0038 0.0085 Estes' (My colour add)

From OR - Mass with no motors 13g

A3-4T - Mass with motors 21.5g
ie, 8.5g motor

A10-3T - Mass with motors 18.3g
ie, 5.3g motor

So, my assumption is that OR isn't loading the new A10-3T .eng file on startup
 

kuririn

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I don't know if this is the issue but there is a check box on the motor selection window that says "hide motors with similar thrust curves" or something like that. If it's checked try unchecking it and see if you can find the new file.
 

Cape Byron

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The .eng file loading has the same file production date as the one from John (October 3, 2000) in the text.

eng file.jpg


Note the launch mass is given as 5.25 g.

I really do think I'm missing something basic here, but I don't know what.
 

BrendanH69

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Wild guess here based on no experience...maybe try re-downloading the file and giving it a new name to differentiate it from the current ones in OR and see if that works.
 

Cape Byron

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Wild guess here based on no experience...maybe try re-downloading the file and giving it a new name to differentiate it from the current ones in OR and see if that works.
Tried that with no luck. Also downloaded the A3T file and put that in the motor curves folder. No joy yet, but thanks all for trying.
 

neil_w

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When I get home tomorrow I’ll look into this.
 

sebastianc

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Perdon por el idioma amigos, he tenido estos problemas con algunos modelos de motores que no estan bien cargados en el OR, hay que revisar todos los datos reales con los archivos del software.
 

kuririn

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Perdon por el idioma amigos, he tenido estos problemas con algunos modelos de motores que no estan bien cargados en el OR, hay que revisar todos los datos reales con los archivos del software.
MS Translator:
Sorry for the language friends, I have had these problems with some engine models that are not well loaded in the OR, you have to check all the real data with the software files.
 

Cape Byron

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MS Translator:
Sorry for the language friends, I have had these problems with some engine models that are not well loaded in the OR, you have to check all the real data with the software files.
Thanks @sebastianc and @kuririn .

Still fiddling in my spare (!) time. I'm a bit surprised that this hasn't come up before. Where's the love for mini engines?
 

neil_w

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I don't know how the A10T motor file got screwed up in OR 15.03, but for what it's worth it's fixed in the new release.

First, go back into the file you downloaded from Thrustcurve.org and change the motor designation to something you'll recognize, say, "A10-new", or maybe even "A10-correct", or whatever.
1628039610659.png


Save and restart OR.

Now, go back into "select a rocket motor", and uncheck "Hide very similar thrust curves". At this point you can click on the pull-down arrow of the "Select thrust curve" selector, and you should see your new A10T option there. Select that and all should be right in the world. You can confirm by looking at the motor "Show details", and you should see the correct launch mass of the motor.
1628039477327.png
 

sebastianc

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Thanks @sebastianc and @kuririn .

Still fiddling in my spare (!) time. I'm a bit surprised that this hasn't come up before. Where's the love for mini engines?
Yo he tenido problemas con los pesos de algunos motores mayores a los A, tambien debo reconocer que tal vez en algunos casos aqui en Argentina no se tengan los ultimos modelos, por eso tengo diferencias.
Hasta ahora de la Marca Estes, esta mal el peso de los D12-5 el peso es mayor en realidad ya que se uso el peso del modelo D12-0. En la Marca Aerotech, los modelos de motores E30-7 y D21-7 son menores los pesos que los motores que tuve en mis manos.
PD: Perdon por el idioma.
 

kuririn

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Ah, I neglected to use the pull down on the select thrust curve selector.
Didn't realize there was more than one.
I assumed that John Coker's file just wasn't showing.
Glad you posted this Ozzie. I'm learning new things.
Neil, question for you.
Could you do the same by going Edit/Preferences/Add then selecting the directory of the downloaded engine file?
I tried that and it didn't seem to show the new file until I read your post regarding changing the name and using the pull down tab.
2021-08-03.png


It works!
(sim 2)
PS Cape Byron you should get a prize for finding a bug that no one else saw. :bravo:
 
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Cape Byron

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I don't know how the A10T motor file got screwed up in OR 15.03, but for what it's worth it's fixed in the new release.

First, go back into the file you downloaded from Thrustcurve.org and change the motor designation to something you'll recognize, say, "A10-new", or maybe even "A10-correct", or whatever.
View attachment 475582

Save and restart OR.

Now, go back into "select a rocket motor", and uncheck "Hide very similar thrust curves". At this point you can click on the pull-down arrow of the "Select thrust curve" selector, and you should see your new A10T option there. Select that and all should be right in the world. You can confirm by looking at the motor "Show details", and you should see the correct launch mass of the motor.
:clapping:
Yes. Yes. Yes!

It works!

Thanks so much for that. It gives me inspired confidence in my sims and a new overall glow like I've been waxed and polished.
 

kuririn

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Yo he tenido problemas con los pesos de algunos motores mayores a los A, tambien debo reconocer que tal vez en algunos casos aqui en Argentina no se tengan los ultimos modelos, por eso tengo diferencias.
Hasta ahora de la Marca Estes, esta mal el peso de los D12-5 el peso es mayor en realidad ya que se uso el peso del modelo D12-0. En la Marca Aerotech, los modelos de motores E30-7 y D21-7 son menores los pesos que los motores que tuve en mis manos.
PD: Perdon por el idioma.
"I have had problems with the weights of some engines greater than the A, I must also admit that maybe in some cases here in Argentina do not have the latest models, so I have differences.
So far of the Brand Estes, this bad weight of the D12-5 the weight is actually higher since the weight of the D12-0 model is used. In the Aerotech Brand, the E30-7 and D21-7 engine models are less weight than the engines I had in my hands.
PS: Sorry for the language."
 
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Cape Byron

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PS Cape Byron you should get a prize for finding a bug that no one else saw. :bravo:
Not really. I thought it was user error, not a bug.

@neil_w should get a prize for everything he's done. And @sebastianc and @kuririn while we're handing out prizes. This is why I support the forum: it's where we find the answers.
 

msjohnso

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I don't know how the A10T motor file got screwed up in OR 15.03, but for what it's worth it's fixed in the new release.

First, go back into the file you downloaded from Thrustcurve.org and change the motor designation to something you'll recognize, say, "A10-new", or maybe even "A10-correct", or whatever.
View attachment 475582

Save and restart OR.

Now, go back into "select a rocket motor", and uncheck "Hide very similar thrust curves". At this point you can click on the pull-down arrow of the "Select thrust curve" selector, and you should see your new A10T option there. Select that and all should be right in the world. You can confirm by looking at the motor "Show details", and you should see the correct launch mass of the motor.
View attachment 475581
Somewhere in my computing archives I have a C program called 'raspinfo' that reads the rasp.eng file format as shown in this thread and produces a one-per-line report showing the total impulse of each motor described in the file. I am also fairly confident that the comments regarding accuracy of TI with only 32 thrust data points are mine. It's been a long time since I fiddled with that program but I have the source code and a DOS executable somewhere on file, if anyone happens to want it...
 

BABAR

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Not a sim expert by any means (I have OR and RS, have yet to actually use them)

I am just mind-simming this with available data.

But is there a chance your stability difference is NOT do to the weight but due to the thrust?


The A3-4T weighs about 13% less than the A10-3T. Given the motor is only a small part of the rocket mass, while YES it will bring the CG forward a smidge, the peak thrust is almost 54% less, I am not not very good at integrating but presuming it takes about 0.2 seconds to clear the rod, the A10 powered rocket has a heck of a lot higher velocity which should make the fins much more effective.

so maybe sim is right. The minimal improvement in CG is overwhelmed by, hmmm, not a CHANGE in CP but a less effective CP because of low velocity?



Yeah, the total impulse of the A3 is slightly more, but the difference is made up during “cruise”, (hey, I made up a new term but it makes sense). “Lift off” is the time the rocket is on the rod or rail, stability calcs by OS, RS, and ThrustCurve are approximately made on basis of speed at the end of rod or rail guidance. What I call “cruise” is the period between rod/rail exit and burnout of thrust, which is followed by traditional term coast which is either delay charge or until electronics detects apogeee.

so A3-4T is the go to for a light rocket if you WANT altitude (something I often don’t need). A10-3T is my go to when
I am insecure getting the rocket stable off the rod.
 

Cape Byron

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But is there a chance your stability difference is NOT do to the weight but due to the thrust?
The large variation in static CG/CP relationship is what I was alerted by and why I went to the Estes motor data first. That's when I asked here, as I expected very static stability change.

With the 'new' A10 data loaded the stability change between A3 and A10 is essentially zero, more like the expected result.

so A3-4T is the go to for a light rocket if you WANT altitude (something I often don’t need). A10-3T is my go to when I am insecure getting the rocket stable off the rod.
Absolutely. Anecdotally I always test a new 13 mm rocket with an A10 before I fly with an A3. The final altitude is slightly less, but the initial thrust is higher with the A10.

BTW, 13 mm motors are great and underrated by a lot of people. Four flights for the price of three!
 
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