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Stability override in OpenRocket?

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Raketenolli

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Since wRASP is no longer compatible with current OSes like Win7 oder Win10, and RASP XP does not include many motors (e. g. Klima from Germany is missing), I would like to abuse OpenRocket to do quick and dirty sims to determine altitude, delay times or speeds off the rail for different weights or engine selections.

Unfortunately, if I create a really simple rocket without fins, the OpenRocket sim takes the instability into account and my vertical speeds are all over the place and the altitude is of course wrong (rocket is doing loopings, obviously).

openrocket_unstable.png

Is there any way to override the stability issues (e. g. through prescribing a CP location), just like one can override mass and CG location?

Or should I rather focus on extending the RASP XP motor library? I always liked wRASP a lot.

Oliver

PS: I added an illustration of the sim behavior for unstable rockets.
 
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mccordmw

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You can override CP in OpenRocket. Just click on and edit the overall rocket model. The override options are there. I always override mass since prime+paint adds more weight. If I have a kit that prescribes the CP measurement, I'll also override that and use the vendor's supplied data. Seems to work better for fat models with lots of base drag.
 

Raketenolli

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Where exactly? I find the options for overriding CG location and mass both for the components and an entire stage, but I can't find anything to override stability margin or CP location.

Oliver
 

mccordmw

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Oops. Sorry. I just opened a file for a stubby 3FNC I have, and I've overriden the CP for the tube airframe, not the whole model.
 

Buckeye

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Oops. Sorry. I just opened a file for a stubby 3FNC I have, and I've overriden the CP for the tube airframe, not the whole model.
How do you override CP, component or otherwise?
 

Rex R

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I too am curious, with OR 15.03 there does not appear to be any method for over riding the center of pressure...just mass and center of gravity.
Rex
 

mccordmw

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Ack. That's what I get for answering at work without having OR up. Mass and cg are overrideable only. Not cp. Sorry.
 

MikeyDSlagle

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Add some fins, then over ride the added mass and Cd of them.
I was thinking this, but a mass massless, dragless fin wouldn't do anything for stability. Right? May as well not even be there.
 

dhbarr

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I was thinking this, but a mass massless, dragless fin wouldn't do anything for stability. Right? May as well not even be there.
It'll be massless, but the drag will still be calculated as with the phantom cone for base drag fudging on shortfats.
 

Buckeye

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The OP wants to vary weight and motors very quickly. Thrustcurve and wRASP are best for this - no need to "build" a rocket in the simulation.
 

rcktnut

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For quick and dirty of everything you ask for, use Thrustcurve motor guide, instead. It the only way to efficiently look at all motors in your rocket.

https://www.thrustcurve.org/guidepage.jsp

I also use wRASP32 on my Win7 machine:

https://www.rocketryforum.com/showthread.php?70394-Rediscovering-RASP


The key word here is rocket, a design right? What good does this do for a rocket without fins and whatever else is omitted? I enter the weights and measured CG of my completed rockets, kits and scratch built alike into the sim and then go from there. Seems like a waste of time entering info into this program when you have no idea of what you are actually going to end up with.
 

Flyfalcons

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The key word here is rocket, a design right? What good does this do for a rocket without fins and whatever else is omitted? I enter the weights and measured CG of my completed rockets, kits and scratch built alike into the sim and then go from there. Seems like a waste of time entering info into this program when you have no idea of what you are actually going to end up with.
Because the original question was how to do a quick and dirty estimation of a motor's performance based on weight and engine selection. Thrustcurve does exactly that.
 

UhClem

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For quick and dirty it is hard to beat the old RASP program. I looked at what I have on my hard drive and it is version 2.62. I see via thrustcurve.org that they have a link to a version 3.2. The split appears to have happened in the early 90's with the version I have being updated by Mark Johnson and Chuck Gibke. The 3.2 version appears to have a couple of changes I like so perhaps I need to spend a little time and see if I can combine the two branches.

I have already cleaned up all of the -Wall cruft from the 3.2 version and it runs.

I am considering adding the ability to look up motors on thrustcurve.org but that might be more effort than it is worth. Not too much since I can borrow most of the code from my changes to Chuck Gibke's stats program.
 

Buckeye

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The key word here is rocket, a design right? What good does this do for a rocket without fins and whatever else is omitted? I enter the weights and measured CG of my completed rockets, kits and scratch built alike into the sim and then go from there. Seems like a waste of time entering info into this program when you have no idea of what you are actually going to end up with.
I am not sure if you are objecting to Thrustcurve or the OP's attempt to spoof OR into making a minimalist rocket.

Regardless, it might be helpful to read up on what the simulators do and the assumptions made. John Coker's site has good info, as will a search on "rocket equations." All that is needed to get a basic 1D flight simulation is motor thrust and burn duration, mass, diameter, air density, and a drag coefficient. Doesn't matter what the rocket looks like. Building a "design" in OR and RS is more of a CAD and visualization exercise than anything else. OR and RS will do some fancy Cd and Cp refinement based on the design details, but in the end, the Thrustcurve answer is usually good enough for simple flight regimes.
 

rcktnut

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I am not sure if you are objecting to Thrustcurve or the OP's attempt to spoof OR into making a minimalist rocket.

Regardless, it might be helpful to read up on what the simulators do and the assumptions made. John Coker's site has good info, as will a search on "rocket equations." All that is needed to get a basic 1D flight simulation is motor thrust and burn duration, mass, diameter, air density, and a drag coefficient. Doesn't matter what the rocket looks like. Building a "design" in OR and RS is more of a CAD and visualization exercise than anything else. OR and RS will do some fancy Cd and Cp refinement based on the design details, but in the end, the Thrustcurve answer is usually good enough for simple flight regimes.


No objections at all. I'm just wondering how mass and Cd is determined without fins and other components. I must admit that 20 years as a BAR this is the first time I've heard of this. All I want to know is how does this help in determining what the altitude, delay length, and speed off the rail I need with a finished rocket sitting in front of me, rather than a imaginary/minimalist rocket with imaginary mass/ Cd with imaginary results? What's the sense?
 

Buckeye

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No objections at all. I'm just wondering how mass and Cd is determined without fins and other components. I must admit that 20 years as a BAR this is the first time I've heard of this. All I want to know is how does this help in determining what the altitude, delay length, and speed off the rail I need with a finished rocket sitting in front of me, rather than a imaginary/minimalist rocket with imaginary mass/ Cd with imaginary results? What's the sense?
Mass is not imaginary. You still need to weigh the model and type in the value.

Cd is taken as a simple, constant, average value over the flight duration. 0.6 or 0.7 is an average value that works pretty good for 90% of shapes that look like a subsonic rocket.

Once you go beyond quick and dirty and start introducing wind, weather, stability, sonic speeds, launch site, and 6 DOF to the simulation, then the Thrustcurve assumptions no longer apply. OR, RS, and RAII allow you to model those complexities more precisely.
 

rcktnut

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Mass is not imaginary. You still need to weigh the model and type in the value.

Cd is taken as a simple, constant, average value over the flight duration. 0.6 or 0.7 is an average value that works pretty good for 90% of shapes that look like a subsonic rocket.

Once you go beyond quick and dirty and start introducing wind, weather, stability, sonic speeds, launch site, and 6 DOF to the simulation, then the Thrustcurve assumptions no longer apply. OR, RS, and RAII allow you to model those complexities more precisely.


I fully understand how the programs work, RS has served me well through the years. I agree with the statement you made, (underlined). Good programs to see how a design with all components or completed rocket will perform. What I don't understand is why someone would want to do sims / check motor performance on an incomplete design, rough idea, whatever you want to call it, even going as far as not having a fin design and trying to tweak the CP. Why would someone want to know the details, altitude etc. on a rough idea with guesstimated mass etc? It is not going to do you any good on a final design with all components, or final build.

Again IMO seems like a waste of time doing quick and dirty, compared to working with a complete design.
 

Flyfalcons

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Sure it does. I was able to plug in the parameters of a kit I have in waiting using its diameter and estimated finished weight, and get a list of motors that'll work for it and expected performance. I can fine tune later using a sim file, but it's not going to be much different than what Thrustcurve spits out. Took about 20 seconds.
 

Buckeye

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Sure it does. I was able to plug in the parameters of a kit I have in waiting using its diameter and estimated finished weight, and get a list of motors that'll work for it and expected performance. I can fine tune later using a sim file, but it's not going to be much different than what Thrustcurve spits out. Took about 20 seconds.
^^^ This. +1

rcktnut, maybe you have your designs dialed in exactly the way you want and build your rocket around a specific motor. Then, yes, the Q&D sims won't be of value to you. I have some fluidity in my designs and like to explore different options, especially the bazillion motors choices out there.

I just ran 348 simulations covering every 38 mm and 54 mm motor in my rocket, sorted by altitude, launch guide velocity, and Mach number. Took 5 seconds as I write this post. Now I can figure out what to buy on Black Friday! Try that with OpenRocket. Can't be done.
 

rcktnut

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Quote Originally Posted by Flyfalcons
Sure it does. I was able to plug in the parameters of a kit I have in waiting using its diameter and estimated finished weight, and get a list of motors that'll work for it and expected performance. I can fine tune later using a sim file, but it's not going to be much different than what Thrustcurve spits out. Took about 20 seconds
.






^^^ This. +1

rcktnut, maybe you have your designs dialed in exactly the way you want and build your rocket around a specific motor. Then, yes, the Q&D sims won't be of value to you. I have some fluidity in my designs and like to explore different options, especially the bazillion motors choices out there.

I just ran 348 simulations covering every 38 mm and 54 mm motor in my rocket, sorted by altitude, launch guide velocity, and Mach number. Took 5 seconds as I write this post. Now I can figure out what to buy on Black Friday! Try that with OpenRocket. Can't be done.





I agree with you guys, and know that will work. What I have in question is the OP's reasoning on entering what he wished to, and getting any results that would be beneficial. Don't see where a length of body tube and a nose cone, no fins, and whatever else is not included, and guessing on the mass would do any good.

A kit with known dimensions /mass, a complete design, or built rocket, fine agree 100%. I decide what I want/design first, kits, scratch, and then sim with various motors. I never built/ designed around specific motors no need to with adjustable delays, and altimeters. Much easier/ cheaper now to do that than 20 years ago! I like scale military, and have lots of designs on file, with all components included.

Thanks guys for the input appreciate it.
 

Buckeye

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What I have in question is the OP's reasoning on entering what he wished to, .
Yeah, one can effectively dumb down the simulator, which the OP was attempting to do. To do so, you would, at a minimum, "build" a generic nose cone, body tube, and fins. That's all that is needed to spoof the software. Then, override mass, override Cd, turn off wind, turn off launch angle, turn off weather variability, turn off any other variability. Voila, you just turned OR into Thrustcurve. Actually, this may be easier with Rocksim, and there is probably an Apogee newsletter explaining how to do it. As some of us were suggesting, all this isn't worth the hassle when Thrustcurve and wRASP are available (and better suited).
 

Flyfalcons

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I agree with you guys, and know that will work. What I have in question is the OP's reasoning on entering what he wished to, and getting any results that would be beneficial. Don't see where a length of body tube and a nose cone, no fins, and whatever else is not included, and guessing on the mass would do any good.

A kit with known dimensions /mass, a complete design, or built rocket, fine agree 100%. I decide what I want/design first, kits, scratch, and then sim with various motors. I never built/ designed around specific motors no need to with adjustable delays, and altimeters. Much easier/ cheaper now to do that than 20 years ago! I like scale military, and have lots of designs on file, with all components included.

Thanks guys for the input appreciate it.
I still don't think you're getting it. The OP wants a generic profile for estimating performance based on different weights. Thrustcurve does exactly that. He's not talking about design.
 

Raketenolli

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Whew, seems like I opened a large can of worms here.

To answer one of your questions:
What I have in question is the OP's reasoning on entering what he wished to, and getting any results that would be beneficial. Don't see where a length of body tube and a nose cone, no fins, and whatever else is not included, and guessing on the mass would do any good.
A finless rocket, thrust vector stabilized for example (semi-scale Black Arrow, anyone? 8 motors in the first stage, 4 motors in the second stage, 1 motor in the last stage ... burn time of 18 seconds, altitude 550 m). Or just out of sheer curiosity.

I caved in and installed RASP XP and manually added the Klima engines to the .eng file. I'll add other motors as required.

Thanks for the lively discussion, though ;-)

Oliver
 

rcktnut

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I still don't think you're getting it. The OP wants a generic profile for estimating performance based on different weights. Thrustcurve does exactly that. He's not talking about design.
I "played" with Thrustcurve for awhile, neat. Still don't know the reasoning behind "playing" with something generic, guessing/ changing mass as opposed to something "sound". The results are useless unless you happen to hit the correct mass of the design you have in mind.
Just my opinion again, seems more productive producing the design, entering the specs of the kit, or specs of a built rocket and then running it thru Thrustcurve.

Guess I still don't get it!
 

rcktnut

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Whew, seems like I opened a large can of worms here.

To answer one of your questions:

A finless rocket, thrust vector stabilized for example (semi-scale Black Arrow, anyone? 8 motors in the first stage, 4 motors in the second stage, 1 motor in the last stage ... burn time of 18 seconds, altitude 550 m). Or just out of sheer curiosity.

I caved in and installed RASP XP and manually added the Klima engines to the .eng file. I'll add other motors as required.

Thanks for the lively discussion, though ;-)

Oliver

No can of worms opened up. Glad this came up, totally new to me after 20 years back in the hobby. By your first post I thought this thread was going to be about finless rocket designs. Excellent discussion!
 

timbucktoo

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It's very useful at launch site using smartphone app. If the motor I planned to use isn't available or if rail length is different from what I simmed in OR you can see predictions on the fly versus pulling out laptop (which myself and most others don't bring to launches)!
 
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