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PropellantHead

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Ok, I've finished the design of my 8" DX3 in Open Rocket (48 pounds w/o motor) and I'm trying to sim a few M and N motors. Below are the results I'm seeing with a variety of CTI and AT motors. They are ordered by predicted apogee and also include the motor designation / total impulse. Do you see anything...odd? Any idea how to fix?

M3400/9995 1688 ft
M2500/9573 2407 ft**
M1890/9852 2717 ft
N2500/13766 3731 ft
M1939/10369 3945 ft
M1845/8093 7517 ft
N2220/10831 9507 ft
N2000/12028 12028 ft

** For the M2500 it sometimes reports a predicted apogee of 8908 ft. for this motor. No rhyme or reason I can detect.

Anybody have any ideas how to fix?
 

OverTheTop

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Is the rocket unstable and yawing about, perhaps due to shifting CG with the different motors? That can blow a lot of energy just be being away from zero AoA for a significant time.
 

Danh

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Change the motor delay to "none" , if it is set to 0 then it will think apogee is at motor burnout.
 

Buckeye

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Well, what do you think the correct answer should be? 99% of these issues are user error (or drunkenness as the case may be)
 

H_Rocket

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A DX3 is a pretty simple air-frame to model. Try it in RAS Aero (I presume you prefer free tools). I have found over time it is actually far more accurate. You just can't do all the sci-fi effects (transitions, pods, tube fins, Fins in weird places, etc.) or pretty colors in the simulation. It should easily handle a 3FNC job like a DX3.
 

NateLowrie

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Somethings wrong with your sim. The M2500 should sim to over 8,000. I also agree you need to check the CP/CG for instability. Check your motor delays and parachute deployments to make sure the sim is not trying to eject the chute right after motor burn out. Also, check your Mass and CG overrides and make sure it's set correctly. Post the sim results or better yet the rocksim file here.
 

rharshberger

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Or in addition to the above, make sure your parachutes deployment settings are to Apogee instead of "first deployment charge of this stage" it will then ignore the motors delay but the sim will still show whatever the optimum delay is.

Sent from my SM-N920V using Rocketry Forum mobile app
 

Zebedee

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Check the stability predictions - OR will chop the flight off at the point it thinks your rocket goes unstable and starts skywriting.
 

PropellantHead

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Is the rocket unstable and yawing about, perhaps due to shifting CG with the different motors? That can blow a lot of energy just be being away from zero AoA for a significant time.
It's quite stable. This is a well established kit and with the M2500, OR shows it as having 1.84 caliber of stability.

With the bigger motors I selected, the altitudes tended to be more what I expected even though the stability goes down to 1.52-1.55.
 

PropellantHead

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A DX3 is a pretty simple air-frame to model. Try it in RAS Aero (I presume you prefer free tools). I have found over time it is actually far more accurate. You just can't do all the sci-fi effects (transitions, pods, tube fins, Fins in weird places, etc.) or pretty colors in the simulation. It should easily handle a 3FNC job like a DX3.
I don't know what that is but I'll look for it.
 

mikec

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If you don't post the file you can't expect anyone to have useful advice.

In my experience OR is close in accuracy to RASAero and easier to use. You have a problem more fundamental than the simulation, but without the file it's impossible to know what's wrong.
 

PropellantHead

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Change the motor delay to "none" , if it is set to 0 then it will think apogee is at motor burnout.
Ding ding ding!!!

That was it! Somehow some of the motors I picked were set to 0...other was set to None. I never change anything soooo... *shrug*


At any rate, all is well now. Thanks!
 

PropellantHead

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If you don't post the file you can't expect anyone to have useful advice.

In my experience OR is close in accuracy to RASAero and easier to use. You have a problem more fundamental than the simulation, but without the file it's impossible to know what's wrong.
I was planning to post them but after reading the initial comments here, the problem was identified and fixed.
 

manixFan

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(OK, so the fix was posted while I was writing this and it was the Zero delay issue. But since I had already posted it I'll just leave it.)

Check/plot your CG/CP during the flight. It may start out stable and but become unstable during flight as CG shifts and then OR terminates the flight. OR has preset plots that you can use to check stability during motor burn or you can do your own. Could be the reason the altitudes are so low.


Tony

here's one I did for a +Mach flight, line to watch is the green one which shows stability margin. You can see this one increases but I've had plots that decrease as propellant burns and CG shifts:
Mach-stability-check.png
 
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Buckeye

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Ding ding ding!!!

That was it! Somehow some of the motors I picked were set to 0...other was set to None. I never change anything soooo... *shrug*


At any rate, all is well now. Thanks!
FWIW, I set all my motors to an artificially high delay, like 30 seconds. This is to ensure that they reach apogee and begin descent. This method is a hold over from the days when Rocksim's optimal mass calculation got confused if the rocket "stopped short" before reaching the max possible apogee.
 

MikeyDSlagle

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Looks like problem has been solved, but like Rich said, just set the first ejection event to deploy at apogee or apogee plus one or whatever and ignore the delay on your motor. OR will tell you the optimum delay. Use that number to pick the motor delay in OR then sim it again to check deployment speed. Pick/drill delay accordingly.
 
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