How accurate is OpenRocket?

I have an Estes Ascender 2" I have flown several times using a Altimeter Two to record data. The OpenRocket predictions of altitude are high by 10% or so, and the ground hit speed is higher than the Altimeter Two data by about 33%.

Is this typical of OpenRocket?

Dan

Is open rocket set to the correct launch sure elevation?

Any program is only as good as the input data. Lots of the physical data are easily confirmed (diameters, weights, etc.) but variables such as the coefficient of drag parachute effects are difficult to determine with any certainty. If you trust your altimeter information, you can use that to "play" with values in OR to get it to agree with the altimeter.

Once you've done that, you can check it against a different motor and continue to fine tune it until your getting an acceptable agreement.

Ravenex said:

Is open rocket set to the correct launch sure elevation?

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Yes it is. I have also very carefully weighted each component as I entered them into OpenRocket.

That's actually not the most accurate way to input weight (or CG.) It can be used in the design stage to suggest the stability but it really is necessary to weigh the rocket after it's completed with everything on board except the motor and determine the center of gravity in that configuration to do the final input using the mass and CG location overrides. At the very least, you'll be correcting inaccuracies for finishing weight, bonding weight and location. Beyond that, you probably have the wrong CD. Nothing you did wrong. Nothing OR did wrong. It's the single most difficult parameter to determine and is best done by using the above for weight and CG location and adjusting CD manually to match observed behavior.

dhkaiser said:

Yes it is. I have also very carefully weighted each component as I entered them into OpenRocket.

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Peter Olivola said:

That's actually not the most accurate way to input weight (or CG.) It can be used in the design stage to suggest the stability but it really is necessary to weigh the rocket after it's completed with everything on board except the motor and determine the center of gravity in that configuration to do the final input using the mass and CG location overrides. At the very least, you'll be correcting inaccuracies for finishing weight, bonding weight and location. Beyond that, you probably have the wrong CD. Nothing you did wrong. Nothing OR did wrong. It's the single most difficult parameter to determine and is best done by using the above for weight and CG location and adjusting CD manually to match observed behavior.

Click to expand...

Interesting, thanks. How does one adjust CD manually?

dhkaiser said:

How does one adjust CD manually?

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You can't do that directly in OR, but you can change the component finish and fin cross-section to better match the altitudes you're getting. 10% is pretty good, though, you shouldn't expect a lot better than that.

Other factors are the motors themselves have a tolerance, so 2 launches with the same rocket with the same motor type, especially from different lots, will give different results.

Wind/weather cocking will affect the actual altitude - and needing to consider winds at different altitudes which will be different.

I agree with mikec - if you are getting within 10% that is pretty good

Thanks guys, I appreciate the advice.

try setting your 'component finish' to regular paint(if it isn't already), that usually gets the altitude within spitting distance.
Rex

dhkaiser said:

I have an Estes Ascender 2" I have flown several times using a Altimeter Two to record data. The OpenRocket predictions of altitude are high by 10% or so, and the ground hit speed is higher than the Altimeter Two data by about 33%.

Is this typical of OpenRocket?

Dan

Click to expand...

I would guess both the hardware (AltimeterTwo) and the software (OpenRocket) are somewhat less than 100% accurate.

Once you have several AltimeterTwo readings with the same rocket/motor combination, then you can start to tweak your sim in OpenRocket so it matches better.

How accurate is your sim in OR as far as rocket dimensions, weight, paint finish, and launch conditions?

les said:

Other factors are the motors themselves have a tolerance, so 2 launches with the same rocket with the same motor type, especially from different lots, will give different results.

Wind/weather cocking will affect the actual altitude - and needing to consider winds at different altitudes which will be different.

I agree with mikec - if you are getting within 10% that is pretty good

Click to expand...

+1 to all these

and adjusting CD manually to match observed behavior.

Click to expand...

Anybody have data showing how well the adjusted CD repeats itself for different flights and different motors?

dhkaiser said:

I have an Estes Ascender 2" I have flown several times using a Altimeter Two to record data. The OpenRocket predictions of altitude are high by 10% or so, and the ground hit speed is higher than the Altimeter Two data by about 33%.

Is this typical of OpenRocket?

Dan

Click to expand...

On apogee and velocity predictions with a rounded fin leading edge rocket, Openrocket and Rocksim are virtually identical. On rockets with significant areas of flat fin leading edges, Openeocket is much more accurate:

My discovery of the radically different altitude predictions between Rocksim and Openrocket for flat leading edges:

https://www.rocketryforum.com/showt...R-1-2-altitude-of-Rocksim&p=701621#post701621

My experiments:

https://www.rocketryforum.com/showt...R-1-2-altitude-of-Rocksim&p=709474#post709474

dhkaiser said:

I have an Estes Ascender 2" I have flown several times using a Altimeter Two to record data. The OpenRocket predictions of altitude are high by 10% or so, and the ground hit speed is higher than the Altimeter Two data by about 33%.

Is this typical of OpenRocket?

Dan

Click to expand...

First, 10% altitude error isn't bad. Your launch-to-launch variability is likely higher than that.
Second, did you weigh the rocket before you launched it? Predicted launch weights are not always accurate.
(If your rocket was a tad overweight you would be short on altitude and high on landing speed. Not saying this happened but it was worth remarking.)

BTW, predicted CG's should be always verified before launch. CG placement can make the difference between a stable and unstable rocket. Everyone worries about CP; they should worry more about CG.

As others have noted, Cd is hard to predict accurately. In particular, the Cd/speed curve before subsonic Cd plateaus is hard to predict. Therefore, flights that spend a large percentage of time in this region are hard to predict.
(Oddly, supersonic Cd's tend to be better predicted.)
Stability corrections, which are largely random, affect Cd as well.

In general (hate to tell you), altitude predictions within 20% are par for the course.

-LarryC

As for your ground hit velocity, Cd of the parachute is configurable in the Parachute Config Dialog. Defaults to 0.75

Doug