Apparent big discrepancies between Open Rocket & Thrust Curve...

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Tim51

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Hi All

This is probably a case of a bad rocketeer (i.e. me) blaming his software tools but I'm getting big discrepancies in predicted results from OpenRocket sims vs Thrust Curve. For example, Thrust Curve simulates my specs for a 57.4 mm diameter rocket with a dry weight of 737g reaching an impressive 3223m / 10,574ft on a CTI 644-J94 (38mm of course). The same rocket with the same motor on OR achieves not even half that - 1410m / 4626ft. I've double checked all relevant points of comparison, including finish, dry weight etc.

Otherwise there is nothing particularly unusual about my design - basically a Loc Vulcanite file, but with a VK 5:1 nosecone and slightly modified fins.

Can anyone suggest where I maybe going wrong, please..?

--
UKRA #1895 L1
 
What Cd did you use for Thrustcurve? What fin cross-section did you use in OR. If you used square that makes a huge difference in OR.
 
Also, OR allows you to set the rail length, whereas thrustcurve assumes a 3' rail. Check your delay times in OR; if it shows you deploying your chutes too early, that will limit your altitude.
 
What Cd did you use for Thrustcurve? What fin cross-section did you use in OR. If you used square that makes a huge difference in OR.

Thanks for the swift reply. I didn't even know there was a drag coefficient adjustment option with Thrust curve! I was assuming it was 0.75. I input the data on the TC 'Rocket Motor Guide Page', then selecting the J94. As for the fins, in OR I selected rounded.
 
Also, OR allows you to set the rail length, whereas thrustcurve assumes a 3' rail. Check your delay times in OR; if it shows you deploying your chutes too early, that will limit your altitude.

Thanks. The delay times could well be the issue.. I'll adjust and see what happens.
 
Look at your velocity at deployment. If it is really high, then you probably are deploying before apogee.
 
By tweaking the deployments (I'm imagining a drogue at apogee and main at 500') I raised the OR apogee estimate to 2437m / 7995', but it's still a sizeable discrepancy. Obviously I appreciate the Thrust Curve page is a rough 'n' ready reckoner rather than a precision simulation.
 
You assumed a perfect finish on you rocket in thrustcurve, so the Cd used is 0.3 and the calculated apogee is 3223 meters.

If you use average finish in thrustcurve, the Cd increases to 0.6 and the calculated apogee drops to 2211 meters.

If you use a rough finish in thrustcurve, the Cd increases to 0.9 and the calculated apogee drops to 1767 meters.

OR calculates the Cd based on your design inputs, and calculated a Cd which is greater than 0.9.

You need to read the output files of your sims to make sure that the same assumptions were used in both sims. If so the results should be the same or very close.

Bob
 
You assumed a perfect finish on you rocket in thrustcurve, so the Cd used is 0.3 and the calculated apogee is 3223 meters.

If you use average finish in thrustcurve, the Cd increases to 0.6 and the calculated apogee drops to 2211 meters.

If you use a rough finish in thrustcurve, the Cd increases to 0.9 and the calculated apogee drops to 1767 meters.

OR calculates the Cd based on your design inputs, and calculated a Cd which is greater than 0.9.

You need to read the output files of your sims to make sure that the same assumptions were used in both sims. If so the results should be the same or very close.

Bob

Many thanks for the detailed run through and clarification, Bob. I'll revisit those points again. The finish settings I'd used in OR were smooth (20 µm)
for the nose cone and fins and regular (60 µm) for the airframe.
 
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