RS Predicted Flight Altitude

[Steven88]

Good evening. I recently flew my Loc Big Nuke 3E at Airfest 27. With an AT L850 it had a projected altitude on RS of over 9,000 feet. However, both altimeters only recorded 6,400 and some feet. They were within 40 feet of each other. I am wondering why RS was so far off on the predicted altitude? I have heard that it tends to predict high but cannot imagine it being that far off. I did go into the launch settings and set the altitude for the launch site and a few other settings but did not get real technical with that aspect of it. Is there anything else I can do differently to make my sims more accurate? Thanks for the help

 

[user 30299]

Can you post your RS file ? Did you weigh the rocket and locate the CG, and use them in the sim?

 

[Steven88]

Yes I did. I’ll try to get it posted, maybe give me until this evening. There was a breeze when I launched and it flew pretty straight until it was pretty high then it angled south. I’m wondering if it lost some altitude by doing that, but surely not 3000’.

 

[Steven88]

Does the file need to be in ork format?

 

[Steven88]

Here is the requested file. Sorry it took so long.

 

[sebastianc]

Tengo el mismo problema con el open Rocket simuló una aventura de 310 m y alcanzo un apogeo de 271 m

 

[OzHybrid]

OR says 8204 ft. If it windcocked, 6400 is probably about right allowing for error. Are the empty weights and paint finishes correct?

 

[user 30299]

I think the issues is the your Cd; Coefficient of Drag. RockSim always has a very liberal calculated Cd. I always start with a fixed Cd of 0.6 with Loc rockets in that size class. When I used that Cd in your sim file it came in at 6800 ft. for the L850 motor. Over the years, thru multiple launches of rockets I designed in RS, I started to dial in the starting Cd numbers depending on the build material, paint job, diameter, etc.

 

[Steven88]

OR says 8204 ft. If it windcocked, 6400 is probably about right allowing for error. Are the empty weights and paint finishes correct?

The empty weights are correct but I did nothing to adjust the the paint finishes

 

[Steven88]

I think the issues is the your Cd; Coefficient of Drag. RockSim always has a very liberal calculated Cd. I always start with a fixed Cd of 0.6 with Loc rockets in that size class. When I used that Cd in your sim file it came in at 6800 ft. for the L850 motor. Over the years, thru multiple launches of rockets I designed in RS, I started to dial in the starting Cd numbers depending on the build material, paint job, diameter, etc.

 

[Steven88]

Ok. I will adjust cd. Thanks for the tip!

 

[Bat-mite]

The empty weights are correct but I did nothing to adjust the the paint finishes

Note, I did not look at your sim, so this may be irrelevant.

Paint finish has a HUGE impact on altitude. A "polished" finish will get you a ton more altitude than "smooth paint" or "unfinished". But as stated, these are just contributors to Cd. Override your Cd and finish becomes irrelevant.

To my experience, "polished" is near impossible to achieve. https://www.rocketryforum.com/threa...-higgs-farm-square-rocket.144808/post-1792909