How to adjust RockSim settings to match actual performance

[Smatt]

Hello,

We are experiencing a situation where the rocket's actual apogee is less then the RockSim simulations by 50 to 100' after inputing the settings for finish, weather, altitude etc. We've checked that the weight and GG are the same (amazingly close). The variance of the actual apogees for the same motor etc and slightly different winds, temp and barometric readings is under 50' for 7 launches. I.e., how does one calibrate RockSim to simulate the actual performance?

Thank you!

Smatt

 

[Coop]

50 to 100' is pretty good, all things considered, I'd say. There's going to be a +/- range in not only your motor and igniter, but also in your altimeter. The only thing you may be able to do to more accurately predict results is calibrate the altimeter, if the option exists for whatever model you have. Could also be atmospheric conditions at your launch site.

Is there an imperative where your sim must be closer than 50-100' to practice? That really isn't all that much, when you take everything in, is why I ask. Unless you're flying real low to start with... What actual altitudes are you getting?


Later!

--Coop

 

[RandyT0001]

Have you tried to adjust the Cd override of the rocket in the simulation so that it matches actual flight results?

 

[Smatt]

This is part of a TARC competition effort to achieve 750' apogee. Right now the team makes an allowance for coming up with the weigth to add to the booster section by simulating the rocket in RockSim to go to 810 feet in calm conditions. This works pretty good with the first two actual launches in 1 to 3 mph winds achieving 755 and 739 (based on using a rocksim model apogee of 810' to establish the weighting of the overall rocket). The tricky part is when the wind picks up to 4 to 7 mph, the RockSim modeling doesn't relate to what actual occurs (the actual rocket apogee drops 30 feet lower then the prior launches to 708') while RockSim indicates that the rocket will actually go higher with the same weighting for the early rockets. They overlayed the first 3 seconds of launch data from the altimeter and saw that the last launch had a quicker jump off the pad then the first two and then later in the launch, the three launches where within 10' of each other at 4 seconds (approximately 500' feet) into the launch...so were trying to get the RockSim to 1) accurately predict the rocket's performance and 2) understand why the RockSim suggested that the rocket would climb higher in greater wind conditions under the same weighting (which it didn't as it went quite a bit lower). I feel there must be a setting or something we don't know about in RockSim.

Thanks!
Smatt

 

[Smatt]

Randy,

Thank you for asking. No we haven't tried adjusting the overall Cd. They have entered in each components CG and finish etc and come up with a pretty accurate CG for the rocket. I wasn't sure if they should adjust the overall Cd. If they do adjust the Cd, will they see a change in the Center of Pressure on the RockSim Model (to know if they rocket is stable or is such a concern not signficant)? I guess my question really is, when the overall Cd adjustment is used does it uniformally apply to the rocket surface area or is it just applied to the RockSim design's Center of Pressure? The rocket appears to have a solid stability between 1.5 and 1.9 body diameters in the RockSim design.

Thank you for response,
Smatt

 

[RandyT0001]

Changing the rocket's drag coefficient, the Cd, does not affect rocket stability. Stability depends on the relationship between the rocket's center of gravity (CG) and its center of pressure (CP). A stability margin of 1.5 to 1.9 calibers is sufficient. Go to Apogee Components website and look through back issues of their newsletter. One details the method of adjusting the Cd in the simulation to match test flight results.

In RocSim's flight profile wind flows from the left to right. Set the wind conditions to custom wind speed, make the wind about 9-10mph. In the starting state window adjust the launch angle from 0 degrees to -(minus) 15 degrees and finally to 15 degrees for three different flights of the rocket using the same rocket-motor combination. After that set the launch angle and vary the wind speed for three different settings (0mph, 9-10 mph, 18-19mph). You should be able to see how wind speed and launch angle affects rocket performance.

 

[les]

Another issue could be wind speeds at higher altitudes versus ground airspeed for your actual flight. You don't have to get up very high before you encounter a cross wind with a different speed. You can sometimes get an idea of this by watching the drift while coming down on the parachute/streamer. I don't believe Rocksim accounts for this.

 

[gdjsky01]

Did you check the rounding of the fins?

That said, I myself do adjust the CD until things match up.

Still given altimeter variance, simulation variance, and atmospheric variance, 50' to 100' is pretty darn good! :wink:

However yes, I get your point about this being for TARC and all...

 

[ckreef]

Adjusting the Cd to match the simm apogee to the altimeter. I'm going to try that once the motors finally arrive and I have some data to work with. TY

 

[Smatt]

Randy,

Thank you for the suggestion to go Apogee's website...I found three articles of particular applicability (I've listed them for future reference for anyone searching this topic):

https://www.apogeerockets.com/education/downloads/Newsletter130.pdf
https://www.apogeerockets.com/education/downloads/Newsletter249.pdf
https://www.apogeerockets.com/education/downloads/Newsletter208.pdf

These are going to take a day or two to read through carefully and apply but seem to be spot on what we have we need. We will certainly try the full spectrum of wind conditions. Thank you! Steve

 

[Smatt]

Les,

We agree. We will definitely have to consider estimating the wind speeds higher up by watching people launching ahead us. The students need to come up with a clever way to estimate the wind speed from the ground.

Thank you Les,

Steve

 

[Smatt]

Jeff,

I'm in Manhattan Beach as the sponsor for the middle school TARC team. We have a garage full of 7th and 8th graders trying to figure things out so I appreciate the coaching. We will make the Cd change and look at what happens and see if the data from past launches lines up...

Thank you,

Steve