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Understanding Rocksim and actual flights

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kcobbva

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I'd love some education where I seem to be falling short. I recently flew my highest flight to 9490ft AGL and max velocity 1350. I'm ecstatic and completely happy with the outcome. However, my rocksim simulations put me at 10,947. That's quite a difference. Futhermore Rocksim shows a max velocity of 1198.8 which I busted. I've input the CG and Weight of the project end the end overriding (without the weight of the L1000 motor. I set the humidity, launch altitude, wind, etc as close as possible to the day of the launch at LDRS and even AFTER the launch come up above 9490. Am I missing something? Can someone educate me? I'll attach the Rocksim file, the altimeter readout etc. I want to learn and hoping someone can show me where I'm not providing accurate data, failing to input data properly etc. Again, I couldn't be happier with the flight at LDRS, and to be honest I got REALLY LUCKY because NumbNuts forgot to secure the quicklink on the nosecone and it separated during the main ejection. However, the descent landed just beside the nosecone within two feet! WOW! Again I would be grateful in learning where I'm not accurately depicting the proper representation in the sim. I even tried to account for the 808 camera shroud in the file as a pod on the outside of the rocket. Thanks in advance and I look forward to any thoughts, recommendation, education!!

Here's the sim
LDRS36 Rumble Bee Flight Altimeter.jpg
Attached is the rocksim file
View attachment Level2-FG4in nose avbay 5.2oz.rkt

And Here's the video from the flight. Go straight to 2:17 to see the flight. I see a lot of spinning which I didn't see under lower impulse motors, but it seems to level out before apogee. Is that due to Mach Speed and drag? Again I welcome anything to gain deeper understanding!
[video=youtube;ofXBOtmNH9I]https://www.youtube.com/watch?v=/ofXBOtmNH9I[/video]
 
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Nytrunner

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I've heard that Rocksim doesn't predict past Mach speed well. Have you flow this rocket previously to adjust the Cd? Rocksim allows you to.adjust it In an.attempt.to reconcile sim with flight performance.
 

kcobbva

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I've heard that Rocksim doesn't predict past Mach speed well. Have you flow this rocket previously to adjust the Cd? Rocksim allows you to.adjust it In an.attempt.to reconcile sim with flight performance.
I have flown 5 time with this one. I'm honestly growing and not sure how to adjust for the cd; but am open to understand/learn. I have all data from every flight previously as well.
 

markkoelsch

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How were you measuring velocity? If it was barometric you cannot trust it.

Rocksim is ok for low Mach. That said, it tends to be a little optimistic.

Also, remember that there is variation in motor performance. When motors are tested usually 3 are tested and an average is derived. You could have one that was a bit below average.

Also, what did you have the finish set too? If you set it to polished you are telling the computer a lie. The polished finish is almost impossible for folks to achieve.

You have wind in the ground, humidity, temperature. Do you have launch rail angle? Also, as others have mentioned the winds aloft were moving pretty fast, and nothing other than Rocksim Pro can account for that.

So, there are a variety of issues.

You could use the manual Cd override. I would try to do it for a day with little wind so you can get a good, straight flight and good altitude data. Take that info into a sim and compare. Adjust the Cd until it is close to your recorded altitude. Realistically, if you are within 5-10% that is about as close as you can expect for a lot of reason included above.
 

markkoelsch

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One other oddity about Rocksim is nose cones. It is largely based on surface area versus aerodynamics.

I did some careful experiments. If you have a 5:1 Von Karman you will be close if you make it a 5:1 Cone of the same mass.
 

kcobbva

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How were you measuring velocity? If it was barometric you cannot trust it.

Rocksim is ok for low Mach. That said, it tends to be a little optimistic.

Also, remember that there is variation in motor performance. When motors are tested usually 3 are tested and an average is derived. You could have one that was a bit below average.

Also, what did you have the finish set too? If you set it to polished you are telling the computer a lie. The polished finish is almost impossible for folks to achieve.

You have wind in the ground, humidity, temperature. Do you have launch rail angle? Also, as others have mentioned the winds aloft were moving pretty fast, and nothing other than Rocksim Pro can account for that.

So, there are a variety of issues.

You could use the manual Cd override. I would try to do it for a day with little wind so you can get a good, straight flight and good altitude data. Take that info into a sim and compare. Adjust the Cd until it is close to your recorded altitude. Realistically, if you are within 5-10% that is about as close as you can expect for a lot of reason included above.
Now I'm getting closer! Polished Finish! Man my name is NOT Nathan. I dropped it to gloss. This dropped the sim to 9832. That I'm much closer to and is absolutely more respective of the build. I think i did a good job, but not a polished job. Dropping to Matte returns 9297 which is only 200 ft lower than what I flew. I firmly believe you hit the nail on the head here! Thank you very much. I knew I couldn't do much about the upper winds, but the flight went pretty darned straight. I've flown with hidden upper winds on this one before and saw it take the arch about 4k and keep going to 5.5k AWAY from us. I think the video shows it did a pretty decent ascent and the descent is where it really started to go.

As for mach, I only have the data out of the Perfectflite alimeters, but both of them are within a few feet/s of each other maximum velocity wise. I have nothing else telling me the velocity.

And the nose was a 5:1 Ogive 4" standard fiberglass from madcow.

Thank you again!
 
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markkoelsch

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You are welcome. Have been using Rocksim for about twenty years, and I have been a beta tester for versions 8&9. I have a pretty good handle on how it works.
 

Buckeye

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Yes, Mark covers all the RockSim bases with valid points. Be careful with overriding the Cd. Real Cd is not constant (it varies with speed), and all the problems associated with your sim/flight comparison cannot be explained with just one number. Moreover, thrust profiles are allowed pretty high standard deviations, so detailed comparisons really need to be done on a statistical basis, not a single observation.

Keep in mind that altimeters don't measure the "real world" either. There is a lot of data filtering and assumptions behind the scenes. As mentioned, velocity derived from barometric data is dubious. Standard Day conditions are usually used without temperature corrections, leading to altitudes that are probably too low. See the last page of the Raven manual, for example.

https://www.featherweightaltimeters.com/uploads/Raven_Users_Manual_2014May20.pdf

Sims and altimeters are complementary tools. If they agree within 20% and you are happy with the flight, then consider it a success!
 

markkoelsch

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Exactly, the Cd override can work well if you are flying similar motors and velocity. That said, there is enough variation in all these different items that if you get within 10% consistently you are doing well.

This is just the way that it is.
 

kcobbva

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Thanks guys. I really appreciate the input.
 

ksaves2

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Exactly, the Cd override can work well if you are flying similar motors and velocity. That said, there is enough variation in all these different items that if you get within 10% consistently you are doing well.

This is just the way that it is.
I'll second that. I've had a few times when I was within 3% but generally I think all the errors just so happened to throw the RS estimate in that direction!
I don't think it had anything to do with my "skill" at all. Just a comedy of errors that just happened to lead to a correlation in a few instances.
Remember, It's only a hobby. When making predications as to whether a waiver could be busted, I believe one will have to allow more of a leeway with
higher speed and higher flying rockets than with the sub-Mach lower altitude stuff with Rocksim. This means if getting within 1000 feet of a high altitude waiver
might have to think twice about the chosen motor if using the standard Rocksim. Kurt
 
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Buckeye

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Right. Claims of correlation are usually happy coincidences. "Everybody trusts the measurement except the guy that did it. Nobody trusts the simulation except the guy that did it."

Think about what you want the tool to do. For me:

Simulations help me:
  • choose a motor
  • choose a delay
  • don't break the waiver
  • need a Mach delay or not
  • land in the recovery area
  • estimate max acceleration, velocity, and altitude

Altimeters help me:
  • control my deployments
  • determine how well the delay or altimeter algorithm predicted apogee
  • confirm descent rates
  • estimate max acceleration, velocity, and altitude
  • record data to play around with afterwards
 

markkoelsch

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I would also argue the same can be said about Open Rocket. Rasaero is a pretty good if you input correctly as far as performance.
 

Buckeye

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I use all the currently-supported software, and they have their own quirks and usefulness that you need to understand to get the most out of them. In my humble opinion:

Ease of use (Highest to lowest): Thrustcurve, Rocksim, OpenRocket, RASAero
Accuracy (Highest to lowest): RASAero, OpenRocket, RockSim, Thrustcurve

Hmm, no surprise there is an inverse relationship here. No free lunch.
 
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Coop

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Glad you were able to get it. I was going to suggest looking at your finish, but Mark beat me to it --and he forgot more about this program than I've ever learned.

I generally take the first flight or three with a grain of salt from the sims... then adjust once I know how she behaves by adjusting the Cd. I usually just set my finish to matt(e) --their misspelling, not mine-- on all components at first, and then maybe over to gloss when I see how she behaves. I'm not normally getting near a waiver ceiling so that's hardly ever my concern --but tree lines and drift rates from a given altitude are...

Later!

--Coop
 
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