Experiences with OpenRocket supersonic simulation

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Sampo

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Hi,

In a recent thread, Adrian pointed out that on his Mach 2 flight OpenRocket had predicted the flight altitude much better than RockSim. Until now I've assumed the OpenRocket supersonic simulation to be inferior to that of RockSim.

I'd like to hear what experiences other people have had with supersonic rockets. What was the actual flight altitude, max. speed, OpenRocket prediction and RockSim prediction? (I can also sim them myself if you send a design file.) Preferably with the actual mass of the rocket weighed and updated to the design.

If you have it, throw in the RASAero prediction as a benchmark - that's likely by far the most accurate. Thanks!

Cheers,
Sampo N.
 
Subscribed and should be interesting. Thank you Sampo for looking into this.

Greg
 
Hi,

Anyone have any supersonic flight data they'd care to share...?

Regards,
Sampo N.
 
Sampo,

Please excuse my ignorance, but could you explain why OR's and RS's supersonic predictions should differ at all? Or may be, if it's not too much to ask for, you can even summarise and explain how much OR differs from RS with regard to prediction and modelling. I read your thesis, and understood the general principals behind OR's modelling, but I'm not that knowledgeable to work out myself dereferences between the two tools.

Thank you.
 
Anyone have any supersonic flight data they'd care to share...?
Sorry, I missed this thread earlier.

Here's my fastest flight to date. Only about mach 1.6 but I think OR did a great job. I did some RASAero sims but didn't save the results and they were a bit optimistic (surface finish makes a big difference, I tweaked my OR file to better match flight data, but not in an unrealistic way -- "polished" will probably never describe one of my rockets.)

Actuals in the plots are from a Raven 2 (raw barometric altitude and integrated velocity). Altitude in feet, velocity in feet/sec.

The OR files is in the ZIP file; this was on a K300.

alt.jpg

velocity.jpg

View attachment orsim.zip
 
Please excuse my ignorance, but could you explain why OR's and RS's supersonic predictions should differ at all?
In a nutshell, determining the drag coefficient as a function of velocity and angle of attack is hard to do, especially for supersonic velocity.
 
Hi,

Please excuse my ignorance, but could you explain why OR's and RS's supersonic predictions should differ at all?

Like mikec said, supersonic drag simulation is no trivial task. When I started writing OR as my master's thesis I hadn't studied aerodynamics at all :) so I had a lot to learn. Subsonic simulation is relatively straight-forward; there's a number of different constituents to the drag, each of which can be estimated using various methods.

In supersonic flow there's a lot of really weird things going on, including shock-waves, stagnation areas and those colliding with different areas of the rocket. For example the area of a fin that is affected by a shock-wave has a different lift than other parts.

I investigated these methods for a while, and decided it was just too much work at the time to figure out the "proper" way to computing the supersonic drag, so I settled for some rough approximation methods. The only comparison data I had was literature wind tunnel data, which doesn't provide that good info on the actual effect on altitude. I've also assumed the RS methods to be better since it's all commercial and pricey. :)

Cheers,
Sampo N.
 
I don't have any actual supersonic flights to convey (yet), but my experience with the simulation of my maximum effort 54mm MD, Rocksim only gives ~21,000ft and ~2300f/s, Open Rocket running the same file gives upwards of 29,000ft and ~2600f/s.

I think I have read that Rocksim has issues with how it calculates nose cone drag, and that seems to fit, when I use a conical cone in place of the VK the Rocksim numbers are closer to OpenRocket.
 
Hi,
I have a suprising stiuation. In OR calculated 17672 feet (5390m) but altitude reached up to 20112 feet (6134m). Do you have any idea? I don't have Rasaero or RS data. RRC2+ and Entacore AIM USB and AltimeterTWO were onboard. Three of them have the similar altitude.
Motor Cesaroni N2900.
If you have any idea, share please.
Can

Flight and AltimeterTWO video links are below.


 

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  • OR SS.jpg
    OR SS.jpg
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In OR calculated 17672 feet (5390m) but altitude reached up to 20112 feet (6134m). Do you have any idea?
Could easily see that much difference (14% high) if weight, surface finish and/or fin cross-section was different. Not much of a surprise.

This thread was inactive for 8 years before you posted.
 
Could easily see that much difference (14% high) if weight, surface finish and/or fin cross-section was different. Not much of a surprise.

This thread was inactive for 8 years before you posted.
I realised. I didn't want to open a new thread.
All OR data is similar to rocket. Only the fin cross-section, I have choosen airfoil from OR but the original fin was semi diamond. Refrees checked before competition. Target was 20K feet. And we did it very close. Refree altimeterTWO was 20023 feet.
I want to predict as close as I can.
Thank your answer.
 
To get as accurate an OR sim as possible, you must ensure that all form factor variables as well as ACTUAL rocket weight and CG are represented in your sim.

From there, you need to be sure that the actual launch conditions are set as close as possible in the launch parameters.

After that, it's a matter of test flights to tweak the sim variables to match your empirical evidence. Most of us usually get within 5%, some sims can be as accurate as 1%.

Bottom line is that a window of about 5% on average is pretty good for altitude/flight path due to the variability of the motor performance and winds aloft.
 
That was a competition flight. Refrees checked all one by one OR and rocket. I suppose that OR is not reliable on transsonic. Max speed occured about 1.3 Mach. I corrected Atmospheric conditions and OR add 1000 feet more. I am approaching the real altitude step by step. There was no wind and temperature was about 95F. Launch site altitude is 3000 feet ASL.
We are planning to join SA Cup IREC on 2021. I want to make exact calculation of altitude.
I will try to calculate by RS, Rasaero and CFD analisys.
 
The one supersonic rocket I had data on flew consistently higher than OR predicted, even with a polished surface (which it wasn't). However, it was *extremely* predictably over altitude by a fixed percentage, IIRC ~8%. With that percentage correction applied to the prediction, it was within 50 feet of the corrected predicted altitude to 13,500 feet or so.

For any rocket, you'll get kind of close with the OR sim, but you'll want to tune that via test flights if you're in an altitude competition.
 
I am dealing with model rockets since 1983. These rockets were up to E class.
In 2018 an HPR competition started in Turkey. In 2018 we joined 5000 feet category and won the race. In 2019 we joined 10K feet and we were second. Both of them were under 0.8 M. We have no opportunity to make test flights. Joining to race by reports and OR files. Both rocket flew 10% lower than OR.
This year we joined to 20K feet category.
It was our first supersonic (mostly transsonic) flight and flight was +10% higher.
It is impossible to buy HPR motors and test flight in my country as well as most of the European countries.
In this competition (Teknofest Rocket Competition) there is a limited catalogue and you have to choose one frimary and one secondary motor. There are four therotical computating stages. 516 teams made applications. 82 of them have called for launch event. The organization checked all parameters of our real rockets. And less than 50% of the teams get the launch.
By the way where is Sampo :) This is a very usefull thread. I want to thank to him :p
I like TRF and I read most of them. I learned alot.
Thank you for your answers and patience for me (and my bad English).
 
I am dealing with model rockets since 1983. These rockets were up to E class.
In 2018 an HPR competition started in Turkey. In 2018 we joined 5000 feet category and won the race. In 2019 we joined 10K feet and we were second. Both of them were under 0.8 M. We have no opportunity to make test flights. Joining to race by reports and OR files. Both rocket flew 10% lower than OR.
This year we joined to 20K feet category.
It was our first supersonic (mostly transsonic) flight and flight was +10% higher.
It is impossible to buy HPR motors and test flight in my country as well as most of the European countries.
In this competition (Teknofest Rocket Competition) there is a limited catalogue and you have to choose one frimary and one secondary motor. There are four therotical computating stages. 516 teams made applications. 82 of them have called for launch event. The organization checked all parameters of our real rockets. And less than 50% of the teams get the launch.
By the way where is Sampo :) This is a very usefull thread. I want to thank to him :p
I like TRF and I read most of them. I learned alot.
Thank you for your answers and patience for me (and my bad English).
Sampo only occasionally stops by, in the 4 or so years I have been a TRF member he's only popped in once or twice if I recall correctly.
 
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