Altitude and Velocity are OVER Simulation Values

[SammyD]

Hello everyone!

I ran into an issue that I'm hoping someone can help with. I've never experienced this before but at a recent launch in Bayboro, NC (Oct 14-15, 2017) a new scratch-built rocket traveled to an altitude and velocity that were different (substantially different in Flight 2) than the Open Rocket simulation in two successive flights. I'm using redundant altimeters that reported to within 1 foot of each other on altitude and within 1mph of each other on two different flights.

I have a new 4" Patriot that I scratch built made of BlueTube airframe and motor mount, 1/4" birch plywood fins and a LOC PNC-3.9 nosecone (more on this later). I'm not going to bore you with the OR design file as EVERYTHING has been checked and rechecked for accuracy!

Here is how the flights broke down from Stratologger CF altimeters:

Maiden Flight: CTI K-520, Simulation said 7545' and 696mph
Actual readings: Altimeter 1: 7600' and 661mph,
Altimeter 2: 7601 and 662mph. (Yes, I realize this speed is LOWER than the simulation, but see Flight 2)

Flight #2: CTI K-570, Simulation said 8514' and 765mph
Actual readings: Altimeter 1: 8916' and 890mph
Altimeter 2: 8917' and 890mph.

Here's is where I need help: Would a rounded nosecone like the one I'm using, a LOC PNC-3.9, cause the variations in altitude and velocity? It seems like I've read somewhere that a blunt nosecone like the one on the LOC 3.9 can/will cause air to move over the airframe more easily, thus giving me greater speed and altitude. It seems like I read that is the reason why the front end of so many jet aircraft have a big, bulbous nose on them: better aerodynamics...

Is this what is happening here? The ONLY variance in my design file and the actual rocket are the shape of the very tip of the nosecone. OR only allows a pointed nosecone (Ogive shape), but my nosecone has a bulbous end about the size of a quarter.



Had to drop a launch photo in here too
CTI K-520 in full burn at the end of the 1010 rail...:

 

[djkingsley]

I would double check the weights of the components in the simulationin, or override the entire estimated mass properties with the actual weight and cut of the rocket.

 

[Kallahan11]

The maiden flight was within 1% altitude and 5% speed simulated, that tells me your sim is really good. One thing to note is that the second flight went supersonic while the first did not. Your second flight is within 5% altitude wise but the speed does look funky, are the altimeters using barometrics for speed? that can give wonky readings past mach, but wouldn't effect the apogee detection which seems fine. What altimeters did you use?

Also where were you launch locations, if flight 2 too place in a field 1000' above flight 1's elevation the reduced air density could effect these things.

 

[Nathan]

My Frenzy XL used to consistently fly much higher than sim altitudes until I realized that I had the fin cross sections set to square in OpenRocket. I changed fin cross sections to airfoil and that made a big difference. Now the sim altitudes are pretty close to actual altitudes.

 

[Nytrunner]

That range of error isn't bad for OR.

Anything from surface finish, fin profile, launch elevation, and atmospheric conditions can jog the numbers a little.
You could try doing a compound nose cone (round cone to the diameter of your actual nose, with an ogive transition for the rest of the profile), although this may freak OR out even more.

Elliptical shapes such as airliners and the space shuttle noses are quite effective at Subsonic speeds. Transonic and super/hyper are where pointy things shine.

 

[markkoelsch]

That is not variation to worry about. A motor can have more variation than that.


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[SammyD]

I would double check the weights of the components in the simulationin, or override the entire estimated mass properties with the actual weight and cut of the rocket.

Done that. I use ACTUAL weights for my simulation - always have to add a bit here and take away a bit there to be sure that my CG lines up with actual test of CG, and I always make sure my total weight is consistent with the simulation and vice versa...

 

[SammyD]

The maiden flight was within 1% altitude and 5% speed simulated, that tells me your sim is really good. One thing to note is that the second flight went supersonic while the first did not. Your second flight is within 5% altitude wise but the speed does look funky, are the altimeters using barometrics for speed? that can give wonky readings past mach, but wouldn't effect the apogee detection which seems fine. What altimeters did you use?

Also where were you launch locations, if flight 2 too place in a field 1000' above flight 1's elevation the reduced air density could effect these things.

Launches were at the same field from the same location a day apart on a Sat/Sun launch weekend, but you make a good point about exceeding Mach. Altimeters do use barometric pressure (Stratologger CF units), so that could be an issue...

 

[SammyD]

That is not variation to worry about. A motor can have more variation than that.


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I considered this too. Note that the second flight was with another K-motor, but it was a 2060K-570 (5-grain) versus a 1711K-520 (4-grain), so it had higher average impulse, higher peak thrust (201lbs versus 139lbs), a longer burn time (3.6s versus 3.3s), higher total impulse (2060Ns versus 1711Ns), but the rocket just screamed off the pad! I was sure it was going to break mach when it was 100ft off the pad - maybe it was a hot batch from CTI

 

[Steve Shannon]

Yes, rounded or elliptical nosecones perform better at subsonic velocities than conical, ogive, etc. nosecones.
Those differences between simulations and actual are not very significant. I would replace the nosecone in the simulation with one that has a more blunt or rounded point and I would decrease the drag coefficient until sims are approximately the same as actual or very slightly more.


Steve Shannon

 

[Buckeye]

I considered this too. Note that the second flight was with another K-motor, but it was a 2060K-570 (5-grain) versus a 1711K-520 (4-grain), so it had higher average impulse, higher peak thrust (201lbs versus 139lbs), a longer burn time (3.6s versus 3.3s), higher total impulse (2060Ns versus 1711Ns), but the rocket just screamed off the pad! I was sure it was going to break mach when it was 100ft off the pad - maybe it was a hot batch from CTI

This is not the variation (K570 vs. K520) mark was referring to. An individual motor burn can vary a lot from its stated average thrust curve given to the simulation, resulting in 10%-20% altitude discrepancies.

Pat yourself on the back for a simulation well done. It doesn't get any better.

Peak velocity measurements from altimeters are not very reliable. Your simulation is a better predictor. So, you may have tickled Mach on Flight #2.

Also, Day 2 might have been of different temperature and barometric pressure from Day 1, which can alter the flight a few percent more.

 

[DavidMcCann]

Dude my "perfect" sims aren't this close.

 

[OverTheTop]

Given that your maiden flight was within 1% and 5% according to a earlier post, I would say your sims are fantastic.

As my sig says "Everybody's simulation model is guilty until proven innocent". Innocence proven

 

[markkoelsch]

This is not the variation (K570 vs. K520) mark was referring to. An individual motor burn can vary a lot from its stated average thrust curve given to the simulation, resulting in 10%-20% altitude discrepancies.

Pat yourself on the back for a simulation well done. It doesn't get any better.

Peak velocity measurements from altimeters are not very reliable. Your simulation is a better predictor. So, you may have tickled Mach on Flight #2.

Also, Day 2 might have been of different temperature and barometric pressure from Day 1, which can alter the flight a few percent more.

Thank you for clarifying my point. Most people do not understand the amount of variation there can be in motors. This can have a variety of effects as seen.

In general, if your sims are within 5% of altimeter readings consider that sim good to go and not in need of further tweaking. Heck, unless you are going for records or have a rocket/motor combo that might be pushing the waiver 10% is probably good.

Another question, did you adjust the sim to reflect actual launch conditions? Wind, temperature, air pressure, and rail/rod angle are the big ones. Heck, I have taken to doing that, and measure the angle with my phone at the pad-at least when it will be a substantial altitude for the launch site.


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[SammyD]

Thank you for clarifying my point. Most people do not understand the amount of variation there can be in motors. This can have a variety of effects as seen.

In general, if your sims are within 5% of altimeter readings consider that sim good to go and not in need of further tweaking. Heck, unless you are going for records or have a rocket/motor combo that might be pushing the waiver 10% is probably good.

Another question, did you adjust the sim to reflect actual launch conditions? Wind, temperature, air pressure, and rail/rod angle are the big ones. Heck, I have taken to doing that, and measure the angle with my phone at the pad-at least when it will be a substantial altitude for the launch site.


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I think we're interpreting the variations in motors the same way: I said I might have gotten a few out of a CTI "hot batch". Nonetheless, no, I don't make all the sim changes for barometric pressure, launch angle, temp, etc. so I suppose I shouldn't be concerned with those numbers. Like others have said in previous posts to this thread, my sims look good. Thank you for that!

Most of my flights are usually within something like a 2% of actual altitudes and velocity after checking my flight logs going back 5 years. That is why this one kinda freaked me out! Thank you to all of you for putting my concerns to rest. I think I'm going to tweak some of my day-of-flight parameters to see how that changes my simulations.

Again, THANK YOU to all who replied and gave me valuable (as always) insight into this fascinating thing we do..................

 

[Nick@JET]

I think your sims are very close. I had a big rocket fly within 200’ of sim on an M1939, then 3500’ OVER sim at Airfest on an O3400, glad it was at AF with a big waiver - now that is a sim that was off!
No help I know just sharing some experience


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