openrocket sim inversion

I am never surprised when my simulations do not perfectly match real flights.

However, usually they do a good job of telling me which motor is going to perform how in a given rocket. Specifically, if I simulate motor A to 3500 feet and motor B to 3000 feet at similar speeds, even if they are both off by 500 feet I would expect motor A to go higher in reality, and the percentage error to be the same.

Last launch I had motor B go higher, by a few hundred feet! I thought it might be to higher initial thrust and underestimating total rocket weight in the sim, but I added half a pound and the apogees slid by a similar amount.

Are my simulation expectations reasonable or am I simply asking for too much accuracy? Would Rocksim likely be more accurate?

Can you post the sim and tell us which motor is "A" and which is "B"?

Without seeing the sims it is hard to tell.

Good point. Shoot, I will not have that computer for a few days. I bet, if you take the stock LOC Nuke Pro Max file from Apogee Rockets site and run it on H100 imax and H120 red will show the effect, though. Maybe add some weight to the nose or payload. The simulator shows more altitude on H100 but I saw the opposite.

Attached.

Let's go through the usual questions. Did you weigh the rocket? Did you input wind and other weather conditions? Launch rod angle?

Frankly, these two sims are identical for all practical purposes, so it is not surprising the numbers flip-flop on you. The H100 is very low thrust and the rocket can easily weathercock on launch day, thus losing altitude more so than the H120. Also, the magnitude of discrepancy easily falls within motor data error or variation.

It's normal. Don't sweat it.

I'm more familiar with RockSim than I am OR. I'll get that out of the way first.

That said, a computer simulation will give values for flight parameters based upon the internal equations and user input. Sometimes, it's the first that's a problem... sometimes it's the second.

In RockSim, you can change the finish to set defaults, or enter your own Cd, CG, CP, and reference diameter values --among others, as well as launch conditions: temperature, wind speed, launch site elevation, etc. While I weigh assemblies and components, I update a completed virtual model with measured values where I'm able, and after flight, if there's serious discrepancy --such as rocket went much higher/faster than expected-- I'll attempt to reproduce the observed results with the computer sim. Sometimes, it's just getting the weather conditions right and launch elevation accurate. Others, it's changing the finish to something more polished (I will typically set to "matt" (sic) during initial building, as I find this is often a more realistic finish, but sometimes, I have to go to "gloss," or even "polished." Or maybe changing the leading edge finish from "rounded" to "square," something along those lines. And yes, that's all in RockSim. If OR has the same capabilities (which I think it does), you may want to try that...

It may help to keep a log of the launch site weather and conditions at time of launch as well...


Later!

--Coop

Buckeye said:

Let's go through the usual questions. Did you weigh the rocket? Did you input wind and other weather conditions? Launch rod angle?

Frankly, these two sims are identical for all practical purposes, so it is not surprising the numbers flip-flop on you. The H100 is very low thrust and the rocket can easily weathercock on launch day, thus losing altitude more so than the H120. Also, the magnitude of discrepancy easily falls within motor data error or variation.

It's normal. Don't sweat it.

Click to expand...

Yes, I think I was looking at it the wrong way. I was expecting all my flights to be off by roughly the same percentage, in the same direction, but in reality they are both within reasonable percentage of their simulations.

Thanks to all for the explanations!

Like Coop said, there are MANY variables. I use OR, and my sims to 7000' are usually within about 75' - 100' of actual altitude, with lower altitude flights being closer the lower the flight. Then again, I pore over the simulation and every detail of the design and actual build to make sure that what I'm launching matches what I've simulated down to the smallest available detail.

Some of the more common things that catch some of the folks that I fly with when I look at their designs versus the actual rocket are 1) fin material, 2) fin thickness, 3) body tube thickness (and weight that can be an override function), 4) failure to account for construction adhesive and paint masses, 5) actual weight of the installed motor (it's not always accurate in the databases), 6) mass, material, and drag of other components like parachutes, shock cords/hardware, bulkhead plates/hardware, installed electronics, etc., and a host of other items.

If you get your finished rocket's weight and CG simmed correctly, you'll likely be VERY close to simmed altitudes with actual flight, all other things like wind, launch angle, etc being equal.

S

Not necessarily surprising. Environmental differences and other variables can alter the actual flight performance.

The Pro38 286H100-15A has more total impulse (281 Ns versus 261 Ns) than the Pro38 261H120-14A so ideally it should go ~9% higher all else being equal. Additionally the H120 has more thrust and a shorter burn time than the H100 so it will go faster and therefore have more drag losses.

However real flights can vary greatly from the sims. Was the rod angle identical, and was the weather conditions (wind speed, barometric pressure, and temperature) identical for both launches? A rocket with a higher rod velocity and acceleration will be less susceptible to weather cocking and it will fly on a more vertical trajectory.

You didn't provide any flight data so we can't really determine why there was a difference. What were the observed differences in altitudes? What altimeter did you use and did you get your apogee from the beeps or by examining the altimeter trace? A data glitch or an early deployment could make a difference. Also did the motors perform as per the thrust curves you used in the sims? CTI has observed an issue with accelerated burn rates for their red propellant recently. The burn time is only about 75% of the expected value. This in turn generates a much higher thrust for a shorter period of time, reducing the gravity loss proportionally. If you look at the acceleration and velocity curves obtained from the altimeter analysis, you may find out that one of the motors performed quite differently in flight from what you predicted in your sim, and that could explain what you observed.

Bob