rocsim simulations

So, I programed a AMW parrot into rocsim, and some of the altitude predictions seem way off. Like over 5000 ft on a G-33. What could be causing this? I've noticed that some of the sims lately seem to be way off, and am a little concerned that I might be getting the wrong delay because of it. Any ideas?View attachment parrot.rkt

I loaded the parrot.rkt into Open Rocket and a Pro29 143G33-9A shows 4030 ft, which would be about right for 4.4 oz rocket on a 143 Ns G motor.

Just as a quick check I simmed it with G64W in both Open Rocket and WRASP and both programs showed about 3400 ft. WRASP did give its typical warning about being too close to Mach speed and the sim may be inaccurate.

I fired it up in Rocksim 9. A couple notes- out of the box I did your sim and got 5400'. That seemed very optimistic. I dug into your details. First thing- I would never select polished as it is nearly impossible to achieve. Once I did that it dropped to 4800'. Next, you have no rail buttons on it, which I added. This dropped it to about 4200'. Throw in that Rocksim tends to be a little optimistic (it tends to underestimate base drag a little) I think right about 4000-4100 is reasonable with about a 9 second delay.

On other note- you are cutting it quite close from a stability point with this motor. You have no motor retention yet, which will add weigh to the base of the rocket. You might want to add some nose weight- maybe a couple ounces of epoxy in the nosecone. That would get you to a little over a 2:1 margin.

It actually uses a fly away rail guide system from my friend Bill, he sells them through AMW. Work quite well. It also has an aeropack 29mm retainer, guess I forgot to put that one on. Talking with Robert from AMW, he thinks that the rocket won't get anywhere near those types of altitudes on that motor. I'm a bit concerned here, as these huge discrepancies could cause some problems with the delay time.

kzimmerman said:

It actually uses a fly away rail guide system from my friend Bill, he sells them through AMW. Work quite well. It also has an aeropack 29mm retainer, guess I forgot to put that one on. Talking with Robert from AMW, he thinks that the rocket won't get anywhere near those types of altitudes on that motor. I'm a bit concerned here, as these huge discrepancies could cause some problems with the delay time.

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If you input correctly the mass/CG (not what rocksim thinks the mass is, but the actual mass measured by you), launch site conditions, and drag model, and the motor burns like its thrust curve says it should, then your simulation will be very good. Certainly good enough to pick a delay time. For a sport flight, it probably doesn't matter if your delay is 9 sec or 7 sec.

What are the huge discrepancies? You have multiple, consistent answers based on data and physics, and then you have some opinions. I choose data every time.

In this case, the data that I'm getting from rocsim doesn't seem to match up with my common sense evaluation, plus some other very experienced fliers who have a pretty substantial amount of experience with this kit. I'm going to double check the mass and cg, hopefully I'll get some better answers after that, plus I'll change the components to match a none polished finish. It's pretty smooth, but certainly not polished. I simmed it in openrocket, and the result there was a little over 1000' difference. So there is some discrepancies in the data. We also don't know the data for the impact of the fly away rail guides on performance yet, somebody richer than me is going to have to do that......

You have yet to fly it so your common sense evaluation has little or no basis. I would trust a sim before trusting an opinion assuming the sim is accurate.

All that said, I think a 10 second delay seems a reasonable starting point.

While I haven't seen your Parrot rocket, I have used RockSim for some time. Just going on the AMW specs your RockSim Model is grossly underweight. AMW specs say the Parrot is 30" long and weighs 12 oz - that's 340.2g. Your RockSim model is only 128.792g. That single difference can easily account for the unrealistic altitudes your simulations show.

RockSim default component weights must be compared (and revised if needed) with actual As-found component weights in a kit. In addition, your model does not account for the weight of epoxy (noted in another earlier reply).

Weigh your rocket (without any motor) and compare that weight to the 128 g of your simulated model. Any weight difference must be accounted for in the model before it can even begin to reflect real life altitudes. Also check the as-built CG and compare it to the RockSim model.

Also, you can do some test flights using lower impulse motors in order to get a better drag coefficient dialed in for Rocsim. But, for sure, weight your finished rocket and get rid of the overly optimistic 'polished' settings in RS.

We have flown the rocket. Mostly on F's, that's one of the reasons I was using common sense to say, this data doesn't seem right. I've changed the finish on the components, and I'm going to weigh the rocket when I get to my shop. Also find the cg, etc. The shops at my moms, much better than the little tiny space I have in my condo. Consequently, I haven't weighed it yet, but that will come soon enough. I guess I assumed that rocsim would calculate the weight a little better, and didn't really notice it. Lesson learned, eh?

Rocksim is only as good as what you enter. There are a lot of things to take into consideration when making an accurate sim. Sure overall dimension are important. Weight of individual components is also important- the database contains a lot of materials, but many types of materials might have a different value. Should include mass of epoxy, hardware, recovery gear, and any electronics.

I am often accused of over engineering things, but I've learned that it doesn't hurt to verify information from software. RockSim is a nice program, but it needs some tweaking and operational enhancements, which I've shared with Apogee over the past two years.

I use its component Mass Override feature when a part's measured weight deviates by more than 1% from RockSim's calculated weight. But when you override the default mass value it is important to also input the component's CG. If you use the program's default of "0" RockSim assumes the CG is at the very front of the component, which will ultimately give you an unrealistic CG for the rocket model.

Whatever you chose to do, it is critical that one at least compares the actual rocket weight to a simulation program's calculated value. If they are not very close, then do not expect to realize the simulation results.

markkoelsch said:

Rocksim is only as good as what you enter. There are a lot of things to take into consideration ...Should include mass of epoxy, hardware, recovery gear, and any electronics.

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Very true! I use a spreadsheet to keep track of these weights and as a check-off list as I enter them into RockSim.

Yeah, Rocsim doesn't estimate any adhesives or finishing materials. So it's nice that it does some estimates of the other materials, but you really need to weight the rocket yourself once it's built as well as 'force' in the real center of gravity. Once you've done a flight then back in your drag coefficient and things will be closer overall.

I weigh every component, all the epoxy, and hard ware. Only way it will be accurate.