"Normal" Number of Rocksim Simulations

[COSTransplant]

Ok, so I am a NOOB at this, but I finally got my 48" scratch built model to fly straight up to 1,700 feet and come straight back down, but it took me 87 launches to get this far and I am sure that I have not optimized everything that I need to optimize. And, I can only get the 1st stage of the rocket to light, not the 2nd stage. Is it normal to have to run that many sims to get to an at least partially optimized flight?

 

[Banzai88]

It would help tremendously if you would post the sim file so that it can been looked at.

 

[KenECoyote]

Plus please post pics of your rocket! (We kinda like that here )

EDIT: Disregard if you haven't actually built it yet.

 

[COSTransplant]

Ok, here is the file! Again, I am a NOOB to Rocksim, so any thoughts you can provide woud be greatly appreciated. And BTW, Cooper is the name of the pup that we just lost, so everything I build will be Coopers. I have the nosecone, payload fairing and transition all as one piece -- which would be more accurate -- but I don't know how to get that into the Rocksim system yet. Right now those parts are in there as three seperate pieces.

 

[neil_w]

On import to OpenRocket it looks like it's basically unstable, or marginally so a best. CP is waaaaaay forward, because there are no fins in back. That is, assuming the import to OpenRocket worked correctly. Is this what it's supposed to look like? If not it will be a bug report for OR.



If this is wrong, please post a picture of the side view of the rocket.

 

[COSTransplant]

Oh no, this is what it is supposed to look like, so you are good there. And yes, it shows as unstable -- but I've not figured out how to correct that yet. I did try larger fins to change the CP, but then it would only tumble. I think my scale (in my brain) is off, and maybe my fins are too large overall? Maybe I should be adding weight in the nose, or get all the little parts installed and run again? Maybe different fin shape / placement, etc. I want to use a flip fin can for the 1st stage. So many variables!

 

[KenECoyote]

With no fins in the rear and instead in the middle and forward, it's highly unstable and CP is waaaay forward, so to make it stable, you'd need to add a lot of weight to the nose to move the CG up enough (1-2 calibers).

Look at the classic stable rocket (ex Alpha)...fins at the rear bottom move the CP far back enough that it's well behind the CG. When you move fins forward, it becomes less stable since the airflow isn't hitting the rear to stabilize the rocket (also think axis of motion). Tim at Apogee has a good video about this where he demos with a wind tunnel and rocket on a string.

Easiest solution is to move the booster fins back. (Edit and make them bigger.

 

[COSTransplant]

I suspect that I may be focused on the wrong thing here, which is the altitude that I can achieve. I have been moving the fins, running a sim and checking the altitude achieved when I should be focused on a 1.0 stability rating. It sounds like I need to put more work into the effects of CG and CP on stability. Thanks!

 

[waltr]

If you are new to using a simulator I recommend starting with a simple 'classic' rocket to learn how to use the sim.
Then modify and see what the sim tells you.

Later go back to this design.
For that nose section, only way I know to do this in a sim is separate parts like you did. You can use Mass over rides to get the mass of the 'real' parts.

 

[mtnmanak]

I would also recommend a read through of Harry Stine's "Handbook of Model Rocketry" to get a basic understanding of CP and CG theory and how the control surfaces (and their placement) affect stability. That book is required reading (IMO) for scratch building rockets. If you understand the underlying theory, you will understand what is happening in the simulator.

 

[ghostfather]

Have you ever played darts? It helps me to visualize a rocket like a dart.
You've usually got a heavy nose with the dart point, and feathers at the back.
Have you ever tried to throw a dart, feathers first? It tumbles around, and is unstable.

Your rocket looks like a dart being launched backwards, feathers first.
Just try moving the big fins back to around the motor, you'll see that it becomes stable.
It's just a sim, it doesn't cost you anything to play around.