Weird Results in Openrocket?

I'm working on a project in a computational science class that I am in. I am using 3 different nose cone shapes and simulating each of them with different lengths to find optimum nose cone lengths. However, I'm running into weird numbers when I'm simming them.


A conical nose cone 3.1 inches wide and 1 inch tall sims to a bit over 7K.
An ogive nose cone with the same weight and length and all sims to almost 13K.


The rocket is staying subsonic throughout the whole flight.


Does anyone have any ideas how I can get more realistic results? I would just live with it except it's saying that an optimum Ogive nose cone is about a quarter of an inch tall for a 3" diameter nose cone. And just off of a back of the hand sim I KNOW that's wrong.


Any advice?

Thanks all!

Matt

I can't vouch for the numbers of OR, but I could suggest you post your .ork files for us to look at.

You may have to load another motor because these are custom thrust curves I made.

View attachment Test Rocket.ork

Bump?

This is a perfect problem for RASAeroII and you don't even have to put a motor in the rocket to solve it.

You should have to set a boundary conditions however. Perhaps.

AF Diameter: 3"

Total length: 36"

NC Aspect ratio: .5, 1, 2, 3, 4, 5.

NC length + AF length ==> 1.5 + 34.5, 3 + 33, 6 + 30, 9 + 27, 12 + 24, 15 + 21. Respectively.

NC Types: conical, ogive, vonKaman.

Simply input each rocket and look at the Cd vs Mach Number 0-3. The Cd only depends on the shape of the rocket, nothing else.

This will answer your principal question.

Bob

THANK YOU. That is exactly what I am looking for. Thank you so much

I did a quick optimization in OR on your rocket and didn't see any odd behavior, it said the optimal V-K length was about 15 inches. I suspect that if a nearly-flat nose cone behaves oddly it's just a bug; it's not like that is a realistic case that people are actually interested in.

I've long suspected that OR's handling of nose cone shape drag was a little off, but not by a lot and I haven't had the time to dig into it.

I've also seen OR be suspiciously optimistic with Ogive nosecones. E.g. a 4" MD rocket with a 22" VK = 56K feet while swapping it for an 11" Ogive = 64K feet....

mikec said:

I did a quick optimization in OR on your rocket and didn't see any odd behavior, it said the optimal V-K length was about 15 inches. I suspect that if a nearly-flat nose cone behaves oddly it's just a bug; it's not like that is a realistic case that people are actually interested in.

I've long suspected that OR's handling of nose cone shape drag was a little off, but not by a lot and I haven't had the time to dig into it.

Click to expand...

Yeah, like this.....


View attachment 283261