Can OpenRocket simulate this?

[Marc_G]

Hi folks,

Here's my latest build; is there some way to simulate it in OR? Note the shaved nosecones...





Also, is there a library somewhere of OR components I can import?

Marc

 

[Pantherjon]

You might be able to 'fake' OR by using a transition to simulate the shaved cones..

 

[Peter Olivola]

Yes, just treat it as a transition. The CD will be off but not by enough to cause a problem.

 

[Marc_G]

OK, I will have to figure out the way to transition from three small tubes to one big one. anyone got an example to share?

 

[Peter Olivola]

For Barrowman and flight sim purposes treat it as a single tube of the minimum diameter that would contain the clustered tubes. The interference drag offsets the missing material.

 

[KerryQuinn]

I agree with Peter.

Attached are two OR models.

Rocket1 uses Peter's approach where there is a transition to a "circumscribed" lower "fake" tube with an internal cluster.

Rocket2 uses the same transition section, but I shrunk the diameter of the lower "fake" tube so that the rocket resembles more of your original intent - one tube on top, three on the bottom. I lengthed the bottom "engine mount tubes" so they run all the way up to the transition section.

I made them both about the same mass and simulated a launch on 3xC6 engines.

Both simulations are predicted to be similar (in this case between 1300 & 1400 feet apogee).

-Kerry

View attachment rocket1.ork

View attachment rocket2.ork

 

[Marc_G]

Cool! Thanks!

I'm still learning the software and this helps a lot!

Marc

 

[KerryQuinn]

You are welcome.

The reason that both Rocket1 and Rocket2 attain about the same altitude may not be immediately obvious.

Rocket2, while it LOOKS more like your real rocket the simulation is not at all an accurate mathematical representation of your rocket - this is because the three lower tubes are modeled in OpenRocket at "Inner Tubes" - meaning that they do NOT contribute to skin drag nor do they contribute to the CP calculation. They are really just there as "visualizations" and they DO add mass to the model so the impact the CG calculation.

So when you run Rocket2, your are not really performing a very good simulation of your actual launch. Rocket1, while it doesn't look quite like your real rocket, is a much better math model since the "fake" tube DOES add skin friction drag and does contribute to the CP (and CG) calculations.

The reason Rocket2 sim results is close to the Rocket1 is that the primary drag contribution is coming from the transition section - which is the same in both rockets. (And hopefully the drag of this transition approximates the drag of the 3-partial nose cones in your actual rocket.) The primary contributor to the CP calc in both Rocket1 and Rocket2 is from the fins, which have similar areas in the two models and are located a similar distance behind the CG.

If you want to make the best predictions of your rocket's behavior your should use the Rocket1 approach. The Rocket2 approach is really just for fun - and for appearances.

-Kerry