# Simulating short, wide rockets with a mass-less transition cone

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#### MetricRocketeer

##### Well-Known Member
TRF Supporter
Hi everyone,

I have heard of the technique of simulating short, wide rockets by placing a transition cone having no mass, and with the forward end having a near-zero diameter and the aft side having a diameter equal to the rocket caliber multiplied by pi. I am sure that you highly experienced and knowledgeable rocketeers have also seen this technique.

I know that I have seen it in Apogee's Peak of Flight Newsletter.

So, I have two questions please:

• Do people think that adding such a transition improves simulations?

• I don't remember seeing anyone doing this with OpenRocket. Actually, so far on The Rocketry Forum, I don't remember seeing anyone doing this with RockSim either. Why not? Or is it because people don't think that this transition improves simulations.

Thank you.

Stanley

#### Jeff Lassahn

##### Well-Known Member
TRF Supporter
I have heard of people doing this with both OpenRocket and RockSim. Anecdotally it sometimes give results people like.
Theoretically, I don't think this technique is very well founded.
The Apogee article justifies this as an attempt to model base drag. But the solutions without the cone are already assuming that flow separates at the base of the rocket (you can force the calculation to assume the flow doesn't separate by adding a very short transition down to zero diameter at the tail, and notice how much that moves the CP forward).
I suspect the actual reasons OpenRocket puts the CP of wide rockets too far forward are because the "thin body" approximation used in these calculations don't work well for blunt noses, and that ignoring lift from the body tube is less accurate the more surface area the body has. The cone at the base trick doesn't really address either of these problems, but maybe it gives a correction of roughly the right size sometimes.

This is a subject where we need both better theory and better experimental results to compare against.
Sorry not to have a more definitive answer.

#### Kelly

##### Usually remembers to get the pointy end up
I've used this technique in OR. I'm guessing that it improved the simulation, because without the transition OR said the rocket would be marginally unstable; with the transition OR said it would be fine. In the real world the rocket flew fine, with no indication of instability.

I tend to agree that this technique feels like a bit of a 'hack'. I wish someone with a firm understanding of the fluid dynamics involved would come up with a better way of modeling base drag (in short/stubby rockets, and in saucers/spools/etc.) and get these incorporated into the modeling software.

#### neil_w

##### Good at some things
TRF Supporter
I've done it in OR. You have to remember that it's purely for CP calculation; once you confirm it's in a good place and want to sim the flight you need to remove it.

#### MetricRocketeer

##### Well-Known Member
TRF Supporter
Hi neil_w,

Why do you have to remove the transition before you run the simulation? What would be the consequences of not removing it?

Stanley

#### neil_w

##### Good at some things
TRF Supporter
Tons of extra drag.

The base drag compensation is solely for the purpose of calculating a more realistic center of pressure.

#### Kelly

##### Usually remembers to get the pointy end up
To expand on that, I think OR does two types of calculations. One of them is a Cp calculation, using the Barrowman equation or whatever. This doesn't give a quantity for the drag, just analyzes the various components and determines the location. These equations don't understand the effect of base drag on Cp. The second calculation is the Cd calculations, which determine the overall drag coefficient, but not the location. This does take into account the base drag. So, simulating this with a transition would get you base drag from the actual rocket, plus base drag from the transition.

#### Cape Byron

##### The BAR formerly known as Skippy-2
TRF Supporter
I find it makes me a little more confident of my Cp position prior to that first test flight. Removing the transition probably gives you a more accurate altitude simulation.