Send me examples of the OpenRocket Base Drag Hack working

[Jeff Lassahn]

I'm working on ways to make the OpenRocket stability calculations work better for stubby rockets. I've got some ideas of how to proceed, but I need real-world test data to validate the calculations.

So I'd like any examples of OpenRocket designs people have made that meet these criteria:
1. OpenRocket says the design is unstable if you don't do the "base drag hack" but says it's stable if you do.
2. the rocket has been built and flown, and is actually stable in flight
3. the Center of Gravity has been checked on the actual flight model and matches the location in the OpenRocket sim.

Cool stories about other situations where the base drag hack has resulted is cool/interesting/horrific things happening are also welcome, but I'm really hoping to get a collection of designs meeting these criteria so I can use them for software testing.

 

[Cape Byron]

Can you define 'unstable' for your project? Is it <1 calibre of stability?

Conventional fins or tube fins (or a combination)?

I have some stuff that may be of use.

 

[Jeff Lassahn]

I'm thinking unstable means a negative stability value not just less than one caliber.
Any kind of fins you've managed to use are cool with me.

Thanks!

 

[Cape Byron]

I'm thinking unstable means a negative stability value not just less than one caliber.
Any kind of fins you've managed to use are cool with me.

Thanks!

Okay, I've got nothing handy with a negative stability. Lots of <1 but no negatives.

 

[rharshberger]

I use the hack quite often to reduce the noseweight requirement in V2 rockets, haven't had one go unstable yet using it.

Okay, I've got nothing handy with a negative stability. Lots of <1 but no negatives.

Same here, less than 1 caliber but never in negative category.

 

[Jeff Lassahn]

Interesting, I wouldn't think a rocket like V2 with its tail cone design would be a candidate for base drag stuff. Can you show me an example?

 

[rharshberger]

Interesting, I wouldn't think a rocket like V2 with its tail cone design would be a candidate for base drag stuff. Can you show me an example?

Let me see if I can find the file, its a Estes V2 file that I tweaked to get around the fins on transitions issue in 15.03



I think this is the one I used, just add the base drag phantom cone trick an you will see what I get. The rocket was never showing unstable but it has very low stability yet flies great, in any conditions I have flown it in so far and I have downscaled it to BT50, BT55, BT60. By modifying the fins a bit I also use it for 4" Loc V2.

 

[lakeroadster]

A Fat Boy and Big Daddy are examples of kit rockets that have very low stability numbers, but that the simulations look great with the base drag hack.

 

[Charles_McG]

The Ute booster of my Ute Tomahawk sims at 0.03 cal. But not actually negative. And it coasts fine after staging. It was bashed out of a Big Daddy.

 

[Jeff Lassahn]

Those sound like they would be useful test cases. Does anyone have accurate models of the geometry and center of gravity?

 

[Charles_McG]

Here's my Ute Tomahawk.

 

[BigMacDaddy]

Here is a Super Baby Bertha... Does not look stable on E12 but definitely flies stable...