Saucer Shape Model Rockets - Sims

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joeman
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Saucer Shape Model Rockets - Sims

Postby joeman » Sun Aug 28, 2016 1:53 pm

Hello,
I have a model rocket (that looks like a UFO). Matthew Small from Cairns was kind enough to give my kids one each

I know the "rocket" is stable. I've flown in before and I got it before. However I wanted to find out its theoretical height/velocity.

I looked for an OpenRocket Sim, but have been unable to find one....so I tried to do up a "best approximation" Sim...just so I could get some indicative figures on altitude, velocity...

So I created a rocket with very large nose cone (to represent the top part of the Saucer shape). I couldn't invert the nose cone to make the bottom section of the saucer, so I put in some fins to try and represent some of the aerodynamic force that the bottom section would have. I then had the motor offset from motor retainer tube, so its weight was closer to the "top" of the saucer rocket. I was able to get it somewhat stable....CG above the CP.

Sims then showed about 90m. If I reduced the 'nose cone' to about 2.5cm...its estimated altitude increased dramatically. This makes sense to me because this "saucer" rocket has a lot of drag.

What are other people's thoughts on this?

Would anyone possibly have a simulation for something like a "ufo" rocket?

Cheers

Joe
L1 - Callisto (H133) - 11-Jan-2016 - LCO
L2 - March Fly (1633K940) - 18-Mar-2018

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SpaceManMat
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Re: Saucer Shape Model Rockets - Sims

Postby SpaceManMat » Sun Aug 28, 2016 4:51 pm

You need to put a tail cone on it. Use a transition to do this.
QRS: 124
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joeman
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Re: Saucer Shape Model Rockets - Sims

Postby joeman » Sun Sep 04, 2016 12:26 pm

Thanks for that idea Mat.

I tried doing that. The 'leading section' (top) of the saucer was fine. But the trailing section (bottom) didn't work as expected.
See screenshot. It has the CP well ahead of where I'd expect it to be (and where it is). It definitely is stable.

Perhaps I'm doing something Openrocket doesn't expect or cater for. Some assumption isn't being satisfied...

Joe
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saucer.png
L1 - Callisto (H133) - 11-Jan-2016 - LCO
L2 - March Fly (1633K940) - 18-Mar-2018

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Jordz
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Re: Saucer Shape Model Rockets - Sims

Postby Jordz » Sun Sep 04, 2016 4:33 pm

Looking at your components in the top left id say theres probably an easier and more accurate way to do it. Id say the extra transition in your component list is whats throwing the measurements out.

If you go:
- Nose cone
- Body Tube
- Transition

you will probably have more joy with the sim. Set the body tube to be the OD of the saucer and 0cm long, set you nose cone and transition to the appropriate lengths and set their shapes to basically anything other than conical which looks right.

Let me know if it works out better.

matthew
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Re: Saucer Shape Model Rockets - Sims

Postby matthew » Wed Sep 07, 2016 5:52 pm

Joe, you're looking to simulate base drag. Apogee Newsletter 154 has an article from Bruce Levison that discusses ways this can be done. In a nutshell, add a forward facing conical, er, cone, to the aft of the rocket (or saucer). It should be pi diameters long, and one diameter at its base. It should come to a point, and basically be poking the entry of the motor tube. This can be used for saucers, spools, and short, fat rockets like an Estes Fat Boy (basically, anything less than 10:1 length to diameter).
Matthew Small
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TRA #11466 L3
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