Base Drag: How does a fat rocket know it’s fat?

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BABAR

Builds Rockets for NASA
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Say a BT-80 rocket is 10 inches long, that’s a fat bottomed rocket with a lot of base drag.

but if it’s 10 FEET long it is a SuperRoc.

how does it know?

or is it all relative, both have the same base drag, but the base drag is much greater in the shorter rocket relative to the other forces on the rocket?
 

mbeels

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It calculates its "B.D.I" (Base Drag Index) and consults a table to determine if it is:
  • Super Roc
  • 3FNC Rocket
  • Fat Rocket
  • Saucer
 

dhbarr

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Fluids hate turning sharp corners. When they do, they create a lot of turbulence that shows up as drag.

On a long rocket, that drag is relatively small compared to the overall drag on the fins and nose and body. On a short rocket the base drag is a much more significant component of the overall drag.

I left out some other kinds of drag, but that's the basics as far as I understand them.
 

boatgeek

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There's probably an additional effect where on a short rocket the airflow has barely sorted itself out after the nosecone transition before the fins and base drag mess everything up again. This would be even more so on a saucer. On a superroc, the flow has lots of time to stabilize around the rocket body before you hit it with the fins and base drag.

The two rules of hydrodynamics:
Don't surprise the water
Put it back nicely when you're done
 

Funkworks

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Here’s one way to put it:

The fins of a short rocket fly into the turbulence caused by the nose.

The fins of a long rocket fly into air smoothed out by the long airframe.
 
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neil_w

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The fins of a short rocket fly into the turbulence caused by the nose.

The fins of a long rocket fly into air smoothed out by the long airframe.
Fill in the blank:

"The fins of a ____ rocket fly into the turbulence caused by all the excess design crap all over the rocket".

Asking for a friend.
 

boatgeek

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Fill in the blank:

"The fins of a ____ rocket fly into the turbulence caused by all the excess design crap all over the rocket".

Asking for a friend.
“The fins of an awesome rocket...”

Incidentally, how good are your OR sims in predicting altitude? I would consider plus or minus 5-10% to be normal sim errors.
 

neil_w

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Incidentally, how good are your OR sims in predicting altitude? I would consider plus or minus 5-10% to be normal sim errors.
Haven't flown enough altimeter flights to get a good handle on it, to be honest, and only a few of my rockets have payload bays suitable for altimeter inclusion anyway. I know it is possible to just hang an altimeter from the shock attachment point, which ought to be good enough at least for apogee measurement, but I haven't tried that yet.

I've gotten in the habit of creating two separate models for many of my rockets, one for construction and finishing, and one for flight. I continue to tweak my flight models as I think of better ways to do things, but really I need to fly and then adjust according to the real data.

What's that you say? Actually "fly" a rocket? What a novel concept. 😬
 

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