# Rotation of bomb

1. May 1, 2014

### gm3

I'm working on a project that will find the coefficient of drag on bombs with various added drag features by modeling flow over the bomb at various angles of attack using CFD.

The problem is that I'm not sure if bombs with fins rotate while they are falling due to gravity, and if so, how I can relate the velocity of the flow to the rotation rate of the bomb.

Since the fins on a bomb are flat and not curved like an impeller, do they simply keep the bomb flying on a predictable path?

2. May 1, 2014

### 256bits

Same purpose as an the feathers or fins of an arrow. They keep the nose pointing in direction of travel and prevent the arrow ( bomb ) from tumbling head over heal. In this way drag is minimized along the streamlined surface of the projectile, travel distance ( especially for the arrow ) and velocity would be maximized upon reaching the target. Thus time from release to impact would be minimized which could be a desirable feature as could velocity upon impact.

A weather vane works upon the same principle.

3. May 1, 2014

### gm3

Excellent. Thank you.

4. May 1, 2014

### Bobbywhy

Some bombs are meant to spin as they fall towards their target. Others not; it all depends on the bomb and its purpose. Here are some examples:

Fixed fins, stability only: Paveway IV ( GBU-12 added GPS ) and the GBU-54

Fins that cause the bomb to spin:
https://share.sandia.gov/news/resources/news_releases/wind_tunnel/#.U2LllGdOVgU

Grid fins (or lattice fins) are a type of flight control surface used on missiles and bombs in place of more conventional control surfaces, such as planar fins. These fins increase the aerodynamic drag.
http://en.wikipedia.org/wiki/Grid_fin

Hundreds of images of various types of bomb fins:

A technical paper on the subject:
JOURNAL OF THEORETICAL
AND APPLIED MECHANICS
48, 1, pp. 27-44, Warsaw 2010
MODEL OF GASODYNAMIC CONTROL SYSTEM FOR
GUIDED BOMBS

5. May 1, 2014

### Staff: Mentor

So after reading the great responses and links, can you summarize when a ballistic bomb design would be better served with rotation versus not?

And can you extrapolate to air-to-air missles, and why some may use rotation? Which ones do?

6. May 1, 2014