Bomb Fin Advantages

[KJ_Lesnick]

I've noticed that the RAF and possibly the USN (early on) used bombs that had the fin diameter the same as the bomb diameter and had a circular shaped ring surrounding the fins.  The USAAF on the other hand had bomb-fins that were larger in diameter than the bomb and used a box-fin.

I'm curious why the RAF and USN chose the circular ring and a smaller diameter fin and why the USAAC/F chose the larger diameter fin.

[PR19_Kit]

Because with circular and smaller fins you can get more bombs in the same piece of bomb bay...........

[KJ_Lesnick]

PR19_Kit

Because with circular and smaller fins you can get more bombs in the same piece of bomb bay...........

What I'm wondering is why did the USAAF use the large box-fin arrangement?

[PR19_Kit]

Perhaps because :-

a) They hadn't thought about the space issue, or

b) They thought THEY knew so much better than anyone else?

[KJ_Lesnick]

PR19_Kit

Perhaps because :-

a) They hadn't thought about the space issue, or

b) They thought THEY knew so much better than anyone else?

I wouldn't be surprised if they didn't factor space in much.

I am curious if they thought larger fins would produce more stability and the box-shape would produce less rolling movement and keep it more stable on pitch and yaw.

Didn't the RAF design the bombs with fins that allowed a spiral motion to build up?  Such a shape would produce good ballistic stability...

[Old Wombat]

Repeating my PM to KJL:

In all honesty - I've no idea.

Possibly in the belief it provided more stability to the bomb?

However, using the short-&-round-fin, simply building in a very slight angle to the fins would cause it to spin, thus giving even greater ballistic stability. This wouldn't work on the box-fin because the angular shape doesn't lend itself to spin.



An addendum to that:

The box-fins were, however, probably slightly quicker & easier to manufacture, as you only needed to make 4 flat plates with 45^o angles at the end & bolt/weld them to the ends of the fins. The circular type required the more complex process of tube manufacture & a frame to keep the fins equally separated whilst drilling bolt holes or welding them on.

[KJ_Lesnick]

Old Wombat

Repeating my PM to KJL

LOL, I generally ask questions to two different people if I can to see if I'm on the right track... I didn't mean to be repetitive in anyway

The box-fins were, however, probably slightly quicker & easier to manufacture, as you only needed to make 4 flat plates with 45^o angles at the end & bolt/weld them to the ends of the fins.

That actually is a good point...

[rickshaw]

I would suggest it was a combination of factors:

The USAF had the luxury of planning big planes and so, big bombs (physically, rather than just mere weight) were designed into the aircraft design.  Americans always like to do things big, remember?

The US Navy and the RAF however, were always facing a space problem.  The US Navy had to carry the bombs on their carriers and fit them into smaller aircraft, so therefore a smaller physical size for a given bomb weight was to their advantage.  The RAF faced a different problem, they had to get the maximum tonnage onto their targets in Europe with the least number of bombers, so smaller physical size to their bombs worked well for them.

Then we have what I'd call "fashion".  You'll note that most air forces in Europe used drum finned bombs on at least some of their bombs.  Why?  'cause everybody else did, of course!   

I've often wondered what sort of experimental work was done on bomb design in WWII, particularly amongst the smaller air forces.  Was much time spent in the wind tunnel or was it more a case of guess work?

Finally, Kendra, in answer to your question about the utility of fins on bombs, obviously the larger the fin, the greater effect it would have on controlling the bomb's path.  However, as experience showed, bomb fins, no matter what their size of shape, often had little effect.  Bombs would often fly in unpredictable directions, once they left the bomb bay.  They might even tumble and collide, which would then send them haring off in other directions again.

So I suspect that putting fins on bombs, while it looked good and was fine in theory often didn't quite work out the way intended in practice.

[PR19_Kit]

You may have been thinking of the Barnes Wallis designed 'earthquake' bombs that had fins specially aligned so that they spun. The 5 ton 'Tallboys' and 10 ton 'Grand Slams' both dropped at supersonic speeds and spun to ensure their accuracy when used with the SABS bomb sight.

[Old Wombat]

You may have been thinking of the Barnes Wallis designed 'earthquake' bombs that had fins specially aligned so that they spun. The 5 ton 'Tallboys' and 10 ton 'Grand Slams' both dropped at supersonic speeds and spun to ensure their accuracy when used with the SABS bomb sight.

This bit I was well aware of &, although I didn't (& still don't) think the smaller bombs had this feature, it could well have been added to the short-&-round/drum-fin type without too much difficulty to increase their accuracy (but I'm pretty sure Bomber Command weren't that concerned about greater accuracy from these weapons). However, in a dive-bombing or fighter-bombing role, such a feature may have increased accuracy enough to make them even more effective because, in a diving attack, the bombs would have begun spinning almost immediately after release & the improved ballistics would have meant a more predictable trajectory.



Guy

[PR19_Kit]

Sorry Guy, I didn't mean that you may have been thinking of them, I meant that Kendra might have been.

I've not heard of spin fins being added to otherwise 'normal' bombs either. Apparently the need for them on the Tallboys and Grand Slams was found during the initial trials when the prototypes tended to tumble when they reached M 1.0, presumably due to the shock waves. I found that in my Hayne's Dambusters Owners Manual. 

[Captain Canada]

I thought the old bombs had the round rings for strength. And when you're in the field in combat conditions they were easier to handle, rolling etc. But that was just my thinking, no idea of the reality of it.

[Old Wombat]

I've not heard of spin fins being added to otherwise 'normal' bombs either. Apparently the need for them on the Tallboys and Grand Slams was found during the initial trials when the prototypes tended to tumble when they reached M 1.0, presumably due to the shock waves. I found that in my Hayne's Dambusters Owners Manual.  

I remember it (or parts of it, anyway) from Guy Gibson's Enemy Coast Ahead.

(Incidently, & of absolutely no consequence whatsoever, he & the singer/actor Guy Mitchell were the inspiration for my parents giving me my name. )



Guy

[eatthis]

You may have been thinking of the Barnes Wallis designed 'earthquake' bombs that had fins specially aligned so that they spun. The 5 ton 'Tallboys' and 10 ton 'Grand Slams' both dropped at supersonic speeds and spun to ensure their accuracy when used with the SABS bomb sight.

them big buggers were supersonic in freefall?
thats some serious kinetic enrgy even without explosives in it   

[kerick]

That's why they burrowed so deep into the ground before exploding.
I've heard of weapons loaders during bombing competitions closely watching over their bombs to make sure no one "accidently" bent one of the fins on the bombs, thereby throwing its accuracy off. Perhaps the fins on more modern weapons were more effective.