Allied/Axis Bomb-Shapes

[Zipper730]

I was very recently reading some USAAC documents on tests in the late 1930's of round bomb "fins" -which the Air Corps had been using since WWI - vs the box fin. The AAC's findings were that the box fin made the bombs more stable and allowed for greater accuracy, a very important aspect to the Air Corps and its daylight bombing philosophy.

From what I was reading from this site, I got...

In 1921 the War Department convened a Bomb Board to conduct an extensive program for testing bombs against various kinds of structures and surfaces. The tests, running over a period of two years, provided data that guided the Ordnance Department and the Air Corps through the 1930s, Ordnance engineers strengthened demolition bomb cases by forging them as nearly as possible in one piece, with a minimum of welding, and substituted for the long fins of World War I short box fins that gave greater stability in flight. Uniformity of fragment size of the fragmentation bomb was achieved by encasing the body in rings cut from steel tubing or in wound steel coil. For low-level bombing, experiments with means of delaying the action of the fragmentation bomb sufficiently to permit the airplane to get to a safe distance before the bomb detonated produced a parachute attachment in place of fins. The parachute slowed descent and caused the bomb to strike the ground with its axis nearly vertical so that the fragments tended to be scattered above ground instead of being buried. Collaboration with the Chemical Warfare Service developed bombs that could be filled either with a fire-producing substance or with gas or smoke. The filling was the responsibility of the Chemical Warfare Service, the case of the Ordnance Department. The case had thin walls like the demolition bomb but had a burster tube running down its center. Shortly before the United States entered World War development of the incendiary bomb became entirely the responsibility of the Chemical Warfare Service.

I'm not sure what types of fins they evaluated from

1. The old, long annular fin
2. The short box-fin
3. All the designs in between

You'd think a cylindrical fin of proper aerodynamics (the cross-section of the fin) would work better because the airflow over the bomb is circular, and the airflow over the fin is circular, plus a box-fin is made of an upper & lower, bottom & top, as well as four fins at the junctions, where as an annular fin has the annular ring (one part), and the X-fin arrangement as before.

 

[tyrodtom]

From what I was reading from this site, I got...
I'm not sure what types of fins they evaluated from

1. The old, long annular fin
2. The short box-fin
3. All the designs in between

You'd think a cylindrical fin of proper aerodynamics (the cross-section of the fin) would work better because the airflow over the bomb is circular, and the airflow over the fin is circular, plus a box-fin is made of an upper & lower, bottom & top, as well as four fins at the junctions, where as an annular fin has the annular ring (one part), and the X-fin arrangement as before.

That ring was or box section purpose was to brace the fins.
The major cause of inaccuracy in a bomb is the fins vibrating or moving around, it's easier to brace the fins, and stronger, with just a straight piece of steel between the fins.
The US bombs were quite squatty, the fins were mostly in turbulent airflow.
A curved piece between the fins would just change it's curvature, and let the fins still move around unless you made it very thick and heavy..

Also box fins meant the bombs were easier to handle on the ground once the fins were installed. You could set them on a trailer, still had to tie them down, but not as much trouble keeping them in place as you would have with a round fin at the back.

You'll notice on about any aircraft that has a visible external brace of a flying surface, that brace, is straight, not curved.

What they did later when aircraft got faster was get rid of the brace between the fins entirely, they made the fins stronger.

 

[Shortround6]

The box fins had an advantage (theoretical?) in that they were super caliber. That merely means that the span of the fins (over the diagonal) was greater than the diameter of the bomb and the last few inches of the fins should be operating in undisturbed air.

US bombs tended to short squatty things with a rather abrupt taper at the rear end.

Airflow over the fins of a circular or drum shape of equal or lesser diameter than the bomb body is going to be turbulent.
The British used a much more gradual taper on their bombs

At least in the early part of the war. Please note the two british bombs are not to scale. The 250lb bomb was 10.2in in diameter while the 500lb bomb was 12.9in and the 500lb bomb was about 25% longer with standard fins.

 

[wuzak]

Those would be the GP bombs, with about 25%-30% charge-to-weight ratio.

The MC bombs, introduced early in the war, were more like US GP bombs.

500lb MC bomb
500lb Mk.II Bomb (M.C. = Medium Capacity) by Lord Throplebury of Winklesham

This also appears to be the long/standard tail.

IIRC, the 500lb MC had a diameter of 17.75", nearly 5" greater than the GP equivalent.

 

[wuzak]

These MC bombs (500lb and 1,000lb) have the short tails, though it is hard to see from that angle. Also there is the 4,000lb MC bomb. Seems they didn't bother with a 2,000lb MC bomb.

https://i1.wp.com/www.junobeach.org/wp-content/uploads/2014/04/arms_air_bomber_aircraft_3.jpg

Aircraft, Bombs and Radars

 

[Zipper730]

That ring was or box section purpose was to brace the fins.

Didn't know that

The major cause of inaccuracy in a bomb is the fins vibrating or moving around, it's easier to brace the fins, and stronger, with just a straight piece of steel between the fins.

Didn't know that. I thought the ring had some aerodynamic benefit. It makes sense that they'd be in turbulent airflow, but aircraft have tails in the rear too and they clearly have a benefit, so I figured if the ring shape was used, the airflow would still be cylindrical (the cross section of the bomb) rather than cubic (the fins).

A curved piece between the fins would just change it's curvature, and let the fins still move around unless you made it very thick and heavy.

Are we talking about the curve of the fins? Also, how much did the RAF's annular ring cost over our box-fins?

As for the removal of the braces, that makes a lot of sense, and to be honest, a lot of the bombs that we'd carry later on kind of had the same look as the tallboy, just smaller.

The box fins had an advantage (theoretical?) in that they were super caliber. That merely means that the span of the fins (over the diagonal) was greater than the diameter of the bomb and the last few inches of the fins should be operating in undisturbed air.

You still do require more space in the bomber to carry them.

Did they produce any significant ballistic accuracy improvement?

 

[tyrodtom]

If you'll look at the American AN M64 bomb in Shortround6's post and the British Mk II bomb in Wuzak's post you'll notice both are 500 lbs bombs, with very similar bomb bodies, but different fins.

Just the British bomb is quite a bit longer, the more tapered tail might have produced less turbulence, and might have been a more accurate bomb
But the longer length might have meant fewer bombs could be carried in some bomb bays.

A lot of war material America made during WW2 might not have been the best that could have been made.
But whatever they made, they made in massive amounts.
A lot of the massive production came from designing products so they were simple to produce.

I don't like quoting Stalin, but I think he said " quantity has a quality all it's own "

 

[Shortround6]

Unless you could drop both bombs at the same time from the same plane and do so repeatedly and measure the miss distances (or drop a crap load (thousands) of each type and plot the miss distances) you are not going to get a good comparison. There are too many variables for small scale testing to sort out if the bombs are dropped on different flights even on the same day.

 

[Reluctant Poster]

RDX, cyclonite or hexogen is invented in Germany. British took part in the invention of the method for desensitising RDX by mixing it with small amount of wax. Germans did it by mixing RDX with graphite. British actually invented Torpex (Torpedo explosive) which was mixture of RDX, TNT and aluminum powder and about 50% more brisant than TNT, but that explosive was not in use prior to mid 1942. It was used in MC bombs 500-4000lbs, HC 4000 lbs and both DP bombs. US version of RDX based mixture was Comp B present in some number of US GP bombs. Germans in their bombs used Trialen which had only about 15% of RDX.

A poorly worded post on my part. I was intending to convey the point you are making ie the development of explosives based on RDX as well as methods of production that greatly increased output. Torpex was actually preceded by Minol in torpedoes and depth charges

 

[alsaad]

Related to box shaped fins...box and shrouded fins give more stabilizing surface for the same length and span. i.e. making bomb overall length shorter, reducing longitudinal vibrations in the airflow thus making trajectory smoother.

 

[Dimlee]

I don't like quoting Stalin, but I think he said " quantity has a quality all it's own "

He did not. Another misattributed quote.
He could say/write something similar of more general meaning as "quantity turns into quality", since that was typical for Engels and Hegel, whose works were studied by all Communists at that time.

 

[Milosh]

Did any of the bombs under 2000lb have fins that caused the bombs to rotate in flight?

 

[Zipper730]

This is something confusion here, there are two figures here for the 4000 lb light-case AN/M56: Two different sources list 34.25" as diameter, the other listing 36" (3'). One source was from 1944 (which lists dimensions), the other from 1950 (which lists a picture)

The thing is usually the overall body diameter (not the fin diameter) seems bigger than basic cylindrical body (the basic bomb-body shape minus all the stuff that allows it to be mounted to the aircraft). I'm not certain which figure is correct, as it seems to be the same model depicted.

 

[Zipper730]

I was very recently reading some USAAC documents on tests in the late 1930's of round bomb "fins" -which the Air Corps had been using since WWI - vs the box fin. The AAC's findings were that the box fin made the bombs more stable and allowed for greater accuracy, a very important aspect to the Air Corps and its daylight bombing philosophy.

So, they were doing tests all the way into the 1930's?

 

[Zipper730]

I should seriously start putting together my excel graph on this.

 

[Zipper730]

I'm curious if I should do it by nation or by type and mass?

 

[GrauGeist]

By nation would provide a good contrast of types, then you can do the weights and types as a sublist within each nation's list.

And don't forget to include the US's 55 gallon drum bombs that had plywood fins!

 

[Zipper730]

I never heard of the 55 gallon drum bomb

 

[pbehn]

I never heard of the 55 gallon drum bomb

I have a feeling that I know what it looks like.

 

[GrauGeist]

I never heard of the 55 gallon drum bomb

Used on some occasions in the Pacific.

Posted (with pic) about it a while back, here:
Picture of the day.