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"Circular Error Probability" of WWII bombs?

Discussion in 'Air War in Western Europe 1939 - 1945' started by Wild Turkey, Nov 2, 2014.

  1. Wild Turkey

    Wild Turkey New Member

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    Definition: http://en.wikipedia.org/wiki/Circular_error_probable

    The bombs of WWII were much less aerodynamic than the "low drag" bombs of Vietnam and later.

    Did that affect the accuracy of the drop?

    If, for example, a dozen WWII GP bombs were dropped from a stationary balloon at 20,000 ft AGL how large an area would their impacts cover? What about 30,000 ft?

    Since the bombs approach the "Sound Barrier"at 20K and break it from 30K does this change their accuracy?

    Would the "Low Drag" modern bombs be more accurate?

    Did the loss of accuracy due to wind shear, etc and the supersonic flight characteristics both contribute to the change to low level bombing in Japan?

    Yes, I'm a retired Physical Science teacher who still plays with physics for fun.
     
  2. CAC

    CAC Ace of Spades

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    Newtonian physics should answer your problem...The factors that change accuracy would be:
    Shape of the bomb (drag)
    Height at which it is dropped: Thinner air = less drag quicker fall.
    Speed horizontally: A straight or parrabolic drop.
    Weather conditions: Wind for example (though needs height to have time to act against the bomb in any real way.)

    Modern bombs would be more accurate if only becasue the scope or spectrum of factors would be less.

    If the same bombs (shape and size) were dropped from the same height, from the same location (geo-synchronis), all bombs would be affected the same on their decent...they should drop almost on top of eacgh other...
    Just my two cents.
     
  3. TiredOldSoldier

    TiredOldSoldier Ace

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    I would expect the low drag bombs to be less affected by weather, but no idea if there is a significant difference, all other factors would be the same. Possibly internal rather than external stowage have an influence, as the bomb doors may introduce some buffeting, but pictures of drops from B17 (ww2 internal), B52 (low drag internal) and A6 (low drag external) do not show visible differences.
     
  4. Martin Bull

    Martin Bull Acting Wg. Cdr

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    This is strictly a two penn'orth as I am absolutely no academic.......

    But I do know that Barnes Wallis grappled with this problem to produce the 'Tallboy' and 'Grand Slam' bombs which targeted specific structures such as V-weapon sites, railway viaducts etc. The bombs were machined to very strict tolerances, the tailfin angles were carefully calculated and the bombs were dropped from high level in order that they would 'go supersonic'. Their use also depended on perfect weather conditions, a stabilized bombsight and no inconvenient interruptions - such as the Luftwaffe.
     
  5. Sheldrake

    Sheldrake Member

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    The shape may have another affect other than drag. Asymmetry may result in a Bernoulli effect as the bomb drops resulting in a force acting on the bomb in one direction or another. Imagine dropping a "bomb" shaped like a paper dart. This might be the reason for careful machining, and control surfaces were used as part of the Fritz X precision guided bomb.



    The probable error of a bomb was less significant than the errors introduced by the bomb aimer taking account air speed, wind sped and direction etc. Bomb sights such as the British SABS , the German Lofte and US Nordon removed some of these errors.

    The other way to reduce these errors was by dive bombing. The bomb could be released on a neat vertical trajectory with a high enough velocity to minimise the effect of wind.

    The reason for low bombing in Japan compared to high level in Germany was that the German air defences were very efficient and flying at low level would have been suicidal. The USAAF were trying to high industrial targets with some precision. In Japan they were trying to set fire to Japanese cities against a feeble night air defences.. .
     
  6. mcoffee

    mcoffee Son-of-a-Gun(ner)

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    The change to low-level bombing over Japan was due to the newly encountered jet stream commonly found at high altitude over that country. The result of those encounters were that the B-29s would have either a very fast or very slow ground speed over the target - outside the range that the bomb sights were capable of dealing with. The resultant wind shear as the bombs fell out of the jet stream also affected accuracy.

    The most widely used USAAF bomb - the 500 lb GP - had a subsonic striking velocity. The 1000lb and 2000lb GP bombs had a low supersonic striking velocity.

    The document "Terminal Ballistics Data, Volume I" may be of interest.
    http://cgsc.cdmhost.com/cdm/ref/collection/p4013coll8/id/2342

    It provides range and trail data for various bombs from various altitudes, and differential effects of wind on range. From those charts it is possible to infer the effects of wind and air density on bombing accuracy.

    If you Google "terminal ballistics data wwii", you will find links to Volumes II and III.
     
  7. lwd

    lwd Ace

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    I would expect that to have it's greatest impact if there are winds particularly variable (with height) winds. In support of this the Norton bomb sight was apparently very (some would say incredibly) accurate when tested in the US Desert SW it proved less so over Europe and in the Pacific.

    Given that they were higher drag was their terminal velocity really in excess of the speed of sound?
     
  8. bronk7

    bronk7 Well-Known Member

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    would a subsonic bomb cause less damage than a low supersonic, same weight?
     
  9. lwd

    lwd Ace

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    Damage to what? The shock wave would be capable of inflicting some damage but that might be immaterial compared to the bomb itself. A supersonic bomb should have better penetration but if you are talking an HE bomb then the greater velocity might result in it breaking up on impact or malfunctioining in some other way. In short it depends on what you want to damage and what you are using to damage it with.
     
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  10. mcoffee

    mcoffee Son-of-a-Gun(ner)

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    To put what LWD said in slightly different terms, a bomb traveling at a greater velocity has more kinetic energy upon impact, not counting the chemical energy released in the explosion. Whether or not the greater kinetic energy is important depends on the type of target.
     
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  11. phylo_roadking

    phylo_roadking Member

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    ...although at the start of the war the Luftwaffe for example worked to error radii of some thirty metres...which is quite a lot when tryinmg to hit a small target!

    In the two massed Stuka attacks on the bunkers of the Mlawa Line in Poland on September 2nd, NONE of the German bombs hit any of the anti-tank gun-armed concrete bunkers they were aiming for. The gunners were shocked/demoralized for up to 45 minutes after the attack...but due to the wet ground in front of the Line, the German forces on the ground weren't able to make use of this.

    By the summer of 1940, this error radius had been halved...mostly due to experience (and boy did they have experience by then!) but also in late August and September 1940 by the extra training carried out by Richthofen's Stukas in preparation for Sealion. But it was still a 15-metre error radius...
     
  12. harolds

    harolds Member

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    Since WW2 bombs were mass produced, there certainly had to be some differences in individual bomb's manufacture that effected accuracy. As an example, target bullets are manufactured to much higher standards than regular hunting bullets and are much more accurate, all things being equal.
     
  13. lwd

    lwd Ace

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    Yes and no. If you standard for accuracy is say, hits on target for a man sized s[SIZE=medium]ilhouette at 100 meters then you probably wouldn't see much if any difference in accuarcy. If it's the number of x's on standard competition target at the same range then you likely would. If the errors introduced in the manufacturing are small compared to the aiming errors then it may actually give you a better chance of landing one on target.[/SIZE]
     
  14. harolds

    harolds Member

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    That was my next point. If your target for "precision bombing" is a factory plus the area around it, and you have a bomb group or even a bomb division all toggling their bombs when they see their leader's bombs drop, accuracy of the bombs is kind of moot.
     
  15. bronk7

    bronk7 Well-Known Member

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    much thanks for reply and as always, very interesting, enjoyable, and informing to read....you are spot on to my question...I was thinking of the damage to a semi-hardened target with, lets say, a 1000lb AP bomb?..also, is there a big difference in damage for a Tallboy going subsonic or low supersonic, due to its bigger mass??
     
  16. bronk7

    bronk7 Well-Known Member

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    15 meters, with a Stuka? I guess that would be right, because that would explain the number of the hits scored on the carriers at Midway [moving targets ]
     
  17. phylo_roadking

    phylo_roadking Member

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    ...remembering, though, that a USN aircraft carrier or an RN destoyer is a different prospect in scale to...say...a tank or a bunker/pillbox ;)
     
  18. bronk7

    bronk7 Well-Known Member

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    but it's moving, constantly changing directions, unlike a bunker..
     
  19. phylo_roadking

    phylo_roadking Member

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    That's the awkward thing...no they're not ;)

    Off Norway and Dunkirk...and later off Crete, it was found that an RN destroyer could avoid fall of ordnance repeatedly given THREE circumstances, all of which had to apply...

    Searoom and depth of water to allow free manouvering...

    Sufficient AA munitions...

    Properly trimmed; in the case of Dunkirk, Crete, and the departure of various heavily-loaded destroyers from Cemntral Norway...any troops etc. on deck HAD to be got down below the water line and trimmed port and starboard or a vessel as "small" as a destroyer could roll like a barrel :(

    (It also required a VERY skilled captain and excellent deck watch, which was not axiomatic - not all commanders were created equal)

    If one or more of those circumstances was lacking - the LW won the exchange. Apart from anything, destroyers had very thin skins ;) Even near misses could badly damage them from shell fragments/splinters.

    Carriers...apart from having a really much larger turning circle...have this painful habit of having to turn into the wind and steaming full speed in a straight line when flying off or recovering aircraft :( At times therefore it could be easier to hit than others.
     
  20. harolds

    harolds Member

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    Does anyone know how IJN, RN and USN destroyers compared as to amount of AA ordinance that could be stored. Referring to the post above mentioning "Sufficient AA munitions..." This could be described using total rounds, rounds per gun or even total weight of shells that could be thrown in a given time.
     

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