Iron Bombs
The advent of modern jet fighter bomber which carried under-wing stores put an end to the WWII era aircraft bombs. They were found to be drag inducing and inaccurate for delivery. In light of these facts, Ed Hinemann of Douglas Aircraft undertook to design a new bomb family the MK series in 1955 (Ref.1) for the A-4 Skyhawk. The MK series consists of four sizes from MK-81 weighing 250 pounds to MK-84 weighing 2000 pounds. They are a two part design- the main explosive container of steel looks like a large bullet and tail assembly carrying the fins. It is a tribute to the soundness of his designs that these are still in inventory and have been copied by all major air-forces.
Modern iron/dumb bombs are sleek and aerodynamic shapes resembling aircraft fuel drop tanks. They need to have the right drag coefficient so that they are not slowed down during descent contributing to the error leading to undershoot. On the other hand , they should not have too much lift or on separation they could raise up and hit their delivery aircraft. Again lift contributes to the error at target, leading to overshoot. Lift in lateral axis also contributes to the cross range errors. In addition the shape has to have static margin ie the center of pressure has to be behind the center of gravity or else will lead to tumbling - static instability. This is achieved by adding fins to the tail section.
Having examined the static factors lets look at the dynamic factors. Spinning leads to dynamic instability - leading to under or overshoot. Adding the fins leads to other kinds of dynamic instability. Hence they have to be just right. After looking at clear weather operations we have to look at non-linearites due to gusts etc which tend to make the dynamic instability problem complicated. Here the mass moment of inertia and the center of gravity come into play. Without getting too technical and explaining the intricacies of dynamics of rotating bodies, suffice it to say tight controls have to be exerted on the fin shape and the overall mass and cg of the contraption, for it to behave as per design.
After achieving all this tight controls what can we expect it do? A blast over-pressure of 5 psi is sufficient to destroy most soft skinned vehicles. A 2000 lb TNT based device had destructive radius of 400yards (Ref. 6) for fragments. The blast radius is smaller.
Jet aircraft with Russian style avionics can hope to release these with an accuracy of 35-40mils. That means from a height of 3000m, the CEP of these is about 105m -120m (35*10^-3*3000). To get better you have to fly in lower and get targetted by short range IR guided SAMs. Infact during the '71 war, IAF pilots flying in low dropped 500kg iron bombs with great accuracy at Tezgoan airfield (Ref. 2). It also means you need heavier bomb load to assure target destruction. Using modern digital navigation systems aided by GPS systems it is possible to achieve 6-7 mils (Ref. 3) i.e. a CEP of 18m- 21m from 3000m. Thus a smaller bomb load is enough to ensure target destruction.
Iron bombs are generally considered useful against area targets. Generally they are made of forged steel casing and filled with high explosive. The amount of filling is based on the targets it is meant to be use against. Nose and tail mounted fuzes are based on target at hand. The later are for defeating hard target while the former are more general purpose. Details of types of iron bombs and fillings as these can be researched on Internet for example Ref. 5. To get an idea of Indian advances in this field DRDO has an article in its Techfocus magazine where in two major types of iron bombs and fuzes are mentioned (Ref.4).
A smart bomb on the other hand is like a controlled glider and achieves its error corrections from some form of guidance. These are good for point targets.
1. Christopher Chant, Pictorial History of air warfare, Octopus Books, London, 1979.
2 'Thunder over Dacca', http://www.bharat-rakshak.com/IAF/History/1971War/Dacca.html
3. "A Propensity for Conflict" , Ken Brower, Janes Special Reports No. 14,Feb. 1997, pg. 16-17
4. DRDO Tech Focus February '98, http://www.nic.in/techfocus/feb98/armament.htm
5. FAS primer on dumb bombs-
http://www.fas.org/man/dod-101/sys/dumb/index.html
6. From BAS article: http://www.bullatomsci.org/issues/1991/may91/may91walker.html
"typical bomb is the 2,000 pound Mk-84, developed in the 1950s. The Mk-84
creates a crater 50 feet in diameter and 36 feet deep or, exploding before
it hits the ground, disperses shrapnel to a lethal radius of 400 yards.
[This message has been edited by rupak (edited 09-10-1999).]
Short Note on Iron Bombs
Re: Short Note on Iron Bombs
Great links and info here.<P>A short read through the links will reveal why iron bombs are still important and "concrete bombs" discussed in another thread can only have two possible roles - i.e. validating pinpoint accuracy of smart munition guidance and possibly a very very small role in bunker busting. <P>Iron bombs are definitely WW II vintage in the same sense that all cars are descendants of The Ford Model T. The statement allegedly made by an IAF officer about the use of WW II vintage iron bombs in Kargil is technically correct, but can be taken out of context to suggest the use of obsolescent weapons.<P>The links that ramana has ferretted out are worth preserving.<p>[This message has been edited by shiv (edited 09-10-1999).]