By Pierre’Piet’ MICHIELS
Many thanks to
Alain BODY, 260th Compagnie Munitions, for the documents and
Andrew LEEMING, for his scientific expertise.
When looking at a cookie drop picture, one notes it is falling almost flat but rapidly front heavy because the rear section is lighter, effectively making the cookie front heavy, so that the bomb falls mainly on the front detonating pistols. 2 side detonating pistols assuring eventual programmed barometric detonation. The 4,000 lb Mk II reference notes state that the average vertical velocity of the cookie is 750 ft/second or 250 m/sec, the terminal velocity being of just over 400 mph or 700 km/h.
Simply through is velocity and accumulated kinetic energy, the single hit of a 4,000 lb “cookie” can reduce to rumble an acre or so of built surface. But there’s more.
Looking at a cross section of 4,000 lb Cookie, one can sees that there is a 1 centered suspension bracket for hoisting the cookie with a cable in the bomb bay. There are 2 more hoisting brackets (front+rear) to hang the cookie in the bomb bay. And there are 4 circle crutch pads (2front-2rear) to block the cookie in its position in the bomb bay.
There are 1 or 3 (depending on Mk.I or Mk.II models) front fuses, armed by pulling the pin(s) at release, which is "automatic" as the bomb falls, the pin being fixed to the aircraft by a cable. Fuses on the front of the cookie are placed to ensure complete detonation throughout the large mass of AMATOL explosive (the bomb is 80 cm across and 220 cm long). Reference notes state that“...at least one of the pistols in the nose fuzing positions fires its detonator ......so as to ensure the detonation of the main filling as a whole”. There are 2 more optional barometric fuses on each side at the rear, also armed by pulling the pins at release.
The 4,000 lb high capacity design was little more than a cylinder full of explosives, it was unaerodynamic and did not have fins at the origin. When further fitted with a nose spoiler and a drum tail the British "blockbuster" bomb was to fell straight.
These bombs were designed for their blast effect, to cause damage to buildings - specifically to blow roof tiles off, so that the smaller 4 lb (1.8 kg) incendiary bomb could reach the building interiors and ignite wooden debris such as flooring, furniture and fittings. These high capacity bombs were used only by the RAF, being too big to fit in the bomb bays of other allied aircraft. The 4,000 lb (1,800 kg) "cookie" was regarded as a particularly dangerous load to carry. Due to the airflow over the detonating pistols fitted in the nose, it would sometimes explode even if dropped, i.e., jettisoned, in a supposedly "safe" unarmed state. Recommended safety height above ground for dropping the 4,000 lb "cookie" was generously set at 6,000 ft; any lower and the dropping aircraft risked being damaged by the explosion's atmospheric shock wave.
TNT (Tri Nitro Toluène) is insensitive to shock and friction, so is very stable. But it can be detonated by a pressure wave from a starter explosive (explosive booster). TNT has then a detonation velocity of 6,900 m/sec (25,000 km/hour).
The diagram in the reference notes refers to “exploders”: the pistols push in on impact, then the shock detonates the primary explosive (mercury fulminate or lead azide) in the exploders, then the shock/pressure wave detonates the TNT (added with ammonium nitrate in the AMATOL), generating heat to decompose the TNT (and ammonium nitrate) and also blasting the hot gases faster and further.
The role of the exploders is to expand temperature in the mixture of TNT (explosive) and ammonium nitrate forming together AMATOL, as TNT is very stable at normal temperatures and only starting to decompose (detonate) at its boiling point of 240°C. Sublimation is a physical property of a solid substance changing immediately from a solid (low volume) into a gas (high volume). Picture ice turning immediately into vapour, without becoming liquid water first: 18 grams of ice turning instantly into 22.8 litres of gas. It would be like a party with friends where an ice cube in a gin-tonic would instantly turn into a polypin of Ale: The table would be ravaged and the party ended…
As to summarize: high explosion power + short time of gas expansion = high air pressure, thus blast effect.
As to estimate the volume of gas generated by the detonation of a 4.000 lb bomb “Cookie”, the calculation is as follows:
If the mixture is 60:40 TNT/ammonium nitrate.
Then Molecular weight of the mixture (making the assumption that the densities of the two materials are the same) = 136 + 32 = 168 grams.
So, 168 grams of Amatol will generate 22.4 litres of gas. Assuming that all solids sublimate into gases by efficient combustion, and not into solid black carbon particles of smoke, and assuming Standard Temperature and Pressure (STP) of 20°C and 760 mm of mercury atmospheric pressure but, of course, an explosion is of hot, burning gases at low pressure (hot gases expand) so the volume should be much, much higher.
So, one pound (1 lb) of Amatol is 0.45 kg
Assuming that the metal casing of a Cookie is 250lb and assuming that the weight of Amatol is (4000 - 250) 3750 lbs:
Explosive velocity, also known as detonation velocity or velocity of detonation (VoD), is the velocity at which the shock wave front travels through a detonated explosive. This is of partical importance for a Cookie. But brisance is different, expressing the shattering capability of a high explosive determined mainly by its detonation pressure. Brisance is only of practical importance for determining the effectiveness of an explosion in frgamenting shells, bomb casings, graneades, structures, and the like.
Chemistry of AMATOL
The “ring” is made from six carbon atoms.
Upon detonation, TNT decomposes as follows:
2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C
2 C7H5N3O6 → 3 N2 + 5 H2 + 12 CO + 2 C
The 7 or 12 CO and the 7 or 2 C (soot) show that the explosion is not very efficient – otherwise the CO (carbon monoxide) and solid C (carbon) would form high volume CO2 (carbon dioxide). More Oxygen is needed for an efficient explosive reaction. More oxygen is provided by the ammonium nitrate (NH4NO3) in the synergistic mixture of AMATOL. AMATOL, by adding Ammonium Nitrate is much more explosive (in less time) than TNT as it provides the O directly (at the very beginning of the ignition process). So procuring higher blast.
TNT is 50% deficient in oxygen. AMATOL is oxygen balanced and is therefore more effective than pure TNT when exploding underground or underwater.
But AMATOL has a little lower explosive velocity and correspondingly lower brisance than TNT but is cheaper because of the lower cost of ammonium nitrate.
Note: Ammonium nitrate leaves no residue when heated: NH4NO3 → N2O + 2H2O (nitrous oxide and water vapour - both gases). Ammonium nitrate is not, on its own, an explosive, but it readily forms explosive mixtures with varying properties when combined with primary explosives such as asides. Ammonium nitrate decomposes into the gases nitrous oxydes and water vapor when heated (not an explosive reaction); however, it can be induced to decompose explosively by detonation. As an illustration airbags in cars use azides : Older airbag systems contained a mixture of sodium azides (NaN3), KNO3, and SiO2. A typical driver-side airbag contains approximately 50-80 g of NaN3, with the larger passenger-side airbag containing about 250 g. Within about 40 milliseconds of impact, all these components react in three separate reactions that produce nitrogen gas.
Pierre MICHIELS, july 2017.