#### @Chemist

*"a RPG delivers very roughly a million times that amount of energy" ... kW is a unit of power not energy... If you meant 90 kJ are you seriously suggesting that a RPG delivers 90E9 J ? For comparison TNT has an explosive yield of ~4 MJ/kg.*

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**Please show your workings ! (90E9 J is the rough yield from > 20 tonnes of TNT) That's some RPG. Shoulder launched ?*

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I am old now (and possibly senile), but when I did this sort of thing as an explosives chemist for MoD over 40 years ago the rough rule of thumb was 1 cubic inch of composite HE generates 7million HP when it goes bang.

Many of us get confused about energy power and and the rate of doing work. I can get confused with units because when I was at school, we learnt ft-lb/sec, then c.g.s, then m.k.s; and then, when I was working, SI units. So, to see if I am in the right ball park, a few numbers:-

Petrol (which is a lot more energetic/Kg than TNT as it has no oxygen contained within it) has 8 MJ/L or 44.4 MJ/kg - In a 90 Kw car, that 1 kg would allow it to travel very roughly 15 km in 15 mins - The engine uses that 44 MJ in 900 seconds or very roughly 0.05 MJ/s (It doesn't because petrol engines are ~30% efficient).

TNT only has 4.7 MJ/kg. The more powerful composite high explosives would have >5 MJ/kg. A RPG warhead contains ~0.2Kg of explosive so that would be ~1 MJ. The velocity of detonation of composite is ~8000 m/s, and the maximum distance the detonation front has to travel within the charge is ~0.1 m. So the explosion takes 0.1/8000 sec or 1/80000 sec, therefore the RPG warhead explosion generates 1 x 80000 MJ/s or 80 GJ/s. Your 90E9 J value (90 GJ) has no time units.

The calculation based on the MoD rough rule of thumb:-

1HP = 7.46 E-7 GJ/s, so 7 million HP = 7E6 x 7.46 E-7 GJ/s or ~5 GJ/s. The warhead contains roughly 0.2kg of explosive with a density of ~1.5 which is ~130mL or roughly 8 cubic inches, so the total yield is 8 x 5 GJ/s or ~ 40GJ/s - Obviously this is about half of the "theoretical value" but allowing for differences in explosive material, velocity, and energy transfer; it is pretty close.

The secret of the large amount of destructive power in a small device is the very short time that it takes to be generated (The velocity of detonation within the charge being 8000 m/s). So, no you don't need >20 tonnes of TNT you only need 0.2 kg. RPGs can be particularly effective as they rely on the "shaped charge" effect, where most of the charge's energy is used to project a similar mass of copper in a jet with a tip velocity of ~10km/s and a temperature of ~500C into the target.

High explosives are generally classified as such if they have a detonation velocity of >3000 m/s. Gunpowder (black powder) as used in fireworks only has a deflagration velocity of 600 m/s and does not 'explode' unless confined.