This is really a vast subject but in short:
Generally speaking, RDX, explosive mixed with TNT to form Torpex, is rated by itself as giving a detonation wave of 8700m/s, while TNT yields 6900m/s.
By itself, RDX is therefore about 21% more power than TNT, but combine the two and they compliment eachother.
The RDX is too sensitive to use on its own in most applications, but mixing RDX powder into molten TNT allows the TNT to desensitize it. TNT is relatively insensitive, and powerful on its own, but it often does not detonate cleanly and is therefore not quite so efficient; with RDX in the TNT matrix, the efficiency of detonation is increased.
The advantage, at least in warheads and depth charges, is that the power of the explosion can be increased without materially increasing the weight of the warhead.
Adding aluminium powder to the explosive increases the generation of heat and increases the brisance by allowing the explosive to convert itself into gas pressure more rapidly.
The choice of the British and US navies in adopting aluminized RDX/TNT explosives was simply to increase the power of a given weight of explosives in a warhead without having to adopt a whole new delivery system, ie, design and produce a larger more powerful torpedo body.
As a HBX type explosive, it became pretty much a standard in post war years both as an contact and proximity type explosive because of its ability to rapidly generate maximum pressure and shock waves.
During WW2, in British service at least, Torpex began to replace earlier explosives from about 1943, with 18in arial torpedos having having a higher priority than the larger 21 inchers of surface ships and subs.
The percentage of explosive in Torpex was 40-45% TNT, 40-45% RDX, and 10-20% aluminium powder.
Keep always in mind that more powerful does not always mean more effective. Effectiveness depends largely on the application you are applying the explosive to. Brisance is the ability of the explosive through heat, shock and pressure to destroy structures it is in contact with.
In air, the radius of equivalent overpressure is proportional to the cube root of the charge weight, so doubling the damage radius (using overpressure as a measuring stick) requires multiplying the charge weight by eight.
The situation in water is much more complex, but basically quite similar. In a nutshell, changes of even 15%-20% in charge weight tend to have relatively little effect on the amount of damage observed.
