Chuklehead : The thing to understand is that redundancy is not all it's cracked up to be. Better to have one system unlikely to fail than two dodgy ones one to catch the frequent failure of the other... don't think about having a spare tailrotor think instead of making one that can withstand the fabled test chicken. SIMPLEX theory is quite good. What do you want in your car a strong reliable braking system or a marginal one with a spare for when the main one doesn't work?
Instead of wasting payload on carrying a spare engine/gearbox and it's EXTRA fuel , spend it on critical components.
Engines are simple to duplicate and it is obvious to the layman that it sounds like a good idea... but what does the layman know? They think a helicopter falls out of the sky on engine failure, they also don't appreciate that engines are not allways a good thing... they explode catch fire and cause other problems .. so doubling the exposure to that is not such a great idea either.
This being shot at nonsense is just that - in Vietnam very few engines were hit despite taking fire often. The performance loss from carrying 2 engines is worse than the payload and performance loss, as Crab quite rightly points out in the Lynx, Sea king and to some extent the THREE ENGINED Merlin. (rubbish reliability/serviceability ratio etc chronically ineffective in Bang/Buck)
Sure good thing the Chinook was there (needing both engines)
When you need both engines to perform in a twin at altitude you are twice as exposed to engine failure than in a single - as well as carrying less life-saving supplies etc
It is not correct to say that higher DA favours 2 engines quite the opposite ref: AS350B3 compare to the almost useless AS355. Engine failure is not the problem in the B3 (and neither was it in the Gazelle - which would have done a better job bang per buck in Afgan surely?)
Crab: as for the single engined 380 you do have a good point and that is a matter of scale; the bigger the aircraft gets and the smaller the proportion of payload that is spent on carrying the spare engines around, the more it makes sense to carry 'redundant' engines around with you, like the payload rational for carrying a defribulator when you have 600 pax, small proportion of weight, high chance of it being useful. In the 380 also it's engine off landing characteristics are not as good as a helicopter. A 160kt arrival in a 300 ton aircraft is not going to be pretty. Landing gently at no airspeed (even at sea) is not such a bad thing.