Guvnor- although I realise that the L1011 is PERFECT, the answer from 1.3Vstall gives your answer.
The Tristars were a panic buy (which also subsidised BA pre-privatisation!) They are great transport aeroplanes (on the military runways which may actually be long/strong enough to take them) but the aerodynamics make them lousy tankers.
They should have been used soley as transports with the VC10 tranpsorts being modified to tanker spec in the mid 80s when the deathstarts arrived.