a shroud closely surrounding a high thermal inertia rotor is provided with a support structure having low thermal inertia characteristics . the support structure is selectively exposed to different temperature mediums during transient operation by way of a thermal actuated valve so as to cause a rapid growth thereof during periods of engine acceleration and a slow shrinkage thereof during periods of deceleration of the engine . in this manner the clearance relationship between the shroud and enclosed rotor is maintained at a minimum during periods of steady - state and transitional operation to thereby increase the efficiency of the combination .