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Context. The sizes of dust grains in the interstellar medium follows a distribution where most of the dust mass is in smaller grains. However, the re-distribution from larger grains towards smaller sizes especially by means of rotational disruption is poorly understood. Aims. We aim to study the dynamics of porous grain aggregates under accelerated ration. Especially, we determine the deformation of the grains and the maximal angular velocity up to the rotational disruption event by caused by centrifugal forces. Methods. We pre-calculate porous grain aggregate my means of ballistic aggregation analogous to the interstellar dust as input for subsequent numerical simulations. In detail, we perform three-dimensional N-body simulations mimicking the radiative torque spinup process up to the point where the grain aggregates become rotationally disrupted. Results. Our simulations results are in agreement with theoretical models predicting a characteristic angular velocity ω disr of the order of 10 8 − 10 9 rad s −1 , where grains become rotationally disrupted. In contrast to the theoretical predictions, we show that for large porous grain aggregates ( 300 nm) ω disr does not strictly decline but reaches a lower asymptotic value. Hence, such grains can withstand an accelerated ration more efficiently up to a factor of 10 because the displacement of mass by centrifugal forces and the subsequent mechanical deformation supports the build up of new connections within the aggregate. Furthermore, we report that the rapid rotation of grains deforms an ensemble with initially 50:50 prolate and oblate shapes, respectively, preferentially into oblate shapes. Finally, we present a best fit formula to predict the average rotational disruption of an ensemble of porous dust aggregates dependent on internal grain structure, total number of monomers, and applied material properties.

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