Pellets injected into a fusion plasma (for refueling as well as disruption mitigation) are accelerated by the so-called pellet rocket effect. The non-uniform plasma heats the pellet ablation cloud asymmetrically, producing pressure-driven, rocket-like propulsion of the pellet. This effect was shown in experiments to significantly modify the pellet trajectory, and projections for reactor scale devices indicate that it may severely limit the effectiveness of pellet injection methods. We present a semi-analytical model of this process by perturbing a spherically symmetric ablation model, accounting for asymmetries stemming both from plasma parameter gradients [1] and an asymmetric plasmoid shielding [2] caused by the drift of the ionized pellet cloud [3]. This model is then used to model pellet trajectories in scenarios representative of present medium sized tokamaks and reactor grade devices. For high temperature, reactor relevant scenarios, we find a wide range of initial pellet sizes and speeds where the rocket effect severely limits the penetration depth of the pellet. In these cases, with relatively large and slow pellets, the plasma parameter profile variations dominate the rocket effect. On the other hand, for small and fast pellets, plasmoid shielding induced asymmetries dominate, and the total rocket effect is also less pronounced. [1] N. Guth *et al*, *Phys. Rev. Lett.* **134** 035101 (2025) [2] N. Guth *et al*, submitted to *J. Plasma Phys.* (2025) [3] O. Vallhagen *et al*, *J. Plasma Phys.* **89** 905890306 (2023).