This study is designed as a Phase II multi-center trial to determine the feasibility of achieving a high rate of event-free survival (EFS) at 2 years post transplant using pre-conditioning hydroxyurea (HU) with a conditioning regimen that consists of a combination of Thymoglobulin/Cyclophosphamide/Fludarabine/Thiotepa with post-grafting high-dose cyclophosphamide in patients with severe SCD who have HLA-haploidentical donors. EFS is defined as survival without a qualifying event. This is a single arm study in which participants will be enrolled into one of two strata. The first stratum will be restricted to children who have stroke and 40 children will be enrolled in this stratum. The second stratum will consist of adult patients with severe sickle cell disease and 40 participants will be enrolled in this stratum.