ZONA proposes a Unified Gas Kinetic Scheme (UGKS) to cover the full Knudsen number range from the continuum flow to free molecular flow that can simultaneously exist in the jet nozzle flow of the spacecraft. In the UGKS solver, the BGK model of Boltzmann equation is solved directly with finite volume method based on the Discrete Ordinate Method (DOM). In the UGKS computation the convective terms and collision term is computed in one step with a multi-scale scheme without splitting. The UGKS also closely couples the update of macroscopic conservative variables with the update of microscopic gas distribution functions in one step. At the continuum flow regime, the UGKS recovers the Navier-Stokes solutions with much larger time step than regular direct BGK solver where UGKS essentially becomes a shock-capture scheme. In the rarefied flow regime, UGKS recovers the direct BGK method up to the free molecular flow. With the UGKS, the jet-like flow can be simulated with a unique flow solver. Furthermore, a two-species chemical reaction will be incorporated in UGKS. The UGKS will be applied to various test cases whose results will be validated with others' computational results and available experimental data to verify its accuracy and computational efficiency.