A Crewed Mission to Apophis Using a Hybrid Bimodal Nuclear Thermal Electric Propulsion (BNTEP) System

Abstract

A BNTEP system is a dual propellant, hybrid propulsion concept that utilizes Bimodal Nuclear Thermal Rocket (BNTR) propulsion during high thrust operations, providing 10's of kilo-Newtons of thrust per engine at a high specific impulse (I sp ) of 900 s, and an Electric Propulsion (EP) system during low thrust operations at even higher I sp of around 3000 s. Electrical power for the EP system is provided by the BNTR engines in combination with a Brayton Power Conversion (BPC) closed loop system, which can provide electrical power on the order of 100's of kW e . High thrust BNTR operation uses liquid hydrogen (LH 2 ) as reactor coolant propellant expelled out a nozzle, while low thrust EP uses high pressure xenon expelled by an electric grid. By utilizing an optimized combination of low and high thrust propulsion, significant mass savings over a conventional NTR vehicle can be realized. Low thrust mission events, such as midcourse corrections (MCC), tank settling burns, some reaction control system (RCS) burns, and even a small portion at the end of the departure burn can be performed with EP. Crewed and robotic deep space missions to a near Earth asteroid (NEA) are best suited for this hybrid propulsion approach. For these mission scenarios, the Earth return ΔV is typically small enough that EP alone is sufficient. A crewed mission to the NEA Apophis in the year 2028 with an expendable BNTEP transfer vehicle is presented. Assembly operations, launch element masses, and other key characteristics of the vehicle are described. A comparison with a conventional NTR vehicle performing the same mission is also provided. Finally, reusability of the BNTEP transfer vehicle is explored.Nomenclature D x L = diameter by length ΔV = delta-velocity or change in velocity He-Xe = Helium-Xenon klb f = thrust (1000 pounds force) km/m/kg = kilometers/meters/kilogram kW e /kJ = kilowatts electrical/kilojoules NASA = National Aeronautics and Space Administration psi = pounds per square inch s = seconds t = metric ton (1 t = 1000 kg) UO 2 -W = Uranium dioxide-Tungsten UC-ZrC = Uranium carbide-Zirconium carbide composite