Atmospheric entry of Mars-return nuclear-powered vehicles due to accidental termination of operations

Abstract

Atmospheric E n t r y of M a r s -R e t u r n Nuclear-Powered Vehicles Due to Accidental Termination of Operations A b s t r a c tThe entry of nuclear reactors into Earth's atmosphere resulting from an accidental or inadvertent abort of a space vehicle powered by nuclear-thermal rockets is investigated. The study is made for a typical piloted Mars mission vehicle incapacitated by an accident or malfunction during the Earth-arrival phase of the Marcretnrn journey due to simultaneous, multiple failures of its component systems. A single accidentfabort scenario resulting in three entry possibilities is considered for a nominal hyperbolic in-bound approach velocity of 8 kmfsec.The most severe case involving a direct entry (Le., flightpath angle = -90') is then analyzed over a broad range of approach velocities extending to 12 kmfsec to include sprint-type missions. Calculations include aerodynamic pressures, deceleration g-loads, ground-impact velocities, convectivefradiative heat transfer rates, total heat loads, and equilibrium wall temperatures. The results indicate that the severe surface heating, stagnation pressures, and g-loads are greater than 150 kW/cmz, 300 atm, and 800-g, respectively. The wall heat transfer rate exceeds the value that can be accommodated by a carbon heat-' 1 9 shield through radiation equilibrium prior to sublimation at 5500 K. These conditions are beyond our previous experience in crew safety, structural design, and thermalprotection. New safety/ahort strategies and technology development are required to ensure crew survival and survival, containment, and retrieval of the radioactive components.Projected frontal area of NTP or reactorlengine, m2. Aeroassisted space transfer vehicle. Drag coefficient of NTP or reactorfengine. Hyperbolic approach velocity squared, (km/sec)2. Earth crew capture vehicle. Acceleration due to gravity (9.8 m/sec2). Low Earth orbit. Mars excursion vehicle. = Mass of NTP or reactorfengine, kg. NTP = Nuclear-thermal propulsion system (radiation shield, engines, reactors). RTG = Radioisotope thermoelectric generator. STS = Space Transportation System (Space Shuttle). T = Time measured from a specified reference point, sec. TEI = Trans-Earth injection into hyperbolic inbound transfer orbit at Mars departure. TMI = Trans-Mars injection into hyperbolic outbound transfer orbit at Earth departure. T,d = NTP hnrn time for start of Vd, sec. Vd = Velocity dispersion of NTP at break-up of TEI vehicle, mfsec. V, = Entry velocity, kmfsec. Vhuo = Hyperbolic approach velocity of TEI vehicle, ". kmfsec.p = Ballistic coefficient (m/CdA), kgfm'. ye = Entry flight-path angle, deg (negative downward).