Artificial gravity vehicle design option for NASA's human Mars mission using 'bimodal' NTR propulsion

Abstract

Recent human Mars exploration studies at NASA have focused on a split mission approach involving predeployment of surface and orbital cargo elements followed by piloted missions with long surface stays (-500 days) and “l-way” transit times of -6 to 7 months. In the event an aborted landing or major surface system failure forces an early return to the crew transfer vehicle (CTV), astronauts could spend the entire mission duration (-900 days) in a weightless environment. An artificial gravity CTV design capable of countering the potentially debilitating physiological effects of “zero gravity” is described which uses “bimodal” nuclear thermal rocket (NTR) propulsion. With its high specific impulse (Isp -850-l 000 s), attractive engine thrust-to-weight ratio (-3-i 0) and demonstrated feasibility, the NTR is the most promising propulsion technology for future human exploration missions to the Moon, Mars and near Earth asteroids. Because only a minuscule amount of enriched uranium235 fuel is consumed in a NTR during the primary propulsion maneuvers of a typical Mars mission, engines configured for both propulsive thrust and modest power generation (referred to as “bimodal” operation) provide the basis for a robust, “power-rich” stage enabling a propulsive Mars capture capability for the CTV. A common “bimodal” NTR (BNTR) “core” stage powered by three -15 thousand pounds force (klbf) BNTRs supplies 50 kWe of total electrical power for crew life support and an active refrigeration system enabling long term, “zero-boiloff” liquid hydrogen (LH2) storage. On the piloted CTV, the bimodal NTR core stage is connected to the inflatable -----------------------------------------------------------------------*Ph.D./Nuclear Engineering, Senior Member AIAA ‘*Aerospace Engineer, Member AIAA “TransHab” crew module via an innovative, spinelike “saddle truss” (approximately 22 meters in length) which is open underneath to allow easy jettisoning of the “in-line” LH2 propellant tank following the trans-Mars injection (TMI) burn. The CTV then initiates vehicle rotation at o 4 revolutions per minute (rpm) to provide the TransHab crew with a Mars gravity field (-0.38 g E) during the outbound transit. A higher rotation rate (w 6 rpm) can provide -0.8 gE on the return leg to help reacclimate the crew to Earth’s gravity after their -500 day stay at Mars. In addition to supplying artificial gravity and abundant power for the crew, a Mars architecture using BNTR transfer vehicles also has a lower total launch mass, fewer transportation system elements and simpler mission operations than competing “non-nuclear” chemical and solar electric propulsion (SEP) options. INTRODUCTION AND BACKGROUND Over the last 3 years, NASA’s intercenter Mars Exploration Study Team has been evaluating a split cargo / piloted mission approach for sending humans to Mars in the 2014 timeframe. Payload masses have continued to be refined and updatedl, and a variety of space transportation technology options have been examined*,s. In the FY98 reference mission profile, the crew traveled to Mars under “zero gravity” conditions and landed on its surface in a common transit / habitat module integrated into an aerobraked lander configuration. Two cargo flights preceded the piloted mission and were used to predeploy surface assets and a separate transfer stage for returning the crew to Copyright