Stanley K. Borowski

Artificial Gravity Vehicle Design Option for NASA's Human Mars Mission Using "Bimodal" NTR Propulsion

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

2001: A Space Odyssey Revisited—The Feasibility of 24 Hour Commuter Flights to the Moon Using NTR Propulsion with LUNOX Afterburners

The prospects for 24 hour commuter flights to the Moon, similar to that portrayed in 2001: A Space Odyssey but on a more Spartan scale, are examined using two near term, high leverage technologies: liquid oxygen (LOX)-augmented nuclear thermal rocket (NTR) propulsion and lunar-derived oxygen (LUNOX) production. Iron-rich volcanic glass, or orange soil, discovered during the Apollo 17 mission to Taurus-Littrow, has produced a 4 percent oxygen yield in recent NASA experiments using hydrogen reduction. LUNOX development and utilization would eliminate the need to transport oxygen supplies from Earth and is expected to dramatically reduce the size, cost and complexity of space transportation systems. The LOX-augmented NTR concept (LANTR) exploits the high performance capability of the conventional liquid hydrogen (LH2)-cooled NTR and the mission leverage provided by LUNOX in a unique way. LANTR utilizes the large divergent section of its nozzle as an afterburner into which oxygen is injected and supersonically combusted with nuclear preheated hydrogen emerging from the engines choked sonic throat, essentially scramjet propulsion in reverse. By varying the oxygen-to-hydrogen mixture ratio, the LANTR engine can operate over a wide range of thrust and specific impulse (Isp) values while the reactor core power level remains relatively constant. The thrust augmentation feature of LANTR means that big engine performance can be obtained using smaller, more affordable, easier to test NTR engines. The use of high-density LOX in place of low density LH2 also reduces hydrogen mass and tank volume resulting in smaller space vehicles. An implementation strategy and evolutionary lunar mission architecture is outlined which requires only Shuttle C or in-line Shuttle-derived launch vehicles, and utilizes conventional NTR-powered lunar transfer vehicles (LTVs), operating in an expendable mode initially, to maximize delivered surface payload on each mission. The increased payload is dedicated to installing modular LUNOX production units with the intent of supplying LUNOX to lunar landing vehicles (LLVs) and then LTVs at the earliest possible opportunity. Once LUNOX becomes available in low lunar orbit (LLO), monopropellant NTRs would be outfitted with an oxygen propellant module, feed system and afterburner nozzle for bipropellant operation. Transition to a reusable mission architecture now occurs with smaller, LANTR-powered LTVs delivering ~400% more payload on each piloted round trip mission than earlier expendable all LH2 NTR systems. As initial lunar outposts grow to eventual lunar settlements and LUNOX production capacity increases, the LANTR concept can enable a rapid commuter shuttle capable of 24 hour one way trips to and from the Moon. A vast deposit of iron-rich volcanic glass beads identified at just one candidate site located at the southeastern edge of Mare Serenitatis could supply sufficient LUNOX to support daily commuter flights to the Moon for the next 9000 years!

Realizing "2001: A Space Odyssey": Piloted Spherical Torus Nuclear Fusion Propulsion

A conceptual vehicle design enabling fast, piloted outer solar system travel was created predicated on a small aspect ratio spherical torus nuclear fusion reactor. The initial requirements were satisfied by the vehicle concept, which could deliver a 172 mt crew payload from Earth to Jupiter rendezvous in 118 days, with an initial mass in low Earth orbit of 1,690 mt. Engineering conceptual design, analysis, and assessment was performed on all major systems including artificial gravity payload, central truss, nuclear fusion reactor, power conversion, magnetic nozzle, fast wave plasma heating, tankage, fuel pellet injector, startup/re-start fission reactor and battery bank, refrigeration, reaction control, communications, mission design, and space operations. Detailed fusion reactor design included analysis of plasma characteristics, power balance/utilization, first wall, toroidal field coils, heat transfer, and neutron/x-ray radiation. Technical comparisons are made between the vehicle concept and the interplanetary spacecraft depicted in the motion picture 2001: A Space Odyssey.

Nuclear Thermal Propulsion (NTP): A proven growth technology for human NEO/Mars exploration missions

The nuclear thermal rocket (NTR) represents the next “evolutionary step” in high performance rocket propulsion. Unlike conventional chemical rockets that produce their energy through combustion, the NTR derives its energy from fission of Uranium-235 atoms contained within fuel elements that comprise the engines reactor core. Using an “expander” cycle for turbopump drive power, hydrogen propellant is raised to a high pressure and pumped through coolant channels in the fuel elements where it is superheated then expanded out a supersonic nozzle to generate high thrust. By using hydrogen for both the reactor coolant and propellant, the NTR can achieve specific impulse (Isp) values of ~900 seconds (s) or more - twice that of todays best chemical rockets. From 1955-1972, twenty rocket reactors were designed, built and ground tested in the Rover and NERVA (Nuclear Engine for Rocket Vehicle Applications) programs. These programs demonstrated: (1) high temperature carbide-based nuclear fuels; (2) a wide range of thrust levels; (3) sustained engine operation; (4) accumulated lifetime at full power; and (5) restart capability - all the requirements needed for a human Mars mission. Ceramic metal “cermet” fuel was pursued as well, as a backup option. The NTR also has significant “evolution and growth” capability. Configured as a “bimodal” system, it can generate its own electrical power to support spacecraft operational needs. Adding an oxygen “afterburner” nozzle introduces a variable thrust and Isp capability and allows bipropellant operation. In NASAs recent Mars Design Reference Architecture (DRA) 5.0 study, the NTR was selected as the preferred propulsion option because of its proven technology, higher performance, lower launch mass, versatile vehicle design, simple assembly, and growth potential. In contrast to other advanced propulsion options, no large technology scale-ups are required for NTP either. In fact, the smallest engine tested during the Rover program - the 25,000 lbf (25 klbf) “Pewee” engine is sufficient when used in a clustered engine arrangement. The “Copernicus” crewed spacecraft design developed in DRA 5.0 has significant capability and a human exploration strategy is outlined here that uses Copernicus and its key components for precursor near Earth object (NEO) and Mars orbital missions prior to a Mars landing mission. The paper also discusses NASAs current activities and future plans for NTP development that include system-level Technology Demonstrations - specifically ground testing a small, scalable NTR by 2020, with a flight test shortly thereafter.

High Power MPD Nuclear Electric Propulsion (NEP) for Artificial Gravity HOPE Missions to Callisto

The following paper documents the results of a one‐year multi‐center NASA study on the prospect of sending humans to Jupiter’s moon, Callisto, using an all Nuclear Electric Propulsion (NEP) space transportation system architecture with magnetoplasmadynamic (MPD) thrusters. The fission reactor system utilizes high temperature uranium dioxide (UO2) in tungsten (W) metal matrix “cermet” fuel and electricity is generated using advanced dynamic Brayton power conversion technology. The mission timeframe assumes on‐going human Moon and Mars missions and existing space infrastructure to support launch of cargo and crewed spacecraft to Jupiter in 2041 and 2045, respectively.

NERVA-Derived Concept for a Bimodal Nuclear Thermal Rocket

The Nuclear Thermal Rocket is an enabling technology for human exploration missions. The “bimodal” NTR (BNTR) provides a novel approach to meeting both propulsion and power requirements of future manned and robotic missions. The purpose of this study was to evaluate tie‐tube cooling configurations, NTR performance, Brayton cycle performance, and LOX‐Augmented NTR (LANTR) feasibility to arrive at a point of departure BNTR configuration for subsequent system definition.

“Bimodal” Nuclear Thermal Rocket (BNTR) Propulsion for an Artificial Gravity HOPE Mission to Callisto

This paper summarizes the results of a year long, multi‐center NASA study which examined the viability of nuclear fission propulsion systems for Human Outer Planet Exploration (HOPE). The HOPE mission assumes a crew of six is sent to Callisto. Jupiter’s outermost large moon, to establish a surface base and propellant production facility. The Asgard asteroid formation, a region potentially rich in water‐ice, is selected as the landing site. High thrust BNTR propulsion is used to transport the crew from the Earth‐Moon L1 staging node to Callisto then back to Earth in less than 5 years. Cargo and LH2 “return” propellant for the piloted Callisto transfer vehicle (PCTV) is pre‐deployed at the moon (before the crew’s departure) using low thrust, high power, nuclear electric propulsion (NEP) cargo and tanker vehicles powered by hydrogen magnetoplasmadynamic (MPD) thrusters. The PCTV is powered by three 25 klbf BNTR engines which also produce 50 kWe of power for crew life support and spacecraft operational needs....

A spherical torus nuclear fusion reactor space propulsion vehicle concept for fast interplanetary travel

A conceptual vehicle design enabling fast outer solar system travel was produced predicated on a small aspect ratio spherical torus nuclear fusion reactor. Initial requirements were for a human mission to Saturn with a>5% payload mass fraction and a one way trip time of less than one year. Analysis revealed that the vehicle could deliver a 108 mt crew habitat payload to Saturn rendezvous in 235 days, with an initial mass in low Earth orbit of 2,941 mt. Engineering conceptual design, analysis, and assessment was performed on all major systems including payload, central truss, nuclear reactor (including diverter and fuel injector), power conversion (including turbine, compressor, alternator, radiator, recuperator, and conditioning), magnetic nozzle, neutral beam injector, tankage, start/re-start reactor and battery, refrigeration, communications, reaction control, and in-space operations. Detailed assessment was done on reactor operations, including plasma characteristics, power balance, and component design.A conceptual vehicle design enabling fast outer solar system travel was produced predicated on a small aspect ratio spherical torus nuclear fusion reactor. Initial requirements were for a human mission to Saturn with a>5% payload mass fraction and a one way trip time of less than one year. Analysis revealed that the vehicle could deliver a 108 mt crew habitat payload to Saturn rendezvous in 235 days, with an initial mass in low Earth orbit of 2,941 mt. Engineering conceptual design, analysis, and assessment was performed on all major systems including payload, central truss, nuclear reactor (including diverter and fuel injector), power conversion (including turbine, compressor, alternator, radiator, recuperator, and conditioning), magnetic nozzle, neutral beam injector, tankage, start/re-start reactor and battery, refrigeration, communications, reaction control, and in-space operations. Detailed assessment was done on reactor operations, including plasma characteristics, power balance, and component design.

Modular Growth NTR Space Transportation System for Future NASA Human Lunar, NEA and Mars Exploration Missions

The nuclear thermal rocket (NTR) is a proven, high thrust propulsion technology that has twice the specific impulse (Isp ~900 s) of today’s best chemical rockets. During the Rover and NERVA (Nuclear Engine for Rocket Vehicle Applications) programs, twenty rocket reactors were designed, built and ground tested. These tests demonstrated: (1) a wide range of thrust; (2) high temperature carbide-based nuclear fuel; (3) sustained engine operation; (4) accumulated lifetime; and (5) restart capability – everything required for affordable human missions beyond LEO. In NASA’s recent Mars Design Reference Architecture (DRA) 5.0 study, the NTR was selected as the preferred propulsion option because of its proven technology, higher performance, lower IMLEO, versatile vehicle design, and growth potential. Furthermore, the NTR requires no large technology scale-ups since the smallest engine tested during the Rover program – the 25 klbf “Pewee” engine is sufficient for human Mars missions when used in a clustered engine configuration. The “Copernicus” crewed Mars transfer vehicle developed for DRA 5.0 was an expendable design sized for fastconjunction, long surface stay Mars missions. It therefore has significant propellant capacity allowing a reusable “1-year” round trip human mission to a large, high energy near Earth asteroid (NEA) like Apophis in 2028. Using a “split mission” approach, Copernicus and its two key elements – a common propulsion stage and integrated “saddle truss” and LH2 drop tank assembly – configured as an Earth Return Vehicle / propellant tanker, can also support a short round trip (~18 month) / short orbital stay (60 days) Mars reconnaissance mission in the early 2030’s before a landing is attempted. The same short stay orbital mission can be performed with an “all-up” vehicle by adding an “in-line” LH2 tank to Copernicus to supply the extra propellant needed for this higher energy, opposition-class mission. To transition to a reusable Mars architecture, Copernicus’ saddle truss / drop tank assembly is replaced by an in-line tank and “star truss” assembly with paired modular drop tanks to further increase the vehicle’s propellant capacity. Shorter “1-way” transit time fast-conjunction Mars missions are another possibility using this vehicle configuration but, as with reusability, increased launch mass is required. “Scaled down” versions of Copernicus (sized to a SLS lift capability of ~70 t – 100 t) can be developed initially allowing reusable lunar cargo delivery and crewed landing missions, easy NEA missions (e.g., 2000 SG344 also in 2028) or an expendable mission to Apophis. Mission scenario descriptions, key vehicle features and operational characteristics are provided along with a brief discussion of NASA’s current activities and its “pre-decisional” plans for future NTR development.

Human Exploration and Settlement of the Moon Using LUNOX-Augmented NTR Propulsion

An innovative trimodal nuclear thermal rocket (NTR) concept is described which combines conventional liquid hydrogen (LH2)‐cooled NTR, Brayton cycle power generation and supersonic combustion ramjet (scramjet) technologies. Known as the liquid oxygen (LOS)‐augmented NTR (LANTR), this concept utilizes the large divergent section of the NTR nozzle as an ‘‘afterburner’’ into which LOX is injected and supersonically combusted with nuclear preheated hydrogen emerging from the LANTR’s choked sonic throat—‘‘scramjet propulsion in reverse.’’ By varying the oxygen‐to‐hydrogen mixture ratio (MR), the LANTR can operate over a wide range of thrust and specific impulse (Isp) values while the reactor core power level remains relatively constant. As the MR varies from zero to seven, the thrust‐to‐weight ratio for a 15 thousand pound force (klbf) NTR increases by ∼440%—from 3 to 13—while the Isp decreases by only ∼45%—from 940 to 515 seconds. This thrust augmentation feature of the LANTR means that ‘‘big engine’’ perform...