Roger X. Lenard

Small fission power systems for Mars

A Mars surface power system configuration with an output power of 3 kWe and a system mass of 775 kg is described. It consists of a heatpipe-cooled reactor with UN fuel coupled to a Stirling engine with a fixed conical radiator driven by loop heat pipes. Key to achieving this low mass is the use of a highly radiation-resistant multiplexer for monitoring and controlling the reactor, as well as radiation resistant generators and motors. Also key is the judicious placement of shields to prevent radiation scattered from the Martian surface and air from damaging the reactor controls. Several alternate configurations also are briefly looked at, including a moderated reactor with UZrH fuel and a reactor using 233U instead of 235U. The moderated reactor system has essentially the same mass as the baseline unmoderated UN system and yields the same radiation shielding requirements. The 233U reactor is significantly smaller and yields a system mass about 228 kg lighter than with 235U, but part of this weight reductio...

Small fission power systems for NEP

Two nuclear electric propulsion (NEP) power system configurations are presented, each with an output power of 50 kWe and a system mass of about 2500 kg. Both consist of a reactor coupled to a recuperated Brayton power conversion system with a fixed conical radiator driven by loop heat pipes. In one system the reactor is gas-cooled with the gas directly driving the Brayton power conversion system. In the other the reactor is heatpipe-cooled with a heat exchanger between the reactor and the Brayton system. Two variations are described briefly with powers of 100 k We and 150 kWe. The mass scales approximately with the square root of the power.

A gas-cooled reactor surface power system

A human outpost on Mars requires plentiful power to assure survival of the astronauts. Anywhere from 50 to 500 kW of electric power (kWe) will be needed, depending on the number of astronauts, level of scientific activity, and life-cycle closure desired. This paper describes a 250-kWe power system based on a gas-cooled nuclear reactor with a recuperated closed Brayton cycle conversion system. The design draws upon the extensive data and engineering experience developed under the various high-temperature gas cooled reactor programs and under the SP-100 program. The reactor core is similar in power and size to the research reactors found on numerous university campuses. The fuel is uranium nitride clad in Nb1%Zr, which has been extensively tested under the SP-100 program. The fuel rods are arranged in a hexagonal array within a BeO block. The BeO softens the spectrum, allowing better use of the fuel and stabilizing the geometry against deformation during impact or other loadings. The system has a negative t...

Recent Progress in Silicon‐Based MEMS Field Emission Thrusters

The Indium Field Emission Thruster (In‐FET) is a highly characterized and space‐proven device based on space‐qualified liquid metal ion sources. There is also extensive experience with liquid metal ion sources for high‐brightness semiconductor fabrications and inspection Like gridded ion engines, In‐FETs efficiently accelerate ions through a series of high voltage electrodes. Instead of a plasma discharge to generate ions, which generates a mixture of singly and doubly charged ions as well as neutrals, indium metal is melted (157°C) and fed to the tip of a capillary tube where very high local electric fields perform more‐efficient field emission ionization, providing nearly 100% singly charged species. In‐FETs do not have the associated losses or lifetime concerns of a magnetically confined discharge and hollow cathode in ion thrusters. For In‐FETs, propellant efficiencies ∼100% stipulate single‐emitter currents ⩽10μA, perhaps as low as 5μA of current. This low emitter current results in ⩽0.5 W/emitter. C...

An Ultra‐Lightweight, High Performance Carbon‐Carbon Space Radiator

Propulsion systems for deep space exploration that rely on nuclear energy require innovative advancements in radiator technology, both materials and construction technique, to meet the demands associated with high rejection temperatures. A five fold reduction in radiator specific mass is achievable and will be needed to meet the demanding challenge of space exploration. Our development of a carbon‐carbon (C‐C) based radiator design unaffected by long term exposure to high temperature and radiation has wide ranging application, for both small and large power conversion systems. Our results stem from a NASA SBIR program focused on demonstrating thermal performance in a high temperature carbon‐carbon (C‐C) radiator configured with titanium water‐ heat pipes, using approximately 500 K water for the working fluid. However, joining strategy and material choices employed are appropriate for very high temperature alkali fluids. Distinct design advantages of carbon‐carbon material are its low density, unlimited li...

Proposed Follow -on Mini -Mag Orion Pulsed Propulsion Concept

This program led to considerable analytical and experimental ef fort that provided tantalizing, but inconclusive compression results. This paper discusses our proposed follow -on effort. This follow -on effort would provide more analytical detail while exploring the benefit of a smaller fission package and higher yield fraction. The yield fraction (estimated at 10% of the fissionable material from MCNP results) could be increased with a deuterium -tritium fusion boost. We propose to investigate how this approach would improve the system by increasing yield with a small er package and allow for a greater amount of thrust for a given quantity of fissionable material. We will investigate how the magnetic bottle will change under these conditions as well. Finally, we will discuss how this may affect the Z -drive mechanism.

Technical aspects of a commercially viable in‐space transportation system

An in‐depth study and evaluation of a commercially‐viable in‐space transportation system was conducted between the Sandia National Laboratories and the INTRASPACE Corporation. Historical mission profiles were evaluated and a series of missions potentially profitable for a first‐generation space tug were identified. The subset of missions from the available manifest was 60%. A survey of available space tug technologies was performed with several requirements: 1) Technology aggregates must be demonstrated; 2) Systems must be represented by a substantial industrial base; 3) The integrated system concept must be of tractable risk for a commercial venture; and 4) The consequential system must be commercially profitable. A definite mission manifest and technical solution emerged which represented a commercially profitable system. Alternative technologies were evaluated including conventional propulsion technologies, solar electric and solar thermal technologies. None of the alternatives were profitable based on...

A TRICARBIDE FOAM FUEL MATRIX FOR NUCLEAR THERMAL PROPULSION

A team from Ultramet Inc., Sandia National Laboratories and the University of Florida has been developing a new high temperature, tricarbide fuel matrix consisting of ZrC, NbC and UC using an open-cell reticulated foam skeleton. The new fuel matrix is the product of a NASA STTR project funded through Marshall Space Flight Center. The new fuel is envisioned for use in nuclear thermal propulsion systems, bi-modal reactors and terrestrial high temperature gas reactors and builds on the tricarbide fuel research in the former Soviet Union. The new fuel is mechanically robust and very efficient given its extremely high surface area, higher melting point, minimal thermal stresses, and much reduced pressure drop compared to conventional fuel types. The matrix is anticipated to operate at temperatures as high as 3000 K with minimal hydrogen erosion. The foam is an engineered material in which the porosity, size and thermal conductivity of the ligaments can be controlled independently to meet specific requirements. In this article, we review the vapor phase alloying of the tricarbide to produce a homogeneous solid mixture of the constituents using TaC as a UC surrogate with the option of additional diffusion barrier layers as required. A credible reactor design and results of a new MP-CFD modeling capability using detailed simulation of the porous media are presented along with new FEM CFD simulations of a complete fuel element and core assembly. A new CVI reactor for the deposition of the tricarbide containing UC is now operational at the University of Florida. This article presents our recent experiences with UC depositions and results from hot hydrogen testing of these materials at 3000 K.

Technology needs for asteroid and comet trajectory deflection of a Tunguska-sized object using fission propulsion

Recent studies of Near Earth Object Interceptions (DOC, 1993) have shown that impact of the Earth by a civilization-killing sized asteroid are rare. However, some have publicly stated that impact of the Earth by a smaller asteroid, ∼100m diameter, such as the one impacting near Tunguska, Siberia, in 1908, occur approximately twice per century (Young, 1999). While such objects will not necessarily result in widespread societal dislocations, such objects are sufficiently energetic to destroy a very large city, such as Los Angeles or New York. Consequential earthquakes and fault disruptions can result in further damage and loss of life. Displacing the trajectory of a Tunguska-sized asteroid, estimated to be <100m in diameter so that it will convincingly miss the Earth is not a trivial venture. If the asteroid is stony in nature, in composition, it will weigh 20–30 million kg. Depending upon when and where the asteroid is discovered, a velocity increment of ∼10cm/s is necessary to impart to the asteroid in order for it to convincingly miss the cis-lunar system. The technology requirements for system a system, based on fission propulsion are examined, and a strawman concept is developed.