This program has provided seven new materials having excellent radiation resistance and the requisite mechanical properties. Figure 15 (NASA NSO 71-3465) indicates their excellent condition even after radiation exposure equivalent to the dose expected on a nuclear stage from 10 hours of NERVA operation at full power.

Advanced Nuclear Stage Structural Concepts-As a result of prior years' work, considerable data are now available to designers for choosing suitable components and materials for use in a nuclear-propelled stage. Accordingly, NASA support in this technology area is being minimized, and new work is being started to provide answers to questions generated from mission operations and stage definition studies. In general, the technology goals arise from the criterion of long-life/reusability for a nuclear stage operating in the relatively near-earth regime, i.e., Earth orbit to lunar orbit. More specifically, such technology includes an examination of structural designs and configurations to permit maximum use of the available volume of the space shuttle cargo bay for delivery of nuclear stages to Earth orbit.

Stage Sensors-The long-life, reuse requirement (3 years in Earth orbit) demands new approaches for measuring stage systems performance in critical areas such as propellant leakage, tank stresses, tank punctures from meteorities, and radiation exposure. NASA programs are now underway to improve stage instrumentation, of both the nuclear and non-nuclear variety. Breadboard neutron and gamma ray detectors have been designed. Assembly and test of such detectors are planned for the coming year.

Long Term Cryogenic Storage-The problems of long-term cryogenic storage, including propellant conditioning, insulation systems, etc., has been thoroughly studied by others in NASA-especially the high-performance insulation systems. We have concentrated our efforts on development of a flight-type hydrogen reliquefier which will shortly be ready for ground testing.

The multiple-tank nuclear stage configurations under investigation have a large number of "plumbing" connections compared to a single large tank with the equivalent volume. Cryogenic insulation systems have now been developed to such an extent that these "plumbing" connections have now become the major source of heat transfer into the propellant. An investigation will be underway shortly to analyze this problem, and arrive at a means of reducing the heat input through both the piping connections and structural support members.

ADVANCED PROPULSION CONCEPTS

While NERVA represents a major advancement in space propulsion, nuclear energy can potentially provide even greater improvements in performance of space missions beyond that possible with NERVA. In technical terms, nuclear processes yield the highest known specific energy (energy per mass of reactants) releases. Theoretically, nuclear energy could produce a specific impulse of 1 million seconds; however, there are major technical problems that prohibit the attainment of the ultimate in performance. The existence of this vast potential for nuclear propulsion stimulates programs to extend technology and to explore the feasibility of new concepts for utilizing nuclear energy to the maximum practical extent.

Structural limitations associated with solid fuel elements restrict the specific impulse of the NERVA-type system to 1000 seconds. As we have reported for many years, the nuclear rocket program has conducted research and studies to ascertain the feasibility and performance potential of nuclear fission reactors in which the fuel is in the gaseous state and for which the potential specific impulse is as high as 5000 seconds. These systems are classed as gas-core nuclear rockets. Two concepts are receiving attention at this time, the coaxial-flow and light-bulb reactors (fig. 16, NASA NPO 70-878 and fig. 17, NASA NPO 70-928). The coaxial-flow reactor (fig. 16) consists of a large nearly spherical cavity surrounded by a moderator-reflector system. Vaporized uranium would be centered in the cavity, held there by the action of the hydrogen propellant flowing through the porous walls of the cavity. Heat generated in the fissioning uranium plasma would be transferred to the hydrogen by thermal radiation. The light-bulb reactor (fig. 17) consists of several cylindrical cavities each containing a transparent wall of fused silica used to separate the gaseous uranium from the hydrogen propellant. (In contrast to the coaxial concept, no uranium would be carried away with the hydrogen stream.) Thermal radiation must pass through the transparent wall in order to heat the hydrogen to desirable temperatures.

In both concepts, we are dealing with very difficult questions of feasibility and in past years we have detailed the NASA-sponsored research programs involved in getting answers. While this work is broad in scope it is limited to studies of fundamental problems. Basic studies are conducted to define the emission and absorption of thermal radiation in uranium and hydrogen plasmas at high temperatures and pressures. The use of ultra-fine particles to "seed" hydrogen to

COAXIAL-FLOW GAS-CORE
NUCLEAR ROCKET

make it opaque is the subject of experimental programs and considerable effort is devoted to providing electrically heated plasmas hot enough to explore thermal problems of gas-core reactors. Other research includes fluid mechanics, radiation effects on transparent materials, system analysis, and stability of high-density plasmas.

Progress in these gas-core programs has been encouraging within the context of the limited funding applied in this area. Plasmas have been produced with equivalent radiating temperatures hotter than the Sun. In a small but significant first step, seeded hydrogen has been heated for the first time in a laboratory by thermal radiation alone to temperatures in excess of 3550° F. Small sca e tests of a coaxial-flow reactor configuration have been conducted with inductionheated plasmas, and a porous wall, and vaporization of a solid to simulate formation of a uranium plasma. In the area of systems studies, analyses have shown a coaxial-flow gas core reactor may be capable of achieving a specific impulse in the range of 5000 seconds.

Other concepts besides these gas-core reactors are being studied in the nuclear rocket program. A small NASA supported program is underway to investigate a dust-bed reactor concept and its potential for a high-power density. A unit such as this would be useful in applications where high thrust and moderate engine weight are essential. In addition, we began sponsorship of some research at the Lewis Research Center into that area of plasma physics related to the production of propulsive thrust from a controlled fusion reaction. This step appeared appropriate in view of the progress reported recently in the fusion research programs throughout the world.

Recently reported advancements in pulsed lasers and predictions that fusion plasmas could be produced thereby have generated interest in this form of energy production. LASL has begun studies and research into the means by which a laser-ignited fusion reaction could be applied to propulsion and power generation. Indications are that high specific impulses could be produced at high thrust levels.

It is not possible to say at this time if any of the advanced systems described above can be successfully developed. The technology is extremely difficult. In spite

of these basic uncertainties, we believe that research into the feasibility of these advanced concepts should continue at a modest level of funding. Through support of this research, breakthroughs may be stimulated or we may find the job in some areas to be less difficult than we now expect. Even if this does not happen, the very nature of these advanced propulsion concepts places the research at the frontiers of technology in high-temperature plasmas, radiant heat transfer, lasers, fluid mechanics, materials, etc. The output of this research will be beneficial in many areas even if a new propulsion capability is not produced.

SPACE NUCLEAR ELECTRIC POWER PROGRAM

The overall goal of the space nuclear electric power program is to provide the technology for a limited number of compact power systems capable of meeting future space program needs. Future missions will need electric power ranging from watts to megawatts and lifetimes up to ten years. The systems must be compact, light-in-weight, reliable through all mission phases, and cost effective. To meet these difficult goals requires long lead times.

To carry out our technology program in the low-power range, up to a few kilowatts, we have selected radioisotope energy sources in combination with promising power conversion systems such as thermoelectric and Brayton cycle. In the medium-power range, a few tens of kilowatts, the program considers the uranium zirconium hydride (UZrH) reactor, also with Brayton cycle and thermoelectric generator conversion systems. For high-power systems, i.e., above about 100 Kw, reactors with thermionic and liquid-metal Rankine conversion systems are under consideration.

During the past year, we have completed the planned technology work on the SNAP-8 mercury-turbogenerator system designed for use with the UZrH reactor except for the combined system test with a reactor. Figure 18 (NASA NSO 713475) shows the maximum and total test hours which have been achieved on the major components of this system. As indicated in the letter from Dr. Low to this Committee of October 9, 1970, the primary objectives in this program have been largely achieved, and the mercury Rankine system can be made available within a relatively short time if mission timetables for a space station and base should become advanced from what now appear likely.

MERCURY RANKINE POWER CONVERSION SYSTEM
COMPONENT LIFE DEVELOPMENT

RADIOISOTOPE THERMOELECTRIC GENERATORS

A radioisotope thermoelectric generator (RTG) is a device that converts heat directly into electricity without moving parts by means of the familiar thermocouple effect. Heat, produced by the decay of radioisotope fuel, flows through thermocouple elements made of specialized (semiconductor) material and causes an electrical driving force to arise in them.

In this area, the AEC funding provides for development and delivery of RTG's for flight missions and supports most of the technology work. The NASA funding in the Office of Advanced Research and Technology supports a limited program to develop the technology needed to integrate advanced RTG's with spacecraft in future outer planet missions.

RTG MISSION SUPPORT

During FY 1971, the performance of RTG's in on-going NASA missions were monitored on a continuing basis. The SNAP-19 generators aboard the Nimbus-III weather satellite continued to deliver useful power to the spacecraft after 22 months in orbit. The SNAP-27 RTG, which was placed on the moon by Apollo 12 Astronauts Alan Bean and Charles Conrad on November 19, 1969, continued to de'iver the full power and heat requirements of the Apollo Lunar Surface Experiment Package (ALSEP-1) even after completing its design life of one year's lunar operation. A second SNAP-27 RTG was placed in operation on the lunar surface on February 5, 1971 by the Apollo 14 crew for extensive scientific studies of the Fra Mauro area. Concurrent ground testing of electrically heated SNAP-19 and SNAP-27 generators was continued at the Jet Propulsion Laboratory. These tests provide a basis for comparison with the performance of the units in space and also provide detailed data on the thermoelectric behavior during very long time periods under thermal-vacuum conditions.

In support of the Pioneer F/G and Viking Mars Lander projects, thermoelectric generator experiments were performed at the Jet Propulsion Laboratory (JPL) and at Goddard Space Flight Center employing electrically heated, modified SNAP-19 units built of the same thermoelectric and structural materials. Thermal and electrical experiments were conducted at simulated mission conditions for stowed and deployed generators. Thermal simulation of one of the Pioneer RTG's, when stowed inside the vehicle shroud during the pre-launch and launch phases of the mission is shown in figure 19 (NASA NSO 71-3484).