Electrophysics

Miniature Pressure Transducer.-During research on the effect of pressure on the energy levels of semiconductors used in electronic communication systems, it was found that a gallium antimonide crystal had outstanding characteristics as a transducer for converting pressure variations into corresponding electrical signals. As a result, a device was fabricated which was small enough to be inserted into the artery of a dog to record the cardiovascular pressure. It is anticipated that this device can also be used to sense the very small changes in acceleration and pressure in space flight.

Structure of the Arc Discharge.—Arc discharges are essential subsystems of magnetohydrodynamic type gaseous plasma power generators and electrical propulsion engines both of which have great potential for spacecraft use. In research on the mechanisms by which the flowing working gas interacts with the arc discharge, it was found that the arc discharge in a cross-flow configuration consists of a pair of counter-rotating plasma vortices about which are both a viscous and a thermal boundary layer. The working gas receives its momentum through the viscous boundary layer, and the thermal boundary transmits the heat. From this new knowledge of arc-gas interactions, it should be possible to improve designs for plasma generators and accelerators.

Through the nuclear rocket program, work continued to move toward the achievement of the four key goals, with the primary objective being the development of the 75,000-pound-thrust NERVA engine. Major components of the 35-50KW SNAP-8 power conversion system were further tested, nuclear electric power research and technology efforts received continuing stress, and work in the area of space power technology pressed for significant advances in developing space electric power systems.

The Nuclear Rocket Program

The nuclear rocket program, a joint endeavor of the National Aeronautics and Space Administration and the Atomic Energy Commission, is intended to provide a significant increase in propulsion capability for future space activities. The program has four key goals: to provide the basic technology for nuclear propulsion systems, to develop a NERVA engine of approximately 75,000 pounds thrust for flight applications, to extend the technology of graphite reactors and engine system components as a basis for improving nuclear rocket performance, and to investigate advanced concepts.

During this period, the program made substantial progress toward completing the technology goals. Engineers and scientists completed the tests initiated in the latter part of 1967 on the cold-flow ground

experimental engine (XECF) in Engine Test Stand No. 1 (reported in Eighteenth Semiannual Report). They also completed the hot tests of the Phoebus-2A reactor, the most powerful nuclear rocket reactor ever operated, at the Nuclear Rocket Development Station in Nevada (NRDS). In addition, significant advances in fuel-element technology were made and demonstrated in laboratory tests. These advances indicate that much longer operating durations can be achieved.

Preliminary work on the design of the NERVA engine continued. This work makes use of the output of the NERVA technology program. Also continued were efforts to define facility modifications required to test the NERVA reactor and engine (using the existing facilities at NRDS). Test Cell "C" is to be used for all reactor testing. NERVA engine testing is to be accomplished in Engine Test Stand No. 1 now being activated for the first "hot" tests of the NERVA ground-experimental technology engine.

Status of Reactor Technology

Most of the goals established for the reactor technology phase of the program were achieved when the NRX-A6 reactor completed its full-power endurance test (December 1967). Additional goals included exploring increased specific impulse and power density and scaling up the reactor system to higher power levels.

With the test of the Phoebus-2A reactor and the significant advances made in fuel-element technology, all of the remaining reactor technology goals were satisfied.

Phoebus-2A Reactor.-The Phoebus-2A reactor (Fig. 5-1) was the twelfth nuclear rocket reactor to be tested at NRDS. Designed and developed by the Los Alamos Scientific Latoratory, the Phoebus2A also was the most powerful nuclear rocket reactor ever to be tested at NRDS.

The Phoebus-2A test program was originally planned to demonstrate high-power reactor performance for direct application to the 200,000 pound thrust NERVA-2 engine. When the proposed NERVA engine thrust level was reduced to 75,000 pounds, the Phoebus-2A no longer had this purpose. It remained a useful tool, however, for demonstrating higher power density and for obtaining data on reactor design concepts being considered for use in the 75,000 pound thrust NERVA engine. The Phoebus-2A test program also included experiments for obtaining data relating to a new method of reactor control. The major experiment of the Phoebus-2A test program was conducted on June 26, 1968. The reactor was operated for about 32 minutes at significant power, about 12 minutes of which were at a power level above 4000 megawatts. The peak power reached was about 1200 megawatts. The test was terminated as planned when the dura

tion limits set by the available propellant and water storage were reached. Additional experimentation at intermediate power was being planned. Analysis of data from this experiment was in progress at period's end.

Fuel Element Materials Research.-The initial duration objective for nuclear rocket fuel elements was achieved in December 1967 when the NRX-A6 reactor was operated for 60 minutes at full power (approximately 1100 megawatts). (The NRX-A6 reactor test program was described in the Eighteenth Semiannual Report.) Emphasis in

fuel-element materials research then shifted toward cyclic testing and testing at higher power densities and temperatures.

Laboratory tests of improved fuel elements have provided test durations of more that 100 minutes with reasonable corrosion results. These tests were conducted at temperatures much higher than achieved in the NRX-A6 reactor. The substantial impact of high specific impulse on space vehicle performance makes increased-temperature reactor operation most desirable. Improved high-temperature fuel-element performance is therefore a constant goal.

As described in the Eighteenth Semiannual Report, the program for improving fuel-element performance uses electrically heated corrosion furnaces for corrosion testing. This mode of testing however must be checked periodically through full-scale reactor tests at NRDS. Investigations at Los Alamos have made it apparent that a smallersized reactor, requiring a smaller number of fuel elements would be the most economical approach to satisfying this requirement. As a result of LASL's investigations two such reactors, the Pewee-1 and Pewee-2, were being fabricated.

The Pewee-1 reactor was in the final stages of assembly, with testing scheduled for the fourth quarter of 1968. The minor modifications to Test Cell "C" required to meet Pewee test conditions were defined. and the work to incorporate these modifications was scheduled to follow the Phoebus-2A test program.

The test schedule and objectives for Pewee-2 reactor test reactor were being defined.

Status of Engine System Technology

The nuclear rocket program continued to stress completion of the engine system technology effort and the NERVA flight engine development activities.

The principal objective of the engine system technology effort is to accumulate data and experience on which to base development of the NERVA flight engine. Most of the work that remains relates to testing the hot XE ground experimental engine in Engine Test Stand No. 1. Assembly of this engine was largely completed during the period. Most of the engineers and scientists responsible for this XE design and development effort were transferred to the NERVA engine design work.

The XECF Test Series.-The most significant accomplishment in the engine system technology program during the period was the conduct of the XECF (experimental engine, cold flow) test series in April. This was the first time a nuclear rocket engine had been tested in the new test stand, ETS-1, in a downfiring position. (Fig. 5-2.) The major objectives of the series were to verify that ETS-1 was ready for hot