propellants indicates that several techniques may be used to effect multiple starts and thence provide more versatile operation. Special propellant burners which can, with a small amount of propellant, simulate the combustion-induced oscillations of the largest solid motor have been developed. These burners are being used to study the various types of combustion instability present in large motors and are contributing to a better understanding of transient combustion behavior.

Nondestructive inspection and test techniques can now indicate the presence and size of defects such as cracks or voids in the propellant grain and separations between the grain and the insulator or between the insulator and the casing.

Continued activity in the thrust vector control area has provided valves which offer promise of meeting the severe requirements for hot gas bleed control with the current solid propellants. The most promising configurations are being tested and should lead to a suitable flight system in the near future.

Hybrid rocket systems generally have a solid fuel grain and a liquid oxidizer. The hybrid technology program of the Department of Defense is being followed and design studies of its possible use for launch and spacecraft propulsion are in process.

Earlier the role of development of advanced subsystems as an important part of the advanced research and technology program was mentioned. In chemical propulsion there are three such developments. The first is the M-1 engine, figure 191, having a thrust of 1,500,000 pounds are using oxygen and hydrogen. This engine was

started in 1962 by the Office of Manned Space Flight and transferred to the Office of Advanced Research and Technology the first of this year. This engine will have a useful place in future space missions and its development is being continued under Lewis Research Center management.

The second advanced system is an experimental high energy engine using fluorine and hydrogen. A program is underway to demonstrate the use of this high energy propellant combination in a pump-fed flight-weight engine. An existing RL-10 oxygen-hydrogen engine, figure 192, was selected as an engine of convenience because it could be used with the hydrogen fuel system undisturbed. Modifications were required only on the oxidizer side of the system. The work underway involves experiments on the oxidizer pump with emphasis on seals, valves, combustion and cooling, and finally, on operation of the complete system. The performance gain expected is about 20 units or more of specific impulse which, combined with greater propellant density, results in a considerable increase in payload. The work is being carried out by the Pratt & Whitney Aircraft Division of United Aircraft Corp., West Palm Beach, Fla., and at the Lewis Research Center in Cleveland, Ohio.

The third advanced system is a large solid propellent motor, 260 inches in diameter. The work was initiated by NASA and the Air Force in June 1963. Two contractors were selected to design, build, and static test 260-inch diameter solid propellent motors weighing almost 2 million pounds and generating 3 million pounds thrust. The Department of Defense has managed this effort until recently but they have requested that NASA take over and finish the work.

Chemical propulsion is limited by the energy release of chemical reactions and by the relatively heavy atoms and molecules to be accelerated by gas expansion. In nuclear propulsion a reactor heats to very high temperatures a working fluid of low molecular weight, usually hydrogen, and a performance severalfold greater than chemical rockets is possible. This increased performance is of great interest for deep space penetrations of large payloads, for example, manned exploration of the nearby planets or even for closer missions such as a lunar logistic ferry. Figure 193 illustrates a typical manned spacecraft concept using nuclear rocket propulsion. Such a spacecraft may be one-third to one-tenth the weight of the chemically propelled spacecraft required to do the same job. Studies of such concepts are employed to guide research along the most fruitful lines.

The NASA and the Atomic Energy Commission are engaged in a joint program to develop the technology necessary to use nuclear rocket propulsion for space missions. The program consists of two parts: (1) advanced research and technology on reactors and on nonreactor flow components and subsystems utilizing hydrogen as a working fluid, and complete system simulation; and (2) two projects, KIWI and NERVA. The projects are aimed at the development of an experimental nuclear rocket engine. The NERVA engine depends on the KIWI reactor technology developed at the Los Alamos Scientific Laboratory and NRX reactors (NERVA reactor experiment), which are part of the NERVA project itself. The NERVA project, which will be conducted initially at the 1,000 megawatt power range, will

emphasize reactor testing with sufficient experimental engine operation to understand system design and operating characteristics. It will determine operating temperatures and durations as well as restart capabilities which may be required for various missions.

Some of the major reactor problem areas are listed by figure 194. Thus far, the results of the reactor program have shown good promise for the employment of uranium-loaded graphite fuel elements for nuclear rockets. In addition to high-temperature fuel elements, substantial progress has been made in reactor startup and controllability with liquid hydrogen, the attainment of uniform radial core temperature, and in mechanical support of the core. Much effort has been concentrated in this latter area over the last year. In November of 1962, the test of the KIWI-B4A revealed structural vibration problems in the reactor. Figure 195 outlines the series of major reactor development tests starting with the 1962 one. Extensive component and subsystem tests were run during 1963 along with cold-flow reactor experiments. The latter revealed that the structural vibration problem is related to pressure differences and leakages within the core and that proper control of these factors should eliminate structural vibrations. The KIWI and NRX reactor program schedule includes two additional cold-flow reactors and four hot reactor tests. These tests should determine if the reactor design now favored is both suitable and capable of development. NRX reactor tests will continue through 1965 and beyond, exploring limits of power level, temperature, duration, and recycle.