The objective of the joint NASA/AEC nuclear rocket program is to provide a significant increase in propulsion capability for future space activities. To achieve this objective a number of key goals must be attained: providing the basic technology for nuclear propulsion systems; developing a NERVA engine of approximately 75,000 pounds thrust for flight applications; extending the technology of graphite reactors and engine system components, thus establishing the basis for improving nuclear rocket performance; furnishing the technology for a nuclear flight stage; and investigating advanced concepts.

During the second half of 1968, NASA and AEC made further progress toward the identified technology goals, Highlights included completion of tests on the high-power Phoebus-2A reactor and the Pewee-1 reactor, and the initiation of tests on the ground-experimental engine (XE). Laboratory tests revealed significant advances in fuel element technology, indicating that the high temperature and power-density levels required for the NERVA flight engine have been exceeded.

Work continued on the preliminary design of the NERVA engine based on the data produced by the NERVA technology program. Facility modifications required to test the NERVA engine using the existing ETS-1 engine test stand at the Nuclear

Rocket Development Station were further defined. Test Cell "C" will be used for NERVA reactor testing.

STATUS OF REACTOR TECHNOLOGY

The endurance goals established for the NERVA reactor technology phase of the program were accomplished with the fullpower endurance demonstration of the NRX-A6 reactor (December 1967). The test of the Phoebus-2A reactor and the advances in fuel-element technology satisfied all the remaining reactor technology goals.

Phoebus-2A Reactor

The Phoebus-2A (Fig. 5–1) reactor was designed and developed by the Los Alamos Scientific Laboratory (LASL) for a reactor test program to provide technology for high-power, high-temperature rocket reactors (19th Semiannual Report, p. 102). Although this purpose was no longer primary when the thrust level of the proposed NERVA engine was reduced to 75,000 pounds, the Phoebus-2A test program continued to be used to obtain data on reactor technology. The major experiment of the Phoebus-2A test program was conducted on June 26, 1968, (19th Semiannual Report, p. 102). On July 18, the reactor was restarted for a series of experiments at low and intermediate power levels to provide essential performance data for a wide range of operating conditions. The reactor operated over a range of power levels up to 3670 megawatts, and for a total time of approximately 30 minutes. The tests were highly successful and all required data were obtained.

Fuel Element Materials Research

The results of tests of improved fuel elements (19th Semiannual Report, p. 104) indicated that the temperature and thus specific impulse performance requirements for the NERVA flight engine have been achieved.

The Pewee Reactor Program

Two major milestones were achieved in the Pewee reactor program. The first involved completing modifications at Test Cell "C" for Pewee-reactor testing. This work consisted primarily of installing a new liquid hydrogen feed-system turbopump and making minor changes in certain lines and valves to accommodate the reduced-flow requirements. The second milestone achieved was the completion of power tests on the Pewee-1 reactor. (Fig. 5-2).

The major experiment of the Pewee-1 test program was conducted in December. During it, the reactor was operated at significant power levels for about 1-1/2 hours. Two separate cycles at power levels over 500 megawatts consumed more than 40 minutes of the operation. Such power levels were about half those of previous KIWI and NERVA technology reactors. The reactor oper

ated stably and reached a temperature of 4140°F (4500°R), the highest operating temperature yet achieved in the nuclear rocket program. In addition, power densities were higher than ever achieved before. The December test was the second power operation of the Pewee-1; the first was conducted in November at part power for 40 minutes to determine the overall operating characteristics of the Pewee reactor design.

STATUS OF NERVA ENGINE SYSTEM TECHNOLOGY

In the NERVA engine system technology phase, major emphasis was placed on preparing for testing of the ground-experimental engine, the XE, at the NRDS. (Fig. 5-3)

The XE engine is an experimental system designed to duplicate the arrangement of components and to function like a flight engine. Earlier, a "breadboard" engine consisting of similar components, arranged for convenience on a reactor test car, was tested in the first demonstration of a nuclear rocket engine operating as a self-contained power plant. The breadboard engine was tested with the exhaust nozzle pointing up, as in previous tests of reactors, and the hot hydrogen exhaust was expelled directly into the atmosphere.

The XE engine is designed to be tested in the downfiring position and under simulated altitude conditions to approximate the operation of a rocket engine in a space environment. ETS-1 at NRDS provides these test conditions. (The checkout of ETS-1 is described in the 19th Semiannual Report, p. 106.)

As planned, the XE engine was installed in ETS-1 in October. Preliminary checks and tests conducted to assure that the engine and stand were ready for operation were completed in early December, and then the first experiments in the XE engine test program—an initial criticality check and calibration run-were conducted.

After these experiments, the engine was temporarily removed from the test stand to the Engine Maintenance, Assembly, and Disassembly (E-MAD) building. Following other test events at the site, the engine was reinstalled and testing resumed.

STATUS OF NERVA ENGINE DEVELOPMENT

The NERVA development effort will provide a flight engine with approximately 75,000 pounds thrust and a specific impulse of 825 seconds, capable of performing a wide variety of advanced missions likely to be called for in the future space program (19th Semiannual Report, p. 106).

Preliminary planning and design work for development of the engine continued; areas of primary interest were determining engine requirements, evaluating design alternatives, and conducting preliminary design studies. The primary engine requirements are high performance, manned rating, high reliability, and safety. Other design requirements include the ability to withstand a space vacuum for relatively long periods both before and after