SPACE VEHICLE DESIGN CRITERIA
(METEOROID ENVIRONMENT FOR EARTH ORBIT AND CISLUNAR SPACE)

craft provide us with six circuits having identical transmitters, receivers, power supplies, and data systems.

Continued operation and testing of these systems until they fail in space will provide us with an opportunity to obtain expected lifetime and efficiency data on many standard-type circuits. We have, therefore, extended the operation of all three Pegasus spacecraft beyond their planned cut-off times to obtain experience on spacecraft systems reliability.

Radiation Dose Prediction.-An important factor in manned space missions is assessment of the radiation hazard to be encountered. Attempts to predict the radiation hazard in space are plagued by uncertainties in the environment and biological effects, as well as in the penetration of radiations through shields. Efforts are being made to minimize the uncertainties in each of these areas since it is anticipated that space missions may eventually require exposures nearing safe limits.

Over the past several years a significant part of our space vehicle radiation shielding program has involved the development of accurate analytical techniques for predicting electron penetrations through shields. The tortuous paths which electrons travel through shields coupled with the production within the shield of X-rays (bremsstrahlung) which are even more penetrating than electrons, complicate the problem.

Because of the complexity of the problem, it is important that the analytical techniques be carefully checked experimentally. Figure 294 shows typical comparisons between theory and experiment. The curves on the upper left are for the energy distribution of bremsstrahlung (X-rays) escaping a shield while those on the lower left are for the penetrating electrons. The agreement between theory and experiment is seen to be quite good, giving confidence in the use of the analytical techniques.

These analytical techniques have been used by the Manned Spacecraft Center to generate data for use in manned flight programs. These data have been used, along with environment data obtained external to the vehicle, to calculate the radiation dose within the GT-4 and GT-7 spacecraft during their missions for comparison with measured values. The comparisons, shown on the right of the

which was used to measure the dose was not capable of discriminating between the two components. However, since the calculation for protons in the thinly shielded Gemini spacecraft is straightforward, the good agreement is indicative of the accuracy of the electron calculation. Such comparisons increase our confidence that through our efforts in this very difficult area, we are reducing the uncertainty surrounding the penetration of radiations through shields.

Refinement of these techniques will continue during FY 68 with increased emphasis on engineering applications of the data.

Now let me turn to energy conversion systems-propulsion and space power. The propulsion programs-chemical, nuclear, and electric-continued to make excellent progress in developing technology for future boosters and spacecraft. Advances in Large Turbopumps and Thrust Chambers.-During the year, several significant advances were achieved during closeout work on the M-1 engine, the world's largest (1.5 million pounds thrust) engine using liquid hydrogen and liquid oxygen propellants. The huge turbopumps were successfully tested by the Aerojet-General Corporation and demonstrated major advances in pump technology, figure 295. This technology will be very useful for new engines that may be developed in the future.

The eight-stage liquid hydrogen pump has three times the capacity of any known rocket fuel pumping system. It delivers 60,000 gallons per minute and is driven by a gas turbine developing 90,000 horsepower. The single-stage liquid oxygen pump delivers 22,000 g.p.m. It is driven by a 27,000 horsepower turbine. The two pumps were tested over a wide range of conditions. Results were consistent with predictions and no mechanical problems were encountered. The efficiencies obtained were high and consistent with the number of stages involved. The M-1 thrust chamber also operated several times successfully during 1966. The large uncooled test chamber incorporated a special baffled injector shown in the upper left of figure 296, to eliminate the possibility of destructive combustion oscillations. Ignition was by means of four torches of fluorine and hydrogen which react spontaneously. Start-up was very smooth and high speed

As can be seen from the figure, the 1965 Apollo meteoroid design criteria are conservative as compared with this newly recommended guideline. The data obtained from the three Pegasus satellites have been heavily utilized in the formulation of the new criteria, along with data from the Explorer XVI and XXIII satellites, and ground observations of natural meteors.

Spacecraft System Reliability.-It now appears that the three Pegasus spacecraft will yield a bonus in an area other than meteoroids, in that they offer through continued operation an opportunity to obtain important data concerning systems design and reliability of spacecraft systems operating for long periods of time in space. The systems of these spacecraft were designed and qualified to operate for a minimum of one year each. As of April 1967, Pegasus I had been in orbit and operating for 26 months, Pegasus II for 23 months, and Pegasus III for 21 months, for a combined total operating life of almost 6

SPACE VEHICLE DESIGN CRITERIA (METEOROID ENVIRONMENT FOR EARTH ORBIT AND CISLUNAR SPACE)

craft provide us with six circuits having identical transmitters, receivers, power supplies, and data systems.

Continued operation and testing of these systems until they fail in space will provide us with an opportunity to obtain expected lifetime and efficiency data on many standard-type circuits. We have, therefore, extended the operation of all three Pegasus spacecraft beyond their planned cut-off times to obtain experience on spacecraft systems reliability.

Radiation Dose Prediction.-An important factor in manned space missions is assessment of the radiation hazard to be encountered. Attempts to predict the radiation hazard in space are plagued by uncertainties in the environment and biological effects, as well as in the penetration of radiations through shields. Efforts are being made to minimize the uncertainties in each of these areas since it is anticipated that space missions may eventually require exposures nearing safe limits.

Over the past several years a significant part of our space vehicle radiation shielding program has involved the development of accurate analytical techniques for predicting electron penetrations through shields. The tortuous paths which electrons travel through shields coupled with the production within the shield of X-rays (bremsstrahlung) which are even more penetrating than electrons, complicate the problem.

Because of the complexity of the problem, it is important that the analytical techniques be carefully checked experimentally. Figure 294 shows typical comparisons between theory and experiment. The curves on the upper left are for the energy distribution of bremsstrahlung (X-rays) escaping a shield while those on the lower left are for the penetrating electrons. The agreement between theory and experiment is seen to be quite good, giving confidence in the use of the analytical techniques.

These analytical techniques have been used by the Manned Spacecraft Center to generate data for use in manned flight programs. These data have been used, along with environment data obtained external to the vehicle, to calculate the radiation dose within the GT-4 and GT-7 spacecraft during their missions for comparison with measured values. The comparisons, shown on the right of the