volumes of hydrogen than hydrogen-oxygen rockets. Thus, although more compact rockets using hydrogen-fluorine can and likely should be built, the prospect for vastly improved chemical combustion rockets is not good. The notion that rockets are terribly inefficient as a means of space transportation is not due to the inherent inefficiencies of the rocket process. It is due to the fact that our needs for space transportation are conditioned by the planet on which we live and the inherent energy packaging limitations of our only easily utilized source of energy — chemical energy. The exhaust velocity achievable is roughly half of the velocity required to achieve Earth orbit. The basic rocket equation makes it quite clear that, even for zero structural weight, it is only possible to carry approximately 12% of launch weight to orbit. By the time currently practical structural weights are included, we have today a shuttle which places less than 1.5% of its launch weight on orbit. Better structures are obviously possible in the future and one could expect the payload delivered to reach a major fraction of the zero structural weight limit. To get around the latter, however, greater energy concentration in the exhaust than is achievable with known chemical reactions is mandated. One way to attempt to increase rocket performance is to make use of nuclear reactors to heat the working fluid stream. A number of test firings of such rockets were made in the Nerva program in the early 1960s. Although great amounts of energy are available from nuclear reactions, the need to create better rockets by concentration of larger amounts of energy per unit mass of exhaust leads directly to higher temperatures. These temperatures are limited with conventional nuclear reactors to the melting point of the materials in the reactor. At the maximum temperatures of the best materials available, the exhaust velocity, using hydrogen as the working fluid, is only about twice that of the best chemical rockets. The reactor and its shielding will obviously be much heavier than a conventional chemical rocket. The performance gains over hydrogen-oxygen, hence, are not overwhelming. A much better nuclear rocket could be achieved if the reaction were made to take place in gaseous materials. This would remove the temperature restrictions, but quickly leads to the question of containment of the fission products. At the temperatures envisioned, however, virtually all of the energy can be transmitted to the working fluid stream by high temperature radiation. Thus, it is actually possible to design nuclear rockets where the energy is transmitted from fission plasma to the working fluid through a transparent wall. It is possible, at least theoretically, to obtain perfect containment of the fission products. The performance of these “light bulb" nuclear rockets, as analyzed two decades ago, gave much higher exhaust velocities and substantially better thrust/weight ratios than solid core nuclear rockets could achieve. The weight of rocket fuel and working fluid which must be carried for a single stage to orbit transport vehicle is shown as a function of rocket exhaust velocity in Fig. 1. It is seen that gaseous-core nuclear rockets would drive the required fuel weight down to only 25% of launch weight. The payload weight data in Fig. 1 come from shuttle orbiter and hypersonic glide rocket studies (I). The curve is for vehicles capable of Earth return. One could do better with an orbit-to-orbit-only vehicle, but the upper zero structural weight curve cannot be exceeded. Figure 1 makes it quite clear that rockets are not basically indecent, but rather that we have not yet learned how to build decent ones. The energy efficiency of the combustion rocket is remarkably high. This is due to the fact that the rocket cycle works over a wide temperature range. Combustion temperatures are much higher than in other devices and as the exhaust expands
RkJQdWJsaXNoZXIy MTU5NjU0Mg==