do. Figure 2.4 shows the problem. The plotted points correspond to Hohmann transfers. Shorter trips require more propulsion at both ends of the journey. The check list of missions, Section 4, shows that 263 metric tons is a good selection. A large size is unnecessary, and a smaller size would require more refuelling. A few missions would be precluded, except by multiple retanking on orbit. A fair question at this point is whether another means of propulsion might lead to a more economical or a more capable rocket. It has often been stated that nuclear propulsion is mandatory for manned exploration of the planets. The chemically powered space ship already described in this section shows that such is not the case. For any mission in the solar system, nuclear propulsion has marginal advantages and disadvantages relative to chemical propulsion. Such advantages as may exist would hardly justify the cost, delay, and uncertainty of a development program. Solid rockets are sometimes advocated as expendable first stages. However, it is difficult to show that any expendable first stage is competitive with the increase in size required to give an equivalent capability to a single-stage-to-orbit, hydrogen-oxygen rocket. Microthrust electric propulsion to the planets is interesting. It has been proposed for one-way trips to the outer planets, but it is not really competitive with the chemical rocket described in this section. It has also been proposed for manned round trips to Mars. If the ratio of total mass of the rocket to the jet power could be made less than about 11 kg/kw, electric propulsion combined with chemical propulsion would reduce the round trip time to less than 500 days, and make it possible to start the trip in either direction almost at will. However, such a ratio is definitely outside the limits of present engineering knowledge. Almost within the limits would be 19 kg/kv. This capability is worthless for Mars trips, but would cut the round trip time to any of the outer planets, including Pluto, to about five years. The possibility presented for manned trips to the outer planets in future decades may or may not be considered a justification to continue a low-level development program in electric propulsion. 24-215 O-78-52 29 b Two other proposals are academically interesting. One is the ORION rocket. The ORION was intended to use a succession of nuclear explosions behind a massive steel plate to accelerate a space ship mounted forward of the plate on air cushions. Such a method is not economical for small cargos. However, for enormous space ships, carrying perhaps 500 people, the proposal is attractive. The market · for 500-passenger space ships would appear to be dependent on findings which must await exploration by such means as the chemical rocket described in this section. The other academically interesting proposal is to find raw materials for the manufacture of propellants on the moon and on the planets. Nuclear-powered chemical transformation equipment would be: installed on the planet to manufacture propellant. Such support would enhance the cargo capacity of interplanetary rockets 10 to 20fold, or provide for repetitively reusable interplanetary space ships. Again, this proposal depends on finding environments, raw materials, and a transportation market. It may or may not be interesting after a few decades. 3 TRIP OPPORTUNITIES Trip opportunities are defined by orbit mechanics and propulsion capabilities. This section will be concerned with actual orbit mechanics requirements, and assumed propulsion capabilities, the latter, however, being consistent with presently existing engineering knowledge, as presented in Sections 2.1 and 2.2. 3.1 TRAJECTORIES In 1958 it was possible to calculate trajectories of particles moving in orbits around gravitational centers, given the orbit elements. However, there was at that time no way to specify the elements of the most advantageous trajectories for round-trip expeditions to the planets. For each opportunity it is necessary to calculate a matrix of trajectories, and to select from among these. The calculations were so tedious by the methods then available improved methods had to be developed. The first step toward i 30 improvement was the presentation of a paper on the "Lambert Theorem" in January 1959 by R.H.Battin, "The Determination of Round-Trip Planetary Reconnaissance Trajectories", J. Aerospace Sci., Vol.26, No.9, September 1959, pp. 545-567. The second step was the present-r ation of a paper in November 1959 by J.V.Breakwell, R.W.Gillespie, and S. Ross, "Researches in Interplanetary Transfer", ARS J., Vol. 31, No. 2, Feb. 1961, pp. 201-207. The paper showed how to calculate the matrices quickly and cheaply in computing machines, and to present the data graphically to make it usable. Copies of the magnetic tapes on which the rachine computation was coded were furnished free to all who wanted them by Lockheed Missiles and Space Company. There followed the publication by S. Ross et al, "Planetary Flight Handbook", Vol.3, Parts 1,2, & 3, SP-35, National Aeronautics and Space Admin-...... istration, 1963. Meanwhile and subsequently, progress has continued until now, in 1969, it is possible to formulate a continuing program of planetary exploration based on a complete summary of trajectory requirements. Every 1.60 years there is an opportunity to make a round-trip, stopover expedition to Venus, characterized by a return-trip, flybyabort option at Venus. All trips are uniformly alike. The roundtrip duration is between 410 and 420 days, including a stopover of any desired duration between 0 and about 20 days. Every 2.13 years, on the average, there is an opportunity to * make a round-trip, stopover expedition to Mars. At each opportunity one can choose either a conjunction-class or an opposition-class trip. The conjunction-class trips are uniformly alike, easy, and of long duration, typically 950 days for the double-Hohmann trip each time. The opposition-class trips vary in difficulty and trip duration through a 16-year cycle, and require more propulsion capability then the conjunction-class trips, but typically require only 450 to 650 days, depending on which modification is used, and which opportunity is considered. The opposition-class trips can be further classified into direct trips, and perihelion-zaneuver trips. Of particular interest is the Venus-swingby trip, in which the perihelion mancuver is 31 executed, not by rocket propulsion, but by the exchange of momentum with Venus through gravitational coupling; during a near passage by that planet. It is possible to size a launch rocket, and to design the space craft and its contents, such that a single launch of a standard roc ket will make possible a circular-capture, round-trip expedition to either Venus or Mars at any opportunity. This is described in Section 2, above. " The trip data for Venus is shown in Tables 3.1.1-a and 3.1.1-b¦ Data for Mars is given for the year 1978, in Table 3.1.2. Table 3.1.3.. summarizes data for Mars through a complete cycle of oppositions, from 1971 to 1988, inclusive. The numbers in the left column of each square are remaining mass fractions, in porcent, for ideal propulsion stages having a specific impulse of 444 seconds, appropriate to hydrogen and oxygen. They correspond, from the top downward, to departure from a low, circular, earth parking orbit, circular capture at the planet, and departure from the capture orbit to return to earth. Where appropriate, a perihelion aneuver is included. The numbers in the right hand column correspond to eccentric capture at the planet, and departure from the eccentric orbit to return to earth. The trajectories tabulated in the tables have not been selected by the usual criterion of minimizing the mass before departure from earth parking orbit, but, rather, to permit standardization of the stages of the space ship. To assist in preparing and using the tables, charts were prepared for converting hyperbolic excess speeds (EMOS,.. earth mean orbital speed) to mass fractions (M/M). Three such charts are shown as Figures 3.2.1, 3.2.2, and 3.2.3. With a minimum of practice, the user can evaluate trajectory data in terms of propulsion capabilities at a glance, without the usual laborious calculations. 3.2 SCHEDULES AND MASSES Trajectory data are necessary, not only to design the reference rocket, but to determine the production rate, the launch schedule, |