« AnteriorContinuar »
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.
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
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 machine 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-maneuver trips. Of particular interest is the Venus-swinghy trip, in which the perihelion maneuver is
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 spacecraft 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
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,
NOMINAL TRIPS TO VENUS THROUGH ONE SYNODIC CYCLE OF 8.00 YEARS
.35 .62 .16 .38 .69 .28
The composite of worst maneuvers is for the short stopover trips.
In every case, the outbound trip of the short stopover expedition can be substituted for the outbound trip shown for the long stopover expedition, thus providing the flyby abort option in every case.
Observe that the outbound trip of the short stopover expedition is in each case identical with the outbound trip of the round trip flyby expedition. They could have been chosen otherwise, of course, but with only small advantage, mainly in respect to "window" width.