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8-CYLINDER GILLESPIE ROCKET
MASS TABULATION, MUTUALLY ADJUSTED FOR MARS AND VENUS SHIPS.
ROUND TRIP TO VENUS, CIRCULAR CAPTURE; 3 PEOPLE, OPEN-CYCLE ECOLOGY.
145-17-280 DAYS; MASS FRACTIONS, 0.851 x 0.35 & 0.38
8-CYLINDER GILLESPIE ROCKET
MASS TABULATION, MUTUALLY ADJUSTED FOR HARS AND VENUS SHIPS.
ROUND TRIP TO MARS, CIRCULAR CAPTURE; 3 PEOPLE, OPEN-CYCLE ECOLOGY.
AND CAPTURE AT LAPS; ONE STAGE FOR DEPARTURE FROM MARS.
385-100-385 DAYS; MASS FRACTIONS, 0.702 x 0.41 & 0.41. ́
LOADED SHIP, METRIC TONS ON EARTH PARKING OBIT (CIRCULAR)
SELECTED SAMPLE SHIP MASSES, HUTUALLY ADJUSTED
ONE 3-STAGE SHIP, CIRCULAR CAPTURE AT EITHER.
ONE 3-STAGE SHIP, CIRCULAR CAPTURE AT EITHER、
TWO 2-STAGĖ SMIPS, CIRCULAR CAPTURE AT VENUS
TWO 3-STAGE SHIPS, CIRCULAR CAPTURE AT EITHER
ONE 2-STAGE SHIP, ECCENTRIC CAPTURE AT MARS (950-DAY TRIP); ROUND-TRIP VENUS FLYBY
SATURN V ROUND-TRIP VENUS FLYBY
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.
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