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a“ f i ——— Pd ti To summarize the Sart thus far: the stare of space travelers existing at plane¥s attached to neighboring stars is very much greater than the change of space-traveling Martians. The one can be viewed almost as a certainty (if the assumptions are accepted), whereas the other is very Slight indeed. CK In obder to estimate the relative changes that visitors from Mars or star X could come to the Farth and act like "flying objects" some discussion of characteristics of space ships is necessary. To handle the simple case first, a trip from Mars to Earth should be feasible using a rocket-powered vehicle. Onee here, the rocket would probably use more fuel in slowing down for a landing . than it did in initial takeoff, due to Earth's higher gravitational force. A rough estimate of one-way performance can be found by adding the so-called "escape velocity" of Mars to that of the Farth plus the total energy change (kinetic and potential) used in changing from one planetary orbit to the other. These are 3.1, 7.9, and 10.7 miles per second, respectively, wlving a tctal required performance of 20.8 miles per. second for a one-way flight. Barring a suicide mission, the vehicle would have to land and replenish or else carry a 100% reserve for the trip home. Let us assume the Martians have developed a nuclear, hydroren- propelled vehicle (the most efficient basic arrangement that has been conceived here on Earth) which uses half its stages to get here and the remaining stages to return to Mars, thus completing a round trip without refueling, but slowing down enough in our atmosphere to be easily visible (i.e., practically making a landing). Since it is nuclear powered, gas temperatures will be limited to the maximum OPeral ee temperatures that materials can withstand (heat must transfer rom the pile to the gas, s0 cooling can't be used in the pile). The hizhest melting point compound of uranium which we can find 1s uranium carbide. It has a melting point of 4560°R, Assume the Martians are capable of realizing a gas temperature of 45000R (= 2500°K), and that they also have alloys which make high motor pressures (3000 psi) economical. Then the specific impulse will be I = 1035 seconds and the exhaust velocity will be c = 33,400 ft/sec (reference 5). Calculation shows that using a single atane for each leg of the four- ney would require a fuel/gross weight ratio of 8.96 (for each stare) too high to be practical. Using two stages wach way (four alto-zether) brings the required fuel ratio down to’.81, a Walue that can be realized. If, by the development of strong alloys, the basic weight could be kept to 10% of the total weight for each stage, a residue of 9% could be used for ‘Rikanas A four stage vehicle would then have a gross weight (190) 15,000 times as zreat as the payload: thus, if the payload a. 2,000 pounds, the gross weight would be 350 million pounds at initial takeoff (Earth pounds). Declassification Authority: NND 57565