Those tanks will be jettisoned before the second maneuver is undertaken, the latter requiring tankage for 492 tons. This will likewise be jettisoned before departure from the satellite orbit around Mars. 222 tons of propellants suffice for this departure and, when the corresponding tankage has again been disposed of, the final tankage for entering the terminal orbit around Earth need hold but 134.5 tons.
The above will give you a rough idea of the general dimensions of a space ship for a round trip to Mars."
"I have described only a part of the proposed journey, namely that between the two satellite orbits concerned. To effect safe and satisfactory landings on and departure from the surface of Mars poses an entirely novel set of problems. These cannot be solved by the vessels hitherto discussed. It would be worse than spendthrift to land all those propellants on the surface of Mars and then be faced with the tremendous power requirement for overcoming Martian gravity. Not only that, but the construction of the vessels utterly unsuits them to operate in any kind of atmosphere. They have no fuselages or hulls, nor are they winged, and their rocket motors wholly lack the power to lift them from the Martian surface. For that reason, we propose to make our landings in special craft which we shall call "landing boats," the space ships themselves continuing to orbit around Mars at the altitude of 1,000 kilometers. They will complete this orbit in about two hours and twenty six minutes at an orbital velocity of 3.14 km/sec.
"Now, the problem of descending from this orbit to the Martian surface is not dissimilar to that of descending from Lunetta to Earth. That is to say, the landing boat will decrease its velocity from that of its mother ship. This will throw it into an unpowered elliptical path touching the upper layers of the Martian atmosphere after one half of a revolution around the planet. Such a landing boat must of course be equipped with wings and controls permitting it to produce negative lift, in order to force it into a circular path within the atmosphere. The drag will then slow the boat down. The wings will eventually produce the positive lift required for a glide and a normal landing in airplane fashion.
"But a landing of this nature on Mars is accompanied by two novel problems when compared to its terrestrial counterpart. One is due to the Martian atmosphere being, at surface level at least, markedly less dense than that of Earth. This will diminish the obtainable aerodynamic lift. It is indeed fortunate that this drawback is to some extent counterbalanced by the weakness of Martian gravity, which decreases the weight to be supported by the wings. The atmospheric pressure at the surface of Mars is but one-twelfth of that on Earth, or not far from it. This, of course, reduces the load-carrying capacity of any given wing panel to one-twelfth at any given speed close to the surface.
But gravity at the surface of Mars is but 38 % of that of Earth, and this likewise reduces the weight and the required lift for any given landing boat to 38 %.
"This is of great importance in the matter of landing speeds. Landing speed is the lowest speed at which the craft can be held in the air by the application of every possible aerodynamic expedient, such as flaps, slots, extreme angle of attack, etc. The figures indicate that any given landing boat will have just about double the landing speed on Mars that it would have on Earth. That means that our wings for a landing on Mars will have to be about quadruple their normal area, if we propose to maintain the same landing speed as on Earth.
"That, however, is but one half of our problem. Here's the other. When our Sirius-class ships return from Lunetta's orbit, we're accustomed to their being almost empty, for their propellants are nearly exhausted by the climb. This makes the ships light on their return to the atmosphere. The conditions of the landing craft for Mars will encounter this situation in reverse, for they will be obliged to later depart from the surface of the planet.
To do this they must land with an adequate propellant supply."
Spencer grinned at his audience. "Mr. Hansen has praised the intelligence of the Martians very highly," he said. "But my engineers refuse to work on the basis that they'll have hydrazine and nitric acid in quantity ready for us… So, the landings will inevitably be made with rather heavily loaded craft.
"Of course this handicap is somewhat mitigated by the feeble gravity of Mars. This not only tends to decrease the effect of the mass of the landing boats and to reduce their landing speed, it also facilitates their return to the satellite orbit.
"It requires a terminal velocity of 8.26 km/sec for a Sirius-class vessel to reach Lunetta from Earth, and this must be gained in opposition to the powerful gravitation of the Earth. To reach the satellite orbit in which the space ships will be circling around Mars, however, our landing boats will have to be accelerated only to 3.70 km/sec and will be opposed only by that planet's much weaker gravitational field. Even the maneuver of adaptation by which the flight of the landing boats is made to conform to that of the circling space ships requires less power near Mars. 180 m/sec suffices in this case, versus 460 m/sec for a Sirius-class vessel approaching Lunetta. Another important factor is given by the fact that the takeoff weights of the landing boats also amount to only 38 % of what they would be on Earth, and thus we can lift off an initially much greater mass with a reasonable rocket thrust and impart to it to a very satisfactory initial acceleration. The large area wings which will be required for safe landings must, of course, be abandoned on Mars before departure.
"The combination of advantageous circumstances referred to permits the use of a single-stage rocket to rise from Mars to the satellite orbit. This is in contradistinction to the triple stagers required to solve the analogous problem on Earth.
"But it is not my desire to bore you with too many details about the rather involved problem of the landing boats and their design and construction. Here are their main features in a nutshelclass="underline" Each boat will have a 200 ton thrust unit, deviating from the conventional type only in so far as its expansion ratio will be somewhat reduced by the not inconsiderable density of the Martian atmosphere. In other words, the gases will not expand down to as low a terminal pressure as in the rocket motors used in our space ships.
That means that we shall lose some exhaust velocity; the rocket motors of the boats will achieve exhaust velocities of about 2,600 m/sec versus 2,800 m/sec in vacuum.
"With full tanks, the boats will weigh 200 terrestrial tons at departure from the satellite orbit. This weight will decrease to 185 tons by the expenditure of propellants used to throw them into the elliptical landing path. These 185 tons will effectively amount to only 70 tons in the Martian field of gravity. The landing payload of 12 terrestrial tons will be decreased by 7 before departure from Mars, and both wings and landing gear, which together weigh about 40 terrestrial tons, will be abandoned. This will reduce the takeoff weight on Mars to but 138 terrestrial tons, namely 52.5 Martian tons. Their 200 tons of cut and dried rocket thrust is, therefore, capable of giving them a very respectable acceleration, permitting them to lift five tons of payload back to the satellite orbit of the space ships.
"Let us drop the landing boats and return to the loads the space ships will have to cope with.
"The initial weight of each landing boat, namely 200 tons, must be considered pure payload for those space ships which hauled them into the satellite orbit around Mars. But after the boats have served their purpose of landing and have returned their crews and payloads to the circling and waiting space ships, they have completed their tasks. Their value does not justify the expenditure of the propellants required to return them to Earth.