Nor, as a matter of fact, is it practicable to return even their mother ships. Both, therefore, will simply be abandoned in the satellite orbit around Mars. The crew will return to the orbit of departure in the remaining ships, and from that orbit crews and load, if any, will be taken off by the good, old, reliable Sirius-class.
"If we assume that the landing boats will be transported to the Martian satellite orbit in mother ships of performance characteristics similar to the space ships designed for the round trip, you may note from my tabulation that the weight of such a space vessel after the second propulsive maneuver, that is, entrance into the Martian orbit, will be 410 tons.
The tabulation also shows that 222 tons of propellants are required to depart from the Martian orbit and a further 134.5 tons must be used to enter a satellite orbit around the Earth. Therefore, such of the space ships as are to return home will be obliged to devote an overwhelming portion of their load-carrying capacity at arrival near Mars to propellants for the two homeward bound maneuvers, while these vessels to be abandoned can employ the major portion of the available 410 tons for payload. This reveals the trick by which we are able to get the 200-ton landing boats into Mars's orbit at all, despite our otherwise very tight situation with respect to weight.
"The empty weight of a mother ship on arrival in its Martian satellite orbit is about 15 tons, for it consists only of power plant, empty plastic tanks and a light connecting structure, once it has completed its one-way task. If we subtract the weight of a landing boat, 200 tons, plus the 15 tons from the available 410 tons, we get 195 tons for useful load of various sorts; and believe me, we can use it! So you will easily see that the trick of abandoning some of the space ships near Mars frees us to a large extent from our weight limitations."
"Let me now cover the technical overall planning of the voyage.
"We anticipate that the expedition will consist of seven passenger ships for the round trip, and three cargo vessels to be abandoned in Mars's orbit. Each of the latter will bear a landing boat plus 195 tons of useful load. This gives us a total of three landing boats and 585 tons of cargo in addition to the load carried by the passenger ships. The crews will be 70 in all.
"You have heard that the one-way trip to Mars will require 260 days. To this will be added a stay on Mars of 449 days, which I shall later explain. Let us refer to this as "waiting time," although the expedition will then fulfill its actual purpose. The waiting time will be followed by the return trip which will also require 260 days.
"Obviously the expedition as a whole can draw on the supply of the three cargo vessels during the trip out and during the waiting time. On the return voyage, the passenger vessels, bearing the personnel and whatever Martian cargo may be appropriate, will be on their own.
"This enables us to determine the payload requirements for the passenger ships. Each of the seven ships will carry a crew of ten on the way home. Those ten people have to be provided with food, drink and utility water, as well as breathing oxygen, for 260 days.
Adding all these loads to the empty weight of the ship proper, we come to a total of 41.5 tons. Now I told you that, in effect, we based our calculations on a terminal weight of each passenger ship of 50.5 tons after completion of the last propulsive maneuver. So we still have a leeway of 9 tons per ship, or a total of 63 tons, which we have so far made no attempt to assign specifically. There are, of course, many uses for this reserve which are desirable although not vital. For example, busy bees will be extremely convenient, not only for visiting between ships, but also in the event of one or another ship getting into difficulties and requiring help. Long distance radio, borne on the trip out by the cargo vessels, could be taken over by one of the passenger ships for the run home. I can assure you that those 63 tons of available capacity for the trip back will gradually be filled up with various things as our plans and program take more definite shape. The final decision on the utilization of this reserve may be left to the discretion of the Command of the expedition.
"The situation is envisaged as being even somewhat more favorable than I have here represented to you. We may well assume that accurate navigation will render the 10 % propellant reserve after propulsive maneuvers 2, 3 and 4 somewhat excessive. But we cannot exactly predict just how great this excess will be after each application of power and the corrective maneuvers that may follow it. Hence we propose to equip both passenger and cargo vessels with four reserve propellant storage tanks, each of 15 cubic meters capacity. Prior to any new maneuver, when it is desired to jettison the tankage of the previous one, any remaining liquid can be drained into the reserve tanks.
"During the earlier power applications, when large quantities of propellants are involved, accurate work on the part of the navigators and the control equipment makes possible the saving of very considerable quantities of propellants. If these savings be later applied, when the ships are relatively light, they mean large reserves for load carrying capacity or for changing velocity. It will largely be up to the Command of the expedition to apply these reserves at the times and in the manners which will do the most good, enabling them to choose whether they will bring back more payloads or save the reserves against possible unexpectedly large velocity change requirements.
"Let me now discuss the payload distribution of the cargo vessels. You know that, aside from the landing boats, there are still available some 195 tons aboard each vessel.
We shall have to deduct from this the weight of the oxygen, water and food required for the voyage out and the waiting time together, 709 days. Each ship will require 25 tons of oxygen in liquid form, 35 tons of water and 20 tons of food. In addition, there's a reserve of 20 tons of water in the event that one entire cargo ship should be lost; but we still have 95 tons of available capacity. It would be a long task to convey to you all the articles which will comprise these thrice 95 tons, but here are the most important:
• Two heavy-duty radio sets for communication between the expedition and Lunetta.
• A large reflecting telescope with which to reconnoiter the surface of Mars from the satellite orbit. Such reconnoitering is a sine qua non before an attempt is made at landing.
• Three complete spare rocket power plants, each being applicable to either passenger or cargo vessels.
• Two spare tanks of each size, in the event of any of the regular ones springing a serious leak, will be kept ready at all times together with a lavish stock of all sorts of spares. Our damage control and repair equipment will even include two small lathes.
• There will be gravity cells, the busy bees for intership communication, as well as other things…
And now, gentlemen, I must touch upon another point in my discussion — the timing of the expedition. It is, perhaps, the most critical problem of space shipping, and particularly so for a voyage through the solar system.
"We would profit but little by attaining Mars' orbit unless he is at the right time at the point where we intercept it. The same applies to meeting the Earth amid the vastnesses of space on the return trip. Let me now present the timing problem, in somewhat simplified guise, by assuming that the orbits of both Earth and Mars are circular. In actuality, of course, both orbits are elliptical — that of Mars in particular displaying a very considerable eccentricity — and when we perform the actual computations, this is, of course, considered.
"Mars makes a complete, 360° circle around the Sun in 687 days, equivalent to a segment of arc of 0.524° per day. Earth, on the contrary, takes only 365 days for a complete encirclement of the Sun and therefore accomplishes 0.987° in a single day. In view of the 260 day length of the trip to Mars, the latter must, at the moment of departure, be at a point 260 times 0.524° — 136° — ahead of the intersection of the path of the ships with the orbit. When observed from a hypothetical point on the Sun, this intersection will lie 180° behind the point of exit of the ship's elliptical path from the terrestrial orbit.