The big wheel shaped space station was a necessary adjunct to the development of plans for both the Lunar and Mars expeditions; the crews constructing the Lunar and interplanetary space ships naturally had to have a "home" to stay in, in between work periods. In this case, a big spacewheel would use an annular parabolic solar collector that focuses light onto a pipe carrying mercury to generate vapor to drive a turbo-electric generator. Photovoltiac cells were simply not considered a viable energy source at the time the station was designed.

Professor Schwartz has pointed out a number of defects in the design of the wheel; his criticisms are to a certain extent valid. That is not the point. The point is whether or not any of the project could have been made to work. The giant three stage rocket ships used to re-supply (and initially supply the building components of the wheel) would have worked (albeit, surely after some redesign work, revealed by test flights and other engineering investigation).

The giant wheel may have been difficult to manage, especially for the very reasons that Dr. Schwartz has pointed out; however, I do not believe that the project would have been impossible as proposed (only, perhaps, resulting in some headaches in the management of the operation of the station).

What we are most interested in pointing out, starting with the next installment, is how the rocket engineers dealt, in their calculations, with the low specific impulse of the fuel they chose: Nitric Acid.

In part two, we pointed out that the specific impulse of Nitric Acid and Hydrazine was so low that it would have been impossible to build a rocket that could travel between the earth and the moon; this is of course, absolutely true. Here is the comment, from "Conquest of the Moon" that re-iterates this point:

"It is commonly believed that men will fly directly between the earth and the moon, but to do this, we would require a vehicle of such gigantic proportions that it would prove an economic impossibility".

However, launching an expedition from earth orbit would save the expedition (or expeditions) a necessary velocity change of nearly five miles per second!! Even better than that, they could "cheat", and pick up even extra velocity from the poor performance fuels by launching an extremely long power maneuver (escape maneuver) because they did not have to lift against gravity; they could load up with as much fuel as they wanted to and run the engines as long as they wanted to, there being no gravity to fight!

Now, it is time to explain what is meant by the terms "specific impulse" or "total impulse".

Any kind of fuel can be made to generate any amount of thrust. But for how long? How useful would a fuel be that could generate a million pounds of thrust for only three seconds? I think you can see my point. Any given fuel must be able to generate a required amount of thrust for as long as is required to develop the desired velocity for the vehicle. One could even build a giant rocket, like the resupply rockets, using ordinary black powder for the first stage booster, and make it develop enough thrust to lift the ship; but if the black powder fuel only burned for a few seconds, the vehicle would immediately fall back onto the launching pad. What we need are fuels that will generate the required thrust for the required lengths of time to be useful.

I like to explain specific impulse this way: let us say that one has constructed a little test motor to test how efficient rocket fuels are: the little motor can be adjusted to give one pound of thrust as efficiently as it can (it will have a variable plug in its nozzle so that the exhaust velocity can be kept as high as possible for each fuel).

Now, this little motor is connected to a fuel tank (and oxidizer tank) that will hold only one pound of fuel. Now, we are ready to see how long each different pound of fuel will generate a pound of thrust. This duration of burn time we will call "specific impulse". As it turns out, various fuels have different impulse times.

The specific impulse of any fuel varies directly in proportion to its exhaust velocity. Here is a chart depicting the various exhaust velocities of some different fuels; though Nitric Acid and Hydrazine is not depicted on the chart, we already know the exhaust velocity for this combination (7,400 feet per second; in our little test motor, this would yield a burn time of only 230 seconds).

One can now understand why the re-supply ships had to be so large; the specific impulse for Nitric Acid and Hydrazine is relatively low. I imagine that the planners of the coming space age chose these fuels as the "initial" fuels of choice for a variety of reasons relating to their convenience in handling.

Any vehicle that would have to accomplish all of the velocity changes required for an interplanetary flight, if it had to leave the earth directly, would have been impossibly large (indeed, one wonders if the re-supply ships as designed could have even be made to be practical).

Lunar and interplanetary flights starting from earth orbit (the higher the better) would gain two very significant advantages: they could start with the initial speed generated by their orbital situation; and: they could manage power maneuvers of very long duration, albeit starting at low accelerations. Only by these strategies could Nitric Acid and Hydrazine be practical for a Lunar or interplanetary flight.

Illustration from: "Rockets, Jets, Guided Missiles and Space Ships" by Jack Coggins and Fletcher Pratt (Random House: New York, 1951)

We have seen that a departure directly from the earth for an expedition for Mars, using Nitric Acid and Hydrazine as fuels, simply would have been impossible; the size of the rocket required would have been beyond the engineering capabilities of even today's aerospace engineers. It is a proposition we would never have to consider anyway: we have liquid hydrogen and oxygen as fuels now, with their almost double specific impulse over Hydrazine and Nitric Acid.

But Wernher Von Braun and Willy Ley wanted to show how we could go to Mars before hydrogen/oxygen propulsion had been developed. The key to the project was launching from a high earth orbit. Instead of requiring a departure speed of almost 8.88 miles per second to begin the "escape leg" of the half ellipse to Mars, the ships would require a power maneuver yielding only 1.59 miles per second (for a total departure speed from the 10,075 mile high orbit of "only" 5.99 miles per second).

In the above illustration, we see the ships, complete, or nearly complete, after their assembly in earth orbit, near the space station. In the far distance may be seen the space station, here shown, curiously, with four spokes connecting the hub with the wheel, rather than only two, as depicted in the detailed close-up illustration published in part III (take special note of this, Doktor Schwartz).

The ships have been assembled twice: once in a hangar on earth, and then again, up here in the 10,075 mile high two hour orbit. The ship in the foreground is of course a deep space ship only; it has no provision for landing anywhere on any planet. I would particularly call your attention to the next ship in the view; it contains (is mostly comprised of) both the landing vehicle (the giant glider), and on its forward part, and the ascent stage which will carry the astronauts up from Mars to rendezvous with the orbiting deep space ship (in the foreground) which will return to earth.

This giant glider and its return to Martian orbit ascent stage is the most problematic part of the entire expedition, engineering and operational wise. We will return to this aspect of the project later and discuss it in detail. It is interesting to note that, because of the presumption of the very thin air of Mars, the designers gave this glider the astonishingly low wing loading area of 24,500 sq. ft. (2280 square meters)- this, of course, to keep wing loading low enough to allow a "low" landing speed on the Martian plains- we shall see that their choice of a "low" landing speed was way too high for a safe landing on Mars.

It is this single feature, the method of landing, that would have doomed the expedition to a circum-Martian orbital survey of the planet only; the astronauts would have surely been disappointed, keenly, at finding out that the terrain would not have allowed an airplane style landing speed of 120 miles per hour!

Please note the very large spherical fuel tanks on the ship constituting the glider/ascent stage; they contain the bulk of the gross weight of the ship before departure for Mars. These are, in fact, the tanks containing the earth orbit departure fuels. Both ships are identical in weight; for the obvious reason that they need to fly in tandem, to stay together.

In fact, both ships, after exhausting all of the propellants in the large spherical tanks, will weigh almost exactly 500 tons each! What a grandiose scheme! But this scheme was trifling compared to Von Braun's 1953 proposal in "The Mars Project", which I will comment on before we leave the subject. I gave a copy to Lt. Col. William Welker a long time ago. From The Exploration of Mars is a comment about this work:

"In 1953, one of the authors of the present volume published a scientific study investigating the feasibility of a voyage to Mars with chemical rocket fuels ("The Mars Project", by Dr. Wernher Von Braun, Urbana, Ill.; University of Illinois Press, 1953). The study dealt with a hypothetical expedition of no less than seventy members traveling in ten rocket ships, and it arrived at the conclusion that such an expedition was feasible, theoretically at least, if only someone could be found to foot a fuel bill equivalent to that of TEN BERLIN AIRLIFTS! The backbone of the undertaking was to be a fleet of three stage orbital rocket ships identical with those described in "Across the Space Frontier". In 950 ferry flights, shuttling back and forth between the ground base and the orbit of departure, these rocket ships were to carry the prefabricated parts of the Mars ships, their fuel, the expeditionary equipment, and the crews to the orbit, where the Mars vessels would be reassembled and whence they would finally depart for the Red Planet.

The following may be considered a revision of this study. Although it envisions an expedition of only twelve men traveling in two ships, the total propellant requirement- a good yardstick for the over-all logistic effort- is only 10 per cent of that found in "The Mars Project". This enormous saving is due solely to a superior over-all plan, for the specific assumption made with regard to rocket engine performance and construction weight factors have NOT been altered.

At right is an illustration of the chart of the power track for the two Mars ships as they depart earth orbit, for their voyage to Mars. The chart is very intuitive, in its presentation, and needs little commentary for one to understand it. It is interesting to note the advantage, for Nitric Acid and Hydrazine, by departing from an orbit 10,075 miles above the Earth: they may accelerate the ships at only 0.212 "G" at the beginning of a 948 second burning time, finishing at only 0.710 "G" at the end of the powered track for departure from the two hour orbit.

Illustration from: "Rockets, Jets, Guided Missiles and Space Ships" by Jack Coggins and Fletcher Pratt (Random House: New York, 1951)

"Each of the two Mars ships, ready for departure, will weigh 1,870 tons. Initially, each ship is powered with twelve rocket engines, using hydrazine as fuel, and nitric acid as oxidizer. The twelve engines develop a combined thrust of 396 tons. This first power maneuver, the departure from earth orbit, lasts 948 seconds. During this time the twelve engines gulp up 1,370 tons of propellants, or just about 73% of the ships' initial weight. The initial acceleration is low- just about one fifth of normal gravity. As a result of the weight loss caused by fuel consumption during the first power maneuver the acceleration gradually climbs to about seven tenths of a G. By the moment of power cut-off, both ships have climbed from their original orbital altitude of 1,075 miles to 1,965 miles, and their speed has increased from their orbital velocity of 4.40 miles per second to 5.99 miles per second.

This first power maneuver, properly timed and guided, puts the two ships directly in the unpowered voyaging ellipse to Mars. With their weights reduced to about 500 tons, they will faithfully follow this prescribed elliptical path like "two comets in formation" until, 260 days later, their flight paths will approach the Martian orbit."

The 1956 Mars Expedition Proposed by Von Braun and Ley

Part 1: Mission Introduction
Part 1: Mars Departure: 10,075 Miles Above Earth
Part 1: Power Maneuver for Martian Orbit