"You don't have to be a rocket scientist to... "

You can fill in the blanks as you choose. Rocket science has come to be the standard of difficulty against which other professions and activities are compared.

The odd thing is that rocket science is not difficult. Rocket engineering, I will be the first to admit, is enormously complicated and pushes today's technology to the limit. However, the basic science that lies behind rockets is very old and very simple. Isaac Newton would have been comfortable with it, back in 1680, and it was about then that he formulated his third law of motion that lies behind all rocketry: "Action and reaction are equal and opposite."

A rocket achieves its thrust by ejecting material. The push of the ejected material (a high-temperature jet of heated gases in all chemical rockets) provides an exactly equal push to the rocket in the opposite direction.

This sounds so obvious that you might wonder if anyone ever doubted it. They did. Many people had the idea that a rocket moved by pushing against the air. This led to such famous statements as a 1920 editorial in the New York Times: "...after the rocket quits our air and starts on its longer journey, its flight would neither be accelerated nor maintained by the explosion of the charges it might then have left... That Professor Goddard, with his chair in Clark College and the countenancing of the Smithsonian Institution, does not know the relation of action to reaction, and of the need to have something better than a vacuum against which to react - to say that would be absurd. Of course, he only seems to lack the knowledge ladled out daily in high schools."

To its credit, the New York Times did publish a retraction of that editorial - fifty years later.

Launching systems to space is, even today, a difficult business, but not because of any scientific mysteries. To see the nature of the problem, imagine that you have a rocket engine that will provide steady and substantial thrust at the cost of very little fuel. We build a rocket using the engine, stand it on the launch pad, and switch on. What happens?

The answer, unless the total upward thrust exceeds the weight of the rocket, is: absolutely nothing. The whole thing will simply sit there. Earth's gravity provides an effective "downward thrust" equal to the rocket's total weight, and unless the upward thrust exceeds that weight, the rocket will not move one inch. Even when the upward thrust is more than the weight of the rocket, much of the fuel is simply going to counteract the downward force provided by the Earth itself.

Now it is clear why astronauts are trained to accept high accelerations. The faster that the rocket can burn its fuel, the higher the thrust will be, and the less fuel will be wasted before reaching orbit. Once we are in orbit, it is a totally different ballgame. Except for atmospheric drag effects on low orbits, no fuel at all is needed to maintain a spacecraft there. Hence the old maxim of spaceflight: "Once you are in orbit you are halfway to anywhere."

The efficiency of a rocket in launching itself and its payload to orbit is critically dependent on the speed of the exhaust jet. And here we come to the central tragedy of space launches for the past forty years. Everything we have ever launched into space has used chemical fuels. With such fuels, the exhaust velocity of the jet that pushes a rocket on its way is limited. The best fuel in general use today, a liquid hydrogen/liquid oxygen combination, provides an exhaust velocity of only a few miles a second.

Do we, in principle, have anything better than this? We certainly do. We have sources of energy vastly more compact and powerful than chemicals, sources that could provide exhaust speeds at least ten times as high. Unfortunately for our hard-working rocket engineer, those sources all contain the forbidden word "nuclear."

It is hard to overemphasize the ways in which our aversion to all things nuclear ties the hands of the rocket designer. I like to illustrate it with a fable. In the year 1850, the government of the United States called together its leading engineers and made the following statement:

"We foresee increasing opportunities for interstate commerce, and to encourage that development we propose to build a number of bridges across major waterways throughout our land. As a first move, we are soliciting your proposals to build a Brooklyn Bridge, a San Francisco Bay Bridge, and a Verazzano Narrows Bridge. Although we recognize that these will challenge to the limit the state of engineering design and construction, we are requesting fixed price proposals."

And then, as the companies are about to disperse to prepare their proposals, the government representatives add a final rider: "By the way, the general populace believes that the use of iron, steel, and other metals can be environmentally dangerous. We will therefore not consider any proposal which involves the use of such substances."

Substitute "nuclear" for "metal," and you have today's situation when it comes to access to space. Can you imagine the Brooklyn Bridge made entirely of wood? The road to space for the past forty years has been built with the equivalent of wooden bridges.

Even NASA's most advanced proposed system, a single-stage-to- orbit spacecraft able to make multiple journeys to space with rapid turnaround, relies on chemical rockets. It is also, sad to say, a system in deep trouble that in its present form will probably never be completed and flown.

It was Winston Churchill, in times of war, who said, "Give us the tools, and we will finish the job." In these times of peace, you don't have to be a rocket scientist to see that without the tools, any job is going to be much harder and may be impossible. Rocket scientists, today, are being denied the right tool.

If we ever want easy, inexpensive access to orbit and to the rest of the solar system, we will someday have to change our attitudes. We will, uneasy as this thought may make you, have to accept and employ nuclear technologies.

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