Unfortunately, the reality of the situation was rather different. Not only did the Orion propulsion system have a great many flaws, but its advocates have tended to dramatically overstate itspotential benefits. First, let's look at what some of the actual Orion proposals promised. There were actually several different proposals throughout the lifetime of the Orion project, but they fall into two classes. Note that the Orion project was originally classified, and some of the specifics of the research into some Orion technologies remain classified to this day. The proposals and the estimates of their capabilities, however, are now publicly available. Before we look at the proposals I should mention that one way to define the efficiency of a rocket is a value called its ISP. ISP is essentially a measure of the efficiency of a rocket. A higher ISP means that the engine can produce more acceleration for a given amount of fuel. Thus a higher ISP rocket can take the same payload from point A to point B using less fuel, or (typically) in less time. Orion engines also have a high thrust, which means they can produce a lot of acceleration quickly rather than just over a long period of time. This is useful for escaping the Earth's gravity.
Solvency – Heat
Orion will fail – fission creates exhaust that melts the rocket
One possibility was to use a reactor to generate electrical power to run the ion rockets conceived by Goddard and actually built on a small scale by Soviet researchers between the world wars. But ion rockets could only be used in a vacuum and with their small thrust were not likely to be practical and useful until humankind was well established in space. To be of value in the near future, nuclear reactors had to be used to heat a propellant (or, more exactly, working fluid) that would be expelled through a rocket nozzle. The higher the temperatures the reactor developed, the greater the exhaust velocity. Potentially, the most potent reactors would run so hot that their cores were molten or even gaseous. Gaseouscore reactors might attain a specific impulse (the measure of the effectiveness of a rocket fuel or engine) as high as 3,000 to 7,000 seconds. (The absolute limit of chemical propulsion was under 600 seconds.) But such reactors were technically far distant, and it would be hard to prevent the costly and dangerous loss of fissioning reactor fuels through the rocket exhausts; the contamination problem meant that they could only be used well out in space. Although the so-called light-bulb or closed-cycle type of gaseous-core reactor might avoid that problem, even studying the problems inherent in this sort of reactor was difficult. 3 Only solid-core reactors, running at much lower temperatures, were likely to be practical in the near future; but since they developed a specific impulse as high as 1,300 seconds, they still offered higher performance than chemical combustion.
Nuclear propulsion causes the rocket to melt – its also inefficient
So how does each of the present concepts for nuclear propulsion work? The principals are simple but the execution can be complicated. NTP works on the same concept as a hydrogen rocket. The material that makes thrust is heated by a heat source. In this case it is a nuclear reactor. The sheer energy this system can produce when properly managed can exceed that of normal rocket systems. Unfortunately this type of propulsion is highly inefficient as the temperatures needed to make it truly effective would actually melt any known material now used to make rockets. To prevent this, the engine would have to lose 40% of its efficiency. The other approach is Nuclear Electric Propulsion. This works on the concept of using electrical power to heat the rocket propellant. The main design concept now in use for this type of propulsion is the Radioisotope Thermoelectric Generator. The generator is powered by the decay of radioactive isotopes. The heat generated by the isotopes is captured by thermocouples which convert this heat to the electricity need to heat rocket propellants. This technology is currently being used by NASA deep space probes like Voyager and Cassini.
Nuclear propulsion melts the rocket – causes failure
Stern 5 (David P., MA in physics @ Hebrew U in Jerusalem, 4/29/5, http://www-istp.gsfc.nasa.gov/stargaze/Sstern.htm) JPG
Heavy elements such as the uranium used in generating nuclear power are extremely concentrated sources of energy. A few pounds of nuclear fuel can produce as much as thousands of tons of coal or oil--or of high explosives, in the case of nuclear bombs. Harnessing nuclear energy for spaceflight therefore seemed a natural direction to explore. But nuclear spaceflight is not easily accomplished. Space rockets require not just energy, but also mass, matter ejected backwards, of which nuclear fuel provides very little.The limiting factor in the operation of rockets is not a shortage of energy but the high temperature at which they operate. Ordinary rocket nozzles already run red-hot: adding extra energy to the fuel would raise the temperature, perhaps beyond what the metal can take.