Radiation pressure is one of the oldest known physical phenomena. You have seen it work if you have ever noticed a little glass enclosed gadget that jewelers often place in their shop windows. This is the "Crookes radiometer," a device more that 100 years old. Inside a small glass globe four metal vanes, black on one side and silver on the other, rotate on an axis, although there is apparently no motor driving them.

Light supplies the power. It radiates a "push" that varies from the surface of one vane to that of the one next to it. Now, where could the Saucer engineers get enough light to propel their missile through space?

I envision a motor somewhat like a fluorescent lamp. An inner core is filled with fissionable material, probably a gas. An outer tube surrounding the core contains a fluorescent material.

The fissionable gas activates the fluorescent material, causing light. Light exerts pressure -- a propulsive push or thrust -- against a heavily shielded curved reflector. The push propels the missile.

The activity of the fluorescent material is controlled by increasing or decreasing the amount of fissionable gas in the inner core. And fluorescent material is pumped into the outer tube as it is used up. This is greatly oversimplified, and certain major problems remain.

There is, for instance, no known substance out of which the tubes could be constructed. Any material we can compound would disintegrate under the fissioning process. I would also hesitate to say what kind of shield would completely protect the Saucer crew from radiation effects.

The theory of a radiation pressure motor has been discussed with experts at a number of universities. Some scoffed. Some were encouraging. Since our present investigations of atomic power are in the primer stage, however, I think it's early to say the idea is impossible. Let's simply admit that if the Saucer people utilize anything like this principle, they have probably refined and perfected it.

While I have my neck out really far, I'd like to add that a Saucer would use three sets of motors.

The main set would be used to launch it and propel it on its space voyage. These motors would be placed in one segment of the edge of the disk. A second set, probably located in the flat under surface of the disk, would be used to sustain it in flight while it hovered or prepared to land.

The third would be a small set to control roll and tilt. Since radiation from the main power plant might be great, I should imagine that the crew would be confined to a segment of the leading edge of the disk.

This would leave a large area of the midsection for fuel storage tanks, food supplies and other equipment.

One case, described by both Shalett and Keyhoe, involves a disk conforming to this pattern.

At 2:45 a.m., on July 24, 1948, an Eastern Airlines DC-3, enroute to Atlanta, Georgia, from Montgomery, Alabama, sighted a "brilliant, fast moving object" about a mile away. Captain Clarence S. Chiles, an ex-Air Transport Command flier, and Pilot John B. Whitted, formerly a B-29 pilot, both observed it clearly.

They agreed that it was about 100 feet long, shaped like a cigar. It was wingless. When the object passed them, at about eye level, they saw two rows of "windows" along the fuselage. These glowed with a blinding white light. A dark blue light ran the length of the shape, along the underside. There was a red orange flame exhaust which rocked the DC-3 at the missile veered off and zoomed out of sight.

It seems to me that rather than a cigar shaped fuselage, what the pilots may have seen was a disk, edge on. The two rows of ports would be the vents of the main power plant. The blue light came from the belly motors, thrusting downward to enable the disk to operate at the slow speed of 500 to 700 miles per hour which the pilots estimated. The flame I am less sure about. Possibly the fact that it was traveling within the Earth's atmosphere (the DC-3 was at 5,000 feet during the encounter) made exhaust particles visible. The jolt it imparted to the DC-3, however, is not surprising. I should say that the plane felt a light energy blast, a product of radiation pressure motor operation.
Radiation pressure
From Wikipedia, the free encyclopedia.
Electromagnetic radiation exerts a pressure upon any surface exposed to it. If absorbed, it is the energy flux density divided by the speed of light. If the radiation is totally reflected, the radiation pressure is doubled.

For example, the radiation of the Sun at the Earth has an energy flux density of 1370 W/m2, so the radiation pressure is 4.6 μPa (absorbed) (see also Climate model).

The radiation pressure from the Sun against an absorbing sheet with a mass of 0.8 g/m² is equal to its weight with respect to the Sun's gravity, independent of its distance to the Sun. For example, at the Earth this weight is 4.6 μN/m² and this gravity 6 mm/s².

The fact that electromagnetic radiation exerts a pressure upon any surface exposed to it was deduced theoretically by James Clerk Maxwell in 1871, and proven experimentally by Lebedev in 1900 and by Nichols and Hull in 1901. The pressure is very feeble, but can be detected by allowing the radiation to fall upon a delicately poised vane of reflective metal (Nichols radiometer).

It may be shown by electromagnetic theory, by quantum theory, or by thermodynamics, making no assumptions as to the nature of the radiation, that the pressure against a surface exposed in a space traversed by radiation uniformly in all directions is equal to 1/3 the total radiant energy per unit volume within that sphere. For black body radiation, in equilibrium with the exposed surface, the energy density is, in accordance with the Stefan-Boltzmann law, equal to 4σT4/c; in which σ is the Stefan-Boltzmann constant, c is the speed of light, and T is the absolute temperature of the space. One third of this energy is equal to 2.523×10−16T4 J/m3, which is therefore equal to the pressure in pascals. For example, at the boiling point of water (T = 373.15 K), the pressure only amounts to 5 micropascals (about 3 pounds per square mile). Such feeble pressures are, nevertheless, able to produce marked effects upon minute particles like gas ions and electrons, and are important in the theory of electron emission from the Sun, of cometary material, and so on (see also: Yarkovsky effect, YORP effect).

Sources for the above information include the van Nostrand Scientific Encyclopedia (3rd edition).

In stellar interiors the temperatures are very high. Stellar models predict a temperature of 15 MK in the center of the Sun and at the cores of supergiant stars the temperature may exceed 1 GK. As the radiation pressure scales as the fourth power of the temperature, it becomes important at these high temperatures. In the Sun, radiation pressure is still quite small when compared to the gas pressure. In the heaviest stars, radiation pressure is the dominant pressure component.

Solar sails, a proposed method of spacecraft propulsion, would utilize radiation pressure from the sun as a motive force.

http://en.wikipedia.org/wiki/Radiation_pressure]