Hill witnessed a quartet of UFO's maneuver in the sky in 1952. His trained eye, having observed dozens of rocket launches and aircraft flight tests, could judge the velocity and G-capabilities of the objects. He had been involved in virtually every highly-classified high-performance rocket and aircraft program in the United States. He knew immediately what he saw could not be of earthly construction
However his supervisor, Bob Gilruth (Director of NASA Houston during the Apollo moon landing project), enforced an official policy that UFO's did not exist. NACA/NASA resources could not be spent investigating them. The best that Hill could manage was to be NASA's informal clearing house for UFO sightings. He also managed to instigate NASA projects on hovering platforms that allowed him to study the performance of UFO-type systems.
Hill saw another UFO, this time a classic 'mother ship', in 1962. He used his comprehensive knowledge of physics and aerodynamics to analyze the flying saucer from an engineer's point of view. Four years after his retirement from NASA in 1970, he began a meticulous final engineering analysis of the UFO. This was only published posthumously in 1995, available currently as Unconventional Flying Objects: A Former NASA Scientist Explains How UFOs Really Work.
The scientist analyzed every physically possible method of UFO propulsion and compared them with the observed performance. This included the behavior in flight and hovering, the glow seen around and below the objects, and the reported physical effects. He went through the entire electromagnetic spectrum and the all known families of subatomic particles. Among the non-starters were impulse power, magnetic fields, electrical fields, neutrino and photon propulsion. In the end he was able to rule out all possibilities except one.
Hill compared his analysis to that of the atom envisioned by Ernest Rutherford in 1911. Rutherford, using Newtonian physics, was able to describe an atom consisting of protons and neutrons in a nucleus with electrons spinning in orbits around it. Later 20th Century Einsteinian and quantum mechanics would show Rutherford's model to be simplistic and the atomic reality to be much more complex. But the point was that Rutherford could make a correct first approximation of the atomic structure using the physics he knew. Similarly, Hill believed he could mathematically describe the UFO field without necessarily knowing the future physics that would provide the complete understanding (and allow earthlings to construct such a device).
Hill's analysis completely described the commonly-described characteristics of the UFO:
UFO's can only be propelled by an engine that produces a static field that repels all mass. This field follows the inverse-square law of gravity or photon fields, and is focused to provide propulsive force. Hill showed that using this field the amount of energy to move a UFO to any particular position above a gravitational source (such as the earth) is only equal to energy required to lift it to that height - a very small value.
This field has an electromagnetic component or expression in the range between x-rays and gamma rays. This explains the radiation effects described in many cases on those who get to close to hovering UFO's, while at the same time is consistent with the lack of residual radioactivity at UFO landing sites. It is also the precise energy wavelength that creates the glow which surrounds UFO's when in operation. This is a result of excitation of air molecules, and shifts from dull red through orange, yellow, and then to intense welding-arc white as the electromagnetic component shifts from soft x-rays up to the lower portion of the gamma-ray spectrum. The plasma generated by this wavelength of radiation is cold - consistent with the lack of intense heat reported in UFO close encounters.
The nature of the UFO field means that saucers have to maneuver in the manner described in the sightings but quite different from that of lifting, thrust-driven aircraft. Saucers tilt in the direction of the force to be applied to accelerate or decelerate. In cruise, the slight saucer tilt is opposite that of a wing producing lift. This slight tilt is necessary to overcome both air drag and the negative lift a saucer creates:
Hill showed that the UFO's he and others had observed and could easily accelerate or decelerate at over 100 G's. This meant that reported 'instantaneous' changes of direction and 90 degree turns by flying saucers were no such thing. It was quite possible for a UFO to brake to a complete stop, and then accelerate again to the same speed in any direction, in under one second. This would appear as an 'instantaneous' maneuver to the human eye but in fact violate no laws of physics or inertia.
Hill studied in detail the necessary arrangement of field generators within spherical and elliptical objects necessary to produce the necessary propulsive force, as well as make the G-forces imperceptible to the occupants and exert the necessary force to move air away from the moving saucer. Aerodynamics was Hill's specialty, and he provided the mathematics that showed why saucers produce no sonic boom, can move within the atmosphere at thousands of miles per hour with little friction and no perceptible noise. In fact, he showed the induced pressure drop of the UFO field would cool rather than heat the saucer while moving within the atmosphere at tremendous speed. This explained why saucers, unlike the space shuttle or high-speed aircraft, were not red hot when landing after high speed runs.
Investigating long-range maneuvers using the UFO field near a planet, Hill determined the optimum trajectory was high-speed and followed a ballistic path. This was consistent with reports that UFO's would depart near-vertically at tremendous acceleration and speed at the end of a sighting.
The UFO static field would be cyclic in nature, and this would produce a mathematically definable movement of air molecules when a UFO was hovering. Hill determined that this cyclic air movement, and its vibration effects on objects near a UFO, was the source of the sound heard when UFO's were hovering The UFO 'hum' was not the sounds of machinery within the UFO but rather the effect of the field on the environment.
Finally, Hill studied interstellar travel by UFO's. This would involve a close approach to a massive nearby gravitational object (e.g. the sun of a solar system) to obtain maximum utilization of the UFO gravity drive. He calculated that assuming an approach within 23 million miles of the Earth's sun, and using an initial acceleration of 1000 G's, a UFO could reach 70% of the velocity of light by the time it reached the orbit of Pluto. By that time the gravity-repulsive UFO drive would be nearly ineffective. Further acceleration to near-light speed using impulse power would however be relatively easy with advanced technology. Thanks to relativistic effects, the occupants would experience only a few months of travel time when traveling between solar systems.
Of course, decades of time would have passed on earth and their home planet during the travel period. Hill imagined that the large cylindrical 'mother ships' might be the actual home of the extraterrestrials.
A little reflection shows that such a ship would be the focus of their lives and the location of their friends and families, if such existed in their culture. So it would be irrelevant in their way of existence if everyone they had known back at the home planet, if they had one, had aged and died while they were away. A human equivalent would be when earthlings move permanently in space rather than time from one state or country to another. Those left behind are a closed chapter of a life once known. For star-hopping extraterrestrials, 'you can never go back' would have a tougher physical reality than for humans.
Hill produced a complete explanation of the UFO phenomenon using known physics. There was no need for recourse to other dimensions, faster-than-light-travel, or mystical explanations. The diversity of UFO types and occupants reported convinced the engineer that they represented visitors from a large number of worlds. He assessed their behavior as being that of scientists on study missions, taking environmental samples, even abducting humans and animals for tests, but avoiding contact and interaction with humans.