Hiller experimented with and then abandoned small pulse jets. His next idea revealed that a "hot-cycle pressure-jet" system that used the rotor blade as a both combustion chamber and exhaust pipe. This proved more efficient and produced more thrust that Doblhoff's cold-cycle system. But Hiller soon recognized that the greatest weight savings and propulsive efficiency could be gained by mounting engines directly onto the tips of the main rotor blades. Jet turbine technology was not sufficiently developed, so Hiller began to consider the ramjet.

Although extremely promising on paper, the HOE-1s and YH-32s proved troublesome during operational tests. During mountaintop takeoff and landings conducted by the Army, gusty winds easily upset the helicopter but it was so small that personnel attempting to steady the aircraft risked a significant impact hazard from the main rotor. During operations at night, the ramjets produced a brilliant flaming exhaust ring that was visible for miles. This was hardly suitable for a military environment, and the effect prompted numerous reports of UFO sightings whenever the aircraft operated near populated areas. In addition to polluting the visible spectrum, the ramjet helicopter was extremely noisy.

In addition to these irritations, another serious operational flaw was exposed. The voracious ramjets consumed an enormous amount of fuel. They gulped ten times the amount used by a piston engine of comparable output. At full power, the ramjets burned nearly 272 kg (600 lb) of fuel per hour, yet the helicopter carried only 136 kg (300 lb) of fuel. The engines could operate for only half an hour including engine start-up and shutdown. This limited endurance was exacerbated by a maximum airspeed surpassed by many automobiles of the day. The Hornet's never-exceed airspeed was a mere 114 kph (71 mph). This crippling velocity and poor endurance combined to gave the helicopter an absolute maximum range of less than 56 km (35 miles). Hiller appreciated the aircraft's limitations, but he believed the idea would prove itself as soon as reliable turbine engines could be found to replace the ramjets. But the matter was further complicated because turbines required special centrifugal fuel pumps engineered to withstand several thousand times the force of gravity as they rotated on the ends of the Hornet's main rotor at more than 500 rpm. The technical knowledge and expertise to build such pumps lay years in the future.

The Army and Navy never deployed their YH-32s and HOE-1s operationally, but they did utilize them in some interesting tests. Flying helicopters in forward combat zones during the Korean War sparked interest in arming these aircraft to provide fire support for army ground units. . The Army tested the two helicopters in 1957 at Fort Rucker, Alabama with a wide variety of weapons, including missiles, rockets, and recoilless rifles. The large blast radius at the back of the recoilless rifles had necessitated the change from the tail rotor to twin rudders. Army officials considered these tests successful, and believed that the experiments proved the viability of the helicopter as a stable weapons platform. However, the performance of the YH-32A was so marginal that any further role was unthinkable, and the ULV project became the Hiller ramjet helicopter's swan song.

Source: National Naval Aviation Museum