TEAL RAIN

In the 1970s, DARPA initiated project Teal Rain, focused on improving unmanned aircraft engine performance and design, and its Praeire and Calere programs, which demonstrated the ability to carry significant payloads for operationally required durations. From 1980 to 1982, DARPA's TEAL RAIN program investigated advanced concepts for High Altitude Long Endurance (HALE) UAVs to perform reconnaissance, surveillance, and target acquisition missions. The objective was to find ways to keep UAVs aloft for days, even weeks. TEAL RAIN investigated nuclear-, solar-, and microwave-powered motors, as well as exotic materials and designs. These were unfettered, technology-push studies seeking to generate new ideas. This program successfully demonstrated very long-endurance and high-altitude operation but failed to receive support to move into acquisition.

The Global Hawk and Predator unmanned aerial vehicles (UAVs) were prominent in Operation Enduring Freedom in Afghanistan and Operation Iraqi Freedom. DARPA started on the concept of a high altitude, long-range, extended loiter unmanned system in the 1970s with the TEAL RAIN program. After a number of significant technical breakthroughs, the Global Hawk high altitude endurance UAV transitioned from DARPA to the Air Force in 1998. The Tier 2 Predator medium-altitude endurance UAV evolved directly from DARPA’s AMBER and Gnat 750-45 designs and was operationally deployed in the mid-1990s.

Development of Praeire and Calere led to the development of Aquila RPV. The Philco-Ford RPV had exchangeable modular payloads, the RPV carrying the daytime TV-laser target designator configuration called PRAEIRE, and the same RPV carrying a lightweight FLIR and laser target designator combination, called CALERE. The propulsion system was an adaptation of an engine that had been used in lawn mowers.

DARPA has invested in UAV development since the 1960s, a time when the concept of using UAVs engendered widespread skepticism within military circles. The agency’s foresight and unflagging interest in UAVs, which included research on structures, propulsion, guidance, sensors, communications and autonomy, helped fuel a revolution in unmanned aircraft that continues to this day.

At the same time, DARPA began work on the first small, low-observable, remotely piloted vehicle (RPV) for the Navy. And later that decade, DARPA funded the development of small, airborne sensor technologies for use in UAVs, including miniaturized stabilization systems, moving target indicators, radars and anti-jam data links — all of which contributed to increased platform survivability at greater ranges.

A crucial characteristic about several of these accomplishments, which holds true for many DARPA programs, is that it took a long time from an idea’s conception to its use by the U.S. military. DARPA has shown itself very willing to repeatedly tackle hard technical problems, even in the face of previous failure, if the technology offers revolutionary new capabilities for national security. Patience and persistence are required for those who pursue high-risk technology, but they are often rewarded with extremely large payoffs.

After the failure of high altitude, long endurance UAVs like the Air Force’s Compass Cope in the mid-1970s, DARPA in the late 1970s began a project called “Teal Rain” investigating high altitude engine performance. Based on that work, they commissioned Israeli inventor and iconoclast Abraham Karem to design a less ambitious project than Compass Cope, but one with medium altitude (15,000-25,000 feet), long loiter capability. Still concerned about the cruise missile threat, the Navy took over partial sponsorship of the $40 million program as it showed promise for the same mission its ill-fated “over the horizon” (OTH) UAV was slated to fill—long range Harpoon target acquisition.

Under the Teal Rain joint program with the U.S. Navy, DARPA funded the development of the first endurance unmanned aerial vehicle (UAV), Amber, which in 1988 flew for more than 38 straight hours and reached an altitude of 25,000 feet. The Amber demonstration featured innovations in many technologies (digital flight controls, composite materials, microprocessors, and satellite navigation) and led to the Gnat 750 and the Tier 2 Predator. DARPA also supported development of the Global Hawk, a related high-altitude UAV system. These platforms have been transformative with respect to warfighting and ISR (intelligence, surveillance, and reconnaissance) capabilities.

The Predator unmanned aerial vehicle (UAV) grew out of Amber, a program that grew out of DARPA's Teal Rain program for advancing technologies for High Altitude Long Endurance (HALE) UAVs. The UAV that became Amber was proposed to DARPA in 1978 by its developer, Abraham Karem, who owned a firm called Leading Systems, Inc. [LSI] The PM he initially briefed did not pursue the idea, but DARPA Director Robert Fossum heard the presentation, over-ruled this rejection, and funded it out of his own office’s funds.

Karem successfully developed and demonstrated a UAV called Albatross. DARPA then in 1984 began a program for Amber, a scaled-up version of Albatross. In 1984 Leading Systems Inc. (LSI) received a contract to develop the Amber medium-range tactical surveillance UAV. Amber was a classified reconnaissance UAV, which was flown in November 1986 — just two years after the initial DARPA contract. However, Amber was used only in small numbers (by the CIA), and, with no subsequent DoD business, Karem’s firm, Leading Systems, Inc., went into bankruptcy and was sold to General Atomics. After a decade of delay, OSD pushed renewed interest in HALE UAVs and Amber was modified under a DARPA program to become Predator,

In 1988 Grumman Aerospace Corporation was awarded a patent [4,786,008] providing an effective nuclear powered unmanned aircraft that is capable of remaining in flight for extended periods of time, for example intervals exceeding six months--without the need for refueling. Such a nuclear powered unmanned aircraft (drone) may be preprogrammed to survey the landscape or may be manually directed to do so; and video images as well as instrumentation readings could be forwarded over an emergency communication link to a central government facility to assess damage. It is understood that nuclear powered aircraft are undesirable during peacetime in view of the substantial danger that such an aircraft poses when flying over civilian populations. However, such objections become academic after a nuclear attack has been experienced.

Unlike previously disclosed nuclear reactor aircraft designs, the Grumman invention utilized a sealed hot gas reactor loop for powering turbomachinery connected to propellers which power the drone. The utilization of propellers is advantageous since their efficient operation permits the aircraft to remain aloft for a protracted period of time without refueling. Unlike prior art nuclear aircraft which exhausts heated gas in order to remove waste heat from a reactor, the present invention incorporates radiators under the skin of the aircraft wings which radiate waste heat to space. The reactor utilized in the present invention is a gas-cooled helium reactor. As helium gas passes through the reactor, it is heated; and the heated gas travels through a closed loop to the turbomachinery which drives the aircraft propellers. The heated gas then travels to radiator tubes to radiate the remaining waste heat to space. The resulting cooled helium gas then undergoes compression and is returned to the reactor to complete a Brayton cycle.

The Grumman invention avoided the use of liquid metal reactors because these metals are normally solid; and in order to use them as a circulating liquid, extremely high temperatures must be maintained which would present material design problems for the present aircraft which must be light and durable in order to sustain protracted flight. Although the loop for carrying the heated helium gas is intended to be completely sealed, it is inevitable that the light gas will diffuse slowly through seals. Accordingly, a helium storage reservoir 29 is included within the fuselage to replenish any lost helium gas on a continual basis. Although the reactor was described as operating with helium gas, it can also operate with a helium-argon gas mixture to decrease helium permeability thereby decreasing the chance of gas leak through small fissures, cracks and seals.

The Grumman design also accomplished the discussed objective where other conventional types of reactors would fail. For example, the obvious choice of a water-cooled conventional reactor would be inappropriate for the present invention inasmuch as the relatively low heat transferability of water would require a huge radiator surface resulting in an impractically large wing area.