The U.S. Navy was in the LTA "business" from 1917 to 1962. In April 1917, the year the United States entered World War I, the Navy accepted its first LTA aircraft, the non-rigid airship ("blimp") D-1. This was the progenitor of 245 manned LTA craft acquired by the Navy. These were in addition to several tethered observation balloons (operated by the Navy mostly from ships) during World War I. Navy LTA development between the world wars included non-rigid blimps and rigid airships. Two of the rigid airships, the Akron and Macon, were "flying aircraft carriers," each capable of storing, launching, and retrieving four fighter aircraft.
The Navy stressed the development and procurement of non-rigid airships. During World War II, the Navy's LTA strength reached 168 blimps, which were engaged in coastal patrol and convoy escort, mainly in the Atlantic and North African theaters. Although the blimps scored no U-boat sinkings-and one blimp was shot down by a U-boat-the airships did make a major contribution as a deterrent to submarine attacks and carried out other reconnaissance missions.
Navy interest in LTA continued into the Cold War era with the procurement of 56 blimps of advanced design, initially for anti-submarine warfare, and subsequently for the airborne early warning mission. The latter were for use on the seaward extension of the early warning network built to warn of a Soviet air attack against the United States. In this role, the radar-configured airships were to supplement EC-121 Constellation-type aircraft, surface radar picket ships, and fixed offshore radar stations. In 1962 the Navy's airship program was terminated because of fiscal constraints as other Navy aviation and missile programs (including the Polaris deterrent system) were accorded higher priorities.
The U.S. Navy carried out a limited LTA research and development effort from 1975 to 1990, oriented primarily to maritime patrol and cruise missile defense for surface combatants. A prototype airship, the Sentinel 1000, was procured for tests. A 1985 agreement between the Navy and Coast Guard transferred responsibility for tethered aerostats to the latter service. The Coast Guard operated several aerostats and support ships until 1992, after which the program was transferred to the Army and promptly disbanded by the Army.
The balloon is a free floater that is capable of carrying heavy loads to high altitudes. The weather balloon is a common example. Balloons are used by the National Aeronautics and Space Administration (NASA) and other research activities to venture above the stratosphere and higher. Payloads of 8,000 pounds borne aloft to an altitude of 134,000 feet and higher are not uncommon. The military application for balloons is limited, however. Subject to the atmosphere's prevailing winds, free- floaters are unable to maintain position over an area, a capability necessary for persistence.
Aerostats achieve persistence over an area of interest. The aerostat offers the advantage of retaining a position and the ability to extend the horizon. Today, aerostats are used for advertising, communications relay, surveillance, and other ISR missions. They range from those used at a tethered altitude of several hundred feet with a light payload, to those that can ascend to 15,000 feet and accommodate a 2200-pound payload.
Today's manned airships can operate in benign areas. Present payload ranges are about 3,000 pounds and maximum altitudes are less than 10,000 feet. The altitude extends the horizon for ISR. Current airborne endurance is limited by both crew endurance and fuel capacity.
Current U.S. operations, ashore and afloat, and especially for the GWOT, have demonstrated the need for persistent ISR for specific areas. LTA vehicles, whether tethered (aerostats) or free floating (airships) are capable of remaining in the same location for prolonged periods and providing a persistent presence. Similarly, commanders, whether afloat or ashore, have always sought to extend their horizons-to see farther or "over the next hill." Tethered or free-floating LTA vehicles can provide the ability to carry ISR, EW, and communications-relay systems to greater altitudes than can presently be reached for ships or tactical units, and can be directly responsive to the needs of ship or unit commanders.
At the same time, potential ISR, EW, and communications-relay payloads for LTA vehicles are lighter and require less power than their predecessors. Airships can also help to mitigate "urban canyon" effects on ISR, communications, and navigation aides Persistence can be achieved relatively easily with current LTA technology to altitudes up to about 10,000 feet. Although higher altitudes for ISR/EW/communications relay systems are highly desirable, reaching them is relatively difficult and presents technological challenges.
In many respects, LTA vehicles represent a potentially cheaper option for providing certain capabilities than satellites or Unmanned Aerial Vehicles (UAVs). LTA vehicles also offer the potential to lift and transport heavy payloads over long distances directly to the vicinity of the warfighter - "from fort to foxhole." This mode of transport can eliminate transfer points (e.g. ship to pier to road convoy), the vulnerability of road convoys, and need for intermediate depots.
LTA vehicles are capable of ascending to altitude and increasing the line of sight to the horizon. This figure shows how the line of sight is increased as altitude increases. In a typical environment, line of sight (for communication or surveillance) is limited to less than 20-25 nautical miles. Line of sight can be increased to over 100 nautical miles by ascending to less than 10,000 feet, an altitude easily attainable by the aerostats available today. It also demonstrates the value of moving to higher altitudes since line of sight can be increased to over 300 nautical miles by ascending to or above 60,000 feet.
Altitude also provides the ability to see and achieve line of sight over both urban and geographic terrain features. At altitude, an aerostat can provide communication connectivity to a low flying helicopter hidden by terrain or urban structures and provide surveillance of threats, which might otherwise be hidden by the environment.
LTA vehicles have relatively large volumes when compared to aircraft, an attribute that offers the potential for very large, internal antenna apertures. Furthermore, platform non-recurring costs of LTA vehicles are relatively low when compared to aircraft procurement. Similarly, LTA infrastructure costs are potentially low relative to the costs of other forms of aviation, both manned and unmanned. LTA vehicles also permit quiet operation, an attribute that enhances survivability and may create advantages for psychological warfare.
Most of the airships currently flying are similar in size to the Goodyear airship, which is approximately 100 feet long, contains 200,000-300,000 ft3 of helium, and operates at altitudes below 10,000 feet. Existing aerostats fall into a similar size range. High-altitude airships will require a significant increase in size to reach the altitudes desired. For example, NASA's high-altitude balloon requires 40 million cubic feet of helium to fly to an altitude of 130,000 feet.
- The US Army's "Rapidly Elevated Aerostat Platform (REAP)" system is a lightweight aerostats only 9.5 meters (31 feet) long and with a sensor payload weighing 16 kilograms (35 pounds). The entire system is capable of being deployed by a Hummer light truck, and has seen service in Iraq.
- The US Army's "Rapid Deployment Initial Development (RAID)" system uses small TCOM 15M aerostats, with a length of 15 meters (49 feet) and a sensor payload of 90 kilograms (200 pounds). It has been deployed in small numbers to Afghanistan.
- The US Army later deployed a "Persistent Threat Detection System (PTDS)" to Iraq. It uses mid-sized Lockheed Martin 56K aerostats, with a length of 33.4 meters (109 feet 6 inches) and a sensor payload of 225 kilograms (500 pounds).
- The US Marines have deployed their own aerostat system, the "Marine Airborne Re-Transmission System (MARTS)" to Iraq, though as its name implies it is intended for radio relay and not surveillance. It uses TCOM 32M aerostats, with a length of 32 meters (105 feet) and a relay payload weighing 225 kilograms (500 pounds).
- The US Navy has committed to the development of a sea-based aerostat surveillance system with a payload capacity of 225 kilograms (500 pounds).
- The US military acquired a set of radar aerostats for defense of forces in the field under the "Joint Land Attack Cruise Missile Defense Elevated Netted Sensor (JLENS)" program. The contract for JLENS was awarded to Raytheon in 1998, with the first system delivered in 1999. They feature TCOM 71M aerostats carrying a Raytheon payload with surveillance and targeting radars. Plans were to acquire up to a dozen systems. South Korea, Kuwait, Saudi Arabia, and the United Arab Emirates also operate aerostat radar systems.
- "Tethered Aerostat Radar System (TARS)" began in 1980, when the USAF used an aerostat to keep an eye on Cuba from the Florida Keys. In 1984, the US Customs Service began to put up an aerostat surveillance system to spot smugglers trying to get into the US from the south, while the US Coast Guard began to put up aerostats along the Gulf of Mexico for maritime surveillance. Initially, the network used aerostats built by General Electric, but they proved too leaky and were replaced by "71M" aerostats built by TCOM Corporation of Columbia, Maryland. The 71M aerostats were 71 meters (233 feet) long and carried the AN/TPS-63 surveillance radar system. They were replaced in turn in the late 1990s by more capable "420K" aerostats from Lockheed Martin Tactical Defense Systems (TDS), carrying the Lockheed Martin L-88A surveillance radar. The 420K aerostats are 63.5 meters (208 feet 6 inches) long, they are about 50% bigger in volume than the 71M aerostats they replaced, and are built under subcontract by ILC Dover Corporation. Floating at an altitude of 3,000 to 4,500 meters (10,000 to 15,000 feet), the aerostats can spot low-flying intruders and water surface traffic.
Airspace deconfliction, already a major concern for military commanders, will be exacerbated by the presence of LTA vehicles. The relatively low speed of airships and the fixed (tethered) operating mode of aerostats must be addressed for both combat and behindthe- lines areas. Similarly, while helium is readily available in the United States, the establishment of an LTA program will require the development of an infrastructure to acquire, inspect, store, and transport the gas to LTA facilities.
A large number of aerostats have been built and are in operation, making it relatively easy to procure and evaluate them further. Presently, the airship community is limited to relatively few commercial vehicles. Thus, airship operations would require the development of a procurement and evaluation process, establishment of a training program and personnel career-planning programs, building of a logistics structure, and other steps.
Because of their large surface areas and lack of aerodynamic control surfaces, LTA vehicles are affected greatly by weather and winds-less a concern for the airship, which can maneuver out of weather, but still a major consideration for all forms of LTA vehicles. Takeoffs and landings of LTA vehicles, both manned and unmanned, can be difficult because of wind and weather conditions. Airships can, under certain circumstances, avoid unfavorable weather and, if necessary, land at alternative locations. Aerostat landings are more problematic because they must be winched down in the event of unfavorable weather or winds; bringing down an aerostat from 10,000 feet could take up to two hours.
Historically, rigid airships have been susceptible to severe atmospheric turbulence. The primary options for avoiding high winds and atmospheric turbulence are flying the airships to a safe location or securing them in hangars designed to withstand hurricane-force winds. This will likely continue to be the dominant threat to LTA vehicles.
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