Archive for December, 2023

Aviation History, Part III

December 23rd, 2023

Authors: Relly Victoria Virgil Petrescu & Florian Ion Tiberiu Petrescu

Airships have been proposed as a potential cheap alternative to surface rocket launches for achieving Earth orbit. JP Aerospace has proposed the Airship to Orbit project, which intends to float a multi-stage airship up to mesospheric altitudes of 55 km (180,000 ft) and then use ion propulsion to accelerate to orbital speed. At these heights, air resistance would not be a significant problem for achieving such speeds. The company has not yet built any of the three stages.

NASA has proposed the High Altitude Venus Operational Concept, which comprises a series of five missions including manned missions to the atmosphere of Venus in airships. Pressures on the surface of the planet are too high for human habitation, but at a specific altitude the pressure is equal to that found on Earth and this makes Venus a potential target for human colonization.

The advantage of airships over airplanes is that static lift sufficient for flight is generated by the lifting gas and requires no engine power. This was an immense advantage before the middle of World War I and remained an advantage for long-distance or long-duration operations until World War II. Modern concepts for high-altitude airships include photovoltaic cells to reduce the need to land to refuel, thus they can remain in the air until consumables expire.

The disadvantages are that an airship has a very large reference area and comparatively large drag coefficient, thus a larger drag force compared to that of airplanes and even helicopters. Given the large frontal area and wetted surface of an airship, a practical limit is reached around 130–160 kilometers per hour (80–100 mph). Thus airships are used where speed is not critical.

The lift capability of an airship is equal to the buoyant force minus the weight of the airship. This assumes standard air-temperature and pressure conditions. Corrections are usually made for water vapor and impurity of lifting gas, as well as a percentage of inflation of the gas cells at liftoff. Based on specific lift (lifting force per unit volume of gas), the greatest static lift is provided by hydrogen (11.15 N/m3 or 71 lbf/1000 cu ft) with helium (10.37 N/m3 or 66 lbf/1000 cu ft) a close second. At 6.13 N/m3 (39 lbf/1000 cu ft), steam is a distant third. Other cheap gases, such as methane, carbon monoxide, ammonia and natural gas have even less lifting capacity and are flammable, toxic, corrosive, or all three (neon is even more costly than helium, with less lifting capacity). Operational considerations such as whether the lift gas can be economically vented and produced in flight for control of buoyancy (as with hydrogen) or even produced as a byproduct of propulsion (as with steam) affect the practical choice of lift gas in airship designs.

In addition to the static lift, an airship can obtain a certain amount of dynamic lift from its engines. Dynamic lift in past airships has been about 10% of the static lift. Dynamic lift allows an airship to “take off heavy” from a runway similar to fixed-wing and rotary-wing aircraft. However, this requires additional weight in engines, fuel and landing gear, negating some of the static lift capacity.

The altitude at which an airship can fly largely depends on how much lifting gas it can lose due to expansion before stasis is reached. The ultimate altitude record for a rigid airship was set in 1917 by the L-55 under the command of Hans-Kurt Flemming when he forced the airship to 7,300 m (24,000 ft) attempting to cross France after the “Silent Raid” on London. The L-55 lost lift during the descent to lower altitudes over Germany and crashed due to loss of lift. While such waste of gas was necessary for the survival of airships in the later years of World War I, it was impractical for commercial operations or operations of helium-filled military airships. The highest flight made by a hydrogen-filled passenger airship was 1,700 m (5,500 ft) on the Graf Zeppelin’s around-the-world flight. The practical limit for rigid airships was about 900 m (3,000 ft), and for pressure airships around 2,400 m (8,000 ft).

Modern airships use dynamic helium volume. At sea-level altitude, helium takes up only a small part of the hull, while the rest is filled with air. As the airship ascends, the helium inflates with reduced outer pressure, and the air is pushed out and released from the downward valve. This allows an airship to reach any altitude with balanced inner and outer pressure if the buoyancy is enough. Some civil aerostats could reach 100,000 ft (30,000 m) without explosion due to overloaded inner pressure.

The greatest disadvantage of the airship is size, which is essential to increasing performance. As for size increases, the problems of ground handling increase geometrically. As the German Navy changed from the P class of 1915 with a volume of over 31,000 m3 (1,100,000 cu ft) to the larger Q class of 1916, the R class of 1917, and finally the W class of 1918, at almost 62,000 m3 (2,200,000 cu ft) ground handling problems reduced the number of days the Zeppelins were able to make patrol flights. This availability declined from 34% in 1915, to 24.3% in 1916 and finally 17.5% in 1918.

So long as the power-to-weight ratios of aircraft engines remained low and specific fuel consumption high, the airship had an edge for long-range or -duration operations. As those figures changed, the balance shifted rapidly in the airplane’s favor. By mid-1917, the airship could no longer survive in a combat situation where the threat was airplanes. By the late 1930s, the airship barely had an advantage over the airplane on intercontinental over-water flights, and that advantage had vanished by the end of World War II.

This is in face-to-face tactical situations. Currently, a High-altitude airship project is planned to survey hundreds of kilometers as their operation radius, often much farther than the normal engagement range of a military airplane. For example, a radar mounted on a vessel platform 30 m (100 ft) high has radio horizon at 20 km (12 mi) range, while a radar at 18,000 m (59,000 ft) altitude has radio horizon at 480 km (300 mi) range. This is significantly important for detecting low-flying cruise missiles or fighter-bombers.

The most commonly used lifting gas, helium, is inert so presents no fire risk. Modern airships have a natural buoyancy and special design that offers a virtually zero catastrophic failure mode. A series of vulnerability tests were done by the UK Defence Evaluation and Research Agency DERA on a Skyship 600. Since the internal gas pressure was maintained at only 1–2% above the surrounding air pressure, the vehicle proved highly tolerant to physical damage or to attack by small-arms fire or missiles. Several hundred high-velocity bullets were fired through the hull, and even two hours later the vehicle would have been able to return to base. Ordnance passed through the envelope without causing critical helium loss. In all instances of light armament fire evaluated under both test and live conditions, the airship was able to complete its mission and return to base.

High-altitude platform station

High-altitude platform station (short: HAPS) is – according to Article 1.66A of the International Telecommunication Union´s (ITU) ITU Radio Regulations (RR) – defined as “a station on an object at an altitude of 20 to 50 km and at a specified, nominal, fixed point relative to the Earth”.

Each station shall be classified by the service in which it operates permanently or temporarily.

A HAP can be a manned or unmanned airplane, a balloon, or an airship. All require electrical power to keep themselves and their payload functional. While current HAPS are powered by batteries or engines, mission time is limited by the need for recharging/refueling. Therefore, alternative means are being considered for the future. Solar cells are one of the best options currently being used under trial for HAPS (Helios, Lindstrand HALE).

Whether an airship or an airplane, a major challenge is the ability of the HAP to maintain station keeping in the face of winds. An operating altitude between 17 and 22 km is chosen because in most regions of the world this represents a layer of relatively mild wind and turbulence above the jet stream. Although the wind profile may vary considerably with latitude and with the season, a form similar to that shown will usually obtain. This altitude (> 17 km) is also above commercial air-traffic heights, which would otherwise prove a potentially prohibitive constraint.

Since HAPS operate at much lower altitudes than satellites, it is possible to cover a small region much more effectively. Lower altitude also means much lower telecommunications link budget (hence lower power consumption) and smaller round-trip delay compared to satellites. Furthermore, deploying a satellite requires significant time and monetary resources, in terms of development and launch. HAPS, on the other hand, are comparatively less expensive and are rapidly deployable. Another major difference is that a satellite, once launched, cannot be landed for maintenance, while HAPS can.

One of latest uses of HAPS has been for radiocommunication service. Research on HAPS is being actively carried largely in Europe, where scientists are considering them as a platform to deliver high-speed connectivity to users, over areas of up to 400 km. It has gained significant interest because HAPS will be able to deliver bandwidth and capacity similar to a broadband wireless access network (such as WiMAX) while providing a coverage area similar to that of a satellite.

High-altitude airships can improve the military’s ability to communicate in remote areas such as those in Afghanistan, where mountainous terrain frequently interferes with communications signals.

One of the best examples of a high-altitude platform used for surveillance and security is Northrop Grumman RQ-4 Global Hawk UAV used by the US Air Force. It has a service ceiling of 20 km and can stay in the air for continuous 36 hours. It carries a highly sophisticated sensor system including radar, optical, and infrared images. It is powered by a turbofan engine and is able to deliver digital sensor data in real-time to a ground station.

Another future use that is currently being investigated is monitoring of a particular area or region for activities such as flood detection, seismic monitoring, remote sensing and disaster management.

Perhaps the most common use of high-altitude platforms is for environment/weather monitoring. Numerous experiments are conducted through high-altitude balloons mounted with scientific equipment, which is used to measure environmental changes or to keep track of the weather. Recently, NASA in partnership with The National Oceanic and Atmospheric Administration (NOAA), has started using Global Hawk UAV to study Earth’s atmosphere.

Due to the height, more than 90% of atmospheric matter is below the high-altitude platform. This reduces atmospheric drag for starting rockets. “As a rough estimate, a rocket that reaches an altitude of 20 km when launched from the ground will reach 100 km if launched at an altitude of 20 km from a balloon.” Such a platform has been proposed to allow the usage of (long) mass drivers for launching goods or humans into orbit.

The United States Department of Defense Missile Defense Agency contracted Lockheed Martin to construct a High-Altitude Airship (HAA) to enhance its Ballistic Missile Defense System (BMDS).

An unmanned lighter-than-air vehicle, the HAA was proposed to operate at a height of above 60,000 feet (18,000 m) in a quasi-geostationary position to deliver persistent orbital station keeping as a surveillance aircraft platform, telecommunications relay, or a weather observer. They originally proposed to launch their HAA in 2008. The airship would be in the air for up to one month at a time and was intended to survey a 600-mile (970 km) diameter of land. It was to use solar cells to provide its power and would be unmanned during its flight. The production concept would be 500 feet (150 m) long and 150 feet (46 m) in diameter. To minimize weight. it was to be composed of high strength fabrics and use lightweight propulsion technologies.

A subscale demonstrator unit for this project, the “High Altitude Long Endurance-Demonstrator” (HALE-D), was built by Lockheed Martin and launched on a test flight on July 27, 2011, to demonstrate key technologies critical to the development of unmanned airships. The airship was supposed to reach an altitude of 60,000 feet (18,000 m), but a problem with the helium levels occurred at 32,000 feet (9,800 m) which prevented it from reaching its target altitude, and the flight was terminated. It descended and landed at a speed of about 20 feet per second in a heavily forested area in Pennsylvania. Two days after the landing, before the vehicle was recovered from the crash site, the vehicle was destroyed by fire.

A stratospheric airship is a powered airship designed to fly at very high altitudes 30,000 to 70,000 feet (9.1 to 21.3 kilometers). Most designs are remote-operated aircraft/unmanned aerial vehicles (ROA/UAV). To date, none of these designs have received approval from the FAA to fly in U.S. airspace.

Stratospheric airship efforts are being developed in at least five countries.

The first stratospheric powered airship flight took place in 1969, reaching 70,000 feet (21 km) for 2 hours with a 5 pounds (2.3 kilograms) payload. On December 4, 2005, a team led by Southwest Research Institute (SwRI), sponsored by the Army Space and Missile Defense Command (ASMDC), successfully demonstrated powered flight of the HiSentinel stratospheric airship at an altitude of 74,000 feet (23 km). Japan and South Korea are also planning to deploy HAAs. South Korea has been conducting flight tests for several years with a vehicle from Worldwide Aeros.

The Integrated Sensor is Structure (ISIS) was a program managed by the United States Air Force (USAF) Research Laboratory to research the feasibility of using an unmanned airship as a high-altitude aerial reconnaissance and surveillance platform. It is sometimes called Integrated Sensor is the Structure, as a fundamental innovation was the use of the airship structure as the sensing component of a state-of-the-art radar system.

In 2006, contracts were awarded to Raytheon for the development of a large-area, light, Active electronically scanned array antenna which could be bonded to the structure of a blimp, Northrop Grumman for antenna development, and Lockheed Martin for the development of the airship. As proposed, the 450-foot (140 m)-long surveillance airship could be launched from the US and stationed for up to 10 years at an altitude of 65,000 feet (20,000 m), observing the movement of vehicles, aircraft, and people below. At that altitude, the airship would be beyond the range of most surface-to-air and air-to-air missiles. The airship would be filled with helium and powered, at least in part, by solar-powered hydrogen fuel cells.

On March 12, 2009, the USAF announced that it had budgeted $400 million for work on ISIS. In April 2009, DARPA awarded a $399.9 million contract to Lockheed Martin as the systems integrator and Raytheon as the radar developer for phase three of the project: the construction of a one-third scale model, which would remain in the air for up to a year. The ultimate goal was to provide radar capable of delivering persistent, wide-area surveillance tracking and engagement of air targets within a 600-kilometer area and ground targets within a 300-mile (480 km) area, according to DARPA. The model blimp was to have radar coverage of about 7,176 square yards (6,000 square meters) and be tested at an altitude of 6 miles (9.7 km) above the ground. The contract initially awarded $100 million to the two companies, with the rest to follow in phases, with a completion date of March 2013.

As of 2012, the development of the airframe had been delayed to focus on “radar risk reduction”. The United States Department of Defense ended the program in 2015. $471 million had been spent from 2007 through 2012.

Mystery airships or phantom airships are a class of unidentified flying objects best known from a series of newspaper reports originating in the western United States and spreading east during late 1896 and early 1897. According to researcher Jerome Clark, airship sightings were reported worldwide during the 1880s and 1890s. Mystery airship reports are seen as a cultural predecessor to modern claims of extraterrestrial-piloted flying saucer-style UFOs. Typical airship reports involved unidentified lights, but more detailed accounts reported ships comparable to a dirigible. Reports of the alleged crewmen and pilots usually described them as human-looking, although sometimes the crew claimed to be from Mars. It was popularly believed that the mystery airships were the product of some inventor of genius who was not ready to make knowledge of his creation public. For example, Thomas Edison was so widely speculated to be the mind behind the alleged airships that in 1897 he “was forced to issue a strongly worded statement” denying his responsibility.

It has been frequently argued that mystery airships are unlikely to represent test flights of real human-manufactured dirigibles as no record of successful sustained or long-range airship flights are known from the period and “it would have been impossible, not to mention irrational, to keep such a thing secret.” To the contrary, however, there were, in fact, several functional airships manufactured before the 1896–97 reports (e.g., Solomon Andrews made successful test flights of his “Aereon” in 1863), but their capabilities were far more limited than the mystery airships. Reece and others note that contemporary American newspapers of the “yellow journalism” era were more likely to print manufactured stories and hoaxes than modern news sources, and editors of the late 1800s often would have expected the reader to understand that such stories were phony. Most journalists of the period did not seem to take the airship reports very seriously, as after the major 1896-97 have concluded, the subject quickly fell from public consciousness. The airship stories received further attention only after the 1896-97 newspaper reports were largely rediscovered in the mid-1960s and UFO investigators suggested the airships might represent earlier precursors to post-World War II UFO sightings.

The best-known of the mystery airship waves began in California in 1896. Afterward, reports and accounts of similar airships came from other areas, generally moving eastward across the country. Some accounts during this wave of airship reports claim that occupants were visible on some airships, and encounters with the pilots were reported as well. These occupants often appeared to be human, though their behavior, mannerisms, and clothing were sometimes reported to be unusual. Sometimes the apparent humans claimed to be from the planet Mars.

Historian Mike Dash described and summarized the 1896–1897 series of airship sightings, writing:

Not only were [the mystery airships] bigger, faster and more robust than anything then produced by the aviators of the world; they seemed to be able to fly enormous distances, and some were equipped with giant wings… The 1896–1897 airship wave is probably the best investigated of all historical anomalies. The files of almost 1,500 newspapers from across the United States have been combed for reports, an astonishing feat of research. The general conclusion of investigators was that a considerable number of the simpler sightings were misidentification of planets and stars and a large number of the more complex the result of hoaxes and practical jokes. A small residuum remains perplexing.

The Sacramento Bee and the San Francisco Call reported the first sighting on November 18, 1896. Witnesses reported a light moving slowly over Sacramento on the evening of November 17 at an estimated 1,000-foot elevation. Some witnesses said they could see a dark shape behind the light. A witness named R.L. Lowery reported that he heard a voice from the craft issuing commands to increase elevation in order to avoid hitting a church steeple. Lowery added, “in what was no doubt meant as a wink to the reader” that he believed the apparent captain to be referring to the tower of a local brewery, as there were no churches nearby. Lowery further described the craft as being powered by two men exerting themselves on bicycle pedals. Above the pedaling men seemed to be a passenger compartment, which lay under the main body of the dirigible. A light was mounted on the front end of the airship. Some witnesses reported the sound of singing as the craft passed overhead.

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