Definition: Flight at altitudes higher than most flights but lower than orbital flight; roughly between 50,000 feet (9 miles) and 100 miles.
Significance: High-altitude flight has often been the frontier of aviation technology, meteorology, astronomy, and aerial reconnaissance. Many tasks can still be done more cheaply, or can only be done, in high-altitude flight.
High-Altitude Characteristics
The meaning of the term “high altitude” has changed over the years. Balloonists struggled to reach altitudes between 20,000 and 30,000 feet, yet by the last third of the twentieth century these altitudes were routine for commercial and military jet transports. The only constant is that the frontier always lies at the current definition of high altitude.
Decreasing pressure is the most important feature of high altitude. Most of the earth’s atmosphere is in the troposphere, roughly the first 40,000 feet from the surface, and 99 percent of the atmosphere is below 127,000 feet. This has many implications. For high-speed jet aircraft, lesser air density allows greater speed, reduces heating problems, and allows greater engine efficiency until the available oxygen is too dilute to support combustion. For rockets, which require no external oxidizers, there is no limit except available fuel and oxidizer.
For slower aircraft utilizing maximum lift for minimum energy, progressively less air density requires progressively wider wingspans, bigger control surfaces, cleaner aerodynamics, or more power to lift the same payload. For lighter-than-air (LTA) craft, such as balloons and dirigibles, decreasing air density with increasing altitude means there is less lift available per unit volume, so LTA craft must be larger to carry a given payload to higher altitudes.
For living creatures, such as human crewmembers, a low-pressure (hypobaric) environment can be deadly. For instance, at about 18,000 feet the total air pressure is halved from that at sea level, and the amount of oxygen available to the body is similarly halved. The result is hypoxia (low oxygen) with progressively more severe symptoms as pressure declines: euphoria, headache, nausea, irritability, confusion, unconsciousness, and death. Aircraft crews can compensate for low pressure by breathing a greater percentage of oxygen. However, above 49,000 feet even pure oxygen does not have sufficient pressure to sustain life, so crews must have either pressurized cabins or pressure suits.
Flying above much of the atmosphere means that much of the radiation usually stopped by the atmosphere will impact the craft. The lack of atmosphere allows clearer astronomical observations at light wavelengths stopped by the atmosphere, such as infrared. However, increased radiation in ultraviolet and shorter wavelength bands can attack a number of plastics that might be used in aircraft structures, and flight crews are subject to higher doses of ionizing radiation than people on the ground.
Cold is another feature of high altitude. A rough formula is that in temperate zones every thousand feet of altitude is equivalent to traveling 75 miles farther from the equator. Temperatures drop steadily with altitude in the troposphere, stay the same orevenrise slightly in the lower stratosphere, and then become somewhat irrelevant as declining air density begins to approach vacuum. Cold is not a serious problem for supersonic craft, for which avoidance of overheating is the prime concern. However, it can be life-threatening for slower craft.
Lastly, every mile of altitude yields roughly 33 miles of line-of-sight to the horizon. This has great importance for airborne radars and communications platforms. A radar plane flying at 40,000 feet has a range 260 miles, compared to 520 miles for a craft at 80,000 feet. In communications, an aircraft holding position or flying in tight circles to stay nearly in the same place can replace satellite communications service at lower cost and allow for ground stations that use much less power. They can also be put in place or upgraded more quickly than can satellite launches.
There are three types of high-altitude craft: highly efficient propeller and jet craft, supersonic jets and rockets, and lighter-than-air craft, such as balloons and dirigibles.
The U-2 and Its Competitors
The Lockheed U-2, sometimes called the Dragon Lady, is the most famous formerly secret high-altitude aircraft. In the early 1950′s, during the most intense part of the Cold War, the U.S. intelligence community wanted a spy aircraft that could fly higher than any interceptors in the Soviet Union. Kelly Johnson at Lockheed proposed radically reconfiguring an F-104 Starfighter as a glider body with an 82-foot wingspan and a jet engine so it could fly at 70,000 feet. Johnson and his “Skunk Works” flew the first craft in August, 1955. On July 5, 1956, a U-2 flew over Moscow, the Soviet capital. Although the Soviets protested, they could do nothing about the over flights, and the United States denied its existence.
However, the U-2 flew slowly, turned slowly, had not been designed for stealth by minimizing its radar and infrared signature, and Soviet antiaircraft missiles improved. On May 1, 1960, the Soviets downed a U-2 one thousand miles inside their border, and captured the pilot, precipitating a major diplomatic incident. Another U-2 was shot down during the Cuban Missile Crisis in 1962, and the U-2′s were pulled back from well-defended areas. However, they continued to be used into the twenty-first century as high-flying signal-intelligence craft, obtaining data without crossing into hostile territory, and as conventional reconnaissance craft once air superiority was achieved, as in the Gulf War of 1991.
This long life required a series of upgrades. The most important was the U-2R, beginning in 1967, which was a larger, stealthier aircraft that accomodated a two-person crew, a fourteen-hour maximum mission operations time, and a ferrying range of 8,000 miles. The civilian U-2 is the ER-2, which has done mapping and atmospheric sensing for several decades.
The most recent U-2 competition has come from two planes from the company Scaled Composites: the Raptor and the Proteus. Both use lightweight composite materials and advanced aerodynamics pioneered by Burt Rutan, designer of the nonstop world-circling Voyager. The remotely controlled Raptor is a propeller-driven slower competitor, but it is stealthier than the U-2, and it can linger over an area for forty-eight hours. It demonstrated an 8,000-mile flight range in 2001.
The Proteus is a direct, cheaper competitor to the U-2, with jet propulsion, a 2,000-pound payload, a fourteen-hour operations length, and an operational altitude of nearly 70,000 feet. As with its shape-changing namesake in Greek mythology, the Proteus can be configured for several other missions. Most important, it is a telecommunications repeater station, and for this mission, the Proteus demonstrated stable flight at 55,000 feet in late 2000.
A new altitude record of 96,500 feet was set on August 13, 2001, by the Helios, a robotic flying wing designed by Paul MacCready of AeroVironment. (Mac-Cready had also designed the human-powered Gossamer Condor.) Although its payload is only 220 pounds, the Helios is direct competition for the Scaled Composites’ Proteus repeater stations. Helios has solar cells for daylight power and for electrolyzing water into hydrogen and oxygen for nighttime fuel-cell power. Consequently, Helios can fly for six months at a time.
Lighter-Than-Air Craft at High Altitudes
Balloons were the first craft capable of reaching high altitude. On December 1, 1783, Jacques-Alexander-Cesar Charles made the first flight in a hydrogen balloon and also made the first high-altitude flight, limited only by the uncomfortable cold he encountered. For the next 120 years, balloons were the only means of observing the atmosphere.
Swiss balloonist Auguste Piccard demonstrated the first pressurized cabin on May 27, 1931, when he and an assistant reached 51,793 feet, making them the first fliers ever to reach the stratosphere. More importantly, they discovered that cosmic rays increased with altitude, proving that they came from somewhere in space rather than the other suggested source, radioactivity within the earth.
American and Soviet flights from the 1930′s through the 1960′s carried personnel and instruments to steadily greater heights and developed many technologies later used in the space race. In fact, on May 4,1961, the American Stratolab Vreached an altitude of 113,700 feet with an open gondola, testing space suits in near-space conditions for the Mercury orbital-flight program.
After the 1960′s, improved robotic instrumentation allowed LTA craft to shed the weight of the balloonists and their life support gear. By the late twentieth century, the National Aeronautics and Space Administration (NASA) began using super-pressure balloons for relatively small payloads in balloons weighing several tens of pounds. These balloons are different from zero-pressure balloons that expand when warmed by the sun and contract at night. When warmed at high altitude, zero-pressure balloons must vent excess helium to prevent bursting. This gas loss limits mission duration to only several days. With stronger materials, super-pressure balloons keep the same maximum shape even when warmed. Because no gas is lost such balloons can operate for months, and some of these balloons have circled the globe several times. By the early twenty-first century, NASA began flying large super-pressure balloons in a program called the Ultra Long Duration Balloon (ULDB), with balloons carrying several tons of instrument payload. These balloons compete with spacecraft for carrying astronomic payloads because they are cheaper, turnaround time is shorter, and awkward pay-loads can be accommodated that might not fit in a rocket or aircraft fuselage.
Dirigibles have greater difficulty reaching high altitudes because the volume of buoyant gas needed to lift the payload as well as a body structure, engines, and control surfaces can become truly immense compared to the weight of payload being carried. Yet, dirigibles can fly slowly enough into the wind to remain stationary over one spot for weeks, ideal for communications repeating stations. Thus, by the early twenty-first century, Sky Station International was building a dirigible to compete against those of AeroVironment and Scaled Composites.
Supersonic High-Altitude Craft
The most important supersonic high-altitude craft was the North American X-15 rocket-propelled research airplane, used from 1959through 1968 to test materials and aerodynamics at speeds as great as 6.7 times the speed of sound (Mach6.7, or 4,520 miles per hour) and altitudes as high as 354,000 feet. Lessons learned from these tests were later applied to the space shuttle and many supersonic airplanes.
High-altitude supersonic flight development reached a peak in the early 1960′s and then languished until the beginning of the twenty-first century. As noted, supersonic craft operate best in high-altitude regimes because thinner air causes less heat through friction and allows greater efficiency. Higher altitudes had also been a general direction of military flight since World War I (1914-1918).
These two trends led to the North American XB-70, planned as a heavy bomber flying at Mach 3 and a flight ceiling of70,000 feet. The XB-70 flew in the early 1960′s. By 1964, the Soviet Union responded with the Mach-2.8 Mikoyan MiG-25 interceptor.
After the 1960′s, other developments intervened. First, intercontinental ballistic missiles (ICBMs) were widely deployed. ICBMs could deliver bombs much faster than could aircraft. Furthermore, they did not require the expensive high-temperature alloys and vast amounts of fuel in operational training. Second, increasingly effective surface-to-air missiles caused large military aircraft to switch from high-altitude flight to flying low while ducking around missile sites. The XB-70 was never produced in volume, and the F-25 became largely a high-speed reconnaissance craft. The Lockheed SR-71 Blackbird was the best high speed reconnaissance craft, with a maximum speed of more than Mach 3 (2,200 miles per hour and a maximum altitude of 90,000 feet).
The XB-70 also demonstrated that no matter how high supersonic transports flew, sonic booms were a major irritant to people on the ground. The booms and the cost of heavy fuel use both limited the market for commercial supersonic transports, such as the Concorde and the similar Tupolev Tu-144.
However, research has continued to develop better supersonic craft as the first stages for launch into orbit because oxygen carried by rockets weighs eight times as much as hydrogen fuel whereas jets get their oxygen from the atmosphere. Also, there have been reports of secret military craft with speeds of Mach 5 through Mach 10 and flight ceilings of 148,000 feet.
