Over the past century, exploitation of the air domain’s speed, vantage, maneuverability, flexibility, and range changed the nature of warfare. Militarily, operating in the air domain provides hypersonic speeds, vantage: the ability to see not only over the next hill but also over the horizon. It provides maneuverability unencumbered by mountain ranges, roads, river crossings, or rocky shoals at sea. The air domain is physically linked to every other domain, thus providing flexibility in operations, while its range provides an avenue for access anywhere in the world, anytime.
Today, from a military perspective, the degree to which the United States can exploit the air domain in its favor to find and hold at risk
any target (fixed, mobile, hardened, and deep inland) anywhere on the globe is a key differentiator that makes it a military superpower.
Attributes of the Air Domain
The Department of Defense defines the air domain as “the atmosphere, beginning at the Earth’s surface, extending to the altitude where its effects upon operations become negligible.” At its most fundamental level, the atmosphere is composed of air, a mixture of gases consisting of 21 percent oxygen, 78 percent nitrogen, and 1 percent argon, carbon dioxide, and other gases. The composition of air is perhaps its most
extraordinary and important characteristic because it determines the very nature of the domain and dictates what can and cannot be
done in it and drives the characteristics of the platforms that fly through it.
Because these gases have mass, the distribution of the atmosphere is not uniform. For example, due to gravitational effects, nearly 50 percent of the atmospheric mass is contained below 18,000 feet at the equator, 90 percent is contained below 52,000 feet, and 99.99 percent is contained below 330,000 feet or an altitude of 100 kilometers.
The atmosphere is divided into several layers that are of varying degrees of significance to military operations.
• The lowest level, the troposphere, varies in height from the surface to 60,000 feet at the equator to 30,000 feet over the poles. All-weather occurs in the troposphere. The top of the troposphere, called the tropopause, is the “cap” where summer thunderstorms flatten out to form an anvil shape. In the troposphere, the wind blows west to east in the Northern Hemisphere and east to west in the Southern Hemisphere. Temperature decreases by about 3.5 degrees Fahrenheit with every 1,000 feet of climb. Wind speed changes significantly with altitude, averaging 75 mile per hour from the west at 35,000 feet over the central United States in winter to as much as 200 miles per hour in the strongest jet streams.
• Above the troposphere is the stratosphere, which extends to about 180,000 feet. The stratosphere is where the ozone layer is located, and it is free from clouds and weather. Wind diminishes significantly with altitude in the stratosphere. Most of today’s military operations occur in the troposphere and the stratosphere.
• Above the stratosphere at an altitude of about 34 miles is a region of the atmosphere that has proven easy to transit but difficult to operate in persistently. In this region, there is enough air to cause drag and surface heating but not enough to support aerodynamic control or air-breathing engine combustion.
• Sitting above the stratosphere, extending to 260,000 feet, is the mesosphere. Here, meteors burn up due to atmospheric heating. The ionosphere, which causes high-frequency radio waves to bounce off the atmosphere enabling long-range amateur radio operations, begins in this region.
• Above the mesosphere lies the final layer of the atmosphere, the thermosphere, which extends to as much as 600 miles above the Earth depending on solar activity. Atmospheric drag caused by gases in the lower portion of this layer limits the lowest unpowered, stable satellite orbit to roughly 120 miles.
The characteristics of air and the atmosphere make five modes of access to the air domain possible: lighter-than-air flight, heavier-than-air flight, missiles, ground-fired or sea-fired projectiles, and the electromagnetic spectrum
Lighter-than-air flight is achieved by trapping gases lighter than oxygen and nitrogen, like hydrogen or helium, or heated air in a
sealed casing. Because the gas inside the casing is lighter than the surrounding air, the lift is produced. The volume of air contained in that casing, coupled with the characteristics of the gas inside, determines its lifting ability. This allows exploitation of the air domain using hot air balloons, gas-filled balloons, or powered airships (dirigibles and blimps). Lighter-than-air aircraft can provide persistence and relatively
heavy lift, but this means of access is both slow and heavily affected by weather.
Although the speed of heavier-than-air platforms made them dominant over their lighter-than-air brothers, a role remains for balloons and powered airships today. Tethered balloons (aerostats) extending up to 14,000 feet line the U.S. border with Mexico and have been used in Iraq to provide persistent surveillance coverage. Powered airships used by the logging industry to extract harvested timber from remote areas could provide a slow-speed, heavy-lift logistics option for military purposes.
High-altitude balloons also offer military utility as a backup to space-based capabilities like communications satellites.
Heavier-than-air flight, on the other hand, uses aerodynamic forces to produce and sustain lift. Aerodynamic lift is produced by moving an airfoil (wing) through volume of air or fluid. Design differences between the upper and lower surfaces of the airfoil force the air to move faster across the upper surface as the wing is propelled through the air. This creates an area of lower pressure on the top of the wing
that generates lift. There are other factors involved, but if one produces enough aerodynamic lift to overcome the force of gravity, then
a heavier-than-air machine can fly.
There are two other forces at play in the creation of aerodynamic lift: the thrust required to propel a wing though the air to generate lift
and the drag that the wing creates through the process of creating lift. Thus, balancing the problems of lift, gravity, thrust, and drag makes
flight possible using vehicles that are powered (airplanes, cruise missiles, helicopters, tilt rotors, and quadcopters) and unpowered (towed
gliders, lifting bodies, and air-delivered guided munitions).
Aircraft provide a reusable form of access to the air domain and offer an incredible degree of flexibility with regard to speed, range, payload, and endurance for military operations.
Missiles use the brute force of expanding, burning gases provided by liquid-fueled or solid-fueled rocket engines to overcome the effects
of gravity and gain access to the air domain. As the vehicle accelerates, it takes on aerodynamic characteristics and can be controlled using aircraft-like control surfaces until it reaches the mid stratosphere. Above this altitude, small thrusters or gimbaled engines controlled by guidance systems allow the highest levels of precision in movement and endgame placement.
Missiles deliver high-speed effects in both the air and space domains without the risk associated with manned flight, but there are trade-offs. Lift is created on the sheer power of their engines, making this form of access markedly less efficient than winged aircraft. Moreover, missiles used for attack or defense are not reusable; an aircraft can return to base and reload with ordnance, but a missile is a one-time shot.
Projectiles like bullets, mortars, rockets, and bombs use a controlled explosive charge, propellant, or the momentum gained by a parent platform to overpower the aerodynamic effects of weight and drag temporarily in order to enter and transit the air domain. Aimed downward, air-launched munitions provide an additional and incredibly potent axis of fire against land-based and sea-based targets. Aimed upward, ground-fired projectiles provide a low-cost, effective way to deny an enemy use of the air domain in a limited area. For example,
the vast majority of aircraft losses in Vietnam were due to anti-aircraft artillery rather than surface-to-air missile defenses.
Today, new technologies like electromagnetic rail guns can fire projectiles from land-based or sea-based platforms at hypersonic speeds to attack other surface targets or defend against low-flying, supersonic cruise missiles and high-speed ballistic missile warheads.
Finally, the electromagnetic spectrum provides a less obvious but equally powerful method of accessing the air domain to enable, disrupt, or deny air operations. This includes the use of voice and data communications to direct and employ forces; optical, infrared, laser, and
radar-based sensors to detect objects in the air domain and guide weapons; high-power lasers to deny optical sensors or to attack incoming
aircraft, missiles, or bombs; high-powered microwaves to disrupt the operation of airborne vehicles and weapons; electromagnetic decoys to confuse an opponent’s systems; and modern jamming techniques to deny, disrupt, or spoof radars, communication, and space-based navigation systems like the Global Positioning System (GPS).