by Richard Sheffield
What Is Stealth Technology?
The use of the word stealth in the aerospace industry is a fairly recent occurrence. But it falls under a larger program that the military has been studying for a number of years—Low Observability, or LO, technology.
Most of the emphasis has been placed on the ability of stealth or LO aircraft to avoid radar detection, and rightly so, but to be truly stealthy, six areas must be taken into account.
- The Infrared, or heat, signature of the aircraft
- The Acoustics, or how much noise the aircraft makes
- Visual appearance, since the human eye is still a good detection device
- Smoke emitted; smoky engines point a large finger at the aircraft
- Contrails, or vapor trails left by the engines that must be reduced
- Radar Cross Section, or how well the aircraft can be seen on radar
Although all the others are important, dependence on high-tech radar for air defense makes the Radar Cross Section, or RCS, by far the most important area.
Radar Cross Section
Radar operates by sending out radar waves. When these waves strike an object, some of these waves are reflected back to the radar receiver and show up as a pip on the screen. The nature of these waves makes them operate in much the same way as bright sunlight on a highly polished surface such as an aircraft. No matter where you stand, there's some part of the aircraft that will reflect the sunlight directly at you. This is known as glinting. So when a normal aircraft is hit by radar waves, there is always some surface on the aircraft that will reflect these waves strongly back to the receiver.
Figure 1-1. Glinting and Facetting
Curved surfaces “glint” and reflect some of the radar energy back in all directions. Facetted surfaces reflect most of the radar energy in a single direction.
These surfaces may be broad, flat areas such as vertical stabilizers, which reflect most of the waves back in one direction. Or they may be curved surfaces such as wing edges or the aircraft body, which reflect the waves more or less equally in all directions, some of them glinting directly back to the receiver. These waves may also get stuck in cavities such as engine inlets. In this case, they'll resonate and send waves back in all directions.
In the early stages of radar development, researchers looked for a standard way to measure the ability of different objects to reflect radar waves. A standard call called the Radar Cross Section, or RCS, was developed.
The RCS for an object is calculated by first determining the amount of radar energy reflected by the object. Then calculations are made to determine the size of a reflective sphere that would reflect the same amount of radar energy. The area of a disk with the same diameter as the sphere is then the RCS for the object. The RCS is referred to in square meters.
It's important to note that the RCS for an object is not directly linked to its size but to how well it reflects radar waves. Broad, flat surfaces are very efficient reflectors. An F-15 Eagle fighter shows about 25 square meters of surface area when viewed broadside. But its RCS could be as high as 400 square meters, due to the design of the aircraft. Large numbers like this occur because reducing the aircraft RCS was never taken into account during the design of the airframe. When reducing the RCS is a major design criteria, amazing performance improvements can be obtained.
Jet fighters typically have a frontal RCS of about 6 square meters. Frontal RCS numbers as low as .01 square meters have been seriously discussed when experts talk about the stealth fighter. This would present roughly the same radar signature as a medium-size bird.
It might seem incredible to be able to reduce the radar signature of similar aircraft by so much. But once researchers discovered what made some aircraft reflect radar so well, they could set about reducing those items.
There are some treatments such as radar-absorbing paint and materials that can help to reduce RCS in normal aircraft, but the huge reductions in RCS are gained in changing the way we design airframes. Since the size and weight of the aircraft are not primary factors in RCS, the designer has some latitude in the way the aircraft is laid out. But some assumptions must be made up front. One assumption is that RCS from all angles cannot be reduced—there must be tradeoffs. If the main area of concern is the RCS when seen from the front and side, the RCS from top or bottom might not be so good. But that's a good assumption since few radars can look straight up or down. Another assumption: If this aircraft is going to depend on not being seen as a defense, it will not need to be as maneuverable as one that must hide in the terrain. This, too, gives the designer room to work.
With these items in mind, the designer can get to work. He must reduce broad, flat surfaces that would reflect back to the source. He must also eliminate as many right angles as possible. These usually occur where two items such as the wings and body meet. These right angles cause radar waves to be reflected from one surface to the other and then right back to the receiver, greatly increasing RCS. This is not to say that flat surfaces cannot be used, just that care must be taken to ensure that radar waves are not reflected back to their source.
The designer must also think ahead. If this is to be an attack aircraft, it must be able to carry weapons. If you hang a bunch of bombs and missiles under your carefully designed stealth aircraft, you're wasting time. The weapons will have as high an RCS as an entire airplane. So, the weapons must be carried internally until they're needed. This also goes for fuel. Since most fighters carry about 30 percent of their fuel in external tanks, these must be internalized as well.
Another method of reducing the RCS is the use of RAM, or Radar Absorbing Materials. These materials are composed of carbon and certain iron compounds, or salt-related polymers. These compounds have the ability to take the energy from a radar wave and convert it to heat. This heat is then easily dissipated by the aircraft. When these compounds are combined with nonreflective resin epoxies, they produce a material that's stronger than steel and 30-percent lighter than aluminum. This material can then be used as the skin of the aircraft or in the internal support structure.
Figure 1-2. Two Radar Absorbing Materials
All these items must be juggled to create an aircraft capable of avoiding radar detection, carrying enough weapons to be worth the trip, and possessing enough range to get somewhere useful. No easy task.