March 5, 2013
Engineers who work in aviation learn to be risk averse. Change tends to happen slowly, through evolution rather than sudden breakthroughs. When a new idea comes along, it’s usually tested for years before being introduced to the fleet, and even then it usually debuts with the military—they have ejection seats, after all. Some engine components are tried out in ground-based turbines for electrical power plants before working their way into aircraft engines, a practice GE says it pursued for its latest engines.
One maxim of jet engine design is that higher power comes at a cost of greater heat. Unfortunately, heat melts metal. So anyone who can make an engine run hotter and still survive will be able to tweak more thrust out of the same amount of fuel. People have been working at the problem for years.
Early in the game, engineers tried alloys that could survive the 2,000+-degree-Fahrenheit gas that meets the first set of turbine blades. But the gas, coming straight from the combustor, put such stress on the turbines that they only lasted tens of hours. The metal would soften to a point where the rapidly spinning blades elongated, and their tips began to rub against the tip seals on the engine’s outer wall.
By combining metals such as nickel, chromium and even more exotic elements from the periodic table, engines could be made to run hotter and survive. Later, the blades were made from crystals grown in such a way that the metal’s grain aligned with the centrifugal force, lending greater strength. Another improvement was cooling the blades with tiny passages that carried cold air to the leading edge, using air from the engine’s compressor to supply the cooling flow. That stole some power from the compressor, but the investment paid off in higher combustion temperatures and improved power and efficiency.
Research into the use of ceramics in the engine’s hot section began decades ago, starting with ceramic coatings on combustors and parts of the turbine section. A NASA technical memorandum (89868) dated May 1987, authored by Gerald Knip, Jr. at NASA’s Lewis Research Center in Cleveland, Ohio, describes “revolutionary materials” applied to subsonic jet engines. It describes ceramic composites that overcome the typical brittle quality of ceramics by using reinforcing fibers in much the same way that carbon fiber reinforces modern composites.
GE and the Air Force are now going all in with advanced ceramics, which are incorporated into a research engine called ADVENT for ADaptive Versatile ENgine Technology. In tests, the engine is reported to have run hotter than any engine ever built. Ceramic matrix composites, or “CMCs,” made from silicon carbide matrix and fibers, make it possible for the engine to tolerate gas temperatures of 2,400 degrees and achieve a reported gain in fuel efficiency of 25 percent. With fuel prices so high, that kind of progress, following decades of materials research, couldn’t come at a better time.
May 16, 2012
Yesterday the Defense Department sharply curtailed flying in the Lockheed Martin F-22 Raptor due to pilot reports of disorientation similar to the symptoms of oxygen deficiency. Although the DoD has been aware of the situation for a while, recent TV broadcasts may have given the issue a new focus while a supplemental oxygen system is being readied as backup.
The Raptor isn’t engaged in combat anywhere, but it will proceed with a deployment to the Mideast for joint maneuvers. Training flights are to be restricted to a distance allowing for immediate landing. Other types of aircraft will handle long-range patrol missions in Alaska.
The decision may not accurately be described as “grounding” the airplane, but it comes very close.
May 11, 2012
Early May was not a good time to be a public affairs officer for the F-22 Raptor. Both ABC and CBS investigated the airplane’s troubles with a system that provides enriched oxygen to the pilot at high altitude. A CBS “60 Minutes” segment extensively interviewed two Air Force pilots who went public with their decisions to refuse to fly any more missions in the F-22, which is not currently engaged in combat.
The pilots say they have experienced symptoms that appear to indicate hypoxia, or a lack of oxygen that occurs at high altitude. One told the CBS interviewer that he recognized the early onset of hypoxia but became so hampered by his lack of ability to focus that he was unable to recall where the control was to activate the emergency oxygen supply.
Older aircraft supply oxygen to the crew from pressurized tanks, which are heavy and can explode if struck by a projectile. With newer aircraft like the F-22, the Air Force has been moving to systems based on a molecular sieve principle, in which nitrogen can be separated from ambient air by forcing it under pressure to adsorb onto a substance to which the nitrogen adheres, thereby providing air that is higher in oxygen content. So far, it’s not clear whether the system employed by the F-22 has a fundamental design problem, or if the airflow from the system is being contaminated somehow.
Air Force flight surgeons suggested that pilots wear sensors that clip onto one fingertip and read body oxygenation levels; the devices are familiar to hospital patients whose vital signs are monitored continually by automatic machines. But if contamination rather than an actual lack of oxygen is causing the symptoms of hypoxia, such sensors would not help. Charcoal filters were installed to cleanse the pilots’ airflow of any possible contaminants, but the filters only added to the problem when pilots began inhaling black dust from the charcoal.
So far, one pilot’s death in a Raptor crash is being blamed on the suspect oxygen system. Pilots are especially wary because hypoxia has bewildering symptoms, among which is a sudden feeling of elation. Its onset may reduce a pilot to a conscious but dysfunctional state within one or two minutes, and all military air crew members are trained in chambers that simulate oxygen deprivation at high altitude by pumping the air out while trainees try to write words or recognize pictures of common objects.
As a result of the controversy, the utility of the F-22 itself is being questioned, mostly due to its sophistication and cost. Reports say that one flight hour in one of the fighters costs the Air Force north of $40,000. And critics say the Raptor was designed to fight a foe that doesn’t exist. Interestingly, the two pilots interviewed by CBS both praise the airplane and love to fly it. But it’s hard to love anything when you’re starting to pass out.
You can watch the “60 Minutes” segment below: