Article by AI based on youtube video transcript: The Insane Engineering of the F-117 Nighthawk

The Birth of Stealth

Two F-117s are approaching their target over Baghdad. They carry two GPS-guided missiles aimed at the assumed location of Saddam Hussein. This mission may have revealed the stealthy aircraft to radar below. The F-117 Nighthawk's journey began on the desk of Ben Rich, the new director of Skunk Works. He was given a Russian scientific paper that held the key to a new generation of aircraft. The development of Soviet radar-guided missiles had shifted the balance of the battlefield. The US needed a new strategy.

Countering the Soviet Threat

The Soviets invested heavily in radar-guided surface-to-air missiles like the SA5. These missiles could reach 125,000 ft and carry nuclear warheads. The US Air Force realized they could be wiped out in just 17 days in a war with the Soviets. Two counter-strategies emerged: low-altitude terrain hugging aircraft like the B1 Lancer, and stealth technology. Dennis Overhauser, a Skunk Works radar dome expert, found the key to unlocking stealth design in the Russian paper.

The Hopeless Diamond

The paper provided a method to calculate the diffraction of electromagnetic waves, a crucial element of stealth design. This led to the development of the Echo1 program, which allowed for the analysis of simple shapes for stealth capabilities. The program birthed the "Hopeless Diamond," a faceted diamond-shaped template that lacked the components of a flyable aircraft. This template was to be molded into a functional aircraft by the best engineers.

Defining the Mission

The first step was to define the F-117's mission profile. Stealth was a priority, so external hardpoints were not an option, limiting its weapons capacity. The F-117 was to be a precision bomber, armed with two laser-guided missiles. It featured infrared sensors for targeting and could also carry GPS-guided missiles. The F-117 was to be a subsonic aircraft to avoid sonic booms and minimize its infrared signature.

Engine Selection and Design

The General Electric J85 engine was chosen for the half-blue demonstrator and later upgraded to the General Electric F404. The engine inlets were covered with a radar-reflecting grid to minimize radar signature. The grid spacing was critical to impede radar transmission. The inside of the inlet was lined with radar-absorbing material (RAM). The F-117 also had unique secondary intake doors to increase airflow at lower speeds.

Overcoming the Ice

The engine inlet grid posed a significant challenge for ice buildup. To combat this, the F-117 was equipped with inflight anti-icing equipment, essentially a windscreen wiper that sprayed a glycol-based anti-icing mixture. The engine outlet was also uniquely designed to minimize heat and radar signatures. The exhaust was cooled and distributed over a wider area using a flat rectangular duct and a "Platypus tail" covered in heat-absorbing ceramic tiles.

Flight Control Challenges

The unusual exhaust setup caused instability issues during flight testing. The inward-angled thrust vectors of the engine nozzles often differed, causing differential thrust and sideways movement. This was exacerbated by the flight computer misinterpreting the sideways movement as sideslip, leading to a positive feedback loop of instability. The problem was solved by using a beta vane to accurately measure sideslip.

Designing the Wings

The F-117's wings were subject to unique constraints due to the stealth requirements. The 72.5-degree sweep angle, determined by the Echo1 program, severely limited lift. The flat faceted design of the airfoil further reduced lift and introduced aerodynamic flutter issues. To compensate for the reduced lift, the wings were made large, with extensions increasing the wing area to 72 square meters.

Stability and Control Issues

The extreme wing sweep of the have blue demonstrator caused pitch instability at high angles of attack. To counter this, a control surface on the Platypus tail automatically deflected downwards. However, this led to a crash during testing. The F-117 replaced the inward canted fins with a V-tail and enlarged the fins for better control. The Platypus tail was removed, and a second outboard elevon was added to each wing.

Operational Limitations and Fuel Capacity

The F-117's poor lift-to-drag ratio limited its maximum takeoff weight. Its fuel tanks, located above the weapons bay and in the wings, held 8.2 metric tons of JP8 fuel, giving it a range of 1,720 km. This necessitated frequent aerial refueling, especially for long missions like the deployment to Saudi Arabia for Operation Desert Storm. A proposed carrier-based version, the F-117N, was rejected.

The Legacy of the Nighthawk

The F-117 was a groundbreaking achievement in aviation technology, but its limitations were apparent. The US Air Force recognized its limited shelf life and began developing the F-22, a true fighter aircraft with advanced stealth capabilities. The F-117's success in the Gulf War, where it effectively neutralized Iraqi air defenses, solidified its place in history. However, the emergence of counter-stealth technologies led to its eventual retirement.