Recently, the United States Air Force (USAF) announced the award of Exosonic, Hermeus and Boom supersonic small commercial contracts for the development of a potential supersonic transport aircraft.
Boom Supersonic said in a September 8 statement that the company`s contract funding will fund the exploration of configurations based on the Overture aircraft that the company is developing for commercial supersonic flight. Overture will use carbon fiber composite materials to maintain strength at high temperatures, which is better than the aluminum alloy used in supersonic Concorde aircraft. According to a statement from the company, the expansion and contraction of composite materials under supersonic conditions is much smaller than that of metals, allowing Overture to conceptually be able to fly at higher speeds.
Earlier, Boom announced that its supersonic demonstrator XB-1 will be launched on October 7, 2020. XB-1 is the world`s first self-developed supersonic jet. It will showcase the key technologies of Overture and Boom commercial airliners, such as advanced carbon fiber composite structure, computer-optimized efficient aerodynamics, and efficient supersonic propulsion system . Analyzing the materials of the built XB-1 also allows us to further understand Overture.
Materials for making supersonic aircraft
Wood is perishable, fabrics are too fragile, steel is too bulky, titanium is too expensive, and aluminum cannot withstand the heat all the time, so carbon fiber composites stimulate innovation.
In fact, the history of aircraft materials—what parts are made of—is like a chronicle of modern engineering.
Today's airplanes include a variety of materials, many of which were not widely used or not feasible at all 50 years ago. For the past two years, crazy engineers have been testing materials for supersonic flight. They have combined innovative and traditional materials on the supersonic demonstration aircraft XB-1.
The material of XB-1 is selected based on dozens of factors. In every step of construction, engineers have to balance performance, cost, strength and weight requirements. For every pound of weight lost in the design, one pound of fuel will be added, making the XB-1 fly longer at supersonic speeds. Therefore, the analysis of each material and its subsequent components is methodical. Materials also need to be tested over time to see how they perform; throughout the life of the aircraft, each component must maintain its mechanical properties, which means that after 500 hours of flight, the performance should be as good as after 50 hours of flight.
For supersonic aircraft, the material must also provide thermal resistance due to the unique problems of aerodynamic heating. When flying at supersonic speeds, the air on the plane is compressed more, which generates more heat. Since all metals expand and lose their strength when heated (when the speed exceeds Mach 5, most metals will melt or become very soft, and then bend and lose their properties), so the choice of materials for supersonic aircraft must Consider aerodynamic heating.
On the basis of maintaining strength and heat resistance, the main materials of XB-1 are as follows:
Carbon fiber composite material
Most of the XB1's fuselage structure and some brackets are made of lightweight carbon fiber composite materials, which have strong strength and low CTE (coefficient of thermal expansion). Their expansion rate is closer to that of titanium, which is also the key material of XB-1. Composite materials have been shown to withstand corrosion and wear, making them an element of choice for the fuselage structure.
In the design process of the Concorde, the composite materials were not fully tested and proven in aeronautics, although the aircraft did include them. The Concorde was designed in the 1960s. Its manufacturing materials range from unique aluminum alloy and steel to stainless steel honeycomb and resin-bonded glass fiber (a composite material). These materials can withstand extremely high temperatures, but not all are lightweight (which requires more fuel). We can only imagine how Concorde engineers would use today's composite materials to improve weight and thermal challenges.
Titanium
Titanium has super strength and is also compatible with carbon fiber composite materials. These two materials have similar thermal properties and have relatively close expansion speeds, making them ideal partners for supersonic aircraft manufacturing.
The XB-1`s main landing gear bulkhead is made of 66 pounds of 4-inch thick titanium plates. It is one of the materials with the highest strength-to-weight ratio in XB-1, and the bulkhead will bear most of the load at landing speed. When the XB-1 lands, the bulkhead will absorb 112,000 pounds of force from each landing gear. For strength requirements, most of the rear fuselage is made of titanium.
Although titanium is ideal in these specific situations, it is also possible to use this metal throughout the entire aircraft manufacturing. The reason is simple, cost! Titanium is extremely difficult to obtain, so it is very expensive, currently an average of US$40 per pound, while aluminum is about US$1 per pound (the two prices vary greatly due to market conditions and the way the metal is purchased).
Titanium is also difficult to manufacture. However, titanium skin aircraft do exist: the most famous of these is the Lockheed Martin SR-71 Blackbird. During the development of the aircraft, titanium skins were part of the spy aircraft family, and special titanium tools had to be manufactured, because ordinary metal tools would break the more brittle titanium. Coincidentally, SR-71 contains some of the earliest carbon fiber composite materials used in aircraft manufacturing.
Ultem 9085
Ultem (polyetherimide) 9085 is a thermoplastic material that can not only be 3D printed, but is also strong, light, and flame retardant. In XB-1, most of the tertiary stents, clamps, gaskets, ducts and fuel shut-off devices are produced in-house, using 3D printing Ultem 9085.
By allowing rapid design iterations, 3D printed Ultem 9085 can save time and money. Before the advent of 3D printing technology, complex aircraft parts were processed from a solid material, which usually required expensive, laborious and time-consuming efforts.
Finally, although XB-1 has many newer materials, it also includes tried and tested materials such as aluminum, stainless steel, rubber, brass, bronze, acrylic, and copper. Each one is very suitable for the specific purpose of each component and provides the required strength, heat resistance, durability, weight, processability and supersonic flight performance. These materials need to undergo the first flight test of Boom's XB-1 on October 7 this year.