The era of hypersonic flight had arrived. Countries are developing future hypersonic Spaceplanes , enabling intercontinental travel at very high speeds, that could cut the journey times from the UK to Australia from the current duration of around 20 hours to as little as two hours. They shall also provide revolutionary military capability like prompt global strike, launch on demand, satellite servicing and anti satellite missions. The Military of United States, Russia and other countries are developing sixth-generation fighters that may be capable of achieving hypersonic speeds. There is global race to develop Hypersonic Missiles such as US HTV-2 and X-51, Chinese WU-14, Russian Yu-71, that travel at least five times the speed of sound (Mach 5) or more. These vehicles can fly along the edge of the space and can glide and maneuver to the targets.
NASA categorizes speeds between Mach 5 ( 6125 kilometers per hour) to 10 Machs as Hypersonic. When moving at such velocity the heat generated by air and gas in the atmosphere is extremely hot and can have a serious impact on an aircraft or projectile’s structural integrity. That is because he temperatures hitting the aircraft can reach anywhere from 2,000 to 3,000 °C. The structural problems are primarily caused by processes called oxidation and ablation. This is the when extremely hot air and gas remove surface layers from the metallic materials of the aircraft or object traveling at such high speeds.
The Airframe of Hypersonic Vehicles like SR-72 ( Lockheed Martin reconnaissance drone with strike capability) , must include advanced materials to stay intact while subjected to high dynamic loads, and to withstand the extreme aerodynamic heating of hypersonic flight, as air friction alone would melt conventional materials, rites Dora E. Musielak, Ph.D. University of Texas at Arlington.
Jet engines also need to work in temperatures up to 1,500°C and under high-stress conditions. They use materials such as ceramic matrix composites, high-temperature metals, and carbon-fibre composites. Since fuel-efficiency and performance increase with engine temperatures, new materials that can tolerate these temperatures must be tested before they are used in the engines, MTS stated. MTS Systems is working on the development and demonstration of new, advanced materials testing technologies, as part of a cooperative research and development agreement (CRADA) signed with the US Air Force (USAF). The testing of these materials will ensure safety, reliability and improved performance, according to the statement.
To combat the problem materials called ultra-high temperature ceramics (UHTCs) are needed in aero-engines and hypersonic vehicles such as rockets, re-entry spacecraft and defense projectiles. Lightweight carbon composites used in hypersonic vehicles can withstand temperatures of up to 2900 degrees Fahrenheit.
Hypersonic platforms are ultimately limited by the capacities of the materials available. A prime example of an aircraft’s failure due to inadequate materials is the test flight of the hypersonic Falcon HTV-2, which, according to DARPA, crashed into the ocean after “unexpected aero-shell degradation” due to excessive thermal and structural loads from the brutal shock waves present at such high Mach numbers.
Now Researchers have discovered that tantalum carbide and hafnium carbide materials can withstand scorching temperatures of nearly 4000 degrees Celsius. In particular, the team from Imperial College London discovered that the melting point of hafnium carbide is the highest ever recorded for a material. Being able to withstand temperatures of nearly 4000°C could pave the way for both materials to be used in ever more extreme environments, such as in heat resistant shielding for the next generation of hypersonic space vehicles.
Materials Critical for successful hypersonic flights
Hypersonic flights, re-entry, and propulsion vehicles, regardless of their design, require maneuverability of materials against high temperature erosion in excess of 2400°C. “It’s all about the heat,” said hypersonics expert Brad Leland. At speeds of Mach 5 and higher, aerodynamic friction can heat an aircraft’s exterior enough to melt steel. Advanced materials, recently in development, are necessary.
For example, atmospheric re-entry vehicles, rockets and scramjet powered air-breathing hypersonic cruise vehicles primarily encounter high pressures and heat flux on the leading edges due to air stagnation and shock waves. Also the respective propulsion systems experience high temperature exothermic combustion reactions to produce thrust.
Melting temperature is only one of the many properties required of the materials. As most engine and hypersonic leading edge applications will involve exposure to oxidizing fuels or aero-heating, all non-oxide materials will undergo oxidation to form some combination of solid, liquid, or gaseous reaction product. It is the oxidation behavior that is one of the primary constraints associated with the selection process of UHTC materials. Additionally, the adverse environmental effects include detrimental effects of chemical reactions, mechanical stresses and radiation.
Ultrahigh temperature materials (UHTM)
There is growing interest in developing ultrahigh temperature materials (UHTM). These are materials with temperature capability greater than 1650 °C and able to withstand extreme erosive / corrosive environments. UHTM should possess high strength at high temperatures, oxidation resistance, ablation resistance, thermal shock resistance.
These materials will be required for hypersonic air breathing vehicles, hyper speed cruise missile, hypersonic aircrafts, re-usable launch vehicles to protect leading edges and nose cones that experience very high temperatures (> 2000 °C). Refractory diboride composites like ZrB2, HfB2 etc and multilayer coatings of HfC, SiC on C-C composites are most promising UHTM candidates.
World’s most heat resistant material found
Researchers from Imperial College London in the UK discovered that the melting point of hafnium carbide is the highest ever recorded for a material. Tantalum carbide (TaC) and hafnium carbide (HfC) are refractory ceramics, meaning they are extraordinarily resistant to heat.
Their ability to withstand extremely harsh environments means that refractory ceramics could be used in thermal protection systems on high-speed vehicles and as fuel cladding in the super-heated environments of nuclear reactors.
However, there has not been the technology available to test the melting point of TaC and HfC in the lab to determine how truly extreme an environment they could function in. The researchers developed a new extreme heating technique using lasers to test the heat tolerance of TaC and HfC.
They used the laser-heating techniques to find the point at which TaC and HfC melted, both separately and as mixed compositions of both. They found that the mixed compound (Ta0.8Hf0.20C) was consistent with previous research, melting at 3,905 degrees Celsius, but the two compounds on their own exceeded previous recorded melting points. The compound TaC melted at 3,768 degrees Celsius, and HfC melted at 3,958 degrees Celsius. The findings may pave the way for the next generation of hypersonic vehicles, meaning spacecraft could become faster than ever.
Dr Cedillos-Barraza said: “The friction involved when travelling above Mach 5 – hypersonic speeds – creates very high temperatures. So far, TaC and HfC have not been potential candidates for hypersonic aircraft, but our new findings show that they can withstand even more heat than we previously thought – more than any other compound known to man. This means that they could be useful materials for new types of spacecraft that can fly through the atmosphere like a plane, before reaching hypersonic speeds to shoot out into space. These materials may enable spacecraft to withstand the extreme heat generated from leaving and re-entering the atmosphere.”
Examples of potential uses for TaC and HfC could be in nose caps for spacecraft, and as the edges of external instruments that have to withstand the most friction during flight.
Ultra-high temperature ceramics (UHTCs)
The trajectories for hypersonic space vehicles demand high thermal resistance for the base material to sustain operating temperatures in range 1873-2673 K. These conditions are beyond the capabilities of many engineering ceramics and have motivated researchers to pursue the development of UHTCs.
Ultra-high temperature ceramics (UHTCs) are promising materials for potential application in thermal protection system (TPS) under these extreme thermal and chemical environments. The past few decades have seen phenomenal research on Ultra High Temperature Ceramics (UHTCs), which are commonly defined as materials with very high melting temperatures (~3273 K) and ability to perform under extreme thermal environments.
Columbia’s hypersonic vehicle used black, grey and white tiles on the nozzle. White tiles (1 inch-thick) were used to protect regions such as upper wings and fuselages; black tiles (1-5 inch-thick) were used to protect lower side and nose area and super absorbent gray tiles were used to protect nose tip and leading edges of the Columbia’s wings. These UHTCs are selected for application on the basis of distribution of temperature in various parts of the vehicle. The root cause for the Columbia disaster was the crack initiation due to foam impact in the wing that allowed destructive superhot gases to penetrate the wing, and consequently melted and disintegrated during the reentry of the shuttle into earth’s atmosphere.
Recent research is also focused on the development of Hafnium and Zirconium based boride ceramics using advanced ceramic processing techniques for leading edge sections of hypersonic vehicles which experience extreme thermo-mechanical loading. Novel additives are being added to the ceramics to enhance oxidation resistance and thermo-mechanical properties. Carbon nanotubes (CNTs) have shown to improve the fracture toughness of the composites through a range of toughening mechanisms, like CNT pull-outs, crack bridging, and crack deflection.
DARPA’s Materials Development for Platforms
DARPA’s Materials Development for Platforms (MDP) program seeks to compress the timespan between designing tough materials that can withstand harsh environments and having them used on actual military equipment, from an average of 10 years or longer to just two and a half years. The program plans to develop methodology and toolset for the rapid development of advanced materials that are lighter, stronger and more resistant to stress, heat and other harsh environmental conditions.
The program intends to focus its initial efforts on development of materials for Hypersonic vehicles, that travel at more than five times the speed of sound resulting in shell temperatures of several thousand degrees Fahrenheit—hot enough to melt steel. The goal is to prove the MDP concept by developing, manufacturing and independently testing various new structural material elements for outer shell within two and a half years.
To achieve its goal, the DARPA program intends to establish a cross-disciplinary model that incorporates materials science and engineering, Integrated Computational Materials Engineering (ICME) principles and the platform development disciplines of engineering, design, analysis and manufacturing.
HRL Laboratories, LLC (formerly Hughes Research Labs) will be developing new materials for hypersonic vehicles that aim to reduce the weight and cost of vehicle aeroshells while withstanding the extreme environment encountered during hypersonic flight under DARPA’s Materials Development for Platforms (MDP) program. Led by Dr. Tobias Schaedler, HRL’s team aims to combine innovative additive manufacturing techniques with new high temperature materials.
“Sandwich panels are used throughout the aerospace industry for lightweight, load-bearing structures, but their use in high temperature applications has been limited by the availability of structurally robust high-temperature cores and scalable fabrication techniques. Our goal is to solve these challenges and develop ceramic sandwich structures that enable weight savings across a wide range of high temperature applications.” —Dr. Schaedler
Determination of micro-structural characteristics through simulation and testing
The development of UHTM requires an in-depth understanding of the structural stability of the ceramic by evaluating micro-structural characteristics, determination of high temperature mechanical properties, study of shock-material interaction under different experimental conditions (shock wave velocity, number of pulses, environment and pulse duration) etc.
“Through simulation and testing, we are able to analyze properties of microstructures, using this knowledge to develop new materials with new properties,” said research scientist Jason Poleski. “This isn’t just about using the latest in heat-resistant materials. In order to manage heat dissipation, we need to consider both materials on a microscale as well as the vehicle topology in order to create a vehicle that won’t melt under the heat caused by wind resistance at Mach speeds.”
High-temperature material properties measurement methods for hypersonic vehicle systems
PSI has been awarded a Phase I SBIR contract from the US Air Force Research Laboratory to determine the high-temperature properties of thermal protection system materials and structural materials utilized in the development, design, fabrication and testing of hypersonic vehicles.
Test vehicle flights aimed at understanding the structural integrity of hypersonic demonstrators have been performed from about Mach 5 to 20, but these vehicles have not been recovered. The post-test forensics is difficult due to the lack of the high temperature properties of the thermal protection layer and the structural material strength. In order to properly design hypersonic test vehicles and the weapon systems that evolve from them, it is necessary to better understand the materials in their proper thermal environment. The goal of this effort is to determine these high-temperature properties by a combination of experimental laser testing and modeling. PSI has achieved prior success in such evaluations using a well-characterized thermal source and matching as many experimental test results as possible as a function of the experiment time duration.
The acquisition of validated high-temperature properties for thermal protection materials and structural materials will lead to rapid improvements in the design, development and testing of new hypersonic weapon systems. The recommended hypersonic material database development will be of keen interest to the Air Force, DoD and DARPA as the development of hypersonic vehicles accelerates. NASA and commercial corporations will also find the database and the innovative material data acquisition process very useful as they develop hypersonic technology for earth and other planetary ventures.
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