In February of 1949, the V-2 rocket was the first hypersonic flight to reach a maximum speed of Mach 5. Despite this achievement, the re-entry of the V-2 rocket into the atmosphere caused the vehicle to burn completely, leaving only charred remnants to be found. Since this initial attempt at hypersonic travel, Researchers have continued to work towards making this phenomenon into a reality.
What is hypersonic speed?
Hypersonic speed can be evaluated by a number of parameters involving its air compressibility, shock waves and expansions, however its property of achieving a Mach number of 5 or greater is its most notable characteristic. A Mach number describes the ratio of the object’s speed to the speed of sound, which is usually 330 meters per second (m/s) or 760 miles per hour (mph). To put this astounding level of speed into perspective, a normal flight from Miami to Seattle takes approximately six hours and 40 minutes, however a hypersonic flight could reduce that travel time to about 50 minutes or less.
CNTs in Aircrafts
Of the various different technologies, shapes and materials that Aviation Engineers have incorporated into aircrafts over the last several decades, the utilization of carbon nanotubes (CNTs) has been particularly advantageous for this industry. Capable of a strength that is 200 times greater than that of steel combined with superb elasticity, CNTs can withstand temperatures of up to 400 °C.
It is particularly important that aircrafts are composed of materials like CNTs that are able to withstand aerodynamic heating, a process in which the kinetic energy of the air compressing onto the traveling aircrafts high speeds is converted into heat.
BNNTs vs. CNTs
As a close analog of CNTs, boron nitride nanotubes (BNNTs) exhibit strength qualities, however BNNTs possess a much wider band gap as greater resistance to oxidation as compared to CNTs. Additionally, BNNTs can survive in ambient temperatures as high as 850 °C, without any degradation occurring until the temperature is 900 °C or above. When present within metals, BNNTs maintain this thermal stability as compared to CNTs which are more susceptible to corrosion under these conditions.
BNNTs for Hypersonic Flights
A recent research initiative conducted by Engineers from Binghamton University and the United States Air Force evaluated the effects that high temperatures have to the overall structure and mechanical properties of BNNTs. In situ Raman and optical spectroscopy measurements determined that the BNNT microfibrils are capable of surviving in ambient temperatures as high as 1000 °C, without causing any type of substantial weakening in the BN bond strengths.
Atomic force microscopy (AFM) and high resolution transmission electron microscopy (TEM) measurements also confirmed the survival of BNNTs in ambient temperatures up to 850 °C. The Binghamton study confirmed the overall stability of BNNTs in high temperature environments, thereby inviting Aviation Engineers to look further into the potential of this material to be used for future hypersonic aircraft technologies.
The Future of BNNTs
While the advantages of replacing aircraft materials with those that involve BNNTs, the current cost of BNNTs is approximately $1,000 per gram. The high prices of BNNTs therefore limit its practicality to be used in large-scale aircraft materials.
Despite this reality, it is important to note that when CNTs were first introduced into the market about 20 years ago, they also had a market value that was close to the current price of BNTs. As more research has been devoted to improving the feasibility in mass-producing CNTs, its current rate can range from $10-20 per gram.
Further advances into understanding the properties and applying this technology into various projects is expected to relieve some of the costs of this material, thereby improving the reality of a hypersonic travel option.
- “Quantitative Characterization of Structural and Mechanical Properties of Boron Nitride Nanotubes in High Temperature Environments: X. Chen, C. Dmuchowski, et al. Scientific Reports. DOI: 10.1038/s41598-017-11795-9.