The era of hypersonic flight had arrived. Hypersonic refers to aircraft, missiles, rockets, and spacecraft that can reach speeds through the atmosphere faster than Mach 5, which is near 4,000 miles per hour. The hypersonic wind tunnel is used to test flight characteristics in a hypersonic region of Mach number 5 or more.
Hypersonic research have uncovered some of the lingering elements that continue to plague vehicle aerodynamics, viz. Limited capabilities of ground testing facilities for the simulation of hypersonic flows. Other challenges are the limited aerothermodynamic flight test database. The stringent access restrictions to existing databases and the limited verification efforts of computational fluid dynamics (CFD) aerothermodynamic codes against ground test data.
The hypersonic regime introduces a number of flow attributes such as: extremely high turbulence, pressure, temperature, density, vorticity, and energy, thin shock layers, viscous interactions, entropy layers, changes in vehicle stability and control; and physical-chemical gas changes such as ionization, dissociation, equilibrium effects, and other molecular phenomena.
“The physics are enormously difficult at hypersonic speed,” aerospace engineer Steven Beresh of Sandia’s aerosciences department said. The air and gases react differently than at subsonic speed; materials are put under extreme temperatures and pressure; and there is the added challenge of guidance mechanisms also needing to withstand those pressures. “We have some information, but not enough information,” he said. “We’ve mostly been dealing with re-entry vehicles. Before, the idea was to just have the vehicle survive; now, it needs to thrive. We’re trying to fly through it.
“To really make an impact in hypersonic research, it requires a collaboration between people who understand the hypersonic vehicle, people who understand the fluid dynamics, people who understand the measurement science and people who understand the computer simulations,” said Daniel Richardson, a mechanical engineer in diagnostic sciences. “That’s how you can begin to understand the underlying physical phenomena.”
A hypersonic wind tunnel is designed to generate a hypersonic flow field in the working section, thus simulating the typical flow features of this flow regime – including compression shocks and pronounced boundary layer effects, entropy layer and viscous interaction zones and most importantly high total temperatures of the flow.
Hypersonic wind tunnel testing presents many challenges for scientists and engineers who aim to recreate realistic flight conditions in an observable test apparatus. Chief among the complications of ground testing is matching the aerodynamic conditions with inexpensive and rapid turnaround testing.
The need for improved, cost effective instrumentation hardware and interpretive techniques seems to be essential for advancing hypersonic flight technology. Furthermore, collaboration through CFD, ground testing, and analytical modeling will greatly assist in data interpretation.
In addition to database creation, fundamental fluid dynamics analysis of hypersonic flow motions constitutes another essential aspect of aerodynamic research. Specific topics include boundary layer transition in hypersonic flight and boundary layer effects around vehicles that directly impact surface heating.
According to a study mandated by the FY2013 National Defense Authorization Act (P.L. 112-239) and conducted by the Institute for Defense Analyses (IDA), the United States had 48 critical hypersonic test facilities and mobile assets in 2014 needed for the maturation of hypersonic technologies for defense systems development through 2030. These specialized facilities, which simulate the unique conditions experienced in hypersonic flight (e.g., speed, pressure, heating),53 included 10 DOD hypersonic ground test facilities, 11 DOD open-air ranges, 11 DOD mobile assets, 9 NASA facilities, 2 Department of Energy facilities, and 5 industry or academic facilities.
In its 2014 evaluation of U.S. hypersonic test and evaluation infrastructure, IDA noted that “no current U.S. facility can provide full-scale, time-dependent, coupled aerodynamic and thermal-loading environments for flight durations necessary to evaluate these characteristics above Mach 8.”Since the 2014 study report was published, the University of Notre Dame has opened a Mach 6 hypersonic wind tunnel and at least one hypersonic testing facility has been inactivated. Development of Mach 8 and Mach 10 wind tunnels at Purdue University and the University of Notre Dame, respectively, is ongoing. In addition, the University of Arizona plans to modify one of its wind tunnels to enable Mach 5 testing by early 2021, while Texas A&M University in partnership with Army Futures Command plans to complete construction of a kilometer-long Mach 10 wind tunnel by 2021.
The United States also uses the Royal Australian Air Force Woomera Test Range in Australia and the And¸ya Rocket Range in Norway for flight testing. In January 2019, the Navy announced plans to reactivate its Launch Test Complex at China Lake, CA, to improve air launch and underwater testing capabilities for the conventional prompt strike program.
In addition, in March 2020, DOD announced that it had established a “hypersonic war room” to assess the U.S. industrial base for hypersonic weapons and identify “critical nodes” in the supply chain. Initial findings are to be released in mid-2020.
The Naval Research Laboratory (NRL) Hypersonic Wind Tunnel is a long-duration mid-size aerodynamics test facility capable of real-time altitude and speed variation. The range spans sea level to over 30km and Mach 1.3 to 6+ in a 12” x 12” x 24” test section. Pressurized air is stored in stagnation tanks, then forced through a convergent-divergent planar nozzle where it reaches a sonic condition at the throat. The flow then expands to supersonic speeds in the test section at the desired density altitude or turbulence level and Mach number.
The NRL Hypersonic Wind Tunnel offers the unique capability of varying flight conditions – the Mach number, Reynold’s number, density altitude, and dynamic pressure in real time. This capability enables continuous aerodynamic testing along a flight path as opposed to a discrete point flight condition. The NRL tunnel offers low cost operation and rapid test turnaround. Recharging the air tanks from empty takes less than 90 min.
U.S. Army Hypersonic Test Center
The Texas A&M Engineering Experiment Station, headquartered at the RELLIS campus, has already signed a $65 million cooperative research agreement with the Combat Capabilities Development Command (CCDC)-Army Research Laboratory (ARL). In addition, the 86th Texas State Legislature appropriated $50 million to Governor Greg Abbott to transfer to the Texas A&M Engineering Experimental Station (TEES) for Army Futures Command efforts.
“Texas A&M and the RELLIS campus will become a nexus for collaboration and high-tech testing in service to our nation’s security,” said Elaine Mendoza, chairman of the A&M System board. “Today’s vote will bring hundreds of millions worth of private investment to Central Texas as these facilities come to life. Simply put, this is where American defense contractors will want to set up shop if they want to work with the U.S. Army Futures Command.”
The facility is intended to aid in the Austin-based U.S. Army Futures Command’s modernization mission by allowing soldiers from Fort Hood and the A&M Corps of Cadets to test high-tech prototypes as they are being developed. The $200 million center will also aim to support the U.S. Army’s desire to quicken its procurement process.
The complex will feature the nation’s only kilometer-long hypersonic facility to help determine the best design and materials for the fastest vehicles ever built. It will also have laboratories, runways, underground and open-air ranges and a resilient network of sensors and systems for experimentation, data collection, analysis and storage.
Engineers make modifications at Arnold AFB hypersonic propulsion unit
The APTU facility, which is capable of producing test conditions from Mach 3.1 to Mach 7.2, can support a myriad of test setups: propulsion, material, structures, store separation, and directed energy lethality / survivability.
The Aerodynamics and Propulsion Test Unit (APTU), a blowdown wind tunnel designed for aerodynamic testing of supersonic and hypersonic systems and hardware at true flight conditions, recently experienced problems with a certain electrical component.
A team at Arnold Air Force Base (AFB) has made software modifications to detect unsafe conditions, avoid unnecessary downtime and prevent damages. The software modifications addressed an uncommanded runaway of a rectifier, an electrical device that transforms alternating current into direct current. Electrical engineer Adam Webb said: “A runaway is when the output current increases significantly above the set point value.” The software modifications avoided unplanned test termination, fuel system damage and unscheduled test repeats.
Webb said: “A repeat test at APTU can be expensive and could cause additional degradation to the test article.” Webb enhanced the logic used in the programmable logic controllers on the units to handle uncommanded runaways, which enabled him to detect the part at fault. Instrumentation data and controls engineer Gareth Penfold used a spreadsheet-based method to track equipment that needs calibration.
APTU group manager for test operations and sustainment at Arnold AFB Sharon Rigney said: “Gareth leveraged a previous spreadsheet method of tracking instruments into a fully functional computer database format.” The database format includes an inventory of spare instruments available substitutions of failed items or items that require calibration. The new database will also track warranty dates and time-sensitive data needed for engineering decision-making.
Russia reportedly conducts hypersonic wind tunnel testing at the Central Aero-Hydrodynamic Institute in Zhukovsky and the Khristianovich Institute of Theoretical and Applied Mechanics in Novosibirsk, and has tested hypersonic weapons at Dombarovskiy Air Base, the Baykonur Cosmodrome, and the Kura Range
China has a robust research and development infrastructure devoted to hypersonic weapons. USD (R&E) Michael Griffin stated in March 2018 that China has conducted 20 times as many hypersonic tests as the United States. China tested three hypersonic vehicle models (D18-1S, D18-2S, and D18-3S) each with different aerodynamic properties in September 2018. Analysts believe that these tests could be designed to help China develop weapons that fly at variable speeds, including hypersonic speeds. Similarly, China has used the Lingyun Mach 6+ high-speed engine, or “scramjet,” test bed to research thermal resistant components and hypersonic cruise missile technologies.
Lingyun-1 Hypersonic Cruise Missile Prototype
Chinese constructing world’s most high speed Hypersonic Win Tunnel
For example, in March 2018, China’s state media announced construction on an 870-foot wind tunnel capable of simulating conditions from Mach 10 to Mach 25. Scheduled for completion in 2020, it will join existing wind tunnels able to simulate environments from Mach 5 to Mach 9. The U.S., by comparison, has Mach 5 to Mach 9 wind tunnels, but they are smaller than the Chinese tunnels, and capable of tests lasting only a few seconds.
As per Chinese media reports, China is working on the construction of the world’s most high-speed wind tunnel to test a future hypersonic spaceplane theoretically capable of Mach 25. The 265 meter shall be one of the largest in its category globally and is taking shape at the Chinese Academy of Sciences Laboratory of High temperature gas dynamics. It would be used to test missiles, space launch vehicles etc.
A wind tunnel would allow China to test hypersonic weapons before conducting real flight. The advances in Computational fluid Dynamics ( CFD) modelling on high speed supercomputers has reduced the demand of wind tunnel testing but the modelling results are still not completely reliable and wind tunnels are used to verify CFD predictions.
India is also developing an indigenous hypersonic cruise missile as part of its Hypersonic Technology Demonstrator Vehicle program and successfully tested a Mach 6 scramjet in June 2019. India operates approximately 12 hypersonic wind tunnels and is capable of testing speeds of up to Mach 13.
Satish Dhawan Wind Tunnel Complex Commissioned at VSSC
In the quest to reduce the cost of access to space and to extend the frontiers of space exploration, ISRO has ventured into Reusable Launch Vehicle (RLV) and Re-entry missions, Air-breathing propulsion technology demonstration and Interplanetary missions. These missions encounter design criticalities at Hypersonic Mach number regime and need rigorous aero-thermodynamic characterisation at these Mach numbers. In order to cater to the above need, Industrial type Hypersonic Wind Tunnel and Shock Tunnel have been established at Vikram Sarabhai Space Centre (VSSC).
Hypersonic intermittent blow down-type wind tunnel is a test facility in the ground to simulate flight conditions of space vehicles by blowing air at hypersonic speeds, for short periods at regular intervals, instead of blowing air continuously.
The different subsystems are high pressure system, wind tunnel system which consists of Pressure Regulating valve (PRV), Heater, Settling chamber, Nozzle, Test section, the Diffuser (DIFF), After Cooler (AFCL) and Vacuum Chamber (Vac).
Air is compressed and stored in the high pressure system. It is released through a pressure regulating valve to create the desired pressure in the settling chamber. Heater is used to heat the air while passing through the heater bed to avoid liquefaction when it is expanded through the nozzle to get high Mach numbers. The pressure in the settling chamber is controlled by the proper operation of the control valve so that flow through test section meets the Mach number and mass flow rate specified for the test conditions.
The first phase of the facility, commissioned in 2012, with Mach 6 system and Shock Tunnel, has been extensively utilised and tests are being carried out for the current programs of ISRO. The second phase of Hypersonic Wind tunnel consists of the realisation of Mach 8, 10 and 12 nozzles, Heater-II system, Hot Shut-off Valves, Cooling system and associated subsystems.
Realisation of these systems involved intricate design and analysis, high precision machining, heavy engineering hardware realisation, fabrication of high temperature performance materials, high temperature and high pressure valves realisation, development of state of the art Cored Bricks as heat storage media, realisation of massive 15-5 PH forgings and high temperature heating modules. Integration of these systems was carried out meticulously, performance assessment was made and trial runs were conducted to calibrate and validate the tunnel systems.
With capability up to Mach 12 operation, Hypersonic Wind Tunnel is the third largest in the world in terms of hypersonic flow simulation capability over a wide spectrum. The tunnel has the capability to simulate flow field conditions at Mach 6, 8, 10 and 12 of nominal nozzle exit diameter of 1 metre with Reynolds number ranging from 1 to 80 million per metre. The tunnel is pressure-vacuum driven with high pressure storage system of 300 bar and vacuum system of 10-2 mbar capacity. Regenerative storage heater system is used to preheat the compressed air up to 1550 K before it is expanded through the nozzle to Hypersonic Mach number. The state-of-the-art technology is used for data acquisition and control of the tunnel system. Programmable Logic Controller (PLC) based system works on Dual redundant Hot Standby concept and monitors more than 100 input parameters and controls more than 60 events. The shock tunnel uses combustion driver and has the capability to simulate free stream velocities up to 4.5 km/s and a maximum Reynolds number of 3.3 million per metre.
The realisation of the above facilities paved the way for the indigenous development of technologies, in the area of Cored Bricks, Hot Shut off Valves and massive 15-5 PH forgings. Hot Shut-off Valves, the most critical valve in the tunnel circuit with simultaneous application of high pressure and high temperature of 110 bar and 1550 K respectively, have been indigenously developed. High temperature Heater system demanded high purity Alumina Cored Bricks, of low dust characteristics and high temperature thermal shock resistance, as heat storage media. These were jointly developed by ISRO and Indian industries. Massive 15-5 PH forgings with integral flanges are realised for the high pressure shock tubes designed to withstand 1000 bar and associated fatigue cycles.
The major systems of the tunnel are designed to meet the requirement of five blow downs per day. It consists of 500 valves, 2 km pipelines, 40 numbers of electric motors and 35 fluid pumps. These facilities have been indigenously designed, developed and made with the support of Indian Industries.
This Integrated Hypersonic Wind Tunnels facility has been commissioned by Chairman, ISRO / Secretary, DOS, recently during March 2017. The entire complex, consisting one metre Hypersonic Wind Tunnel, one metre Shock Tunnel and Plasma Tunnel was named as “Satish Dhawan Wind Tunnel Complex” as a tribute to Prof. Satish Dhawan, who has made very significant contributions in the field of wind tunnels and aerodynamics.
In addition to the Woomera Test Range facilities one of the largest weapons test facilities in the world Australia operates seven hypersonic wind tunnels and is capable of testing speeds of up to Mach 30. France operates five hypersonic wind tunnels and is capable of testing speeds of up to Mach 21. Germany operates three hypersonic wind tunnels and is capable of testing speeds of up to Mach 11. The Japan Aerospace Exploration Agency operates three hypersonic wind tunnels, with two additional facilities at Mitsubishi Heavy Industries and the University of Tokyo. Other countries including Iran, Israel, and South Korea have conducted foundational research on hypersonic airflows and propulsion systems, but may not be pursuing a hypersonic weapons capability at this time.