Although the number of hypersonic weapons and aircraft development programs is increasing, engineers must innovate so that testing can enter combat systems such as aircraft and weapons

Flying above Mach 5 is an old idea. In the 1950s and 1960s, American hypersonic flight research programs (such as the X-15 rocket plane) pioneered the testing of this concept. Intercontinental ballistic missiles that re-enter the atmosphere at hypersonic speeds were also developed around the same time.

In the decades since, several hypersonic vehicles have been developed for research, and then they have been put on hold by engineers. But in the past five years, mainly in the United States, Russia, and China, interest in flying at speeds exceeding 3,000 mph (4,800 km/h) has revived. 

Commercial companies such as Reaction Engines in the UK have made substantial progress in the development of air-breathing engines for the first time. It is hoped that in the future, these engines will enable "space planes" to fly over the stratosphere and above at hypersonic speeds. However, most of the actual testing activities for hypersonic flight are still focused on single-use weapon systems. 

There are at least 18 military hypersonic projects in the world. Market analysts estimate that between 2015 and 2024, the U.S. government will spend nearly $15 billion to develop hypersonic technology and weapon systems.  

There are currently two main types of hypersonic weapon systems. The hypersonic gliding vehicle (HGV) is launched from a rocket, possibly an existing intercontinental ballistic missile, and then glides to the target. Hypersonic cruise missiles are powered by high-speed air-breathing engines (also known as scramjets) after acquiring the target.

The first test phase of these two systems is extensive ground testing. There are few facilities capable of testing hypersonic concepts and technologies. Northrup Grumman's gas and heat research and testing facility in Ronkonkoma, New York is one of them. 

Dan Cresci is the chief engineer responsible for testing services at the facility and has worked in Northrup Grumman's supersonic and hypersonic fields for 35 years.

For decades, engineers like Cresci have been conducting hypersonic tests in the form of rocket and rocket auxiliary power propulsion experiments. According to Cresci, since then, people's interest in and research on hypersonic and aspirated propulsion has "fell from time to time." However, many projects have one thing in common-they have been in the research and development stage, focusing on advancing. 

"The difference is that now we are closer to actually developing these technologies into combat systems—weapons, spacecraft, and manned aircraft," Kressi said. "In the testing field, this means that we are conducting longer-duration tests at a higher speed and testing various technologies simultaneously in one test.

"We have solved many of the technical challenges surrounding propulsion, and we are continuing to test materials, control and guidance systems. We have six different units, each with the ability to focus on different aspects of hypersonic flight ground testing."

The test reproduces as much as possible the conditions that will be experienced during the flight. The data from the test is used to improve the model that predicts the performance of hypersonic flight.

The scale of the test is also different, from running the entire engine in the cell to subjecting the material sample to the extremely high temperature and pressure airflow from the nozzle. The test usually runs for 30 to 120 seconds. 

"Our goal-one of everyone's goals-is to run the test as long as possible, because the operating system will have many minutes or even longer flight times," Cresci said.

For propulsion testing, engineers are measuring the same key parameters as they were 20 or 30 years ago: pressure, temperature, air flow, fuel flow, and thrust. Material testing usually focuses on thermal structural interactions. If the material is ablated, engineers will look for and measure deformation modes.

Simulating these hypersonic test environments requires high-temperature combustion of hydrogen, oxygen, and air from a complex factory control system and a large number of storage tanks. In order to simulate high-altitude flight, the Ronkonkoma site also has a large vacuum ball, similar to the cabin used to test the spacecraft. The recently added ejector system increases the running time of the vacuum ball. 

One of the fundamental changes in testing that took place during Cresci's career was the substantial increase in the amount of data generated. "Over the years, the measured density may have increased tenfold," Cresci said. "The accuracy is significantly improved, and we can compare CFD [computational fluid dynamics] predictions before testing faster. 

"This means that the iterative process of development can proceed faster-we have a much better understanding of things like flow fields and engine response in the hypersonic field.

"We have always wanted to have higher fidelity testing, so the focus of investment is to upgrade the instrument and data acquisition network to obtain a higher data rate. All this is for take-off. Ground testing always generates more data than flight testing. , The cost and planning of flight test will increase a lot."

Paul Cook is the director of missile systems for Curtiss-Wright Defense Solutions (CWDS), a major supplier of instrumentation and data acquisition solutions for hypersonic projects. 

Flight testing of weapon systems, including hypersonic systems, is carried out in stages-early short-range flight test vehicles, long-range flight test propulsion, and longer flight test propulsion and guidance. 

A B-52 carried a prototype of the hypersonic AGM-183A air-launched rapid response weapon in its first captive flight on June 12, 2019 (Image source: US Air Force)

Cook said: "Many hypersonic missile projects are now in a very early stage, so they are using our instruments to measure internal temperature, pressure, acceleration and other parameters. Later guided flights collected more data that is usually telemetry."

"The hypersonic flight technology has changed, but the instruments used for testing have not changed much. What is changing is the ability to transmit data during flight."

"Commercial test engineers like to record their data, and in the military, for security reasons, they like to transmit data." In business, they want the vehicle to return, and in the military, the vehicle usually does not return. " 

Axon/ADAU: The next-generation compact data acquisition unit from Curtiss-Wright for flight testing

The instruments provided by CWDS are off-the-shelf, but also highly configurable. System designers can choose from thousands of different digital and analog signal conditioning modules, which can be reconfigured to measure specific test points. "There is no set of measurements that all our customers want, so the modules must support their different needs," Cook said.

CWDS can provide the entire integrated system, including recorder, instrument package, guidance data interface, transmitter and tracking system. According to Cook, customers are increasingly adopting this option of the entire system to take advantage of the expertise of CWDS.

The FTI used in the hypersonic flight test must be more robust than the FTI used in the traditional test. Twenty years ago, early hypersonic test aircraft would be made of ablated materials. "Vehicles fly at relatively low altitudes at very high speeds, so air heating is important," Cook said.

"Today, due to advances in materials, air heating is no longer a problem, which enables the latest hypersonic aircraft to achieve faster speeds and longer distances."

However, heat is still a problem in missiles. When integrated into a flight test vehicle, the instrumentation and data acquisition package must be as small and light as possible. "Thanks to advances in integrated circuit and packaging technology, our instrumentation system is 80% smaller than it was 20 years ago," Cook said.

"But with it comes thermal challenges, especially when something is running at very high data rates. You must have expertise in thermal management to solve these problems."

To further exacerbate the thermal challenge, the amount of data processed is expected to continue to grow. "The data explosion will be uncontrollable. We are studying 40GB/S links instead of 20MB/S. The demand for data is so high that we have to develop very high-speed technology. We will only be able to use higher frequencies. Transmission, so we can transmit data with a wider bandwidth. This is a very exciting time."

Curtiss-Wright's Compact Collision Protection Memory (CPMM) unit is used for data storage in destructive testing

All the hypersonic FTI customers of CWDS are conducting short-distance test flights while studying how to collect data from longer flights. Potential solutions include dedicated 5G links to the launch area and laser-based satellite communications. "The challenge is that higher data rates will require higher-speed infrastructure," Cook said.

"Contractors will rent time on these networks to collect their data. In order to let you know the data rate-we adjusted the system to use a 100GB interface to record the data they generate today."

As engineers redesign systems to reduce size, reduce costs, and fly further, the CWDS missile market is growing rapidly, not only in the hypersonic field, but also on all platforms. However, a large number of tests have also been carried out in the space launch system. 

Cook said: "The commercial launch industry is growing as fast as Hypersonics to support the launch of high-data-rate satellites and to supply space stations-this market is also exploding. We are currently working with six launch companies," He said.

  "But in the next ten years of flight tests, the missile market will still be our leading product."

Hypersonic flight may be an old idea, but it has new ambitions-test facilities, instruments and data will play an important role in achieving them.

The upper skin of Talon-A is made of Out-of-Autoclave Bismaleimide carbon composite material, which is a high-temperature thermosetting resin (photo source: Stratolaunch)

California-based Stratolaunch is developing test platforms to provide easier and cheaper hypersonic flight tests, the first of which is Talon-A.

The Talon vehicle will be launched from the Stratolaunch carrier, which has a wingspan of 385 feet (117 meters) and was originally developed as a satellite aerial launch platform.

Talon-A is a reusable, autonomous, liquid rocket-powered, hypersonic vehicle with Mach 6 capability. It has a length of 28 feet (8.5 meters), a wingspan of 14 feet (4.3 meters), and a launch weight of approximately 6,500 pounds. (2,948 kg). The aircraft is designed to provide hypersonic flight test conditions for more than 60 seconds, and then glide back to land autonomously on a conventional runway.

Talon-A is scheduled to be put into use next year and will be able to perform 12 missions a year.

The Holloman high-speed test track of the U.S. Air Force is used to test ejection seats and parachutes, as well as hypersonic weapons (Photo: U.S. Air Force

Holloman Air Force Base in New Mexico has a unique facility. It is the longest hypersonic sled test track in the world. 

The Holloman High Speed ​​Test Track (HHSTT) is operated by the U.S. Air Force's 846th Test Squadron and is a monorail nearly 10 miles (16 kilometers) long. During the test, engineers used sleds equipped with instrumented test items to launch repetitive rockets along straight and horizontal tracks at speeds of up to 6,400 mph (10,300 km/h). Some tracks are equipped with sprinkler systems to simulate rainfall.

The sled test establishes a link between laboratory investigations and full flight tests by simulating selected parts of the flight environment. HHSTT was upgraded last year and plans to add new braking systems, fiber optic control and data systems, and new ski designs in the next ten years. 

The final test of HHST's new system and sled is scheduled to be carried out this summer, starting with weather impact tests, such as simulated rain. Testing of the US Air Force’s Air Launched Rapid Response Weapon (ARRW) is expected to begin next year.

Hermeus is working with NASA to develop an air-breathing hypersonic engine, the core of which is the GE J85 engine (photo source: Hermeus)

SABRE aspirated engines have been under development since the 1980s, and key components have been verified in the past few years (photo: reaction engine)

All existing hypersonic systems use liquid or solid rockets as propulsion systems and carry fuel and oxidizers.

The reusable hypersonic aspirated rocket engine has been studied for about 50 years, but the operating system has not yet been deployed in the military or commercial. Air-breathing rocket motors get the oxygen needed to burn fuel from the atmosphere instead of from the oxidizer. This opens up opportunities for more efficient operation and use of hydrocarbon fuels. 

Four companies are developing air-breathing rocket engines for hypersonic air transportation and travel. American start-up company Hermeus is testing the improved GE J85 engine. The engine will form the core of a turbine-based combined cycle propulsion system, powering its proposed 20-seat, Mach 5 passenger aircraft

Pratt & Whitney recently confirmed that it is developing a hypersonic engine called Metacomet, which is based on the J58 engine, which powers the SR-71 Blackbird spy plane, which reached a speed of around Mach 3.4 in the 1970s. Record speed.

At the same time, Reaction Engines is testing key components of its SABRE (Synergy Aspirated Rocket Engine) before the first fully integrated engine test. So far, the pre-cooler, heat exchanger and hydrogen have been verified.

Finally, Australia-based Hypersonic conducted a wind tunnel test and plans to start hardware-in-the-loop testing of its Spartan scramjet propulsion system next year. Engines with Mach 12 capability will be used in the space shuttle to put satellites into orbit.

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Ben has served as a reporter and editor for the past 20 years, covering technology, engineering, and industrial fields. He originally wrote articles on topics ranging from nuclear submarines to self-driving cars to future design and manufacturing technology. Before becoming aerospace test editor in 2017, he was the editor of a leading engineering magazine in the UK.

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