ARNOLD AIR FORCE BASE, Tenn. -- Thanks to advancements made in the wake of U.S. Air Force captain and test pilot Chuck Yeager shattering the sound barrier in 1947, a new generation of aircraft was pushing the speed envelope like never before by the mid-1960s.
The benefits of such progress were obvious but also gave rise to new concerns. Engineers were working with never-before-seen speeds and configurations. The question of whether munitions could be safely released from faster-than-ever aircraft needed to be answered, especially with America in the midst of the Vietnam War.
The weapons themselves were also changing. The use of gravity-guided “dumb bombs” was giving way to guided bombs, allowing increased accuracy.
To aid in the development of new weapons and to ensure stores could be released safely from high-speed aircraft, a new wind tunnel was constructed at Arnold Air Force Base. That addition to the Flight Systems Combined Test Force reached a major milestone earlier this year.
2018 marks 50 years of service for the wind tunnel that has come to be known as 4T.
The idea for a 4-foot transonic wind tunnel dates back to the early 1960s. Officials at Eglin Air Force Base, Florida, where store separation is managed, recognized that with the advent of the jet age, the development of new weapons that could be safely separated from aircraft and accurately strike the intended targets was critical.
“That was the motivation for Eglin Air Force Base wanting to get more insight, more information,” said Dr. Edward Kraft, AEDC Fellow and University of Tennessee Space Institute Associate Executive Director of Research. “So they engaged AEDC [leadership] about whether AEDC could put together a wind tunnel to look at these configurations of interest and give them better insight into the safety and the ability to hit a target. That was the genesis of it.”
1st Lt. Roland H.A. West, with the help of others in the U.S. Air Force Development Division and AEDC personnel, prepared and submitted in 1965 a brochure which detailed the need for a new wind tunnel and the plans for its construction at Arnold AFB. In the document, West wrote the development of a 4-foot wind tunnel would provide an economical capability for conventional weapons development:
“Addition of a 4ft x 4ft test section utilizing the existing Propulsion Wind Tunnel Plenum Evacuation System compressors and operating over the Mach Number range of 0.3 to 1.5 has been determined to best satisfy the requirement for conventional war armament development testing.”
According to West, weapons of the time would need to be redesigned and new ones developed for use in subsonic and transonic speed regimes to meet then-current and projected requirements.
At the time AEDC had at its disposal only two tunnels in those speed ranges – a 1-foot transonic wind tunnel and a 16-foot transonic wind tunnel. West pointed out issues existed with both.
West wrote the 16-foot tunnel, known as 16T, was designed for testing large- or full-scale articles. A tunnel of this size was not needed for the testing West had defined. There was also the issue of cost. In the mid-1960s, the operating cost of 16T was approximately $4,000 per hour.
Then there was the matter of availability. West noted 16T was booked solid for the foreseeable future. This made the scheduling of weapons testing more difficult at a time when a rapid response was desired.
The 1-foot tunnel, on the other hand, was too small to meet the meet the testing needs outlined by West. He indicated that this facility was designed to test small-scale models and instrumentation, such as aircraft pilot tubes.
West felt something in between – a 4-foot transonic tunnel – would aid in the quick and economical development of armaments.
West estimated the total cost of 4T would be $1,150,000 with $65,000 for the design process and $1,085,000 to build.
The proposal worked.
The AEDC Engineering Support Facility began the final design on 4T in February 1966. Construction of the facility was authorized on Aug. 15, 1966, and construction was completed around 15 months later on Nov. 22, 1967.
Kraft, who was a co-op student at AEDC from 1964-67, had a front-row seat to the inception of 4T. During that time, he worked in the facilities engineering branch at the Propulsion Wind Tunnel alongside those who designed and oversaw the construction of the tunnel.
“It was a pretty sophisticated wind tunnel for its time,” Kraft said.
The 4-foot transonic wind tunnel was described in technical reports as a “closed-loop, continuous flow, variable-density tunnel.” The continuous-flow aspect of 4T allowed the tunnel to have greater run times than blowdown tunnels. To save money, an individual compressor system was not installed for 4T. Instead, the new wind tunnel was connected to the existing Plenum Evacuation System compressors in the PWT facility. The PES powered 4T, and the new tunnel adopted the data systems used for 16T and the 16-foot supersonic wind tunnel, known as 16S.
Despite being built on what he called a “shoestring” budget, Kraft said 4T introduced two very important technologies. The first was its Captive Trajectory System, the six degrees-of-freedom system that allowed for the simulation and study of weapon and store separations.
“That was new to AEDC,” Kraft said. “I think some other people in the country had played a little bit with the idea, but nobody had really put one into an operational wind tunnel.”
The second technology was the variable porosity walls found within the tunnel. These allowed operators to control wave attenuation and blockage effects on a test article.
“I kind of marvel at it,” Kraft said. “Two new technologies were put into a low-cost wind tunnel that has served our nation extremely well for 50 years.”
The initial calibration of 4T began on Dec. 6, 1967, around two weeks after the completion of construction.
It was determined that the operation of 4T was feasible in the Mach range from 0.1 to 1.4, with good Mach number distributions obtained up to and including Mach 1.2.
It was during the initial calibration period that 4T personnel managed to squeeze in their first user test. That test began on Jan. 29, 1968, and continued through Feb. 9, 1968.
The first test at 4T was performed at the behest of the Air Force Armament Laboratory and involved the testing of a Hard Structure Munition missile model at transonic speeds. Similar tests on the HSM had previously been conducted in the 8-foot transonic wind tunnel of what was then known as the Cornell Aeronautical Laboratory and 16T at Arnold Air Force Base.
The first store separation test at 4T was conducted in April 1968. Tests were conducted to determine the dynamic and static stability characteristics of the AGM-12E missile and to investigate the separation characteristics of the missile from the inboard wing pylons of the F-105 aircraft. Among other conclusions, this test would ultimately determine that the store would separate from the inboard pylon without jeopardizing the aircraft.
After its first decade of operations, 4T was credited with saving millions of dollars and eliminating the need for thousands of hours of flight testing in development of payloads for military aircraft.
“This important facility and the personnel responsible for its operation have made a truly major contribution to the mission of the Armament Development and Test Center and the increased combat effectiveness of the Air Force,” Maj. Gen. Howard M. Lane, then-commander of System Command’s Armament Development and Test Center based out of Eglin AFB, said in early 1978.
Within its first 10 years, 4T had been used in a number of test programs for the Air Force, Army, Navy and NASA. These tests supported the development of stores and certified their use with numerous aircraft, including the F-105, F-4, A-7 and A-10.
The 4-foot wind tunnel has continued to evolve over the years.
Nozzle blocks were completed in 1971 allowing 4T to exceed its original maximum Mach number of 1.35 up to Mach 2.0. From November 1988 to April 1989 a flexible nozzle was installed. Mach numbers greater than 2.0 were made possible through the installation.
Among other improvements was the development in the early 1980s of a grid technique for analyzing store separation, replacing the laborious captive trajectory mode method. With the grid technique, the weapons system was placed under the parent aircraft in a grid arrangement. Data was then collected on the store at different orientations in the space beneath the aircraft. Kraft said this provided greater efficiency in store separation testing and analysis.
In the late 1980s, the Computational Fluid Dynamics team at Arnold developed a method of simulating stores released from aircraft at high transonic speeds. At around the same time, investments brought a store separation graphics analysis program. This allowed personnel to complete wireframe models of test articles and see a 3-D store trajectory display within moments of a test.
Data systems have also continuously improved over the years, said Kraft, who witnessed much of the upgrade work at 4T. He returned to Arnold in 1971 to begin work as a full-time engineer and served as assistant branch manager for 4T in 1980.
“They constantly kept the tunnel at the leading edge,” he said. “It’s still the best store separation wind tunnel in the country and, probably, in the world.”
The demand for 4T has been increasing since 2012 and is expected to remain strong for the foreseeable future because it remains a cost-effective facility for missile and store separation tests, according to Flight Systems CTF Operations Officer Edward Mickle.
“We’re still receiving customer calls for this wind tunnel on a regular basis,” he said.
Lt. Col. David Hoffman, director of the Flight Systems CTF, said the reason behind the recent uptick of work at 4T is the nation is developing systems to answer near-peer competitors and prepare for the next generation of warfare. This includes weapons testing and integration work, as well as the development of systems within the hypersonic regime.
The wind tunnel is expected to continue its significant contribution to the development of hypersonics. Although operations at the hypersonic level cannot be conducted within 4T, Hoffman said the facility provides important groundwork before a test article is moved to a facility with a higher Mach capability, such as the AEDC Von Kármán Gas Dynamics Facility.
“4T will play a role in the hypersonics surge, even though the speed regime is not what you would consider hypersonic, because the tunnel is foundational to that work,” Hoffman said.
4T has maintained the ability to offer such contributions thanks largely to more recent investments and upgrades to the tunnel.
The PES supported 16T, the soon-to-be-reactivated 16S and 4T during the first decades of 4T. By 1993, an Independent Drive System for use exclusively by 4T was completed, along with a modified compressor. The IDS allowed for 4T operations up to Mach 1.3 without reliance on the PES. Operations requiring Mach numbers greater than 1.3 still require use of the PES.
“That has really been a boon for the ability to run 16T and 4T concurrently during simultaneous operations,” Mickle said.
More recently, 4T received additional upgrades through U.S. Air Force Test Investment Planning and Programming (TIPP). The facility was shut down for around a year-and-a-half beginning in October 2012 to allow for the modernization effort.
During this time, the CTS mechanical and control systems were upgraded to significantly increase position accuracy. System load capacity was also increased by around 40 percent, allowing 4T users to accommodate larger models.
Other modifications and improvements, including a new flexible nozzle and the strengthening of the test section wall, enabled testing up to Mach 2.5.
The data acquisition system for 4T was also updated through TIPP. This included a switch to the same software installed in VKF around that time, essentially “marrying” the data systems. With this, customers wanting to test flight systems from subsonic to transonic speeds in 4T could bring the model to VKF to continue testing on the article from transonic to hypersonic speeds.
“We have the same data system, the same screens. We built those things the same, so you can transition from one tunnel to the other, and we have done that recently where we’ll build a model in VKF and run it in 4T then go right to the hypersonic VKF,” Mickle said. “That is an efficiency that is nowhere else in the nation.”
This data system allows for data time stamping at the point of acquisition, as well as higher data acquisition and processing rates. Because 4T and VKF systems are practically identical, personnel are able to work 4T or VKF, providing efficiency and flexibility.
Flight Systems CTF Technical Advisor Scott Meredith said efforts have recently been made to make analytics part of the deliverables provided to test customers. Analysts in the test facility provide real-time data analysis and have developed and use tools to help customers make decisions to optimize tests.
“That information is more valuable the sooner you get it into the decision-makers’ hands,” Meredith said. “Ten business days after end of testing, we’re getting quick-look reports in customer and program offices’ hands to help with rapid decision-making for the test programs. We then follow up with a more detailed technical report on every test that comes through our facilities.”
Those in the Flight Systems CTF agree that a half-century after its first test 4T continues to fill a much-needed niche in medium-scale transonic and supersonic testing.
“That vision from 1967 is still correct,” Mickle said. “There was some good foresight put in. There was some really good thinking put in. And it’s still being used.”