Hypersonic CF4 Tunnel

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1275
Building 1275

Center: Langley Research Center
Location: Hampton, Virginia
Year Built: 1960
Historic Eligibility: National Register Eligible
Important Tests: Shuttle Orbiter, Meteoroid Studies, Crew Rescue Vehicles



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Contents

[top] History

Constructed in 1960 in the Hypersonics Physics Test Area (HPTA), this facility was originally the named the Impact and Projectile Test Unit. It was later known as the Entry Vehicle Flow Simulation Apparatus and most recently as the Hypersonic CF4 Tunnel.

The facility has been used for research on reentry materials for spacecraft including tests of blunt-base models, simulation of real-gas effects during reentry, evaluation of effects of damage to Shuttle Orbiter on temperature characteristics, evaluation of proposed X-33 and X-38 Crew Rescue Vehicles, and development and application of a unique holographic flow visualization technique for test analysis.

The Hypersonic CF4 Tunnel was designed to simulate flow field characteristics encountered by space shuttle-type vehicles during re-entry. The basic concept of the facility operation was to release high pressure tetrafluoromethane gas (CF4) heated up to 1000-degrees F through a converging-diverging nozzle and a 20-inch test section into large evacuated spheres. The gas pressures were regulated by using a combination of flow control valves and a slat bath heater temperature. Gas was compressed to 5000 psig by a centrifugal pump and stored in a bottlefield.

The test section area included the settling chamber, nozzle and piping, the 20-inch test section, diffuser and a heater. The settling chamber was 12-inches long and had a 7-inch inner diameter. Electric strip heaters were used to preheat piping, the settling chamber and nozzle to help maintain the gas temperature and reduce thermal stresses. The nozzle throat was 0.42 inches and expanded to 20-inches in diameter at the entrance of the test section. Mass flow through the test section was determined by the pressure and temperature of the gas. A heat exchanger, utilizing was as the cooling medium, reduced the gas temperature prior to entering the spheres. The system consisted of 2 spheres, 3 blowers, and 4 pumps which was capable of reducing tunnel pressure down to 1/4 mm of mercury.

The Model Support and Injection System vertically injected the model into the test section at a predetermined time when the tunnel flow reached the desired pressure and temperature. The subsystem, installed in 1990, consisted of a hydraulically injected, horizontally translated frame. This replaced a pneumatically injected system in which the yaw was mechanically adjusted prior to each run and could not be changed during the run. The new system was also design to dampen out and reduce model oscillations more quickly than in the old system.

As part of the Center's revitalization initiative, NASA determined that the facility's capabilities were no longer needed and the facility was demolished in 2017.

[top] Building Photos

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[top] Impact and Projectile Range

At the time of construction, the existing gun facility was moved from the Impact Basin to this new building. Apparently, the range had originally been constructed in 1958. The 1964 Facility Resume describes this part of the facility as consisting of a 190’ x 30’ range with 3 smaller adjoining rooms. Range #1 was 105 feet, and Ranges #2 and #3 were 18 feet each. Projectile velocities were determined by photographing the projectiles at two stations and recording the time lapse.

By 1966, the range included a 20mm light gas gun, a .22 caliber shock compressed light gas gun, a .22 caliber accelerated reservoir light gas gun, a .22 caliber rifle, and a 20mm powder gun. A lithium electro-thermal gun and a 4th range had been added by 1974.

In 1966, the Meteoroid Research Group proposed a gun with the capability of accelerating small projectiles to high velocities. Two stations in the gun would determine the projectile speed and size. The target was instrumented to provide time of impact and perforation. The investigation of meteoroid hazards was necessary for testing flight detectors and for impact damage studies. The gun allowed the testing of projectiles in the size range of greatest concern in designing flight experiments and in reducing spacecraft meteoroid damage.

These tests were validated with the return of the Long Durations Exposure Facility (LDEF). The LDEF concept evolved from a spacecraft proposed by NASA Langley Research Center (LaRC) in 1970 to study the meteoroid environment.

See interview with Donald Humes on the development of the light gas gun and his research on meteoroid impacts. Interview includes demonstration of the operation of the gun shown below. Report of studies and short video of LDEF also available.


[top] Photos

ShieldGun with Shield on RightGun along TrajectoryGun Close-upConnection to PumpPumpPressure GaugesConnection to Gauges


Materials Used and Sample Plates After Impact Tests

PelletsPistons and DiaphramsCasingsShell Loading AreaLoading EquipmentTest Plates After ImpactCopper Test Plate with Actual Meteoroid Impact from LDEFCopper Test Plate with Actual Meteoroid Impact from LDEF


Test Results

Don Humes with gun in 2013Selection of Test PlatesFlare SampleStarburst SampleClear Sample Shows Impact Detail


Personnel

1959 Bill Kinnard and Impact Test1963 Willis Ward1963 Fay Moore1963 Don Humes


[top] Arc Heat Materials Jet

The Arc Heated Materials Jet became operational in 1961. This facility was used for the studies of aerodynamic effects on materials at supersonic speeds and high temperatures.


Report: A Magnetically Rotated Electric Arc Air Heater Employing a Strong Magnetic Field and Copper Electrodes. Robert F. Mayo, William L. Wells, and Milton A. Wallio. NASA TN D-2032, November 1963.


[top] 20-Inch Hypersonic Arc-Heated Tunnel / CF4 Tunnel

20-Inch Hypersonic Arc-Heated Wind Tunnel or HAHT was completed in 1962. This tunnel allowed the testing of reentry materials.

In 1972, the tunnel was converted to 20-Inch Hypersonic CF4 Tunnel for the study of heat transfer, pressure distribution, and force tests of entry vehicles. The facility was a blowdown wind tunnels that utilizes high-molecular-weight gases to produce normal shock density ratios comparable to those produced by high-speed entry vehicles, particularly blunt bodies.

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Aerothermodynamic Flight Simulation

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Aerothermodynamic Experimental Methods

The CF4 tunnel was one of several hypersonic wind tunnels used to replicate the effects that space shuttles experience during reentry. The CF4 tunnel was located in building 1275 on NASA Langley’s campus. The CF4 tunnel was constructed during the beginning of computational fluid dynamics, which is a method that used computers to simulate flights. The CF4 tunnel was considered revolutionary because of its use of tetrafluoromethane, instead of air, to simulate travel patterns of high-speed aircrafts. Tetrafluoromethane is a chemical gas that contains one carbon molecule and four fluorine molecules. Although the CF4 tunnel primarily used tetrafluromethane, the tunnel had the capability to use any gas to simulate chemistry effects.

The CF4 wind tunnel was a major contributor to space shuttle research. One of the most important factors in space shuttle research is the presence of sound. Sound can be described as the transfer of vibrations. Space shuttles fly at speeds that are faster than the speed of sound. These speeds include subsonic, transonic, supersonic, and hypersonic speeds. Aerodynamicists use Mach as a system to measure the speed ratio of aircrafts. One Mach is equal to about seven hundred miles per hour. Subsonic speed is any speed that is less than Mach 1. If a speed is equal to Mach 1, the speed is considered to be transonic. Any speed that is greater than Mach 1 but less than Mach 3 is classified as a supersonic speed; high-supersonic speeds are faster than Mach 3 but slower than Mach 5. Lastly, speeds that are faster than Mach 5 are considered hypersonic. Hypersonic shuttle vehicles have big, blunt noses and small wings. Supersonic shuttle vehicles have pointed noses and small wings. Designers create various types of shuttle models in order to understand how certain design features alter the amount of sound caused by the vehicle. In addition to sound, designers strive to have the most minimal amount of mass for space shuttle vehicles. Weight is another important factor to consider when building a space shuttle. The amount of heat that a space shuttle generates directly correlates to its weight. Therefore, if a space shuttle has less mass, the vehicle will generate less heat; this allows the shuttle to carry more material through orbit.

One of the primary purposes of wind tunnels is to design a vehicle that will be able to fly at exponentially high speeds. The CF4 tunnel was comprised of a reclaimer, a CF4 compressor, high-pressure gas cylinders, vacuum pumps, an electric loader, and a test section. Most wind tunnels contain a large vacuum on one side of the tunnel, which causes strong amounts of pressure on the other side of the tunnel. Wind tunnels can create air speed through fans, however, fans are very expensive to use. Although air pressure is an important factor in wind tunnel testing, heat is required to run a test; air speed is not enough to run a test, by itself. During performance tests, wind tunnel models are exposed to a flow of high-speed gas. The tests measure force and temperatures to determine the characteristics space shuttle vehicles would experience while in flight. Initially, space shuttle models are tested in inexpensive facilities, such as the Mach 6 tunnel and the CF4 tunnel, to heat the models. Afterwards, the models are tested in more expensive facilities, such as the 8 foot high temperature tunnel, in order for the models to experience temperatures closer to atmospheric temperatures. Eventually, the models are tested through full scale flight.

In order to conduct a test in the CF4 tunnel, the first procedure was to install the test model into the test section; the model was not in the facility at the beginning of the test. The next step was to setup the instrumentation of the test (e.g. pressure measurements, heat transfer measurements). Finally, one must configure the facility to be able to complete a test. A test in the CF4 tunnel usually lasted about twenty seconds. Afterwards, the technicians used the high-pressure bottle field to reclaim the gas. In order to reclaim the gas, it must be turned into liquid and cleansed of all impurities; the cycle needs to be repeated a few times. Normally, the CF4 gas was reclaimed about four times per day. Tetrafluoromethane molecules have a specific size that allows the gas to mimic the scale of the aircraft.

Space shuttles are designed experimentally. Engineers work tirelessly to understand the environment in which space shuttles fly. Atmospheric conditions vary in heat and pressure, consequently, researchers utilize wind tunnels to determine how to enable shuttle vehicles to safely reenter the atmosphere. Many problems arise when space shuttle vehicles travel through these atmospheres, including generation of heat.

An example of a re-entry obstacle is the pitch up anomaly, which was the result of a real gas effect. As the first flight space shuttle re-entered the Earth’s atmosphere, the nose of the shuttle continuously rose in pitch; consequently, the shuttle experienced real gas effects. Ideal air can be defined as a mixture of gases that obeys the Ideal Gas law of Pv=RT. Real air is a mixture of gases that follows the Real Gas law of Pv = ZR T. In these formulas, p represents pressure, v represents volume, and RT represents temperature. Real air creates less pressure than ideal air, which makes the space shuttle harder to control. Most aircrafts use slats instead of flaps. The shuttle only had sixteen degrees of body flap to control the shuttle. Therefore, engineers did not know the amount the shuttle would lift. The only way to bring the shuttle back down was to take manual control of the shuttle and augment the shuttle through the Reaction Control System.

A Reaction Control System can be described as a series of small rocket motors on the Space Shuttle that are used to control the Shuttle’s orientation at the beginning of the vehicle’s return to earth from the Space Station. Similar to airplanes, the Shuttle’s flaps helped control its flight path. During the high altitude phase of flight, there was a minimal amount of air to allow these control flaps to work. Mission control deflected the body flap of the shuttle, in order to sustain the vehicle in a stable flight. The RCS jets were responsible for propelling the vehicle. When the space shuttle returned from orbit, there was a significant amount of pushing that was required from the RCS jets. Subsequently, the special nature of the CF4 test gas allowed the CF4 tunnel to reveal the reason these jets were overworked during reentry. Another factor to consider is the role of control authority. Control authority can be defined as the maximum amount of achievable output forces that thrusters can provide. Space shuttles are typically advised to not have a significant amount of control authority. During this time, computers were not used, consequently, the CF4 test results became vital to the design of space shuttles. Designers continued to use the CF4 tunnel to test vehicles and search for potential danger scenarios, before the vehicle’s first flight.

Throughout hypersonic travel, the speed at which oxygen molecules travel causes them to dissociate; therefore, the chemical reactions from these oxygen molecules alter how space shuttles fly. Researchers were aware of similar chemistry effects for many years, however, computational fluid dynamics were not able to duplicate the chemistry effects. The CF4 and wind tunnels allowed researchers to measure chemistry effects through experiments. A unique aspect of the CF4 tunnel was its ability to recreate high normal shock density ratio and low gamma ratio specific heats of real gas. Normal shock density ratio measures the increase in the gas density of shock waves. Gamma specific heat is a type of specific in which the value is determined from the state of the gas; gamma often appears in the equations for shock waves. The CF4 tunnel enabled researchers to compare data between the Mach 6 air tunnel and the CF4 tunnel.

When operating any hypersonic facility, the gas needs to have a high temperature. Temperature and pressure are exchanged for velocity; consequently, the gas must start at a high temperature. In any scenario where there are high pressures and high temperatures, there is a risk of something exploding or burning. Tetrafluromethane is heavier than air, therefore, the gas would often lie close to the lower places within the CF4 tunnel. When tetrafluromethane experiences moisture, it becomes corrosive; consequently, the CF4 gas would often damage the pipes around the tunnel. Stainless steel reacts with CF4 gas, therefore, the CF4 tunnel switched to Inconel tubing. Inconel is an alloy that is composed of iron, nickel, and chromium. Inconel is often used in high temperature atmospheres because of its durability. The Inconel tubing significantly improved the quality of test gas and data.

One of the unforeseen challenges with the CF4 tunnel was measuring temperature on the surface of the test models. When measuring thermography on a test model, the color components that the model emits are temperature dependent. CF4 gas absorbs infrared rays, therefore, researchers needed to use a certain type of thermography that would spread a luminous coating on test models and illuminate the model with ultraviolet light. DiFulvio made the suggestion to use a thermographic phosphorous technique for test models. In order to create test models, a mold has to be made from plastic. Afterwards, the ceramic model is created from the mold. The phosphorous coating improved the thermographic image data from the tests. Another unexpected challenge was the gas flow disturbance within the nozzle of the test section. Disturbances were often focused towards the center of the nozzle, which would disrupt the test models. However, The CF4 tunnel was also able to generate a really low Reynolds number, which helped Tom Horvath simulate characteristics of the wake generated behind the capsule of a space shuttle; this was extremely important in the analysis of the Gemini mission. Also, The CF4 tunnel allowed researchers to determine the difference between shock detachment distance and measured pressures, heating, and aerodynamic characteristics.

The CF4 tunnel was highly utilized in the Columbia Accident Investigation Board’s final report. The Columbia Accident occurred on February 1, 2003. The incident included the loss of Space Shuttle Columbia and the seven members of its crew. The accident happened due to a breach in the thermal protection system on the leading edge of the left wing on Space Shuttle Columbia. Shortly after the accident, experimental testing was conducted in the CF4 tunnel. The testing simulated a broad spectrum of thermal protection system damage to the Orbiter windward surface. The CF4 tunnel was able to recreate several of these complex failure scenarios, such as the damage on the leading edge of the left wing, to provide synergistic aerodynamic/aerothermodynamic wind tunnel measurements. When the forensic phase of the Columbia accident investigation began, there was an extensive examination on the condition of recovered debris from the ground.

The forensic investigation strongly emphasized wing leading edge damage as one of the primary causes of the Columbia accident. The results of the experimental tests were used to refute the following hypothesized forms of thermal protection system damage: gouges in the windward thermal protection system tiles, breaches through the wing near the main landing gear door, and protuberances along the wing leading edge that produced asymmetric boundary layer. Boundary Layer Transition is the phenomenon that occurs when smooth, laminar airflow changes into a disturbed, turbulent flow; the transition causes surface temperatures to increase by several hundred degrees Fahrenheit. Space shuttle Columbia experienced these boundary layer transition effects, which contributed to the loss of the shuttle. The CF4 tunnel played a vital role in discovering these results.

The CF4 tunnel was specifically designed to simulate chemistry effects in an inexpensive and safe manner. Apollo and Gemini were also tested in the CF4 tunnel. The Gemini program helped identify that the capsule returned 10 miles from where it should have landed. Real gas effects caused the vehicle to experience a different pressure field on the shell than it would normally experience at ideal gas. When researchers saw what happened to Gemini’s capsule, they wanted to find a gas that would provide simulation parameters. Tetrafluromethane enabled researchers to simulate gamma effects of high altitudes in reentry, which helped researchers identify the cause of the capsule landing.

Also, The CF4 tunnel was extremely involved in Return to Flight. During the Return to Flight, mission control found a protrusion in the space shuttle. Mission control needed to know what caused the protrusion and what would clear the shuttle vehicle for re-entry. Tunnel operators were able to conduct subsequent testing in the CF4 tunnel. While shuttles were in space, CF4 researchers, such as Horvath, would remain at the mission control center. CF4 tunnel operators remained on stand-by, in case any immediate testing needed to be conducted.

The CF4 tunnel was a very dangerous machine to operate. Throughout the CF4’s existence, many filters collapsed, including a lead filter that exploded within building 1275. At the beginning of the CF4 tunnel operation, lead bath heaters were used to heat the gas in order for hypersonic flight to be sustained in the test section. Lead helped the CF4 tunnel hold heat at a constant temperature; lead was also very inexpensive to use. Coils within the lead bath heaters leaked, which caused a large bubble of gas in the heater. After a while, the bubble erupted and spread lead around the facility. John Micol made the decision to use molten salt bath heaters. Eventually, the CF4 used molten tin, instead of molten salt.

Many other facilities use electrical resistance heaters. Electrical resistance heaters require a significant amount of time to reach the necessary level of heat. However, CF4 test time is only around 24 seconds; therefore, the CF4 gas needed to heat quickly. In order to help the CF4 tunnel maintain a constant temperature throughout test times, researchers worked on installing a gas fire heater to help control temperature. However, the installation was not successful. Eventually, natural gas furnaces cracked, causing carbon monoxide to leak; consequently, the CF4 tunnel was shut down.

The CF4 tunnel was the only tunnel of its kind, in the world. The crew who operated the facility grew very close throughout the time of the CF4 operation, due to long hours and extensive research. “We have a mission, we have to do this” said Difulvio, as he described the determination of those who worked with the CF4. Testing conducted in the CF4 tunnel helped researchers understand dangers that space shuttles experience when the shuttles reenter the atmosphere. Ultimately, the CF4 tunnel enabled the United States to help astronauts return to space, in a timely manner.

Write up by summer student Todd Smith, 2014.


Report: Use of Tetrafluoromethane to Simultate Real-Gas Effects on the Hypersonic Aerodynamics of Blunt Vehicles. Robert A. Jones and James l. Hunt. NASA TR R-312, June 1969.


[top] Tunnel Photos

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[top] Model Photos

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Shuttle

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[top] Personnel Photos

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[top] Chemical Kinetic Shock Tube

A Chemical Kinetic Shock Tube became operational in 1966 for high-temperature chemical kinetic studies of gas phase reactions. The stainless steel tube with a 3 ½-inch diameter was composed of a 19-foot test section, a 4-foot buffer section, and 3-foot driver section. The tube was capable of producing temperatures up to 4000 degrees K and pressures up to 45 psi. Test conditions could be varied by the molecular weight of driver and buffer gases.


[top] Films

Donald Humes Interview, November 2012.

Shuttle Reusable Surface Insulation

Impact Studies 1

Impact Studies 2

Shuttle Test 143 - CF4 Tunnel

X37 Test #137 - CF4 Tunnel


[top] Documents

General Building

1960s Building Layout

1990 Resumes for All Facilities


20-Inch Hypersonic Arc-Heated Tunnel

1965 Brief Description of Langley's 20-Inch Hypersonic Arc-Heated Tunnel (TM X-1130)

Resume: 1966


20-Inch Hypersonic CF4 Tunnel

Resumes: 1974


Arc-Heated Materials Jet

1965 Brief Description of Arc-Heated Materials Jet Building

Resumes:1964, 1966


Chemical Kinetic Shock Tube

Resume: 1974


Impact and Projectile Range

1966 Proposal to Build Gun

Light Gas Gun Notebook Informative binder about the Langley Miniature Light-Gas Gun assembled by Light-Gas Gun designer Don Humes.

Resumes: 1964, 1966, 1974

Aeroballistic Range Facilities and Their Peripheral Equipment at Langley Research Center. John D. DiBattista. n.d.

Acoustic Emission Signals in Thin Plates Produced by Impact Damage. William H. Prosser, Michael R. Gorman, and Donald H. Humes. in Journal of Acoustic Emission, Vol 17, June 1999, pp. 29-36.

Dodging Bullets: Skylab and the Langley Miniature Light-Gas Gun. Robert C. Moyer. 2013.

Meteoroid and Debris Impacts on the WF/PC I Radiator

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