Thermophysics Facilities – FAQ
Frequently Asked Questions
What is an arc jet and how does it work?
The Arc Jets are hyper-thermal test facilities that support the testing of thermal protection materials for any program, system or vehicle that’s going at very high velocity in an atmosphere, anywhere in the universe. An arc jet is a device in which gases are heated and expanded to very high temperatures and at supersonic/hypersonic speeds by a continuous electrical arc between two sets of electrodes. The gases (typically air) pass through a nozzle aimed at a test sample in vacuum, and flow over it, producing a reasonable approximation of surface temperature and pressure and at gas enthalpies experienced by a vehicle on atmospheric entry.
What is a heat shield?
The material on the exterior of a spacecraft that protects it from the high temperatures experienced during atmospheric entry from space. Temperatures from 2000° F to 5000° F can develop on the outer surface of the heat shield, yet the heat shield keeps the spacecraft below 200° F. A few examples of NASA spacecraft that used heatshields are: Space Shuttle, Apollo, Mars Exploration Rover landers, Mars Pathfinder, Mars Science Laboratory, Mars Viking, Galileo probe, Pioneer-Venus probes.
What is an arc jet facility used for?
Testing heat shield materials in an environment that simulates the high temperatures experienced during atmospheric entry at high speeds (spacecraft speeds from 15,000 miles per hour to over 100,000 mph).
What unique environment does an arc jet produce?
High temperature hypersonic flows (Mach > 5) of gases in a tunnel that impinge on a sample of heat shield material about the size of a small bowl. The response of the material to the high heating and high temperature is observed during an exposure lasting as short as a few seconds or as long as 30 minutes, depending on what is desired. Measurements that are made are typically heat flux, material temperature, surface pressure, gas temperature, test gas composition and velocity.
How long have you been using arc jets at Ames?
The Ames Arc Jets began in the 1950’s with the founding of a permanent facility in 1961. A breakthrough patented design in 1964 by Stine, Shepard and Watson of NASA Ames produced a high-enthalpy constricted-arc heater which enabled TPS development for Mercury and Apollo missions. We’ve been in business for over sixty years.
What programs have been in the Ames Arc Jets in the past?
We have supported almost all of the customers who have flown through the atmosphere all the way from the Apollo through the space shuttle, X-33, X-34 and on into the current programs like the X-37 (the USAF shuttle). In the space science world we have supported Pioneer, Viking, Galileo, Mars Pathfinder, Mars Exploration Rover (MER) and Mars Science Laboratory (MSL), which is currently on the surface of Mars; all of these programs are part of our heritage. The Stardust space capsule, which returned to earth in January 2006, entered our atmosphere at extremely high velocity (over 28000 mph) and that heat shield was tested in our facility. A significant portion of testing is still ongoing for the NASA Orion spacecraft as well as commercial space ventures. In the future we intend to be a big contributor to the space exploration program. We are a part of a large number of the proposals going in for that program and we hope to develop some new materials and tests for those folks.
How does an arc heater compare to other ways of simulating a space entry environment
In thermal protection system (TPS) designs using new materials or concepts, it is difficult to predict the temperature distribution, heating, and failure levels etc. through analysis or calculation only. Therefore, ground testing becomes the best method. Given the fact that it is practically impossible to simultaneously simulate all of the flight environmental parameters in any ground test facility, there are three options for ground tests for TPS development. These are radiative (lamps, lasers), combustion-driven wind tunnels, and arc plasma facilities. In a radiative facility, lasers or lamps are the source of heat and simulate the aerodynamic heat levels. However, they do not reproduce the hypersonic flow environment (gas composition, temperature and pressure, boundary layer type), which inhibits a duplication of ablation response in a real sense. Combustion facilities can duplicate the hypersonic flow aspects but cannot simulate relevant gas composition and temperature. The arc jets do reproduce very closely the true hypersonic flow, gas composition, heat flux, and temperatures over the test samples.
Where else can I go to do these types of tests (your competitors)? Or Who else can do this type of testing for me?
There are five facilities in the world of this type, although we are not necessarily competing as much as complementing them. Within NASA, there is only one facility of power greater than 3 MW still in operation and that is at Ames Research Center. The arc jets at Johnson Space Center, another NASA facility, was closed in 2014. The JSC 10-MW arc heater was relocated to Ames and shares the Aerodynamic Heating Facility bay with the 20-MW Ames arc heater.
Another facility with similar power capability is in Tullahoma, Tennessee at the USAF Arnold Engineering Development Center referred to as the AEDC. That facility however is optimized to support the Air Force mission, which means it is basically for re-entry technology, ballistic re-entry and as such it tends to be high pressure, low enthalpy missions. Ames facilities have been developed to support the NASA missions, which are high enthalpy, low pressure so we are actually complementary to each other. In fact, we do experiments here that are later sent to AEDC depending on the configuration or mission being tested.
The third facility is the Italian facility operated by the Italian research organization known as CIRA. They call their facility the Scirocco facility. Scirocco is actually based on the design at Ames here forty years ago. In fact, we had a patent for our particular type of arc heater technology, referred to as the segmented constrictor technology, and this technology is now used in all of these types of facilities all over the world. When the Italians were designing their facility, they employed a small company in Mountain View here to have their facility designed and fabricated and checked out for them. When the company did that, they copied our arc heaters and scaled it up slightly. It is presently the highest-power arc jet facility now, but they are relatively new to the technology and have neither the experienced staff that we do nor the wealth of adaptors such as specialized nozzles we have built up over the years.
Other facilities include a 6-MW arc jet operated by Boeing in St. Louis and a 3-MW facility located at DLR in Köln (Cologne) Germany.
How busy are you?
We run, on average, 300 days a year for all three operational facilities. This number has gone as high as 600 days but our optimum usage is 400 days. We can take on more customers at any time. One reason is the three test-leg configuration mentioned previously which allows us to complete fairly complex test series within a relatively short time frame. Also, we are constantly working on getting the arc heaters to higher and higher levels of productivity. For example, we recently added a five-arm model insertion system to one of our facilities that allows up to 100 test exposures per week in less complex test projects.
What is an aeroballistic range?
In simplest terms an aeroballistic range is essentially a wind tunnel that works in reverse. Rather than blowing air over a fixed model to measure it’s aerodynamic characteristics, in an aeroballisitic range a small scale model is launched and flown through still air to measure it’s aerodynamic characteristics.
When is it better to use an aeroballistic range instead of a wind tunnel?
An aeroballistic range is particularly useful for simulating very high speed flight. Typically it is much more challenging and costly to move a column of air at say 7.0 km/s (23,000 ft/sec) than it is to shoot an aeroballistic range model at this same speed.
What are the primary benefits and drawbacks (pros and cons) of aeroballistic testing vs. wind tunnel testing?
The benefits of aeroballistic testing are true flow chemistry and no model mount (sting) induced effects. In other words extremely accurate data. The drawbacks are small model size (scale) and short test duration (typically milliseconds).
What is the difference between a light-gas gun and a regular gun?
A light gas-gun uses a gun powder charge to shoot a plastic (polyethylene) piston into a tube filled with hydrogen. As the piston travels down the tube the hydrogen is compressed, hence raising it’s pressure and temperature. The resultant high pressure and temperature hydrogen is used to accelerate the projectile down and out the gun barrel. Light-gas guns can achieve velocities up to 7 times faster than most typical guns.
The Ames Vertical Gun Range can be oriented to shoot projectiles at angles ranging from horizontal to vertical, why is this important?
Being able to vary the impact angle of the projectile with respect to gravity is extremely important when studying the physics of crater formation processes, and debris tracking.
How do I go about establishing a test agreement with you?
We have a procedure that lays out very clearly that we want to be involved with you early on and that collaboration is at no additional cost, or in other words it’s factored into the rates we have been discussing. We want to help you design your test articles and decide which facilities should be used for them. We’d like to partner with you all along that route so that when you produce a test plan, it includes our experienced recommendations and it will produce the results you require. See our “Contact Us” page for whom and how to contact use with your testing needs. While you are there, download the “Test Planning Guide“, we can help you write a test plan, set up the schedule for your tests, and then work with you throughout the tests and eventually write a test report for you.
What’s the typical cycle time to get a test series planned and executed?
We would suggest you contact us three to four months before you believe you will need the tests completed. Sometime we can get less complex tests properly planned and completed in as little as a few weeks. It depends in large part on your experience with this type of testing.
How can I get more detailed information on your facilities and how to get started with you on a test program?
One of the best written sources is the Test Planning Guide, available on line in PDF format. You can also contact us by e-mail at ARC-thermo-physics@mail.nasa.gov or by calling us at +1 (650) 604-6166.
I was reading over the heat shield document on nasa.gov, and I am curious to know.. space shuttles and modules usually have some sort of viewing ports or windows, if you will… what kind of material do you use for that? What stops the “glass” from melting in re-entry?
The high heating experienced by spacecraft when entering the atmosphere is caused by a high-pressure bow wave in front of the ship. This strong shock wave is caused by the craft flying at supersonic speeds, even hypersonic speeds. Hypersonic is roughly greater than Mach 5. The shock wave is where the atmosphere is rapidly compressed by a factor of 50 to 100, depending on the speed of the vehicle. Because of this rapid compression the gas is heated to high temperatures, as high as 6000 K or more. This hot gas then impinges on the front of the spaceship, and transferring heat to the surface. That is why it has to have a heat shield.
One thing you will notice about human spacecraft is that the windows are all located on the back surface, or leeward side, of the spacecraft. This side experiences lower high heat transfer compared with the windward side, and so it does not reach as high a temperature. This is because the pressure is much lower, at least two orders of magnitude lower (1/100 or less pressure) on the back side. The hot gas on the windward side expands to the leeward side, which means the pressure drops quickly, and so does the gas temperature. For this reason the windows are always located on this “cooler” side. You should know that even on the cooler leeward side if there were any exposed metal surface the metal would melt within seconds. So windows must still be able to withstand high temperatures, say about 1000 C. So Shuttle windows are made from a high-temperature quartz glass that can withstand heating and cooling without cracking. The same explanation applies to the Russian Soyuz and to NASA’s new spacecraft called Orion that is under development.
It may not seem like the windows on the Space Shuttle are on the leeward side, but remember that as it flies through the peak heating portion of the reentry the nose of the shuttle is pitched up from horizontal by 40 degrees. At that angle the pilots are not looking toward the direction that the spacecraft is moving, but more “up” towards space. Therefore the hot shock-heated gas bears only on the underbelly of the Shuttle where there are no windows, and not on the upper surface where the windows are. After the Shuttle passes through the hottest region of its flight it pitches down so that the pilots can see where they are flying by looking through the windows.