ARNOLD AIR FORCE BASE, Tenn. – In May 2011, NASA formally announced that the Orion crew capsule is the spacecraft that will carry humans back into deep space.
Before Orion is ready for crewed missions to an asteroid and Mars, aerothermal testing is being conducted at AEDC’s Hypervelocity Wind Tunnel 9 on a 4-percent scale model of the Orion crew capsule.
“We’re using the Orion model that we used when we went into AEDC’s Tunnel 9 facility in 2006,” said Adam Amar, NASA’s acting Orion aerothermal lead and system manager. “We’re looking at the heat-transfer rate on the heat shield of the Orion capsule.”
Joe Coblish, lead projects manager at Tunnel 9, said, “The Orion capsule has to have a properly sized thermal protection system (TPS) to successfully travel through the harsh environment encountered during atmospheric re-entry. Ground testing serves a critical role in providing high-quality data to validate computational tools being used today to develop advanced hypersonic configurations.”
According to Amar, who is NASA’s principal investigator for the project, the ground test will support a flight test, tentatively scheduled to take place at Kennedy Space Center, Fla., in 2013.
The goal for the current test is succinctly stated – investigate laminar stagnation point heat transfer augmentation on a blunt body.
Air flowing close to the surface of a vehicle, also known as the boundary layer, can be characterized into three categories: laminar, transitional and/or turbulent. A laminar flow can be best described as smooth and well behaved. This flow field in general is easier to numerically compute.
According to Coblish, a turbulent flow fluctuates randomly which tends to mix the flow close to the body of the vehicle; a turbulent boundary layer can therefore be more challenging to compute. Transitional flow is the region when the flow changes from laminar to turbulent.
Hypersonic boundary layer transition is currently not well understood and is the subject of active research being conducted within the aerospace scientific community and has been supported by AEDC Tunnel 9 over the past several years.
“Heat transfer data collected during the 2006 Tunnel 9 Orion crew capsule tests were acquired over a wide range of test conditions covering all three flow regimes,” Coblish explained. “These data compared well with NASA’s computations and were used to confirm the requirements for the Orion capsule TPS. However, one area that that didn’t compare well to computer predictions was in the stagnation point region of the vehicle while it experienced laminar flow.”
In the stagnation point region of any body, the flow stagnates or comes to zero velocity. From that point, the flow turns and begins to accelerate along the surface of the vehicle.
“We found that the laminar heat-transfer rates in this stagnation point region were consistently elevated above laminar flow computer predictions,” Coblish said. “This augmented stagnation point heating was not only observed in data from Tunnel 9, it was also found in data collected in NASA Langley’s Mach 6 and 10 tunnels as well as Cal Tech’s T-5 shock tunnel. This phenomenon is therefore not unique to Tunnel 9.
“NASA is designing the vehicle to handle some severe flight parameters during re-entry. They have therefore accounted for the augmentation found in the ground test data in the current TPS design requirements for the vehicle and I don’t expect the results from this test will affect the system significantly.”
Coblish said NASA is “still very interested” in understanding the science associated with this augmentation.
“While we’re using the Orion test article in this investigation, the problem is not Orion-focused,” he said. “It’s on blunt re-entry bodies; we saw similar augmentation during our Mars Science Laboratory (MSL) capsule testing that occurred right before the 2006 Orion test.”
Coblish explained that the team’s first goal during the test will be to collect an initial baseline of data to compare directly back to the original 2006 Orion crew capsule data set. He also pointed out that several aspects of the current test at Tunnel 9 will differentiate it from the previous work accomplished there.
“We’re going to be collecting data similar to what we had during the first 2006 entry, however at much higher data acquisition rates,” he said. “By sampling the data faster, we can investigate unsteady surface temperature, surface pressure and surface heat transfer in an attempt to gain a better understanding of the flow field surrounding the stagnation point. In addition, we’ll be collecting high-speed flow field Schlieren imagery in this region to observe any flow field unsteadiness.”
NASA and AEDC engineers suspect another factor unique to wind tunnel testing may also be a cause of the elevated stagnation point heating rates on the test vehicle; specifically, unsteadiness due to free stream pressure fluctuations or what is commonly known as “tunnel noise.”
Dan Lewis, ATA project engineer for the Orion crew capsule test program at Tunnel 9, said, “We will be collecting free stream tunnel noise data in parallel to the data collected directly on the Orion. With the tunnel noise data we’re hoping we can see a correlation between tunnel noise and the high-frequency surface and imagery measurements on and around the Orion model.”
Besides the original coaxial thermocouples sensors which are used to acquire the heat flux on the Orion’s heat shield, other novel measurements on the test article will be added by AEDC personnel to assist in the investigation. Specifically flush-mounted pressure sensors will look at surface pressure fluctuations on the heat shield, as well as a new heat flux sensor called an ALTP (atomic layer thermopile) sensor.
“The ALTP sensor has the potential to measure high frequency heat flux not possible with traditional heat flux sensors used in Tunnel 9,” Lewis said. “This sensor has never been run in Tunnel 9; however, they have been successfully utilized in high-enthalpy facilities located in Europe. We are extremely interested in evaluating the performance of these sensors in Tunnel 9’s test environment. AEDC will continue to provide the highest quality data set possible to our customers.”
Lewis added, “In addition to the new surface-mounted sensors, we are also using laser diagnostics during the tests to interrogate any fine particulate we may have in the approach flow. Laser-Induced Incandescence (LII) is a new technique being deployed for the first time during this test. We constantly try to advance our understanding of our facility whenever possible as well as investigate new measurement techniques that we can deploy on customer’s tests. AEDC has even successfully partnered in the past with NASA, DoD and academia to further these advancements. This continual improvement is imperative for AEDC to continue to provide the highest quality data set possible to our customers.”