AEDC's APTU supports first direct-connect hypersonic engine test program

  • Published
  • By Deidre Ortiz
  • AEDC/PA
ARNOLD AIR FORCE BASE, TENN. --  Arnold Engineering Development Complex (AEDC) is modifying its Aerodynamic and Propulsion Test Unit (APTU) in preparation for the first ever direct-connect tests of larger scale scramjet engines.

"Ground tests of scramjet engines at this scale have not been conducted at APTU or any other facility," said Kevin Holst, a propulsion analysis engineer.

Once the upgrades are completed, the facility will be fully capable of supporting the Air Force Research Laboratory's (AFRL) Medium Scale Critical Components (MSCC) direct-connect test program.

Sean Smith, an AEDC Air Force project manager, stated the engines tested will have 10-times the airflow rate of an X-51A Waverider.

"The goals of the MSCC program are to improve performance, operability and thermal management, enhance the understanding of governing physics and evaluate design and analysis tools," he said.

The upgrades to APTU, AEDC's hypersonic blowdown freejet test facility, will include a new fuel heater, three direct-connect nozzles and enhancements to the facility control system.

The new fuel heater will have automated temperature control rather than manual. Other features of the new heater will be its dedicated 12 megawatt power supply and improved surface temperature measurements.

Brett Boylston, a facility analysis engineer, explained that in hypersonic vehicles, the fuel serves as a dual-purpose coolant.

"Due to the extreme stagnation temperatures experienced by the vehicle in flight, active cooling is required by not only the avionics and electrical systems, but also the vehicle's airframe," he said. "The fuel is circulated through the vehicle as part of the thermal management system before going to the engine's combustor. The endothermic fuel may undergo cracking as heat is absorbed. The reaction caused by cracking the fuel provides an additional heat sink due to the endothermic nature of the fluid."

Boylston added that cracked products also provide a more reactive fuel for the combustor relative to the base fuel.

"During ground testing, a fuel heater is required to simulate the heat loads of the vehicle and engine before the fuel is delivered to the combustor," he said. "Its original four-coil heater used manual control to adjust power to each heater to modulate temperature."

The new control system will be fully automated and provide stable and consistent flow through each coil with multiple coils active. This new system will also provide repeatable flow rates and temperatures, minimize startup time and allow for selectable residence times.

A real-time computer simulation of the fuel heater has been developed that supports the control system development. The model consists of modular blocks, or subsystems, that simulate the dynamics of the flow of fuel through the fuel heater. This model will also be used with a larger existing APTU facility simulation before each test to ensure that control system inputs are correct for the planned test and that the facility hardware is configured correctly.

"Individual components models such as pipelines, valves, sources and sinks were developed," Boylston said. "The components can be combined in a modular way so that any system can be modeled and is not specific to the APTU fuel heater."

The control strategy developed will ensure safe operation and rapid follow and temperature transients with minimal overshoot.

In determining the requirements for the new direct connect nozzles for APTU, multiple computational fluid dynamics (CFD) tools were used and AEDC's previous high temperature nozzle designs were examined. Three new direct-connect nozzles are required to support the MSCC test program. The nozzles, with an air-injection distortion generator (DG), are designed to provide flow to an isolator representative of the flow seen in flight.

These nozzles, in conjunction with a flow distortion generator, will provide flow directly to the isolator throat, replacing the engine inlet.

"Direct-connect testing is atypical at APTU," Holst said. "Tests are normally conducted using freejet nozzles, which provide flow into the test cell at desired conditions. Direct-connect testing was selected for the MSCC program as a more efficient method to gain knowledge regarding medium scale scramjets."

AEDC's APTU project team has performed facility checkout test runs to calibrate multiple venturi tube installations in the APTU high pressure air, isobutane and liquid oxygen systems.

The objective of the checkout runs for the venturi tube calibrations was to determine the venturi tube flow coefficients, characterize line pressure and temperature effects, and characterize system measurement uncertainties.

Dr. Doug Garrard, the lead analysis engineer for APTU, stated that because all of the flow will be entering the scramjet propulsion system under test, the exact total mass flow rate must be known.

"Because of the in-place calibration tests, mass flow rates into the MSCC propulsion system under test will be defined accurately and precisely with a stated uncertainty," Garrard said.

The heated fuel system (HFS) will be activated and calibrated by executing specific cold-flow and hot-flow tests. Cold-flow tests will characterize valves and control systems, while hot-flow tests will mimic conditions required during MSCC testing.

Once this plan is completed, Holst said the HFS operations will be well understood.

"The flow and temperature controls will be tuned, the chemical makeup of the fuel will be characterized and the fuel distribution flow measurement venturis will be calibrated," Holst said.

During the checkout of the direct connect nozzles the total pressure and total temperature at the isolator entrance will be determined and the combustion air heater ignition process for each nozzle will be refined.

"The Design of Experiments approach will be used to illustrate the operational envelopes [of the MSCC program]," Smith said.

This approach deals with planning, conducting, analyzing and interpreting controlled tests to evaluate the factors that control the value of a parameter or group of parameters.

AFRL started its Robust Scramjet Program in 2003 to improve scramjet propulsion system capability in the areas of scaling, operability and durability. The program's mission was to guide critical technologies toward a hypersonic propulsion system capable of speeds from Mach 3.5 up to Mach 8.

In 2011, AFRL began working on its MSCC program exploring first generation larger scale scramjet engine characteristics.