The Role of Turbomachinery in Modern Hypersonic Cycles

In the coming age of hypersonics, a variety of engine types and cycles are being innovated and worked on. Yet turbomachinery remains unique in its ability to use a single airbreathing engine cycle to carry an aircraft from static conditions to high speeds. One of the largest limitations of turbomachinery at hypersonic speeds (Mach 5+) is the stagnation temperature, or the amount of heat in the air as it is brought to a standstill. While material improvements for turbomachinery are made over time which increases the effective range of temperatures steadily (Figure 1), this steady rate means that the ability of these materials to allow use at stagnation temperatures of more than 1600K remains unlikely any time soon.

Figure 1 Material Improvements Over Time
Figure 1 Material Improvements Over Time

This is the limiting point for traditional turbojet cycles, as Mach 5+ speeds result in temperatures far exceeding these limitations, even for the compressor. However, improvements in cryogenic storage of liquid hydrogen has allowed the concept of precooling, using the extremely low liquid temperature of hydrogen to cool the air enough to push this Mach number range, as well as improve compressor efficiency. To drive the turbine, the exhaust gas and combustion chamber can used, heating the hydrogen and reducing the nozzle temperature for given combustion properties. This has the added effect of separating the turbine inlet temperature from the combustion temperature, reducing limitations on combustion temperatures.  This type of cycle can reduce the inlet temperatures underneath material limits.

Figure 2 Hydrogen Precooling Turbojet in AxCYCLE
Figure 2 Hydrogen Precooling Turbojet in AxCYCLE

Using AxCYCLE™ (Figure 2), it is possible to see how this type of cycle can perform across a wide range of flight conditions going from sea level static conditions, to Mach 6+ at 30 km. The design pressure ratios for the compressor, heat exchanger efficiencies, and fuel flow rates, can all be changed to see how they affect thrust. However, analyzing a cycle piece by piece can prevent the user from easily seeing how changes to individual components change the entire performance of the machine. For example, fuel flow rates can change combustion properties, as well as affect how much the compressor air is cooled (because of the precooler), and the temperature into the turbine (because of regenerative cooling).

Figure 3 AxCYCLE CEA Combustion
Figure 3 AxCYCLE™ CEA Combustion

To combat this issue and find the best performance on a system level, wrapper tools such as AxSTREAM ION™ (Figure 3) can be used to communicate inputs and outputs between individual components. Utilizing this type of tool, engineers can expedite and automate these complex tasks, ultimately generating maps with varying inputs. Once this map has been generated, it can be used to study how the fuel flow rate and regenerative cooling effectiveness can affect how well the turbine performs, while also making sure that the nozzle and turbine don’t exceed material temperature limits. Additionally, we can also get data on thrust, specific fuel consumption, and even how high the hydrogen needs to be pressurized to power the compressor by running and analyzing the cycle at each setting used.

Figure 4 AxSTREAM ION Map of Fuel Flow Rate and Regenerating Cooling Efficiency Settings
Figure 4 AxSTREAM ION™ Map of Fuel Flow Rate and Regenerating Cooling Efficiency Settings

Figure 4, a map created in AxSTREAM ION™, shows the chosen fuel flow rate against the regenerative cooling efficiency shows lines of constant required pressure. Here, we can see that the highest possible pressure needed is 12 bar. While the heat exchanger has an obvious effect of improving the turbine performance by heating the incoming air, we can also see fuel flow rate increasing the required pressure up to a point, eventually lowering after 1.2 kg/s. This effect is driven by how as fuel flow increases, the temperature of the fuel decreases for a constant amount of heat, worsening turbine efficiency. However, after a point, the increases in mass flow provide enough power to counteract this effect. Using this knowledge, it is possible to redesign the turbine to minimize the necessary size, to lower weight and drag penalties.

By running this type of analysis for various conditions and settings, it is possible to optimize a complicated turbomachinery cycle for use at Mach 6+. With this type of analysis, it is possible analyze various settings to monitor nozzle temperatures, compressor and turbine inlet temperatures, changing cycle parameters so that at high altitudes and high Mach numbers, these temperatures are kept under the limits of current materials available. This type of performance and cycle monitoring can be further used to create component and cycle designs allowing turbomachinery to be safely used in engines intended for use in hypersonic ranges.

Increase in operational temperature of turbine components. After Schulz et al, Aero. Sci. Techn.7:2003, p73-80.