SR-72, “Son of the Blackbird,” recently made CNN headlines when Lockheed Martin announced the hypersonic plane. Blazing through the air at Mach 6 or greater, it would be the fastest plane since the X-15, which utilized rockets to break Mach 6. Unlike the X-15, the SR-72 would use a turbine based combined cycle, “TBCC,” (see below), a meshing of turbine engine and scramjet, to break into the hypersonic range. However, the design challenges of making these two technologies work in tandem are immense.
The transition from turbine power to scramjet acceleration is not an easy one. In the configuration shown, the turbine is stacked above the scramjet. It appears that the turbine inlet closes when the scramjet inlet opens. It sounds simple, but it’s not at high speed. Inlet design is vital for both types of propulsion. Even in modern commercial jet engine technology, if the airflow coming though the inlet is heavily distorted, the engine can stall. The TBCC has the double the complexity with the dual airflows to the turbine inlet and scramjet inlet. According to Dave Majumdar, DARPA’s Blackswift program developed a method to transition from turbine to scramjet. Yet, due to the program’s cancellation in 2008, it is doubtful that the method was ever tested.
Another challenge is the gap between Mach 2 and Mach 5 that neither propulsion technology is able to fill. Military turbine engines can barely reach Mach 2, even using afterburners (which is basically converting the end of the engine into a rocket by injecting lots of fuel). USNI cites turbine technology as one of the reasons Blackswift was cancelled. The turbines couldn’t propel the vehicle fast enough for the scramjets to ignite. Fortunately, the article continues, Lockheed and Rocketdyne joined together to combat this problem five years ago. SR-72 may be the result of this partnership.
Even with faster turbine technology, current scramjets don’t ignite till Mach 5. That’s extreme for turbine engines. One solution is reducing the scramjet ignition speed, according to a NASA overview. Then the turbines would not have to be improved quite as drastically. However, it’s another technology that requires money and development. Scramjet technology may require even more money to improve than turbine technology. Testing scramjets often requires rockets and/or B-52’s (as I discussed in my Scramspace article), which are not typically cheap.
Money is often the biggest hurdle for aerospace technology. Blackswift was never completed due to budget cuts. On the bright side, each new hypersonic plane idea solves part of the design challenges. Blackswift developed a transition method. Maybe SR-72 will develop and demonstrate Mach 3+ turbine technology. Each development is a piece of the puzzle; eventually, the aerospace community will have the whole picture. Whether the SR-72 is the final version remains to be decided.