In May 2009 the Defense Advanced Projects Research Agency (DARPA) announced the kick-off of its Vulcan program by awarding eight-month system requirements development contracts to four contractors:  Alliant TechSystems (ATK), General Electric (GE), Rolls Royce, and United Technologies Corporation (UTC).  The Vulcan program aims to develop a “constant volume combustion” (CVC) engine that will accelerate a vehicle from rest to Mach 4+.  (Some reports say the program aims to achieve Mach 6, which would be equal to a true airspeed of about 3,700 at a flight level of 50,000 feet, or a true airspeed of about 4,600 MPH at sea level.  In other words, Vulcan aircraft will be very fast.)  According to DARPA, the Vulcan engine will integrate an existing production turbine engine (such as a F100-229, F110-129, F119 or F414 engine) with a CVC engine.  DARPA says that the Vulcan engine will “enable full scale hypersonic cruise vehicles for intelligence, surveillance, reconnaissance, strike, or other critical national missions.”  A DARPA briefing calls the Vulcan engine “a game changer.”

This concept has been described as the integrated combination of a turbo jet and a scramjet.  Current engines, including advance turbine engines, operate on the principle of constant pressure combustion (CPC).  According to DARPA, CVC engines “offer potentially very significant performance improvements over conventional cycles, have the ability to operate statically through high Mach numbers and offer significant design flexibility.”  In particular, CVC engines should be able to achieve a 35% improvement in thermal efficiency when compared to traditional CPC engines.  However, an aircraft needs to attain Mach 2+ before the CVC engine can begin to operate.  Thus, the need to combine a traditional turbine engine with the CVC engine in order to provide continuous thrust from take-off through the higher Mach velocities.  DARPA states that a key objective of the program will be ”to integrate the turbine engine into the Vulcan engine system with minimal modification to the turbine engine, operate the turbine engine from rest to its upper Mach limit and cocoon the turbine engine when it is not in use during flight.”  DARPA’s vision is that the turbine and CVC engines will share a common inlet and nozzle.

There are, of course, technical challenges that will need to be overcome.  As DARPA itself notes, the challenges include “efficient detonation initiation, low total pressure loss detonation initiation devices, low total pressure loss air valves, thermal management systems, efficient nozzles, and control systems.” Among the other challenges is deciding on the appropriate configuration for the CVC engine, which could consist of pulsed detonation engines (PDEs), Continuous Detonation Engines (CDEs), or another “unsteady CVC” method.

Flight International magazine (FlightGlobal.com) reports that the Vulcan configuration decision will be made in December 2009.  According to a FlightGlobal.com report,

The Vulcan's configuration will be selected from a number of different options, some of which were presented at the American Institute of Aeronautics and Astronautics' 45th Joint Propulsion Conference held in Denver, Colorado. What is decided is that the engine will use a turbojet and a constant volume combustion engine (CVC) in combination. Examples of CVC are Continuous Detonation Engines (CDE) and Pulse Detonation Engines (PDE). The basic operation for a CDE or PDE is a propellant injected into a tube, open at one end and the fuel is ignited. The detonation wave travelling down the tube generates the thrust.

After selection of the configuration, the program will begin an eighteen-month Phase 2 to include preliminary design, trade studies, and a demonstration of the CVC components.  Phase 3 is forecasted to last another eighteen months and will include detailed designs, simulations, and tests of the CVC engine components.  Phase 4 (another 18-month phase) will include fabrication, assembly and test/demonstration of the integrated Vulcan engine.  If all goes according to plan, by 2014 the Vulcan engine should be proven and demonstrated technology.

At that point, we should know whether the Vulcan engine is truly the “game changer” that DARPA believes it can be.