Aviation Week.com had two articles on the above hypersonic concept vehicle, on June 13th and June 16th. Aerojet’s website didn’t seem to have any news on the TriJet, as of this writing. The following two schematic figures, from Aviation Week, are small and not very clear, but they are all one can get at the moment.
They work on the age old principle that if you cannot get something to go fast enough, add rockets.
If you want to go hypersonic (greater than Mach 5 as a rule of thumb), the modus operandi is to do it in one of two ways. The first is to use rockets to accelerate continually from standstill (Mach 0) to the required speed. However, rockets are costly, have to be fitted out with their own fuel and oxidizers (making long distance rockets very large in size), are not fuel-efficient (low specific impulse), cannot be shut down easily once started and have other control issues arising from staging (moving between stages). (Recently, variable speed, multi-start liquid fuelled rockets made an appearance in the aerospace arena.) The second approach is the air-breathing mode which is to use combined cycle engines (aka hybrid engines), i.e., motors consisting of a combination of two or more engines that transition as the vehicle builds up speed. Turbojets can go from Mach 0 to a top speed of about Mach 2.5. Ramjets typically start beyond Mach 1 and become more efficient between Mach 3 and 5.5. Thus, the turbojet is required to get the vehicle up to ramjet starting speed before the ramjet can takeover. This dual-mode was observed in the SR-71 Blackbird’s Pratt & Whitney J58 engines.
Ramjets have an upper velocity of Mach 6. Ramjets are most commonly observed in missiles, such as the US’ Talos missile which uses a solid rocket motor to push it to ramjet starting speed. Ramjets technology is fairly well established and practically achievable. Scramjets, on the other hand, still have many hurdles to clear. Scramjets only start between Mach 4 to 5 depending on inlet design. Scramjet starting is the biggest hurdle, even if the starting speed is attained because many things can still go wrong, such as improper fuel ignition (see the article entitled “X-51A’s Second Flight Test Fails” ) or inlet unstart caused by boundary layer build up within the engine.
The transition phase from one engine type to another are critical periods of engine operation. This is why the Aerojet’s TriJet design uses a rocket (possibly running on hydrogen peroxide) to boost the ramjet’s thrust, in a combination called rocket-augmented ejector ramjet. An ejector is a device in which a high speed flow pulls a slower flow along with it. Thus, the rocket exhaust flow pulls the ramjet flow with it thereby increasing the thrust significantly.
The aircraft would take off on turbine power, then ignite the ejector ramjet to push through the transonic drag rise and accelerate to the take-over speed for the dual-mode ramjet. But the elegance of the concept is how the operating envelopes of the three engines overlap to provide seamless propulsion.
Read the two articles at the top to get a better understanding of the concept vehicle’s operations and a few more schematic diagrams. Here is another excerpt.
The TriJet combines the two options into one by melding a turbine engine and rocket-augmented ejector ramjet (ERJ) with a dual-mode ramjet (DMRJ) to achieve the final push to hypersonic flight. In a basic sense, the vehicle will take off using turbine engine power, which can be partially augmented by rocket-assisted thrust from the ejector and dual-mode ramjet. At higher speed, the turbine engine is shut down and closed off, with the ejector ramjet taking over. The exhaust from the ERJ is ducted through the nozzle of the DMRJ to form an aerodynamic choke. This enables the DMRJ to produce more thrust and accelerate the vehicle to greater speeds until the ERJ can be closed off, and the dual-mode ramjet converted into a scram jet.
The concept is attracting U.S. Air Force interest as a possible pathway to a high-speed intelligence, surveillance and reconnaissance (ISR) or strike aircraft, says Mel Bulman, Aerojet’s chief engineer of advanced propulsion and mission architecture.
The aircraft can take off using just the turbojets or all three engines (turbojet, rocket and ramjet) for higher payloads. After getting airborne, the aircraft is refueled from a flying tanker. After topping up the fuel tanks, all the engines are throttled in stages to reach supersonic cruise speed.
The advantage that Aerojet’s concept has is that all the engine types they included in the design are already in use and established. Thus, it is mainly a matter of integration. This idea could possibly reach flight tests in a handful of years. However, I am very skeptical if the Air Force will bite since the US Senate has been forced to cut costs and military spending. (See article on US Senate cutting funding for the ONR’s railgun.)