GUY NORRIS / LOS ANGELES
USAF studying concept that will provide hypersonic performance using conventional fuels and powerplants
Studies have begun in the USA of an engine concept which injects water into the engine inlet, allowing a vehicle to accelerate from a sea level standing start to Mach 6-plus using a single powerplant.
The concept, called SteamJet by inventor Vladimir Balepin, is being proposed for potential applications ranging from hypersonic cruise missiles and propulsion of launcher first stages, to supersonic miniature air-launched interceptors (MALI) and low-cost experimental hypersonic test vehicles such as the X-43 reusable space vehicle demonstrator.
At present, to reach Mach 6 speeds weapon developers have to use a combination of turbojet or a rocket motor combined with a ramjet.
Balepin, a former Russian Central Institute of Aviation Motors rocket scientist who worked in Japan and France before joining Montana-based MSE Technology Applications, says: "Real applications cannot be disclosed, but potential uses can include missiles such as ship or air-to-surface missiles and research vehicles".
The SteamJet principle involves an engine inlet flow conditioning system which consists of a water-injection device and air/water heat exchanger located in front of the compressor of a conventional turbojet.
Balepin says the injection system reduces the air stagnation temperature of the incoming flow, and keeps the compressor at a more efficient operating point.
This, he says, allows Mach 6-plus speeds to be reached using conventional hydrocarbon fuel and current turbomachinery which are normally restricted to speeds of around Mach 3.
Although other modified turbojets have been demonstrated at higher speeds, all have been adapted with exotic materials and pre-coolers mounted upstream of the compressor.
Most, such as the Japanese ATREX engine, have used hydrogen fuel for cooling, rather than conventional hydrocarbon fuel.
The US Air Force Research Laboratory's (AFRL) Propulsion Directorate has performed preliminary studies of the concept using its in-house developed ENGAGE++ analytical software against a reference vehicle design and performance estimates developed under contract by Boeing.
Based on a conventional take-off vehicle weighing 15,000kg (33,000lb), and a transition from turbojet to ramjet operation at Mach 4, the study showed the SteamJet used less fuel than a turbo-ramjet.
The results also illustrated that, even at Mach 6, where the cycle features a low specific impulse, the StreamJet vehicle retained usable fuel for sustained cruise.
Balepin says the AFRL study proves the benefits of the SteamJet's higher thrust-to-weight ratio, compared to a ramjet. The simpler installation makes it a better candidate for future hypersonic research vehicles, he says.
Several simple gas turbines have been identified as potential testbeds, although all would require the addition of afterburners to enable them to transition to supersonic flight. They include engines made by Allison Advanced Development, Teledyne Continental and Williams International.
Balepin says General Electric has also shown interest in using its J85 engine, but adds that this is currently "a bit big for initial demonstrations", and has a higher than ideal initial compression ratio as currently configured. He adds that MSE is "looking for further funds" to help perfect the concept.
Source: Flight International