A concept explored by the power generation industry 50 years ago is being dusted off by NASA as a possible way of enabling jet engines to operate to higher speeds, to power hypersonic vehicles and reusable spaceplanes.
The magnetohydrodynamic (MHD) energy bypass engine would electromagnetically extract energy from air entering the inlet, slowing the flow and allowing the turbine engine to operate to a higher Mach number.
The electricity generated could be used to power aircraft systems or to electromagnetically accelerate the engine's exhaust flow, increasing thrust. One concept developed in Russia, called Ajaxa, was outlined in a paper in 2001 by its authors Claudio Bruno at the University of Rome and Paul Czysz at St Louise University, Missouri.
NASA has begun studying MHD energy bypass engine concepts under its revitalised hypersonics research programme, aimed at developing technology for a two-stage space access vehicle. The MHD energy bypass cycle could make it easier to design a combined-cycle propulsion system by allowing a turbine to accelerate the vehicle to hypersonic speed where a scramjet would take over.
"The idea is to extend the operating range of turbomachinery to higher Mach numbers," says Isaiah Blankson, a senior scientist at NASA Glenn Research Center. "We would put a device ahead of the engine to ionise the flow, extract the energy then put it back in the combustor."
By slowing the flow entering the inlet, the operating range of existing turbine engines could be extended to M7, from today's maximum of around M3. Extracting 30-40% of the total energy of the flow would reduce its speed by a half to three-quarters, he says, allowing the turbine engine to operate at M2.8.
The MHD energy bypass cycle was investigated, but not implemented, by the power generation industry in the USA and elsewhere in the 1950s and 1960s as a way of increasing electricity output at peak times. At that time, ionisation was achieved by seeding the flow with cesium. Later electron beams were proposed.
NASA's idea is to exploit recent advances in high-voltage pulse power systems to increase ionization efficiency. "We need a very efficient way of ionizing the flow field. If we can increase ionisation efficiency by 40% we can be very competitive with classic over-and-under turbine-based combined-cycle engines."
Over the next two years, NASA Glenn plans computational and experimental work aimed at sizing an MHD energy bypass engine capable of M7 to see if it is a feasible way of powering the air-breathing first stage of a reusable spaceplane.