I keep thinking on high bypass rocket engine powered VTOL design and making changes on it. My latest proposal is to use an axial rocket engine inside the engine duct. This approach would reduce the air drag and give more time for the bypass air to heat up.
In order to reduce the cross section of the turbopumps, axial pumps would be used. Axial pumps would be cascaded to increase the pressure output. This comes handy while the engine would produce the maximum thrust at takeoff and the cruising thrust would be much lower. When the plane is cruising, some of the cascaded pumps would be bypassed to lower pressure. Lower fuel pressure results lower output pressure which gives more time for the bypass air to burn the unburned fuel and heat up.
The combustion chamber and the nozzle would also be cylindrical in shape to align with aerodynamics of the rocket engine. The combustion chamber and the nozzle would have aerodynamic heat exchanger fins to transfer heat to the bypass air. There would be no regenerative cooling for the propellant.
Very long tubular design of the ducted engine requires it to be placed much forward from the wing. This allows clearance on the ground when the engine and the attached wing are rotated during VTOL. In order to reduce weight on the wings, most of the flight control parts will be removed. The wing rotates as a hole with the engine, therefore there is no need for most of these controls.
The plane would use liquid methane as fuel and LOX. The pressure of these tanks would be used to rotate the wings and the engine. This reduces the need for heavy motors and their controllers.
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