Tuesday 9 March 14:30 Blue Phoenix 500- an Engineer’s dilemma by Ken Wheeler RCEA.
A sequel to the talk on Bluebird K7 given a few years ago this presentation dealt with the various factors effecting the design of a water speed contender with a design point speed of 500 mph; that is 223.6 m/s. (Mach 0.65) The current record is held by Spirit of Australia at around 317 mph and a quantum leap is thought necessary to justify the expenditure of around £2 million which would be needed for such a venture with the considerable expenditure on CFD (computational fluid dynamics) which is par for the course for such hazardous adventures these days. At 500mph there is little point in spending more to achieve an even higher speed due to the onset of transonic effects as a supersonic shock wave ahead of a craft travelling at Mach 1.0 would dig a trench in the water with attendant consequences. Initially all depends on the availability of a suitable engine and several alternative radical formats were examined using such engines as the Viper and the Adour
In the end a prescriptive design is based on K7 but with four shoes, a sleeker aerodynamic hull, a more forward engine position with pitch stability enforced by the aft butterfly tail assembly. In this manner the centre of mass (CM) is located forward of the pitching centre of pressure (CP) giving weather vane static stability with the tailplane also providing powerful damping forces aiding dynamic stability. This is completely contrary to previous craft such as K7 when aerodynamic aids were disallowed. Here, pitch stability was attained by countering the aerodynamic pitching moment (taken for convenience about the transom) by the weight moment about the same point. With this arrangement with the CP also forward of the CM such a craft is only statically stable in pitch within a narrow range determined by the craft’s angle of attack to the air freestream.
The engine selected was the Orpheus as on K7 with the available thrust augmented by fitting a limited life afterburner. The augmentation ratio required is 1.5. The Orpheus was picked because it is a basic turbojet with good thrust /weight/size ratio it will tolerate abuse and unlike modern bypass engines requires no substantial logistic support in the field. Another factor to be taken into account when calculating the aerodynamic drag (values normally ignored below a speed of Mach 0.3) is the air density compression, which increases to 23% above ambient at Mach 0.65.
Maximum velocity is attained when the total drag intersects the thrust curve at a specific velocity (in this case the design point speed of 223.6m/s or 500 mph) and if the craft had no rudder fitted it just makes it. The immersed rudder is a principle source of drag and can absorb up to 2000hp at 500 mph. A possible solution is to use a retractable rudder at low speeds and fit vectored thrust to the afterburner exhaust
With a speed through the measured kilometre of 223.6 m/s consideration has to be given to the run out distance, which could be in excess of 10 miles after shutting down the engine and could result in beaching the craft. Additionally, the driver would like some sense of control over the craft’s deceleration. To this end a retractable waterbrake is proposed of wedge cross-section immersed across and below the transom. This theoretically will reduce the enormous turbulence caused by the circular rod design used on K7 as a waterbrake.
Ken Wheeler Member