by Ben Evans and Mark Chapman
A massively important aspect of the BLOODHOUND SSC development has been the design evolution of the intake duct for the EUROJET EJ200 jet engine. The size and centrality of the intake duct clearly has huge implications for the packaging of the rest of the vehicle. The roll of the intake is to deliver an airflow from the freestream to the engine such that it arrives at the compressor face (the compressor being the first stage of the engine) under conditions of sufficient ‘quality’ that the engine will perform satisfactorily and at maximum efficiency.
The ‘quality’ of airflow delivered to a jet engine via an intake duct can be measured in terms of a simple set of variables. These include total pressure distortion, which is a measure of the magnitude of the variations in the total pressure (or total energy) across a plane slightly upstream of the engine compressor face, swirl, which is a measure of the mean deviation from a purely axial flow upstream from the compressor face, and flow stability, which is the degree to which the flow in the duct will fluctuate. All of these quantities vary with mass flow rate through the duct, or strictly speaking, a mass flow function (controlled by the throttle setting applied by Andy in the cockpit) which varies with mass flow rate, pressure and temperature. Clearly, the methodology for analysing how all of these variables vary with flow function and freestream Mach number is a complex one and in a short time scale can only be tackled using the methods of computational fluid dynamics (CFD).
The initial vehicle concept developed by Ron Ayers and Glynne Bowsher was configured for a twin intake ‘bifurcated’ duct as shown in Figure 1 (left).
It quickly became apparent that consistently achieving a stable, smooth flow through this bifurcated arrangement was not straight forward. A typical highly turbulent flow pattern through the bifurcated duct is shown in Figure 2 (left).
Soon in the car design development process, the transition to a single intake duct configuration was made with intake position above the cockpit canopy as shown in Figure 3 (left).
This had a massive improvement in the quality of flow arriving at the engine compressor face, shown in Figure 4 (left).
The external duct flow around the vehicle and the internal duct flow are closely coupled. In fact, the geometry of the nose and cockpit canopy are of fundamental importance in terms of efficiently delivering the air supply efficiently to the EJ200. The final shape of the cockpit canopy and intake have been specifically designed such that, at supersonic speeds, a two-shock system is established ahead of the duct. This decelerates the flow to the subsonic condition necessary for the intake duct, whilst minimising losses in total energy and hence maximising engine performance. An example of this shock system is illustrated in Figure 5 (above).
With the early work highlighting the difficulty in getting the correct flow to the engine, the packaging of the engine and duct has been fundamental to the shape of the car.
The simplest route to steadying the flow is to make it turn more gradually, this can be done in three ways;
1. Pushing the engine further back in the car, which in turn will increase the length of the duct, and therefore make the bend shallower - this can be done to a point. However, moving a ton of engine rearwards is bad for the weight distribution, and needs to be matched by an increase in wheelbase.
2. Raising the engine in the car. This straightens the duct, but at the cost of vehicle stability - the ton of engine now sits higher and makes the car more top heavy.
3. Lowering Andy. The original seating position was based on Thrust SSC, by sitting Andy lower in the car the duct height can be reduced, having the same effect as raising the engine, but without the impact on stability.
To test its effectiveness, the "Lowering Andy" option was taken to its most extreme and showed promise with greatly improved flow to the engine, but at the expense of comfort and visibility. Andy was so low that the front body needed a deep scallop for him to see between the front wheels, shown in Figure 6 (left).
To get Andy back to a more upright seating position the duct internals were reshaped following a parametric study looking at a combination of intake aspect ratios, offsets and controlling the expansion of the flow. Put simply, the trick is to expand it quickly (to slow down the velocity), and turn it late with the slower flow.
All this work was based on a rectangular duct, as this gives the cleanest intake flow, and simplify the analysis. Having proven an acceptable flow was possible, the next stage has been to progress with a duct whose shape follows the intake canopy. The more triangular shape works to our advantage and actually helps the flow around the bend of the intake. From an external aerodynamics perspective it's a much neater looking package (Figure 7, left).
The current ongoing work is in finalising a size for the intake to match an engine design point. The engine can only gobble up the air at a certain rate, so if the intake is too large, then at higher speeds excess air will spill around the outside of the intake causing drag. However, match the intake size for high speed operation, then at low speeds (where there isn't significant ram effect) the engine will be starved of air, and won't be able to operate at full power. One avenue that is being pursued is to design an intake sized for an intermediate speed. We'd accept the spill drag at higher Mach numbers, but have additional auxiliary intake ducts that open at low speed to allow full engine power to be applied even with the car stationary. This can be taken a step further with the auxiliary intakes becoming exhausts and spilling excess intake pressure at the higher speeds. This may prove a better way of managing the spill drag.
One thing I'll touch on very briefly is that the duct shape not only influences the engine performance and external aerodynamics, but also has a significant effect upon the packaging within the car. The area under the duct is the only space available to house the APU piston engine and the HTP and jet fuel tanks, so some compromise has had to be made on duct geometry to maintain a realistic space. Even so, as the duct has dropped the flanks of the car have had to swell slightly to maintain the volumes required.