What it takes to go 1000mph

The star components of the Bloodhound SSC car are obviously the engines. Not content with the one jet engine that Richard Noble used on Thrust2, or even the two jet engines used on Thrust SSC, Bloodhound has 3 engines.



Engines
The first engine which will be used by Bloodhound on its runs is a jet engine from a Eurofighter Typhoon, the primary fighter jet of the Royal Air Force. The team have been lent three of these jet engines by the Ministry of Defence, and they are the only engines of this type not in the possession of a national government. As a result the exact specifications of the engine are kept under tight wraps; however we do know that it produces around 13,500lb of thrust, or around 20,000lb with the afterburner lit. In other words it’s an extremely powerful engine, however in the Bloodhound car its role is to power the car from standstill to 300mph. Even powered by a jet engine it won’t be a fast acceleration, with 0-100mph expecting to take around 15 seconds. A fast Ferrari will do the same acceleration in half the time, while the Bugatti Veyron (one of the fastest road cars in the world) takes a little over 5 seconds.


It is what happens after 300mph that sets Bloodhound SSC apart from anything before it. At this point the rocket engine is engaged. Releasing 25,000lb of thrust and running for a 20 second burn, on top of the 20,000lb provided by the jet engine, the car will literally rocket to 1000mph, reaching the mark just 40 seconds after beginning its run. Over this 40 second period the car will have travelled around 6 miles.

Rear view of Bloodhound showing the jet engine and rocket engine outlets. Images by Curventa and Siemens.

The rocket providing this thrust is created by Daniel Jubb who, at just 26 years of age, is one of Europe’s top rocket engineers and sports one of Europe’s most resplendent moustaches. With his company Falcon Projects, Jubb has developed the largest hybrid rocket ever designed in Europe. The engine, measuring 45cm in diameter, contains 1130kg of a synthetic rubber called HTPB as a solid fuel. When fired, liquid hydrogen peroxide is injected into the rocket chamber where it reacts with silver coated nickel discs, producing water, oxygen and immense amounts of heat (around 600°C). This heat causes the solid fuel to spontaneously combust, which provides the actual thrust. This type of rocket is able to be easily shut-down by stopping the flow of hydrogen peroxide into the reaction chamber, allowing an important safety feature.

Daniel Jubb and the 45cm rocket designed for Bloodhound. Image by Bloodhound http://www.bloodhound.ssc

And the third engine? That is a Formula 1 racing engine provided by Cosworth, and provides no thrust for the car at all. Instead, Bloodhound SSC uses this Formula 1 engine as a pump for the liquid fuel powering the rocket engine. During its 20 second burn time the rocket will require 963 litres of hydrogen peroxide fuel, or 48 litres per second. Considering a conventional fire hose will deliver around 6 litres of water per second, the need for a super-powered fuel pump is obvious. In addition to running the fuel pump, the Cosworth F1 engine provides power for the electrical and hydraulic systems.

The Cosworth CA2010 F1 race engine alongside the full size BLOODHOUND SSC Show Car. Photograph by Flow Images

Together, the rocket engine and jet engine will give Bloodhound SSC a total power output equivalent of 135,000bhp (brake horsepower). As a point of comparison, your daily car probably has between 100-250bhp, and a fast road car has around 400-500bhp. A Formula 1 engine, such as the Cosworth running the fuel pump and auxiliary systems in Bloodhound SSC has around 750-800bhp.

Artists impression of Bloodhound SSC with both engines engaged. Image by Curventa and Siemens

But power alone won’t ensure success, and arguably the most important design considerations are given to the body and aerodynamics.


Aerodynamics

The design of Bloodhound SSC has been carried out using computational fluid dynamics, or CFD. Computational fluid dynamics, as the name suggests is a computer based simulation of how fluids (which includes air) flows around objects. By computing how the air flows around the car’s shape engineers can calculate how much drag, lift or downforce the shape is producing. From this, they can also calculate its stability, and beginning to develop the shape until it has the characteristics they seek.


CFD is used by many industries, such as aerospace in the designing of aircraft, and more related, Formula 1 teams use CFD to design their cars. In fact, one team (Marussia Virgin Racing) is using only CFD to design their car, while the other teams use CFD and then re-examine the results using physical wind tunnels. The CFD demands of designing a car such as Bloodhound are so great that the computers responsible for running the programs have more computing power than the English Meteorological Office.

A CFD simulation of Bloodhound SSC. Image from Evo Magazine, copyright of Swansea University, Curventa and Siemens

The aerodynamics of a car such as Bloodhound are absolutely vital. Any moving object transmits sound waves, which travel away from the body. When these waves meet air molecules, it effectively acts as a warning to the air that an object is moving and it needs to start moving out of the way. When the object is moving at the same speed, or faster than these sound waves (i.e. supersonic), the air doesn’t get a warning an object is moving towards it, and the sound waves that normally transmit this warning begin to literally bunch up in front of the moving object. These bunched up waves create shockwaves, which people call the ‘sonic boom’. As these shockwaves form, the air stops moving smoothly over the surface of the car but are constantly changing with every shockwave.


Further complicating supersonic aerodynamics is not the flow of air below the speed of sound and above the speed of sound, but rather the flow of air over objects at the speed of sound. As an object goes through the speed of sound, some air will be flowing just above the speed as sound and some will be flowing just below the speed of sound. This creates conditions in which the airflow is highly unstable, with the number and position of shockwaves constantly changing, meaning everything about the aerodynamics of the car is constantly changing as it passes through this transonic point. A large part of the design of Bloodhound SSC has been centred on how these shockwaves travel over the car, and also interact with the ground surface. Sonic shockwaves have the potential to seriously affect the car, so understanding them and how they form around the car is crucial.


The body of the car needs to be designed to be completely neutral in downforce and lift. A shape which gives downforce will push the wheels into the ground, causing more drag and slowing the car. Conversely, a shape which causes lift will have the obvious effect of the car being extremely unstable and potentially taking off. In fact, one early shape was calculated to result in 12 tons of lift. Considering the car weighs 6.5 tons, this would have a spectacularly undesirable outcome.

Bloodhound SSC after aerodynamic development. Image by Curventa and Siemens

Since the first design sketches were produced in 2008, Bloodhound has been through more than 13 different configurations. One of the most serious problems with some of the initial designs was the layout of the engines, which had the rocket engine placed above the jet engine. When the rocket was triggered, this configuration forced the nose violently downwards. The simple solution was to swap the engines, with the jet engine now above the rocket. However this then meant that the rocket would in fact dig a trench in the ground behind the car, necessitating a slight nose-down angle of the rocket to raise the plume.


Traditionally, land speed record cars have had a large fin to provide directional stability – i.e. to help keep the car running straight. One interesting feature of the Bloodhound SSC car is its surprisingly small vertical fin. The problem is that a fin too large will be overly affected by crosswinds, so a balance needs to be struck. The Bloodhound engineers have developed their car in such a way that the car is stable enough to be able to downsize the fin and reduce the dangerous effects of crosswinds.

Bloodhound SSC from the side, highlighting the small size of the vertical stabiliser. Image by Curventa and Siemens

The engineers have now developed the design to a point where they believe all the characteristics of the car will be ideal. Chief engineer Mark Chapman explains, “What we’ve got is zero lift at Mach 1.3 (990 mph), although there might be some instability on the way to that speed.” The nature of the car and the speeds at which they will be travelling at means that the car will be unstable at some speed, and the challenge was to make it unstable at low speeds but steady at high speeds, when stability is critical. Stability when the car reaches the speed of sound is one of the most vital characteristics of a car such as this, as the car is encountering air traveling at the speed of sound, as well as above and below the speed of sound. As the car is encountering air at different speeds it places different forces on the car, and it is at this transition phase when the car needs to be at its most stable to overcome these buffeting forces.


Next article will continue looking at the technical requirements of a land speed record car and the design features of Bloodhound SSC.

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