How do you build a car that can drive at 1000 mph?
By: Emily Dark
Tags: Top Gear, Engineering, Cars, Bloodhound Ssc, Physics

Figure 1: AMRC, Sheffield University [1]

In 1898 the first land speed record (LSR) was set by the Jeantaud Duc Profilée which hit the mighty speed of 39 mph, with a whopping 36 horsepower electric engine. Since then we have made huge strides. With the introduction of jet engines, Sunbeam broke the 200 mph barrier in 1927. After many attempts, in 1997 the British broke the sound barrier with Thrust SSC (Supersonic car) reaching 763 mph, but this was just the beginning[2]. The newest project, Bloodhound SSC is not afraid to push the boundaries. The aim of this project is to exceed 1000 mph, almost 4 times faster than the Bugatti Chiron, but it also aims to inspire children and students to learn about and get involved in engineering.

Testing the aerodynamics of this would normally be carried out in a wind tunnel, however this is very expensive and you cannot get some of the aspects of the model correct for supersonic speeds, for example the rotation of the wheels would not be taken into account in a static tunnel. This is where Computational Fluid Dynamics (CFD) comes in. CFD is used to solve the Navier-Stokes equations that describe the aerodynamic flow around the SSC. The Bloodhound team can then make design decisions to change and test the aerodynamics of the vehicle. As well as using CFD to reduce the aerodynamic drag there are many other challenges that have required a lot of innovative ideas, such as, the immense temperature the engine reaches and creating wheels with no tyres[3].

Testing in Africa

Bloodhound will be driven by Wing Commander Andrew (Andy) Green OBE who is used to travelling at incredible speeds in RAF fighter jets. Andy broke the sound barrier back in 1997 with Thrust SSC and so he is keen to break his own record.

The testing has been taking place at the Hakskeen Pan desert in Northern Cape, South Africa, which is a very hard and flat open area, perfect for going supersonic. Even with something as exciting as Bloodhound, the Land Speed Record does still have a few rules. The vehicle must have four wheels and they must always be in contact with the ground. The speed is then measured as an average of two runs across a mile, with there being no longer than one hour between finishing one run and starting the next[4]. This ensures that the vehicles have good braking systems, and are able to have similar pit stop operations to a normal car such as wheel changes, welding and refuelling. Completing two runs also eliminates the advantage or disadvantage of the wind or a slight incline/decline in the track.

In 2017 “low speed” testing took place, where Bloodhound reached about 250 mph and then in late 2019 the first high speed tests were run reaching 628 mph. These tests are vital to ensure that the engineers can gather data on all aspects of the car such as the way the wheels work with the desert floor, the CFDs and the brake systems[3].

Vehicle Design

Wheels

The wheels are one of the most interesting components of the vehicle. Due to the RPM they have to be strong enough to withstand 50,000 times the force of gravity at the rim whilst holding a 7.5 tonne vehicle[5]. But they still need to be lightweight to reduce their inertial force. Instead of tyres, alloy metals are used because the rubber would be ripped apart instantly. The alloys were tested by firing high speed particles at them and looking at the severity of the damage caused. Precision engineering was then used to ensure that they were completely even to avoid disturbances in the airflow[6].

Engines

Bloodhound is of course no regular car and its three engine system is no exception. It is fitted with a jet engine from Rolls Royce which provides around nine tonnes of thrust which sits on top of a zero emissions Nammo rocket[5]. The Nammo has a catalyst which breaks down Hydrogen Peroxide into steam, oxygen and a large amount of energy, which gives an extra 6 tonnes of thrust. And last but not least there was originally going to be a V12, similar to that in a McLaren F1, which would pump the fuel to the Nammo. However, with the advancements in electric motoring, a 500 HP electric motor is going to be used instead which can be recharged within about one minute, perfect for that quick turnaround time[7].

Temperature

Travelling at these high speeds will inevitably create a lot of heat from air friction, the engine and the rocket. Bloodhound, as a showcase for engineering, has used 3D printing to make a few of their parts, one of which is the titanium tip at the front of the vehicle. Temperatures can reach 130°C at the nose, due to shockwaves (see CFD Section), and the tip helps to reduce the heat exposed to the carbon fibre body. The Nammo rocket will create a temperature which is twice of that inside a volcano, 3000°C.

Figure 2: UK Motor and Top Gear [7]

These large temperatures obviously are not ideal and so we need some way of cooling it down. There are large cooling tanks which absorb the heat, mainly from the jet engine. In a test run one of the water tanks burst and so they had to add an expansion tank in case of extra fluid. This led to the addition of a sports water bottle[7] with a pipe stuck into it seen in figure 2.

Braking System

How do you stop something going at 1000 mph? Bloodhound uses a combination of 3 braking systems. First the throttle is turned off and then airbrakes come out either side of the vehicle. They double the drag of the car by increasing the area, but this has to be done very carefully to avoid the airbrakes being asymmetric and causing the vehicle to change course. A parachute is then deployed along with a second if required, and finally when we reach 250 mph the traditional wheel brakes are used[5].

Computational Fluid Dynamics (CFD)

Lead Engineer Ben Evans and the rest of the team at Swansea University have been the main contributors to the CFD work carried out on the Bloodhound project. CFD is used to model the airflow around Bloodhound and determined the shape and design of the car based on those models. The equations used to do this are the Navier-Stoke’s equations which describe the density, velocity and pressure of airflow by five partial differential equations. These need to be solved computationally rather than numerically and if someone does devise a proof of how to solve them there is a prize of $1million[8]. These equations are solved through a process of discretisation. This involves splitting the area around bloodhound into tiny elements called a mesh. Each element has an approximated solution which, when put together with a software package, forms a complete solution and a visual representation, which can tell us everything about the airflow around the car[9]. Data is gained from test runs by 159 little holes all over the vehicle which are pressure sensors. This data can then be compared with the model and simulations can be run in order to optimise the vehicle.

Going supersonic brings with it the problem of shockwaves which CFD also takes into account. Shockwaves occur when something disturbs the airflow and that causes the air to compress, creating a sudden increase in both temperature and pressure. The main areas these shockwaves occur is at the tip, front wheels, jet engine air intake and the rear wheels.

There were two options for the front wheel placement, either staggered, to allow for a narrower vehicle body giving less drag, or parallel to one another. After simulations the parallel formation was chosen for the ease of steering which overcame the small difference in drag[9].

Figure 3: Aerodynamic Design Paper [9]

The jet engine air intake is designed to give maximum thrust when there is maximum drag and it uses the cockpit as an intake ramp. The cockpit decelerates the air to around 500 mph as it must be sub sonic when it reaches the engine[7]. Figure 3 shows the pressure distribution and the velocity streams modelled through CFD and you can see the main areas where the flow has been compressed; over the cockpit to the intake as well as the rear wheels[9]. Bloodhound has a single intake which differs from Thrust, the current record holder, which had a twin intake. This will reduce the drag considerably as Thrust was very wide and created large shockwaves from many different areas of the vehicle.

Figure 4: Bloodhound Education [5]

The front of the car is made of carbon fibre, mainly to ensure the safety of Andy Green because it is lightweight but very strong. This enables it to take the aerodynamic load of approximately 11.8 tonnes which is equivalent to two elephants. The cockpit has been made to fit Andy perfectly, even the steering wheel is 3D printed titanium, fit to his hands. The windscreen of the cockpit is so strong that a 1 kilo piece of debris travelling at 1000 mph would just bounce off it[7].

The back of the car is mainly made of metal with the jet engine sitting on top of the Nammo rocket and the V12 (soon to be an electric motor). The decision to put the jet engine on top may seem strange at first, as it is the heaviest part of the car and you want the centre of gravity to be as low as you can. There are two main problems with putting the engine on the bottom is that the intake is at the top of the car and you would have to bend and run it through the centre to reach the engine. And as the jet engine runs continuously, if the Nammo was at the top, when it fires up, the back of the car would kick up and when its turned off the back would drop down. By having the engine at the top, the centre of gravity only increases its height by about 10 cm and there is much less disturbance when firing up the rocket[7].

Future of the Land Speed Record

Bloodhound was planning to attempt 800 mph in 2021, however this is being rescheduled due to Covid-19 causing uncertainty in the economy and sponsors being careful with their investments. Hopefully we will see a new record being set in 2022 and eventually breaking that 1000 mph barrier10.

If Bloodhound reaches the 1000 mph, where do we go from there? Is there an absolute limit on the speed of a car?

Here P[engine] is the power supplied by the engine, F[D] is the drag force, v is the top speed, C[R] is the coefficient of rolling resistance, m is the mass of the car, p is the density of, in this case, air, C[D] is the drag coefficient and A is the frontal area of the car. When a car is going at its top speed all the power from the engine is being used solely to overcome the rolling resistance and the aerodynamic drag of the vehicle which are the two terms on the right hand side respectively. From research, there is not a theoretical absolute limit for a top speed, but as we find new materials and possible engine types the LSR could keep increasing for a while. When the Jeantaud Duc Profilée set the 39 mph record, they didn’t expect us to be thinking about beating 1000 mph only 120 years later.

Conclusion

Inspiring a generation of engineers and beating the current land speed record are the two aims of Bloodhound and they have definitely achieved one of these so far.

There are many challenges to overcome in creating a car that is capable of going at 1000 mph. Accelerating a 7.5 tonne machine with a jet engine, a V12 and a Nammo rocket. The immense temperatures each section of this car is going to be subjected to and ensuring the strongest, most heat resistant materials are being used. The aerodynamic drag and shockwaves being reduced as much as possible through simulations and modelling by computational fluid dynamics. Creating a safe 3-brake system of airbrakes, parachutes and wheel brakes. And being able to do it all over again in an hour.

Even if Bloodhound does not manage to break the existing LSR it has been a source of inspiration and a learning tool for many young people, and been a brilliant showcase for British engineering.

References

1. AMRC, University of Sheffield, Bloodhound SSC supersonic show case for engineering

2. Redbull, 2020, https://www.redbull.com/gb-en/history-of-land-speed-record-cars

3. Bloodhound LSR, 2020, http://www.bloodhoundlsr.com/

4. International Automobile Federation, 2020, https://www.fia.com/fia-world-land-speed-records

5. Bloodhound Education, 2020, https://bloodhoundeducation.com

6. Innoval Technology, https://www.innovaltec.com/bloodhound-blog/

7. UK Motor and Top Gear, 24/04/20, https://uk.motor1.com/news/414494/bloodhound-lsr-top-gear-walkaround/

8. Clay Mathematics Institute, 06/11/20, https://www.claymath.org/millennium-problems/navier–stokes-equation

9. H. Aref, S. Balachandar, 2018, A first course in computational fluid dynamics

10. Evans, B.J. et al., Computational fluid dynamics applied to the aerodynamic design of a land-based supersonic vehicle, Numerical methods for partial differential equations, 27, 1, pp. 141–159, (2011)

11. BBC, 2020, https://www.bbc.co.uk/news/science-environment-51715582