The mission is easily stated in a few, highly comprehensible and simple words: To drive a car at 1000 mph. That’s it. Even a child can understand this very familiar concept of driving a car. The challenge, however, is no child’s play. What exactly is this about? Breaking a world record. Not a big ask. Yet, this is about pushing our boundaries in engineering, technology, science and mathematics. It is the equivalent, if you will, of sending a rocket to the Moon but instead of going up, you send the vehicle horizontally, bound to the ground, thrust forwards like an arrow piercing its way through air. This is about pointing the metaphorical arrow of adventure towards the metaphorical target of awesomeness. What is all this about, you ask? There is this daunting project called Bloodhound SSC and it is the challenge a group of engineers have taken up to see if they can drive a car at 1000 mph. This is about 1.3 times the speed of sound! For a car, that is unheard of, so far. SSC, thus, stands for super sonic car. A car moving faster than sound. Sounds simple yet it is full of all sorts of challenges. What can be so demanding about travelling at 1000 mph? To find out, we need to dig into the physics of motion and materials.
Who better to open the chapter on motion than our old friend Isaac? 325 years ago, Newton gave us the laws of motion which he stated so eloquently, albeit in Latin. In plain English, they translate to:
1. Unless you push it, it won’t move and it will keep on moving unless to make it stop.
2. The more you want it to change the way it’s moving, the harder you have to try.
3. For every push there is a pull.
Granted, these are not the exact laws of motion. But, they give you a taster of what they are about: moving objects and the force related to their motion. In physics, this is called mechanics. Over-simplification is bad. So I’ll give a better description of what these laws are about. Let’s start with the most obvious one.
An inanimate object will not move unless some external agent causes it to do so. You won’t expect a ball to start rolling on its own accord. You won’t expect a coin to toss itself up in the air all by itself. You won’t expect an apple to fall off a tree just because it was in a mood to do so. These objects won’t move unless there is something making them move. An extension to this reasoning is as follows: if some object is already in motion, you won’t expect it to stop moving all by itself. Some thing must have caused it to stop. That thing, that external agent, is what we call force. Motion is related to force. This is the basis of mechanics. Force is at the root of motion. Through observations and experiments, we can deduce what I’ve described above: you don’t expect some object to move all by itself; there must be a force causing it to do so. If you want to move a car you need something to provide the necessary driving force and I’m not talking about the driver. To make the car move at a speed of 1000 mph, you can bet that you will need a considerable amount of force to do so. This is where the second law of motion comes into play.
What the second law tells us is, if you want to change how something is moving, that is, if you want to accelerate it or make it slow down or change its direction of travel, then you have to use force to do so. The greater the change you want to make to that motion, the more force you will need to do so. We tend to equate Newton’s second law of motion with the following well-known statement: F = ma. For it is true that the greater the acceleration a required, the more force F needs to be exerted on the object of mass m. But this is not the whole story. For more details on this, please see my post on motion called Skydiver. The car is to be accelerated from stationary to 1000 mph in about 42 seconds! This is a huge change in the state of motion of the car. From rest to 1.3 times the speed of sound in such a short period of time requires a lot of force. Assuming the mass of the car doesn’t change too much during that sudden acceleration, the force required would be the product of the mass of the car and its acceleration. In reality, however, the mass of the car will change because it is burning fuel as it accelerates. The mass will decrease as it uses up the fuel needed for thrusting the car at such high-speed. Thus, the F = ma equation will not be accurate in determining how much force will be needed to push the car from rest to a supersonic speed. Nevertheless, the magnitude of the force required for such a task is monumental!
One hurdle that the Bloodhound SSC will have to overcome is air resistance. Air resistance is a resistive force, just like its name suggests. There are different kinds of resistive forces. Some are helpful, others aren’t. Resisting motion is not always a bad thing. For example, when you are walking, your footwear is pushing against the floor as much as the floor is pushing against your footwear, in the opposite direction. This resistive force which the floor is applying on your footwear is what is propelling you forward. Had the floor been less resistive; in other words, had the floor been more slippery, the more difficult would it be for you to walk. You would most likely skid and slip. Flat shoes on an ice rink do not provide enough grip, that is to say, the contact between the flat shoes and the ice do not result in resistive forces and therefore walking on ice is like climbing a slippery slope: it’s hard. You’d slip and fall. What you need is something that provides some grip, some resistance and therefore allows you to push your feet back without slipping and therefore move forward. In such cases, like walking and climbing, resistive forces are in fact helping the motion. This is in fact an example of the application of the third law of motion. The force acting on the footwear is equal in magnitude but opposite in direction to the one acting on the ground. This pair of forces is of the same nature and act along the same line but on different bodies. The tyres of the car should have enough grip so that the friction between the tyres and the ground is significant enough to propel the car forward. Friction, an example of resistive force, is helping the car achieve its goal.
An example of how a resistive force impedes motion is air resistance. You can experience that even at slow speeds. If you drop a sheet of paper, for example, you will notice how it takes its time to reach the ground. It doesn’t plummet like a plum dropped from the same height. The reason the sheet of paper gradually makes its way downwards, twirling and drifting as it does so, is because of its large surface area over which air underneath it is pushing against it. If it had a smaller surface area, there would be less air pushing against it and thus less resistance to its motion. That way, the sheet will fall quicker to the ground. Try this little experiment: instead of letting go of the sheet of paper with its largest surface area parallel to the ground, let go of it with that surface area parallel to the wall. Observe how it drops quicker as it slices through the air. In this case, there is hardly any resistance to its motion. The greater the surface area, therefore, the more the object will experience air resistance. If you want something to slice through air as quickly as possible without experiencing too much air resistance, then you need to make it such that it has the least surface area pushing against air as it moves through it. No wonder, then, that the Bloodhound SSC is designed almost like an arrow. Its tip is as pointed as possible so that it encounters the least air resistance as possible. There are other factors, other than surface area, that determine the amount of air resistance encountered by a moving object. The texture is one of them. The smoother the object is, the less air resistance will be. Close inspection of the bodywork of the Bloodhound SSC shows how smooth it is. You will also notice that it has fins sticking out of its sides. These are other features that allow air to flow smoothly over and around it as it dashes towards its world record. The more efficient it is at moving through air the more aerodynamic it is. Sports cars and airplanes are designed to be as aerodynamic as possible. The aquatic equivalent of this is hydrodynamic. Submarines and torpedoes and such are designed to be as hydrodynamic as possible. Water, like air, is a fluid. And moving through fluid can be as tricky as a bee flying through treacle. Precisely: it can be a sticky business. For it might appear that air is most innocuous but it can be quite the contrary when it comes to moving through it at high speeds. Hence why it is such a challenge to come up with a design which is at the same time practical and aerodynamic and easy to build.
Now, the challenges are not just about being able to get the car to accelerate fast enough to 1000 mph but also about being able to sustain the acceleration and the high-speed. As the car moves faster and faster, the more air resistance it will encounter. As a result, the pressure air will exert against the car will increase. If the body of the car is not strong enough to resist that pressure, it could collapse. Or at least, it will buckle. So, the car has to be made with a material strong enough to hold its shape under such huge pressure. It is not just about using a strong and heavy material as the more mass you add to the car, the more force you will need to accelerate it. So here engineers have to be clever in choosing the right material. It has to be light yet strong. In such cases, carbon fibre and other such composites do the job very well. They might be reinforced with other light materials but the overall goal is to ensure that the material with which the body of the car will be built is strong enough to withstand high air pressures and to be lightweight. The catch, though, is as follows. It cannot be too light either for it might simply fly off when air resistance gets too high. It has to have some weight to keep it grounded. But here is where the fins are included into the picture. They are not there just to make the car look sleek. Their purpose is to direct the incoming air so that the flow of air over and around the car causes an overall downwards force pushing the car to the ground. This is what prevents the car from flying off like a leaf being blown away by a gust of wind. This is similar to the spoilers and fins on sports cars and Formula 1 cars. Because they are made with light materials, they have to be kept grounded. Lowering the centre of gravity is not enough to keep it grounded (see how this helps in keeping an object stable: Equilibrium).
Besides the designs and materials, the actual engineering of the heart of the SSC is crucial. How efficient it can be in burning its fuel to produce maximum thrust is a definite challenge for the engineers and technicians. What type of fuel to be used is another important question to be addressed? Ordinary petrol will not do: it does not have enough ‘juice’ to give the SSC the required push. As such, the SSC will use rocket fuel. Or, in other words, it will ‘burn’ hydrogen in oxygen to generate the necessary thrust. It is not dissimilar from a rocket except for the fact that this land-locked rocket will travel horizontally as opposed to vertically.
As you can imagine, there are all sorts of challenges to be met and overcome, right from the conception of the project, its implementation and, eventually, its realisation. Hopefully by the end of all this endeavour, we would have broken more than the sound barrier; we would have surpassed the barriers or limits that currently define the extent of our knowledge in such fields as mathematics, science, engineering, technology and design. What is certain is that, unless we have such dedicated people who are motivated to push our boundaries and challenge our capacity to achieve more, progress will remain an abstract concept. Scientists are the inspirational heroes of our society. We need to support them as much as they support and sustain the progress of society. Science is humanity’s foundation upon which a future is built.