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Fastest car on earth!

So you want a build a car that will go 1000mph. Have you any idea how fast a 1000mph is? Do you have any idea just how nuts that is? That’s a mile every 3.6 seconds!

For every breath you take, you will have covered a mile! Every second you must push a column of air a mile long out of your path. That’s 4.1 tonnes of air. Not only does the air weigh a lot, but it is also very reluctant to move. The faster you try to push it out of the way, the harder it resists. The force opposing the movement of the car through the air is called drag.

The faster you go, the more drag you experience. What’s more if you double your speed the drag doesn’t just double, it goes up by a factor of 4. So if your 100 Horse Power car will do 100mph you need at least 400 Horse power to do 200mph. To do 400mph you would need at least 1,600 Horse power. At 1000mph you need at least 10,000 Horse Power just to overcome the drag. In fact, to get up to those sorts of speeds in a reasonable distance you need a lot more power than even that!

Such power outputs become impossible to deliver by driving the wheels of any car. Imagine trying to put 10,000Hp through two wheels! It would be 5000Hp per wheel. Any wheel only has a few tens of square cm’s in contact with the ground. All that would happen would be that you would get wheel spin, there is just not enough friction with the ground to push the car along against the drag. The wheel driven land speed record is 470 mph and this really is just about as fast as anyone will ever go in this kind of car, to go faster than that you need to use the air, not fight it, you need a jet!

Jets provide thrust by sucking in air at the front and pushing it out the back at a much higher speed. People sometimes think that jets work by pushing this air against the still air around them, in fact air is much too squashy and awkward to let you do that. Jets work by using Newton’s Third Law of motion. As the air is pushed out of the back at huge speeds the engine recoils in the opposite direction carrying the car with it. Action and reaction are equal and opposite. The more air you push out the back, the more force pushes the engine in the opposite direction.

A typical jet squirts well over a tonne of air per second out of the back, which gives a lot of push! People sometimes think that jets work by pushing this air against the still air around them, in fact air is much too squashy and awkward to let you do that. Jets work by using Newton’s Third Law of motion. As the air is pushed out of the back at huge speeds the engine recoils in the opposite direction carrying the car with it. Action and reaction are equal and opposite. The more air you push out the back, the more force pushes the engine in the opposite direction.

A typical jet squirts well over a tonne of air per second out of the back, which gives a lot of push! It’s still not enough to get up to a 1000mph though, so strap on a rocket motor as well, for extra thrust!

Rocket car

A rocket motor works in a similar way to a jet squirting hot gas out the back, but it doesn’t need to suck in air from the front because it carries its own oxygen in the fuel to make it burn.

A rocket motor is a just a controlled explosion. The difference between a stick of dynamite and a stick of wood is just the speed of burning. If you had a stick of dynamite 5 miles long and lit one end it would burn to other end in just under a second! When it does this it turns from a small lump of solid into a wall of gas moving at over five miles per second. The trick with a rocket motor is to control the rate of burning and keep pumping in high explosive at just the right rate to keep the wall of gas moving for a long period of time in the right direction. Easy!(Not!). Even NASA gets it wrong sometimes and ends up with an uncontrolled explosion. So let’s strap a rocket motor under our jet.

We can now provide enough push to get up to 1000mph in a reasonable distance of about 15 miles! Sound travels at about 760mph at normal sorts of air pressures and temperatures. So to achieve 1000mph you have to drive right through the sound barrier.

Why is the speed of sound a barrier?

Ah, this all has to do with waves!

Sound waves are made from regions of high and low pressure as the air molecules vibrate. If the source of the sound is moving the waves can get all squashed up in the direction of movement. If the car is travelling at the speed of sound, the high pressure parts all become superimposed one on top of the other, and a big high pressure wave is produced! This is what you hear as a sonic boom. The more important effect is that it changes the way the air flows around a car and can reduce stability. This effect needs to be taken into account and minimised!

Breaking the sound barrier

One of the biggest problems with ultra-high speed cars is control. In an ordinary car the friction with the ground is quite high compared to the air resistance. Turn the wheels and the car will follow. At these kinds of speed, air resistance is massively greater than the friction with the ground. Turn the wheels and you go straight on. It’s very much like driving an ordinary car on ice.

So you need to have two control systems, one for low speeds up to about 400mph(!!!) like a conventional car and one for high speeds like an aircraft. What’s more they must change over seamlessly as the driver increases speed. Not an easy problem to solve. Wheel steering at low speeds, steering by rudder, like an air craft at high speeds.

The next problem to solve is keeping the car on the floor. This vehicle needs to stay being a car and not turn into an aircraft at unexpected moments. Any object moving through the air at these kinds of speeds is subject to huge forces. If the front end was to lift even slightly, the force of the air resistance could flip it over in an instant.

This is to be avoided at all costs. A system is needed to produce a down force that will keep the car stuck to the ground. But, it also needs to very sensitive. If you produce too much down force the car can dig into the ground creating problems that are nearly as bad, and at best, increase the rolling resistance so that it slows you down.

The surface you are going to run across is also crucially important. It would be great to have a nice smooth tarmac runway to use. Unfortunately, the longest runway of this type is only 2.8 miles long. This is the runway that the space shuttle uses to land at Kennedy space centre. At 1000mph the car would use it up in less than a second. So you need a really long flat bit of natural terrain.

Pendine Sands

In the early days beaches were used, notably Pendine sands in Wales or Daytona Beach in America. As speeds reached 300mph even these colossally long stretches of sand were not long enough and record breaking moved to the Bonneville Salt Flats in Utah, USA. Here a 20 mile plus stretch of dead flat salt can be found. The bed of an ancient dried up lake. For over 35 years all the great land speed records were set here. It does however have a few problems. The surface is very hard. To get to a thousand miles per hour the car is going to need a lot of down force to keep it stable, it will need very thin, metal tyres to withstand the stress. The problems with vibration would be massive.

So, find a slightly softer flat place! This is the Hakskeen pan in South Africa. Here the surface is made of fine dry mud. Hard, but not as hard as the salt of Utah. It should produce less vibration.

These are just a few of the problems! No one has ever travelled so quickly, so close to the ground before. The really tricky parts may turn out to be solving the problems that have not been foreseen. This is the real purpose of trying to achieve anything as extreme as this; it’s what you learn along the way that really counts! I think the Bloodhound team may be learning quite a lot!

All engineering problems have solutions, but this a really, really, really big challenge!

Follow progress here: http://www.bloodhoundssc.com/

Project

Make a rocket powered car.

Remember to design a rocket you will need a chassis with wheels, a propulsion system and some means of controlling its direction.

The first thing to work on is the propulsion system. What method of producing thrust are you going to use? Vinegar and baking powder can produce high pressures. So can Diet coke and Mentos! Water rockets? There are many ways of generating lots of shove!

Decide how much propellant you are going to use. You need to achieve the ideal compromise between weight and thrust to achieve the greatest speed.

Once you have decided on how much propellant, you need to decide on the chassis and wheels. It needs to be as light as possible but still be able to support the motor. Light vehicles may be able to run on cardboard wheels. Heavy devices may even need a skate board chassis.

Think about streamlining. Your model may well go fast enough to gain from a properly streamlined body.

Think about the surface you can use. A smooth indoor surface would be ideal but would it be long enough to get up to top speed?

How are you going to give it directional stability? Fins? A remote control system?

How are you going to measure its top speed?

Let us know how your attempt went. Send us a video if possible.

Good luck!

The big questions:

Why is the speed of sound a barrier?

What are the main problems involved in a making a car go fast?

Why is drag so much more important at high speeds than low speeds?

Why are very fast cars always powered by jets or rockets?

How does a jet push itself forward?