Land speed record cars are built to travel faster than ordinary vehicles can safely or efficiently go. Many record attempts use jet engines or rocket motors instead of sending engine power through the wheels. This matters because at extreme speed, traction limits, air resistance, stability, and heating become the main engineering challenges.
The vehicle is more like a low flying aircraft that must stay on the ground than a normal car.
Key Facts
- Thrust comes from accelerating mass backward: F_thrust = mass flow rate x exhaust speed change.
- Newton's third law explains propulsion: exhaust is pushed backward, so the vehicle is pushed forward.
- Rocket thrust can be estimated by F = m_dot v_e + (p_e - p_a) A_e.
- Aerodynamic drag grows with speed squared: F_d = 1/2 rho C_d A v^2.
- Power needed to overcome drag grows with speed cubed: P = F_d v.
- Wheels on jet and rocket land speed cars mainly support load, steer, and measure speed, not provide driving force.
Vocabulary
- Thrust
- Thrust is the forward force produced when a jet engine or rocket motor accelerates gas backward.
- Jet engine
- A jet engine takes in air, compresses it, mixes it with fuel, and expels hot gas backward to create thrust.
- Rocket motor
- A rocket motor carries its own oxidizer and fuel, so it can produce thrust without needing outside air.
- Aerodynamic drag
- Aerodynamic drag is the resistive force from air pushing against a moving object.
- Stability
- Stability is the ability of a vehicle to keep its intended direction and attitude without flipping, sliding, or lifting off.
Common Mistakes to Avoid
- Assuming the wheels drive the car forward is wrong because jet and rocket record cars get their main forward force from exhaust thrust, not tire traction.
- Ignoring air resistance is wrong because drag becomes enormous at land speed record velocities and can dominate the force and power requirements.
- Treating jet and rocket propulsion as the same is wrong because jets need atmospheric oxygen while rockets carry oxidizer onboard.
- Thinking more thrust always means a safer run is wrong because higher acceleration can worsen stability, heating, tire stress, and steering control.
Practice Questions
- 1 A rocket land speed car has an exhaust mass flow rate of 18 kg/s and an exhaust speed of 2200 m/s relative to the car. Ignoring pressure effects, what thrust does it produce?
- 2 A car travels at 340 m/s through air with density 1.2 kg/m^3, drag coefficient 0.20, and frontal area 1.1 m^2. Use F_d = 1/2 rho C_d A v^2 to estimate the drag force.
- 3 Explain why a jet or rocket land speed record car still needs wheels even though the wheels do not provide the main propulsive force.