By Rachel Gould - Engineering Student @ Jesus College, Cambridge
Starship is a fully reusable rocket under development by SpaceX. The project aims to create the most powerful rocket ever developed, capable of launching satellites into low Earth orbit, and eventually take cargo and humans to Mars. The vehicle is a huge 120m tall (nine meters taller than Saturn V) and could be around 5000t by mass when stacked and fueled.
The rocket has made headlines in the last 18 months through its high-altitude flight tests. The 50m tall top stage launches to around 10km altitude and performs a “belly flop” at apogee (the point at which it is furthest from the earth), rotating itself horizontally. 500m from the ground, two to three engines reignite and the rear flaps fold in, flipping it upright again, and the vehicle performs a controlled landing.
This maneuver has never been tried before by the space industry and is extremely difficult to execute, but the belly flop maneuver is surprisingly a safe and effective way to return the vehicle to earth to be used again.
The logic behind the action is relatively simple. An object falling from space will accelerate due to the Earth’s gravity until it reaches its terminal velocity – where the downwards force from gravity balances the force upwards from air resistance. The larger the air resistance, the lower the terminal velocity, as the object will accelerate less before the forces balance.
By “belly flopping” Starship increases its bottom surface area from 70m² to 545m², significantly reducing the vehicles terminal velocity. This reduces the fuel and peak deceleration required to bring the vehicle to a stop (it will be travelling slower), and reduces the maximum heat experienced by the vehicle due to friction.
Implementing this is much more of a challenge. Firstly, the optimal altitude for the final flip must be chosen. This can be optimized for fuel use, safety of onboard passengers, possibility of engine malfunction and thrust to weight ratio. Additionally, the huge air resistance felt by 545m² of stainless-steel causes extreme heating. SpaceX have developed a heat shield made up of hundreds of hexagonal ceramic tiles, which will cover the half of Starship which faces down during the descent phase. This heat shield protects the body from peak temperatures of over 1400 degrees Celsius that it would experience during reentry to the Earth’s atmosphere.
To pull off such a complex maneuver is a hugely impressive engineering feat. A reusable vehicle of this size with the capability of being scaled will hugely expand our horizons in the spaceflight sector.