Launch, land, repeat - reusable rockets explained
Reusable rockets can slash the cost of getting into space. Cathal O’Connell explains how the revolutionary technology works.
The bottom of the ocean is a rocket graveyard – littered with the expended shells of thousands of rockets used to send satellites, and people, to space. These are the relics of a past age.
The Falcon 9, developed by aerospace company SpaceX, means it is now possible to reuse the first-stage of the rocket, by flying it safely back to Earth.
The achievement seems so unbelievable to some that there exists an online community of SpaceX sceptics. A number of their YouTube videos, such as “SpaceX Rocket Camera Landing Footage Fakery”, have been viewed more than a million times.
So just how does the Falcon 9 do it?
Falcon 9 is a two-stage rocket. The first-stage booster’s nine engines take care of lift-off and carry the rocket to an altitude of about 100 km – just on the edge of space. Here the second stage separates and fires its own single engine to take the payload into orbit. The first stage returns to Earth.
Cold-gas thrusters (flip):
The Falcon 9 first-stage booster is equipped with small thrusters near its ‘nose’ that vent nitrogen gas. Shortly after separating from the second stage, a controlled blast ‘flips’ the rocket to prepare it for its return to the ground.
Rocket first stages have historically used every drop of fuel to boost their payload into orbit. Falcon 9’s first stage carries extra. After it flips over, three of its engines reignite – slowing the rocket’s re-entry velocity. These engines fire again as the rocket nears the landing platform. Altogether, the first stage slows from its top speed of 4,700 km/h to a landing speed of just 20 km/h.
The nine ‘Merlin 1’ engines of Falcon 9’s first-stage booster give the rocket the 600 tonnes of thrust needed for lift-off. For stability, the engines are arranged in what SpaceX calls an ‘octaweb’ configuration – one central engine surrounded by eight more. Each engine can alter its angle of thrust, to control pitch, yaw and roll during ascent, and for positioning on descent.
Grid fins (steering):
Grid fins have the lattice look of plastic tennis rackets, except each one is the size of your kitchen table. After the first stage begins its ‘boostback burn’, reigniting three engines to slow its speed, these heat-resistant wings pop out from the sides of the booster. They make minute movements to help steer the rocket towards the landing pad – a bit like how skydivers can change direction with little gestures of their hands.
Unpredictable effects of the environment, such as changes in air pressure, mean the rocket’s precise trajectory can’t be worked out prior to launch. Falcon 9’s onboard computer has to do it on the fly, and in a fraction of a second, before it runs out of fuel. At least a dozen sensors feed it information on the craft’s orientation, position, velocity, acceleration and altitude. “The landing is so fast that no human could react quickly enough to ensure a smooth touchdown,” says Hugh Hunt, an expert in engineering dynamics and vibration at the University of Cambridge.
“If one can figure out how to effectively reuse rockets just like airplanes, the cost of access to space will be reduced by as much as a factor of a hundred. A fully reusable vehicle has never been done before. That really is the fundamental breakthrough needed to revolutionise access to space.”
Elon Musk, CEO of SpaceX.
The four legs, made of strong, lightweight carbon fibre, deploy just before landing. Each leg has a shock-absorbing system to absorb the force of impact. For especially hot landings, a core of non-reusable material crushes on impact – a bit like the way a car is designed to crumple to absorb an impact and protect those within.
Like other rockets, the Falcon 9 launches from sites near the ocean, so when the first-stage booster falls back to Earth there’s nothing beneath it but open sea. Although it would be technically feasible for Falcon 9 to fly back to its launchpad, doing so would take much more costly rocket fuel. It’s much cheaper to touch down at sea and then ship the rocket back.
The landing barges are platforms about the size of a soccer field, equipped with their own suite of sensors that are in constant communication with the rocket. They have been given colourful names taken from spaceships in Iain M. Banks’s sci-fi novel The Player of Games, such as “Just Read the Instructions” and “Of Course I Still Love You”.
Although SpaceX has not released precise figures, a spokesperson has said the cost to refurbish and reuse the stage-one rocket is “substantially less than half” the cost of building a new one. Ultimately SpaceX plans to cut the turnaround time to just 24 hours.
The next evolution of its program, the Falcon Heavy, is a super-heavy lift vehicle using three Falcon 9s capable of lifting more than twice the payload of the next closest operational vehicle – the Delta IV Heavy, made by United Launch Alliance, a joint venture between Lockheed Martin and Boeing – at one-third of the cost.