How Do Rockets Work? Throwing Mass Backwards to Reach Orbit

Newton's third law says that every force has an equal and opposite one, and a rocket is that law made into a vehicle. When the engine throws exhaust gas out of the nozzle at several kilometres a second, the gas pushes back on the engine with the same force, and the rocket accelerates forward. Nothing outside is involved; a rocket works better in a vacuum than in air, since there is no atmosphere in the way. The thrust depends on two things, how much mass is thrown out per second and how fast, and the second matters more: doubling the exhaust speed doubles the thrust for the same fuel, while doubling the fuel flow uses fuel twice as fast.

The engine's job is therefore to make the exhaust as fast as possible. Burning fuel and oxidiser in a chamber at high pressure produces hot gas, and the bell-shaped nozzle lets it expand and accelerate, converting heat into directed speed. The best chemical rockets, burning liquid hydrogen with liquid oxygen, reach exhaust speeds of about 4.4 kilometres a second, and that number, which chemistry sets, is the reason rockets are the shape they are.

Because a rocket has to carry the mass it will throw, and has to accelerate that mass before throwing it, the fuel needed grows exponentially with the speed required. Konstantin Tsiolkovsky wrote the equation down in 1903: the change in a rocket's speed equals the exhaust speed times the natural logarithm of the ratio of its full mass to its empty mass. To reach orbit a rocket needs about 9.4 kilometres a second, including losses to gravity and drag, which with an exhaust speed of 4.4 means a mass ratio of about 8, and with more typical kerosene engines at 3 kilometres a second a ratio above 20. That is why nearly everything on the pad is propellant and why the structure has to be made of the thinnest possible metal. Staging helps: dropping empty tanks and used engines on the way up means the upper stages do not have to accelerate dead weight, and every orbital rocket ever flown has had at least two stages. The main terms:

• •Thrust: mass flow through the engine times exhaust speed, the force pushing the rocket
• •Specific impulse: exhaust speed expressed as seconds, the measure of an engine's efficiency
• •Delta-v: the total change in speed a rocket can achieve, fixed by the rocket equation
• •Mass ratio: full mass over empty mass; higher is better and hard to raise
• •Staging: shedding structure as propellant is used, which multiplies the achievable delta-v

The hard part of spaceflight is not getting high but getting fast. A rocket that climbs straight up to 100 kilometres and stops falls straight back down. To stay up, a spacecraft must be moving sideways so fast that as it falls towards the Earth, the Earth's surface curves away beneath it at the same rate, and it falls around the planet for ever; at 400 kilometres, the height of the International Space Station, that speed is 7.7 kilometres a second, about 28,000 kilometres an hour, a lap of the Earth every ninety minutes. That is why rockets tilt over within seconds of launch and spend most of their burn accelerating horizontally. Astronauts float not because gravity is absent, since it is ninety percent as strong as on the ground, but because they and their vehicle are falling together. Leaving Earth altogether takes 11.2 kilometres a second, and the rocket equation makes every extra kilometre a second expensive.

Solid rockets, the oldest kind, are a tube packed with fuel and oxidiser mixed into a rubbery block that burns from the inside out; they are simple, powerful and cannot be throttled or shut down, which is why they are used as boosters and in missiles. Liquid rockets pump fuel and oxidiser from separate tanks into the chamber, can be throttled, stopped and restarted, and power every crewed vehicle; the pumps, spinning at tens of thousands of revolutions a minute to feed a Saturn V's engines at the rate of a swimming pool every few seconds, are the hardest part to build. Ion engines, used on deep-space probes, throw out a thin stream of charged atoms at 30 kilometres a second or more, giving a thrust no stronger than the weight of a sheet of paper but sustaining it for years; they cannot lift anything off a planet but can reach speeds no chemical rocket can.

The Chinese were launching gunpowder rockets by the thirteenth century, and the rocket's red glare in the American anthem is British Congreve rockets over Baltimore in 1814. Robert Goddard flew the first liquid-fuelled rocket in Massachusetts in 1926, a flight of two and a half seconds; the German V-2 of 1944 was the first to reach space; and its designers, in Soviet and American hands, built the rockets that launched Sputnik in 1957 and carried Apollo to the Moon on a Saturn V whose first stage burned 2,000 tonnes of propellant in two and a half minutes. The recent change is reuse. A rocket whose first stage flies back and lands, as the Falcon 9 began doing in 2015, throws away far less hardware per launch, and the cost of putting a kilogram into orbit has fallen by roughly a factor of ten as a result. The equation has not changed; the accountancy has.

A rocket moves by expelling its own mass at high speed, so that the reaction pushes it the other way, which is the only way to accelerate in a vacuum. Because it must carry and accelerate everything it will throw, the propellant needed grows exponentially with the speed required, which is why rockets are mostly fuel, are built in stages and shed them as they climb. Reaching orbit means reaching about 7.7 kilometres a second sideways, so that the vehicle falls around the Earth rather than back to it.