Rocket

• 1. Introduction

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  A rocket takes its name from the Italian rocchetta, meaning bobbin or little spindle, because its shape resembled the spool that holds thread on a spinning wheel. That humble naming hides what the device can do. A rocket carries everything it burns. It needs no oxygen from the air, so it can fly where there is nothing to push against at all. In the vacuum of space, a rocket engine actually works more efficiently than it does inside the atmosphere. Multistage rockets can reach escape velocity from Earth, which means there is no fixed ceiling on how high they can go. Yet the device began as something far smaller and far older than spaceflight. It frightened an empress at a feast. It lit up the sky over a besieged American fort in 1814. How does a single principle, reaction to expelled exhaust, carry you from a 13th-century Chinese firework to the surface of the Moon? And why, for all its power, is a rocket one of the most wasteful ways to move at ordinary speeds?

• 2. From China To The Moon

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  Gunpowder-powered rockets evolved in medieval China under the Song dynasty by the 13th century, where engineers also built an early form of multiple rocket launcher. According to Joseph Needham, the Song navy used rockets in a military exercise dated to 1245. A reference to 1264 records that the ground-rat, a type of firework, frightened the Empress-Mother Gongsheng at a feast held in her honor by her son, the Emperor Lizong. The Mongols adopted Chinese rocket technology, and the invention spread through the Mongol invasions to the Middle East and to Europe in the mid-13th century.

  The Huolongjing, also known as the Fire Drake Manual, was written by the Chinese artillery officer Jiao Yu in the mid-14th century. It describes the first known multistage rocket, the fire-dragon issuing from the water, thought to have been used by the Chinese navy. Between 1270 and 1280, Hasan al-Rammah wrote a book on military horsemanship that included 107 gunpowder recipes, 22 of them for rockets. Roger Bacon mentioned firecrackers in the Opus Majus of 1267, and around 1405 Konrad Kyeser described rockets in his treatise Bellifortis.

  The Mysorean rockets were the first successful iron-cased rockets, developed in the late 18th century in the Kingdom of Mysore under the rule of Hyder Ali. Sir William Congreve designed a British weapon based directly on those rockets in 1804, using compressed powder. It was the Congreve rocket that Francis Scott Key meant when he wrote of the rockets' red glare, while held captive on a British ship besieging Fort McHenry in 1814. Together the Mysorean and British innovations raised the effective range of military rockets from 100 to 2000 yards.

  William Leitch first proposed using rockets for human spaceflight in 1861, in an essay titled A Journey Through Space. Konstantin Tsiolkovsky conceived the same idea in 1903 and built the body of theory that later spaceflight rested on. In 1926 Goddard attached a supersonic de Laval nozzle to a high-pressure combustion chamber, turning hot gas into a cooler, highly directed jet that more than doubled thrust and raised engine efficiency from 2 percent to 64 percent. Production of the V-2 rocket began in Germany in 1943, designed at the Peenemünde Army Research Center with Wernher von Braun as technical director. The V-2 became the first artificial object to reach space, crossing the Kármán line with the vertical launch of MW 18014 on the 20th of June 1944. Doug Millard, a space historian at the Science Museum in London, called the V-2 a quantum leap, noting the Nazis used slave labour to manufacture the rockets. American crewed programs would culminate in 1969 with the first crewed landing on the Moon, using equipment launched by the Saturn V.

• 3. Anatomy Of A Machine

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  A rocket design can be as simple as a cardboard tube filled with black powder. Making an efficient, accurate rocket is harder. The main difficulties are cooling the combustion chamber, pumping the fuel in the case of a liquid, and controlling and correcting the direction of motion. At its core a rocket needs only three things: a propellant, a place to put it such as a tank, and a nozzle.

  Directional stabilization is what keeps a rocket from tumbling. Devices include fins, vernier engines, engine gimbals for thrust vectoring, and gyroscopes, held together by a structure that is typically monocoque. Rockets built for high-speed atmospheric use add an aerodynamic fairing such as a nose cone, which usually holds the payload. Beyond that core, a rocket may carry wings, parachutes, wheels, or even, in a sense, a person, as with a rocket belt.

  The tall thin shape that takes off vertically is only one form among many. The family includes balloon rockets, water rockets, and skyrockets bought at a hobby store, alongside missiles, rocket cars, a rocket bike, rocket sleds, rocket trains, rocket torpedoes, and rocket-powered jet packs. Ejection seats and launch escape systems are rockets built for rapid escape. The enormous Saturn V used for the Apollo program sits at the far end of that same spectrum.

  Goddard's first liquid-fuel rocket looked nothing like the machines that followed. He put the rocket engine at the top and the fuel tank at the bottom, believing the rocket would gain stability by hanging from the engine like a pendulum in flight. It veered off course and crashed 184 feet from the launch site, no more stable than one with the engine at the base. This mistaken idea is now known as the pendulum rocket fallacy.

• 4. The Chemistry Of Thrust

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  Chemical rockets are the most common type of high-power rocket, creating a high-speed exhaust by burning fuel with an oxidizer. The stored propellant takes many forms. It can be a pressurized gas, a single liquid that breaks down in the presence of a catalyst, called a monopropellant, or two liquids that react on contact, called hypergolic propellants. It can be two liquids that must be ignited to react, such as kerosene, known as RP1, and liquid oxygen, used in most liquid-propellant rockets. It can also be a solid combination of fuel and oxidizer, or a hybrid of solid fuel with liquid or gaseous oxidizer.

  Monopropellants such as hydrazine, nitrous oxide, or hydrogen peroxide can be catalytically decomposed into hot gas without being burned. An inert propellant can instead be heated from outside, as in a steam rocket, a solar thermal rocket, or a nuclear thermal rocket. For small, low-performance jobs like attitude-control thrusters, a pressurised fluid simply escapes through a nozzle. Chemical rockets store a large amount of energy in an easily released form, and the consequences of accidents can be severe.

  Newton's third law is the heart of how the engine works. A chemical reaction begins between fuel and oxidizer in the combustion chamber, and the resulting hot gases accelerate out of the nozzle at the rear. At the top of the chamber the hot gas cannot move forward, so it pushes against the top of the chamber. The opening at the bottom leaves that upward pressure unbalanced, and the remaining pressure pushes the rocket along.

  The shape of the nozzle decides how much force the engine gets. In a convergent section the gases accelerate, reaching Mach 1 at the throat. Beyond the throat, a bell-shaped expansion lets the gases push against the engine and add thrust, roughly doubling the total force compared with a simple tapering nozzle. In liquid-propellant engines, pumps must hold the propellant at a pressure larger than the chamber, typically on the order of 100 atmospheres.

• 5. Mass And Momentum

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  Almost all of a launch vehicle's mass is propellant. The mass ratio is the ratio between a rocket's initial mass and its final mass, and a high ratio means a lightweight, better-performing rocket, for the same reasons low weight helps a sports car. The Saturn V had a takeoff mass of about 3,038,500 kilograms and a mass ratio of 23.1. The Ariane 5 reached 39.9, while the V-2 managed 3.85 and the X-15 only 2.3.

  Specific impulse is one of the most important numbers describing a rocket's performance, measuring net impulse per weight unit of propellant expelled. The greater it is, the greater the net thrust. Effective exhaust velocities can be extremely high, around 4500 metres per second, roughly 15 times the sea-level speed of sound in air. From the conservation of momentum, the faster the exhaust leaves in one direction, the greater the speed the rocket gains in the other.

  The Tsiolkovsky rocket equation gives the delta-v, the total change in velocity a rocket can achieve with no outside interference. Launching from Earth's surface to low Earth orbit takes about 9.7 kilometres per second of delta-v, leaving a sideways speed of about 7.8 kilometres per second at an altitude of around 200 kilometres. About 1.9 kilometres per second of that is lost to air drag, gravity drag, and gaining altitude. Single rockets carrying payloads can manage only a few kilometres per second.

  No single rocket has yet reached the velocity needed for orbit, so orbital rockets always use more than one stage. The first stage of the Saturn V, carrying the upper stages, reached a mass ratio of about 10 and a specific impulse of 263 seconds, giving a delta-v of around 5.9 kilometres per second, short of the roughly 9.4 needed. Staging solves this by shedding empty tankage and engines during flight. The maximum speed from staging is theoretically limited only by the speed of light, but the payload falls geometrically with each added stage.

• 6. The Inefficiency Problem

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  The energy density of a typical rocket propellant is often around one-third that of conventional hydrocarbon fuels, with much of the mass being oxidizer. Inside the engine, near-adiabatic high-expansion nozzles cool and accelerate the gas, reaching an energy efficiency of up to 70 percent thanks to good Carnot efficiency. Engine efficiency, though, is not the whole story.

  Propulsive efficiency is where rockets fall down at ordinary speeds. At low speeds the exhaust carries away a huge amount of kinetic energy rearward, so rockets are extremely inefficient no matter how good the engine is. Efficiency rises as the vehicle speeds up, peaking when the vehicle travels at exactly the speed the exhaust is emitted, since the exhaust would then stop dead in space and surrender all its energy to the rocket. A rocket flying at Mach 0.85 with an exhaust velocity of Mach 10 would reach an overall energy efficiency of about 5.9 percent, against roughly 35 percent for a modern air-breathing jet engine.

  That gap explains why rockets are rarely used for general aviation. Such a rocket would need about 6 times more energy, and with propellant carrying around a third the specific energy of conventional air fuel, roughly 18 times more mass of propellant for the same journey. Rockets earn their keep only where very high speed is required, such as intercontinental ballistic missiles or orbital launch. NASA's Space Shuttle fired its engines for around 8.5 minutes, burning 1,000 tonnes of solid propellant and an additional 2,000,000 litres of liquid propellant to lift the 100,000-kilogram vehicle to an altitude of 111 kilometres and an orbital velocity of 30,000 kilometres per hour. By that accounting the Shuttle was about 16 percent efficient at launching the orbiter.

  The Oberth effect turns speed itself into an advantage. Because kinetic energy rises as the square of speed, a rocket gains more orbital energy from a given delta-v the faster it is already moving. Interplanetary missions exploit this by burning while travelling fast, close to a planetary surface, rather than waiting until the rocket has slowed at altitude.

• 7. Power, Peril, And Price

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  Rocket exhaust generates a significant amount of acoustic energy as the supersonic flow collides with ambient air and forms shock waves. The Space Shuttle generated 180 decibels of noise around its base. To fight it, NASA built a sound suppression system that floods the launch pad with water at rates up to 900,000 gallons per minute, cutting the level from 180 decibels down to 142. Without it, reflected acoustic waves could vibrate the payload and crew severely enough to damage or destroy the rocket.

  Launch escape systems are small, usually solid rockets that can pull a crewed capsule away from the main vehicle in an instant, and they have operated correctly every time they were used. The Soviet Safety Assurance System pulled away the L3 capsule during three of the four failed launches of the N1 Moon rocket, vehicles 3L, 5L, and 7L, saving each capsule from destruction. A successful escape of a crewed capsule occurred when Soyuz T-10, bound for the Salyut 7 space station, exploded on the pad. The Soyuz escape system can produce 20 g.

  Safety reflects the quality of engineering. A cabin fire during a launch rehearsal of the 1967 Apollo I spacecraft killed all three crew members. After the 1986 Space Shuttle Challenger disaster, the physicist Richard Feynman, serving on the Rogers Commission, estimated the chance of an unsafe condition for a Shuttle launch at very roughly 1 percent. The historical per-person-flight risk in orbital spaceflight has been calculated at around 2 to 4 percent.

  Cost, in the end, is dominated by dry mass, not fuel. With liquid oxygen at 0.15 dollars per kilogram and liquid hydrogen at 2.20 dollars per kilogram, the Space Shuttle in 2009 spent roughly 1.4 million dollars on liquid propellant per launch, against the 450 million dollars from other expenses. Expenses of 2,000 to over 10,000 dollars per kilogram of dry weight are common for orbital hardware, while raw materials make up only about 2 percent of the total. Since the early 2010s, new private options for spaceflight services have brought substantial price pressure into the existing market.