Rocket propulsion Summary

  • Last updated on November 10, 2022

The movement of a device by the ejection of matter, without the need for taking on ambient air for combustion.

Most rockets exhaust propellant at high velocities and temperatures. The propellant is produced at high pressure through the release of chemical, nuclear, or electrical energy to the working fluid. In outer space, the propellant escapes from the rocket chamber to a high-exit kinetic energy in proportion to its available energy per unit mass. This explains why the reaction of low molecular weight fuel, such as liquid hydrogen, with liquid oxygen can produce an effective exhaust velocity almost twice that of rockets using solid fuel mixed with oxidizer crystals. Newton’s second law shows that rocket thrust is the product of the effective exhaust velocity multiplied by the mass flow rate. Thus the higher the “effective” exhaust velocity, the lower the mass of propellant required for a given mission. Future long-range space missions may be based on electric, nuclear, or solar energy to increase the effective exhaust velocity up to ten times, thereby reducing required propellant mass by a factor of ten. A quick estimate of a rocket thrust is to multiply the rocket chamber pressure by the smallest flow area of the exhaust nozzle, called the throat.

Common rockets contain at least the following components: engine, nozzle, propellant storage, payload, airframe, and guidance and control devices. Payloads vary widely and include spaceships, instrument packages for upper atmosphere observations, warheads on missiles, artillery projectiles, and fireworks.

History

As early as 600 c.e., the Chinese manufactured black powder, a mixture of charcoal, sulfur, and saltpeter, for use as rocket propellant. In the year 1232, the Chinese used rocket-propelled fire-arrows successfully to defend their towns against hordes of invading Mongols. In the early eighteenth century, Sir William Congreve developed a sophisticated military missile, known as the Congreve rocket, which provided the “rocket’s red glare” observed by Francis Scott Key in 1812 at Fort McHenry.

The first liquid fuel rocket, propelled by hydrogen and oxygen, was designed in 1903 by the Russian scientist Konstantin Tsiolkovsky. Physicist Robert H. Goddard was the first in the United States to succeed in launching a liquid fuel rocket using oxygen and gasoline on March 16, 1926. Within 2.5 seconds, Goddard’s rocket gained an altitude of about 40 feet and a speed of 60 miles per hour.

The Soviet Union was the first nation to achieve spaceflight, with the Sputnik 1 satellite on October 4, 1957, and with the pilot Yuri Gagarin’s flight on Vostok 1 on April 12, 1961. The next crewed spaceflight was made by the United States one month later, on May 5, 1961, in Mercury capsule Freedom 7, piloted by astronaut Alan Shepard. In the 6-by-6-foot capsule, Shepard experienced a gravity load of 7 g’s (seven times his weight) during launch and a gravity load of 1 g during recovery. Rocket flight has remained a continual challenge to crews.

At the end of the twentieth century, rockets had already launched the space shuttle more than one hundred times. Rocket technology development in the last half-century has made a major impact on modern space exploration and warfare strategies. Examples of such technology include the nearly 3,000,000-pound-thrust solid booster rockets developed for the space shuttle and the approximately 100,000 horsepower required by the turbine-driven pumps to pressurize the liquid hydrogen and oxygen for each of the three shuttle main engines. Improving materials and thrust-level control throughout the burn period is an ongoing technical challenge.

Rocket Science

Rockets have been developed for many different purposes and therefore differ widely in dimension, takeoff weight, thrust, range, propellant type, pressure, and temperature. The combustion process itself pressurizes solid fuel rockets, making solid fuel rockets more simple than liquid fuel rockets and capable of higher thrust levels. In the case of the space shuttle, two solid booster rockets are used to launch the vehicle, each with almost 3,000,000 pounds of thrust. Their function is to accelerate the shuttle as quickly as the crew can tolerate to near orbital velocity. From then on, the liquid fuel space shuttle main engines (SSME) continue to provide thrust with almost twice the effective exhaust velocity but only about one-seventeenth the thrust level. Typical fireworks rockets have burn times of only seconds and therefore require exceptionally high acceleration rates or thrust-to-weight ratio. Air-pressure-driven, water-type toy rockets have low exhaust velocities, similar in magnitude to the nozzle velocity of a garden hose.

The most important difference between rockets and jet engines is that rockets do not need to take in air, whereas jet engines require air for combustion and temperature control with mass-flow rates of up to one hundred times that of their fuel-flow rate. Because jet engines must ingest air, they can only operate below 100,000 feet altitude. However, rockets can operate anywhere inside and outside the atmosphere, even under water.

The rocket nozzle is used to accelerate the propellant to high exit velocity. To keep this nozzle small and therefore lightweight, the propellant must be generated at high pressure inside the rocket chamber. The corresponding smaller exit area also minimizes the nozzle thrust loss from ambient air pressure. The use of a low molecular weight propellant at high temperature increases the effective exhaust velocity, thus minimizing the required propellant mass-flow rate.

The most energetic chemical propellants are produced by the combustion of liquid hydrogen with liquid fluoride as the oxidizer. This mixture generates a combustion temperature of 7,200 degrees Fahrenheit and nozzle gas velocities of up to 15,400 feet per second. Less corrosive is a combination of liquid hydrogen with liquid oxygen, which produces a combustion temperature of 5,400 degrees Fahrenheit and nozzle gas velocities of up to 14,600 feet per second.

To pressurize these liquid propellants, very high-horsepower turbopumps are used. For example, the SSME requires both fuel and oxidizer pumps of a delivery pressure of around 8,000 pounds per square inch. With their combined-flow rate of approximately 1,000 pounds per second, this requires almost 100,000 horsepower for pumping per engine.

In contrast, a solid fuel rocket is much more simple to operate and therefore less expensive, as it does not need a pump to pressurize its combustion chamber. To understand the pressurization process, one must realize that the maximum amount of mass-flow which can escape through a rocket nozzle is directly proportional to gas density or pressure inside the chamber. Prior to ignition, the rocket chamber is at ambient pressure. The solid propellant inside is typically a mixture of oxidizer crystals, such as ammonium perchlorate, combined in a synthetic rubber binder, which serves as fuel. The fuel inner surface geometry is designed to adjust the combustion rate and thereby provide the desired thrust/time characteristics. When this solid surface is ignited, the amount of hot gas produced exceeds that escaping out the nozzle. Therefore, gas mass accumulates inside the rocket chamber and increases the gas pressure. This pressure rise continues until it is high enough to allow as much gas to escape out of the nozzle as is being generated inside the combustion chamber. In a fireworks rocket, this operating pressure is reached within a fraction of a second.

Some small liquid-propellant rockets can be operated without a pump, if the fuel and oxidizer are pressurized by a container of high-pressure inert gas. An even more basic liquid rocket uses a monopropellant, such as hydrogen peroxide. This liquid, when brought into contact with a catalyst, transforms into a 1,000-degree-Fahrenheit steam-and-oxygen gas that makes a good propellant.

Liquid propellants have the advantage over solids in that the exhaust velocity is higher and that thrust is controllable with a valve. Thrust control can also be obtained by combining a liquid oxidizer with a solid fuel, which is termed a hybrid engine. Hypergolic propellants such as nitric acid and hydrazine spontaneously combust upon contact, thereby eliminating the need for an igniter.

Rocket staging is an important technology used to increase payload capacity. The dropping off of empty fuel and oxidizer containers reduces weight and drag, which, in turn, reduces the thrust required in the subsequent stage. The disadvantage of rocket staging in launching the space shuttle is that retrieving and refurbishing the solid booster rocket casings adds several months to the launch turnaround time. This means many units are needed for a frequent launch schedule. The cost associated with this arrangement is the main reason for current research efforts to develop a space shuttle replacement in the form of a single-stage-to-orbit vehicle. In such a vehicle, the weight savings normally achieved by the staging process must be replaced by reducing the amount of oxidizer carried on board, necessitating takeoff with an air-breathing jet engine instead of a rocket. The switch to rocket propulsion cannot occur until after the vehicle reaches a speed of Mach 10. Then, a nonstaged rocket is sufficient to continue into orbit. Such an air-breathing jet engine is called a supersonic combustion ramjet, or scramjet, and its technology as yet remains nonoperational.

Long-range space missions in orbital trajectories represent flight in a zero-gravity environment for a majority of the mission. In those cases, a very small thrust supplied over a long time period can be made more fuel-efficient than can the use of chemical rockets. The energy for this type of rocket is supplied by either nuclear or solar energy. Electric energy can be used to heat gas to temperatures of up to 10,000 degrees Fahrenheit in an electric arc and produce a very high exit velocity. Ion rockets are even more propellant efficient. They accelerate charged particles in an electric field to exhaust velocities up to ten times those possible in chemical rockets.

International cooperation in rocket launch systems expanded at the end of the twentieth century. Near Moscow, the Russians have built more than 10,000 rocket engines. Based on the Russians’ experience, Lockheed Martin placed an order for 101 type RD-180 rocket engines. The RD-180 is a single-engine rocket, producing up to 933,400 pounds of thrust and using two combustion chambers, each with its own steerable nozzle. It burns a kerosene-oxygen mixture, with up to 1 ton of oxygen per second at maximum thrust. It can be throttled back to 40 percent thrust level for accurate trajectory control. These rockets were planned for used in the U.S. Atlas III Program, designed to put 9,000 pounds of payload in Earth geosynchronous orbit.

Other applications of rocket technology are in the airbags used for passenger safety in modern automobiles. These bags are filled with the exhaust gases from a small solid rocket, which is ignited when the vehicle experiences a collision.

Bibliography
  • Oates, Gordon C. Aerothermodynamics of Gas Turbine and Rocket Propulsion. Reston, Va.: American Institute of Aeronautics and Astronautics, 1997. An electronic text on the aerodynamic principles of aircraft turbines and rocket engines, featuring a bibliography and index.
  • Sutton, G. P. Rocket Propulsion Elements. 7th ed. New York: John Wiley & Sons, 2001. A comprehensive text on the workings of rocket engines.
  • Turner, Martin J. L. Rocket and Spacecraft Propulsion: Principles, Practice, and New Development. New York: Springer, 2000. A text covering the science of rocket propulsion in spaceflight.

Robert H. Goddard

Jet Propulsion Laboratory

Microgravity

National Aeronautics and Space Administration

Rockets

Russian space program

Spaceflight

Categories: History Content