Rockets work by expelling mass in the form of a high-speed gas. As this mass accelerates out of the back of the rocket, an equal and opposite reaction pushes the rocket forward. This is an application of Newton’s third law of motion.
The high-speed gas that provides a rocket’s push is produced by the rocket engines. Rocket engines generate thrust by burning propellants – specially chosen chemicals that contain a great deal of chemical energy that can be released quickly as heat and gas.
There are several different types of rocket engines, each with advantages and disadvantages. The most commonly used type is the liquid bipropellant engine. This engine burns a liquid fuel with a liquid oxidizer to produce gas that escapes through a nozzle at the rear, propelling the rocket forward.
Some key principles govern the efficiency and capability of rocket propulsion systems:
Exhaust Velocity
The faster the exhaust gas escapes the engine, the more thrust can be obtained from a given amount of propellant. Achieving high exhaust velocities is a key aim in rocket design. The specific impulse measures how efficiently a rocket uses propellant to produce thrust. Engines with higher specific impulses require less propellant to accomplish a given mission.
Thrust-to-Weight Ratio
This ratio is a measure of the thrust produced by an engine divided by the weight of that engine. Rocket stages with higher thrust-to-weight ratios can accelerate faster. Optimizing this ratio allows a rocket to carry more payload for a given amount of propulsion system weight.
Propellant Selection
The best propellants provide high exhaust velocity and can be stored for long periods without deterioration. Many liquid propellant combinations are available with different advantages and disadvantages. Cryogenic propellants like liquid hydrogen and oxygen can deliver very high performance but require special handling procedures to remain cooled to extremely low temperatures.
Nozzle Design
Converging-diverging de Laval nozzles are used to accelerate exhaust gases to supersonic speeds and produce maximum thrust. Careful nozzle design is needed so the exhaust flow remains supersonic as it leaves the engine.
Engine Cycles
Alternative engine power cycles can be used to tap energy from different parts of the engine and optimize performance. Gas generator cycles take some propellant to power the engine’s turbopumps while staged combustion cycles burn all propellant in the main chamber to increase performance.
Engine Configurations
Clusters of smaller engines can provide redundancy and allow a failed engine to be shut down without losing the vehicle. Upper stages often use a single engine for simplicity. Some launch vehicles employ strap-on solid rocket boosters to increase liftoff thrust.
Rocket Propellants
Both liquid and solid propellants are used in rocket engines. Solid propellants are easier to store and handle but liquid propellant engines can be throttled, stopped, and restarted.
Solid Propellants
Solid rocket propellants consist of fuel and oxidizer combined into a solid mixture with the consistency of hard rubber. The propellant must not be so brittle it would crack or crumble, or so soft it would flow like a liquid. Common solid propellants include ammonium perchlorate composite (APCP) which burns smoothly and consistently.
Once ignited, solid rockets cannot be shut down until all propellant is exhausted. Thrust can be variable if the rocket motor casing is designed to erode in a controlled fashion. Spinning the rocket can also smooth out thrust. Solids find frequent use as strap-on boosters and upper stage motors.
Liquid Propellants
Liquid propellant rocket engines feed separate fuel and oxidizer liquids into a combustion chamber where they mix and burn. Common liquid propellant combinations include:
- Liquid oxygen (LOX) and refined kerosene (RP-1)
- LOX and liquid hydrogen (LH2)
- Nitrogen tetroxide (NTO) and monomethylhydrazine (MMH) – hypergolic propellants that ignite on contact
While more complicated than solids, liquid propellants can deliver higher specific impulse. By controlling the flow of propellants, thrust can be varied to manage acceleration forces during launch and flight.
Rocket Engines in Use
Modern launch vehicles employ different types of rocket engines for various stages:
Boosters
Large solid rocket boosters provide most of the liftoff thrust to fling heavy vehicles off the pad. They separate from the vehicle after burnout and parachute back for recovery and reuse. The Space Shuttle used two recoverable solid boosters while the Space Launch System uses upgraded boosters based on Shuttle technology.
First Stage Engines
Powerful engines generate high thrust to continue accelerating the vehicle once clear of dense lower atmosphere. Multiple engines may be clustered to provide sufficient thrust. The Falcon 9 uses nine Merlin engines on its first stage while the Saturn V and Space Launch System use clusters of five F-1 and RS-25 engines respectively.
Upper Stage Engines
After first stage separation, upper stage engines must operate in the vacuum of space. High efficiency through advanced nozzle expansion and propellant combinations provide the specific impulse needed to place payloads in orbit. The Centaur upper stage uses liquid hydrogen and oxygen propellants. The J-2X engine was in development for the Space Launch System upper stage before the program’s cancellation.
Attitude Control Systems
Small thrusters using various propellants are used to orient vehicles in space and provide steering by gimbaling main engines. Cold gas, solid propellant, and monopropellant thrusters have all been employed. The Space Shuttle used both hypergolic and monopropellant reaction control systems.
The Future of Rocket Propulsion
While the fundamentals of rocket propulsion are well established, advances continue to improve performance and efficiency. Use of lightweight materials, additive manufacturing, and advanced analytical tools can optimize designs. New propellants like liquefied natural gas (LNG) and methane are under consideration. Concepts like air-breathing hybrid engines, linear aerospike nozzles, hybrid rocket engines, and single-stage-to-orbit vehicles could provide revolutionary improvements but require mastering significant technical challenges not yet achieved.
