Section 6 EO C232.02 – IDENTIFY THE CHARACTERISTICS OF ROCKET ENGINES

Every method of propulsion relies on Newton’s third law, which states that for every action there is an equal and opposite reaction. This is most obvious when the original action affects an object that is close in size to the object that the reaction affects, such as when a swimmer pushes a floating object. In that case, the swimmer is pushed backward when the object is pushed forward.

However, when the object to be pushed is as large as the Earth, as in the case of a person taking a step forward, it is not so obvious that the Earth moves in the opposite direction when the step is taken. Yet the tiny motion of the Earth is in the opposite direction. The difference in the amount moved is proportional to the difference in weight between the Earth and the walker, so that the reaction is equal, as well as opposite.

In that same way, a wheeled vehicle such as an automobile pushes on the Earth when it begins its journey. Since the automobile is much smaller than the Earth, the smaller mass of the automobile moves much more than the great mass of the Earth. The swimmer, the person walking and the wheeled automobile are all relying on traction to propel them forward.

Newton’s third law of motion also dictates the movement of propeller-driven aircraft and jet aircraft. The forward motion of aircraft depends on pushing gases backward instead of pushing the Earth backward. A propeller pushes air backwards and this is called prop wash. A jet engine ejects hot exhaust gases backwards. To move in any direction, all objects and all creatures, whether living or artificial, must push matter of some sort in the opposite direction.

Newton’s third law of motion states that for every action there is an equal and opposite reaction. The third law can be correctly interpreted to mean that for every desired reaction there must be an equal and opposite action.

A balloon rocket would work in outer space. The air that is ejected from the balloon would produce the same opposite and equal reaction in space that it does in the Earth’s atmosphere, except that form drag from the atmosphere would not slow the balloon’s travel. The balloon rocket’s performance would be improved in space, without the form drag of air.

A reactive propulsion system can operate by ejecting any material. However, the higher the speed of the ejected material, the greater the resulting propulsive force will be. To raise the velocity of ejection, material is most often heated to create pressure. This has been the preferred solution since Hero used steam to operate his toy Aeolipile (pronounced A – O – lipile).

Show the cadets a slide or distribute handouts of Aeolipile in Figure B-1.

Hero of Alexandria invented a rocket-like sphere called an Aeolipile, in about 150 BC, which used steam as a propulsive gas. Hero mounted it on top of a water kettle. A fire below the kettle turned the water into steam, which travelled through pipes to the Aeolipile. Two L-shaped tubes on opposite sides of the sphere allowed the steam to escape and so gave thrust to the sphere that caused it to rotate.

Heating of the material to be ejected most often involves combustion in contemporary rockets although other methods could be used. Rocket combustion systems operate in space because they are self-contained and require no atmospheric oxygen.

Show the cadets a slide or distribute handouts of Combustion in Figure B-2.

Combustion in a rocket engine or a jet engine requires the rapid oxidization of fuel. A jet engine gets access to oxygen by drawing it from the surrounding air, so that a jet’s range is limited to the atmosphere. A rocket develops thrust in much the same way as a jet, but a rocket carries its own oxygen supply. Rocket engines and jet engines both have nozzles to generate thrust.

Show the cadets a slide or distribute handouts of a rocket nozzle in Figure B-3.

A rocket engine uses a nozzle to accelerate hot exhaust to produce thrust as described by Newton’s third law of motion. The amount of thrust produced by the engine at any given moment depends on both the amount of gas ejected each second and its velocity. These are determined by the rocket nozzle design.

A rocket works in outer space because it brings everything it needs with it.

The Earth’s atmosphere is mostly nitrogen. Oxygen is only a fifth of the atmosphere’s composition. Therefore, simply storing air for combustion would waste most of the storage space on unreactive nitrogen. To make good use of storage space, oxygen is stored in more pure forms, including liquid oxygen, or LOX. This gives the rocket engine the ability to operate for a longer period in outer space.

There are three main categories of rocket engines; liquid rockets and solid rockets.

Liquid rocket propellants, the fuel and the oxidizer, are stored separately as liquids and are pumped into the combustion chamber of the nozzle where burning occurs.

Solid rocket propellants, both fuel and oxidizer, are mixed together to form a composite fuel and then packed into a solid cylinder. Under normal temperature conditions the solid rocket propellants do not burn until exposed to a source of heat provided by an igniter. Once the burning in a solid rocket starts, it proceeds until all the propellant is exhausted.

With a liquid rocket the pilot can stop or modify the thrust by turning off the flow of propellants; but with a solid rocket, the casing must be destroyed to stop the engine.

Liquid rockets tend to be heavier and more complex because of the pumps and storage tanks. The propellants are loaded onto the rocket just before launch. A solid rocket is much easier to handle and can sit for years before firing.

Show the cadets a slide or distribute handouts of Figure C-1 (Solid Rocket) and Figure C-2 (Liquid Rocket).

Solid rocket engines are used on air-to-air and air-to-ground missiles, on model rockets and as boosters for satellite launchers, including the space shuttle’s two solid rocket boosters (SRBs).

Liquid rocket engines are used in the Space Shuttle’s main engines to place humans in orbit, on many robot missiles to place satellites in orbit and on several high-speed research aircraft.

In a solid rocket, the fuel and oxidizer are mixed together into a solid propellant, which is packed into a solid cylinder. A hole through the cylinder serves as a combustion chamber. When the mixture is ignited, combustion takes place on the surface of the propellant. A flame front is generated which burns into the mixture. The combustion produces great amounts of exhaust gas at a high temperature and pressure. The amount of exhaust gas that is produced depends on the area of the flame front and engine designers use a variety of hole shapes to control the change in thrust for a particular engine. The hot exhaust gas is passed through a nozzle, which accelerates the flow. Thrust is then produced according to Newton’s third law of motion.

In a liquid rocket, stored fuel and stored oxidizer are pumped into a combustion chamber where they are mixed and burned. The combustion produces great amounts of exhaust gas at high temperature and pressure. The hot exhaust is passed through a nozzle, which accelerates the flow. Thrust is produced according to Newton’s third law of motion.

Show the cadets a slide or distribute handouts of Figure C-3 (Liquid System Rocket).

There are many parts that make up a liquid-fuelled rocket. For design and analysis, engineers group parts which have the same function into systems. There are four major systems in a full scale rocket: the structural system, the payload system, the guidance system and the propulsion system.

Although space-age rocketry is often considered to be in its early stages, there are many launch vehicles to explore. One example each of American, Russian, European and Chinese launchers follow:

American Launch Vehicle – Ares

NASA currently has many launchers that they can match to particular missions. For manned space flight after the space shuttle program, the Ares series of rockets has been developed.

Show the cadets a slide or distribute handouts of Figure D-1 (Ares I Launch Vehicle).

Ares I is an in-line, two-stage rocket configuration topped by the Orion crew vehicle and its launch abort system. In addition to the vehicle’s primary mission – carrying crews of four to six astronauts to Low Earth Orbit (LEO) – Ares I may also use its 22.5-tonne payload capacity to deliver resources and supplies to the International Space Station, or to “park” payloads in orbit for retrieval by other spacecraft bound for the Moon or other destinations.

The Ares I first stage is a single, five-segment Reusable Solid Rocket Booster (RSRB) derived from the Space Shuttle Program’s reusable solid rocket motor, which burns a specially formulated and shaped solid propellant.

The Ares I second, or upper, stage is propelled by a J-2X main engine fuelled with liquid oxygen and liquid hydrogen.

Show the cadets a slide or distribute handouts of Figure D-2 (Ares V Launch Vehicle).

The first stage of the Ares V vehicle relies on two, five-segment reusable solid rocket boosters for lift-off. The twin solid rocket boosters of the first stage flank a single, liquid-fuelled central booster element.

The central booster tank delivers liquid oxygen and liquid hydrogen fuel to five RS-68 rocket engines. The RS-68 engines serve as the core stage propulsion for Ares.

Atop the central booster element is an interstage cylinder, which includes booster separation motors and a newly designed forward adapter that mates the first stage with the Earth Departure Stage. A J-2X main engine fuelled with liquid oxygen and liquid hydrogen propels the Earth Departure Stage, the same J-2X engine as is used in the Ares I upper stage.

Russian Launch Vehicle – Proton

Show the cadets a slide or distribute handouts of Figure D-3 (Proton Launch Vehicle).

The Proton engines burn a liquid fuel called hydrazine (UDMH) with an oxidizer called Nitrogen Tetroxide. Nitrogen Tetroxide and UDMH burn when they come in contact, without any ignition, so they are said to be hypergolic.

The Proton launch vehicle is currently used for national programs and commercial launches of foreign satellites. Proton is designed as a tandem launch vehicle available in three-stage and four-stage options.

European Launch Vehicle – Ariane 5

Show the cadets a slide or distribute handouts of Figure D-4 (Ariane Launch Vehicle).

Ariane 5’s cryogenic main stage is referred to as the EPC from its title in French, Etage Principal Cryotechnique. The EPC is essentially composed of an aluminum tank with two compartments: one for liquid oxygen and one for liquid hydrogen. Both propellants are produced at plants located inside Europe’s Spaceport in French Guiana.

Weighing 37 tonnes each when empty, the SRBs (Solid-Rocket Boosters) provide 1100 tonnes of thrust, roughly 92% of the total thrust at liftoff.

Chinese Launch Vehicles – Changzheng (Long March) Rockets

Show the cadets a slide or distribute handouts of Figure D-5 (Changzheng [Long March] Launch Vehicles).

The main stages and the booster rockets of Long March rockets use liquid storable propellants with hydrazine (UDMH) as fuel and nitrogen tetroxide as the oxidizing agent—the same hypergolic system used by the Proton rocket discussed above. The upper stages of Long March CZ-3A and CZ-3B use liquid hydrogen (LH2) as fuel and liquid oxygen (LOX) as oxidizer.