- ¿What is a rocket?
- The Tsiolkovsky Rocket Equation
- Liquid-fuel Rockets
- Liquid-fuel Rocket Engine
- Solid-fuel Roquets
- Orbital and Escape Velocity
- Historic Rockets
- The Future: Space Launch System
¿What is a rocket?
Rockets consist mainly of a propellant, a place to put propellant (such as a propellant tank), a rocket engine and a nozzle. They may also have some directional stabilization devices (such as fins, vernier engines or engine gimbals for thrust vectoring, gyroscopes).
Rockets are accelerated by explosive thrust in the opposite direction. Some kind of fuel is burned with an oxidizer, or propellant, in a combustion chamber, creating hot gases accompanied by high pressure.
Rockets are generally classified as either solid or liquid. They produce thrust by burning propellants and expelling the combustion products out of the engine.
A rocket is different from a jet engine. A jet engine cannot function in space because it is an “air-breather.” Although jets and rockets both employ Newton’s law of action and reaction, the jet needs to draw in air from the atmosphere to burn its fuel. This limits the altitude of a jet plane.
Whether powered by liquids or solids, rocket systems are propelled by gas pressure resulting from fuel combustion. The force driving them forward is called thrust, which is the forward or upward force produced by the engines of a plane or a rocket.
If the rocket system (the liquid or solid booster plus payload) is to move upward against gravity, the thrust must be greater than gravity’s counteracting downward pull.
The ratio between these two values is called the thrust-to-weight ratio of the booster. At liftoff, the Saturn V thrust-to-weight ratio was 1.2, enough to allow the Eagle to land on the Moon surface.
As propellants burn, the thrust-to-weight ratio increases, and the booster continues to accelerate at higher rates. By exerting more force, or thrust, to a given payload, it will achieve a much higher velocity before thrust termination.
The Tsiolkovsky Rocket Equation
The equation relates the Δv (the maximum change of velocity of the rocket if no other external forces act) with the effective exhaust velocity ve and the initial and final mass of the rocket.
- Δv: is the maximum change of velocity of the vehicle (with no external forces acting).
- m0: is the initial total mass, including propellant.
- mf: is the final total mass without propellant, also known as dry mass.
- ve: is the effective exhaust velocity.
- ln: refers to the natural logarithm function.
To achieve the greater and greater velocities required to send a given mass farther and farther out into space, higher thrust must be exerted or it must be applied for a longer period of time.
Concerning rockets, mass does not remain the same; it decreases. As each kilogram of propellant is burned, the mass of the ascending vehicle becomes one kilogram less, and large rocket engines may burn hundreds of tons of propellant in seconds.
As a result, the same thrust has much less mass to accelerate. The decrease in weight allows an increase in acceleration. This principle is used in the concept of “staging”.
A space launch vehicle usually consists of a number of sections known as stages. After the first rocket stage accelerates the entire vehicle and finally burns out, its tanks and motors are discarded because they are no longer needed and add unnecessary weight that slows acceleration.
The same happens in the second and subsequent stages. The payload is finally all that remains, having been accelerated to the necessary velocity.
Liquid- and solid-fuel rockets
Liquid- and solid-fuel rockets each have special capabilities, advantages, and applications. Liquid rockets are often preferred for space missions because of their more efficient use of propellants. For example, the Russian Soyuz uses liquid fuels.
Because of their high thrust and simplicity, however, solid-fuel rockets are also used with space launch vehicles. Some launch vehicles, such as the space shuttle, combine both liquid engines and solid motors.
On the other hand, the oxidizer for liquid rockets is usually pure oxygen chilled to 90 K (-183°C) so that it condenses into liquid oxygen.
Liquid Propellant Rockets
Liquid Propellant Rockets Engines
When the liquid rocket engine is fired, high-speed pumps force the propellants into a cylindrical or spherical combustion chamber.
The fuel and oxidizer mix as they are sprayed into the chamber. There they ignite and expand, pushing equally in every direction. A relatively small hole, called the throat, exists at the bottom of the chamber and leads to an exhaust nozzle.
The expanding gases pushing in all directions inside the chamber push through the throat and out against the widening sides of the nozzle. As a given volume of gas reaches the edge of the nozzle, it spreads out over a much greater area than at the throat, so that the pressure on the edge of the nozzle is less than the pressure within the combustion chamber.
Liquid propellant engines have a number of advantages over solid propellant engines. A wider array of propellant combinations are available for different applications. Some of these require an ignition system and others simply ignite on contact.
Adjusting their flow into the combustion chamber adjusts the amount of thrust produced. Furthermore, liquid engines can be stopped and restarted later. It is very difficult to stop a solid propellant rocket once it is started, and thrust control is limited.
Solid Propellant Rockets
Another kind of rocket has come into wide use as part of military weapon systems and spaceflight boosters. This is the solid-propellant rocket. The space shuttle and other space boosters use both solid- and liquid-fueled rockets as a part of their “stack”.
It was discovered that mixing a chemical oxidizer and fuel, often aluminum powder, with a polymer (polymers are the basis of synthetic rubber and plastics and include substances like nylon) to provide the needed oxygen with the polymer resulted in a substance with a consistency similar to peanut butter.
This substance could be poured into forms and baked into a rubbery solid material that burned furiously when ignited and created large volumes of gases, producing a great thrust.
The perforation, a carefully designed hole through the propellant, is formed when the fuel is cast to fit into the rocket’s cavity. Varying the size and shape of the perforation determines the rate and duration of combustion, which controls the thrust.
The upper end of the rocket is closed off and capped with a payload section or recovery parachutes. The lower end of the rocket is constricted with a narrow opening called the throat, above a larger cone shaped structure, called the nozzle. By constricting the opening, the throat causes the combustion products to accelerate greatly as they race to the outside (second law). The nozzle aims the exhaust straight downward so that the rocket travels straight upward.
The propellant in solid rockets is packed inside the insulated case. It can be packed as a solid mass or it may have a hollow core. When packed as a solid mass, the propellant burns from the lower end to the upper end.
Depending upon the size of the rocket, this could take a while. With a hollow core, the propellants burn much more rapidly because the entire face of the core is ignited at one time. Rather than burning from one end to the other, the propellant burns from the core outward, towards the case.
Velocity is a critical factor. The velocities needed for specific space missions were calculated long before spaceflight was possible. To put an object into orbit around Earth, for instance, a velocity of at least 8 kilometers per second (18,000 miles per hour) must be achieved, depending on the precise orbit desired. This is called orbital velocity.
Orbital velocity is attained when the vehicle is moving in the right direction, fast enough to miss Earth as it falls back. The actual resulting course keeps it in space. In effect, it is falling continually around Earth.
To break away from Earth’s gravity for distant space missions, a velocity greater than 11 kilometers per second (25,000 miles per hour) is required. This is called escape velocity.
The largest military research project of the Third Reich was the development of a rocket at Peenemünde. They called it Aggregat 4, but it was known under the name V2 (Vergeltungswaffe 2). With the A4, researchers created the building blocks for liquid-fuel ballistic missiles.
In the nose cone was the explosive material, behind it lay the control system and two fuel tanks. The propulsion system with the thrust nozzle formed the tail of the rocket.
In 2006, while I was in Berlin, I visited the German Museum of Technology (Deutsches Technikmuseum) where I can learn a lot about the V2 rocket. Next you can see some pictures I took there.
Soyuz TM-31 launcher
It was designed to be reusable as were the solid rocket boosters. A new external tank was needed for each mission. Inside a cavernous payload bay were science laboratories, space probes, telescopes, or Earth-sensing systems.
Many shuttle payloads consisted of components for the International Space Station. At the end of a shuttle mission, the orbiter reentered Earth’s atmosphere and glided to an unpowered landing on a runway. The first space shuttle flight took place in 1981 and the last of its 135 missions concluded in 2011.
On the outside are huge solid-fuel motors with a combined thrust of more than 3 million pounds-force. When these burn out, they fall away from the central core, which then continues the mission.
The middle part is itself a two- or three-stage vehicle, but liquid-fueled. In addition to mass, other factors affect velocities necessary for space operations.
The higher a rocket goes from Earth, the thinner the atmosphere becomes, decreasing the amount of friction or drag imparted on the rocket. In space there is very little if any atmosphere and essentially no friction or drag. The pull of gravity becomes less the farther you move away from Earth’s center. This effect is not significant for satellites orbiting a few hundred miles above Earth, but it becomes important if the spacecraft is to be sent thousands of miles into space. Packaging the energy of a rocket vehicle into stages that can be discarded as they burn out has been the secret of launching into space. The number of stages may vary from two to five or even more.
The Atlas V family are the latest evolutionary versions of the Atlas launch system. Atlas V uses a standard common core booster (CCB), up to five solid rocket boosters (SRB), an upper-stage Centaur in either the Single-Engine Centaur (SEC) or the Dual-Engine Centaur (DEC) configuration, and one of several payload fairings.
The main engine of Atlas V launch system delivers more than 860,000 pounds-force of thrust at liftoff using liquid Oxygen and liquid Kerosene as an Oxidizer and fuel.
When missions demand additional thrust at liftoff, Atlas integrates up to five Solid Rocket Boosters on the Atlas V 500 series vehicles with a thrust of 380,000 pounds-force (total thrust of 1,900,000 pounds-force)
Some notable payloads derivered by Atlas V are:
- Mars Reconnaissance Orbiter
- New Horizons
- Solar Dynamics Observatory
- Boeing X-37B
- Mars Science Laboratory
Delta IV Heavy
The Delta IV Heavy is an expendable heavy lift launch vehicle, the largest type of the Delta IV family. It is manufactured by United Launch Alliance and it was first launched in 2004.
The Delta IV Heavy employs two additional Common Booster Cores (CBCs) as liquid rocket boosters (LRBs) to augment the first-stage CBC, each one with a thrust of 702,000 pounds-force.
The main engine of Delta IV heavy delivers more than 702,000 pounds-force of thrust at liftoff using liquid Oxygen and liquid Hydrogen as an Oxidizer and fuel.
The Space Launch System
A new and different kind of rocket is needed as NASA prepares to extend its mission beyond low-Earth orbit and out into the solar system.
The Space Launch System (SLS) will be used for Earth orbital flights and long-range missions to places like asteroids or Mars and its moons. The SLS rocket will be the most powerful launch vehicle in history and it is being developed in two phases:
- Heritage hardware (components from previous rockets) is being used to build a heavy-lift rocket for development testing from 2017 to 2021. It will lift up to 70 metric tons of payload. This rocket will make two lunar flybys carrying an Orion spacecraft, the second with a crew.
- The advanced SLS rocket will lift up to 130 metric tons including equipment, cargo, scientific experiments, and/or the Orion spacecraft into deep space.
These configurations share many other elements and subsystems, as well – such as engines that use liquid oxygen and liquid hydrogen as fuel – so that each costs less to design, build, and launch, without sacrificing the performance required to get the job done. What will they do?