How rockets take off
These devices burn most of their fuel in the first few minutes of flight , while trying to overcome Earth’s gravity.
Writers and inventors have dreamed of exploring the universe beyond Earth for hundreds of years, but the true challenges of traveling into space only became apparent in the 19th century. Experimental balloon flights showed that Earth’s atmosphere thins rapidly at high altitudes, so even before motorized flight became a reality, engineers already knew that wings, propellers, and other devices that create a forward force. Forward or upward pushing against a surrounding medium – such as air – are of no use in that environment.
Furthermore, combustion engines – steam and gasoline engines generate energy by burning fuel in the oxygen in Earth’s atmosphere, in a chemical reaction called combustion – would also fail in the absence of air.
Fortunately, a device had already been invented that solved the problem of generating force under such conditions: the rocket .
Initially used as weapons or to produce fireworks , rockets generate a force in one direction – thrust – according to the principle of action and reaction: the exhaust gases released by explosive chemicals leave their rear at high speed; as a result, they move in the other direction, regardless of the surrounding environment.
The key to using rockets in space is to transport another chemical called an oxidant . This is capable of playing the same role that oxygen plays in Earth’s air, allowing fuel to burn.
The first person to study the potential of rockets for space travel in detail was a Russian schoolteacher and amateur scientist named Konstantin Tsiolkovsky , who published his findings in 1903. This researcher realized that launching represented one of the greatest challenges. – the moment when the rocket carries all the fuel and oxidant that allow it to reach space – since, under these circumstances, the weight of the engine is the maximum possible and a great thrust is needed just to move it. However, once the rocket starts, it loses mass, so its weight is reduced and the same amount of thrust has a greater effect in terms of acceleration.
Tsiolkovsky came up with several designs, and concluded that the most efficient configuration was a vehicle that had to be launched vertically and had several phases . In essence, each of them was an autonomous rocket that could lift the stages above it for a certain time, before running out of fuel, separating and falling. This strategy, which is still widely used, reduces the amount of “dead weight” that must be carried into space.
Tsiolkovsky devised a complex equation that revealed the thrust force that any maneuver in this direction would require, as well as the specific thrust – that is, how much thrust is generated per unit of fuel – required for a rocket to reach its target. Thus, he realized that the thrusters of his day were too inefficient to power a rocket ship, and argued that ultimately, liquid and oxidizing fuels, such as liquid hydrogen and liquid oxygen, would be necessary to reach orbit. and beyond. Although he did not live to see his work recognized, Tsiolkovsky’s principles still underpin all of today’s rocketry.
Taking flight
To pass through Earth’s atmosphere and into space, rockets must delicately balance and control powerful forces .
For a rocket to generate the necessary thrust, a controlled explosion that arises as a result of a violent chemical reaction between a fuel and an oxidant is used. Gases from the expanding explosion are expelled through the rear of the rocket through a nozzle. It channels the hot, high-pressure gas created by combustion into a stream that escapes from the rear at hypersonic speeds, plus five times the speed of sound.
Isaac Newton’s third law of motion states that every action has an equal and opposite reaction, so the action force driving the exhaust from the rocket nozzle must balance with an equal and opposite force pushing the rocket forward. Specifically, it acts on the top wall of the combustion chamber, but since the engine is an integral part of each stage, it actually acts on the entire rocket.
Although the forces acting in both directions are equal, their visible effects are different, due to another of Newton’s laws. It explains how objects with greater mass need more force to accelerate them by a certain amount . That is, while the action force rapidly accelerates a small mass of exhaust gas to hypersonic speeds every second, the reaction force produces much less acceleration in the opposite direction, given the much greater mass of the rocket.
As the rocket gains speed, keeping the direction of motion closely aligned with the direction of thrust is critical: gradual adjustments are needed to steer the rocket into an orbital path, but noticeable misalignment can cause the rocket to spin out of control. Most rockets, including those in the Falcon and Titan series, as well as the Saturn V, are steered using gimbal motors, mounted so that the entire rocket motor can rotate and vary the direction of its thrust from moment to moment. other. Other steering options include the use of external fins to deflect exhaust gases as they exit the engine. This is most effective with solid-fuel rockets, which lack a complex engine, and with auxiliary engines, that is, small booster rockets mounted on the sides of the stage in question.
This is how rocket engines work
Rocket engines are very complex machines that are subjected to enormous heat and pressure .
Modern rocket engines have come a long way since their predecessors were used centuries ago to make fireworks. However, the relatively simple solid rockets, which are most often used as boosters to provide extra thrust at launch, are still based on the same principle: igniting a tube containing a mixture of fuel and oxidant. Once ignited, a solid rocket will continue to burn until its fuel is exhausted, but the rate at which it burns, and thus the thrust obtained, can be controlled by changing the amount of surface area exposed to ignition at different times. of the flight. This can be done by packing the fuel-oxidant mixture with a hollow space in the center, along the rocket. Depending on the profile of that space – it can be circular or star-shaped, for example – the aforementioned amount of exposed surface will change as the vehicle moves.
Liquid-fueled rockets tend to be more complex . In general, they have a pair of tanks – for fuel and oxidant – connected to a combustion chamber through a veritable maze of pipes. High speed turbopumps , driven by their own independent engines, are used to supply liquid fuel to the chamber through an injection system. The delivery rate can be increased or decreased, and the fuel can be injected as a jet or a fine spray.
Inside the combustion chamber, an ignition mechanism is used to start it. It can be a jet of high-temperature gas, an electrical spark, or a pyrotechnic explosion. Rapid ignition is critical : If too much fuel-oxidant mixture builds up in the combustion chamber, a delayed ignition can generate enough pressure to explode the rocket – an event that engineers call unscheduled rapid disassembly, or simply a difficult start.
The design of a liquid rocket stage can vary, depending on fuel and other requirements. Some of the most efficient propellants are liquefied gases, such as liquid hydrogen, which is only stable at very low temperatures, around -253 ° C. Therefore, they must be stored in strongly insulated tanks. Some rockets bypass ignition mechanisms, and use hypergolic combinations, which ignite spontaneously upon contact with each other.
Interplanetary travel
Rockets are necessary to explore the Solar System, but how can they go from Earth orbit to deep space?
The first stage of any space flight involves launching a vehicle from the Earth’s surface into a relatively low orbit, about 200 kilometers above most of the atmosphere. Here, gravity is apparently as strong as at sea level, but the friction of the Earth’s upper atmosphere is very low, so if the highest stage of the rocket is moving fast enough, it can maintain a circular path or stable elliptical. In it, the force of gravity and the natural tendency of ingenuity to fly in a straight line cancel each other out.
Many spacecraft and satellites do not travel beyond this low Earth orbit, called LEO. However, those destined to explore the Solar System need an additional push in order to reach the escape velocity, one in which the force of our planet’s gravity has no effect on the spacecraft. The escape velocity at the surface of the Earth – 11.2 kilometers per second – is approximately 50% greater than the typical speeds of objects located in the LEO position , although it is reduced at a greater distance from the Earth.
Probes traveling into interplanetary space are often first inserted into elongated or elliptical orbits by a carefully timed burst of thrust exerted by a thruster located upstream of the rocket carrying them. It can remain connected to the space probe for the remainder of its interplanetary flight. In such an orbit, the distance from the spacecraft to Earth can range from hundreds to thousands of kilometers, and its speed will also vary, although it will be greater when the spacecraft is closer to Earth, at a point called perigee .
However, surprisingly, the momentum required to reach interplanetary space usually occurs when the spacecraft is close to it. This is due to the so-called Oberth effect , an unexpected property of the equations that govern the motion of rockets that comes to say that they are more efficient when they move at a higher speed. For example, burning a spacecraft’s fuel not only allows the engine to harness its chemical energy, but also its kinetic energy, which is greater at higher speeds. In short, the additional rocket thrust required for it to reach escape velocity from a low altitude if it was traveling at high speed is less than that which would be required to escape from a high altitude if it were moving at a lower speed.
Space engineers and mission planners often refer to the term Delta-v, necessary to achieve a specific flight maneuver, such as a change of orbit. Strictly speaking, Delta-v means a change in speed, but experts use it as a measure of the amount of momentum – the thrust force over time – required to perform a maneuver. Generally speaking, missions are planned around a Delta-v budget – how much thrust they can generate for how long using the fuel supply aboard the spacecraft.
Sending a spacecraft from one planet to another with minimal Delta-v requirements involves placing it in an elliptical orbit around the Sun, called the Hohmann transfer orbit. The probe travels along a segment of the elliptical path that resembles a spiral path between the orbits of the two planets, and requires no further thrust along its journey. Upon reaching its target, it may exclusively use gravity to enter its final orbit, or it may require a rocket blast in the opposite direction. This is usually done by simply spinning the spacecraft at a point in space and starting the engine before it can reach a stable orbit.
