Space rockets
A space rocket is a vehicle with a very powerful jet engine designed to carry people or equipment beyond Earth and out into space. If we define space as the region outside Earth's atmosphere, that means there's not enough oxygen to fuel the kind of conventional engine you'd find on a jet plane. So one way to look at a rocket is as a very special kind of jet-powered vehicle that carries its own oxygen supply. What else can we figure out about rockets straight away? They need great speed and a huge amount of energy to escape the pull of gravity and stop them tumbling back down to Earth like stones. Vast speed and energy mean rocket engines have to generate enormous forces. How enormous? In his famous 1962 speech championing efforts to go to the Moon, US President John F. Kennedy compared the power of a rocket to "10,000 automobiles with their accelerators on the floor." According to NASA's, the Saturn V moon rocket "generated 34.5 million newtons (7.6 million pounds) of thrust at launch, creating more power than 85 Hoover Dams."
Forces
Rockets are great examples of how forces make things move. It's a common mistake to think that rockets move forward by "pushing back against the air"—and it's easy to see that this is a mistake when you remember that there's no air in space to push against. Space is literally that: empty space!
When it comes to forces, rockets perfectly demonstrate three important scientific rules called the laws of motion, which were developed about 300 years ago by English scientist Isaac Newton (1642–1727).
A space rocket obviously doesn't go anywhere unless you start its engine. As Newton said, still things (like rockets parked on launch pads) stay still unless forces act on them (and moving things keep moving at a steady speed unless a force acts to stop them).
Newton said that when a force acts on something, it makes it accelerate (go faster, change direction, or both). So when you fire up your rocket engine, that makes the force that accelerates the rocket into the sky.
Rockets move upward by firing hot exhaust gas downward, rather like jet planes—or blown-up balloons from which you let the (cold) air escape. This is an example of what's often called "action and reaction" (another name for Newton's third law of motion): the hot exhaust gas firing down (the action) creates an equal and opposite force (the reaction) that speeds the rocket up. The action is the force of the gas, the reaction's the force acting on the rocket—and the two forces are of equal size, but pointing in opposite directions, and acting on different things (which is why they don't cancel out).
Thrust and drag
The force that pushes a rocket upward is called thrust; it depends on the amount (mass) and speed of gas that the rocket fires and the way its exhaust nozzle is shaped to squirt out that gas in a high-pressure jet. When a rocket's engine develops enough power, the thrust force pushing it upward will be bigger than its own weight (the force of gravity) pulling it down, so the rocket will climb into the sky. As the rocket climbs, air resistance(drag) will try to pull it back too, fighting against the thrust. In an upward-climbing rocket, thrust has to fight both drag and weight. This is slightly different to an airplane, where thrust from the engines makes the plane fly forward, drag pulls the plane backward, and the forward motion of air over the wings generates lift, which overcomes the plane's weight. So a key difference between a rocket and a jet plane is that a rocket's engine lifts it directly upward into the sky, whereas a jet's engines simply speed the plane forward so its wings can generate lift. A plane's jet engines fire it forwards so its wings can lift it up; a rocket's engines lift it up directly.
The faster things move and the more their shape disturbs the air, the more drag they create and the more energy they waste, uselessly, as they speed along. That's why fast-moving things—jet airplanes, high-speed trains, space rockets... and even leaping salmon—tend to be long, thin, and tube-shaped, compared to slower-moving things like boats and trucks, which are less affected by drag.
Escape velocity
Rockets burn huge amounts of fuel very quickly to reach
escape velocity of at least 25,000 mph (7 miles per second or 40,000 km/h), which is how fast something needs to go to break away from the pull of Earth's gravity. "Escape velocity" suggests a rocket must be going that fast at launch or it won't escape from Earth, but that's a little bit misleading, for several reasons. First, it would be more correct to refer to "escape speed," since the direction of the rocket (which is what the word velocity really implies) isn't all that relevant and will constantly change as the rocket curves up into space. (You can read more about the difference between speed and velocity in our article on motion). Second, escape velocity is really about energy not velocity or speed. To escape from Earth, a rocket must do work against the force of gravity as it travels over a distance. When we say a rocket has escape velocity, we really mean it has at least enough kinetic energy to escape the pull of Earth's gravity (though you can never escape it completely). Finally, a rocket doesn't get all its kinetic energy in one big dollop at the start of its voyage: it gets further injections of energy by burning fuel as it goes. Quibbles aside, "escape velocity" is a quick and easy shorthand that helps us understand one basic point: a huge amount of energy is needed to get anything up into space.
Parts of a space rocketA rocket contains about three million bits,of all shapes and sizes, but it's simpler to think of it as being made up of four separate parts. There's the structure (the framework that holds the whole thing together, similar to the fuselage on a plane), the propulsion system (the engine, fuel tanks, and any outer rocket boosters), the guidance system (the onboard, computer-based navigation that steers the rocket to its destination), and the payload (whatever the rocket is carrying, from people or satellites to space-station parts or even nuclear warheads). Modern space rockets work like two or three independent rockets stuck together to form what are called stages. Each stage may have its own propulsion and guidance system, though typically only the final stage contains the rocket's all-important payload. The lower stages break away in turn as they use up their fuel and only the upper stage reaches the rocket's final destination.
Some rockets (the Space Shuttle and the European Ariane) look like a whole bunch of rockets "strapped" together: a fat one in the middle with some skinnier ones either side. The big central rocket is the main one. The thinner rockets either side are what are called booster rockets. They're little more than fat fireworks: disposable engines that provide a thump of extra power during liftoff to get the main rocket up into space.
Rocket engines
The biggest (and arguably the most interesting) part of a rocket is the propulsion system—the engine that powers it into the sky. As we've already seen, rockets differ from jet planes (and other fuel-powered vehicles that work on Earth) because they have to carry their own oxygen supply. Modern space rockets have main engines powered by a liquid fuel (such as liquid hydrogen) and liquid oxygen (which does the same job as the air sucked into a car engine) that are pumped in from huge tanks. The fuel (also called the propellant) and oxygen (called the oxidizer) are stored at low temperatures and high pressures so more can be carried in tanks of a certain size, which means the rocket can go further on the same volume of fuel. External rocket boosters that assist a main rocket engine typically burn solid fuel instead (the Space Shuttle's were called solid rocket boosters, or SRBs, for exactly that reason). They work more like large, intercontinental ballistic missiles, which also burn solid fuels.
A closer look at a scientific rocket
It's not rocket science, even when it is! Rockets might be super complex, but if you think about them carefully, you'll find the bits inside are arranged in a very logical way that soon makes sense. To see what I mean, let's explore a very early rocket design in a bit more detail. It was developed by Robert Hutchings Goddard(1882–1945), an American physicist widely considered to be the father of the modern space rocket.
This artwork comes from a patent that Goddard filed in 1914 for a rocket that could rise to high altitudes and take photos. Remember that this was back in the early 20th century, long before satellites had entered space or astronauts had plodded over the moon.
Goddard's clever idea here was to put a rocket inside a rocket, which is a bit like the modern idea of a rocket with stages. You can see the entire rocket in Figure 1 on the right. The main rocket engine is colored red. You light it with a fuse (14), which burns up and ignites disks of fuel (12). Once all the fuel is burned up and the rocket has reached a fairly high altitude, the second rocket (blue) mounted on top ignites, separates, and fires off even higher. Because the second rocket weighs much less than the first one, a certain amount of fuel will make it rise very much higher into the sky than if that fuel had to lift both rockets together.
The rocket keeps its stability by spinning round at high speed as it flies along, just like a bullet fired from a gun. Figure 3 shows how this happens. It's a cross-section through the rocket at the point marked 3—3 in Figure 1 (where the blue and red rockets meet). Inserts of fuel (16) burn and send jets of hot gas outward at tangents, making the rocket body rotate. Unlike the main rocket engine, the spinning jets are ignited by an electrical circuit shown as 18, 19, and 20, which enables them to fire simultaneously. In practice, you'd fire up these tangential rockets to make the rocket spin around on its stand (Figure 5), on ball bearings (22) and, once it's spinning, light the main fuse (14) to blast it into the sky.
The business part of the rocket—the part that does our useful work—is the payload section on the top. This is shown in Figure 2 on the left. Goddard's rocket was designed for taking photographs from high altitude, so we have a camera (orange, 36) and a gyroscope and induction motor (purple, top) which keeps it pointing in the same direction while the rocket spins.
So it's nothing like as complicated as it looks!
