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Rockets 101: How Exactly Do Rockets Work?

Ever wondered how rockets work? Then here is a basic overview for your delectation.

Human beings have been using controlled explosions to propel objects for many centuries. Often called rockets, today these devices are commonly used as fireworks, signal flares, weapons of war and for space exploration. 

But how do they actually work? Let's take a very brief look.

This article is not intended to be a comprehensive guide as rocket science is, after all, "rocket science."

 

How exactly do rockets work?

You might be tempted to think of rockets acting by simply "pushing themselves through the air." But since rockets can also operate perfectly well in the vacuum of space, this isn't really what is going on. 

They operate, as previously mentioned, using the principle of Newton's Third Law of Motion, often stated as 'for every action,  there is an equal  and opposit reaction'. Rockets, therefore, actually work by taking advantage of momentum --  the power that a moving object has. 

 All things being equal, with no outside forces, a group of objects' combined momentum must stay constant over time. This is encapsulated in Newton's famous Third Law of Motion

To visualize this, imagine standing on a skateboard while holding a basketball in your hands.

If you were to throw the basketball in one direction, you (and the skateboard) would roll in the opposite direction with the same amount of force. The more force exerted in throwing the ball, the more force will propel the skateboard in the opposite direction.

Rockets work in much the same way. By expelling hot exhaust from one end of the rocket, the rocket is propelled in the opposite direction -- just like in the skateboard example. 

Car or airplane engines, including jet engines, need air to work (well, they need the oxygen it contains), and for this reason, they cannot operate in the vacuum of space. Rockets, on the other hand, work perfectly well in space.

But how?

how rockets works types
Source: grizzzley/Flickr

Unlike combustion or jet engines, rockets carry oxidizers with them. Just like the fuel, these can be in either solid, liquid or hybrid form (more on these later). 

The oxidizer and fuel are mixed in the rocket's combustion chamber and the exhaust gases are expelled at high speed from the rear of the rocket. All of this is done in the absence of air --  in fact, unlike cars and airplances, rockets do not have any air intakes. 

RELATED: SPINLAUNCH: WHO NEEDS ROCKETS WHEN YOU CAN USE SPACE CATAPULTS?

The molecules of the rocket's exhaust are individually very small, but they exit the rocket's nozzle very fast (giving them a great deal of momentum). Enough, in fact, to provide a multi-ton object with the momentum it needs to escape Earth's gravity.

What are the main parts of a rocket?

Most modern rockets consist of at least two stages. These are sections of the rocket that are stacked one on top of each other in a cylindrical shell (aka serial staging).

An example of this form of rocket staging is NASA's Saturn V series.  

Other types of rockets use parallel staging. In this case, smaller first stages are strapped to the body of a central "sustainer" rocket. Rockets like NASA's Titan III's and Delta II's use this kind of staging.

Each stage has its own set of engines, which vary in number depending on the design. For example, the first stage of SpaceX's Falcon 9 has nine engines, whereas Northrop Grumman's Antares rocket has two.

The job of the first stage is to get the rocket out of the lower atmosphere. There may or may not be extra side boosters to help out, too.

Because this initial stage must carry the weight of the entire rocket (with payload and unspent fuel), it is usually the biggest and most powerful section.

As the rocket accelerates, it initially  encounters an increase  in air resistance. But as it moves higher, the atmosphere becomes thinner and the air resistance lessens.

This means that the stress experienced by the rocket during a typical launch rises initially, to a peak, and then falls back down. The peak pressure is known as max q

For the SpaceX Falcon 9 and the United Launch Alliance Atlas V, max q is usually experienced at between 80 and 90 seconds of a launch, at an altitude of between seven (11 km) to nine miles (14.5 km)

Once the first stage has completed its duty, rockets usually drop that section and ignite their second stage. The second stage has less work to do (because it has less mass to move) and has the advantage of having a thinner atmosphere to contend with.

For this reason, the second stage often only consists of a single-engine. Most rockets will also jettison their fairings at this stage too (this is pointed cap at the rocket's tip that protects the payload).

In the past, discarded lower sections of the rocket would simply burn up in the atmosphere. But starting in around the 1980s, engineers began designing these sections to be recoverable and reusable. 

Private companies like SpaceX and Blue Origin have taken this principle further and have designed them to be able to return to Earth and land themselves. This is beneficial, as the more parts that can be reused, the cheaper rocket launches can become. 

how rockets work spacex
Source: SpaceX/YouTube

Which fuel is used in a rocket?

Modern rockets tend to use either liquid, solid or hybrid fuels. Liquid forms of fuel tend to be classified as petroleum (like kerosene), cryogens (like liquid hydrogen), or hypergolics (like hydrazine). 

In some cases, alcohol, hydrogen peroxide, or nitrous oxides can also be used.

Solid propellants tend to come in two forms: homogenous and composite. Both are very dense, stable at room temp and are easily stored.

The former can be either a simple base (like nitrocellulose) or a double base (like a mixture of nitrocellulose and nitroglycerine). Composite solid propellants, on the other hand, use a crystallized or finely ground mineral salt as the oxidizer. 

In most cases, the actual fuel tends to be aluminum based. The fuel and oxidizer are usually held together with a polymeric binder that is also consumed during combustion. 

How do rocket launch pads work?

Launchpads, as the name suggests, are platforms from which rockets are launched. They tend to form part of a larger complex, facility, or spaceport. 

A typical launchpad will consist of a pad or launch mount, which will usually be a metal structure that supports the rocket in an upright position prior to blastoff. These structures will have umbilical cables that fuel the rocket and provide coolant prior to launch, amongst other functions. 

They will also tend to have lightning rods to protect the rocket during lightning storms. 

Launch complexes will vary in design, depending on the rocket's design and the operator's needs. For example, the NASA Kennedy Space Center designed the Space Shuttle to attach vertically to a rocket and move it to the launch pad on a massive tank-like vehicle called a "Crawler."

In Russia, rockets were assembled and transported horizontally to the launch pad before being lifted upright in situ.

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