Rockets 101: How Exactly Do Rockets Work?

Human beings have been using controlled explosions to propel objects for many centuries. One such example, rockets, are commonly used today as fireworks, signal flares, weapons of war, and, of course, 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?

Rockets are basically a special kind of engine that burns fuel to create propulsion. In most cases, rockets will convert their fuel payload into hot gases that are expelled out of their rear to propel them in a given direction. 

In this sense, 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. 

In fact, they operate using the principle of Newton's "Third Law of Motion", which, put simply, states that "for every action,  there is an equal and opposite reaction". In this sense, rockets can be said to be taking advantage of momentum --  the force that a moving object has. 

All things being equal, with no outside forces, the momentum of an object, or the combined momentum of a set of objects, must stay constant over time. This is the foundation of Newton's famous law.

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 (and you) 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. 

But there is a bit more to it than that. Other forms of combustion engines, like car or airplane engines, including jet engines, need air to work (specifically, they need the oxygen it contains). For this reason, they cannot operate in the vacuum of space.

Rockets, on the other hand, work perfectly well in space. But how?

Unlike combustion or jet engines, rockets carry their own supply of oyxgen of other oxidizer 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. 

The process works both in the presence of an atmosphere and in the vacuum of space. The actual workings of the rocket usually take place in the absence of air --  in fact, unlike cars and airplanes, rockets do not have any air intakes. 

The molecules of the rocket's exhaust are individually very small, but they exit the rocket's nozzle very fast and in a large quantity 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. Quite an impressive feat when you think about it. 

What is the difference between thrust and lift?

Thrust and lift are two of the fundamental forces of flight and, while related, they are distinctly different forces (the other two fundamental forces being drag and weight). The first, thrust, is any force that moves an aircraft (or rocket) in the direction of the motion. 

It can be created using a propeller, jet engine, or, of course, a rocket engine. In the former two examples, the air is pulled in and then pushed out in an opposite direction -- much like a household fan. In the latter, no air is required to provide the vehicle's thrust. 

Lift, on the other hand, is the careful interaction of aerodynamic forces that keep airplanes, or similar vehicles, in the air. For aircraft, providing lift is the main job of the wings and other lift surfaces.

For rockets, lift is less of an important consideration, as its trajectory and "flight" are more a factor of its propulsion and trajectory of flight as considerations for overcoming drag tend to take precedence. That being said, lift is important for the stabilization and control of the rocket during flight and is usually provided by the fins, nose cone, and body tube. 

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).

One 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. At launch, all of the engines are ignited. When the propellants in the strap-on rockets are extinguished, they are discarded while the sustainer engine continues burning. The Space Shuttle uses parallel staging, while rockets like NASA's Titan III's and Delta II's use both serial and parallel 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 -- which it must also overcome through brute thrust. 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 miles (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 a 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 around the early-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. 

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 may be 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 was moved 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.

And that, rocket fans, is your lot for today. While this introduction has been pretty brief, it should equip you with the basic tidbits of knowledge you'll need to gain a basic appreciation for these amazing pieces of technology. 

There are plenty of resources out there to learn more, and we thoroughly recommend you check them out at your own leisure.