All modern orbital rockets you see launched during a live stream or on television today are known as multistage rockets. But what exactly is rocket staging, and how many stages does a typical orbital rocket have?

The majority of orbital rockets are two-stage vehicles, with the second stage stacked on top of the first. The first stage ignites at launch, propelling the rocket through the densest part of the atmosphere. After two minutes, it separates from the second stage, which takes the spacecraft to orbit.

Whenever one watches any significant rocket launch, the chances are almost 100 percent that the vehicle on the launch pad is a multistage rocket, meaning it has at least two or more stages that are used to carry the craft and its payload into orbit.

Each section (or stage) of a multistage rocket has its own propulsion system while also carrying its own fuel. Each of these stages is utilized at a specific point during the rocket’s ascent to allow the vehicle to reach Space and put the spacecraft in orbit around the Earth.

This process is known as rocket staging, and without it, it would be practically impossible for any launch vehicle to escape the Earth’s thick atmosphere and gravitational pull.

Apollo 11 First Stage Separation
The Saturn V rocket’s first stage separates from its second stage after lift-off from Kennedy Space Center in Florida, USA.

The following sections will describe exactly what rocket staging is, why it is so necessary for any launch vehicle to reach space, and also illustrate how a typical 2-stage rocket utilizes rocket staging during a launch.

What Is Rocket Staging?

For a rocket to reach Space and establish itself in orbit, it needs to escape the Earth’s gravitational pull, clear the thick atmosphere surrounding the planet, and travel fast enough to reach orbital velocity (approximately 28 000 km/h or 17 398 mph) to stay in Space.

To do this, it needs a large amount of thrust for a sustained period of time to lift several tons of spacecraft and fuel while simultaneously accelerating and gaining altitude. This accounts for the immense forces generated by a modern rocket engine.

However, even with powerful enough engines, a rocket uses so much energy and burns fuel so quickly that it will run out of propellant before reaching the required altitude and velocity. Simply adding fuel will add to the rocket’s weight, which will make reaching orbit impossible.

This problem is best illustrated by what is known as Tsiolkovsky’s Rocket Equation or the Ideal Rocket Equation, as demonstrated by the image below:

Tsiolkovsky Rocket Equation
Image with Tsiolkovsky’s Rocket Equation on the left and diagram illustrating it by plotting its mass ratios against its final velocity on the right.

Going into the details of this equation falls beyond the scope of this article, but it highlights how a rocket’s required mass (especially its fuel, which accounts for more than 85% of the rocket’s weight) works against its ability to break free from Earth’s gravity & reach orbit.

This problem is also known as the “Tyranny Of The Rocket Equation.” It means that in order for a rocket to overcome its mass and reach space, it needs more fuel to burn to produce more thrust for longer. But adding fuel will also add more mass to the rocket.

And this is where rocket staging comes in. Before illustrating how rocket staging works, one first needs to define exactly what rocket staging is:

Rocket staging is the process through which a launch vehicle uses multiple stages or sections, each with its own propulsion system and fuel, to reach orbit. The first stage propels the rocket through the densest part of the atmosphere, while the upper stage carries the spacecraft into orbit.

By breaking a launch vehicle up into smaller components, engineers managed to overcome this problem of a rocket’s mass and the amount of propellant needed to reach orbit, working against it.

Each rocket stage consists of its propulsion system while also carrying its own fuel supply. Once the First Stage propellant is used up, the entire section with thrusters and empty fuel tanks separates from the main body and falls back to the planet’s surface.

Soyuz-Rocket-First-Stage
The four strap-on boosters of a Russian Soyuz rocket can be seen attached to the first stage during horizontal assembly. This launch vehicle is a typical example of parallel rocket staging.

The second stage ignites as the First Stage falls away and is able to continue to accelerate and build on the momentum (velocity) of the First Stage but free from the added mass (structure, thrusters, and fuel) of the First Stage.

With each stage building on the momentum of the previous stage while continuing to get lighter as fuel is used up and the spend stages are jettisoned away from the spacecraft, the launch vehicle is able to reach space and put itself in orbit around the Earth.

As mentioned during the introduction, the majority of modern orbital rockets are 2-stage launch vehicles. However, it is possible to add even more stages if the mission dictates it and up to 5-stage rockets have successfully been launched.

The Saturn V rocket, used during the Apollo missions of the 1960s and 70s, was a 3-stage rocket. However, the spacecraft not only had to establish an orbit around the Earth but also put it on a trajectory to the Moon. The vehicle’s Third Stage was used for this maneuver.

Adding stages makes a rocket more complex, though, with a significant number of added connections and components that can fail, which also makes the vehicle potentially more dangerous. As a result, most launch vehicles used for Earth Orbit are 2-stage rockets.

Types Of Rocket Staging

The most common type of rocket staging is known as serial or tandem staging, which many readers will be familiar with. It is not the only type of rocket staging, though. Essentially, there are two types of rocket staging commonly utilized:

  1. Serial/Tandem Staging
  2. Parallel Staging

By taking a closer look at each type of staging, one will get a clear understanding of how each one works, as well as the benefits and drawbacks of the types of rocket staging:

1) Serial/Tandem Staging

Serial or tandem staging is a type of rocket staging where the different sections or stages are stacked on top of each other. While standing on the launch pad, it may be hard to recognize the individual states since they fit together to look like a single unit.

Serial Rocket Staging
Image illustrating how serial staging works on a 2-stage rocket. Image A shows the rocket’s first stage, powering the whole vehicle through the lower atmosphere. Image B shows the first stage separation, where the first stage shuts down and falls away while the second stage ignites and carries the vehicle and payload to orbit.

The first stage is typically much larger and can also consist of more thrusters than the upper stages since it has to do the bulk of the work to propel the rocket through the thickest part of the atmosphere, where the Earth’s gravitational pull is also at its greatest.

(The engine nozzles on a rocket’s first stage are also much smaller or overexpanded compared to the larger underexpanded engine nozzles of the upper stages. Learn more about the difference between sea level and vacuum-optimized engines in this article.)

After liftoff, the rocket’s first stage engines fire for approximately 2 minutes, taking the spacecraft into the upper atmosphere. After its fuel is spent, the first stage separates through pyrotechnic fasteners or a pneumatic detachment system and falls back to Earth.

(Different launch systems separate at different times and at different altitudes, depending on the specific launch vehicle, but on average, separation occurs at around 2 minutes at an altitude of approximately 60-100 km or 37-62 miles.)

After separation, the second stage ignites and continues to carry the vehicle into orbit. Depending on the mission & payload, the second stage will be sufficient to put the spacecraft in the required orbit but may require additional stages for the vehicle’s final trajectory.

Examples of launch vehicles using serial staging include the Atlas V, Delta IV Medium, Antares, Rocket Lab’s Electron, and SpaceX’s Falcon 9 rockets.

2) Parallel Staging

Not all launch vehicles consist of all their stages stacked on top of each other. In some cases, a 2-stage launch vehicle’s payload requirements necessitate additional thrust, or sometimes a rocket is designed to use external thrusters to deliver its payload to orbit.

Parallel Rocket Staging
Image illustrating how parallel staging works on a rocket. Image A shows the rocket’s strap-on and first-stage boosters powering the whole launch vehicle through the lower atmosphere. Image B shows the booster separation, where the strap-on boosters shut down and fall away while the first stage continues to fire and carries the vehicle and payload to orbit or separates from the upper stage, which continues to power the spacecraft.

In this case, additional thrusters are added/strapped to the central core or sustainer. During launch, all the rocket’s thrusters ignite to give the vehicle maximum thrust and lift to gain altitude and velocity as quickly as possible.

(The strap-on boosters are sometimes referred to as stage 0, which makes sense since they are often connected to a 2-stage rocket with serial staging. Not labeling them as first-stage boosters avoids confusion when identifying any particular stage during a rocket’s launch.)

When the strapon boosters’ propellant is used up, they separate from the central core and are ejected to fall back to the planet’s surface. The first stage thrusters continue to fire and carry the spacecraft to orbit or separate from the second stage when its fuel is also spent.

The rockets used for parallel staging are mostly solid rocket boosters, as used on the Space Shuttle & Arianne V vehicles. Solid rocket boosters are much easier to store & transport than liquid-fueled rockets, making them a cost-effective way to increase a rocket’s lifting ability.

(Learn more about solid-fueled rocket boosters and the potential dangers associated with them in this article.)

Sometimes, though, it makes more sense for a rocket manufacturer to use the first stage of their liquid-fueled launch vehicles as strap-on boosters. SpaceX’s Falcon Heavy and ULA’s Delta IV Heavy are examples of rockets using 3 of their first stages in parallel during launch.

In both cases, all three sections connected in parallel are essentially identical liquid-fueled first stages, with the outer two boosters acting as strap-ons. Their launch sequence and separation, though, are identical to that of conventional strap-on solid rocket boosters.

Examples of launch vehicles using parallel staging include the Delta IV Heavy, Space Shuttle, Falcon 9 Heavy, Titan III, Arianne V, and the Soyuz spacecraft.

Up to this point, this article focussed on explaining what rocket staging is, its importance for a launch vehicle to reach space, and describing the different types of rocket staging.

To see how all of this works in practice, one can look at an example of an actual modern two-stage rocket launch sequence using SpaceX’s Falcon 9.

How A Two-Stage Rocket Work (Falcon 9)

The SpaceX Falcon 9 rocket is the perfect example of a modern 2-stage rocket using serial/tandem staging to deliver its payload to orbit. By taking a closer look at its launch sequence, one will be able to better understand how a typical 2-stage orbital rocket works:

  1. A rocket launch to orbit starts months, sometimes years, before the spacecraft actually lifts off from the launchpad, but the Falcon 9 is physically powered up 28 hours before liftoff on the day before the launch.
  2. Fueling the rocket starts about 3 hours before launch. The fuel component, RP-1 (a highly-refined form of kerosene), is pumped in first, followed by the oxidizer in the form of liquid oxygen (LOX). Each component is stored in its own tank in the vehicle.
  3. Seconds before liftoff, the 9 Merlin engines ignite, and if they perform nominally, hold-down clamps release the vehicle, and the rocket lifts off.
  4. The vehicle goes supersonic at approximately 70 seconds after launch, followed in short succession by the spacecraft passing through MaxQ (the point at which the rocket experiences maximum dynamic pressure).
  5. At 158 seconds after launch at an altitude of 80 km (50 miles), the main engines of the first stage shut down through a process known as MECO (main engine cutoff).
Falcon 9 Rocket
A Falcon 9 lifts off from Kennedy Space Center, Florida.
  1. The first stage starts to separate from the second stage 3 seconds after MECO.
  2. 8 Seconds after first stage separation, the Merlin second stage vacuum optimized engine ignites and carries the vehicle’s payload to orbit.
  3. The rocket’s first stage falls back to the planet’s surface and, controlled by its thrusters and grid fins, lands near the launch site or on a drone ship downrange to be refurbished for a later launch. (SpaceX is currently the only company landing and reusing its first-stage boosters.)

There are some differences in the exact time and altitude at which different launch vehicles shut down their first-stage boosters and separate from the upper stages, but the basic procedures and principles followed are identical for all rockets using serial staging.

Conclusion

As illustrated in this article, the majority of modern orbital rockets are 2-stage launch vehicles. More than two stages can be utilized in a rocket, though, depending on the mission and payload. (Up to 5 stages have been used successfully during a launch.)

The post also explained what rocket staging is and why it is crucial for spacecraft to reach orbit, and also highlighted the different types of rocket staging and how each one functions.

This article was originally published on headedforspace.com. If it is now published on any other site, it was done without permission from the copyright owner.

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