Staging is the combination of several rocket sections, or stages, that fire in a specific order and then detach, so a ship can penetrate Earth’s atmosphere and reach space.

The operative principle behind rocket stages is that you need a certain amount of thrust to get above the atmosphere, and then further thrust to accelerate to a speed fast enough to stay in orbit around Earth (orbital speed, about five miles per second).

It’s easier for a rocket to get to that orbital speed without having to carry the excess weight of empty propellant tanks and early-stage rockets. So when the fuel/oxygen for each stage of a rocket is used up, the ship jettisons that stage, and it falls back to Earth. This becomes part of the rocket’s mass fraction—the portion of its fully fueled pre-launch mass that does reach orbit.

In Earth’s atmosphere, a rocket has to contend with drag, which is a force that pushes against a spaceship and—if not thoughtfully allowed for in the rocket design—can prevent it from going any faster, or even tear the ship apart. As the spaceship moves away from Earth and higher in the atmosphere, air density decreases. Out in the vacuum of space, the density is essentially zero, so there is virtually no drag there.

So how does drag impact rocket staging?

• Each stage can use a different type of rocket engine, each tuned for its particular operating conditions, whether that’s in Earth’s atmosphere or beyond.
• Rockets stages are typically stacked or parallel (boosters on the sides of a central vessel). The two-stage rocket is common, but space programs have successfully launched rockets with as many as five separate stages.
• The more stages—and thus stage detachments, or separation events—a rocket has, the more complicated and dangerous a ship becomes. Ships typically feature pyrotechnic fasteners or pneumatic systems to separate rocket stages. Engineering must be precise to avoid catastrophe.
• Stages can fire sequentially or simultaneously, depending upon the rocket and the mission.
• In the ideal rocket equation, as much as 91-94% of the total mass of modern day solid rocket motors is fuel. The remaining weight includes structure, engines, and payload.

The kind of staging astronauts use depends upon their mission needs and what forces they need to overcome.

1. Serial staging. Stages are attached, one on top of the other, or stacked. The first stage ignites at launch and burns through its fuel until its propellants are spent. Now useless dead weight, in a staging maneuver the first stage breaks free from the previous stage, then begins burning through the next stage in straight succession. Depending on the rocket, the second stage may get the payload into orbit or require a third or fourth stage to ultimately deliver it to space. It depends on the individual rocket and mission.

2. Parallel staging. Whereas serial staging involves stacked stages, parallel staging features one or multiple booster stages strapped to a central sustainer, as on the space shuttle. At launch, all the engines ignite. When their propellant runs out, the strapped-on boosters fall away. The sustainer engine keeps burning to put the payload into orbit. With the shuttle, solid rocket boosters are the stages that fall away from the main sustainer, the external tank that fed the main engines. The Titan III is an example of a rocket that uses both serial and parallel staging; it used a two-stage Titan II as the sustainer and added two solid rocket stages as boosters that fell away once they were done, much like the SRBs on the shuttle.

3. Stage-and-a-half: This less common staging has a main core that acts like a sustainer stage and a booster stage that falls away during the flight. This dates back to the Atlas D that launched John Glenn in 1962 and the three Mercury astronaut who followed in his orbiting footprints. At the time, the upper stages of multistage rockets often didn’t fire on time and rockets blew up. To make sure the engines all ignited properly, it made sense to Atlas designers to have all engines ignite while the rocket was still on the launch pad. Dropping the booster that was also sort of part of the main stage was how it dropped the dead weight in flight, making the rocket light enough to put a Mercury capsule into orbit.

4. Single staging. More a dream in development than a current reality, a single stage rocket is a simpler technology that doesn’t require multiple complicated and dangerous stages to get through the atmosphere.

There are multiple staging schemes for rockets, and the number of stages varies depending upon the spacecraft and the mission objectives. Typically, a rocket with three stages will undergo the same process. In serial staging schemes, the first stage is at the bottom of the rocket and is usually the largest. Its primary purpose is to get the spacecraft to a height of 150,000 feet, above most of the Earth’s air.

• Launch location is important. This comes down to physics. As a general rule, rockets launch from as near as possible to the equator, in order to take advantage of the velocity of Earth’s rotation, which is highest at the equator—about 1,000 miles an hour. The more orbital velocity a rocket gets from Earth, the less fuel it requires to reach orbital speed, which increases its efficiency. Not all rockets can take advantage of Earth’s spin—some are designed to send payloads such as satellites into north-to-south orbit, around the poles.
• The ride is intensely physical. G-forces are three times normal and there is rough, high-frequency vibration as the vehicle shoulders its way through the thick air. After two minutes the rocket is high enough that the air has thinned to almost nothing, and the first-stage boosters explode off in a burst of fireworks.
• As velocity increases, so too does drag. This is why flying a rocket through the atmosphere is so hard. To decrease drag, the cross-sectional area of the spaceship needs to be minimized, which is why rocket ships have to be streamlined.
• Lower stages like this typically require more structure than the upper stages. This is because they must bear their own weight as well as the mass of the stages above them that are not yet in use.
• This initial rocket stage needs to rapidly push the rocket into higher altitudes. As such, it typically has a lower specific impulse rating, trading efficiency for superior thrust.

The second stage is second from the bottom of the rocket. Its purpose is to get the spacecraft to orbital velocity and achieve weightlessness.

• The ride smooths out. Above the majority of Earth’s air, the ride gets suddenly smooth—but steadily heavier as the ship burns off fuel and the acceleration grows. The spaceship rolls through 180 degrees to let the communication antennae point at orbiting relay satellites.
• Achieving weightlessness. The ship becomes light enough that it reaches 3G, and the computers ease the throttles back to not overstress the vehicle. Each passing second takes the crew past emergency abort and failure options and improves astronauts’ chances of making it to orbit. And after eight and a half minutes, the stage engines shut down and they are safely there, weightless, in space.
• Ideal acceleration occurs above the atmosphere. Not only do the most efficient rockets have lower areas, but they also do as much of their accelerating (increase in velocity to orbital speed) as possible once they’ve gotten above the atmosphere into areas of lower air density. Once beyond the atmosphere, astronauts can increase the speed without increasing the force of drag because there’s no atmospheric density.
• Since the vehicle is further outside the atmosphere and the exhaust gas does not need to expand against as much atmospheric pressure, this later stage usually has a higher specific impulse rating.

The spent upper stages of launch vehicles are a significant source of space debris remaining in orbit in a non-operational state for many years after use, and occasionally, large debris fields created from the breakup of a single upper stage while in orbit.

Since the 1990s, when an upper stage is spent, astronauts have generally dumped any remaining fuel or discharged batteries, “passivating” them to reduce risks while those stages continue to orbit. Prior to that, many Soviet and U.S. space programs often did not passivate the upper stages after mission completion.