All orbital rockets are multistage launch vehicles that require two or more stages, each with its own propulsion system, to reach orbit. A rocket interstage or interstage adapter forms a crucial part of this setup.

A rocket interstage is the structural component that forms the connection between the lower and upper stages of an orbital launch vehicle. It supports the vehicle’s upper stage while also protecting its vacuum-optimized nozzle and typically houses the components used for stage separation.

Readers who have seen video footage of a Saturn V launch (which carried astronauts to the Moon during the 1960s and 70s) might remember images of the rocket’s first stage falling away after stage separation.

Shortly after separating from its first stage, the interstage of a Saturn V rocket can be seen being ejected from the rest of the launch vehicle.

Shortly after this event, viewers could see a hollow cylindrical-shaped object also separating and falling away from the upper stage. This object is known as a rocket interstage and is a common and crucial component of all orbital rockets.

In the following sections, we take a closer look at a rocket interstage, what it is, and why it plays such a crucial role in any orbital launch vehicle.

What Is A Rocket Interstage?

A rocket interstage is the structural component that forms the connection between the lower and upper stages of an orbital launch vehicle. It is typically a cylindrically shaped shell made from strong, lightweight materials.

(The interstage adapters of the Falcon 9 and Atlas V rockets consist of aluminum cores covered by carbon composites.)

Its diameter is determined by the diameter of the upper & lower stages. If they have a similar diameter, the interstage has a uniform width. If the lower & upper stages have different diameters, the top and bottom half of the interstage will vary to perfectly fit both stages.

Saturn V - Atlas V - Falcon 9 - Interstage
From Left To Right: Interstage adapters of the Saturn V, Atlas V, and Falcon 9 rockets.

The length of an interstage will vary from one launch vehicle to another and depends on several factors, including the weight it has to bear, the number of mechanisms required on the component, and the length of the upper stage’s nozzle extension.

For example, the interstage adapter separating the first and second stage of the Saturn V rocket was approximately 5.5 meters or 18 feet in height, mostly to safely house the nozzle extensions to the second stage’s four J2 engines.

(A rocket’s interstage forms part of its structural system. A rocket primarily consists of four parts/systems. Learn more about these systems and the role each one plays in this article.)

Purpose Of A Rocket Interstage

It may seem obvious and easy to assume that the only function of a rocket interstage is to hold the different stages of an orbital launch vehicle together. However, it has several important functions, which can primarily be summarized as follows:

  1. Securely Hold The Rocket’s Upper Stage
  2. Protect The Nozzle Extensions Of The Upper Stage
  3. Avoid Dangerous Pressure Buildup In Upper Stage Engine
  4. Keep Liquid Propellants In Place
  5. Ensures A Safe Distance After Stage Separation

1) Securely Hold The Rocket’s Upper Stage

The primary purpose of a rocket interstage is to safely secure the upper stage(s) of an orbital launch vehicle to its first/lower stage. This is why the materials used for this section are made from strong, lightweight materials like aluminum, reinforced by carbon composites.

(Both the United Launch Alliance’s Atlas V and SpaceX’s Falcon 9 rockets use aluminum and carbon composites for their respective interstage adapters.)

An interstage can detach completely from both parts of the launch vehicle or stay attached to one after stage separation. It all depends on the launch vehicle and the specific function of the interstage involved.

In the case of the Saturn V rocket, the interstage first separated from the first stage and assisted the second stage in performing a few critical functions before being completely ejected from the rest of the spacecraft.

A Falcon 9 interstage, though, remains permanently attached to the first stage. It houses the grid fins and Reaction Control Thrusters that help to reorientate and guide the first stage back to the planet’s surface to perform a safe landing before refurbishing for a later launch.

Falcon 9 Interstage
The interstage of a Falcon 9 rocket not only supports the upper stage but also houses the mechanisms used for stage separation, re-entry, and landing.

(Learn more about rocket staging, why it is so crucial for any orbital launch vehicle to reach space and the different types of staging in this article.)

2) Protect The Nozzle Extensions Of The Upper Stage

Another crucial function of an interstage is to protect the upper stage’s nozzle (or nozzle extension) during the critical first part of the rocket’s ascent. It not only provides housing for it, but it also creates the space to avoid accidental collision with the first stage.

The vacuum-optimized engine nozzles of a rocket’s second stage are much larger than those on the first stage to cope with the little air pressure present in the upper atmosphere and space. Any damage to this nozzle can be catastrophic and result in mission failure.

The nozzle extension of a Falcon 9 rocket’s upper stage is so large it barely fits inside the vehicle’s interstage, with its widest section almost touching the inner walls of the interstage.

(Learn more about the difference between sea level and vacuum-optimized engine nozzles of orbital launch vehicles and why the nozzles on vacuum-optimized engines are that much larger than those fitted to sea level engines in this article.)

3) Avoid Dangerous Pressure Buildup In Upper Stage Engine

Typically, a safe distance needs to be created between the lower and upper stages after stage separation. This allows the upper stage to ignite safely without unnecessarily dangerous pressure buildup from the hot gases escaping its engine nozzle.

The pressure of the gases escaping the nozzle of a rocket engine is precisely calculated. If the nozzle is still inside the interstage or against the lower stage, a pressure buildup can occur, which can damage or even destroy the upper stage’s nozzle & combustion chamber.

A rocket interstage not only supports and protects the upper stage before separation, but its positioning also automatically helps to create the much-needed space to prevent the lower stage from intervening with the flow of the exhaust gases from the upper stage.

It also houses various mechanisms that further assist in creating a safe distance between the two stages to ensure no unnecessary pressure buildup occurs.

4) Keep Liquid Propellants In Place

The Earth’s gravity, as well as an orbital rocket’s extreme acceleration, helps keep the propellants at the bottom of their tanks, ensuring a reliable fuel flow and combustion.

As soon as the first stage engines shut down, though, a moment of “weightlessness” is induced, causing the fuel to start drifting away from its intended position, which can dramatically impact fuel flow and combustion.

Many orbital rockets overcome this by using ullage motors that fire after stage separation, allowing the upper stage to safely accelerate away and forcing the fuel in the tanks to settle.

In the case of the Saturn V, the ullage motors were situated towards the aft of the interstage. This had the advantage of providing the acceleration to allow the fuel in the upper stage engine to settle and then be jettisoned away with the rest of the interstage to save mass.

There are exceptions to this commonly-used method, though. Anyone familiar with the Soviet/Russian line of launch vehicles will know that the R7 launch vehicle family, with the Soyuz rocket being the most well-known example, does not have a solid interstage.

Saturn V Interstage
The Saturn V interstage housed ullage motors that helped to accelerate the upper stage away and allowed the fuel to settle before being ejected.

Instead of a solid interstage completely encasing its contents, the interstage of a Soyuz rocket has a vented fence-like structure. This is because the rocket’s stage ignites while still attached to the first stage. This method is also known as “hot staging.”

The vented structure allows the hot gases to escape without causing a pressure buildup that may damage the upper-stage engine. It also allows the upper stage to keep accelerating, maintaining momentum & keep the fuel from drifting away from the bottom of their tanks.

5) Ensures A Safe Distance After Stage Separation

A safe and proper separation between two stages during stage separation is also crucial to avoid any accidental collision between rocket parts. To achieve this, an interstage also has several mechanisms, including pneumatic & pyrotechnic devices, to assist the process.

(After the first stage engines shut down, a brief period of “weightlessness” before upper stage ignition may cause the momentum of a larger first stage rocket to drift into and collide with the upper stage, potentially damaging or preventing it from functioning properly.)

As already mentioned, the interstage connecting the Saturn V’s first and second stage contained ullage motors that fired after first stage separation, allowing the second stage to be safely propelled away from the first stage.

On a modern Falcon 9 rocket, the interstage houses pneumatic pushers that separate the first from the second stage, ensuring a safe distance between the upper and lower stage before the upper stage vacuum-optimized engine ignites.

Conclusion

A rocket interstage forms a critical part of any multistage orbital launch vehicle. And since the concept of a “one-stage-to-orbit” launch vehicle has never been successfully implemented and remains highly Improbable for the foreseeable future.

As this article clearly illustrated, a rocket’s interstage not only holds and supports the vehicle’s upper stage but also houses the multiple mechanisms necessary for stage separation and also protects the vehicle’s nozzle (extension).

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