Staging: The one thing common in every Rocket
Technological innovations that have made home comfier, farming efficient, communications faster, dangerous weather conditions predictable; medical care improved have skyrocketed by rockets.
Staging of Rockets: What is it?
A multistage rocket or launch vehicle uses two or more stages, each of which contains its own engines and propellant. Two-stage rockets are quite common, but rockets with as many as five separate stages have been successfully launched.
There are mainly two types of rocket staging:

- Serial Staging –
In Serial Staging, there is a small second stage rocket that is placed on top of a larger first stage rocket. The first stage is ignited at launch and separates from the second stage when its propellants are exhausted.
Serial Staging was used on the Saturn V moon rockets.

2. Parallel Staging –
In Parallel Staging, several small first stages are strapped onto a central sustainer rocket. All the engines are ignited at launch and the strap-on rockets separate when their propellants are extinguished.
Parallel Staging is used on the Space Shuttle.
PSLV-C51 (Polar Satellite Launch Vehicle — C51) Rocket launched by ISRO on February 28, 2021 is a rocket that uses both Serial and Parallel Staging.
Rocket-Facts:
- R-7 Sputnik by the Soviet Union is the 1st rocket to use staging and put a satellite in space orbit.
- All orbital rockets to date are multi-staged.
Why do we require multi-staging?
- Weight & Cost –
To stay in space, an object must reach orbital velocity. To revolve around the earth, the object must be travelling at a speed of 28,000 km/h. To achieve that, a lot of thrust is required. To achieve thrust & velocity, fuel is increased; weight increases, more engines are required; more fuel is required and weight further increases.
Weight is the biggest enemy in rocket science. Staging helps us to throw the empty fuel tank out and remove the extra dead weight.
- Nozzle Size -
Thrust / Area of nozzle outlet = Pressure (Eq. 1)
In conventional chemical propulsions, we experience two kinds of exhaust exit variations.

1. Converging form –
Exit Pressure > Atmospheric Pressure
2. Diverging form –
Exit Pressure < Atmospheric Pressure
For maximum efficiency, the exhaust should come out vertically through the nozzle. Exhaust in sideways direction is wasted and is not economical.
Wondering why? Let us understand.
Assume Engine 1 has a smaller nozzle outlet area than Engine 2, both producing the same thrust.
Using Eq. 1:
(Exit Pressure)Engine 1 > (Exit Pressure)Engine 2
Therefore, for very small nozzle size, P(exit) it is very high and atmospheric pressure is much higher near the Earth’s surface.
So, with the decrease in P(atm), nozzle size needs to be increased. Hence, Staging comes to play.
How does a Rocket separate stages and put something into Earth’s orbit?
Let us go to the 28th of February, 2021 and launch PSLV-C51.
PSLV-C51 is a four-staged launch vehicle with the first and third stages using solid rocket motors & the second and fourth stages using liquid rocket engines.

· Strap-on Motors:
The two strap-ons augment the thrust provided by the 1st Stage during lift-off.
· 1st Stage:
The 20m high first stage, i. e., PS1 ignites. The strap-ons separate followed by PS1 when fuel in them is exhausted.
· 2nd Stage:
Now, PS2 is ignited. The heatshield payload fairing is separated as it is no longer needed. After the fuel in the 2nd Stage is exhausted, it separates igniting the 3rd Stage.
· 3rd Stage:
The 3rd Stage, HPS3 of height 3.6 m now separates when fuel in it is exhausted igniting the 4th Stage.
· 4th Stage:
PS4 is thrusted; it puts the satellite in proper orbit and then separates.
· Payload:
The major satellite in the PSLV-C51 mission, Amazonia-1 or SSR-1 by Brazil separates, opens its solar panels using small thrusters and faces the earth. It is now revolving in the Earth’s orbit. The mission continues and the other 18 satellites are successfully placed in their respective orbits.
Did you know?
The Satish Dhawan SAT in the mission engraved a picture of Shrimad Bhagawad Gita as well as of Prime Minister, Narendra Modi.
The future of Multi-Staging Rockets
- The death of most 3, 4 and 5 stage rockets can be seen because staging adds points of failure. Falcon 1 test launch-3 failed due to a problem in the staging event; if the 4th mission would also have failed, we would not have SpaceX today. We can see simpler two-stage designs more now like the recently launched RH-560 by ISRO on March 12, 2021.
- Newer rockets prefer serial staging over parallel staging as it is more economical. We can see the New Glenn and Starship rockets using only serial staging.
- With the development of ‘Aerospike Engines’ (It is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes.) and other technologies, reusable SSTOs (Single-Stage-to-Orbit) can be brought to use in the future. Such engines vanish the different nozzle size requirements that lead to staging. These also require less fuel than the traditional ones, thus reducing mass.
Fun-facts:
- The Indian ‘Avatar’ Spaceplane is a concept study for a robotic single-stage reusable spaceplane by DRDO (Defence Research Development Organization).
- Skylon, a series of concept designs for a reusable single-stage-to-orbit spaceplane by the UK would probably enter into service by 2022.

Space Explorations by advancements in Rocket Science would for sure bring immense transformation to the world we live in. Who knows, you can be the next one to travel to space.