Hello all, welcome to yet another blog where I’ll be explaining rocket science and breaking the concepts into simple principles of physics. Today we shall get to know how SpaceX’s Falcon9 booster-stage lands with precision. But why only Falcon9?? I mean orbital rockets have been in existence since the 1950s’ but there had been no vertically landing rockets until 2015. It may seem surprising but the concept of vertical landings is pretty old. So, why we didn’t see any of the vertical landing rockets? Hold on, this article will solve all your doubts, so without much further ado let’s dive in headfirst.


How to Land an Orbital Class Rocket...The Dynamic Frequency
Falcon 9 lifting off

The Falcon9 at the time of lift-off stands at the height of 70m and weighs about 550,000 kg. The rocket lifts off as all 9 merlin engines are turned on. The booster burns for about 162s before the engines are turned off. We observe MECO (Main Engine Cut-Off) at about 72.3 km of altitude while the vehicle is traveling at about 7800 km/h. The 2nd stage Merlin engine is turned on after separation to achieve orbit. At the same time, the booster stage turns around with the help of cold gas thrusters/nitrogen thrusters and begins the boost back burn.

Boost back burn:

How to Land an Orbital Class Rocket...The Dynamic Frequency
Booster preparing to land

There is a certain amount of energy required to get the 2nd stage into a desired altitude and velocity after which the booster stage is way off the launchpad. To get back to the launchpad it needs to cancel out the energy gained and regain the same amount of energy and it is almost impossible considering the physics of our planet. The booster can never take enough fuel with it to boost back to the launch site. So instead, SpaceX planned for landing on a drone ship in the middle of the sea. But why and how is boost back done anyways? Boost back is done just to cancel out the excess energy before it re-enters the atmosphere with about 7 times the speed of sound. Now, here kicks in the entry burn. Without an entry burn, the rocket will be torn apart mid-air. This burn slows down the rocket for a safe entry into the atmosphere.

Suicide Burn:

How to Land an Orbital Class Rocket...The Dynamic Frequency
Suicide burn

Suicide Burn is a legit thing in Kerbal Space Program. But apparently, SpaceX doesn’t like it to be called suicide burn. They call it Hover Slam instead, so we’ll also call it Hover slam 😉 (just kidding). Suicide burn is like parking your car like the protagonist in Rohit Shetty films but here it's with rockets. The rocket just heads to the landing site full speed and at a certain point of time before touchdown, the middle merlin engine is turned on which slows the booster to almost 0 km/h just before touchdown. Now there is a reason why it is called suicide burn, it is because if you miss the right time the booster ain’t going to land. If you burn early, you use up the fuel and smash on the ground, if you burn late the booster still crashes. Risky stuff indeed!!

Soft Touchdown:

How to Land an Orbital Class Rocket...The Dynamic Frequency
Iconic touchdown click from the drone ship

The booster nailing the suicide burn ain’t making no difference with defective landing legs. The Falcon9 uses 4 hydraulics-controlled legs.

The legs have 4 major components;

·       The deployment arms

·       The legs

·       The pusher

·       The latches

The deployment arms: 

How to Land an Orbital Class Rocket...The Dynamic Frequency
Telescopic deployment arms

If you have seen a telescope, then you are probably aware of how it extends forward and retracts in, well, that design is called telescopic mechanism. The deployment arms have the same mechanism to deploy the landing legs. Normally, the arms are collapsed, when the booster nails the suicide burn, then only deployment arms extend as pressurized Helium fills up the arm. These arms also work as shock absorbers as there’s a heavy chance of the booster bouncing back after the touchdown.

The legs: 

How to Land an Orbital Class Rocket...The Dynamic Frequency

Probably the sexiest legs ever built, these are the parts that physically touch the ground, they have grooves to house the deployment arms. The legs are normally closed and open with the deployment of the deployment arms.

The pushers: 

How to Land an Orbital Class Rocket...The Dynamic Frequency
Zoomed-in pic of pushers

These components are too small and barely visible to us. As you can see in the images that I’ve attached here, the pushers look like small antenna-like structures. Their main purpose of existence is to kick the legs slightly off the body so that gravity and the main arms can do their respective jobs.

The latches: 

How to Land an Orbital Class Rocket...The Dynamic Frequency
Visible triangular latches

As the name suggests, these small triangular objects stop the legs from deploying during ascent. These open if and only if the booster does a suicide burn.

How to Land an Orbital Class Rocket...The Dynamic Frequency
The Falcon 9 trajectory

These 4 major components work hand in hand to make a near-perfect landing possible. It’s surprising the Falcon-9 booster can land with a landing accuracy of 10m that’s bonkers considering the booster makes a comeback from near orbital velocity. Insane engineering spotted.

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                                                                             -Ayushman Dash

                                                             Chief Space Exploration Writer

The Dynamic Frequency