SpaceX is now quite used to landing their rebuilt rockets with ease. The 23rd of July 2022, marked their 56th first stage landing of the Falcon 9 successfully. Gone are the days of building new boosters for each new mission and here are the days when it just takes days for SpaceX to refurbish and reuse their rockets. Quite a remarkable achievement, isn’t it? All of this may seem easy on paper but is it really in reality? Let’s get one thing straight. It is not easy to have something land on its own using computer systems onboard. The landings of the Falcon 9 look jaw-dropping and staggering from earth but what goes behind it? Let’s find out.

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(Source: Space.com)

It is incredibly difficult to launch a Falcon 9 rocket into space and then return the first stage booster to Earth in a vertical landing, despite the company’s 80 percent success rate for rocket landings. The end of a fourteen-story aluminum-lithium alloy tube that is exploding with fire doesn’t merely find a comfortable landing in a massive empty field. The first stage’s most noticeable features are its cold gas maneuvering thrusters and aluminum or titanium grid fins, both of which are intended to give the first stage some degree of control and agility during its journey both inside and outside of Earth’s atmosphere. A lot of science behind each step.

(Source: Teslarati)
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Onboard Computers

High-accuracy GPS, gyroscopes, and accelerometers are integrated into the booster and are located at both the top and bottom end to accurately determine orientation, location, and velocity. Additionally, the booster has a large number of strain gauges that track forces acting on the structure at important points, including engine thrust. The onboard computer uses this information about the rocket’s orientation, location, velocity, acceleration, and altitude to perform the appropriate flying changes so that the rocket can make a clean vertical landing during re-entry. On GPUs, the computers process several physics equations that are then utilized to compute errors, regulate thrust vectoring, grid fin locations, and cold gas thruster (Nitrogen gas thrusters) durations, as well as optimize the flight path.

(Source: SpaceX)
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Boost-Back Burn

The booster is initially turned end-for-end while performing a boost-back burn. For a landing on a drone ship or near the launch point, the burn changes horizontal velocity. The reentry burn serves as a second chance to fix any reentry path errors while also reducing air velocity to prevent the rocket from being damaged by reentry heat. Hydraulic actuators are used to gimbal the Merlin engines of the Falcon 9’s first stage booster, allowing the rocket to change the direction of its thrust.

(Source: Quora)
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Grid Fins

Waffle-shaped grid fins extend that move the center of pressure of the booster upwards, increasing aerodynamic stability. The grid fins are arranged in the shape of an X-wing, stowed during ascent, and then deployed during reentry. The fins, which have a footprint of just 4 by 5 feet, are nonetheless able to roll, pitch, and yaw the 14-story stage up to 20 degrees to aim for a precise landing.

Landing

In between the re-entry and the landing burn, the booster spends a lot of time falling through the thick atmosphere. During these two stages, the booster’s slowdown from hypersonic speeds to transonic speeds all within a few seconds. The Falcon 9 actively modifies its orientation both inside and outside of the Earth’s atmosphere via thrust vectoring. The landing burn is sequenced with one, three, and one engine(s) running to reduce fuel usage. The Merlin engines are re-ignited when the booster is about to land. Upon re-ignition, landing lights and deployable landings legs are deployed that are a sequenced rehearsal for all booster landings.

(Source: The Verge)
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Deployable Legs

A four-legged deployable landing gear comprised of durable, lightweight carbon fiber and aluminum are fitted on the Falcon 9. Just before touchdown, high-pressure helium is used to deploy the legs. To soften the landing, each leg is equipped with a shock-absorbing system.

(Source: universetoday.com)
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Drone Ship

Launch sites for the Falcon 9 are located close to the ocean. Therefore, the first stage booster is bound towards the ocean when it returns to Earth following separation. These football-field-sized drone ships allow the Falcon 9 to land on the water, especially when a Falcon flight is headed for geostationary orbit. ‘Just Read the Instructions’ and ‘Of Course, I Still Love You’ are two drone ships that SpaceX currently uses during launches from Cape Canaveral.

(Source: Wikipedia)
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Sources

https://www.asminternational.org/web/hts/videos/-/journal_content/56/10192/36857184/VIDEO

https://www.inverse.com/article/33904-how-spacex-lands-a-falcon-9-rocket-in-6-steps

https://medium.com/predict/how-spacexs-rocket-boosters-land-on-earth-c46694a6ef99

Cover: https://www.overtdefense.com/wp-content/uploads/2020/11/spacexrocketreturn.jpg

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