For the past decade, the entire world has had their eyes on SpaceX as they have revolutionized
rocket engineering and space travel. From launching a sports car into orbit, to
the promise of establishing a futuristic colony on mars, their spectacles have generated levels
of public excitement and media coverage that haven’t been seen since NASA’s Apollo
program which ended more than 40 years ago. At the core of their ambitious plans is one
of the greatest technological developments in the history of rocket engineering: reusable
rockets. First announced to the public in 2011, the
SpaceX reusable launch system development program set out to create a new generation
of launch vehicles that would drastically reduce the cost of reaching orbit.
To accomplish this, SpaceX proposed the seemingly impossible task of recovering rocket boosters
using powered-descent. Their goal was to develop a rocket that could
be launched vertically to deliver a payload into orbit, and then return back to earth
with a controlled descent and vertical landing at a pre-determined landing site.
Either on land, or on an autonomous floating drone ship.
In just 7 years, SpaceX was not only able to achieve their goal of creating such a rocket,
but they have proven that their system is both reliable and economical with more than
60 successful launches and 30 successful landings of their Falcon 9 boosters, along with a 100%
success rate since the completion of their experimental testing program.
Or at least that was the case until December 2018, but at least they had a pretty good
run. 17 of their boosters were also re-used on
successive missions, and their unit cost for launching a kg of payload into orbit has been
reduced to just a fraction of the nearest competitor.
But how exactly did SpaceX accomplish this, and how do they manage to land 70 m tall rockets
weighing in excess of ½ a million kilograms precisely on a 50 m wide landing pad after
they are launched more than 70 km into the atmosphere at speeds exceeding 8,000 km/h?
It all comes down to just 2 key things: experience, and ridiculously well-engineered rockets.
Let’s start with experience by taking a brief look at the history of the reusable
launch system development program. The program itself was first announced in
2011, but it wasn’t until late 2015 that SpaceX was able to land a Falcon 9 booster
on land successfully, and it took several years beyond this to achieve a respectable
landing success rate. Before this, SpaceX spent 5 years conducting
experimental landings where they tested their new technologies and learned how to build
better rockets through trial and error. They began with a prototype vertical takeoff
and vertical landing vehicle called Grasshopper, which completed 8 successful flights from
2012 to 2013. Following the initial success of Grasshopper,
SpaceX then equipped their first Falcon 9 boosters for powered-descent and conducted
several soft landings on the ocean surface from 2013 to early 2015.
Unfortunately, these first tests with the Falcon 9 were only able to achieve a landing
accuracy of about 10 km, but this was greatly improved in future tests.
When the first landings on an autonomous floating drone ship were attempted later in 2015, SpaceX
endured a series of public failures as 4 consecutive barge landings failed quite dramatically.
Despite these failures, they obtained valuable data from every single flight, and they used
the failures as opportunities to learn from their mistakes in order to develop a more
robust landing system. SpaceX continued to perform Falcon 9 landing
tests through 2015 and 2016, both on drone ships and on land, and successful landings
became routine by early 2017, with SpaceX deciding to stop referring to their landing
attempts as experimental. From the beginning of 2017 to nearly the end
of 2018, SpaceX maintained a 100% landing success rate with a minimum landing accuracy
of just 10 m. This impressive accuracy represents a 1000-fold
improvement compared to the initial soft-landing tests which were only able to land within
a 10 km radius from the intended target. But how did SpaceX manage to increase the
landing accuracy of their rocket boosters by 10,000% in just 4 years?
Obviously, this wasn’t achieved through experience alone, and so this brings us to
point number 2: ridiculously well-engineered rockets.
When SpaceX performs a rocket launch with the Falcon 9, the rocket separates into two
stages in Earth’s upper atmosphere. The second stage of the rocket carries the
payload into space, while the first stage booster returns to Earth and lands at a landing
site for re-use. The booster is programmed to follow a precise
flight path back to Earth, and it must autonomously perform a series of controlled maneuvers in
order to maintain that path and land vertically on the landing pad.
The exact flight path depends on whether the rocket is landing on a floating drone ship
in the ocean, or on land, and for landings at sea there is the added complexity of ensuring
that the drone ship is in the correct position when the rocket touches down.
However, the greatest engineering challenge by far is building a rocket capable of performing
the maneuvers that are necessary for controlled descent and landing.
After stage separation occurs, the rocket booster re-orients itself and performs a boost
back burn to achieve the proper trajectory towards Earth.
During the descent, it performs a re-entry burn which is used to reduce its velocity.
As the booster approaches the landing site, it re-orients itself again so that it is in
line with the landing pad, it deploys its landing legs, and it performs a landing burn
to bring its velocity to zero as it touches down on the pad.
During the entire flight, from stage separation to landing, the rocket continuously measures
its orientation and velocity, and it adjusts its trajectory accordingly so that it maintains
the correct flight path. To accomplish all of this, SpaceX has implemented
several rocket technologies that were developed and refined through their experimental testing
program, and it’s these technologies that have been pivotal to the development of their
reusable high-accuracy rockets. The six key technologies incorporated into
the Falcon 9 rocket booster are as follows: 1) Thrust vector control.
The merlin rocket engines of the first stage booster are gimbaled using hydraulic actuators
so that the direction of thrust can be adjusted. This is a method of thrust vectoring that
can be used to control the orientation of the rocket both within Earth’s atmosphere
and outside of Earth’s atmosphere where aerodynamic control surfaces such as fins
are ineffective. Thrust vectoring is actually a common technology
that is used for rockets, as well as military aircraft and missiles, however it is absolutely
necessary for the maneuverability of the Falcon 9.
2) Cold gas thrusters. The Falcon 9 is equipped with a total of 8
nitrogen cold gas thrusters that are mounted towards the top of the first stage.
There is 1 pod on each side of the rocket, each containing 4 thrusters.
Like the gimbaled main engines, the cold gas thrusters are used to control the orientation
of the rocket. They are particularly useful for the flip
maneuver after stage separation because of the large lever arm between the thrusters
and the rocket’s center of mass. They are also used to control the rocket at
times during flight when the gimbaled main engines are shut off.
3) Re-ignitable engines. Since the first stage must perform three separate
burns after stage separation, it is necessary for the main rocket engines to be re-ignitable.
The engines of the first stage booster have therefore been designed so that they can re-ignite
in the upper atmosphere at supersonic speeds as well as in the lower atmosphere at transonic
speeds. 4) Inertial navigation and global positioning
systems. The Falcon 9 is equipped with an inertial
navigation system, or INS, that uses several types of sensors to measure the position,
orientation, and velocity of the vehicle. A global positioning system, or GPS, is also
used to measure geolocation. The onboard computer receives data from the
INS and GPS in real-time and checks this information against the pre-programmed flight path.
If the computer detects any deviations from the flight path, then it instructs the rocket
to adjust its orientation and velocity as necessary.
5) Deployable landing gear. In order to perform vertical landings, the
Falcon 9 is equipped with 4 lightweight landing legs that are deployed using high-pressure
helium just before touchdown. Each leg is constructed from carbon fiber
and aluminum, and contains an impact attenuator for particularly hard landings.
The total span of the deployed landing gear is approximately 18 m, and the entire landing
system weighs less than 2,100 kgs. 6) Deployable grid fins.
Four titanium grid fins are mounted at the top of the first stage booster, and are deployed
during the rocket’s descent back into Earth’s lower atmosphere.
The fins are aerodynamic control surfaces that are used for precise control of the rocket’s
position and orientation prior to landing. The four grid fines alone are primarily responsible
for the incredible 10 m landing accuracy of the Falcon 9 first stage booster.
Grid fins were first used on the fifth soft-landing attempt of the reusable launch system development
program in 2015, and iterations on their design were continued through 2017 in order to achieve
the accuracy that we see from SpaceX today. So in the end, SpaceX was able to employ experience
and good engineering to develop a reusable and highly accurate launch vehicle, the Falcon
9. The Falcon 9 is an astonishing feat of modern
engineering, and I hope that it sets a precedent for the future of space travel.
Without a doubt, the development of a reusable launch system has been one of the greatest
technological developments in the history of rocket engineering, and I can’t wait
to see what the future has in store for SpaceX. Or perhaps I should rather say, what SpaceX
has in store for the future.
I really hope that you enjoyed this video, and I hope that I was able to provide some insight into the landing
technology used by SpaceX. Please remember to subscribe if you would
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