spacex and the quest for rapid rocket reusability...4 image 12: elon musk – the founder of spacex...
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SpaceX and the Quest for Rapid Rocket Reusability
We live in extraordinary times! We go out of our houses and
we in automobiles to reach distant destinations and we
catch airplanes to reach the other side of our beautiful
planet Earth
Image 1: McLaren supercar with an airplane’s jet engines in background
Imagine how people lived before we invented the
automobile and airplane. Imagine how people looked at the
first affordable car – Model T that Henry Ford built in 1908
and the first airplane that the Wright brothers invented in
1903.
Image 2: Henry Ford with the Model T – the first affordable car - 1908
Image 3: The Wright brothers with the first airplane - 1903
... And nowadays everybody has a car and everyone can
catch a plane for a rather reasonable amount of money. We
are literally like Greek gods …
Image 4: We live like Greek God from mythology
… We fly resting effortlessly in our comfortable (not on every
airline) seat.
Image 5: Some airplane seats are pretty comfortable
But it is in our human nature to explore new horizons and to
invent our future …
Image 6: Humans are always looking for a new frontier
What about reaching out even further and going one step
beyond. What about space?!
3 Of course, people have already gone to space! We have a
Space Station orbiting the Earth every hour and a half.
Image 7: The International Space Station 420 km above Earth
We have gone from the invention of the first airplane to
going to space and looking at our beautiful planet from the
Space Station in a matter of only 100 years! But how many
people have gone to space in total? – Only a little more than
530 so far! And how many thousands of us do fly on an
airplane each day? What is the cause of this difference? It
comes from the fact that going to space is expensive! Not
only expensive, it’s extremely expensive – to launch even
1kg to space costs tens of thousands of dollars!
Why? Because it
happens so that the
rockets that we build
and the fuel they
consume are
expensive! Why?
Because how many
times do you see any
particular rocket? Only
once! – At its liftoff on
the launch pad!
Then this mighty
rockets that deliver
precious cargo to
space are being
disposed of and they
fall back into the ocean
uncontrollably. Each
time you deliver
something to space,
you need a big space
agency like NASA or
ESA with big
contractors like Boeing
and Airbus to take
your cargo and put it
on top of a giant brand
new unused rocket
that will work for several minutes and then all these millions
will be thrown away in the sea contributing to the
environmental impact we have on our planet! We use our
multi-million dollar rockets, a state of the art machines that
has been created and perfected by the greatest human
minds, only once! No wonder that it is expensive to go to
space! Imagine you had to buy a brand new Boeing 747 each
time you needed to cross the Atlantic Ocean! Not so
financially viable, eh?! Very few people would travel with
airplanes then.
Image 10: Boeing 747 – one of the largest aircrafts produced by man
Well, this is about to change in the future, of course. What
if I tell you that the future is now! We already know how to
launch successfully a rocket and we have mastered it to the
point where astronauts visit and stay in the Space Station on
a regular basis. Rocket launches are still exciting events
today, but for a
little bit
different
reasons than
the rocket
launches in the
60s when every
other rocket
made
spectacular
fireworks show
in a try-and-see-
what-will-happen attempt to go to space. Today we must
learn how to land our rockets back to Earth and I don’t mean
leaving them to just crash somewhere. I mean a controlled
landing of a rocket back on a pre-determined place on the
ground where it can be re-fueled and sent to space again in
a few hours! This might sound like a science fiction now, but
it sure doesn’t to Elon Musk – the founder of Space
Exploration Technologies (SpaceX), a private
entrepreneurial rocket company headquartered in
Hawthorne, California, that develops their rocket Falcon 9
v1.1 to be the first rapidly reusable rocket in the world!
Image 8: lots of money
Image 9: The Soyuz rocket on the Launch pad in Baikonur
Image 11: Rocket failures are relatively rare nowadays
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Image 12: Elon Musk – the founder of SpaceX and Tesla Motors
Mr. Musk is a bold visionary who concluded that if we are
ever to become a spacefaring civilization, we need to have a
rapid and inexpensive access to space! A SpaceX Falcon 9
rocket costs around 60 million dollars to launch and it is one
of the cheaper alternatives out there. What is fascinating,
and this might be counter-intuitive to many, is that most of
the cost to build a Falcon 9 (and any other rocket for that
matter) is going for the construction of the rocket itself and
the fuel for the launch is only 2% of the total cost! So, if we
find a way to bring back our precious rocket from space and
reuse it, we can drop down the cost of space flight 10 times
at least (an order of magnitude)!
NASA reached a similar conclusion to Elon when they built
the Space Shuttle, but it was an incredibly complex machine,
very expensive and it wasn’t really rapidly reusable. Instead,
it needed months of refurbishing after each return to Earth
in order to get it to the state it needed to be in order to fly
again to space. Falcon 9, on the other hand, is a conventional
rocket (at least to the extent that it doesn’t have large wings
like the Shuttle), but that is only on a first sight.
Image 13: The Space Shuttle
Imagine you are an astronaut about to travel with the
Dragon Crewed Spacecraft on top of the Falcon 9 Reusable
(F9R) and it’s launch day today on Cape Canaveral!
Image 14: The Dragon crewed spacecraft and Falcon 9R
You had a breakfast, you suited up in your space suit, and
you took the elevator to the top of the rocket and God, how
big the rocket looks from up here – you are about 60 meter
above the ground, next to the hatch of the Dragon capsule
and you can see kilometers away into the distance. You (and
your 6 crewmates) are about to go to the Space Station and
you will see Earth from the Cupola!
Image 15: Astronauts before departure to the ISS and a look from the Cupola on the ISS
You have paid only 1 million dollars to be on this seat, you
have trained for several years and it’s all about to culminate
now! You have the flight control panels touch screens in
front of you switched on, you make sure all the
communication systems are working, you are doing
communication checks and you listen to some rock’n’roll
music while you sit back and relax, letting the ground team
do their job. Everything has to be checked before the launch
of this state-of-the-art rocket.
Image 16: The interior of the Dragon crewed capsule (left) and the Soyuz spacecraft (right)
Falcon 9 v1.1 is a two-stage launch vehicle (rockets need to
be constructed of 2 or 3 stages to go to space) that stands
about 69 meters tall
with a liftoff mass of
about 506 tons when
flying in its F9R version
with a re-usable first
stage. Falcon 9R Stage 1
is about 43m tall and
3.7m in diameter and
contains an oxidizer tank
located above a fuel
tank (rockets need
oxygen like the one that
we breathe in the air,
but in liquid form and a
lot of kerosene as a
propellant). The oxidizer Image 17: Falcon 9R schematic
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tank utilizes a monocoque structure and is filled with Liquid
Oxygen oxidizer (LOX). The fuel tank features a
stringer/ring-frame for additional strength to the vehicle
and is filled with Rocket Propellant-1 (RP-1) as fuel which is
highly refined Kerosene. The walls of the first stage are
made from aluminum lithium alloy welded together through
special techniques for maximum strength. The LOX feedline
is routed through the center of the fuel tank to supply
oxidizer to the engines. The overall propellant mass in the
first stage is 385 tons! The fueling process begins about
three hours before launch – RP-1 kerosene propellant first,
then the liquid oxygen. The plume that comes out of the
rocket during the countdown is gaseous oxygen being
vented from the tanks, which is why the liquid oxygen is
topped off throughout the countdown.
A few minutes before liftoff it actually starts to sink in that
you are actually going to space now! Terminal countdown
begins at T-10 minutes and all systems are autonomous at
this time. Eight minutes before liftoff the Dragon capsule
goes to internal power and 5 minutes before liftoff the
rocket is standing by itself on the launch pad as the strong
back next to it retracts. Then you can feel the enormous
power of the engines underneath you at T-0.
Falcon 9R sports 9 Merlin 1D engines at its bottom that are
arranged in an “octaweb” configuration (Image 19). Eight
engines are arranged in a circle around a single engine at the
center that is installed slightly lower with its nozzle
protruding the others.
Image 19: Octaweb Merlin 1D engines configuration
The Merlin 1D engine is a gas generator engine. What does
this mean? It produces large volumes of relatively cool gas
that is used to power turbo pumps. The gas generator
exhaust pipes of the individual engines installed on the
perimeter of the first stage are arranged toward the inboard
direction and their flow passes through the gap between the
center and other engines.
With its nine first-stage Merlin
engines clustered together, Falcon
9 can sustain up to two engine
shutdowns during flight and still
successfully complete its mission.
This new design eliminates a lot of
structure that needs to be installed
to carry loads from the engines to
the Stage 1 skin. The original
configuration of the 9 Merlin
engines was a tic-tac-toe and needed a lot of load-
transferring structures, adding to the overall mass of the
rocket.
Image 21: The original and octaweb configurations of Falcon 9’s engines
All nine engines are ignited about three seconds before
launch and reach operational conditions and liftoff thrust of
654 Kilonewtons (enough to balance about 66,700kg) each
giving Falcon 9 a total liftoff thrust of 5,900kN (600,200kg).
The engine’s combustion chamber operates at a high
pressure of 97 bars to generate these enormous trusts that
Image 18: Falcon 9 and 9R launches
Image 20: A LOX and RP-1 turbo pumps with the turbine at the bottom
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lift the rocket off the ground! These pressures are around 40
times higher than the pressures in your automobile tyres!!
If you are interested in the construction of a Merlin 9 engine,
here is a schematic that explains it in brief (Image 22).
You can feel the enormous power during liftoff and ascent
through the atmosphere while you sit comfortably in your
seat inside the capsule. Seventy seconds after liftoff Falcon
9 reaches supersonic speed and shortly after that the rocket
passes through the area of maximum aerodynamic pressure
– max Q. This is the point when the mechanical stress on the
rocket is strongest due to a combination of the rocket’s high
velocity and the resistance created by the Earth’s
atmosphere. During ascent the rocket’s engines are
monitored constantly and computers can shut down any
engine at any time to prevent a disaster (or as they put it
more scientifically – RUD – rapid unplanned disassembly).
Falcon 9 is a safe rocket with a perfect track record (and I am
sure you know that when you trusted SpaceX to take you to
orbit), but rest assured that in the case of an unplanned
shutdown of an engine, the flight computer would re-plan
the ascent trajectory with the remaining engines by
extending their burn. The first stage is equipped with
Reaction Control System that uses nitrogen for three-axis
control of the rocket.
You are already 2 minutes and 40 seconds into the flight on
an altitude of 80 kilometers when the first stage engines are
shut down, an event known as main engine cutoff – MECO.
At this point you are already traveling at 10 times the speed
of sound! (a speed of Mach 10 - this is more than 3,400
meters per second!) The first stage is separated and you
continue your ride to the Space Station with the single
Merlin MVac engine of the second stage. We will leave you
to enjoy your ride and have a safe trip, but we will look at
something very interesting that is about to happen with the
first stage after its separation.
Immediately after the first stage separates from the Falcon,
it starts a very unusual journey back to Earth to a pre-
determined landing site with the idea to be reused many
times afterwards. This is the most expensive part of the
Falcon rocket and if we manage to fly it more than once, it
will reduce the cost of space flight with two orders of
magnitude. So, let’s see what happens during these
extraordinary first attempts of SpaceX to bring back
successfully the first stage to Earth.
Image 23: A schematic of a Falcon 9R landing procedure
Immediately after detachment, the first stage embarks on a
trajectory with a maneuver that will lead it away from the
plumes of the second stage that fires above it. It does that
with an attitude control system that uses cold gas nitrogen
thrusters on the top side section that keep it oriented on the
right path. Remember that the stage’s speed at this point is
around 10 times the speed of sound. This trajectory will
ultimately lead it to an altitude of 140 km oriented with its
nine Merlin engines pointing to Earth (Image 23).
Around 4 minutes and 30 seconds into the mission, the first
stage re-lights a subset of its engines for a boost-back
maneuver (Image 24) that slows the stage down and
controls its downrange travel distance, beginning to target
the planned landing site – either on land or in the ocean. The
duration of the boost-back burn depends on the target
landing site and is also driven by propellant availability for
the return which varies depending on payload mass and
insertion orbit.
Image 24: Falcon 9R’s Stage 1 boost back burn
Image 22: The structure and components of a Merlin 1D engine
7 Heading back into the dense layers of the atmosphere, the
first stage completes its supersonic retro propulsion burn
using three engines that are fired for about 20 seconds
starting at an altitude of 70 Kilometers (Image 25). This burn
in combination with drag in the atmosphere slows the first
stage down from 1,300m/s to about 250m/s.
Image 25: Falcon 9R’s Stage 1 re-entry burn at 70km altitude
The first stage aft section has been outfitted with shielding
material to be able to withstand the re-entry environment
and during atmospheric flight, the stage can maintain an
engine-forward position by its low center of gravity caused
by the heavy engine compartment in the aft.
At the beginning of re-entry the booster expands its four grid
fins for orientation control (Image 26), especially during the
periods of flight when the engines are not running. These
four grid fins are launched in a position stowed against the
uppermost section of the booster near the interstage.
Image 26: Falcon 9R’s grid fins used for landing accuracy
They can be controlled individually to allow for complex
guidance during the atmospheric flight without active
propulsion of the booster back to Earth. Grid fins perform
well in all velocity ranges including supersonic and subsonic
speeds (more than 1700m/s and less than 270m/s) with the
exception of the transonic regime (between 270 and
410m/s) due to the shock wave enveloping the grid. These
properties make them ideally suitable for the Falcon 9 first
stage. The addition of the grid fins to the Falcon 9 rocket
improved landing accuracy to the level of landing the first
stage to a mobile drone ship in the sea.
Image 27: Falcon 9R approaching the landing site in the sea
The Autonomous Spaceport Drone Ship (ASDS) (Image 28)
is a floating landing platform that measures 91 meters by 52
meters.
Image 28: The Autonomous Spaceport Drone Ship (ASDS) named “Just Read The Instructions”
It uses GPS data to keep its assigned position with an
impressive accuracy of only three meters. This high level of
accuracy is essential since the landing legs of the first stage
have a span of 18 meters, leaving just over 30 meters for GPS
errors between the two crafts, sea swell and errors by the
first stage.
The ASDS is outfitted with a water deluge system that
dumps water onto the deck to protect it from the heat of
the engine of the arriving booster. There are numerous
attachment fixtures on the deck structure that allow the
securing of the first stage after the landing on the platform.
Image 29: The ASDS
Homing in on its landing spot, the first stage ignites its center
engine to begin slowing down to a landing speed of just
about two meters per second and make the final cross-track
and range adjustments to land on the platform (Image 30).
8 Ten seconds into the landing burn, the four landing legs are
deployed to put the stage into its final landing configuration
with touchdown just a few seconds later.
Image 30: Falcon 9R’s Stage 1 a second before landing on the ASDS in the sea
Wait a second – landing legs on a rocket? (Image 31) Yes!
Falcon 9 Stage 1 has four deployable landing legs that are
tucked right next to body of the rocket and in the final stages
of landing they are
spread and locked into
landing position to
support the 20 tons
stage on landing. Each
landing leg weighs just
above 500 kg and is
protected by a
special paint.
We live in extraordinary times! You can use the power of
the sun and electricity to drive an electric automobile with
Tesla Motors or the power of the sun, solar panels and
battery storage capacity to power your home entirely on sun
power and completely cut yourself off the electric grid. We
are also witnessing the first steps of the human civilization
to become a space faring civilization the same way like a
hundred years ago we discovered that we can actually
invent flying machines! In a few decades it will be something
usual to go to a spaceport, pay your ticket to lower Earth
orbit or the Moon and go for a weekend holiday or to work
“upstairs”. When you are there, you will see the beauty of
our spaceship Earth – the only place that we have in the
Cosmos! This is a huge perspective changer! You will call a
relative of yours who lives on Mars for a few years now and
you will tell him how beautiful Earth is. All this will become
possible with the invention of reusable rockets such as
Falcon 9. Others will follow in the space sector as in the
energy consumption sector with electric automobiles and
the energy production sector with solar power and storage.
It is high time to start living like intelligent human race, not
like pigs!
A few centuries from now when our home will be the whole
solar system, we will look back and remember these first
puny attempts to get up from the cradle and leave Earth’s
atmosphere and reach to the stars. It is a story for the
books…
Tihomir Dimitrov
The Human Adventures in Space Exploration
www.humansinspace.org
Image 32: SpaceX Landing Complex 1 – the first rocket landing facility in the world
Image 31: Falcon 9R’s landing legs
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