How to land Space Shuttle
How to Land the Space Shuttle... from Space
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How to land the space shuttle from space.
So, quick disclaimer, this talk is only 20
minutes. So I only have time to give you
an intuition for how landing works. If
you actually want to go fly the real
shuttle, please make sure you read the
owners manual. Also you're gonna need a
time machine because the last shuttle
landed over five years ago they live in
museums now they're completely
unflyable.
However, I, like all of you, have been
living in a state of denial for the last
five years. Especially you Steve Feldman.
So, in my world, the shuttle still flies
and we're just gonna use present tense
for this talk.
Alright, so let's get started. Our goal is
to land on a runway at Kennedy Space
Center in Florida, but let's say right
now we're orbiting over South America
traveling over 17,000 miles [an hour] in the wrong
direction.
Well, we can't just turn around. Changing
direction in orbit takes crazy amounts
of energy. So what do we do?
Well, basically nothing. So it turns out
that the earth spins? Which means that
Kennedy Space Center is just gonna come
to us if we wait for it.
So this time around, when we come up to
Kennedy Space Center we're just gonna
stop!
It always does this. So it turns out that
we're still traveling over 17,000
miles-an-hour. To give you some
perspective of how fast that is, the
runway that we're gonna land on is
15,000 feet long. That's about three miles
or maybe 40 to 45 football fields,
depending on what you consider a
football field.
It's one of the longest runways in the
world, but at our current speed we're
going to travel the entire length of it
in just six tenths of a second. We could
get from New York to London in just 12
minutes. So we need to slow down a lot.
Well, the shuttle's got great engines -
plenty of power to slow us down with. So
let's just fire them up again! Well... so this is
kind of embarrassing. See, we're sort of
out of gas. Womp womp. In our defense launch is
like really expensive. Those two boosters
on the side, they burn 1.1 million pounds or
five hundred thousand kilograms of solid
fuel in just two minutes, and then we
just throw them away. That big orange
external tank holds another 1.6 million
pounds, or seven hundred twenty-five
thousand kilograms, of liquid fuel for
the shuttles three main engines, but
after an eight-minute launch, those are
empty too. So we have to ditch it. Bye! All
we've got left are these wimpy little
orbital maneuvering engines, which,
combined, produce less than 1% the
thrust of the main engines. They're not
going to slow us down 17,000 miles an
hour, but there's a trick. We don't
actually have to slow down by that much.
If we slow down by just 225 miles an
hour, that's enough for us to start
falling into the atmosphere where air
resistance can do the rest of the work.
So we perform our de-orbit burn which
lasts about three minutes with our
orbital maneuvering engines. After that,
we're just going to coast for about a
half hour before we reach the atmosphere.
But we can't go in the atmosphere
backwards! First off,
we would look ridiculous, but possibly
more importantly the air resistance is
so great that we would essentially melt.
So we pitch up to 40 degrees angle of
attack.
That's the angle between where your
velocity is taking you versus where your
nose is pointed. At this angle, our easily
meltable aluminum airframe can be
protected by over 20,000 silica tiles, as
well as these reinforced carbon-carbon
panels on the nose and leading edge of
the wings.
Fun fact, the surfaces of the orbiter
which don't get hot are covered by these
thermal blankets as well as a nomex
felt fabric that goes over the wings and
the payload doors. It's really nothing
like a normal airplane, but anyway back
to entry. So, if all went well, we should
hit the first traces the atmosphere at
four hundred thousand feet about 5,000
miles from our landing site. This is all
good, but after a few minutes there
starts to be a little bit of a problem.
We've got wings! And wings generate lift,
and as we get into denser air they
generate so much lift that we're
actually gonna start to go back up, and
skip off the atmosphere.
This is kind of bad. We really want to
keep going down. So, we could just pitch up.
That would create more drag and less
lift, but we risk overheating
overstressing or just outright losing
control of the orbiter. So we can't
change our angle of attack, which means
we can't change how much lift we
generate. However we can change which way
at points. It doesn't have to point up. If we
roll to the right or left, we can point
our lift sideways, instead of up.
Well, this will effectively let us
control how fast we're descending. With a
steeper bank angle, we're going to
generate less upward lift, so we're going
to descend faster.
Conversely, with a shallow bank angle,
we're going to generate more upward lift,
so we're not going to fall as fast.
But that brings up an interesting question
of how fast do we want to descend? Well,
re-entry is basically a big energy
management problem. We have a lot of
velocity, and a lot of distance to cover.
So the goal is to bleed off that
velocity at just the right rate so that
we cover the right distance. If we slow
down too fast,
we won't make it to the landing site, and
if we don't slow down fast enough, we'll
shoot right past the Kennedy Space
Center and crash out in the Atlantic
Ocean, which is also bad. So, in order to
control descent we figured out that we
just need to change our bank angle, but
how do we control deceleration (how fast
were slowing down)? Well, remember that the
whole reason we're slowing down in the
first place is because we're running
into the air. So if we want to slow down
faster,
what we really need is more air, and
where is there more air?
Well, lower in the atmosphere - the
atmosphere gets denser has you go down.
So in a sense we kinda already did
figure out the right tools to control
decceleration, because if we bank
heavier that means we're going to descend
faster, as we already know, so we're going
to reach thick air faster, and the thick
air is going to help us slow down faster.
Conversely, if we bank shallower then
we're not going to descend as fast, so we're
going to stay in the thin air longer,
which means we're not going to slow down
as fast. So, there's just one last problem,
we're kind of starting to turn. This bank
angle thing isn't working out as well as
we originally hoped. So NASA goes to its
engineers and says, "this is a really big
problem. We can't just land in Panama!" And
the engineers say, "well, just turn the other
way.
This isn't rocket science,
and why are you wasting our time
Steve?" So granted this creates kind of
this weavy S-turn reentry path, but it
works. So before we go any further, let's
review what we just learned. So we start
with our deorbit burn and that lasts for
about three minutes. After that we coast
towards the atmosphere and while we do
that, we pick up to 40 degrees angle
attack so our heat shield can protect us.
Once we get into the atmosphere, we
control everything with bank angle. If it
looks like we're going to overshoot the
runway, then we bank heavier, so that we
slow down faster. And if it looks like we're
going to not make it, then we bank less,
so we don't slow down as fast. And also,
every time we get turn too far away from
our target, we just turn the other way in
this series of what's called role
reversals. [laughter] That's what NASA calls it.
This is the reentry... a picture
of the re-entry of STS-135, the last space
shuttle. Something interesting about
these reentry flames:
that's not technically fire, although it
kinda looks like. It's essentially a
really hot gas, that's so hot that
electrons break away from their atoms
and molecules and they start to glow
this soft orange color. It's a different
state of matter called plasma, which, even
if you never heard of it, you've seen it
all the time in the form of neon signs,
lightning, most importantly the Sun is a
big glowing ball of plasma. Now as we
slow down we get less of this plasma, and
we have less heat, so we're less
concerned about melting. But we get more
and more concerned with just falling
out of the air.
We really to transition from spaceship to
airplane. So at 8,000 miles an hour we
start bringing the nose down, lowering
our angle of attack. Then at 1,700 miles
an hour, we switch into a completely
different guidance mode called Terminal
Area Energy Management, or TAEM. Now
we're flying like an airplane. A really
bad airplane. We have no engines, but we
we sort of function like an airplane. We
pitch to control our descent rate. We
bank to turn, and we've also got this
speed brake thing that can open and
close to help us control our airspeed. so
also up until this point we've
been running on autopilot. An autopilot
run by five of these redundant computers,
each with a whole megabyte of memory.
You couldn't even fit a single cell
phone photo on one of these, but it was
pretty good at flying the shuttle. But as
we get towards the runway the
commander takes over manual flying and
this mode is called CSS, for control stick steering.
Not cascading style sheets. Granted, the
shuttle is fly-by-wire, which actually
means that the computers run everything
all the time. Even in CSS, it's really
just the computer pretending to let the
humans fly, just like normal life.
Side note, no shuttle pilot wants to be called
a co-pilot.
That's just insulting. So in the left
seat,
we've got the commander who does the
flying. And in the right seat,
we've got the pilot, not flying.
I'm not totally convinced that NASA
doesn't just do this to confuse the
media, because it works really well, but
back to TAEM. So TAEM actually flies us past
the runway centerline and then around
this imaginary spiral called the Heading
Alignment Cone. If all goes well, we
should be lined up with runway and on
glide slope by 10,000 feet in altitude.
Course, if we were a typical airliner, "on
glide slope" would mean a 3-degree
descent path flown at about a hundred
and sixty miles an hour with a descent
rate of about 750 feet per minute. But
that's not going to work for us. The
shuttle has stubby little wings and a big
fat round nose. It's affectionately
referred to as a flying brick.
NASA astronauts train in a modified
Gulfstream II jet, which, in order to
simulate how unaerodynamic the shuttle
is, flies with his landing gear down and
its engines in reverse. So we're going to
need a bit more brick-friendly glide
slope of 20 degrees flown at 345 miles
an hour would with a descent rate over
10,000 feet per minute. To give you some
context of how fast a descent rate that is,
10,000 feet per minute is about a
hundred and twenty miles an hour. That's
terminal velocity for a skydiver in
freefall. Obviously we can't land like
that, so at 2,000 feet we start pitching
up to bring the nose up in what's called
a preflare maneuver. This trade the
energy that we have in the form of
airspeed in exchange for slowing our
crazy descent rate. The landing gear
comes down at 300 feet. We wait until
this last minute because the gear
creates a lot of drag, and, once lowered
in flight,
it can't be raised again. We cross the
runway just 26 feet, airspeed bleeding
off like crazy.
We touch down at 225 miles an hour, the
drag chute is deployed, the nose gear is
gradually lowered down. Just an hour and
five minutes since we performed our
deorbit burn on the other side of the
planet, we've landed the Space Shuttle.
From space... obviously, where else would
you land it from? [applause]
So I will leave you with what this looks
like from the pilot's perspective,
because I'm a pilot, and I think this is
the coolest thing ever.
Of course, no one I've ever shown it to
also agrees that it's the coolest thing
ever, but i'm hoping Steve will. This is the
night landing of STS-115. We are flying
around the heading alignment cone right
now. We're looking through the pilot's
heads-up display - that's what all the the
green numbers passing by are. On the left
there is air speed. We're somewhere
between 260 and 270 knots. On the right
is altitude. We're passing through 28,000
feet right now. In just a moment, from the
top, you're going to see the east coast
of Florida come into view. That's the lights
near/south of the Kennedy Space Center.
In the very center of the screen there
is a square with kind of a fuzzy diamond
going in and out of that. That diamond
represents guidance. So what the commander is
trying to do right now is essentially
fly that box over the diamond, and that
will keep the shuttle on the right
descent path and around the heading
alignment cone. Also that box is going to
turn into a circle after a little bit...
doesn't matter too much.
Well it matters, but I don't want to
explain it.
At the bottom, which is now disappeared
because the controls have been opened
apparently, there is a thing it says CSS,
and above that it says HDG for heading.
That's the heading alignment cone and to
the right there's a horizontal line with
a couple of triangles pointed at it. The top
triangle represents the speed brake
where it currently is right now. So it's
open about maybe seventy percent, and the
bottom triangle represents where the
computer wants it to be, which is the
same right now. You'll see that making
adjustments as we go,
and it'll make a big adjustment at 3,000
feet (shortly before landing). There's the
runway coming into view, and from 10,000
feet,
I'm just gonna let the astronauts talk
for themselves, because I think it's a
lot more interesting. The main voice that
you're going to hear is the pilot talking the
commander through landing.
Pilot (PLT): "Correcting"
Mission Specialist 2: "Body flap trail."
PLT: "There you go, 9000"
PLT: "Still two and two, look good"
Commander (CDR): "I agree."
PLT: "8000"
CDR: "Little bit of light crosswind on the deck."
PLT: "7000"
PLT: "You look good."
CDR: "I agree."
PLT: "6000"
PLT: "Okay, 5000. My radar's good,
and your radar's good."
CDR: "I agree."
PLT: "I'm gonna declutter down, and I'm
with you at 3... just about 3000."
CDR: "3000. Speed brakes."
PLT: "... speed brakes are moving to
looks like about 27."
CDR: "Okay"
PLT: "Okay, 2000. Preflare. The gear is armed."
CDR: "Copy, preflare."
PLT: "I see you in the preflare.
I see you lagging a little bit. Looks good.
1000. Max speed 313.
700
600
500
400"
CDR: "Gear down."
PLT: "Here comes the gear.
Gear's moving. I show you
coming down on the ball-bar.
You can turn your HUD up a little bit,
if you haven't.
Showing just a little bit high."
CDR: "I agree."
PLT: "Little bit high, there's a hundred feet
255. Plenty of energy.
Correcting nicely. There's 50.
I see the nose coming up.
30, 230.
K, not too high, not too yet.
There we go. We got 22, 10.
You can start setting it down.
There we go. 7, 6, 5, 4, 3
Touch. Here comes the chute."
CDR: "Derotating."
PLT: "And I show you going down at one and a half.
Down at one and a half.
Down at one and a half.
Okay, touch."
PRESENTER: So, remember there's no
engines available, so this is their one
and only chance at landing.
I'd also like to point out that this
video started about three and a half
minutes ago at 37,000 feet. That's a
pretty typical cruising altitude for an
airliner. So just think about the captain
of your airline saying, "ladies and
gentlemen we are beginning our initial
descent into Philadelphia (or whatever).
We'll be on the ground shortly." And by
"shortly", he means three-and-a-half minutes.
But that's the way that the shuttle flew
and that's it. Thank you.
[applause]
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