WEBVTT

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(computerized music)

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- Speed is everything.

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When the distance between
you and your target

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is fixed,

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the only way to get to your target sooner,

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is by going faster.

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Hypersonic flight is going at least

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five times the speed of sound.

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That is Mach 5 or above.

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If you're already going at Mach 5

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you can go from LA

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to Washington

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in about 40 minutes.

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A hypersonic glide vehicle,

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shown in red,

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spends most of its time

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in the atmosphere.

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In contrast,

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an ICBM,

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shown in blue,

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goes through the atmosphere on its way up,

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and on its way down.

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And if you're wondering

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the Shuttle and the Apollo
capsule also only travel

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through the atmosphere

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and spend most of their time in space.

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Traveling through the
atmosphere has big advantages.

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First, as you can see in
the diagonal in the figure,

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a radar can detect an ICBM on its way up.

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That means that you can detect it.

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You have time to react.

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But, you can only detect an
atmospheric hypersonic vehicle

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much later in its trajectory,

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which means much less time to react.

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In other words, you can't
stop what you can't see.

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Another advantage is the speed itself.

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Some have called
hypersonics the new stealth,

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because the faster you go,

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the harder you are to catch.

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And,

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you are traveling in the atmosphere,

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which means you're maneuverable,

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which means you can change trajectory.

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And so you are less
predictable than an ICBM.

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But all of these
advantages come at a price.

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By going so fast in the atmosphere,

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you are now at the mercy
at aerothermal dynamics.

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And the air is not kind to
objects ripping through it

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at hypersonic speeds.

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My own fascination with
going as fast as we can,

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started when I was in elementary school.

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But when I was 22 years old,

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and literally making my final
decision for where to go

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for grad school,

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I

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felt the building rumble

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at Cal Tech,

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because their hypersonic
tunnel had just gone off,

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and that was it.

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I knew that I was going to
spend the rest of my career,

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trying to study aerothermal dynamics,

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born of hypersonic flight.

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So it's twenty years later,

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and I'm still in love,

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and I'm still chasing solutions

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to key aerothermal dynamic challenges.

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Predicting drag and heating,

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measuring the atmospheric environment

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where these vehicles are going to fly,

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and creating new diagnostics and sensors

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to measure what we already
know are key flow parameters.

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For the first challenge,

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drag and heating,

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let's look at two vehicles.

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Now,

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I am a government employee,

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and so I can play favorites,

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so I'm showing you both,

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a Lockheed and a Boeing concept.

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(laughs)

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But if you think back
to the Apollo Capsule,

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you immediately see that these
vehicles are very different.

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For these vehicles,

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high lift and low drag matter.

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They have wings.

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And you need to understand how hot

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their surfaces are going to get

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at every point of the trajectory,

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because

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you don't want to end up like this.

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A burning meteor,

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which clearly was not designed
by an aero dynamicist.

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(audience laughs)

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Surprisingly,

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most of the heating and the
drag to a hypersonic vehicle

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gets set by a thin layer of
air that touches the vehicle.

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We call that thin layer,

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a boundary layer,

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because it's a boundary

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between the solid surface of the vehicle

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and the main airflow around the vehicle.

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And here's a kick,

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that thin layer of air,

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can be well organized,

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what we called laminar,

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or it can be quite
chaotic and disorganized,

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what we call turbulent.

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To give you an example,

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at the end of one meter cone,

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sharp slender at Mach 5,

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a laminar boundary layer is 2 milometers.

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A turbulent boundary
layer is 10 milometers.

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If the boundary is well
organized and laminar,

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it means less heating,

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less drag,

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and so the designers are happy.

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But of course,

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laminar boundary layers
are prone to disturbances,

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from the atmosphere,

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from wind tunnels,

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and eventually

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they become turbulent.

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They act as filters.

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They selectively choose what
disturbances to amplify.

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But we don't want them
to become turbulent,

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because a turbulent boundary layer

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brings with it three to
eight times the heating load

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than a laminar boundary layer has.

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Three to eight times.

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And if you don't know

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where in your vehicle that
jump is going to happen,

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or how big that jump is,

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then you are forced to carry margin,

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which means that you haven't achieved

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an optimized solution for a vehicle.

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What you're seeing here is an actual movie

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taken at a hypersonic wind tunnel

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at Texas A&M,

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of a canonical geometry.

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That color denotes temperature.

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So the hotter it is, it's yellow.

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And you're looking at the boundary layer

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getting hotter and hotter

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as it goes from a laminar
state to a turbulent state

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towards the end of the model

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and towards the end of the test time.

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I think intuitively you can guess

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that the surface beneath
a turbulent boundary layer

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is much hotter

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than the surface under a
laminar boundary layer.

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So the challenge here

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is to predict this heating
and drag to a vehicle.

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While

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we might not have a choice

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on a boundary layer transitioning

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from a laminar to a turbulent state,

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we can try to understand the process

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from basic principals.

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And we have two teams,

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one is going to fly
this geometry next year,

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and another team is flying the
same geometry the year after,

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so that we can test our hypotheses

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against flight data which
is actually very rare.

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The second challenge
has to do with measuring

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the atmospheric environment
where we are going to fly.

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Some have called that range of altitudes

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where we are going to
fly the "ignorosphere".

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because,

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it's higher than where
commercial or military airplanes

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fly,

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and it's of not great concern
to space weather or satellite.

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But think about this,

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for some shapes,

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it is disturbances in the
centimeter or milimeter scale

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either in the density, the pressure,

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the velocity feel in the atmosphere

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that make a boundary
layer trip to turbulent.

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Now do you think that
atmospheric scientists

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worried about predicting
hurricanes and tornadoes for us,

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care about centimeter scales?

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No.

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Because hurricanes and
tornadoes are measured

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in hundreds of meters,

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or kilometers.

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Particulars are another concern,

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irregardless of whether
they're manmade or natural,

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we know that particulars

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are very good trips of the boundary layer.

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Some scientists think

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that it's going to be particulars,

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the main cause to trip us

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and to give us higher heating in flight.

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And finally,

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raindrops coming at you on
your way up or your way down

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at supersonic speeds or higher,

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can cause a lot of damage.

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Personally,

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I think they shouldn't be
called raindrops anymore,

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it's a water attack in your vehicle.

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(audience laughs)

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And so the challenge is to integrate

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what we know of atmospheric
turbulence shown in the left,

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into our highest fidelity calculations.

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So we have two teams,

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two multi-disciplinary teams
deploying high-altitude baloons

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all over the United States right now,

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because they're trying to bring down

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these atmospheric models to the milimeter,

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centimeter scale that we need,

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while at the same time,

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we are trying to adapt
our high computations

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so that they can talk and
they can get this information

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from the atmospheric
models for new simulations

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of hypersonic flow.

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And the last challenge

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is

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how to create measurments.

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How to create diagnostics.

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Because optical diagnostics

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and sensors

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are our eyes

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and ears

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into hypersonic flow.

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The problem is

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that the wind tunnels provide
a very harsh environment.

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So you see I'm petit,

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and this allowed me to
fit in the test section

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of the hypersonic wind tunnel

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where I completed about 200
hundred rounds to do my Ph.D,

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which held the record for many years.

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I am also the first woman
that got a Ph.D in that lab,

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and twenty years later

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there is another woman in the pipeline,

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and she is going to get her
Ph.D in a couple of years.

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After twenty years.

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Anyways,

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after every test,

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which lasted one millisecond ,

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one millisecond ,

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I opened the wind tunnel and
I climb into the test section

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to inspect the model.

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Many times to find out

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that these sensors that I
had carefuly manufactured,

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and I promise you I had
carefully manufactured them,

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and I had put them into the model,

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they had died.

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Or that particulates coming
at the model at Mach 5.5

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had pitted the model,

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and I had to either re-polish it

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or take it and send it
to the machine shops

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so that they could re-machine it.

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With all these challenges,

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it's not easy to just take any technique

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that works in subsonic flow

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and take it to a hypersonic environment.

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So most of the time we either
have to invent new techniques

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or we have to heavily

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adapt

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techniques that work
well at subsonic flow.

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Good news,

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a few years back,

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a young professor
invented a new technique,

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to measure velocity

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in a Mach

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10

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flow.

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So he's measuring speeds

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of more than one kilometer per second.

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And it's not exaggeration to tell you

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that the community had been
waiting for such innovation

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for about 40 years.

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Next,

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we want to measure gas composition

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at hundreds of kilohertz

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hundreds of kilohertz

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and we want to make even
simpler measurements in flight.

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So let's go back to the beginning.

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Speed is everything.

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Because,

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any nation that has an
atmospheric hypersonic vehicle

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can reach targets faster
and with less detection.

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Yes, the problems that
I mentioned are hard,

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and oh by the way they're old too.

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But as we speak right now,

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we have teams of brilliant people,

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with critical mass

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working in the lab,

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working in the field,

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working in front of their computers,

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asking the right questions,

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and finding answers.

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So I am convinced that
we are in the right path

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to clear any aerothermal
dynamic challenges

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to confidently fly in the
atmosphere at hypersonic speeds.

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Thank you.

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(applause)