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Okay, I'm going to talk about what's special about hypersonic flight.
What's different to hypersonic flight than flight in any other flight regime and where do you
do hypersonic flight? Why would you travel so fast in the atmosphere,
where you get very hot and there's lots of friction around?
Well hypersonic flight - by implication you have to be in an atmosphere. Out in space you go
very fast but you don't really call it hypersonic flight unless you're in an
atmosphere. In going very fast for a lot of aerodynamic effects like: lift, drag, heat transfer.
They can be useful to you and they can be a disadvantage.
We'll look first about where do you fly, what's sort of things do you do
when you fly depending on what your mission has a strong influence
on the shape and the design of the flight vehicle you use.
We can categorise certain commonly used flight paths.
The most well known is the re-entry corridor when you come back from outer space
and it's a good idea to stop before you hit the ground because you're travelling very fast.
The cruise corridor: that's when you want to travel at a constant speed and just
travel places fast. The cruise corridor actually hardly exists at the moment because
we don't know how to do it in a sustained manner. And acceleration corridor relates to
when you're trying to accelerate to go through the hypersonic flight regime and get into space.
Again it's not something we do particularly at the moment because
we don't have a propulsion systems to facilitate it. And then there's more likely
the common thing is : sub-orbital 'space hops' - so you want to shoot something up
very fast, get out of the atmosphere and then come back in again at a
some distant location and travel very fast as you do it.
What you might think of a hypersonic flight trajectory actually isn't.
The normal way of getting into space is not to fly hypersonically,
you need to go at speeds in space that are much faster than hypersonic but actually
you're trying to do that acceleration outside the atmosphere. So the space shuttle
for instance, will have a normal Mach number of 25 when it's in
space it's very hypersonic. It leaves the atmosphere by about Mach 3,
it goes straight up and out of the atmosphere as quickly as it can
because it's not a good place to be when you're travelling very fast in an atmosphere -
lots of drag, lots of heat transfer, you're likely to burn up and self-destruct.
Well we've been using re-entry corridors for about 60 years since the first missiles went up
into space. The main function of a re-entry corridor is just to lose
speed and in doing that if you want to survive you got to control
your drag, your deceleration, you can easily be destroyed just
through too many G-forces and also heat transfer; lots of energy around which can
burn you up if you're not careful. There are several categories of ballistic trajectory
the ones we're going to look at here are the so-called ballistic trajectories
where we have only drag and no lift. Most flight trajectories would
actually make use of lift to a certain amount - like the Apollo coming in at a lifting trajectory
but the ballistic trajectories are very good because you can get analytical solutions for them
and we often use a ballistic trajectory as a starting point for a design study
cause you can get quite close to what a flight path might really be.
And you get analytical solutions which is a good thing to have at the start of the design
process, you know roughly where you're flying - which is how high and I guess which
atmosphere you're in - and how fast you're going and then you can have a stab at designing a
vehicle. So ballistic entry relates to using drag forces to slow yourself down.
The three basic categories within a ballistic trajectory : aerocapture is not
actually been done yet but we talk about it a lot and that relates to coming in from
outside of the Earth, and it was so-called hyperbolic trajectory outside of gravity's
influence and you have to slow down enough in the atmosphere
to get captured but not actually to land. Aerobraking can be used to adjust the
capture orbit by ducking down in the atmosphere for a little while
and losing some velocity and then going out again.
You might wonder why would you do that. Well if you want to change your orbit
and you don't use braking, you have to fire up rockets and
rockets are heavy and they have to carry fue. So it's a way of losing the speed
without having a propulsion system to do it. And most commonly used trajectory
of course is the total re-entry where you come in from outside and you stop and
you adjust things to get the right control of velocity along the way through the atmosphere.
Okay, so what sort of vehicle will we like to build for re-entry?
You know traditional aerodynamicists like to make things that look nice and sleek and sort of sexy
and they turned out - you know they don't work very well for re-entry purposes because
the purpose isn't to generate lift and fly long distances with minimum amount of fuel,
it's to stop. So typically re-entry vehicles are characterised by surfaces aligned normally to the flow,
so the force that acts on them is pushing against the direction of motion.
So in any field of engineering,
a good idea as a starting point when you go into a new application is to look at
what people did before and see if it's going to work,. If you don't do that,
you'll likely to reinvent the wheel.
So when we first had the capability to go hypersonically in the 50s,
we looked at what's the traditional aerodynamic shape and it's
like you see on the left there - it's something long and slender looking. And they'd
analyse that, they tested it in wind tunnels, and it really didn't work very well.
You can imagine if you use that long skinny thing as a brake,
the main surfaces you have for braking are friction surfaces so it's like trying to slide
down a rope,
you rub all the skin off your hands through friction and that's not the way to go.
So the big breakthrough came in the 50s when they decided you don't make
long slender things, you make blunt bodies and that's been the way we've
done it ever since. So it's sort of self-evident what you do is blunt, it
pushes the air out of the way, slows you down and the heat transfer is minimised.
Even if you look at something that's quite streamlined like the space shuttle,
on re-entry it flies more like a brake. It comes in at like 40 degrees to the
flight path and most of the surfaces are pointing forward. Obviously it generates
some lift as well which we're not going to analyse in this course, but it's a blunt body
dissipating heat. You see the same vehicle later when it comes in to land,
it flies in a very different mode. Here's an example of a flight vehicle coming through the atmosphere.
All the energy is being dissipated as radiation in the shock layer, you can see it glowing.
And when it lands and people come out, hopefully they survived, you can see
it's a very simple blunt body shape, it doesn't really look sophisticated
but actually technically it's a very advanced design. Getting people back from space
alive and safe, is not easy.
This is a classic design called the sphere-cone configuration,
shown here in a Titan entry capsule.
You want to make the thing as blunt as you can cause that's the most efficient shape
because if you make it blunt, you introduce another problem. All your drag forces are
on the front because your mass has to be behind the front surface so effectively its unstable as
if you try and balance something with all its weight beyond the point of support.
You see those front surfaces are flared back a bit and
by flaring them back, you reduce their efficiency as the aerobrake
a little bit but you're moving the center of drag further downstream so it's
easier to handle the stability. So that's the classic shape we use for re-entry
into the earth or other planets of high speeds as well.