This is part two of my video on sustained hypersonic flight, watch part one to learn
about the difficulties of finding the right material to build a hypersonic plane.
You’re watching FutureNow, videos about the future of tech and society.
Following the materials and heat issue, although not unrelated, is that of what engine do you
use on a hypersonic plane to maximize fuel efficiency and actually achieve the necessary
speed?
There are four kinds of engines one might consider: turbojet, ramjet, scramjet, and
rocket engines which we already discarded as an option at the beginning of this video.
In my video on the comeback of supersonic planes, I discuss why a turbojet engine is
best to achieve supersonic speeds.
However, a turbojet engine doesn’t cut it if you want to fly at hypersonic speeds because
it caps out around Mach 3.5, short of the Mach 5 requirement to go hypersonic.
A turbojet works by taking in air through the front bypassing it through the compressor
which slows the air down from supersonic to subsonic speeds by the time it reaches the
combustion chamber, where it is mixed with fuel and ignited.
This causes the gas mixture to expand rapidly, pass through the turbine which powers the
combustor, and exit through the exhaust which provides the plane with thrust.
The hotter this gas is, the more thrust is provided.
By slowing the supersonic air to subsonic speeds, the compressor is raising its pressure
and temperature, so the faster the plane travels, the hotter the gas gets, the more thrust is
provided.
But the turbine which powers the compressor will fail if temperatures are much too high,
which is what limits a turbojet’s speed to roughly Mach 3.5.
However, if you’re flying fast enough, simply slowing the speed of the air down is enough
to sufficiently heat up the air and provide the necessary thrust.
This means, you no longer need a compressor and by proxy you can get rid of the turbine
as well.
What’s left is called a ramjet engine.
A ramjet has to already be travelling at about Mach 3 to work, because the speed of the air
has to be fast enough to produce sufficient pressure and heat when slowed down.
A ramjet has a conical inlet at the front, which forces the supersonic air to slow down
to subsonic speeds, this is called the “ram effect” hence the name.
From there it’s injected with fuel and burned to create the thrust.
However, eventually the ramjet reaches speeds that create too much heat when slowed down
to subsonic speeds to combust properly.
But once again the answer is, just go faster!
If you only slow the supersonic air a bit and keep it at supersonic speeds instead of
slowing it down to subsonic, you still get some rise in pressure and temperature and
can attempt to ignite the fuel in supersonic air to create thrust.
Which is exactly what the supersonic combustion ramjet or scramjet engine does.
A scramjet is essentially just an inlet that takes in supersonic air, mixes fuel into it,
burns it, and exhausts it to create thrust.
One of the coolest things about ramjets and scramjets are they have basically no moving
parts, you can look right through them from one end to the other with no fans, compressors,
or turbines to get in the way.
But as cool as they are, they have some pretty glaring problems when considering them for
use on a hypersonic plane.
One being that trying to ignite fuel mixed into air flowing at supersonic speeds is like
trying to light a match in a tornado.
Regular jet fuel does not combust quickly enough to work at these speeds and so a scramjet
would need to be fueled by hydrogen.
Hydrogen is highly flammable and can be ignited with very little energy while occupying quite
a small volume of the air.
This makes it a perfect candidate for fueling a hypersonic plane, which needs to mix the
fuel into the air and combust it in under a millisecond before it exists the engine.
As a bonus, burning hydrogen doesn’t produce pollutants like carbon based fuels and so
it’s much more environmentally friendly.
A hydrogen-fueled engine does, however, increase the complexity of the plane as it needs to
be compressed due to the very low density of hydrogen, meaning the fuel tank would have
to be much larger.
Ways of solving this issue include cooling and pressurizing it to the point where it
becomes more of an almost liquid slush than a gas.
Of course the main and most obvious issue with using a scramjet engine is, you can’t
take off from the runway with it.
It’s a useless drag creator until the plane hits Mach 4 or above at which point the air
entering the engine is fast enough to produce thrust.
So basically, there is currently no one engine type that could be used for sustained hypersonic
flight.
NASA and others who are testing hypersonic plane models have been using two or even three
stage systems.
NASA’s X-51A plane is attached to a rocket booster which is carried on a B-52 plane.
The B-52 carries them until nearly supersonic speed around 20,000 feet, then releases the
X-51A with the booster rocket attached which accelerates to about Mach 5 and an altitude
of 100,000 feet, at which point it finally releases the X-51A which can use its scramjet
engine at that point because it’s travelling fast enough.
Of course this is a fairly ridiculous and expensive process akin to a space shuttle
launch.
The real mission here is to have a system that does not require several stages that
are released, but rather one that is self-contained.
The answer is a variable engine that can switch from subsonic to supersonic to hypersonic
as needed.
Much to the United States’ discomfort, the Chinese have made vast advances in hypersonic
flight in the last few years, including in the area of variable engines.
In 2015, a Chinese aviation news organization published an article showing designs for a
turbo-ramjet engine, which was quickly pulled as apparently these design schematics were
not meant to be public.
Or at least that’s what China wants us to think, that’s global political PR for you.
Here’s how the design works based off the schematics they leaked.
As you can see this basically looks like a regular turbojet engine with a big spike on
the front.
In turbojet mode, air flows around the spike and through the wide inlet that directs air
into the turbojet combustor and generally works as normal.
But once it achieves roughly Mach 2 speeds, it transforms into a ramjet engine with some
pretty simple but clever redirectors of air flow.
These air intake ramps on the spike open up and block most of the air flow, forcing it
to enter at higher pressure as I described earlier.
At the same time, these little doors drop down and block off the turbojet core.
And suddenly it’s a ramjet engine, flying with no combustor or turbine, the only moving
parts being the fuel injection happening at the rear.
Of course, this is a ramjet design, so it’s not enough to go hypersonic, so we’re safe
from hypersonic Chinese missiles and bomber planes right?
Well, actually, the very same month in 2015, Professor Wang Zhenguo of the National Defense
Science and Technology University won an award in China for successfully developing China’s
first scramjet engine and made China only the second country after the U.S. to have
completed a successful test flight of a scramjet engine.
This means if they are able to combine these two engine designs and get their turbo-ramjet
to transition into the scramjet they can achieve 0 speed to hypersonic flight with a self-contained
engine system.
And it seems China is not playing around, China announced that they are opening a factory
to build engines to power hypersonic missiles and spaceplanes.
Because these are of course meant for military and space missions, not commercial Earth travel,
they actually are using rockets in these engines and they could become the world’s first
variable cycle engines to fly by 2025.
That’s less than a decade away, which is pretty buckwild for this technology.
Here’s what that engine looks like, and its long-ass but very descriptive name is
turbo-aided rocket-augmented ram/scramjet engine or TRRE.
I’m assuming that’s not what they call it in China.
It’s my understanding this engine could work for achieving basic hypersonic speeds
without the rockets, but of course just going Mach 5 isn’t going to cut it for something
like space travel.
While, as an American, this news is rather scary, I have to say their hypersonic airplane
design is something else.
It looks like this.
And speaking of which, the final major challenge of developing a hypersonic plane is in fact
designing it.
More than any other kind of transport, a hypersonic plane requires multidisciplinary considerations
to be made simultaneously in its design and cannot simply take the approach of designing
the engine, wings, body, etc. of the plane separately and then putting them together
which is how many existing planes were designed.
Every single inch of a hypersonic plane must be carefully considered as it has an effect
on the whole.
The design must balance efficient propulsion, complex aerodynamics including surviving extreme
heat, the structural integrity of the plane, and its controllability.
This is why conceptual designs for hypersonic planes look unlike any existing plane you’ve
ever seen.
This has lead to the creation of a new highly integrated method for optimizing designs called
multidisciplinary design optimization or MDO, which allows design engineers to consider
all these different disciplines simultaneously.
MDO is a whole field unto itself, but the basics of it are engineers take a baseline
design and create a computer model of it.
They run this through computer programs that use some complex math to predict the performance
of the design and then suggest optimizations of multiple areas simultaneously.
There are literally hundreds of billions of potential optimizations than can be made to
this baseline design, so using computers to narrow this down is incredibly important.
Through analyzing the best results and choosing one to become the new baseline and then running
through this process over and over again, designers can eventually land on a design
that works and test a physical model and eventually a prototype.
Obviously something that pretty much everyone would say is a must when designing a plane
is wings.
But this isn’t necessarily true.
In fact, once you’re going hypersonic, wings are very much not helpful in flying.
This is mainly because of the immense pressure a wing structure would have to sustain, which
would require it to be incredibly strong meaning much heavier.
So many hypersonic planes don’t have wings at all or very slight ones.
This kind of design is called a lifting body, because all the necessary lift to keep it
flying is provided by the body of the aircraft.
NASA pioneered this kind of design in the 1960s and 70s with the mission to create a
smaller manned space plane that could actually generate lift during re-entry, rather than
just falling to Earth.
A popular lifting body design is called the waverider, so called because it allows the
plane to actually ride on its own shock wave produced by its leading edges and nose.
The plane must be designed in such a way as to allow the shockwave to attach itself to
the bottom of the plane.
In a regular convex or flat-bottom plane design, air flow would spill around the plane, but
the concave or conical waverider design traps the airflow beneath it, producing more lift
and saving fuel.
Imagine you have a soup bowl and you’re trying to keep it in the air by shooting it
with a strong water hose.
If you shoot the water at the bottom of the bowl, the water will simply flow around the
sides and likely cause the bowl to fall off the stream.
Now consider what would happen if you turn the bowl upside down.
Now the water is curving along the inside when it hits, and keeping it afloat longer.
That’s basically what a waverider is doing with airflow, except the design process is
incredibly complex, because there isn’t just one shockwave it has to account for,
but rather various layers interacting with each other.
And these are the reasons we don’t yet have hypersonic planes or even hypersonic missiles
being produced.
But we’re closer than we’ve ever been and estimates for when they’ll arrive are
as soon as a decade.
How do you think hypersonic technology will change our world?
Let me know in the comments.
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