This episode of Real Engineering is brought
to you by Skillshare, home to over 20,000 classes that could teach you a new life skill. Last month MIT revealed their ion propelled
plane. The product of 7 years of development and
the first of its kind. A plane capable of sustained powered flight
with no moving parts in it’s propulsion system. Just as the Wright Brothers announced to the
world that powered flight was possible, this flight lays down a milestone for ion drive
technology that could pave the way to future investment and development. It has the potential to drastically improve
propulsion technology. Having no moving parts is a benefit that cannot
be understated. Parts can be made lighter as they no longer
need to survive the stress of movement. Reduced stress means reduced maintenance and
costs, but perhaps the most immediate benefit we can garner from this technology is reduced
noise. With no noisy combustion or rotating aerodynamic
surfaces stirring up the air, these planes are like gliding owls. A characteristic military contractors will
be eager to take advantage of. But with current limitations, this may take
some time to come to market. Let’s investigate just how this new technology
works, and where it needs to improve in order to compete with current technology. This technology has been in development for
decades now with many spacecraft already using variations on the idea to achieve highly efficient
thrust systems. These engines work on a similar principle
to the ion propulsion of the MIT plane, albeit in a very different environment that lends
itself to the technology. Take the NSTAR ion drive aboard the now retired
Dawn spacecraft. This spacecraft used xenon as a propellant,
because it has a high atomic mass allowing it to provide more kick per atom, while being
inert and having a high storage density lending itself to long term storage on a spacecraft.[1]
The engine releases both xenon atoms and high energy electrons into the ionization chamber,
where they collide to produce a positive xenon atom and more electrons. These electrons are then collected by the
positively charged chamber walls, while the positive xenon atoms migrate towards the chamber
exit which contains two grids. A positive grid called the screen grid, and
a negative grid called the accelerator grid. The high electrical potential between these
grids causes the positive ions to accelerate and shoot out of the engine at speeds up to
145, 000 kilometres per hour. At that speed even the tiny xenon atoms can
provide a decent bit of thrust, but even still this engine provides a maximum of 92 milli
Newtons of force. About the same force a piece of paper will
exert while resting on your hand. But in the vacuum of space there is no air
to sap away the precious energy we provide. With no drag or friction to remove energy
we gradually build up our kinetic energy and gain speed. The dawn spacecraft weighed about 1220 kilograms
at launch with a dry mass of 750 kilograms after the propellant had been expended, so
lets say it has an average weight between the two of 1000 kilograms. Rearranging the force equals mass by acceleration
equation, we can calculate the acceleration this engine could provide at 0.000092 metres
per second squared. A tiny acceleration, but multiple by a week
(604800 seconds) and our spacecraft is flying at 55.6 m/s. Multiple it by a year and it’s flying at
2898 metres per second, that’s 8.5 mach. The latest generation ion drives, dubbed the
NEXT engine, can produce three times the force and has been tested continuously without stopping
for 6 years straight here on earth. That’s enough force to accelerate that 1000
kilograms to 44651 m/s, 130 times the speed of sound. [1] This is an incredible technology, that will
revolutionise how we explore space in the near future, but here on earth it has a completely
different set of challenges. Here on earth planes pose a completely different
challenge. Air will continuously sap away any energy
we input into our vehicle through drag, and so we need to create an ion drive that can
provide more energy than air can remove while travelling fast enough to achieve flight. Not an easy task and the fact MIT have managed
it is mind blowing. Let’s see how they did it. They first needed to optimize their plane
design for the application. Reducing weight to minimize the energy required
to maintain height, and minimising drag to reduce any energy losses to the air. They did this using something called geometric
programming optimization, which allows designers to specify constraints and design criteria
to a programme which will then find the optimal design. [2] After running multiple computer simulations
they settled on a plane with a 5 metre wingspan and a weight of 2.56 kilograms. It would require a flight speed of 4.8 metres
per second with a thrust of 3.2 Newtons [3]. 3.2 Newtons is vastly more than anything achieved
by NSTAR or NEXT engines, but they don’t work in entirely the same way. Ion drives for space need to carry atoms to
be bombarded, within earth's atmosphere there is no shortage of atoms to ionize and accelerate
and this helps counteract some of the negatives of the drag they also induce. The planes propulsion comes from an array
of ion drives carried below the wing. The positive anode was a thin steel wire,
which helped minimise the drag it induced. While the cathodes were foam aerofoils covered
in thin aluminium, these being light and capable of producing lift to offset their weight. In this case nitrogen is ionized and attracted
across the electric field induced by the 20 thousand volts of electric potential between
them. The nitrogen ions collide with neutral air
molecules along the way to a provide additional thrust. Creating something called ionic wind. Getting that 20 thousand volts of alternating
current is really the most difficult part and the team had to design their own lightweight
high-power voltage converter to step-up the 200 volts of direct current drawn from their
lithium polymer batteries. This energy storage conundrum, as explained
in my electric planes video, is the biggest challenge facing any technology like this. So how does this compare to conventional propulsion
methods regarding thrust to power ratios? A typical jet engine achieves a thrust to
power ratio of 3 Newtons per kilowatt, while helicopter rotors achieve a power to thrust
ratio of about 50 Newtons per kilowatt (N kw-1). This ion propelled plane is estimated to have
achieved a thrust to power ratio of 6.25 Newtons per kilowatt. So, if we could find a way of powering these
devices that didn’t require heavy batteries, could these ion propulsion engines be used? [3] Scaling these propulsion method is not easy,
and individual electrode pairs have their limit in the current they can pass between
them via ion transport, due to limits in voltage and choked flow within the electric field
[4], just as air can become choked within a constricted pipe. This affects something called the thrust density,
which is the area over which the thrust is applied. Jet Engines have a very high thrust density
at over 10,000 Newtons per metres squared, so we can produce a great about of force with
relatively low area. This plane achieved a thrust density of 3
Newtons per metre squared, so it is generating very little force over a very large area. We can just about manage to provide enough
force with 4 3 racks and 2 rows of these electrodes hanging far below the wing, for a plane this
light and slow. The issue here is the same issue that prevents
batteries from being a viable solution for planes, power requirements do not scale linearly
with the mass of the plane, they increase with the square of the mass. While the power requirements to overcome drag
increases with the cube of the velocity. So our ion propulsion power will need to scale
with it, but we cannot simply hang racks and racks of these electrodes beneath our plane. They, along with the structures required to
support them would cause far too much drag, and in turn flight surfaces would need to
scale to counteract the pitching moment this would cause, causing even more drag. This technology is still in its infancy and
there are engineers far more intelligent than I working to figure out ways to apply it. Just think that 114 years ago the Wright Flyer
managed to fly just 35 metres in it’s 11 second first flight. This ion propelled plane managed 55 metres
in 12 seconds, and who knows where we will be in 100 years time. We as a species are continually growing and
learning how to apply these principles. It’s a fundamental part of the human experience
to continuously develop your skillset, just as I have been developing my segue skills
by watching this course by wendover productions about “How to Make an Educational Video
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