How do iron engines
works? The most efficient propulsion
system out there.
Introduction.
People always ask me why we’re stuck with chemical rockets.
Seriously, exploding a bunch of hydrogen or kerosene is the best we can do?
Good news, there are other, exotic science fiction-sounding propulsion systems
out there which use electromagnetic fields to accelerate atoms, allowing their
spacecraft to accelerate for months at a time.
I’m talking about ion
engines, of course, and several spacecraft have already used these exotic
thrusters to perform some of the most amazing missions in the exploration of
the Solar System. I know, I know, chemical rockets seem really primitive. Take
tonnes of liquid or solid fuel, light it on fire with an oxidizer, and then use
the speed of the explosive gases to give you a kick in the opposite direction.
Thanks Newton’s Third Law. But chemical rockets do the trick. Those gases do
give a rocket the kick it needs to get into space.
Because they bring their own oxidizer
with them, they work in the atmosphere and they work in the airlessness of
space. The advantage of rockets is that they can deliver enormous amounts of
energy in short periods of time, the kind of reaction you need to blast tonnes
of cargo off Earth and into space. But they’re incredibly inefficient. A 550
metric tonne Falcon Heavy is carrying almost 400 tonnes of fuel and oxidizer. The
first stage will only burn for 162 seconds, and the second stage will fire for
397 seconds. That gives you a total burn time of about9.5 minutes.
Want to make more maneuvers?
Want to accelerate for days, weeks or even months? Too bad,
you’re out of fuel. Of course, these shortcomings from chemical rockets have
led scientists to search for other forms of propulsion, especially when you’re
out in space, and the one of the most successful so far is the ion thruster.
When you’re working out the rocket equation, an important factor is the
velocity that you’re ejecting your propellant. The most efficient chemical
rocket can throw hot gases out the back at 5 km/s. Ion engines, on the other
hand, can eject individual atoms 90 kilo meters a second. This high velocity
gives the spacecraft a much more efficient acceleration.
The best chemical rockets see a fuel efficiency of about
35%, while ion engines see an efficiency of 90%. So
how do ion thrusters work?
It’s actually pretty
weird, and totally sounds like science fiction. Instead of hot gases, ion
thrusters ejections. These are atoms or molecules which have an electrical
charge because they’ve lost or gained an electron. In the case of an ion
engine, they’re emitting positively charged ions which have lost an electron.
Once you’ve got ions, you can direct them with a magnetic field, accelerating
them into space at tremendous speeds. So where do they get all the ions? The
thrusters create them by generating a plasma inside the spacecraft. They
bombard neutral propellant atoms of some gas, like xenon with electrons. These
collisions release even more electrons from the propellant, turning them into
positively charged ions. This plasma soup of electrons and positively charged
ions has an overall neutral charge. The electrons are held in the chamber,
leading to more ionizing events, while the positive ions are siphoned out
through a grid at the end of the chamber.
As they pass through this grid, high voltage accelerates
them out of the back of the spacecraft at speeds of up to 90 km/s. For each
ionized particle that the spacecraft can kick out, it gets a tiny kick in
return. The whole system is powered by solar panels, so the spacecraft itself
doesn’t need to carry any kind of battery or power system, minimizing the total
weight it has to carry. The big problem is that that kick really is tiny. The
thrust of ion engines is measured in millinewtons, like, thousandths of a
Newton. Hold a piece of paper in your hand, that’s the kind of forces involved.
But they can operate for days, weeks, even months,
accelerating and accelerating long after chemical rockets would have run out of
fuel. So if you’re already out of the gravity well of a planet, they’re very
efficient engines for dramatic changes in velocity. NASA and other space
agencies have actually used ion engines very successfully in a range of
missions. They had been developing this thruster concept for decades but were
never willing to risk it on an active mission where a failure could end it. So
NASA gathered up a bunch of risky technologies, and packages them together as
the Deep Space 1 mission, which launched in 1998. Deep Space 1 was equipped
with 12 different technologies that NASA wanted to test out, including low
power electronics, solar concentrator arrays, various scientific instruments,
and a solar electric propulsion system.
Its engine was run
for enormous lengths of time, allowing it to make close observations of
asteroids and comets, and even Mars. NASA doubled down on the technology of
Deep Space 1, giving its Dawn Mission three redundant ion engines. These
allowed the spacecraft to go into orbit around the asteroid Vesta, make
observations, then break orbit and travel to asteroid Ceres and make even more
observations. And it could still have fuel in the tank to visit even more
asteroids. Just to give a sense of its acceleration, Dawn can go from 0 to 100
km/h in 4 days of continuous thrusting. Ion thrusters were used to carry ESA’s
Smart1 spacecraft from Earth orbit to lunar orbit, and on the Japanese Hayabusa
spacecraft. Ion engines have been tested here on Earth, and successfully
operated for more than 5 years continuously. With these successes, we’re going
to see even more spacecraft equipped with ion thrusters in the future, but ion
thrusters themselves are getting more powerful and resourceful. I said that ion
engines produce very little thrust, but there are some ideas to boost their
output.
The first is dramatically increase the amount of electricity
you’re using to accelerate the ions. Instead of solar panels, NASA considered
creating an ion engine powered by a nuclear reactor. About 15 years ago, NASA
considered a mission known as the Jupiter Icy Moons Orbiter mission. Powered by
the Nuclear Electric Xenon Ion System (or NEXIS) engine, the spacecraft would
be capable of exploring each of Jupiter’sicy large moons in sequence: Ganymede,
Call is to and Europa. The spacecraft would have been launched into orbit in
three separate pieces, which would then be assembled in Earth orbit and
launched off to Jupiter. The spacecraft would use its 8 ion thrusters to study
Call is to and then Ganymede for three months each, and then settle into a
final orbit around Europa. If conditions were right, it could even go into
orbit around Io. Of course, we don’t get to have nice things, and the mission
was cancelled back in 2005. There are other ways ion thrusters can be scaled
up. NASA is testing a high thrust version of ion engines known as the X3 hall
thruster. This engine is capable of blasting out ions, and produces 5.4 newtons
of force.
Again, not much, but remember that previous thrusters top
out in the thousandths of newtons. At the highest power levels, this could be the
technology that will carry human astronauts to Mars, cutting down the flight
times to just a few months. Engineers are planning to run the X3 for 100hour
tests this year to see if it has the same kind of long-term operation as the
smaller ion engines. The coolest idea
I’ve heard recently for ion engines is the idea of an air breathing engine
under development by the European Space Agency. Instead of carrying any
propellant at all engineers at ESA demonstrated that a spacecraft in low Earth
orbit should be able to pul in molecules of air right from the atmosphere, and
then ionize them and blast them back out. Since the spacecraft would be using
unlimited solar electricity for power, and pulling its propellant from the
atmosphere, it could operate without re fueling for long periods. Spacecraft
could operated at lower altitudes, and space stations could remain in low Earth
orbit indefinitely without needing to be re boosted. This is going to be real
game changer.
And not only Earth,
this technology could be used on Mars or Venus, or Titan. Anywhere with an
atmosphere. Ion engines have already made an impact on space exploration, and
in the next few years, we’re going to see more missions equipped with them.
They could even be the engines that carry human astronauts from Earth to Mars
in the coming decades. What do you think about ion engines? Let me know your
thoughts in the comments. Once a week I gather up all my space news into a
single email newsletter and send it out. It’s got pictures, brief highlights
about the story, and links so you can find out more. Go to
universetoday.com/newsletter to signup. And finally, here’s a playlist.
0 comments: