The Race to Mars: Electric Propulsion To Replace Rocket Fuel

Getting There

Mark Watney has an easier time in the film version of The Martian than he does in the book. But both versions of Mark Watney have a much easier time than what an astronaut today, in 2015, would have if they were stranded on Mars. In that case, humanity (not just NASA) wouldn't even have a spacecraft capable of orchestrating a rescue mission. 

Is this what a future Mars spacecraft will look like?

The Hermes spacecraft from the film.
Image Credit: The Martian (2015)

While Watney's predicament is hypothetical, the depiction of his survival on Mars is the most realistic account of near-future space technology ever conjured up—for the simple reason that it's based on technologies either currently under development or actively being used in space exploration today.

NASA is working toward its goal of putting humans on Mars sometime in the 2030's. In Part 1 of this series, I focused on the technologies required to get humans from the surface of Earth into deep space: a powerful rocket and a next-generation space capsule.

But in order to travel through deep space and actually reach Mars, NASA will need a specially-designed engine capable of generating a large amount of thrust without using an absurd amount of fuel.

The Looming End of Liquid-Fueled Spacecraft?

Since the 1960's, manned spaceflight has only been possible by burning flammable liquids as fuel. In space. But now it's 2015, and the green-energy revolution is booming. With major economies around the world pledging to replace fossil fuels with solar panels in the coming decades, the way we generate energy is changing—both on the ground and space. 

Our solar system already has a giant power plant at its center—the Sun. We just need to harvest some of its energy.
Image Credit: NASA/JPL-Caltech/KSC

Let's not get it twisted—this change in space exploration technology isn't predicated by environmental concerns. The space agencies of the world aren't too concerned about emissions and space warming (and many governments aren't worried about global warming, either). The main reason humanity is beginning this shift towards 'green' energy has less to do with the environment and more to do with cost and efficiency

Since everything we launch into space comes with a substantial price-tag, energy-efficiency is naturally of great importance to the aerospace industry. And that's why electric propulsion technologies are poised to become the new norm. 

We're still a long way off from electric rockets

Even today, the propulsion needed to lift a spacecraft from the surface of Earth can only be achieved by burning tens or hundreds of thousands of liters of combustible fuel in a short amount of time. 

The unmanned Apollo 4 launching on the first Saturn V rocket.
Image Credit: NASA
Fact: the Saturn V rocket used 770,000 liters of kerosene in its first stage (plus 1.2 million liters of liquid oxygen), which was consumed in under 3 minutes. This fuel lifted the craft to an altitude of 68 kilometers and a velocity of 2.7 kilometers per second.
That's an average acceleration of 0 to 100 km/hr in 1.72 seconds—faster than any production car. Nonstop. For 3 minutes. 
Also, in a metal tower that weighs 2.8 million kilograms. That's the weight equivalent to 500 killer whales being blasted into space. Space whales. 
The second and third stages would do the rest, eventually bringing the craft to a height of around 200 kilometers and a velocity of nearly 8 kilometers per second.

This explosive power can't yet be replicated by any alternative fuel source, and it's necessary in order to escape the gravity well and atmosphere of Earth. But once a spacecraft gets into space, it needs a different type of propulsion system to get it the rest of the way—one with less fire and more mileage. 

Using traditional means, a mission to Mars would require multiple stages of thrust: leaving Earth orbit on the correct trajectory, entering Martian orbit for an extended stay, leaving Martian orbit on a trajectory back to Earth, and finally re-entering Earth orbit and/or atmosphere with at least the crew-module still intact. That's an unprecedented number of course adjustments for a manned mission. 

But NASA did it with Apollo, right? Sort of. The Apollo capsule only needed to take astronauts part-way to the Moon, and then the Moon's gravity did the rest of the work. The astronauts essentially 'fell' into orbit around the Moon and, when leaving a few days later, initiated a short engine burn that provided just enough of a push to put them back on a trajectory towards Earth. Then they crashed into the atmosphere and softly landed in the ocean. The entire mission was only possible because they stretched the bounds of energy-efficiency, gaining most of their momentum through gravity and using the friction of the atmosphere to slow down during re-entry. 

A functional ion engine undergoing tests.
Image Credit: NASA
Fact: at no point in any of the Apollo missions was the Sun's gravity pulling on the spacecraft more than the Earth or the Moon's gravity.
But in order to get to Mars, a spacecraft will need to traverse through deep space—falling around the Sun, beyond the gravitational influence of Earth or any other planet. 

It took Apollo three days to get from Earth to the Moon. With a distance of only 384,000 kilometers from Earth, that's an average of about 128,000 kilometers in 24 hours—or a relative velocity of 1.5 kilometers per second. The closest Mars ever gets to Earth is 54.6 million kilometers away, which would end up being a 427-day journey for a spacecraft moving at Apollo-speed—about 14 months. In a straight line...

But (and it's a big but), in 14 months, Earth would have completed more than one full orbit around the Sun. In 14 months, Mars would have completed nearly two-thirds of its orbit around the Sun, meaning it would literally be on the other side of the Solar System by the time an Apollo-speed spacecraft reached the place it had been at 14 months prior.

A 427 day Apollo-speed journey would only be possible if the planets didn't move. In fact, at Apollo-speed, no spacecraft would ever reach Mars—the steady pull of the Sun's gravity would eventually slow the spacecraft to such a point that it would fall directly into the Sun and die in a fusion-powered explosion. 

Therefore, the Journey to Mars will require significantly more thrust than was used by the Apollo missions of the 60's and 70's. And the length of the journey will require that the craft be significantly heavier than Apollo, weighed down by the supplies the astronauts will need to survive the duration of the mission. At best, the manned journey to Mars would require an insane amount of rocket fuel in order to reach the red planet in any reasonable amount of time. 

Falling to Mars isn't an option.
We need a spacecraft that can Go there.

Weighing in at 900 kilograms, NASA's Curiosity rover is the most impressive object ever put on another planet by mankind. Its journey to Mars took 253 days, over which time it traveled 566 million kilometers—that's an average relative velocity of 25 kilometers per second, an order of magnitude greater than the Apollo-speed that carried astronauts to the Moon. The reason it traveled so far is because Earth and Mars move around the sun so rapidly that any spacecraft aiming for Mars has to 'catch' the red planet as it moves along.

Fact: no spacecraft has ever gone into orbit around another planet and then returned to Earth.
The Russian/European Fobos-Grunt sample-return mission to a moon of Mars would've become the first, but it failed before it could leave Earth orbit. Space is hard.

A return journey to Earth would require more than twice as much rocket fuel as was used getting to Mars in the first place. Liquid fuel is extremely heavy (and volatile) for the amount of thrust it provides.

As a result, the more fuel a spacecraft carries, the heavier the spacecraft becomes, and therefore the more fuel it needs in order to get anywhere. Suddenly, carrying enough fuel for a return journey to Earth ends up requiring three times as much fuel as would be needed for a one-way trip, most of it spent just getting the return-fuel to the place it's going to be used to return from.

But then an alternative fuel source was found...

Ion Propulsion Changes Everything

Space agencies around the world have been investing in a type of propulsion system that's over 10 times more efficient per kilogram than traditional rocket fuel. Instead of rapidly burning fuel in order to produce a huge amount of thrust all at once, ion propulsion delivers a small, continuous push for months (or years) at a time.

Fact: in The Martian, the spacecraft that ferries humans between Earth and Mars uses ion engines that accelerate the spacecraft at a constant 2 millimeters per second.
That's a 0 to 100km/hr time of almost 4 hours. But, after 6 days of continuous operation, this would equate to one kilometer per second of added velocity. After two months, 10 kilometers per second—and so on. 

Ion propulsion isn't a far-off concept still under development—it's already been in use aboard NASA's Dawn spacecraft, breaking numerous records with its unique propulsion capabilities. In 2014, Dawn became the first spacecraft to have orbited two different objects in our solar system (other than Earth), first with its exploration of the dwarf planet Vesta, and subsequently with its rendezvous and continuous orbit of the dwarf planet Ceres—two different objects located in the asteroid belt between Mars and Jupiter. 

The Dawn spacecraft approaching the dwarf planet Vesta.
Image Credit: NASA/JPL-Caltech

Dawn is also expected to generate the equivalent of over 10 kilometers per second of velocity change over the duration of its mission, more than any other spacecraft in history, by using only 425 kilograms of propellant. To put that in perspective, it required a Delta II rocket loaded with 40,000 liters of fuel to lift Dawn into space with a starting velocity of 10 kilometers per second. But Dawn launched back in 2007, and NASA's been busy.

What's next? NASA's Evolutionary Xenon Thruster (NEXT), of course. Three times as powerful as the previous ion engine and 12 times as efficient as any liquid fuel, NEXT has been under development since before Dawn even left the launch pad. NASA has already completed a test in which NEXT ran for over 5 continuous years at a testing facility, and the agency plans to use the engine to power a future 4,000 kilogram exploration probe in the near future. 

Ion engines are the future of space exploration, and using them will allow NASA to a) free up a significant amount of weight normally reserved for liquid fuel and b) ensure the safety of its astronauts by avoiding the use of combustible liquids and instead using reliable engines that can run continuously for the duration of any mission.

Scaling up

Like traditional engines, ion engines can also be scaled up in size and power output. Magnetoplasma Dynamic Thrusters are currently under development by a variety of different space agencies (Russia has tested a 100-kilowatt version, which is about 50 times more energy than what Dawn has been operating with), a technology that generates plasma for use in propulsion. Similar in concept to ion engines, plasma engines would be powerful enough to get a human crew to Mars in only a few months while using 20 to 30 times less fuel than regular liquid-fueled engines. 

But of course, there's a catch. Since liquid-fueled engines use a chemical reaction to create propulsion, they don't require any energy input in order to run (other than the spark of ignition). But electric engines do—and lots of it.

A theoretical next-generation plasma engine.
Image Credit: Princeton University/Edgar Y. Choueiri

In order to power an electric space engine as powerful as the one seen in The Martian, we would need a spacecraft capable of generating hundreds of kilowatts of power, likely using solar arrays, dedicated exclusively to propulsion. The complication is that, while the International Space Station has enough solar panels to cover an entire football field, those panels only generate about 84 kilowatts of power—not enough to power a large electric engine, let alone the rest of the spacecraft's instruments. 

This is part two of a three-part series. Check the rest out below:


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