HOUSTON — Franklin Chang-Díaz jumps up a handful of stairs and peers through a porthole cut into the side of a silver tanker-sized vacuum chamber. Inside, a bluish-purple light shines, unchanging and constant, like a bright flashlight. “It looks kind of boring,” Chang-Díaz admits. “But that plume is 3.5 million degrees. If you put your hand in there, it would be very bad.”
Honestly, the plume doesn’t look impressive at all. And yet the engine that ignites in the vacuum chamber is potentially revolutionary for two simple reasons: First, unlike gas-guzzling conventional rocket engines, it requires little fuel. And second, this engine could one day push spacecraft to speeds sufficient to open up the solar system to human exploration.
This has long been the promise of Chang-Díaz’s plasma-based VASIMR rocket engine. From a theoretical physics standpoint, the rocket has always seemed like a reasonable proposition: generate a plasma, excite it, and then push it out of a nozzle at high speed. But what about the real-world engineering of actually building such an engine – managing the plasma and its thermal properties, then firing it successfully over a long period of time? That has proved challenging, and it has led many to question the usefulness of the engine.
Sure, say the naysayers, Chang-Díaz is a great guy. Hard worker. Brilliant guy. And at a time when national discourse erodes the value of Hispanic immigrants, his story provides a powerful counterpoint to that narrative. He spoke almost no English at the time and emigrated from Costa Rica to the United States in 1969 to finish high school. Chang-Díaz went on to earn a PhD in plasma physics from the Massachusetts Institute of Technology. Later, as an astronaut, Chang-Díaz flew seven Space Shuttle missions, tying Jerry Ross’s record for most spaceflights by anyone. ever.
All the while, from his early days at Johnson Space Center, when he set up an early Internet connection to work with data from his plasma physics lab in Boston, Chang-Díaz harbored dreams of linking his science background to his career as an aviator. Slowly he developed the theory of a plasma rocket and started building prototypes. All the while, the critics whispered that it just wasn’t feasible.
Only, now it just might. As part of a program to develop next-generation space propulsion systems, NASA awarded Chang-Díaz’s company, Ad Astra, a three-year contract worth $9 million in 2015. doubters wrong. Of course it won’t be easy. Ad Astra must fire its plasma rocket for 100 hours, with a power of 100 kilowatts, The next year.
By February, the company was about halfway through that contract, and Ars is monitoring progress in the lab. So far, the immigrant from Costa Rica seems to be keeping his promise. NASA gave the company an Excellent rating after the first year of the agreement. Still, there is a way to go. During a visit this month, the VASIMR engine fired at 100 kW for 10 seconds and at 50 kW for one minute.
The rocket engine starts with a neutral gas as raw material for plasma, in this case argon. The rocket’s first stage ionizes the argon, turning it into a relatively “cold” plasma. The engine then injects the plasma into the second stage, the “booster”, where it is subjected to a physical phenomenon known as ion cyclotron resonance heating. Essentially, the booster uses a radio frequency that excites and swings the ions back and forth.
As the ions resonate and gain energy, they are spun into a stream of superheated plasma. This stream then passes through a corkscrew-shaped nozzle and is accelerated out the back of the rocket, creating thrust.
Such an engine design offers a number of important advantages over most existing propulsion technologies. Perhaps most notably, unlike chemical rockets, the plasma rocket runs on electricity. Because it flies through space, it therefore does not need huge fuel tanks or a huge reservoir of liquid hydrogen and oxygen. Instead, the rocket just needs some solar panels.
The sun drives both plasma production and the booster that excites the plasma, and the extent to which this is the case can be shifted. When a spacecraft needs more thrust, more power can be put into creating plasma. This process uses more propellant, but it provides the thrust needed to get out of a gravity source, such as orbit. Later, when the vehicle is moving fast, more power can be sent to the booster, resulting in a higher specific impulse and lower fuel consumption.
“It’s like changing gears in a car,” explains Chang-Díaz. “The engine does not change. But if you want to climb a hill, you put more of your engine power into torque and less into revs, so you climb the hill slowly, but you can climb. And if you go on a highway, flat and straight, you shift up. You’re not going to Mars in first gear. That is the problem. That is why we will run out of petrol when we go to Mars with a chemical engine.”
Another advantage of the engine design is that the plasma remains confined in a magnetic field, inside the engine, during combustion. In practice, this should significantly reduce engine wear, which is useful if you’re designing a spacecraft to eventually fly humans throughout the solar system.