ALL SCI-FI

[Gord Green]

NASA’s longshot bet on a revolutionary rocket may be about to pay off

Chang-Díaz's plasma-based VASIMR rocket engine

The rocket

The rocket engine starts with a neutral gas as a feedstock for plasma, in this case argon. The first stage of the rocket ionizes the argon and turns it into a relatively "cold" plasma. The engine then injects the plasma into the second stage, the "booster" where it is subjected to a physics phenomenon known as ion cyclotron resonance heating. Essentially, the booster uses a radio frequency that excites the ions, swinging them back and forth.

As the ions resonate and gain more energy, they are spun up into a stream of superheated plasma. This stream then passes through a corkscrew-shaped nozzle and is accelerated out of the back of the rocket, producing a thrust.

Such an engine design offers a couple of key benefits over most existing propulsion technology. Perhaps most notably, unlike chemical rockets, the plasma rocket operates on electricity. As it flies through space, therefore, it does not need massive fuel tanks or a huge reservoir of liquid hydrogen and oxygen fuel. Instead, the rocket just needs some solar panels.

The Sun powers both the production of plasma and the booster exciting the plasma, and the extent to which it does either can be shifted. When a spacecraft needs more thrust, more power can be put into making plasma. This process uses more propellant, but it provides the thrust needed to move out of a gravity well, such as Earth orbit. Later, when the vehicle is moving quickly, more power can be shifted to the booster, providing a higher specific impulse and greater fuel economy.

"It's like shifting gears in a car," Chang-Díaz explained. "The engine doesn't change. But if you want to climb a hill, you put more of your engine power into torque and less into rpm, so you climb the hill, slowly, but you're able to climb. And when you're going on a freeway, flat and straight, you upshift. You're not going to go to Mars in first gear. That"s the problem. It's why we run out of gas going to Mars with a chemical engine."

Another benefit of the engine's design is that the plasma remains confined within a magnetic field, inside the engine, throughout the burn. In practical terms, this should greatly reduce the wear and tear on the engine — which is useful if you're designing a spacecraft to eventually fly people around the entire Solar System.

Solar electric propulsion

If the concept of using solar power to propel spacecraft sounds familiar, that's because it is. Both the United States and the Soviet Union began doing theoretical work with Hall-effect thrusters five decades ago, and the Russians were the first to develop working models. With this technology, electric power is used to ionize propellant (typically Xenon) and accelerate this propellant into an exhaust plume of plasma to push the spacecraft forward.

NASA has used Hall thrusters on a handful of robotic missions, most notably the Dawn spacecraft, which has explored Vesta and Ceres in the asteroid belt. Three Hall thrusters powered Dawn, operating at a power of 10 kilowatts from the spacecraft's solar arrays and producing a thrust of 90 millinewtons. This thrust is minuscule compared to the thrust of a chemical rocket engine, which might reach 500 newtons. However, unlike the large amounts of propellant consumed by a chemical engine, the Hall thruster sips its fuel and can be fired almost continuously.

The promise of solar electric propulsion, then, is significantly smaller engines, that require significantly less fuel yet can, over time, accelerate to velocities equivalent to conventional rocket engines. For Hall thrusters, there is a limit to the power a single thruster can accommodate and therefore a limit on the ultimate velocity a spacecraft can attain. This constrains their use to pushing cargo around the Solar System; they simply move too slowly for crew.

To attain higher power, it is possible to cluster Hall thrusters together, and that's what NASA plans to do with the Asteroid Robotic Redirect Mission (ARRM). This proposed mission would send a solar-powered spacecraft to snag a boulder from the surface of an asteroid and return it to the vicinity of the Moon, where astronauts could fly up and study it. While Congress and the new Trump administration are likely to scrap this mission, some solar electric propulsion demonstration mission is still likely, with an operating power of 40kW.

Even that moderate amount of power probably approaches the upper limit of capabilities for existing Hall thrusters, which NASA envisions as being useful for flying consumables and hardware to Mars in advance of human landings. For the VASIMR technology, however, 40kw represents a lower limit to its power capacity, and the engine has the potential to scale up dramatically.

Among its other advantages, Chang-D??az's rocket also produces about twice as much impulse per unit of fuel, meaning its uses less propellant to do the same job as Hall thrusters. The problem is that the new technology has never flown, and because NASA has already invested heavily in developing Hall thrusters, an existing community that stands to lose if the plasma rocket works. That's why the relatively small grant to Ad Astra in 2015 was such a big deal.

"NASA has spent a great deal of money on Hall thrusters," Chang-Díaz said. "We are the latecomers. But now NASA is paying attention. They realize this is a rocket that has a lot of performance and a lot of scaling potential to a very high power. Overall, it hasn't been easy to penetrate NASA from the outside. When you disrupt, you disrupt the status quo, which means funding for other programs."

What NASA wants

Why did NASA finally decide to take a chance on Chang-Díaz? The doubters have cynical explanations. One senior manager at the agency noted that the NASA administrator in 2015 was Charles Bolden, a former astronaut who was a crewmate with Chang-Díaz on two shuttle flights in 1986 and 1994. "Charlie was just doing a favor for a friend," this critic said.

But there are believers, too. One of them is Jason Crusan, director of Advanced Exploration Systems for NASA, who is managing the contract won by Ad Astra. In addition to this award in 2015, the agency also provided a similar amount of "advanced propulsion" money to Aerojet Rocketdyne to work on "nested" Hall thrusters, as well as a lesser amount to MSNW to develop another kind of electric propulsion engine. The goal is to create an engine operating in the range of 300kW.

Crusan said NASA wants to bring electric propulsion into the trade space for human transport to Mars. This might mean a combination of a chemical rocket and an electric engine. It might also lead to an even more powerful electric engine. Right now, it may not be clear whether any of the experimental approaches will work, but NASA would like to find out, for a modest investment, before it gets serious about planning human missions to Mars.

"I fundamentally believe this is an area where there's a proper role for government investment," Crusan said. "These technologies may or may not work out. But there's a huge upside."

Proven, powerful electric propulsion would lower the cost of human landings on Mars. NASA already has plenty of experience with chemical rocket engines and in-space propulsion, but such missions would require multiple launches of the Space Launch System rocket to provide all of the fuel needed for a Mars journey. A gas-sipping, electric engine for in-space propulsion would require far less fuel and fewer launches from Earth to pre-position rocket fuel.

To determine whether any of the electric approaches is feasible, the agency has set a rigid requirement of firing a 100kW engine for 100 continuous hours by mid-2018. "At that point in time you either do it or you don't," Crusan said. "This gets rid of a lot of ambiguity, because you can't really game a 100-hour test."

A space truck

After leaving NASA about a decade ago, Chang-D??az set up shop in a warehouse-like building in Clear Lake, just a couple of miles from Johnson Space Center. The building is nondescript, hidden behind a strip mall that includes a Japanese steakhouse and a weight-loss center. The funding from NASA has allowed Ad Astra to upgrade these facilities.

The funds have gone toward improving the vacuum chamber and the system for removing heat from the engine while maintaining the vacuum inside — no easy matter when your engine is throwing off plasma at 3.5 million degrees. (Coincidentally, there have been interesting discussions with the local fire department.) Ad Astra has also built a new prototype of the engine that its engineers believe is capable of firing for the requisite 100 hours.

So far the company has met all of NASA's milestones. It has tested the new plasma generator and begun short firings of the booster. Chang-D??az said the engine's new modifications are coping well with managing the heat generated by the plasma. The 100-hour test looms large, but Chang-D??az retains a boyish optimism. He's 66 now but remains sharp and in shape, as if ready to fly were NASA to call upon his experience again. His life's work has led him to this precipice, and he's ready to jump.

It's been a struggle to keep the company going, but in the last decade or so, Ad Astra has raised $30 million from private investors. And now, NASA's goal of developing a 300kw engine dovetails nicely with Ad Astra's short-term business plans. The company anticipates this initial version of the VASIMR engine as the foundation of a "space truck" to provide transport for cargo near Earth. "Our investors see VASIMR as the engine that can do the trucking between Earth, the Moon, and cislunar space," Chang-Díaz said. "It's a diesel engine."

For this work, Ad Astra has competition from a chemical rocket engine. United Launch Alliance, a well-established rocket builder backed by Boeing and Lockheed Martin, is developing a reusable upper-stage rocket engine called ACES. Unlike conventional upper-stage engines, which quickly burn their propellant and then are discarded, ACES is designed to be re-fillable. The idea is that the engine could deliver a payload to the Moon, return to Earth orbit, and be filled again.

This is a fairly revolutionary idea for in-space propulsion, especially because ACES could form a key part of a closed-loop transportation system — mine ice from the Moon, convert it into liquid hydrogen and oxygen rocket fuel, and then re-fill your spacecraft. However, Ad Astra says it has a key advantage over ACES: fuel economy.

As he tried to explain the difference between a chemical rocket engine and his engine, Chang-Díaz compared a spacecraft to a small fishing boat. "Imagine that you're in the middle of a lake and don't have any oars," he said. "Instead, you carry a bunch of bowling balls. You throw one, and you move a little bit."

At this point, he laughed at the ridiculousness of his analogy but then pressed on. 'You could move your boat this way. But you've got to have a huge amount of bowling balls sitting there in your boat. Or, you could have a high powered rifle with little bullets, and you fire the rifle. You don't have to carry that many bullets. It's a simple action and reaction. When you are a rocket designer, you have a choice on how you make your thrust. You either choose to throw a lot of material at a low velocity, or you throw a little bit of material at a high velocity."

The thrust exit velocity for the best chemical rocket engine is about 5 km/s. For the VASIMR rocket, the exit velocity is closer to 50 km/s. If your material is exiting 10 times faster, you have to expend 10 times less of it for the same thrust. So instead of carrying a bunch of bowling balls in space, why not carry a much larger payload?

Mars in 39 days

For now, the skepticism toward Ad Astra is understandable. Virtually every news headline about the company's efforts to develop a plasma engine have focused on a single, fantastical number —39 days to Mars. While such a low transit time between Earth and Mars is theoretically possible with a much larger and more powerful VASIMR engine, there is one big catch: it would require a nuclear reactor in space to provide enough power to reach Mars that quickly.

NASA has had some abortive attempts, such as the Prometheus project, to develop nuclear energy for in-space propulsion. But due to the politics and sensitivity surrounding nuclear energy, the agency has never gotten very far down the path toward deploying some kind of in-space propulsion system driven by nuclear power.

Based upon current technology, Chang-Díaz figures that large but manageable solar arrays could eventually provide up to 1 megawatt of energy for electric propulsion. But that is the value at Earth's distance from the Sun, and solar energy really falls off beyond Mars. So solar power seems to be good for transport in the inner Solar System. For areas beyond Mars, solar really won't work.

Even with limitless chemical energy for conventional in-space rocket engines, it would still take several months to get humans to Mars, requiring more food for those on board and increasing the risk of harmful radiation doses. Travel times for destinations beyond Mars simply aren't practical.

Looking to the future, Chang-Díaz believes that serious exploration of the Solar System by humans will require more efficient and faster propulsion. Almost certainly, he thinks that will be based on nuclear power. And he's betting on his engine to harness that power and convert it into thrust.

"Eventually, when the nuclear technology comes of age, when there is a space-based nuclear reactor, we'll have the engine ready and developed," Chang-Díaz said. "Then you'll have yourself a nuclear powered rocket ready to go."

Perhaps one day. For now, NASA would settle for a smaller, Sun-powered rocket that can fire continuously, without overheating. After all, it would be a really bad day for a crew on the way to Mars to suddenly find themselves with an engine that just melted.