A Rocket for the 21st Century

Power Player / by Lee Billings /

Former astronaut Franklin Chang-Diaz explains how his plasma rocket engine could revolutionize space travel and why we need nuclear power in space.

Since the dawn of the Space Age in the mid-20th century, humans have visited the Moon, built orbital telescopes and space stations, and sent robots to explore the vast majority of the solar system’s real estate. Today, many developed nations are charting ambitious courses for their space programs. Some aim to send humans back to the Moon; others simply want to create their own national networks of communications and surveillance satellites. The US has developed plans to build a lunar base and to send astronauts to nearby asteroids, perhaps even to Mars. Yet surprisingly, almost all of these past, present, and planned achievements call for using rocket technology largely based on old ideas from the 1920s.

Dr. Franklin Chang-Diaz believes there’s a better, more modern way of doing business in space. And unlike many other starry-eyed space-age dreamers, Chang has considerable clout to back him up. A former NASA astronaut, he’s flown into orbit on a record seven space shuttle missions, including the one that sent the Galileo spacecraft to Jupiter. And for the past 30 years, Chang has been developing a concept that could revolutionize space travel and commerce: an advanced plasma rocket engine called VASIMR. After serving as the director of the Advanced Space Propulsion Laboratory at Johnson Space Center for over a decade, he retired from NASA in 2005 to form Ad Astra Rocket Company, dedicated to developing VASIMR. Ad Astra has since signed agreements with NASA to test VASIMR in space, and has successfully fired a prototype engine at full power on Earth. Seed’s Lee Billings spoke with Dr. Chang about VASIMR, the limitations of conventional rocketry, and his hopes for the future of human space exploration.

Seed: What exactly is VASIMR and how does it work?
Franklin Chang-Diaz: VASIMR stands for “Variable Specific Impulse Magnetoplasma Rocket.” It’s a plasma-based electric rocket engine, so it’s different from conventional chemical rockets, which are propelled by the combustion of rocket fuel. VASIMR isn’t based on chemical reactions. Instead, it uses plasma, which is a gas that’s been heated to extremely high temperatures, temperatures approaching that of the Sun. Because it’s so hot, the plasma can’t be handled with any conventional materials. We have to use superconductors to generate electromagnetic fields to contain the plasma, form it into a jet, and guide it out the back of the rocket engine. VASIMR is meant for use in outer space—it won’t replace chemical rockets for launching payloads into orbit.

Seed: Working with plasma sounds difficult. Why would you ever want to use it in a rocket?
FCD: There is a term in rocketry, “specific impulse,” which measures how efficiently a rocket obtains thrust from its propellant. The higher the specific impulse, the more efficient the rocket, and the less fuel it requires. In general, specific impulse increases as a rocket’s exhaust gets hotter. A good chemical rocket’s specific impulse is on the order of about 500. And the specific impulse of the VASIMR and most other plasma-based rockets is in the thousands, even the tens of thousands. So we’re talking about an orders-of-magnitude performance improvement of the rocket. That’s why we go to all the trouble of working with plasma, because there’s a huge payoff in terms of how much fuel you use to get any given payload from point A to point B in outer space.

Seed: Aren’t there other kinds of plasma engines already? How are they different from VASIMR?
FCD: There are other kinds, yes. In all plasma rockets, you have to produce thrust by accelerating the plasma. Other plasma rockets do this with electric current from metallic grids that are immersed in the plasma. Too much plasma flowing past these grids will make them essentially melt, so you can’t go to extremely high power. You can somewhat get around this by making the grids very large, or making arrays of them, but you’re still limited by grid erosion and damage. This means most plasma rockets are inherently low-power devices.

In VASIMR, however, there are no grids. Its plasma is contained by magnetic fields and heated and accelerated by electromagnetic waves. Since no parts of the rocket are immersed in the plasma flow, you can make the plasma very dense and hot and get much better performance.

Seed: How did you come up with this idea?
FCD: VASIMR is an example of the need for cross-pollination between disciplines to spark new ideas and new technologies. It came from research in controlled thermonuclear fusion, and in particular from a device called a “magnetic diverter,” which was the subject of my PhD thesis when I was at MIT in 1977. I’d always been interested in propulsion, and realized that this technology was suited for rocketry, but there wasn’t much work being done anywhere else. Back then I was always surprised to find that people who were working on fusion and plasma physics weren’t paying attention to what was going on in propulsion research, and vice-versa. It almost looked to me like time had stood still for these folks. They were pursuing old ideas, they weren’t communicating. Things have changed now, of course.

Seed: Have they? We’ve been sending people and machines into space for more than half a century, but we’re still mostly using chemical rockets.
FCD: Well, part of the problem with electric propulsion back then, and to a lesser degree today, is that it’s hard to get enough electricity to power the rocket. Typically, electricity in space comes from sunlight, solar power. That works okay in Earth orbit and other places close to the Sun. But people have to realize sooner or later that, if we’re ever going to explore Mars and beyond, we have to make a commitment to developing high-power electricity sources for space. What we really need is nuclear power to generate electricity in space. If we don’t develop it, we might as well quit, because we’re not going to go very far. Nuclear power is central to any robust and realistic human exploration of space. People don’t really talk about this at NASA. Everybody is still avoiding facing this because of widespread anti-nuclear sentiment.

Seed: What has to happen to make that change?
FCD: In 1958, the first nuclear submarine, the USS Nautilus, was able to actually navigate under the north polar cap and surface on the other side. No other submarine had ever been able to do that before. It was an eye-opener, a game-changer, a paradigm shift. The idea was that nuclear power enabled a completely different class of missions for these types of ships. Now, nuclear submarines are common. Something similar has to happen in space.

In fact, with the power close to what a nuclear submarine generates, you could use VASIMR to fly humans to Mars in 39 days. A chemical rocket makes the trip in eight months. That’s eight months of exposing your astronauts to debilitating cosmic radiation and weightlessness. By the time they get to where they’re supposed to work, they’re gonna be in bad shape—almost invalids! They’ll have to spend a big chunk of their time just recovering from the trip. That’s simply not a smart way to conduct an exploration program. By not addressing the key problems of limited power and propulsion, NASA is forced to work with extremely complicated and expensive mission architectures that are very limited in capability.

Seed: So you believe that in the long run it would be more cost-effective to develop nuclear-electric capabilities in space, even given potential regulatory difficulties?
FCD: Absolutely. People have fears of nuclear power in space, but it’s a fear that isn’t really based on any organized and clear assessment of the true risks and costs. When you send these missions based on chemical propulsion to Mars, they aren’t only going to be extremely expensive, but also extremely fragile. Imagine being on Earth, watching astronauts on an eight-month death trip from which there is no return, all because they made a small mistake or something failed. It would be an agonizing process, and there would be a lot of questions asked if you lost a crew. Well, in space, power is life. You can plan against a lot of contingencies by simply having more power available for a crew to use.

Seed: What’s the timeline for Ad Astra’s plan? How do we get a VASIMR rocket that takes people to Mars in 39 days?
FCD: Once we’ve demonstrated a 200-kilowatt prototype engine operating at full power on the ground, the next step is testing an identical version in space. We’re already testing the prototype unit in our vacuum chamber here in Houston, and we’re designing the actual flight engine, which is called the VF-200. We signed an agreement with NASA last December to actually mount the VF-200 on the International Space Station in 2012 or 2013. Unfortunately, the space station doesn’t have 200 kilowatts to give us. So what we’ll do is use the solar arrays of the station to charge a battery pack that we’ll carry on board, which will allow us to fire the rocket at 200 kilowatts for up to 15 minutes. We’ll do this again and again for months to qualify the engine in space. In 2013 or 2014, we’ll make clusters of 200-kilowatt engines to give us something close to a megawatt of electricity, and deploy them with a very high-powered solar array. This will be a robotic reusable “space tug” that can refuel or reposition satellites, or even send packages to the Moon at a much lower price. By charging for those services, we hope to bootstrap our way into developing a megawatt-class rocket. That rocket would be too powerful to test on the ISS, but it could perhaps be tested on the surface of the Moon where solar power is abundant. Like the ISS tests, we’d fire the megawatt-class VASIMR continuously for a period of one month, then two months, to validate and verify that it could be used on a human mission to Mars.

But once we have this capability, Mars isn’t really the only place that we can go. With a megawatt-class VASIMR, basically we will have access to the entire solar system. Mars is an interesting place, but so are Europa and Ganymede and Enceladus and Titan. These are places where we might find extraterrestrial life. Even with the 200-kilowatt solar-powered VASIMR we could do amazing things. We’re developing a concept to drive it close to the Sun, between Venus and Mercury, where it can get a momentum boost and catapult a probe into the outer solar system at high speed. This would let us deliver a package to Jupiter in one-and-a-half years; otherwise that trip takes about six.

Seed: That all sounds nice, but can you justify any of this in terms of dollars and cents?
FCD: It’s quite simple—we’ve done the calculations. One thing we’d like to do is maintain the ISS in orbit. The ISS has to be reboosted every few months; otherwise it gradually falls and burns up in the atmosphere. These reboosts require about 7 metric tons of rocket fuel per year. How much does it cost to get 7 metric tons of rocket fuel into orbit? $140 million. That’s the bill someone has to pay, each year, just for hauling up the fuel. The 200-kilowatt solar-powered VASIMR can do the same thing with about 320 kilograms of argon gas per year, which still costs about $7 million, but it decreases the price by a factor of 20. Of course, we have to make a little money ourselves, so the price decrease won’t be quite that large, but it can still save NASA a lot of money and net us a handy profit. That’s just one case—there are many others.

Seed: But the larger versions of VASIMR really need nuclear power to perform. What if NASA chooses not to develop space-based nuclear power?
FCD: We would hope that, if not the US, maybe the Europeans, the Chinese, the Russians, or somebody else will develop a nuclear-electric power capability that we can marry up to this rocket. We have to realize that the US is no longer the only player. The US may choose not to do this, but that doesn’t mean the rest of the world will follow—not anymore. We no longer live in a confrontational world like the one that fueled the Apollo program in the 1960s. We live in a world that has to cooperate, to collaborate. The US has a tremendous opportunity to still be the leader here, but if it isn’t, others will be. Information is traveling faster everywhere now; technology has gained a foothold and developed in the nooks and crannies of the planet. The world has changed, and the US no longer has a monopoly on knowledge. We need to collaborate to build a capable space infrastructure so that we can truly explore.

Originally published September 29, 2009

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