VASIMR Plasma Rocket Puts Solar System In Reach

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. _NextBigFuture

The plasma source cell involves the main injection of neutral gas (typically hydrogen, or other light gases) to be turned into plasma and the ionization subsystem. The RF booster cell acts as an amplifier to further energize the plasma to the desired temperature using electromagnetic waves. The magnetic nozzle cell converts the energy of the plasma into directed motion and ultimately useful thrust._NASA
As long as humans are limited to chemical rocket propulsion with specific impulse around 500, we are stuck in this small part of the solar system. But with new rockets that have specific impulses in the tens of thousands, suddenly humans can think about exploring Jupiter's moons, and beyond. The VASIMR plasma rocket is an example of a high specific impulse rocket engine, with very good prospects, particularly when paired with a small nuclear reactor power source.

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_via_BrianWang