An Interview with John Brophy
Meet Dawn's Ion Propulsion System Manager, John Brophy.The following interview took place in Dulles, Virginia on December 14, 2004, between John Brophy and Education and Public Outreach team member Jacinta Behne (Mid-continent Research for Education and Learning—McREL)
McREL: Please share with us your name and your role that you play in the mission.
JB: My name is John Brophy, and I am the Project Element Manager for the Ion Propulsion System for the Dawn spacecraft. I currently work at the Jet Propulsion Laboratory (JPL) in Pasadena, California. “Project Element Manager” is JPL terminology for a person who manages a specific piece of a project. Each project element manager is responsible for a different part of the spacecraft. I manage the Ion Propulsion system.
McREL: What is an Ion Propulsion System?
JB: An Ion Propulsion System is an alternative to the regular chemical rockets that JPL usually flies on deep space missions. In a regular, conventional rocket, performance is limited because there is only so much energy available in the un-reacted propellants carried by the rocket. With an Ion Propulsion System, we use an external source of energy; in this case, the sun. We collect energy using large solar rays, and convert that energy into electricity. We run that electricity over to the thrusters and add it to the propellant, and now we can add as much energy to the propellant as we want. And we can shoot the propellant out the back at a much higher speed and that makes the spacecraft much more fuel efficient. We can go to more interesting, harder-to-get-to destinations by using this propulsion system.
McREL:Can you make the energy as you go?
JB: We collect the energy as we go, but we carry the propellant with us. Before, in a chemical rocket, the spacecraft carried both the propellant and the energy. The energy is carried in the form of the chemical energy of the un-reacted propellants. In the case of ion propulsion, the spacecraft just carries the propellant, and collects the energy from the sun as it goes along.
McREL: Is the Ion Propulsion System new?
JB: Actually, Ion Propulsion Systems have been around for a long time. The first ion thruster was operated in the laboratory at what is now the NASA Glenn Research Center in Cleveland, Ohio in 1959. That thruster, it turns out, performed very well and had a high efficiency. It had a very high exhaust velocity— really attractive performance. It’s very easy to make an ion thruster work well. However, it‘s very, very difficult to make an ion thruster last long enough to be useful. This is a problem, because typically, an ion thruster needs to run for roughly two years continuously in order to be useful enough to do a mission.
McREL: Can you help us understand why the Dawn mission lasts about ten years?
JB: We coast some of the time. Part of the time, we’re around the asteroids doing our science thing. The ion propulsion system has to run for about 5 years of the 10-year mission. That sounds like a long time, and it is. But, we have done a substantial number of life tests on this type of ion thruster. The one that we finished most recently ran over a period of five years and was completed in the year 2003. The thruster ran successfully for over 30,000 hours. Therefore, we are confident that the ion thrusters will be capable of performing for the Dawn mission.
McREL: Let’s talk about distance, solar panels, and propulsion. Can you connect the dots so that a child might understand how all the pieces fit together in this mission?
JB: To run an ion propulsion system, as we said earlier, you need a source of energy. So, that’s why the Dawn spacecraft carries these very large solar panels. These panels convert sunlight into electricity. Dawn will be the highest powered spacecraft that JPL has ever flown for any deep space mission. So, it has very large solar arrays, not only to get to the power that we need at the beginning of the mission, but because we’re going three times as far from the sun as the Earth is, so, we need big solar arrays just to collect enough sunlight that far from the sun.
The propulsion system works like this: we collect the sunlight, convert it to electricity, and that electricity goes into a box of electronics that converts it into the currents and voltages that the ion thruster needs to run.
An ion thruster works in the following way. We take the propellant, which is, in this case, an inert gas called Xenon. Xenon occurs naturally in the atmosphere. It’s a rare gas, but you’re breathing a little bit of Xenon right now. It’s inert, so it doesn’t hurt you. It’s environmentally friendly, and we get it from liquefying air. Some of the Xenon we use comes from the production of liquid oxygen for the steel industry. We collect the Xenon out of the air and carry it onboard the spacecraft.
First, the Xenon is directed into the engines. The ion engine is about the size of a large coffee can, and it’s basically empty inside. There are rings of magnets around the outside of this kind of large coffee-can thing, and those magnets help improve the efficiency by which we change the Xenon atoms into Xenon ions. That’s a process called ionization. We run an electric current through the gas. You can run electric current through a wire, and nothing happens. But when we run electric current through a gas, some of the electrons smash into the Xenon gas and knock off some electrons resulting in positively charged xenon ions.
So now we have this coffee can filled with Xenon gas, a lot of Xenon ions and some electrons, and we take the whole can and we raise its voltage up to about 1000 volts. We have these positive ions inside this positive can. If you remember basic physics, like charges repel each other. Therefore, this positive can is pushing these positive ions out the back of the spacecraft at a really high speed—about 10 times faster than the exhaust that comes out of regular chemical rocket. The faster we push the propellant out the back, the less propellant we need to carry—because we’re getting more “oomph” from each pound of propellant that we’re pushing out. And the energy to make all this happen came from the energy collected by the solar panels.
So, that’s great! We push out the ions at a really high speed, but now we’ve got a problem, because we stripped electrons off of the Xenon atoms to make ions. And, if all we do is push out positive particles from our spacecraft, we leave behind all these electrons that are negative, and the spacecraft will very quickly charge up negative. And it’ll charge up negative to minus 1000 volts, because that’s the voltage that we put on the thruster. So, we have to collect these electrons and inject them into the ion beam. We have to inject exactly the same number of electrons as ions that we’ve accelerated out.
That sounded like a hard thing to do. And, in fact, in the early 1960s, people weren’t sure whether that was possible or not. They actually did a flight test, called “SERT 1,” which is the Space Electric Rocket Test #1, to find out whether we could actually do this process, which is called “neutralizing the ion beam.” And, fortunately, it turned out that we can. Actually, it’s very easy to do and it’s a kind of self-controlling process. So, we inject exactly the same number of electrons as ions. Now, leaving the spacecraft, we have these high-velocity ions and electrons that are kind of drifting along with them. Shooting out the ions, that way, pushes the spacecraft in the opposite direction.
McREL: You used the term “ion beam” and I’ve heard you talk about ion propulsion. Are the two related?
JB: When an ion thruster operates, it produces a beam of ions. That is what we call the “ion beam.” These are ions that have been accelerated from the thruster. They’re coming out at about 40,000 meters per second, so they’re really cooking along here. They go pretty much in straight lines, right directly away from the thruster. So, that’s the ion beam and again it starts out about a foot in diameter. It does spread out somewhat as it moves away from the thruster.
McREL: Why don’t all the missions use ion propulsion? It sounds great.
JB: Actually, I think they all should, but I might be a little biased on this. As I’ve told people, my other job at JPL is to try to get ion propulsion on every mission that JPL flies. Historically, there have been a lot of missions that are easy to do from a propulsion standpoint. So, NASA and JPL have really done most of those missions. Now they’re looking at missions that are harder and need a better propulsion system.
There are also programmatic issues for why ion propulsion, which has been around for a long time, wasn’t used earlier. Until the early 90’s, the cost of the planetary mission programs did not include the launch vehicle, so spacecrafts could use less efficient propulsion systems because they had such big launch vehicles to get them started. They could carry a lot of fuel on board because these big launch vehicles could get them going. However, if you include the cost of the launch vehicle in the cost of the mission, then you want the most efficient propulsion system on your spacecraft, because that makes the spacecraft smaller and the launch vehicle can be smaller and less expensive. Turns out it’s very expensive to get anything into space. A smaller spacecraft needs a smaller launch vehicle, so the cost goes down all around.
In addition, with a traditional chemical rocket, most of what you’re putting into space is fuel that you’re going to burn so it’s like burning money. It makes financial sense to use a more efficient propulsion system, because you put less propellant into space and can do more interesting missions with a smaller spacecraft, which is less expensive to launch. So that’s why people use ion propulsion now, and they weren’t really interested in using it in the past.
McREL: I’ve heard the term “mass margin” —what does it mean and how does it relate to ion propulsion systems?
JB: The “mass margin” is the difference between how much the spacecraft weighs and what the launch vehicle is capable of getting off the earth and in the direction it needs to go—the “launch vehicle capability.” JPL has very specific policies towards mass margins and what kinds of margins you need to carry, depending on what phase you are in the development of the spacecraft. Early on, there is a requirement to have a fairly large mass margin, because you’re not really sure you’ve got the spacecraft very well-defined. And as you get better definition, the mass tends— well, almost always goes up because you forgot things or you need to change things. You need to have enough margin when you start, so that as the mass goes up, by the time you’re done, you still have a spacecraft that you can launch.
Now, the other thing that’s important is what’s typically called, “the payload mass fraction” — that’s how much of the spacecraft mass is actually made up of the instruments that do the scientific investigations. It doesn’t do you a lot of good to get this big, massive spacecraft to somewhere, if there’s very little mass left over for the instruments to do the science of the mission once you get there. So, that’s typically called the “payload mass fraction.”
Again, an ion propulsion system helps the payload mass fractions substantially, because you start with a lot less propellant, so the spacecraft weighs a lot less at lift-off. Most conventional spacecraft tend to be almost all propellant when they first start out.
McREL: How does an ion engine get enough acceleration from such a small amount of propellant?
JB: An ion engine of the type that Dawn is going to fly is about a foot in diameter. We call it a 30-centimeter thruster—it’s 30 centimeters across—because we like the metric system these days. But, it’s about a foot in diameter. And it’s also about a foot long. So that gives you an idea of the size.
At full power, Dawn’s engine is capable of processing about 2,300 watts, equal to a bank of about 23 light bulbs. That’s the full power operating condition for the thruster, and it produces at full power a thrust level that’s really quite low. In fact, people are always surprised at how low it is—a force that’s equivalent to the weight of a single sheet of paper resting on your hand. It takes 2300 watts to give you that level of thrust. You don’t start out accelerating really fast. But this is not a sports car.
As Marc Rayman has said, “This is acceleration with patience.” An earlier demonstration spacecraft called “Deep Space 1”— the first one that used ion propulsion for a deep space mission —could accelerate from 0 to 60 miles an hour in about two days. So, it’s not a real fast accelerating system. But, if you accelerate slowly for a long time, you can get going very fast—in fact, faster than you can if you take a chemical rocket, burn it like mad for a few minutes and then coast the rest of the time. After a year or two, with ion propulsion, you can be going faster, even though you’ve accelerated very slowly, but that acceleration builds up the velocity over time.
McREL: We’re sometimes asked, “I saw Star Wars and it portrayed asteroids as being very close in proximity. Is that a problem for the Dawn mission?”
JB: The mission is going to the main asteroid belt. Unlike what you may have seen in Star Wars, the asteroids are really very far apart. So, there’s very little additional hazard of running into an asteroid. There’s a lot of work being done in the navigation of this spacecraft, actually, to get to the two asteroids that we’re trying to get to. These are, actually, really big asteroids. They’re not the small hunks of rock that you might think of when you think of an asteroid. As far as avoiding the asteroids, the ion propulsion system wouldn’t be very good at doing that, because the acceleration that it produces is very low; but it’s not really necessary, because the asteroids are not very close together.
McREL: What is a protoplanet?
JB: A protoplanet is a relatively large body in space. In this case, the protoplanets Vesta and Ceres, are large asteroids but their formation dates back to the beginning of the solar system, so there’s interest in going to see these to try to learn what the solar system was like back when it was first formed.
McREL: If Vesta and Ceres are as large as planets, why haven’t they been elevated to planets?
JB: A long time ago the planets were the objects that didn’t follow the same patterns in the sky as the fixed stars. People were familiar with seven things that moved in the heavens, and they called those “planets.” They just couldn’t see Vesta and Ceres. If they had, they probably would have labeled them as planets as well.
McREL: Ion propulsion sounds so cool. Why haven’t they used it in science fiction stories?
JB: It turns out there are a number of science fiction shows that have used ion propulsion and it does sound really cool! The first one I’m familiar with was a Star Trek episode called “Spock’s Brain,” where this alien civilization stole Spock’s brain, and then they carted it halfway across the galaxy using an ion propulsion system; and the Enterprise was chasing after them. We’re not quite up to that yet, but that’s the first one I’m familiar with. Then, in Star Wars, the TIE Fighters, which is an acronym for Twin Ion Engines, used ion propulsion as well. Those fighters have really good performance. I wish we had that kind of capability. Ion propulsion is fun and it does sound like science-fiction.
McREL: Are there any other uses for ion propulsion aside from deep space exploration?
JB: There are the other applications for ion propulsion. Right now, many commercial communication satellites use ion propulsion for station keeping. Station keeping is used for satellites in geosynchronous orbit, which means they appear to hover over one spot on the Earth, but they don’t quite stay there because their orbits get perturbed by the moon and the sun. You need to constantly push them back into the right place. Right now, there are many commercial communication satellites that use ion propulsion to continuously push them back into the right place. Many of the TV shows that you watch may be relayed by satellites that use ion propulsion to keep them in the right place.
McREL: How did you come to choose this career, this specialization?
JB: I started in ion propulsion as a graduate student. When I finished as an undergraduate in mechanical engineering, I applied to a number of graduate schools. I got a call from a professor at Colorado State University who said he was doing research in ion propulsion. This was back in the late 1970s and I asked him, “What’s that?” He sent me a bunch of information, and I read through it, and I thought it was really neat. I went there and received a Masters degree in electric propulsion.
After graduate school, I went to work at the Marshall Space Flight Center on a project called “The Solar Electric Propulsion System.” We were going to build a large planetary spacecraft using, in this case, mercury ion thrusters. These are ion thrusters that use mercury instead of Xenon as the propellant. Mercury, as you may know, has a lot of environmental disadvantages and so they were never successful at getting this mission to go. There were a lot of reasons for it, but at least one of them was because they were using mercury. By the early 80s, ion thrusters had been running on mercury in the laboratory for twenty or thirty years, and the switch was made to use Xenon—a much better propellant. The performance of the ion thruster is about the same as it is with mercury but you don’t have the environmental issues.
At about that time, I left the Marshall Space Flight Center and went back to Colorado State University to get my PhD. I worked for the same professor there and tried to better understand how ion thrusters work. I’m almost there. After I finished at CSU, I came to JPL to develop ion propulsion technology, although the prospects in the mid-80s for using ion propulsion on a planetary spacecraft were not real bright. There were still a lot of missions that could be done without ion propulsion, and people joked that ion propulsion was the propulsion system of the future and always would be.
We went to project managers and told them, “Look, you can use this great new propulsion system on your planetary spacecraft. You could make the spacecraft much smaller, it would get you on a smaller launch vehicle, and you wouldn’t have to pay as much for that launch vehicle.” They told us, “We don’t care because we don’t pay for the launch vehicle anyway—that’s paid for by a different part of NASA. Why should we risk our planetary spacecraft on your wacky new propulsion system?” It was a difficult sell, but we continued at JPL and at NASA Glenn Research Center to develop the technology.
We knew we really needed a flight demonstration mission to get over these hurdles. There was an Air Force program in the early 90’s that was going to fly a spacecraft to demonstrate a different type of electric thruster called an arc jet, and we thought we could piggyback on that spacecraft to demonstrate the ion thruster so we could later fly it on a planetary mission. However, the Air Force cancelled that mission.
Then NASA started a new program called the “New Millennium Program,” which was designed specifically to flight-test new technologies. A lot of work was done to demonstrate to the New Millennium Program that ion propulsion should be one of these technologies. The people at the New Millennium Program believed that ion propulsion was the most important technology they could demonstrate, but they couldn’t fly it on Deep Space 1 (their first mission) because they couldn’t afford the solar array. They said, “Give us a free solar array (worth $7or $8 million), and we’ll fly it on DS1.” So, how do you come up with a free solar array? We had also been working with, at that time, the ballistic missile defense organization and they had, for other reasons, been developing advanced solar array technologies. We knew they were looking for a mission to test their solar array. We told them we could give them a free ride into space on Deep Space 1 if they would provide the solar array. They said they would. We told New Millennium we had a free solar array, and they said they would fly the ion propulsion system. They did, and the Deep Space 1 ion propulsion system worked great. That set the groundwork for flying the ion propulsion system now on Dawn.
McREL: What advice would you give to a young person who might be interested in building spacecraft?
JB: The people that build spacecraft are engineers. Scientists figure out how the world works, then engineers take that understanding and figure out how to build machines that can do things. They figure out how to make things that can do really fantastic things, like rendezvous with two asteroids that you can’t even see with the naked eye, or catch up to a comet, or even land on a comet—engineers do that. It turns out the way the world works is very mathematical. So if you want to work in this field, you need a good background in mathematics and all the disciplines that go along with that. If you’re very good in math, all the sciences will be very easy because they’re all strongly based on mathematics. We get to do things that nobody else in the world does. It’s really remarkable to go to work sometimes and go to meetings where they’re trying to figure out how to land on a comet, for example. How many people get to do that? That’s really amazing.
McREL: Were you always good in math? Did you always know you wanted to work on spacecraft?
JB: I was always good in math. I had a 4th grade teacher who saw I was good in math. Math has always been pretty easy. I considered majoring in math when I was an undergraduate, but I grew up in the 60s when we were racing to the moon, and I just loved the space programs. I always wanted to do something with the space program. Being good at math just was a natural way of getting into engineering and then winding up in the space program.
McREL: When there are chunks of time that are not work-oriented, how do you like to spend that time?
JB: You mean people have time when they’re not working!!!! What’s up with them??? When I’m not at work, I spend almost all my time chasing after our kids. My wife, Bobbie, and I have two kids—one in high school and one junior high—and they’re very good in math. My wife actually worked at JPL until our second child was born and then stayed home to care for them. So I spend most of my free time with my family. Yesterday, I spent all day snowboarding with my son so now I’m sore.
McREL: If you could send a message to the next generation of engineers who are now in Grade 3, what would you say?
JB: Hmmm. That’s a good question, and one I didn’t prepare for. I’d say that one thing that makes working at JPL fun is that you get to do stuff that no one else does. You’re not a stockbroker, you’re not flipping burgers somewhere. You get to do things that are really unique. And, to some extent, engineers always get to do that because they’re always building new things. In the future, there are going to be plenty of new things to develop and new machines to make and new places to go. The galaxy is enormous, and in the future, I believe people will move out into space and there will be a lot of new things to develop. Being an engineer has been really good for me. It pays very well and gets you really interested in things to do. Life doesn’t get much better than that.