Craig Kluever’s dream was born as he found himself awestruck in front of a grainy black-and-white television screen watching Apollo 11 land on the moon. He was in kindergarten. As he puts it, “that just made a big impact on me. Of course, the first thing I wanted to be was an astronaut.” Those early dreams of becoming an astronaut turned instead into a pursuit of the science behind the rockets. Today, the MU Professor of Mechanical and Aerospace Engineering works behind the scenes to solve the kind of problems involved in designing space travel—such as how to take off, how to reach a target, and, more importantly, how to return safely to Earth.
Before 1998, all conventional spacecraft missions were powered by chemical propulsion. “This is what we are used to seeing on TV,” says Kluever, for example, referring to Apollo’s missions to the moon or the robotic Mars Exploration Rovers. “You fire a chemical-fuel engine for a very brief period of time, and it provides a lot of thrust, for only a minute or less, and that provides an impulse that sends the spacecraft on to the moon, or Mars, or wherever.” In those cases, 99% of the trajectory is just a coasting flight through the vacuum of space, where gravity is the only force controlling the trajectory. Kluever explains further: “As complicated as that sounds, that’s really a pretty simple problem because all you have to deal with is gravity, and we pretty much know how to model gravity, and so we know how to predict where a spacecraft is going to be.” However simple this technique, he continues, it isn’t very efficient. “If you look at pictures of the Saturn V going to the moon,” he cautions, “you’ll see a tiny capsule at the top and then three stages of fuel. It took that much fuel to get the tiny capsule to the moon.” The rover mission, for instance, that uses chemical propulsion, may only get 10% of the total mass of the rocket to Mars. Electric propulsion is far more efficient, Kluever notes: “theoretically, we could get as much as 60 or 70% of the initial orbital mass to our destination.”
Dr. Kluever first came to this area of research as a graduate student when he had a fellowship with NASA. At the time, he recalls, electric propulsion was brand new technology, and NASA needed predictive computer models to calculate missions, for example to map a trajectory from Earth to Mars. With electric propulsion, “you have an electric engine at the back of the spaceship instead of a chemical rocket. It’s almost like if you shine a flashlight out the back and emit electrons through a magnetic grid; instead of burning fuel and exhausting hot gases out the back, you’re throwing electrons at very high speeds. It’s the momentum exchange that gives you rocket propulsion. So you’ve got this spacecraft already in space, with a continuous thrust of one pound out the back, and it takes a very long time to build up the acceleration needed to go to Mars (or wherever the target may be). It’s a much more challenging problem because you’ve got to figure out how to steer that thrust, how to manage the electrical propulsion system, and then hopefully hit your target. It’s really just much more complicated than the old missions using chemical propulsion.”
The first space mission to test electric propulsion was Deep Space I, launched in 1998. “It had a very modest target,” says Kluever – basically just to fly past an asteroid – “and it was able to complete that mission.” Since then there have been some big plans to send spacecraft to Jupiter (or other outer planets) using electric propulsion. The problem with that plan is not with the technology, Kluever clarifies, but with its funding. “That’s the status of electric propulsion,” he observes. “It’s a very uncertain business right now. These things cycle; sometimes technologies are politically in favor and sometimes not. Right now, electric propulsion is out of favor.”
While this dimension of his research continues, about five years ago Kluever began working with the X-33 program. Once hailed as the next generation space shuttle, this winged vehicle was slated to replace the existing Space Shuttle, although the program has since been canceled. The Space Shuttle works very well, Kluever says, “but it does not have a lot of robustness built in.” That means that if it comes in on a flight path that is too steep, too shallow, or too fast, it will have very limited capabilities for altering that flight path and still making the planned approach for landing. Fortunately the Shuttle hasn’t had any major mechanical failures on the way down, but if those kinds of failures (like a broken rudder) occurred it would have limited maneuverability.”
Hence, he and his team at NASA sought to build robustness into designs for guidance and control systems. “Robust” describes a new guidance system that is more automated and adaptable; a new generation of safer shuttle vehicles that depends on advanced computing power, “so that if some major failure occurred—like the rudders didn’t work and it had limited banking ability, or the elevators didn’t work and it had limited pitching capability—then you could recalculate a trajectory that would still take it to a safe landing.” Kluever’s critical contribution to this project was to figure out how to make the shuttle’s guidance system recognize and steer toward the runway while maintaining the precise amount of energy needed.
Now the hot topic is the Crew Exploration Vehicle, the capsule in which NASA hopes to send astronauts to the moon and to Mars. Kluever is focusing on the atmospheric part of the entry and guidance system of this “Apollo-style” capsule, particularly the Earth return portion of the mission, which will involve a creative maneuver comparable to skipping a rock on the lake. He is also working on the ascent guidance system for the vacuum-flight phase of the Crew Launch Vehicle. “It turns out that the guidance system is very similar to what was used in Apollo,” Kluever explains. “When these crafts come back, there’s no longer any primary propulsion, and they’re just at the mercy of aerodynamics.” In certain scenarios, the astronauts may need to come back to Earth due to a failure, in which case the return trajectory may suddenly not be optimal for the landing site. With this skip maneuver, “the atmosphere slows the vehicle down, with the idea that by doing a skip you can extend the range to almost halfway around the world. So you can essentially enter anywhere in the atmosphere – do an aerodynamic maneuver, skip out of the atmosphere back into space, re-enter again thousands of kilometers down range, and then land hopefully at your target.” It becomes complicated, however, because if the range is very long, then very small errors (in terms of speed or angle) at that skip-out point will result in very large errors down the road. In fact, that first skip could result in a very dangerous trajectory that would be close to escaping the atmosphere, shooting out into space! “If you don’t manage the first skip properly,” Kluever warns, “it could just skip out and never return.” Like his work on robustness, Kluever’s job is to make sure these shuttles return safely to the Earth and, perhaps, inspire another generation of dreamers.
Surprisingly, when I asked him why this research was important, Kluever surprised me. Certainly, he could have extolled the benefits of innovation and discovery involved in space exploration. However, he responded ambivalently with, “That’s the hardest question.” He could cite the many technological advances that were outcomes of the space program (from Teflon and computers to mammograms), advances that impact many lives. But Kluever sees that kind of response as too clichéd. As to whether there’s a direct benefit to sending a person to the moon, “I myself struggle with that question,” he admits. “I don’t want to put myself out of business, and I certainly enjoy working on these problems, but I’ve worked on paper study after paper study, and it’s a tough business to see things through to a mission. I know people at NASA centers who have worked their entire careers and never worked on anything other than a paper study. That’s got to be frustrating, and it makes you think about where the priorities should lie for funding space projects.”
Others in NASA’s Jet Propulsion Laboratory have suggested doing future missions with robotics, which would be a lot cheaper. Kluever elaborates on that idea: “As soon as you put a person in that loop, then all these other things have to be tested and verified, and it becomes so much more expensive, not to mention the added mass (food, water, protection, shelter) to get a person there and back safely. You don’t have to do all that with robotic probes.” Of NASA’s overall budget, which is less than 1% of the national budget, roughly 75% funds the Space Shuttle and the International Space Station, leaving little to support biological, earth science, and robotic missions (to Jupiter, Pluto, and Mercury). “In this day of tight budgets, I’m not sure if that money is justified to send a person to the moon,” says Kluever.