December 28, 2006
The Dawn spacecraft has made its new year's resolution: to leave Earth behind in 2007 and embark upon its celestial voyage of adventure and discovery. (Actually, it was either that or spend more quality time with friends and family. As much as we all like Dawn, I think we can be grateful it made the choice it did.) The spacecraft is well on its way to achieving its goal.
Over the past few months, Dawn has completed all of the demanding environmental tests planned for it at Orbital Sciences Corporation. In the last log, we saw why such tests are so important. Since then, Dawn has been spun, vibrated, and blasted by noise, and careful testing afterwards has verified that it can withstand these insults and still operate as planned.
One of the tests included attaching Dawn to the structure that will connect it to the upper stage of the Delta II 7925H-9.5 rocket so essential to keeping its new year's resolution. Part of the objective of this test was to verify that the spacecraft and the rocket, although manufactured separately, really will fit together when they meet at Cape Canaveral in June. In addition, this test was used to subject Dawn to another special condition it will experience in its mission. Following the burn of the Delta's third (and last) stage, the rocket will relinquish its firm grasp on the spacecraft. The firing of the release mechanism will cause a shock (certainly physical, possibly emotional) to go through the spacecraft as it is freed to operate in space on its own. Feeling this shock is part of the battery of tests the spacecraft has now completed. Continuing with its perfect record, Dawn passed beautifully, demonstrating that it can tolerate the shock and separate cleanly, with no structures impeding its departure from the rocket.
Following all these tests, the two large solar array wings were extended, allowing engineers another test of the deployment system and the opportunity to verify that the delicate cells were still healthy. Each wing extends 8.3 meters (more than 27 feet) and weighs almost 63 kg (139 pounds). The system is not designed to be strong enough to support them under the strong pull of Earth's gravity; of course, when Dawn is in its natural environment of spaceflight, no such force will be exerted upon the arrays. For working in the exotic conditions here on the surface of our planet, a special structure is erected to bear the weight of the wings yet allow them to unfold smoothly. After the tests, the solar arrays were removed, and they will not be reattached until the spacecraft is in Florida.
Now Dawn is being prepared for its departure from Orbital Sciences in Dulles, VA. Next month it will be transported to the Naval Research Laboratory (NRL) in Washington, D.C. for the final phase of environmental tests, all of which will be conducted with the spacecraft in a vacuum. Orbital has the vacuum facilities to accommodate the spacecraft (see the description in the July 29, 2006 log), but this upcoming series of tests will include a brief firing of the ion thrusters, and that requires a different vacuum system. Because NRL has the needed capability and is near Orbital, it was a natural location for this work. Dawn will spend about 3 months there, and the next log will report on the activities, including the operation of the ion propulsion system.
Devoted readers have asked for more information on ion propulsion. This is only one of the important subsystems onboard (see the overview and relative importance of all the subsystems and systems on September 17, 2006 and October 29, 2006), and Dawn will rely upon all of them in order to explore the remote, alien worlds Ceres and Vesta. Over the many years of the mission, we shall have occasion to learn a great deal more about many facets of the engineering and science of this exceptional adventure, but starting in this log, and continuing in the next, we will take a more detailed look at the ion propulsion system.
While most of our audience is, of course, quite familiar with this topic, we should recall that our readership extends to planetary systems that have had little experience with this technology, and it is to them that this material is directed. Although it may be surprising, apparently there are even some readers who did not follow NASA's Deep Space 1 (DS1) mission, which tested ion propulsion and other high-risk technologies to protect subsequent missions from the risk and cost of being the first users of such advanced systems. Dawn is one of DS1's beneficiaries, and being the first spacecraft ever built to orbit 2 target bodies after leaving Earth, it would be effectively impossible without ion propulsion.
Ion propulsion had its origins in solid science, but despite some scientific and engineering work, it resided principally in the fictional universes of Star Trek, Star Wars, and other fanciful stories. DS1 helped bring ion propulsion from the domain of science fiction to science fact.
First let's recall how a propulsion system works. Most conventional systems use high pressure or temperature to push a gas through a rocket nozzle. The action of the gas leaving the nozzle causes a reaction that pushes the craft in the opposite direction. This is what causes a balloon to fly around when the end is opened and the stretched rubber squeezes the air out. Ion propulsion works on the same principle, but the method of pushing the gas out is unique.
The inert gas xenon, which is similar to helium and neon but heavier, is used as propellant. The composition of xenon is simple: each atom consists of a tiny and dense nucleus surrounded by a cloud of electrons. The nucleus is 54 positively charged protons plus about 76 neutral neutrons. (Xenon gas is a mixture of 9 isotopes, meaning there are 9 different values for the number of neutrons. From a low of 70 to a high of 82, the number of neutrons makes only very modest differences in the behaviors of the atoms.) The 54 positive charges in the nucleus are precisely balanced by 54 negatively charged electrons, rendering the atom electrically neutral -- until the ion propulsion system gets in the act.
Inside the ion thruster, an electron beam, somewhat like the beam that illuminates the screen in a television, bombards the xenon atoms. When this beam knocks an electron out of an atom, the result is an electrically unbalanced atom: 54 positive charges and 53 negative charges. Now with a net electrical charge of 1 unit, such an atom is known as an "ion." Because it is electrically charged, the xenon ion can feel the effect of an electrical field, which is simply a voltage. So the thruster applies more than 1000 volts to accelerate the xenon ions, expelling them at speeds as high as 40 kilometers/second (89,000 miles/hour). Each ion, tiny though it is, pushes back on the thruster as it leaves, and this reaction force is what propels the spacecraft. The ions are shot from the thruster at roughly 10 times the speed of the propellants expelled by rockets on typical spacecraft, and this is the source of ion propulsion's extraordinary efficacy.
All else being equal, for the same amount of propellant, a spacecraft equipped with ion propulsion can achieve 10 times the speed of a craft outfitted with normal propulsion, or a spacecraft with ion propulsion can carry far less propellant to accomplish the same job as a spacecraft using more standard propulsion. This translates into a capability for NASA to undertake extremely ambitious missions such as Dawn.
The rate at which xenon is flowed through the thruster is very low. At the highest throttle level, the system uses only about 3.25 milligrams/second, so 24 hours of continuous thrusting would expend only 10 ounces of xenon. Because the xenon is used so frugally, the corresponding thrust is very gentle. The main engine on some interplanetary spacecraft may provide about 10,000 times greater thrust but, of course, such systems are so fuel-hungry that their ultimate speed is more limited.
The force of the ion thruster on the spacecraft is comparable to the weight of a single sheet of paper. So here is an ion propulsion experiment you may conduct safely at home: hold a piece of paper in your hand, and you will feel the same force that the ion thruster exerts. Because the fuel efficiency is so great, the thruster can provide its push not for a few minutes, like most engines, but rather for months or even years. In the weightless and frictionless conditions of spaceflight, the effect of this thrust can gradually build up to allow the spacecraft to achieve very very high speed. Ion propulsion delivers acceleration with patience.
Throughout its mission, Dawn will be farther from the Sun than Earth, but as long as it is less than about twice Earth's distance from the Sun, those huge solar arrays will generate enough power to operate the ion propulsion system at its maximum throttle level. At that setting, the acceleration will be equivalent to about 7 meters/second/day, or slightly more than 15 miles/hour/day: one full day of thrusting would change the spacecraft's speed by 15 miles/hour. That means it would take Dawn 4 days to accelerate from 0 to 60 miles/hour. Perhaps this does not evoke the image of a hot rod, but its parsimonious consumption of xenon lets it thrust for much longer than 4 days.
To put this in perspective, consider a greatly simplified example based upon the remarkable probes NASA has in orbit around Mars now. When they arrived at the planet, these spacecraft had to burn their engines to drop into orbit. While each mission is different, such a maneuver might be about 1000 meters/second (2200 miles/hour) and could consume about 300 kilograms (660 pounds) of propellants. With its ion propulsion system, Dawn could accomplish the same change in speed with less than 30 kilograms of xenon. A typical Mars mission might complete its maneuver in less than 25 minutes, while Dawn might require more than 3 months. If one has the patience, the ion propulsion can be very effective. Now for many missions, the greater complexity and cost of ion propulsion is unnecessary, and it is quite clear that we can get into orbit around Mars without it. But as humankind engages in ever more ambitious missions in deep space, opening our frontiers, revealing otherwise inaccessible vistas, and seeking answers to new and more exciting questions about the cosmos, the tremendous capability of ion propulsion will be an essential ingredient.
By the end of its mission, having operated from its maximum throttle level down to lower levels when Dawn was much farther from the Sun, the spacecraft will have accumulated over 5 years of total thrust time, giving it an effective change in speed of 11 kilometers/second, or well over 24,000 miles/hour. That is about the same as the entire Delta rocket with its 9 solid motor strap-ons, first stage, second stage, and third stage, and it is far in excess of what any single-stage craft has accomplished.
In the next log, we will see how the Dawn mission takes advantage of ion propulsion and how its use makes the profile of the mission different from most interplanetary flights. In the meantime, the spacecraft will use conventional transportation technology to travel to NRL for more rigorous tests in preparation for the challenging mission it has resolved to begin in 2007.
Dr. Marc D. Rayman
December 28, 2006
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