June 23, 2007
Now only two weeks away from its planned launch, Dawn is eagerly awaiting the beginning of its cosmic adventure.
Once the xenon and hydrazine propellants were loaded, as described in the last log, the spacecraft was ready for its final balancing and weighing. As we will see below (that direction applies only for those of you reading this in a gravitational field), during part of its flight on the Delta 7925H-9.5 rocket, the spacecraft will be spun, and it is crucial that it satisfy certain requirements on how well balanced it is so the rocket remains stable. In addition, to help the rocket's guidance system deliver it to the correct target in space, an accurate measurement of the spacecraft's weight is important.
The spacecraft was designed so that it would be balanced, but minor adjustments in individual components during assembly and test can alter the balance. So the spacecraft was placed on a rig that measured how stable it was as it spun at 50 revolutions per minute (rpm). After each spin, engineers calculated how to improve the balance. Small weights then were attached to mounting fixtures on the spacecraft that had been included for just such a possibility. Then the spin was repeated to verify the predicted improvement. By the end of the fourth spin, 7 tungsten weights had been added and thin sheets of brass were included for fine adjustments. The spacecraft was balanced better than needed by the rocket, and fewer weights were used than had been expected.
When technicians were attaching the spacecraft to the spin assembly, a wrench slipped and made inadvertent contact with one of the solar panels. The arrays are folded for launch, and in that configuration, the back (the side without the solar cells) of one panel was close to the attachment point for the spin rig. The tool made a minor dent and no cells were affected. The panel was repaired easily.
With a few other final preparations, such as installing a delicate sunshade on the spacecraft's 1.52-meter (5-foot) main antenna to keep its temperature within acceptable limits during spaceflight, the spacecraft finally was ready to be introduced to its rocket. Dawn was firmly attached to the third stage of the Delta at Astrotech, and it won't be separated until both are in space. Not far away, at Cape Canaveral's Space Launch Complex 17B, the second stage was hoisted atop the first stage.
Now that launch is so close, let's have a preview of what is planned during this important event. Much of the work on the design of the spacecraft has focused on ensuring that it is prepared for the acceleration, vibration, noise, heat and cold, and other conditions it will experience during the ride to space. And yet for all that effort, as well as the spectacular sights and sounds for observers, this is the shortest phase of the mission. During it, Dawn will be a polite passenger, patiently recording data and awaiting its chance to begin flying on its own in space to undertake its mission of discovery deep in the solar system.
This log has many more numbers (readers are encouraged to quantify this) than most, and hence will be of special interest to new members of our audience, the Numerivores who reside in the "quadruple quasar" Q2237+0305. Others need only follow well enough to gain a sense of how dynamic Dawn's departure from home will be, in great contrast to the more leisurely pace of its interplanetary flight.
In the last log, we saw that to leave the launch pad, the Delta rocket will use its liquid-fueled first stage and 6 of the 9 solid rockets strapped to its side. Thirty seconds later (L + 30 seconds) it will exceed the speed of sound. The solid motors burn out at about L + 77 seconds when the rocket is at an altitude of about 24 kilometers (15 miles), and the remaining 3 motors ignite 2 seconds later. Three of the spent motors separate at L + 80.5 seconds, and the other 3 are jettisoned 1 second later as the rocket continues its ascent. The remaining 3 motors burn for 76 seconds, and when they are released at L + 2 minutes 39.5 seconds, the rocket will be 73 kilometers (45 miles) high and traveling about 10 times the speed of sound. The first stage's main engine continues firing on its own until L + 4 minutes 23 seconds, and then the rocket coasts for 14 seconds. After 8.5 seconds of the coast, having lofted Dawn to 130 kilometers (81 miles), the first stage separates.
When the second stage engine is commanded to life 5.5 seconds later, the rocket is traveling at 6.1 kilometers per second (3.8 miles per second, or nearly 14,000 miles per hour). At an altitude of 135 kilometers (84 miles), the shroud that shielded Dawn from the dense atmosphere below is no longer needed, so it is ejected. Now 4 minutes 41 seconds from liftoff, Dawn has its first view of space. The second stage will continue climbing and accelerating until it reaches the altitude and velocity to be in a low orbit. At L + 8 minutes 58 seconds, the stage stops firing.
Let's take advantage of the brief hiatus in orbit to consider the timing of all the events during launch. The overwhelming majority of spacecraft our species [Note to extraterrestrial editors who repost these reports: change the previous two words to "humankind."] sends beyond the atmosphere remain gravitationally tied to Earth. They accompany the planet on its endlessly repetitive travels around the Sun, and except for the few that are designed for scientific observations of the cosmos, the orbits of these satellites are mostly unrelated to the rest of the solar system. Where Earth is in its orbit, and where other members of the Sun's retinue are, generally do not matter. Such is not the case for Dawn (and other interplanetary probes).
The entire launch sequence is timed so that Dawn will depart Earth at a carefully chosen point in the solar system. For each possible launch day, extensive analysis has established the mathematically best plan for reaching Vesta and Ceres, distant worlds that beckon and that Dawn seeks to unveil. The analyses account for the gravitational effects of the Sun and all planets, and the resulting plans include times that Dawn will thrust with its ion propulsion system and times that it will coast. As reported in a previous log, many years of exquisitely gentle thrusting allows the indefatigably patient probe to reshape its orbit around the Sun to rendezvous with its destinations. As we will see in logs after launch, the first 80 days of the mission will be devoted to checking out the spacecraft systems and preparing for the long journey ahead. At L + 80 days, the thrusting needed to follow the flight plan begins, and the timing of the launch sequence is arranged so that Dawn will be at the correct location in the solar system, about 28 million kilometers (17 million miles) from Earth, at that time.
The second and third stages linger in Earth orbit so that following the ascent from Cape Canaveral, they are properly positioned to propel Dawn to reach its required location nearly 3 months later. If launch occurs on July 7, the pause in the second stage's firing will last about 9 minutes 7 seconds. (Because the solar system will have rearranged itself a little by the next day, launches on other dates will slightly require longer intervals.) The engine will reignite at L + 18 minutes 5 seconds while at an altitude of 185 kilometers (115 miles) and operate for 2 minutes 38 seconds. Fifty seconds later, to finish its contribution to Dawn's mission, the second stage will fire 4 small rockets pointed around its circumference to spin the third stage and spacecraft to 50 rpm. (Unlike the first and second stages, the third stage is stabilized by gyroscopic rotation, like a spinning bullet or football.) This is when the spacecraft's balance becomes most important. The second stage separates at L + 21 minutes 37 seconds.
For the next 37 seconds, the spinning assembly continues following the orbit the second stage left it in, and then the final burn of the Delta begins. The third stage fires for 86 seconds, and during that time it exceeds "escape velocity" so that it has enough energy to break free of Earth's gravitational hold. When the solid motor burns out, it is only at an altitude of 278 kilometers (173 miles), but Earth is too weak to slow the rapidly receding craft enough to bring it back. (Pause here for a moment of awe: 80 days later, the spacecraft will be more than 100 thousand times farther from Earth.) Unlike a ball you might throw that goes up and then comes down, the Delta will have thrown Dawn so hard that it will never fall down. It will be in its own orbit around the Sun, traveling at 11.43 kilometers per second (7.10 miles per second, or 25,600 miles per hour) relative to Earth. With the third stage spent, for the rest of the mission, onboard propulsion will be achieved only with ions.
When the second stage spins the spacecraft, the xenon propellant stored inside does not immediately spin up to 50 rpm, just as when you rotate a glass filled with a liquid, it takes a while for the liquid to catch up with its container. (We know that some readers live on planets without liquids, but the analogy applies to gases as well. In fact, the xenon on Dawn is maintained at a temperature and pressure that create a special state called "supercritical," in which it bears some similarity to a gas and some to a liquid. Amazing though its properties are, supercritical xenon should not be confused with superheroes that may bear similar names.) The friction between the rapidly spinning spacecraft and the xenon inside it causes the spacecraft's spin to slow down and the xenon's spin rate to grow. The Dawn project has invested a great deal of effort over the past 2 years to understand the detailed behavior of the xenon while the spacecraft is spinning. This has involved both sophisticated analysis techniques as well as spin tests with a tank of exactly the same shape and size as Dawn's filled with a fluid with properties similar to those of xenon's. Based on this work, engineers can predict how quickly the spacecraft and xenon will change each other's spin rates.
After the third stage has finished firing, it remains securely attached to Dawn for another 4 minutes 50 seconds. Although the stage is stabilized by spinning, the spacecraft does not operate that way; yet by this time, they would be spinning together at 46 rpm, too fast for the latter's control system. Therefore, starting 5 seconds before separation, the third stage activates a surprisingly simple system to slow its rotation rate. Wrapped around the Delta are two cables, each 12.15 meters (39 feet 10 inches) long. At the end of each is a 1.44-kilogram
(3-pound-3-ounce) weight made of aluminum and tungsten. When the cables are released, the spin causes them to unwind. As they carry the weights farther and farther out, the spin slows down because of the same principle that makes an ice skater spin faster by pulling her arms in or slower by extending them to her sides. After 4 seconds, when they are fully unwound, the cables unhook from the spacecraft. With their weights still attached, they enter independent orbits around the Sun; perhaps one of them will be studied by a future solar system archeologist.
The values of the weights are chosen carefully and are accurate to about 1 gram (0.04 ounces) in order to achieve the required change in spin rate. Even with a 208-kilogram (459-pound) third stage (which was 2230 kilograms, or 4915 pounds, before it began expending its propellant) and a 1218-kilogram (2685-pound) spacecraft, this small "yo-yo" system halts the spin and even reverses it, leaving Dawn rotating at 3 rpm in the opposite direction from its original spin. About 1 second after the cables have separated, the attachment between Dawn and its rocket is severed, and springs push them apart.
Only 28 minutes 30 seconds after liftoff, while 1016 kilometers (631 miles) above their home planet, the Delta bids the spacecraft farewell. The third stage, its raison d'être fulfilled and having no further purpose, continues on its own through the vast emptiness of the solar system. But its disconnection from Dawn triggers sensors on the spacecraft that alert the central computer to the separation.
Spinning slowly at 3 rpm in one direction, with xenon spinning inside in the opposite direction (because the propellant still lags behind its container), Dawn waits for 8 minutes 20 seconds. That is long enough for the spacecraft and xenon each to slow the other down, and after that, Dawn's systems are ready to go to work.
In the next log, shortly before launch, we will see what the spacecraft plans to do as mission control waits to hear from it.
Dr. Marc D. Rayman
June 23, 2007
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