TUESDAY, SEPTEMBER 25, 2007
With the final touches now complete on the Dawn spacecraft, the future asteroid orbiter is counting down the remaining hours to blastoff at sunrise Thursday from Cape Canaveral, Florida.
The beefed up United Launch Alliance Delta 2-Heavy rocket, equipped with larger solid-fuel boosters, is scheduled to thunder from pad 17B at 7:20 a.m. EDT (1120 GMT). The morning’s available launch window will extend to 7:49 a.m., giving a 29-minute period for the liftoff to occur.
Weather forecasters are predicting a 60 percent chance of acceptable conditions. Coastal rain showers and cumulus clouds over the launch area will be the chief worries.
The launch readiness review was conducted Tuesday morning. All systems were reported “go” except for one remaining technical concern about the clearance between the satellite’s high-gain antenna and the acoustic blankets inside the rocket’s nose cone during launch. Engineers completed their analysis and managers ruled Tuesday afternoon that there was no problem for flight.
Launch countdown operations will commence Wednesday evening. Retraction of the mobile service tower from around the 12-story rocket should be completed before midnight.
Thursday’s three-hour Terminal Countdown picks up at 4:20 a.m. EDT. The early-morning count includes fueling of the first stage with highly refined kerosene and supercold liquid oxygen propellants.
A standard pause at the T-minus 4 minute point will give launch managers the opportunity to survey the status before countdown clocks progress into the quickly paced final minutes that see the vehicle switch to internal power, the liquid oxygen tank pressurized and systems armed.
The main engine and six of the nine strap-on solid rocket motors ignite at liftoff to begin Dawn’s ride off the planet. The Delta 2-Heavy features slightly larger solid motors, originally developed for the more-powerful Delta 3 rocket.
Arcing eastward over the Atlantic Ocean, the six solid boosters lit on the ground extinguish 77 seconds into flight as the remaining three motors are ignited moments later. The half-dozen spent boosters are jettisoned at an altitude of 14 nautical miles to fall harmlessly into the sea. The air-lit motors burn out and separate two minutes, 40 seconds after liftoff when the rocket is about 39 nautical miles up.
“If you live in the Bahamas, this is one time you can tell your neighbor, with a straight face, that Dawn will rise in the west,” said Dawn Project Manager Keyur Patel of NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “Weather permitting, we are go for launch Thursday morning, a little after dawn.”
Once the solids are discarded, the kerosene-powered main engine will continue pushing the rocket above an altitude of 65 nautical miles. The spent stage then jettisons at T+plus 4 minutes, 31 seconds to let the hydrazine-fueled second stage propel the vehicle the rest of the way into space. The two-piece nose cone that shrouded Dawn during the trek through the atmosphere is shed just seconds after the second stage comes to life.
Nine minutes into the ascent, the second stage completes its initial firing. The Delta settles into a 100-nautical mile parking orbit along a trajectory that cruises above the central Atlantic before crossing Africa and the Indian Ocean. The second stage reignites its engine off the west coast of Australia at T+plus 51 minutes. The burn will last more than two-and-a-half-minutes as the rocket begins a northeasterly trek over Australia.
The solid-fueled kick motor that serves as the Delta’s third stage then spins up and pops free of the second stage to ignite for an 85-second firing to propel Dawn out of Earth orbit.
Dawn should be released from the rocket at 8:22 a.m. EDT, some 62 minutes after an on-time blastoff, while flying northeast of Australia near the Solomon Islands.
“After separation, the spacecraft will go through an automatic activating sequence, including stabilizing the spacecraft, activating flight systems and deploying Dawn’s two massive solar arrays,” said Patel. “Then and only then will the spacecraft energize its transmitter and contact Earth. We expect acquisition of signal to occur anywhere from one-and-a-half hours to three-and-a-half hours after launch.”
this is one time you can tell your neighbor, with a straight face, that Dawn will rise in the west, said Dawn Project Manager Keyur PatelWhat a cut-up that guy is. ;') If he's ever tellin' people that dawn rises in the east, he really needs to consider a new line of work, or adult ed classes, or something.