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There are no updates from SpaceX yet confirming a good parachute deployment.
The unpressurized component of the spacecraft will burn up on re-entry while the pressurized cabin braves the heat and deploys parachutes for a gentle splashdown in the Pacific Ocean.
Dragon's blunt end is covered with Phenolic Impregnated Carbon Ablator, or PICA, a tough thermal protection system designed to withstand a high-speed, fiery atmospheric entry for missions returning to Earth from locations in deep space, such as asteroids, the moon, or Mars.
NASA's Stardust capsule, which returned dust from a comet, used PICA for a heat shield. And the Mars Science Laboratory Curiosity rover used a PICA heat shield during its landing at the red planet in August.
Dragon uses a formula known as PICA-X, which SpaceX says is an improvement over the formula used on Stardust.
Flying high above the Indian Ocean, the Dragon spacecraft is now firing its Draco thrusters for the deorbit burn, committing the capsule for return to Earth. The burn is expected to last approximately 10 minutes.
Dragon is carrying more than nearly 3,500 pounds of cargo from the space station in its pressurized cabin. The capsule is the only robotic cargo freighter able to retrieve equipment from the space station and return it to Earth for analysis or repairs.
About a dozen SpaceX engineers and technicians are at the splashdown site, which lies about 300 miles west of Baja California southwest of San Diego.
The SpaceX crew includes a four-person dive team to help retrieve the capsule after splashdown.
Coming up in a few hours, the Dragon spacecraft will close the door to its guidance, navigation and control bay.
Ignition of the capsule's thrusters is set for 2:08:42 p.m. EDT (1808:42 GMT) for an appoximately 10-minute deorbit burn. Splashdown more than 300 miles west of Baja California is scheduled for 3:02:41 p.m. EDT (1902:41 GMT).
NASA Television coverage of the Dragon resupply flight has ended. No live video is expected of the Dragon's splashdown, but you can check back here for updates on the status of the mission as we receive information.
You can also follow our Twitter feed to get periodic updates.
NASA and SpaceX report they are in good shape for the departure of Dragon with 3,563 pounds of cargo for the return trip to Earth.
The spacecraft are flying over the southern Indian Ocean right now.
The Dragon's laser rendezvous sensor is reported to be operating as planned.
Astronaut Steve Swanson will command the arm to release the capsule at 9:26 a.m. EDT (1326 GMT), while Russian cosmonaut Alexander Skvortsov will assist. The duo will man a communications panel to issue commands to Dragon if necessary.
The crew will monitor the spacecraft until it exits the so-called keep-out sphere 200 meters around the space station.
Under the control of engineers in mission control in Houston, the robotic arm is maneuvering the Dragon spacecraft to a release point about 30 feet beneath the space station. Astronaut Steve Swanson will take over when it is time to release the Dragon.
Dragon's strobe lights have been activated, and everything is progressing as planned this morning.
The gumdrop-shaped space capsule is loaded with about 1.7 tons of cargo, scientific research samples and other gear for the return to Earth.
The items coming back to Earth with SpaceX's Dragon spacecraft include two freezers and refrigerated bags containing blood and urine samples, plus experiments which examined drug-resistant bacteria in space and DNA damage to human cells in microgravity.
A spacesuit in need of repair is also strapped inside the Dragon cargo craft, along with water samples NASA says it needs to complete an investigation into why an astronaut's helmet filled with water during a spacewalk last year.
Grappled by the space station's robotic arm, the Dragon spacecraft will be removed from a berthing port on the outpost's Harmony module early Sunday before it is released at 9:26 a.m. EDT (1326 GMT) to begin the trip back to Earth.
Astronaut Steve Swanson will be at the controls of the space station's 58-foot-long robot arm when the Dragon spacecraft is released. Cosmonaut Alexander Skvortsov will assist Swanson in monitoring Dragon's departure.
The crew has a command panel to send orders to the Dragon spacecraft to quickly fly away from the space station if a problem occurs.
A series of rocket firings will put the Dragon spacecraft on a course to a safe distance away from the space station.
Ground controllers at SpaceX's headquarters in Hawthorne, Calif., will prepare the spaceship for re-entry by closing a door over Dragon's rendezvous sensors and grapple fixture.
A de-orbit burn is set for 2:08 p.m. EDT (1808 GMT), reducing the height of the craft's perigee, or low point in its orbit, into the atmosphere for re-entry.
After deploying three 116-foot-diameter parachutes, the Dragon capsule is expected to splashdown in the eastern Pacific Ocean about 300 nautical miles west of Baja California at 3:02 p.m. EDT (1902 GMT). SpaceX technicians are on standby near the landing zone to recover the spacecraft and ferry it to port on Long Beach, Calif., where it is due to arrive by early Tuesday.
The commercially-built Dragon logistics transporter was developed in a public-private partnership between NASA and SpaceX, which has a $1.6 billion contract for 12 cargo missions to the space station through 2016.
The mission set to conclude Sunday is the third of the 12 resupply missions contracted to SpaceX.
The Dragon spacecraft arrived at the International Space Station on April 20 after a two-day pursuit of the complex following its April 18 liftoff on a Falcon 9 rocket from Cape Canaveral, Fla.
The cargo craft delivered 4,600 pounds of supplies, logistics and experiments, which were unloaded from Dragon's pressurized compartment by astronauts. Two experimental packages mounted inside Dragon's external trunk section were removed by Dextre, the station's robotic handyman.
Ground controllers guided Dextre and the robotic arm to place the experiment boxes on their new homes outside the space station. One of the experiments is a high-definition camera suite, which is streaming live video footage of Earth online for viewing by the public. The other payload is an optical laser communications terminal to demonstrate high-bandwidth data transfers between the space station and a ground antenna on Earth.
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Final berthing occurred at 10:06 a.m. EDT (1406 GMT) as the space station flew 260 miles over Brazil.
While Wakata is at the controls of the Canadian-built robotic arm, Rick Mastracchio is overseeing the Dragon spacecraft's position and status from the space station's windowed cupola module. Mastracchio has a command panel to order Dragon to retreat or abort if a problem develops.
Astronauts Koichi Wakata and Rick Mastracchio are following the rendezvous in the station's cupola.
They are waiting for the space station and Dragon to fly into sunrise.
Spacecraft communicator Jack Fischer said NASA and SpaceX are monitoring an issue with the UHF link between Dragon and the space station causing brief communications dropouts. Officials don't think this is a problem for today's rendezvous, but they continue to study it.
The dropouts are lasting about 10 seconds before Dragon switches to a backup radio link. The cargo craft is programmed to automatically abort its rendezvous if the dropouts last more than 50 seconds, according to a NASA spokesperson.
The UHF link would be used by the space station crew to issue commands to the Dragon in the event of a problem.
Capture is on schedule for 7:14 a.m. EDT (1114 GMT).
Dragon's autonomous approach is guided with inputs from a LIDAR laser sensor and two thermal imagers.
The thermal and laser sensors independently measure the distance between Dragon and the space station to feed into the craft's guidance system.
Dragon's LIDAR sensor works by bouncing laser signals off the space station, creating a 3D map of the complex and giving Dragon's computer data on the craft's distance, closure rate, and orientation.
Today's rendezvous is going according to plan. Live NASA TV coverage begins in about 7 minutes.
The SpaceX-owned spaceship is carrying about 4,600 pounds of cargo to reinforce the space station's stocks of research experiments, crew provisions and spare parts.
Since launching Friday at 3:25 p.m. EDT (1925 GMT), the Dragon spacecraft has fired its Draco thrusters to adjust its orbit to match that of the space station, setting up the final rendezvous sequence.
By about 2:30 a.m. EDT (0630 GMT), the Dragon spacecraft will be about 28 kilometers, or 17 miles, below and behind the space station. At that range, the Dragon should be within range of a UHF communications panel the space station's crew can use to issue simple commands to the supply ship in the event of a problem.
Several height-adjustment and midcourse correction rocket burns will fine-tune Dragon's rendezvous, guiding the ship into position 350 meters, or about 1,150 feet, directly beneath the space station at 5:13 a.m. EDT (0913 GMT).
The Dragon will initially rely on relative GPS navigation data to guide its approach to the space station. Once directly beneath the complex, its computers will switch to laser and thermal sensors.
Dragon carries a laser guidance sensor package and two thermal cameras to aid its final rendezvous with the space station.
Before leaving the initial hold point 1,150 feet beneath the space station, the cargo craft will conduct a 180-degree yaw maneuver to align its grapple fixture with the position of the space station's robot arm.
Soon after beginning its final approach sequence, the Dragon spacecraft will halt again at a hold position 250 meters, or 820 feet, below the space station. This brief hold allows ground controllers to assess the status of the rendezvous and issue a "go" for the Dragon to enter the so-called keep-out sphere, an imaginary circle around the space station in which traffic is tightly controlled for safety reasons.
The timeline calls for the Dragon spacecraft to depart the 250-meter hold point around 5:39 a.m. EDT (0939 GMT), pausing again at a 30-meter hold position before pressing on to a final point about 10 meters, or 33 feet, beneath the space station.
Arrival at the final hold point is scheduled around 6:56 a.m. EDT (1056 GMT).
Astronauts Koichi Wakata and Rick Mastracchio will monitor the final phase of the Dragon's approach, including manning the space station's robotic arm to grapple the free-flying cargo craft. Grapple is scheduled for 7:14 a.m. EDT (1114 GMT). Once the Dragon is firmly snared by the robotic arm, the 58-foot Canadarm will move the capsule into position for berthing with the Earth-facing port on the space station's Harmony module around 9:45 a.m. EDT (1345 GMT).
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"Flight computers continued transmitting for 8 seconds after reaching the water. Stopped when booster went horizontal."
The Dragon spacecraft uses a LIDAR, or laser, instrument to attain position, range and attitude information on the space station. Dragon's rendezvous suite also includes a thermal imager.
"Orbit insertion and Dragon deploy all good. Falcon reentry burn also good. Waiting for landing data from tracking plane," Musk tweeted.
"Last known state for rocket boost stage is 360 m/s, Mach 1.1, 8.5 km altitude and roll rate close to zero (v important!)."
The rocket is now traveling more than 6,000 mph.
And chilldown of the second stage's vacuum-rated Merlin 1D engine has started in preparation for its ignition.
The command to start the ignition sequence for the first stage will be issued at T-minus 3 seconds, triggering the Merlin engines' ignitor moments before the powerplants actually ramp up to full power.
The strongback has retracted into the launch position more than 20 degrees from the rocket.
The second stage thrust vector steering system has checked out and is ready for flight.
The strongback umbilical tower will soon be lowered a few degrees to clear the rocket for launch. The procedure begins with opening of cradles gripping the rocket at attach points, then hydraulics lower the tower into launch position.
The Dragon spacecraft is being placed on internal power at this time.
The terminal countdown autosequence is about to begin at the T-minus 10 minute mark.
Fully fueled for launch, the Falcon 9 contains about 1.05 million pounds of kerosene and liquid oxygen propellants.
Linkup with the complex is scheduled at 7:14 a.m. EDT (1114 GMT) Sunday, when astronauts Koichi Wakata and Rick Mastracchio will grapple the Dragon spacecraft with the space station's robotic arm.
Among the supplies packed inside are two legs for NASA's Robonaut 2 humanoid robot aboard the International Space Station. The legs will be affixed to the robot's torso and head, which launched to the complex on a 2011 space shuttle mission.
Officials plan for Robonaut to eventually be able to go outside the space station to conduct routine maintenance, reducing astronauts' exposure to the the risks of spacewalks.
The Dragon's total internal payload includes 1,576 pounds of science and research gear, 1,049 pounds of crew provisions, 449 pounds of vehicle hardware, and 271 pounds of spacewalk tools.
The pressurized passengers include a vegetation growth experiment, in which red romaine lettuce will be grown aboard the space station. The plants will return to Earth with the Dragon capsule in May for analysis.
Future crews could use such plant growth housings for fresh fruit and vegetables during their six-month expeditions in orbit.
The commercial spaceship's unpressurized trunk carries two NASA payload packages to demonstrate optical communications and high-definition Earth viewing from the space station.
Today's launch marks SpaceX's third operational resupply run to the space station under a 12-mission, $1.6 billion contract with NASA covering cargo services through 2016.
All weather rules are observed "go" for launch at the moment, and officials are optimistic no more storms will develop to threaten liftoff this afternoon.
SpaceX says the first stage retrieval on this mission is purely experimental, and Elon Musk, the company's chief executive, says the rough seas are preventing the boat from getting to the expected touchdown zone.
The rough seas are not expected to be a constraint to launch, as officials monitor conditions over the Cape Canaveral launch pad in hopes of an opportunity to lift off today at 3:25 p.m. EDT (1925 GMT).
We will have the live webcast of the launch on this page beginning at 2:15 p.m. EDT (1815 GMT).
If you are heading out to the beach or Port Canaveral to watch the launch, sign up for our Twitter feed to get occasional countdown updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)
And if you are need tips on picking a good viewing spot, check out this authoritative guide on where to go.
The two-stage rocket burns RP-1 fuel -- a high-refined kerosene -- and liquid oxygen during today's nine-minute launch sequence.
Fueling began at 11:47 a.m. EDT (1547 GMT).
Today's flight marks the ninth launch of a Falcon 9 rocket since debuting in June 2010. It's the fourth mission of the improved Falcon 9 v1.1 version, which made its inaugural launch in September from Vandenberg Air Force Base.
For details on the Falcon 9 v1.1, here is part of our story on the rocket stemming from an interview with SpaceX boss Elon Musk in September:
Musk said the redesigned Falcon 9 is the prototype for a reusable rocket SpaceX envisions could drastically reduce launch costs, decreasing the price of a Falcon 9 flight even lower than SpaceX's advertised rate, which undercuts competing rockets, such as the Russian Proton and Europe's Ariane 5 launcher.
It is this version of the Falcon 9 that SpaceX hopes will safely deliver astronauts to orbit on the way to the space station, beginning as soon as 2017.
Reliability is paramount in the launch business, and cost and schedule are right behind in a matrix of concerns for rocket buyers.
Musk said SpaceX answered these appeals, and added power and efficiency to the Falcon 9's Merlin engines to loft heftier payloads into higher orbits.
SpaceX engineers installed a triple-redundant flight computer in the Falcon 9 rocket, adding another level of confidence in the launcher's avionics. They also wrote new software for the computer, which is based on a flight-proven unit from SpaceX's Dragon cargo-carrying space station freighter.
"You could put a bullet hole in any one of the avionics boxes and it would just keep flying," Musk said.
Designers adjusted the connection points between the Falcon 9's first and second stages, replacing nine hardware interfaces and three spring-like pusher elements - pneumatic devices which ensure stage separation occurs - with three connectors with integrated pushers.
"We go from 12 things that can go wrong to three at the point of staging," Musk said.
The Falcon 9 v1.1 is powered by 10 Merlin 1D engines - nine on the first stage and one on the second stage - each generating 147,000 pounds of sea level thrust. The vacuum-rated upper stage engine, sporting a niobium nozzle to radiate engine heat, produces 161,000 pounds of thrust once out of the atmosphere.
The Merlin 1C engine, used on all five of the Falcon 9's previous missions, was capable of firing with 95,000 pounds of thrust at sea level.
Along with greater performance, the Merlin 1D is easier to manufacture thanks to high-efficiency processes, increased robotic construction and a reduced parts count, according to SpaceX's press kit.
SpaceX upgraded the propellant injection system inside the Merlin 1D, replacing two valves dedicated to fuel and oxidizer with a single unit to improve reliability and save weight.
Musk said the Merlin 1D engine weighs in at less than 1,000 pounds.
"If we don't have the world record for thrust-to-weight ratio, we're very close," Musk said.
Musk's rocket team modified the engine arrangement on the first stage, an effort he said allows engineers to remove aerodynamic manifolds around the perimeter of the rocket.
Earlier Falcon 9s featured a square "tic-tac-toe" layout of the nine first stage engines arrayed in a three-by-three pattern. The Falcon 9 v1.1 uses what SpaceX calls an "octaweb" design, with eight engines surrounding a center engine in a circular pattern.
According to Musk, engineers installed ablative bumpers between the engines to prevent a mishap with one engine from damaging another.
The first stage upgrades also include a heat shield and stretched propellant tanks for the Merlin engines' supply of kerosene and liquid oxygen.
"We put a stronger heat shield at the base of the rocket to better enable the first stage to survive the high dynamic pressure on re-entry," Musk said.
The new Falcon 9 first stage is 60 percent longer but has the same diameter as the Falcon 9's previous version, permitting the rocket to be fabricated with the same tooling already inside SpaceX's rocket factory in Hawthorne, Calif.
There continues to be a 60 percent chance of violating weather rules due to thick clouds, lightning and precipitation in the rocket's flight path.
But Mike Suffredini, NASA's space station program manager, says he is hopeful the launch team will find a hole in the weather to lift off on time this afternoon.
The Falcon 9 rocket rolled out to Cape Canaveral's Complex 40 launch pad overnight and was rotated vertical around 8 a.m. EDT (1200 GMT), according to NASA.
Fueling of the two-stage launcher is set to begin around 11:30 a.m. EDT (1530 GMT).
Liftoff from Cape Canaveral is scheduled for 3:25 p.m. EDT (1925 GMT), weather permitting.
Forecasters predict widespread showers and isolated thunderstorms around the launch site Friday afternoon, with a 60 percent chance conditions prevent liftoff. The main concerns are with thick clouds, lightning and precipitation in the 208-foot-tall launcher's flight path.
If the rocket does not get off the ground Friday, another launch attempt could be made Saturday at 3:02 p.m. EDT (1902), when weather conditions should be improved enough to allow for better then even odds of liftoff.
The mission's first launch attempt Monday was scrubbed about an hour before blastoff after engineers detected a helium valve in the Falcon 9's pneumatic stage separation system was not holding the correct pressure.
Technicians returned the launcher to its hangar and replaced the faulty valve.
Officials spent Thursday evening packing the Dragon spacecraft with refrigerated experiment samples and other time-sensitive cargo as part of the standard "late load" activities before each resupply flight to the space station.
If the SpaceX launch goes on schedule Friday, the Dragon capsule will arrive at the space station early Sunday, with grapple of the automated vehicle by the outpost's robot arm expected at 7:14 a.m. EDT (1114 GMT).
Astronauts Rick Mastracchio and Steve Swanson are preparing for a spacewalk next Wednesday, April 23, to replace a failed computer on the space station's central truss section. The computer stopped responding to commands April 11, threatening to delay the SpaceX cargo flight, but NASA managers approved the resupply mission after determining the space station's critical systems had enough redundancy to safely deal with any further failures.
If Friday's launch is scrubbed, NASA plans to move the spacewalk forward to Sunday, April 20. A Falcon 9 launch Saturday would take a longer three-day route to the space station, arriving April 22 after the spacewalk.
Launch of the 208-foot-tall rocket is set for 3:25 p.m. EDT (1925 GMT) from Cape Canaveral's Complex 40 launch pad.
"On Friday, a dynamic weather pattern will develop as a surface low pressure area forms on the stalled frontal boundary in the Gulf of Mexico," Air Force forecasters wrote in a weather summary. "The exact track of this feature as it transits the Florida Peninsula will have a drastic and varied result in the weather over the spaceport. If the low tracks further north and makes landfall north of the Big Bend area of Florida as models have been trending, unfavorable weather will be much more isolated. If the low tracks closer to Central Florida, thick cloud cover, periods of rain and isolated thunderstorms are likely."
The outlook for Friday predicts showers and thunderstorms, scattered clouds at 3,000 feet, broken clouds at 10,000 feet and an overcast cloud deck at 25,000 feet. Winds will be out of the southeast at 20 to 25 mph with a temperature of 76 degrees Fahrenheit and a visibility of 5 miles.
The main worries for tomorrow's attempt are violating the thick cloud rule, the lightning rule and flight through precipitation.
Conditions on Saturday will be slightly improved, with a 40 percent chance of weather preventing launch.
Significant weather improvements won't come until early next week.
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There is a 60 percent chance weather will violate weather constraints for Friday's launch opportunity at 3:25 p.m. EDT (1925 GMT).
The earliest launch opportunity will be Friday at 3:25 p.m. EDT (1925 GMT).
Among the supplies packed inside are two legs for NASA's Robonaut 2 humanoid robot aboard the International Space Station. The legs will be affixed to the robot's torso and head, which launched to the complex on a 2011 space shuttle mission.
"Robonaut is an example of how we can use robots for reptitive and dangerous tasks in space," said Andy Petro, head of NASA's small satellite technology development program.
Petro said the legs will be unpacked and attached to Robonaut 2 in the coming months, with an eye toward testing the assembled robot within the confines of the space station's pressurized modules in August or September.
"We call them legs," Petro said. "They're not really for walking in the zero gravity environment. They're used for climbing around."
The legs have seven joints to give Robonaut flexibility when moving around the station.
"At the end, instead of feet they have clamping devices to allow them to connect to handrails anod other objects on the space station."
A power backback will be launched to the space station this summer to finish outfitting Robonaut to move around on its own. Right now, the robot can only be powered through an extension cord connected to the space station's electrical supply.
Officials plan for Robonaut to eventually be able to go outside the space station to conduct routine maintenance, reducing astronauts' exposure to the the risks of spacewalks.
The Dragon's total internal payload includes 1,576 pounds of science and research gear, 1,049 pounds of crew provisions, 449 pounds of vehicle hardware, and 271 pounds of spacewalk tools.
The commercial spaceship's unpressurized trunk carries two NASA payload packages to demonstrate optical communications and high-definition Earth viewing from the space station.
Today's launch marks SpaceX's third operational resupply run to the space station under a 12-mission, $1.6 billion contract with NASA covering cargo services through 2016.
We will have the live webcast of the launch on this page beginning at 3:45 p.m. EDT (1945 GMT).
If you are heading out to the beach or Port Canaveral to watch the launch, sign up for our Twitter feed to get occasional countdown updates on your cellphone. U.S. readers can also sign up from their phone by texting "follow spaceflightnow" to 40404. (Standard text messaging charges apply.)
And if you are need tips on picking a good viewing spot, check out this authoritative guide on where to go.
There continues to be an 80 percent chance of favorable weather for launch, with the only threat being the potential of violating the cumulus cloud rule.
The official outlook calls for scattered clouds at 3,000 feet, broken clouds at 25,000 feet, southeast winds at 17 to 22 mph, and a temperature of 76 degrees Fahrenheit.
The forecast for the first stage landing zone a few hundred miles northeast of Cape Canaveral calls for 50 percent cloud coverage, a visibility of 7 miles and isolated rain. Seas are expected to be acceptable for recovery of the stage.
The launch is timed so the Dragon spaceship can reach the space station with minimal fuel usage.
Arrival at the complex is set for early Wednesday morning, U.S. time, with grapple by the space station's robot arm expected at 7:11 a.m. EDT (1111 GMT). Astronauts Koichi Wakata and Rick Mastracchio will be in charge of the monitoring Dragon's automated laser-guided rendezvous and snagging the craft with the Canadian-built arm.
The spacecraft will be placed on an attach point on the outpost's Harmony module a few hours later.
The Dragon spacecraft is currently expected to depart the station around May 8, bringing approximately 1.8 tons of equipment back to Earth with a splashdown in the Pacific Ocean.
The two-stage rocket burns RP-1 fuel -- a high-refined kerosene -- and liquid oxygen during today's nine-minute launch sequence.
The RP-1 loading began around 1 p.m. EDT (1700 GMT), and liquid oxygen loading will begin shortly.
Today's flight marks the ninth launch of a Falcon 9 rocket since debuting in June 2010. It's the fourth mission of the improved Falcon 9 v1.1 version, which made its inaugural launch in September from Vandenberg Air Force Base.
For details on the Falcon 9 v1.1, here is part of our story on the rocket stemming from an interview with SpaceX boss Elon Musk in September:
Musk said the redesigned Falcon 9 is the prototype for a reusable rocket SpaceX envisions could drastically reduce launch costs, decreasing the price of a Falcon 9 flight even lower than SpaceX's advertised rate, which undercuts competing rockets, such as the Russian Proton and Europe's Ariane 5 launcher.
It is this version of the Falcon 9 that SpaceX hopes will safely deliver astronauts to orbit on the way to the space station, beginning as soon as 2017.
Reliability is paramount in the launch business, and cost and schedule are right behind in a matrix of concerns for rocket buyers.
Musk said SpaceX answered these appeals, and added power and efficiency to the Falcon 9's Merlin engines to loft heftier payloads into higher orbits.
SpaceX engineers installed a triple-redundant flight computer in the Falcon 9 rocket, adding another level of confidence in the launcher's avionics. They also wrote new software for the computer, which is based on a flight-proven unit from SpaceX's Dragon cargo-carrying space station freighter.
"You could put a bullet hole in any one of the avionics boxes and it would just keep flying," Musk said.
Designers adjusted the connection points between the Falcon 9's first and second stages, replacing nine hardware interfaces and three spring-like pusher elements - pneumatic devices which ensure stage separation occurs - with three connectors with integrated pushers.
"We go from 12 things that can go wrong to three at the point of staging," Musk said.
The Falcon 9 v1.1 is powered by 10 Merlin 1D engines - nine on the first stage and one on the second stage - each generating 147,000 pounds of sea level thrust. The vacuum-rated upper stage engine, sporting a niobium nozzle to radiate engine heat, produces 161,000 pounds of thrust once out of the atmosphere.
The Merlin 1C engine, used on all five of the Falcon 9's previous missions, was capable of firing with 95,000 pounds of thrust at sea level.
Along with greater performance, the Merlin 1D is easier to manufacture thanks to high-efficiency processes, increased robotic construction and a reduced parts count, according to SpaceX's press kit.
SpaceX upgraded the propellant injection system inside the Merlin 1D, replacing two valves dedicated to fuel and oxidizer with a single unit to improve reliability and save weight.
Musk said the Merlin 1D engine weighs in at less than 1,000 pounds.
"If we don't have the world record for thrust-to-weight ratio, we're very close," Musk said.
Musk's rocket team modified the engine arrangement on the first stage, an effort he said allows engineers to remove aerodynamic manifolds around the perimeter of the rocket.
Earlier Falcon 9s featured a square "tic-tac-toe" layout of the nine first stage engines arrayed in a three-by-three pattern. The Falcon 9 v1.1 uses what SpaceX calls an "octaweb" design, with eight engines surrounding a center engine in a circular pattern.
According to Musk, engineers installed ablative bumpers between the engines to prevent a mishap with one engine from damaging another.
The first stage upgrades also include a heat shield and stretched propellant tanks for the Merlin engines' supply of kerosene and liquid oxygen.
"We put a stronger heat shield at the base of the rocket to better enable the first stage to survive the high dynamic pressure on re-entry," Musk said.
The new Falcon 9 first stage is 60 percent longer but has the same diameter as the Falcon 9's previous version, permitting the rocket to be fabricated with the same tooling already inside SpaceX's rocket factory in Hawthorne, Calif.
Read our full story.
The mission is SpaceX's third operational cargo flight to the space station, coming after successful Dragon delivery missions in October 2012 and March 2013. The Dragon spaceship completed a test run to the complex in May 2012.
Technicians completed the "late load" of time-sensitive cargo and experiment samples into the Dragon spacecraft's pressurized cabin yesterday, and the rocket was erected atop Cape Canaveral's Complex 40 launch pad overnight.
SpaceX and NASA report all systems are in good shape for today's countdown.
Fueling of the Falcon 9 rocket is expected to begin around 1 p.m. EDT (1700 GMT).
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The "late load" cargo includes refrigerated experiment samples and fresh food for the space station's six-person crew. Officials install such materials at the last possible moment because they degrade over time.
The Falcon 9 rocket was lowered from the Cape Canaveral's Complex 40 launch pad around midday to begin the process of packing the mission's final cargo materials.
Overall, the Dragon spaceship will deliver 2.4 tons of food, experiments, spare parts and crew provisions to the International Space Station.
The multiplexer-demultiplexer, mounted on the space station's center S0 truss segment, stopped responding to commands Friday. The computer controls several systems on the truss, including the external thermal control system, rotary joints which point the station's large wing-like solar arrays toward the sun, and the mobile transporter, a rail car used to move the robotic arm.
Mike Suffredini, NASA's space station program manager, said a mission management team meeting Sunday morning confirmed the decision to go ahead with the SpaceX launch Monday.
The cargo mission, known as SpaceX-3 in NASA parlance, is set for liftoff on a Falcon 9 rocket at 4:58:24 p.m. EDT (2058:24 GMT).
"The team concluded the MMT with a go for SpaceX-3, a go to move the mobile transporter to work site 2 this afternoon at about 3 o'clock local time, and to get the new solar array angles on-board so we can protect ourselves for the next worst failure should it occur."
The failed computer box is known as EXT-2. It was a redundant unit and the primary computer continues to function normally.
Moving the mobile transporter clears the way for a spacewalk, tentatively set for April 22, to change out the failed computer. Suffredini said the software code to be uploaded to the space station will automatically put the lab's eight huge solar array wings in the correct orientation for arrival of the SpaceX Dragon cargo ship should the primary EXT-2 computer fail.
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The failed computer, called a multiplexer-demultiplexer, or MDM, provides redundancy for the space station's mobile transporter, part of the outpost's robotics system that allows the Canadian-built robot arm to move along the lab's truss backbone.
The robot arm will be used to grapple the SpaceX Dragon spacecraft when it arrives at the space station. In a statement issued Saturday, NASA said the Canadian-built robot arm has other backup capabilities not affected by the failed computer.
NASA says the failed MDM is known as "EXT-2" and measures 10.5 x 14.9 x 16.4 inches and weighs 50.8 pounds. The computer failed during a routine health check of the device, according to NASA.
"Station program officials, flight controllers and teams of engineers are working to determine whether there is any risk to launching the SpaceX cargo craft Monday," the statement said. "They will evaluate whether the station has enough redundancy to permit the launch to proceed."
NASA says a separate team is planning a contingency spacewalk to replace the failed computer box, but no date for the spacewalk has been set.
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The launch readiness review included presentations from top NASA and SpaceX officials, ensuring the rocket, payload and ground systems are prepared to support launch.
Launch from Cape Canaveral's Complex 40 launch pad is set for 4:58 p.m. EDT (2058 GMT), the instant the space station's ground track passes over Florida's Space Coast.
SpaceX's Falcon 9 rocket will boost a Dragon cargo capsule into low Earth orbit, beginning a day-and-a-half pursuit of the space station culminating with grapple by the lab's robot arm early Wednesday.
The Dragon spacecraft is packed with 2.4 tons of experiments, supplies and hardware for the space station. The mission is the third of 12 flights under a $1.6 billion commercial resupply contract with NASA.
The weather outlook is generally favorable for launch Monday, with Air Force meteorologists predicting a 70 percent of acceptable conditions at launch time.
But an approaching cold front will bring clouds and a chance of rain into Central Florida by Monday afternoon.
"The increased instability will create a greater risk for thunderstorms in Central Florida with the sea breeze and, combined with the westerly flow aloft, could bring electrified anvil clouds over the launch area," forecasters wrote in a summary. "These factors make the primary launch day concerns thick clouds and anvil clouds. Maximum upper level launch winds will be 70 knots from the west at 42,000 feet."
The official outlook calls for scattered clouds at 3,000 feet, broken clouds at 25,000 feet, southeast winds at 17 mph gusting to 22 mph, and a temperature of 76 degrees Fahrenheit.
Stay with Spaceflight Now throughout the weekend for in-depth coverage of the mission.
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Here is the statement from SpaceX:
To ensure the highest possible level of mission assurance and allow additional time to resolve remaining open items, SpaceX is now targeting March 30th for the CRS-3 launch, with April 2nd as a back-up. These represent the earliest available launch opportunities given existing schedules, and are currently pending approval with the Range.Both Falcon 9 and Dragon are in good health; given the critical payloads on board and significant upgrades to Dragon, the additional time will ensure SpaceX does everything possible on the ground to prepare for a successful launch.
Liftoff is set for 4:41 a.m. EDT (0841 GMT) from Cape Canaveral's Complex 40 launch pad at the moment the path of the space station's orbit passes over Florida's Space Coast.
Less than 10 minutes later, the Falcon 9's upper stage will deploy the cargo-carrying Dragon spacecraft to begin a two-day pursuit of the complex.
SpaceX will also try to recover the Falcon 9's first stage in the Atlantic Ocean in a key test of the rocket's reusability.
The preliminary weather forecast calls for favorable conditions Sunday morning, with a 30 percent of violating launch constraints.
"On launch day a weak, fast-moving low pressure system is forecast to traverse the southern Gulf Coast states, bringing cloudiness into northern Florida. There is a slight risk the cloud cover could make it over the spaceport, making the primary launch day concern the thick cloud rule," Air Force meteorologists wrote in a forecast synopsis.
The forecast predicts a few clouds at 6,000 feet and scattered clouds at 25,000 feet, surface winds out of the south at 12 to 18 mph, a temperature of 67 degrees Fahrenheit, and upper level winds out of the west at 100 knots.
If the launch gets delayed 24 hours, the forecast worsens for Monday with a 70 percent chance of weather prohibiting launch.
SpaceX officials plan to hold a launch readiness review Friday to approve final launch preparations ahead of the start of the countdown late Saturday.
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