SpaceX is set to launch its 25th resupply mission to the International Space Station Thursday night from NASA’s Kennedy Space Center in Florida, following a five-week delay to resolve a leak in the Dragon cargo capsule’s propulsion system.
Liftoff of the commercial cargo mission is set for 8:44:22 p.m. EDT Thursday (0044:22 GMT Friday) from pad 39A at Kennedy. A SpaceX Falcon 9 rocket will give the mission a boost into orbit, putting the Dragon spacecraft on course for docking at the space station at 11:20 a.m. EDT (1520 GMT) Saturday.
Astronauts at the space station will open hatches and unpack supplies, experiments and other equipment stowed inside the Dragon capsule’s pressurized compartment. At the end of the mission, the reusable capsule will undock from the station and head for a parachute-assisted splashdown off the coast of Florida in mid-August with several tons of cargo.
The cargo ship is launching with around 5,800 pounds of supplies and payloads, including a NASA climate instrument to be mounted outside the space station.
The Earth Surface Mineral Dust Source Investigation, or EMIT, instrument was developed by NASA’s Jet Propulsion Laboratory. It will be attached to a mounting post outside the space station to measure the mineral content of the world’s desert regions, the source of global dust storms that can impact climate and weather worldwide.
Data collected by the instrument will help scientists learn more about how dust lifted into the atmosphere from deserts impact Earth’s ecosystems and human health.
“This is going to be a really busy mission for us,” said Dana Weigel, NASA’s deputy space station program manager. “It’s packed with a lot of science. The planned duration is about 33 days.”
The mission is the 25th flight to the space station under SpaceX’s Commercial Resupply Services contract with NASA. This flight, named CRS-25, was scheduled for launch in early June, but officials grounded the Dragon spacecraft after finding a leak in the ship’s propulsion system.
SpaceX detected “elevated vapor readings” of monomethyl hydrazine, or MMH, fuel in an “isolated region” of the Dragon spacecraft’s propulsion system during propellant loading ahead of the launch in early June, NASA said in a statement in June.
The fueling of the Dragon spacecraft is one of the final steps to prepare the capsule for flight, and typically occurs just before SpaceX moves the craft to the launch pad for integration with its Falcon 9 rocket.
The Dragon spacecraft has propellant tanks containing hydrazine fuel and nitrogen tetroxide oxidizer. The two propellants ignite upon contact with each other, providing an impulse for the cargo ship’s Draco thrusters used for in-orbit maneuvers.
Each Dragon spacecraft has 16 Draco thrusters, small rocket engines that generate about 90 pounds of thrust. The Draco engines are used for orbit adjustment burns and control the spacecraft’s approach to the space station, then fire at the end of the mission for a deorbit burn to guide the capsule back into the atmosphere for re-entry and splashdown.
SpaceX’s ground processing team, working at the Dragon refurbishment facility at Cape Canaveral Space Force Station, offloaded fuel and oxidizer from the area of the spacecraft with the elevated hydrazine vapor readings.
Benji Reed, SpaceX’s senior director of human spaceflight programs, said the vapor leak in the Dragon propulsion system was caused by “imperfections in the sealing surface where a valve connects into the system.”
Technicians replaced the valve and confirmed the leak stopped, allowing preparations for the CRS-25 launch to resume at Cape Canaveral. SpaceX’s ground team also replaced the four main parachutes already stowed on the capsule “out of abundance of caution,” Reed told reporters Wednesday.
Officials were concerned the vapors from the toxic hydrazine could have interacted and damaged material in the parachutes. Initial inspections of the chutes removed from the spacecraft show no obvious signs of degradation, Reed said, and the parachutes could be used on a future Dragon mission.
Reed said the issue that caused the leak does not affect other Dragon spacecraft in SpaceX’s fleet, including the Dragon crew capsule currently docked at the space station.
The connection in the propulsion system identified as the source of the hydrazine vapor leak had a sealing surface that was “reworked a little bit,” Reed said. But the sealing passed SpaceX’s preliminary testing, and teams didn’t catch the leak until performing a full-up test of the propulsion system before moving the spacecraft over to the Falcon 9 hangar for rocket integration.
“This was something that was caught early on because of the testing that we already do,” Reed said. “This is well before we’re in a situation where we’re stacked on the vehicle on the pad and worrying about going into launch.”
But SpaceX will beef up testing to try to catch similar occurrences earlier, Reed said.
With the leak resolved, SpaceX pressed ahead with launch preparations earlier this month, transferring the cargo capsule to the hangar at pad 39A to meet its Falcon 9 launcher.
SpaceX rolled the fully-assembled launcher to pad 39A on Tuesday, then raised the 215-foot-tall (65-meter) Falcon 9 vertical. Pad teams completed final loading of last-minute, time-sensitive cargo into the Dragon spacecraft Wednesday using the crew access arm extended to connect to the ship’s hatch.
With preparations complete, SpaceX’s launch team at Kennedy plans to switch on the Falcon 9’s control systems and activate the Dragon spacecraft Thursday for final countdown checks. If all parameters are “go” for launch, SpaceX will begin loading super-chilled, densified kerosene and liquid oxygen into the two-stage Falcon 9 rocket about 35 minutes prior to liftoff.
There is a 70% chance of favorable weather for launch Thursday, according to the U.S. Space Force’s 45th Weather Squadron. The main weather concerns are with cumulus clouds that could create a risk for lightning, and flight through precipitation.
After liftoff, the Falcon 9 will head downrange northeast from Florida’s Space Coast, powered by nine Merlin engines generating 1.7 million pounds of thrust. The rocket will shut down its first stage booster about two-and-a-half minutes into the mission, allowing the booster to descend to landing on a drone ship in the Atlantic Ocean about seven-and-a-half minutes after liftoff.
The booster, tail number B1067, is making its fifth flight on the CRS-25 mission. It previously launched the CRS-22 cargo mission last June, launched two NASA crew missions to the station, and hauled Turkey’s Turksat 5B communications satellite into space.
The Dragon spacecraft will deploy from the Falcon 9’s upper stage about 12 minutes after liftoff to begin the day-and-a-half journey to the International Space Station. The Dragon cargo capsule on the CRS-25 mission is launching on its third flight to the station.
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