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SpaceX’s latest Dragon mission will breathe more fire at the space station [arstechnica.com]:

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SpaceX completed its 33rd cargo delivery to the International Space Station early Monday, when a Dragon supply ship glided to an automated docking with more than 5,000 pounds of scientific experiments and provisions for the lab's seven-person crew.

The resupply flight is part of the normal rotation of cargo and crew missions that keep the space station operating. The Dragon spacecraft's cargo haul comprised packages of fresh food, including some 1,500 tortillas, and equipment for numerous research investigations demonstrating 3D printing in microgravity and examining how the human body responds to long-duration spaceflight.

The cargo manifest is typical of most Dragon resupply flights traveling to the International Space Station. What's different with this mission is a new rocket pack mounted inside the Dragon spacecraft's rear trunk section. In the coming weeks, SpaceX and NASA will use this first-of-its-kind propulsion system to begin boosting the altitude of the space station's orbit.

Maintaining control

"The space station's altitude slowly decays over time due to the thin amount of atmosphere still at our altitude," said Bill Spetch, NASA's operations integration manager for the International Space Station. "To counteract that drag, we must occasionally raise the altitude of the ISS."

Responsibility for maintaining the station's orbit has historically been borne by the Russian space agency, which had the sole capability to reboost the ISS after NASA retired its space shuttle fleet in 2011. Russia's Progress cargo freighters often use their own thrusters to raise the lab's altitude or steer it out of the way of space junk. What's more, Progress ships can refill propellant tanks inside the station's Russian command post, giving the outpost the ability to perform its own maneuvers when necessary.

But that is changing as NASA works with SpaceX and Northrop Grumman, the agency's other commercial cargo transport contractor, to modify their Dragon and Cygnus supply ships for reboost missions.

Northrop's Cygnus spacecraft first demonstrated its ability to raise the station's orbit in 2022 [nasa.gov]. Cygnus missions connect with the space station at a berthing port on the bottom side of the complex. In the space station's usual configuration, this location is not ideal for a reboost because it is misaligned with the lab's velocity vector, an imaginary line running through the complex along its direction of travel.

In low-Earth orbit, thrusters raise the station's altitude by adding a small amount of velocity to the lab as it circles the Earth at more than 17,000 mph. The Cygnus spacecraft compensated for its suboptimal position on the ISS by using its steerable main engine, using gimbals to move the engine's nozzle to direct its thrust in the right direction.

SpaceX's Dragon cargo vehicles, on the other hand, are able to dock at the forward end of the space station's long axis. Theoretically, this would make it easier for Dragon to boost the station's orbit, but SpaceX must set aside enough propellant for the spacecraft to travel up to the ISS and then return to Earth at the end of its mission. Dragon's 16 Draco thrusters are also not steerable, but SpaceX demonstrated they could make small adjustments to the station's orbit last year.

When NASA asked SpaceX to modify Dragon for larger reboosts, engineers devised a new propulsion pack to be placed inside the hollow trunk of the spacecraft. This unpressurized compartment is mounted below the craft's pressurized cargo cabin, and it's where SpaceX usually carries larger experiments that are robotically attached to the outside of the ISS.

For this mission, SpaceX installed two additional Draco thrusters into the spacecraft's trunk. The small rear-facing rocket engines are closely aligned with the station's velocity vector, and they're connected to six dedicated propellant tanks in the trunk containing hydrazine and nitrogen tetroxide, combustible fluids that ignite upon contact with one another.

"Our capsule's engines are not pointed in the right direction for optimum boost," said Sarah Walker, SpaceX's director of Dragon mission management. "So, this trunk module has engines pointed in the right direction to maximize efficiency of propellant usage."

When NASA says it's the right time, SpaceX controllers will command the Draco thrusters to ignite and gently accelerate the massive 450-ton complex. All told, the reboost kit can add about 20 mph, or 9 meters per second, to the space station's already-dizzying speed, according to Walker.

Spetch said that's roughly equivalent to the total reboost impulse provided by one-and-a-half Russian Progress cargo vehicles. That's about one-third to one-fourth of the total orbit maintenance the ISS needs in a year.

"The boost kit will help sustain the orbiting lab's altitude, starting in September, with a series of burns planned periodically throughout the fall of 2025," Spetch said.

After a few months docked at the ISS, the Dragon cargo capsule will depart and head for a parachute-assisted splashdown in the Pacific Ocean off the coast of California. SpaceX will recover the pressurized capsule to fly again, while the trunk containing the reboost kit will jettison and burn up in the atmosphere.

While this mission is SpaceX's 33rd cargo flight to the ISS under the auspices of NASA's multibillion-dollar Commercial Resupply Services contract, it's also SpaceX's 50th overall Dragon mission to the outpost. This tally includes 17 flights of the human-rated Crew Dragon.

"With CRS-33, we'll mark our 50th voyage to ISS," Walker said. "Just incredible. Together, these missions have (carried) well over 300,000 pounds of cargo and supplies to the orbiting lab and well over 1,000 science and research projects that are not only helping us to understand how to live and work effectively in space... but also directly contributing to critical research that serves our lives here on Earth."

Future Dragon trunks will be able to accommodate a reboost kit or unpressurized science payloads, depending on NASA's needs at the space station.

The design of the Dragon reboost kit is a smaller-scale version of what SpaceX will build for a much larger Dragon trunk under a $843 million contract signed with NASA [arstechnica.com] last year for the US Deorbit Vehicle. This souped-up Dragon will dock with the ISS and steer it back into the atmosphere after the lab's decommissioning in the early 2030s. The deorbit vehicle will have 46 Draco thrusters—16 to control the craft's orientation and 30 in the trunk to provide the impulse needed to drop the station out of orbit.

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