CRS-33's Dragon capsule isn't your average Cargo Dragon

Buried so deeply in the mission details about the Cargo Dragon launched yesterday that I didn't see it anywhere is that this isn't a standard Cargo Dragon.  This Dragon contains a scale model test system for the one that will enable the ship to kick the International Space Station out of its orbit and send it to its destruction on re-entry.  The modified Dragon with the full-sized engine system is going to be called the US Deorbit Vehicle (USDV) and is expected to be needed in 2030 or the early 2030s. 

You may recall that the contract for the USDV was awarded to SpaceX in June of '24.  The basic problem is that due to various factors, the atmospheric drag on the ISS varies over time and that lowers the orbit over time.  Yes, there's still atmospheric drag at the altitude of the station, primarily because the atmosphere can expand and contract over time.

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.

Historically, Northrop Grumman's Cygnus spacecraft first demonstrated its ability to raise the station's orbit in 2022.  There are a couple of problems with using the Cygnus - the current big problem that gets them out of consideration is that Cygnus can't fly because their launch vehicle depended on Russian engines.  That's the root cause of why SpaceX is flying Cygnus CRS missions to the ISS.  

As always, it's more complicated than that, including that Cygnus has to dock in a different place than Cargo Dragon can, and Dragon actually nets some more positive effects from that.  Dragon, though, suffers most from using small thrusters.  It just simply needs "more power."   

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."

A look inside the Dragon spacecraft's trunk shows six propellant tanks covered in silver insulation and a central helium pressurant tank overwrapped in black carbon. Credit: SpaceX

From the way it's discussed, it doesn't sound like there's a specific planned date for this test.  When a desired time is decided on and all is ready, the Draco thrusters will gently accelerate the massive 450-ton complex.  It's calculated that the reboost kit can add about 20 mph, or 9 meters per second, to the space station's orbital speed. It doesn't sound like much, but they quote analysis saying it's the same speed-up contributed by "one-and-a-half Russian Progress cargo vehicles."  

I'm sure verifying those calculations and their underlying assumptions are what this mission is all about.  After all, they're planning to splash that 450-ton behemoth into deep water.  It would be a Very Bad Thing to accidentally crash it into a big city because the calculations were wrong.