arstechnica.com/space/2024/06/nasa-will-pay-spacex-nearly-1-billion-to-deorbit-the-international-space-station/
Eric Berger's take on how SpaceX will deorbit the ISS. "NASA will pay SpaceX nearly $1 billion to deorbit the International Space Station". In Ars Technica.
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The propellant mass needed to deorbit the ISS should fit into the volume of a Cargo Dragon with room to spare, according to calculations done by people on this forum and r/SpaceX. The mass will require a FH. I don't see the need for a Dragon XL. I'm a bit surprised at Eric being noncommittal about Dragon and including Dragon XL - perhaps he has people who've come up with better estimates of the needs of such a vehicle.
The required thrusters can be placed in the (permanently mounted) trunk and the propellant lines will run through the base of the capsule - there will be no heat shield. A surprisingly small amount of thrust is needed. SuperDracos are far to powerful, even throttled down. A large cluster of Dracos is likely - this also offers the opportunity for differential thrust when pointing the station. I suppose a DracoPlus can be developed easily if needed.
The vehicle needs to auto dock to the ISS and then stay for at least a year as they allow the orbit to degrade before the final 47 m/s deorbit burn.
So a lashed together assembly would not survive for that length of time to 99.5% probability. It also means a cryogenic rocket is not an option.
That is why Dragon XL with additional propellant tanks and aft thrusters is the logical choice.
Eric Berger, from a NASA statement:
"Bill Spetch, operations integration manager for NASA’s International Space Station Program, confirms that the US Deorbit Vehicle will be based on "Dragon heritage" hardware. It will involve modifications of the trunk."
Dragon or Dragon XL would both need a cluster of 32 Dracos (Scott Manley's estimate). Would both need prop tanks inside the ~cabin* with plumbing to them. Dragon heritage could mean Dragon XL but the mention of the trunk makes it pretty certain it'll be a Cargo Dragon. And you know that with 5 years to work with anything SpaceX works on won't be lashed together.
It remains to be seen if SpaceX develops and uses a few EnhancedDracos or uses a large cluster of Dracos. I'll be interested to see if they use differential throttling for maneuvering control.
*Dragon XL has fairly large tanks, afaik, to decelerate to NRHO, but my guess is they'd still be too small.
Yes Dragon XL tanks would definitely be too small and would require extended tanks in the cargo space.
If the actual NASA statement included reference to the trunk then logically it would be a modified Cargo Dragon. I wouldn’t have thought the current trunk had sufficient capacity for either mass or volume but a stretched version would certainly be possible.
I would think they would limit the Dragon changes and store all the deorbit propellant in the trunk. Dragon would still contain the RCS hardware and control the trunk through the Dragons claw.
I wouldn’t have thought the current trunk had sufficient capacity for either mass or volume but a stretched version would certainly be possible.
Just looked it up. The trunk as is has 35m³ volume. A little over half of that should hold the needed propellant for 50m/s delta-v.
But it seems, with the mass of a Dragon capsule they will need a Falcn Heavy to launch the needed propellant mass. But if they need more volume, SpaceX has mentioned the option of an extended trunk before.