The ISS could be disassembled post retro burn to provide smaller pieces for burn-up. If the hull space were filled with a stoichiometric mix of propellant from the deorbiting vessel post-burn, it would autodetonate on re-entry.
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$\begingroup$ If nothing else it would also be easier to identify who debris belong to, according to Outer Space Treaty Article VIII. $\endgroup$– Bojan KogojCommented Nov 5, 2021 at 13:54
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$\begingroup$ Disassembly may benefit burn-up of ISS. If disassembly is desirable, it needs to be done after the re-entry burn. But the ISS should be uncrewed during that period for safety reasons. Demolition is a possible mechanism. Using propellant from the de-orbiting thruster is likely the cheapest and most reliable demolition choice. It is likely the safest in event of a failed detonation. $\endgroup$– WoodyCommented Nov 5, 2021 at 23:31
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This presumes that a discrete vehicle should be docked and use its thrusters to deorbit the ISS. A more obvious approach for a controlled deorbit would be having the ISS use its own thrusters on the Zvezda module, which is one of the ways that its orbit has been maintained over the years. However since these use hypergolic fuel (as do all of the visitors that have provided boosts such as the Space Shuttle's OMS engines) there's no way to pre-mix the propellants.
That aside, it's not obvious that a detonation in the habitable spaces will be helpful in reducing ground-impacting material: the most hazardous parts are dense and strong, such as engine bells and pressure flasks, and these are known to survive rocket explosions. And auto-ignition of the propellant by re-entry heat feels unreliable, relying upon that collection of tubes to hold together in the mounting gale of re-entry until it becomes incandescent.
So probably it's the mundane approach: a modest thruster burn to send the ISS into the "spacecraft cemetry" between NZ and Chile, far from substantial habitations. But if your modest proposal was adopted I hope it's timed for just before ground-level dawn in winter east of a major city such as New York so that many people can see the expanding vapour and debris cloud catching the early sun.
answered Nov 6, 2021 at 0:14
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$\begingroup$ I assume an accurate impact zone will require a decisive delta-v from the deorbit burn. The Zvezda S5.79 thrusters have 890kg of fuel and 302s specific thrust. Can they provide the delta-v necessary for the ISS mass? $\endgroup$– WoodyCommented Nov 7, 2021 at 11:39
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$\begingroup$ space.stackexchange.com/a/24357/14637 has the details: in a nutshell a docked supply vessel was planned and will supply enough thrust to re-enter the station predictably enough for a disposal region. $\endgroup$ Commented Nov 8, 2021 at 10:11
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$\begingroup$ Thanks. Excellent post. It contains a link to NASA's 2021 deorbit plan: nasa.gov/pdf/578543main_asap_eol_plan_2010_101020.pdf $\endgroup$– WoodyCommented Nov 8, 2021 at 14:51
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$\begingroup$ The 2010 deorbit plan was reaffirmed in 2019. However, since then Russia's involvement with ISS has become less clear. interestingengineering.com/… . If a new deorbit vehicle choice is needed, I'm sure Mr. Musk would volunteer to assist. In that case, Methane and oxygen would likely be available for the proposed demolition. $\endgroup$– WoodyCommented Nov 8, 2021 at 15:00
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$\begingroup$ error: "NASA's 2021 deorbit plan" should read "2010 deorbit plan" $\endgroup$– WoodyCommented Nov 8, 2021 at 15:01