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From NASA Solidifies Planning to Deorbit ISS in 2031:

ISS average altitude estimates targeting re-entry early 2021

ISS average altitude estimates targeting re-entry early 2021

Figure 4. shows the ISS end-of-life de-orbit altitude, cargo resupply, and de-orbit plan assumptions. This timeline assumption is reflected in the budget discussion above for de-orbit vehicle procurement.

Question: Why would NASA need to procure a de-orbit vehicle for ISS end-of-life de-orbiting? It's flipped over several times in the past, why not flip over and do its own de-orbit burn?

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From ISS CONTROLLED DEORBIT: CHALLENGES AND SOLUTIONS, Murtazin et al. (2017):

low thrust capability of all available thruster systems is considered to be a main constraint for the ISS deorbit

The idea (from that paper) is to limit the in track debris length to 6000 km (MIR heritage). Doing so requires going from a stable (relatively speaking) orbit to a destructive entry in ~half an orbit (~40 mins), with contingencies planning forcing the final staging orbit to be higher. This is probably why the thrust is the limiting factor considering the ISS is ~3x heavier than MIR was, yet still using the same Progress vehicles.

As @OrganicMarble points out, that paper also concludes (emphasis added):

If the scenario with SM only is selected the low perigee allows only one attempt for the final burn. It is clear that this option can be addressed only hypothetically

Additionally, the paper goes on to state that the minimum "non-propulsive attitude control" altitude is about 230-250 km. Below this altitude the paper estimates an attitude control propellant budget of ~100 kg per day! Consider that a Progress vehicle can then only deliver about 2 weeks worth of attitude control propellant at these use rates.

There are also "orbital mechanics" considerations relating to ground track phasing/repetition as disposal is targeted for a pacific specific region on Earth. More control (thrust) is desirable for managing all of these considerations.

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  • $\begingroup$ Woul increasing the apogee first (making the orbit more eliptical) help make the reentry steeper and reduce the length of the debris trail? OR would it increase reentry speed so much that it would increase the amount af debris reaching the surface too much? $\endgroup$
    – TrySCE2AUX
    Feb 10, 2022 at 10:05
  • $\begingroup$ @kruemi that would cost more fuel (money) $\endgroup$
    – BrendanLuke15
    Feb 10, 2022 at 12:23
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straight OTTOMH:

Why would NASA need to procure a de-orbit vehicle for ISS end-of-life de-orbiting?

The 'de-orbit vehicle' is more of a propellant tanker and extra bunch of thrusters for the required burns plus propellant contingency.

IIRC they saw maybe up to 3 Progress vehicles docked for the deorbit.

For the size and mass of the ISS I think they were thought they needed much more propellant on hand.

It's flipped over several times in the past, why not flip over and do its own de-orbit burn?

They already operate the optimal propellant maneuver flip to swing 180 degrees rather than burn.

So it is not to do with the 'flip'.

Bear in mind that the original deorbit working group was drawn up in 2011, and IIRC this was before OPM was brought into practice, so up until then, burns were used for flipping around.

Now they don't have to, thanks to the OPM that was developed.

So for the altitude drop from 400km to 280 they were going to rely on orienting the SARJ for maximum drag rather than waste propellant.

https://www.nasa.gov/pdf/578543main_asap_eol_plan_2010_101020.pdf

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    $\begingroup$ There are some drawings in here of firing the SM main engines with a Progress docked at the aft end (!) researchgate.net/publication/… I think the true answer is in that paper too: "If the scenario with SM only is selected the low perigee allows only one attempt for the final burn. It is clear that this option can be addressed only hypothetically." $\endgroup$
    – Organic Marble
    Feb 9, 2022 at 23:16
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    $\begingroup$ I didn't think Progress docked was an issue since the engines on the SM are canted outwards (never checked though). I can't remember where I read it but the Progress as de-orbit vehicle was primarily delivering propellant to the SM for the burns, and in itself acting as extra thrusters if/when required. There are also propellant-only supertanker Progress versions which might be what they are assuming to be used for this. $\endgroup$
    – blobbymcblobby
    Feb 9, 2022 at 23:24
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    $\begingroup$ Oh, cool! And what does "operate the optimal propellant maneuver flip" mean? $\endgroup$
    – uhoh
    Feb 9, 2022 at 23:58
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    $\begingroup$ @uhoh ntrs.nasa.gov/api/citations/20130014033/downloads/… I actually noticed it on the real time ISS tracker when the YPR figures went rolling simultaneously. I do wish a detached spacecraft would observe when it does it. They pretty much use them all the time now. $\endgroup$
    – blobbymcblobby
    Feb 10, 2022 at 0:10
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    $\begingroup$ by you it seems :-) space.stackexchange.com/a/54318/12102 $\endgroup$
    – uhoh
    Feb 10, 2022 at 0:32

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