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I heard in a few places downmass is a limiting factor in the ISS national lab capacity. Is that true?

According to NASA's pricing plan, it actually costs more to get downmass than upmass. Why is that?

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    $\begingroup$ To get a mass down from ISS you need a capsule with heat shields and parachutes. All that should be brought up from ground too the ISS before. So downmass should be more expensive than downmass. $\endgroup$
    – Uwe
    Jan 1, 2023 at 20:56
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    $\begingroup$ @Uwe - only if the capsule mass is more than the payload it can safely deorbit. That's the part that confuses me - looking at e.g. the Apollo CM, structure+heatshield+recovery+RCS mass was (barely) less than 50% of the capsule weight. (From en.wikipedia.org/wiki/Apollo_command_and_service_module -> 2681.2/5560 kg). And that had to withstand a lot harder reentry. $\endgroup$
    – TLW
    Jan 2, 2023 at 18:32
  • $\begingroup$ I've edited the question to link directly to NASA as opposed to spacenews.com as the latter obtained the information it posted directly from the former. $\endgroup$ Jan 7, 2023 at 10:44
  • $\begingroup$ Note that trash disposal is not free. The price NASA charges commercial entities for upmass and trash mass are identical, \$20K per kg. Downmass costs \$40K per kg. $\endgroup$ Jan 7, 2023 at 10:47

2 Answers 2

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For ISS, download payload is significantly more limited than upload. Being a scarcer resource, download mass is priced accordingly.

Spacecraft Upmass to ISS Downmass from ISS Notes
Cargo Dragon 3,307 kg 2,507 kg (+800 kg of trash) Trash is stored in the "trunk" which burns up
SoyuzTMA 100 kg + crew 50 kg + crew

Upload capacity is determined primarily by the launch vehicle. A bit more lift means a bit more cargo. But download is limited by the design of the re-entry vehicle. Overloading = instability and burn-through.

Downmass price is aimed at commercial users who want to download manufactured products, verses those who don't. And those users who want their equipment returned in one piece rather than trashed. There is a limited supply of up, down and trash so the price of each resource should be determined by demand. This ensures efficient use of resources. Or so say the "dismal scientists" in economics.

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    $\begingroup$ The Cargo Dragon can also take 800 kg of trash in its disposable trunk. A very important capability. $\endgroup$
    – Schwern
    Jan 1, 2023 at 23:56
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    $\begingroup$ There is (almost) no air in space and (almost) no drag. The ISS is moving horizontally at 27,600 km/h around the Earth in orbit. If you chuck something out the airlock, let's say at 100 km/h, it will now orbit slightly lower at 27,500 km/h and be hazardous space junk. You have to slow it down a lot until its low enough to be slowed by atmospheric drag. Same reason we can't just "throw" our trash into the Sun; you have to cancel the Earth's 100,000 km/h orbital velocity around the Sun. See Can An Astronaut Throw Something From Orbit To Earth. $\endgroup$
    – Schwern
    Jan 2, 2023 at 0:22
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    $\begingroup$ @Woody Most of the cargo freighters (HTV, Cygnus, Progress, etc.) have/had 0 downmass, and are used to dispose of trash. $\endgroup$ Jan 2, 2023 at 1:21
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    $\begingroup$ @ErinAnne You can't pile infinite weight in a vehicle that's going to be destroyed--it has to have enough propellant to deorbit whatever you put in it. $\endgroup$ Jan 2, 2023 at 1:52
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    $\begingroup$ I think as long as it's made clear the difference between downmass and trash, it's a useful addition to the answer; both are similar issues facing the ISS. As Woody's comment demonstrates, I don't think people consider the trash problem. You can't just throw either out the airlock, you need a vehicle and propellant to deorbit both downmass and trash. The difference is one reaches the ground intact and the other does not. But it isn't my answer. $\endgroup$
    – Schwern
    Jan 2, 2023 at 5:10
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I heard in a few places downmass is a limiting factor in the ISS national lab capacity. Is that true?

That is true. NASA has two contractors (Northrop Grumman and SpaceX) plus other additional vehicles that can carry payload to the ISS, but only two of them (SpaceX and Russia) can bring material back to the surface of the Earth. Those vehicles that can return material to the surface can also dispose of trash, as can the Cygnus, the Automated Transfer Vehicle, the H-II Transfer Vehicle, and the Progress. Downmass is a precious resource.

According to NASA's pricing plan, it actually costs more to get downmass than upmass. Why is that?

Because downmass is a precious resource. With a few exceptions, everything that goes up to the ISS must come down, with the "coming down" as either trash or return. NASA appears to have arbitrarily split the cost of going up and coming down as trash (which is incinerated during reentry) 50-50, at \$20K per kg each way. Downmass is limited, so it is charged an extra \$20K per kg.

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