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Given the high availability of ISS Commercial Cargo launches for NASA, it would seem that the aerospace stereotype of "mass reduction is paramount" may be outdated. What is the current limiting factor for (pressurized) cargo throughput per launch?

These are the categories I thought of:

  1. Weight-Constrained: The limiting factor for adding more cargo to a launch is the max payload weight of the launch vehicle.
  2. Volume-Constrained: The limiting factor is max total volume and/or packing efficiency.
  3. Crew Time-Constrained: The limiting factor is the time commitment for crew to unpack visiting vehicles, or the max "shelf life" of the visiting vehicle before undocking for reentry.
  4. Underfilled: NASA doesn't fill cargo volume or mass. This would likely be due to one of a few reasons. 1: Time-sensitive payloads require a faster launch tempo. 2: Risk of launch failure is mitigated by spreading out necessary supplies across semi-redundant consecutive launches. 3: (Unlikely?) NASA lets cargo space go to waste because it already paid but doesn't need the capacity.

To phrase it in a different way, if I wanted to get a package to the ISS, what should I optimize my package for?

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  • $\begingroup$ Note that un undefiled (by weight) cargo can still be compensated for by performing a reboot maneuver $\endgroup$ – Antzi May 30 '19 at 2:58
  • $\begingroup$ @Antzi yes, but I believe that is only for the Progress and ATV spacecraft. $\endgroup$ – CourageousPotato May 30 '19 at 3:21
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    $\begingroup$ Another potential payload problem is weight distribution, if you're sending something that is 40x heavier on the right side than it is on the left, you may have a problem if it's massive enough. $\endgroup$ – Magic Octopus Urn May 30 '19 at 18:50
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The most simplified answer I could give you is to make it as compact and light as it needs to be. To my knowledge there isn't a standard checklist for items being sent to the station, there is obviously a list of items that are allowed in space but most cargo items are picked and vetted on a case by case basis. If you can convince NASA and it's partners that this 100 pound chunk of lead will have some scientific significance on the ISS than it can go. When it comes to perishables, again its case by case, NASA has sent fresh fruit to the ISS at times and Pizza Hut paid Russia to send a pizza. Basically it's like a really big camping trip, send what you absolutely need and just in case, maybe send a few extras but nothing that is not necessary.

To be more specifically here are some size and weight constraints:

Volume

  • Progress Vehicle: 7.6 m3
  • H-II Transfer Vehicle: 14 m3
  • Dragon 1 Cargo: 10 m3
  • Dragon 2 Cargo: ~9.3 m3
  • Standard Cygnus Spacecraft: 18.9 m3
  • Enhanced Cygnus Spacecraft: 27 m3
  • ATV: 48 m3

Maximum Width

Based on the width of the docking adapter used by vehicle.

  • Progress, ATV - SSVP (aka Russian Docking System): 80 cm
  • H-II, Cygnus, and Dragon 1 - Common Berthing Mechanism: 127 cm
  • Dragon 2 - International Docking Adaptor 2: 160 cm

Maximum Weight (pressurized cargo only)

  • Progress: 2,400 kg

  • H-II: 5,200 kg

  • Dragon 1: 3,310 kg

  • Dragon 2: ~6,000 kg

  • Standard Cygnus: 2,000 kg

  • Enhanced Cygnus: 3,200 kg

  • ATV: 5,500 kg

As I was researching this I honestly surprised my self with all the variations that go into this. Didn't even get to unpressurized cargo space that could be stored in the external stowage platforms then brought in through an airlock. Or possible limitations based on the rocket or current available space on the ISS. There are possibly many other limitations that just aren't public yet. I guess this is why there is whole team in charge of the inventory that is up on the ISS and why there's a nice beefy pdf just for the barcode system they have set up. Best bet is if you are actually planning on sending something to the ISS, just reach out to NASA themselves.

I hope this helped, probably didn't answer your questions but I don't think there really is one, like I said in the beginning, what goes up to the ISS is very case by case for approval.

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    $\begingroup$ Thanks for the specs! This helps. My core question is more along the lines of how the logistics end up working out for these cargo launches. If the limit is volume, it would mean I could opt for a heavier design while keeping it compact. P.S. I may or may not actually work at NASA ;). I’ll update the question or add my own answer if I find exportable information about cargo flight ops like this. $\endgroup$ – CourageousPotato May 30 '19 at 4:50
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    $\begingroup$ You CBM vs PMA width numbers cannot be correct. The PMA is narrower than the CBM. $\endgroup$ – geoffc May 30 '19 at 9:55
  • $\begingroup$ Correct the PMA is narrower than the CBM, The PMA was placed on top of the CBM and adapted to the shuttle's docking connection. I didn't include the PMA because none of the current cargo vehicles use it. $\endgroup$ – Seth Kurkowski May 31 '19 at 14:35
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    $\begingroup$ @CourageousPotato Sorry I didn't answer your full question. If I find some time to do some research into the supply chain for the ISS I'll update my answer. $\endgroup$ – Seth Kurkowski Jun 4 '19 at 14:27

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