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The SpaceX Starship (lunar and/or crew-rated version, upper stage only) is expected to have more habitable volume than the International Space Station. On the other hand, the ISS is around 400 tons, while the dry mass of the Starship is more like 100 tons (rounding to one significant digit in both cases). Now, some of that extra mass is likely down to the fact that short, fat cylinders have a better volume to surface area ratio than long, skinny cylinders. But it also seems plausible to me that a lot of the ISS mass is dedicated to functions beyond habitable space and propulsion, the two things that an orbital Starship would do extremely well. (One possible example: docking lots of ships at once.)

Hence my question:

If we put a human-rated Starship in orbit and tried to use it as a space station, what significant engineering capabilities would it lack that the ISS currently has (and takes advantage of)? Also consider whether the capabilities could be met another way: for instance, a Starship-station would not need the capability to run decades-old computer hardware.

By way of analogy: if the question were "What can a Google Datacenter do that a warehouse cannot?", a good answer might mention the need for massive amounts of water and electricity, as well as ventilation and any structural requirements that might be needed to support racks of processors. Essentially, what would Google look for if they were considering renting a building to house their datacenter? In the same fashion, what would a government look for if they were considering hiring an orbital spacecraft to house their people and experiments long-term, and which of these requirements would an off-the-shelf lunar or Martian Starship lack without significant modification?

Charles Staats
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    This is a very strange question. It's like asking what can a LearJet do, that a 10-tonne cement truck cannot. Aside from massing about the same, the two have nothing much in common. – CuteKItty_pleaseStopBArking Apr 26 '21 at 14:56
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    Note that this was the case of the Space Shuttle vs. a new Skylab. – JDługosz Apr 26 '21 at 16:13
  • @JDługosz The Space Shuttle orbiter could not serve as a long-term space station because it was resource-limited -- specifically, the fuel cells could only be kept running for a max of 40 days or so. This comes up often when people discuss possible rescue missions for Columbia. – Charles Staats Apr 27 '21 at 12:04
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    @PcMan Sometimes people (on this site and elsewhere) suggest that one could replace the ISS with a single Starship upper stage. I'm looking for the best arguments I can find that this replacement would not work, or at least is not as simple as it sounds. – Charles Staats Apr 27 '21 at 12:06
  • The best argument is that it is simple. as in stupid. all the Starship has in common with ISS is a rough measure of mass, and total volume. The Learjet and Cement truck analogy is not being snarky, it is a pretty good analogy of the difference between the two objects. Or you might compare a Google Datacenter with a Warehouse. Same size. Both are buildings. Nothing else in common. – CuteKItty_pleaseStopBArking Apr 27 '21 at 12:11
  • @PcMan What exactly is the purpose of the ISS? One naive response is that the purpose of the ISS is to provide a place for humans to live in space. If that's your belief, then habitable volume (together with everything needed to make it habitable, which the HLS will necessarily have) would seem to be precisely the right answer. So you need to either argue that the ISS has a broader purpose, or that the HLS cannot be used easily for long-term habitation. Based on your Data Center analogy, I assume you take the former approach? – Charles Staats Apr 27 '21 at 12:21
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    @CharlesStaats The ISS is primarily a laboratory. People living there are both laboratory workers and experiment subjects. Besides the basic prerequisites for survival, it has hundreds if not thousands of experiments running, stowed awaiting being performed, stowed awaiting return to Earth, stowed awaiting new ideas to run them differently, or performing "extended service" after the basic thesis was confirmed, to simultaneously test their reliability/robustness and aid the astronauts. – SF. Apr 27 '21 at 12:45
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    The funny robonaut? It's an experiment. The CO2 Sabatier process converter to oxygen? Experiment. The box with lettuce? Experiment. The new adjustable-color LED lighting? Experiment. There are relatively few things on board that aren't simultaneously laboratory experiments. Most things that are direct necessities to astronauts are qualified as "experiment in viability of use of x as regular use in space conditions." – SF. Apr 27 '21 at 12:49
  • @PcMan Thanks for the discussion. Let me know if the edited question makes it more clear what I am asking. – Charles Staats Apr 27 '21 at 14:09

2 Answers2

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  • ISS has a lot of truss constructions to attach equipment, such as power plants or scientific apparatuses. Starship would need these as well.

  • ISS has many docks for visiting spacecrafts. Starship, possibly can as well, but it would need to be designed for this.

  • ISS has a modular design that allows to add and deorbit specialized modules for various tasks. Starship would need either re-equipment on Earth or attaching additional modules like a space station.

  • ISS could1 conduct sensitive microgravity experiments by temporarily detaching a module from the rest of the space station.

  • ISS has gyroscopes for spatial stabilization of the station without spending fuel. (They can be added to Starship as well, but it would take space and weight.)

  • Starship has big fuel tanks and engines, which take additional weight and need spending additional fuel for maintaining orbit.

1per comment:

ISS currently has commercial Bishop airlock module that can be detached, but not in free fly. It's still kept by Canadarm, and the purpose mostly experiments in vacuum, not better microgravity (better than on ISS). Free-flying detachable modules were considered for ISS, but there are no solid plans for creating any.

uhoh
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Anixx
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    +1, but a couple clarification requests. Can you give more details on real instances when ISS modules have been deorbited or temporarily detached? I assume by gyroscopes you mean control moment gyroscopes; neat alternative to reaction wheels! I don't like the phrasing of the last point: it sounds like you're really arguing that if you tricked up a Starship to do everything the ISS can, the Starship-station would end up being even more massive. That requires more careful analysis. – Charles Staats Apr 25 '21 at 13:17
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    @CharlesStaats I think the last point might have to be changed to "more cross sectional area" or "more atmospheric drag" - because weight itself does not affect how much fuel is needed to maintain orbit. – jpa Apr 26 '21 at 05:25
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    "Temporarily detaching module" can be some midleading. ISS currently has commercial Bishop airlock module that can be detached, but not in free fly. It's still kept by Canadarm, and the purpose mostly experiments in vacuum, not better microgravity (better than on ISS). Free-flying detachable modules were considered for ISS, but there are no solid plans for creating any. – Heopps Apr 26 '21 at 06:39
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    @CharlesStaats given the age of the ISS (on average and/or per module) I would absolutely not be surprised if a re-do with modern technology would be able to cut the weight/size down significantly, but also not without a massive amount of extra engineering – Hobbamok Apr 26 '21 at 10:47
  • @jpa I think you're right. A higher-mass craft with the same atmospheric drag will require more fuel for the same delta v, but it will also require less station-keeping delta v because the same drag produces less acceleration. And these should cancel out exactly (ignoring possible effects due to the frequency of maneuvers or the extent of orbit change between maneuvers). – Charles Staats Apr 26 '21 at 12:40
  • @Anixx: Would you consider addressing the points that have been raised? In particular, mass doesn't seem to be a significant consideration outside of launch costs, and the ISS doesn't have any free-flying detachable modules as you imply. – Charles Staats Apr 27 '21 at 12:26
  • @Charles Staats I am sure I have read about something like a space station module being detached for the microgravity experiments. I think it was about ISS, but could be Mir. Possibly though it was Soyuz spacecraft that was detached, not a special module. – Anixx Apr 28 '21 at 08:02
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The ISS has a very large set of solar arrays (8 on the US, multiples on the Russian side). As designed, Starship does not seem to have any solar arrays. (May still appear for Lunar and Mars missions, but no sign at this time).

The 2021 Cargo Resupply Missions will be bringing up new arrays to overlay on top of the current arrays (iROSA). The initial arrays provided 160 Kilowatts of power, and the new 6 overlays will provide 120 kilowatts. The lunar Starship is not going to have that much power, just due to available space. The truss on the ISS is designed to make room for the large arrays. The Russian side, with more analogous arrays to Starship still needs to barter power from the US side.

The ISS has a large set of radiators to get rid of excess heat, which Starship does not yet appear to have.

Pure volume, while useful, is not really the critical metric.

geoffc
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  • The lunar starship proposed for HLS is really what I'm thinking of here, and that definitely has solar panels. (And presumably radiators too.) I'll upvote this answer if you add an explanation why using the starship as a LEO space station would require vastly more electricity (and generate vastly more heat) than using the HLS version of starship in the planned way. – Charles Staats Apr 25 '21 at 13:03
  • A deployable set of Solar panel "wings" would be pretty easy to include in the cargo. – Carl Witthoft Apr 26 '21 at 12:54
  • @CarlWitthoft Not the size or power of the ISS. It could be done, but not easily. They could include a nuclear reactor. But not easily. – geoffc Apr 26 '21 at 13:44
  • I still am looking for some explanation as to why the ISS needs so much more power than the HLS. If it's for ventilation or decades-old computers, a Starship-station might not need that much. If it's for scientific experiments and exercise machines, the Starship-station needs would likely be similar. In particular, if you can cite sources showing that power use is a significant bottleneck when designing experiments for the ISS, that would be a strong argument. (The overlays might be there to compensate for decay rather than to provide power beyond original specs.) – Charles Staats Apr 27 '21 at 12:14
  • @CharlesStaats Mir's power was declining. The ISS overlays are to manage decline. But Russians need to barter power from the US. Power and heat are the basic limits for how much you can do in space. Volume is useful and needed, but without power/heat maagement you cannot. – geoffc Apr 27 '21 at 14:56