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I am sure there are a number of safety and engineering considerations, as well as future integration and flexibility to consider, but I can't think of any so far.

When I think of an airlock, I think of something with two doors - one at each end. I don't understand how this works - To deploy, does it open like a clam shell, or does it just open the one air-lock that also mates with the ISS, and deploy from there? It seems that it also requires the ISS's Canadarm 2 to grab hold and move it away from the ISS. I can speculate, but I am wondering what the real reason is. Is it too dangerous to just push them out with springs?

Since everything is in orbit, the usual concept of "away from the spacecraft" is a little more tricky. Does this make it possible to change the direction of the deployment-nudge to something better suited to avoid future collisions?

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above x2: from Nanoracks PDF presentation and Ars Technica.

NanoRacks’ proposed airlock paves way for a more commercial ISS.

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above: NanoRacks’ proposed airlock paves way for a more commercial ISS.

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2 Answers 2

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This design minimalizes R&D work and number of active components. At cost of more complex procedure, the module is made very simple. There's A CBM port, a dome, and valves for venting and repressurization, a port for canadarm attachment, and the "nanoracks", cubesat launchers. That's it - listing in order of device complexity.

Airlock hatch is a fairly complex and expensive device, if not as complex as Common Berthing Mechanism port. But since the CBM was obligatory anyway, the designers decided to cut cost and complexity of the rest, doing away with an extra hatch and "abusing" the two sides of CBM as airlock.

The fact that the robotic arm can point the launchers in any direction giving a minimal initial orbit adjustment is a minor bonus, but the primary benefit is absolute minimization of the cost of the module itself.

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    $\begingroup$ Your logic is inescapable! Keeping things simple when possible is always a smart engineering approach, and gets even smarter when applied to things in space. There will be a remote-controlled mechanism to hold then deploy individual satellites, but it's likely to be similar complexity in either case. The minor bonus of articulated pointing on a per-cube basis could potentially be amplified with stonger "kickers" but that should be a separate question. Sold! $\endgroup$
    – uhoh
    Feb 18, 2017 at 8:06
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    $\begingroup$ @uhoh: personally, I'd believe work time of an astronaut is quite expensive, so I don't really approve of their approach. Plus I'm not sure how CBM will handle dozens of dockings/undockings; it wasn't designed with this type of operation in mind. But it's a common business practice to 'get things running' at a minimal initial cost while increasing operation costs. $\endgroup$
    – SF.
    Feb 18, 2017 at 20:57
  • $\begingroup$ I understand what you mean, but decisions may include factors like keeping resource utilization maximized in both volume and variety, both for PR/political and real engineering considerations. Here resources could include the robotic arm and astronauts' time, and engineering considerations could include expanding experience and data when resources are used in new ways. $\endgroup$
    – uhoh
    Feb 19, 2017 at 1:05
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    $\begingroup$ @uhoh: While such considerations might be made if it was a NASA project, personally, I believe Nanoracks, being a relatively small startup, simply couldn't afford anything more complex. Their only alternative was not doing this at all. $\endgroup$
    – SF.
    Feb 19, 2017 at 1:35
  • $\begingroup$ general fyi for everyone, here is more information on the Common Berthing Mechanism CMB. $\endgroup$
    – uhoh
    Feb 20, 2017 at 1:34
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Above all else, that design choice is purely for the sake of simplicity. It uses the existing Common Berthing Mechanism attach hardware and seals to interface with ISS, which eliminates the need to design a custom system.

From the linked SpaceFlightInsider article:

Once attached to Tranquility, the airlock would be pressurized to allow the hatch to be opened. The inside could then be configured by the crew for a variety of tasks. Once ready for deployment, the hatch would be closed and the airlock depressurized.

The robotic Canadarm2 would then grab the airlock and move it to a deployment angle away from the outpost. After satellite deployment, the arm would then return the airlock to its port on Tranquility.

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    $\begingroup$ I'm missing something, and having my linked article read back to me isn't helping. I'm assuming that the green hemispherical object shown in the first drawings is not the Common Berthing Mechanism itself, but instead is something new. Is the green thing called an air-lock? And if so, why doesn't it have an opening in both ends like "normal airlocks" do? To me it looks like they forgot the other hole, so they have to go through this procedure like some sea creatures do because they only have one opening. I'm missing something that must be obvious to others here. $\endgroup$
    – uhoh
    Feb 17, 2017 at 19:11
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    $\begingroup$ I keep looking at the Nanoracks page - is the airlock actually just a dome with an open bottom with a mating mechanism? Because when I read "airlock" I think of this or this or this. $\endgroup$
    – uhoh
    Feb 17, 2017 at 19:53
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    $\begingroup$ You are correct. The green thing is the airlock, and yes, it is just a dome with an open bottom that mates to the CBM. Very nontraditional. $\endgroup$
    – Tristan
    Feb 17, 2017 at 20:23
  • $\begingroup$ OK Common Berthing Mechanism is not an answer to "why" at all. No matter the design, that's how it would attach. A fixed, "normal" two-port airlock would allow deployment through a second, "output" port, the way satellites are deployed from the ISS now, with no need for a robotic procedure as shown. I think there must have been several engineering and safety considerations involved in the decision to use a one-port airlock plus robot instead of a two-port. "Because it's simpler" only begs the question "in what way does adding the robotic maneuver make things simpler"? $\endgroup$
    – uhoh
    Feb 18, 2017 at 1:36
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    $\begingroup$ @OrganicMarble: I wouldn't count on this too much. The launcher gives the cubesats a couple m/s at most, so if they are launched 2m/s retrograde, prograde or normal relative to ISS really doesn't change their trajectory much. $\endgroup$
    – SF.
    Feb 20, 2017 at 2:16

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