Thursday, November 18, 2010

Gravity for Sale

xGRF Concept Graphic
Back in June, guest blogger, Kirk Sorensen, over at Selenian Boondocks described a cool concept for generating artificial gravity from a tether (and a Canfield joint). I read the paper behind the concept.  Last week when Jon Goff at Selenian Boondocks followed up with this post about a Variable Gravity Research Facility (xGRF) as a Flagship Technology Demonstrator, it reminded me I needed to post some business applications for such a facility as well.

Could such a facility be run commercially? In the discussion below, I will use the International Space Station (ISS) as my example, but the model I present would work equally well with other orbital destinations like a Bigelow Aerospace habitat.

For us non-engineers, think of the xGRF as a Bigelow module (habitable volume) with a large tether attached. Because of the power (almost magic) of the conservation of angular momentum, when the tether is unwound, the station spins. When the tether is re-wound, the tether stops spinning (this is where the engineers shoot me for over simplifying – but you get the idea).
  • Because such a station could be spun at various rates, multiple G-Loads are possible. 
  • Because the station could be despun quickly, the xGRF station is easier to dock with. 
  • Because the station can be despun and respun at a low energy cost, the station is cheaper to operate.

Selling Gravity
Could an entrepreneur run such a gravity facility at a profit? Their profit centers could be both the “gravity service” they offer (a night sleep under gravity) as well as the data they generate from the effects of varying levels of gravity on humans (for use by others in planning long-duration space flights). Those guests staying on an xGRF become both customer and lab rat.

I don’t think it is too ambitious of a goal to return astronauts to earth with NO LONGTERM NEGATIVE EFFECTS from microgravity. Although obviously not achievable currently, I think we are all assuming humanity has to develop this capability someday – the current system in untenable. Is such life-enhancing effects possible through short bursts of artificial gravity? We do not know.

Even if the effects of artificial gravity prove less than completely restorative, as long as you assume the benefit from short bursts of artificially gravity is superior to the current system of significant daily exercise, I believe one could still develop a lucrative market for a gravity service. The option to sleep under artificial gravity could become highly desirable - one of those services that moves from “luxury” to “requirement” in people’s minds very quickly.

So my idea…
Let’s explore the idea of a commercial xGRF with an example: Put an xGRF in an orbit that would allow for frequent trips to the ISS (low transfer times between facilities and low delta-v costs). Astronauts would work in the microgravity environment of the ISS and sleep in the artificial gravity environment of the xGRF with daily transfer tugs moving astronauts between the two facilities. Co-locating an xGRF with the ISS could:
  • DOUBLE the productivity of the ISS as measured in astronauts’ daily “workable” hours (see the tables below for more on how one doubles station productivity) and 
  • Reduce microgravity physiological impacts on astronauts in orbit.

Here are the details:
  • Transfer time between stations should take no more than two hours
  • Astronaut time on xGRF equals 10 hours per day
  • Astronaut time on ISS equals 10 hours per day
  • Three Astronaut shifts of four astronauts per shift
  • Increase ISS crew size from six to eight at any given time (assuming life support could handle 8 on ISS)
  • Allow around-the-clock work on the ISS – including constant experiment monitoring if needed
  • Repurpose current ISS sleeping, exercise, & personal spaces into science and experiment space 
  • Productive Astronaut hours per day on ISS could increase by 100% without any new modules added to the station itself (from 60 productive hours per day with a crew of six to 120 productive hours per day with shift work outlined below)

Table 1 below highlights the productivity of three shifts of four astronauts transferring between ISS and xGRF daily:

Table 2 below highlights the current productivity (note, exercise and sleep times are my estimates only):

  • The political challenges to be allowed to dock with the ISS three times per day are enormous (perhaps too enormous)
  • The logistics of frequent dockings are significant. Note these first two challenges are relevant to my last post about the last mile problem for mico-cargo delivery to these stations. If today’s post highlights how we are struggling to solve frequent deliveries for macro-cargo, how pessimistic should we be regarding micro-cargo deliveries noted in my last post?
  • Allowing a spinning station so close to the ISS (or any orbital station) creates security challenges that must to addressed. There is always a chance the two stations will collide. Do the benefits outweigh the risks?  How can the risks be mitigated?
  • Is two hours really enough time to transfer between stations? If not, does the loss of productivity from longer “commutes” (three hours, four hours?) degrade the idea to the point of being unexecutable?

As with most tantalizing space business concepts, this one falls into the category of, “If I only had a billion dollars…” I do like Jon Goff’s idea of developing a xGRF as a NASA Flagship Technology Demonstrator. Regardless, once commercial station operators have achieved a few more milestones, this concept may be worth a deeper look – adding productivity to our astronauts in orbit and more importantly, improving the quality of life of those working off-world.


  1. Note that the orbits of ISS and xGRF would begin to drift apart almost immediately, unless they were actively maintained by either a tether between the two or propulsion. The commute between ISS and xGRF would eventually become impossibly expensive.

    This is one of the non-intuitive peculiarities of orbital motion. You can't just put two objects next to each other in orbit and expect them to stay put relative to each other...

  2. Tom:

    Agreed. The tug transport is key. Note I left tug transport section of the post really vague because I don’t know how to do it. Keep transfer times to under two hours and keep the delta-v req for any tug low. Great. How do you do that? I am not sure. Definitely one of the show stoppers to solve to make the idea viable.

    Alternatively, one could consider designing stations that contain a microgravity section (for research/entertainment/etc.) and a spun gravity section forcing designers to solve significant vibration issues for any microgravity experiments or manufacturing on board generated by the spin. But those are the options, right? Either (1) have separate space stations for microgravity and spun gravity and shuttle astronauts between the two stations or (2) figure out how to combine both into one facility.

    Great comments. You are right. The tug transport is definitely a MAJOR challenge.


  3. Colin,

    It's been a while since my grad class in orbital mechanics, but I suspect that the precession of the two station orbits with respect to each other will turn out to be cyclical. There should be regular windows during which ferrying between stations is relatively inexpensive. I can't say off hand how often those windows come up. Note, however, that any station in LEO will need periodic orbital boosts to counter drag and such. These boosts could also be used to improve the transfer windows somewhat. Eventually, creative use tethers could be very handy for plane change maneuvers.

    I imagine that stations combining microgravity sections with spinning sections will also be quite useful. Still, we have a lot to learn about the practical dynamics of spinning space stations. I loved the xGRF proposal.