Random Thoughts: µ-xGRF

I’ve previously blogged a few times about Kirk Sorensen’s xGRF (variable Gravity Research Facility) concept as a way to answer the critical questions of how humans respond to hypogravity1. Kirk’s concept involved a lab spacecraft and counterweight connected by a reelable tether. You’d start in a near-microgravity state with the system oriented radially in a gravity gradient stabilized configuration. Then by reeling the tether in, you would change the rotational inertia of the system, causing it to rotate to conserve angular momentum. Et voila! Instant artificial gravity at a rate that’s driven by how far you reel the tether in.

xGRF Spin-up Recap

xGRF Spin-up Recap

The challenge has been that even though this is a relatively cheap way of getting to a variable gravity research facility, it’s still an expensive prospect overall. And the reelable tether concept has some non-trivial technical risk, both in the reeling mechanism, the tether itself, and the dynamics of reeling it in and out and how that needs to be done to do this maneuver without exciting unwanted vibrations, precession etc.

So what if we solved both problems by starting with a µxGRF–a cubesat-scale xGRF tether dynamics demonstration? Cubesats are relatively cheap2, so they might enable a series of progressively more sophisticated demos to retire the key risks for a full-scale xGRF, prove-out on a small scale some of the hardware techniques you’d use for the full-scale system, and generally build the practical (not just theoretical) experience of your team that would eventually build the full-scale xGRF system.

There’s probably a lot of ways of doing this, but my thought would be to have a 6U design with the “bus” taking up 3-4U worth on one end, and the tether reeling motor system being the counterweight on the other end, with solar panels on one end for running the tether reel, and on the bus end for running the bus, and maybe WiFi for communication between the two halves. You’d want to build a series of these intentionally, with the planned expectation that you’re going to have several of them fail miserably along the way. Cubesat tech demos often have the unfortunate result of testing the cubesat bus or avionics or software, not the technology trying to be demonstrated. If you plan a series of several, you can learn from your mistakes, improve things over several iterations, and grow from lessons learned3.

Some of the things you could test with such a system include:

  • Basic tether deployment–unfortunately this is something that hasn’t always gone perfectly.
  • Tether longevity–there are ideas like the Hoytether for long-lived tethers that are capable of surviving multiple MMOD impacts, but as far as I know they’ve never been flown.
  • xGRF spin-up dynamics–While you should be able to simulate the behavior of the system on a computer, actually testing the system to make sure there aren’t subtleties not captured by your simulation is important. This would include things like verifying the actual CONOPS you need to follow to spin-up or spin down without inducing unwanted precession or vibrations.
  • Electrodynamic reboost and maneuvering–one benefit of a spinning electrodynamic tether is that theoretically you should be able to do not just reboost, but also maneuvers such as plane changes. Demonstrating the ability to control this precisely would be very useful. Especially if you could use it to say maneuver relative to some other object, for instance maintaining a trailing orbit that follows the object the cubesat was deployed from. This would enable a full-scale xGRF to station keep in an ISS-following orbit.
  • Adjusting the gravity gradient length–if you reel the tether in or out very slowly in the gravity gradient configuration, you might be able to use those gravity graident torques to null out the spin-up or spin-down of the system. This effectively allows you to transfer some angular momentum between your spacecraft and earth, allowing you to change the length that provides the gravity gradient orientation. If you can make that work that enables you to vary the artificial gravity level somewhat independently of the RPMs/spin radius.
  • Complete reel-in–if the above works, it might be possible to slowly reel the tether all the way back in. This might enable reusing the tether/counterweight model when doing a full-scale xGRF. For instance, imagine you’re using a Cygnus module as your hab, with the xGRF deployer as the counterweight. If you could deploy the system, do your experiment, and then restow it (without causing the system to spin too rapidly), that would allow you to potentially reuse the xGRF deployer over multiple missions.
  • Simulated tether break recovery–You could also intentionally fail the tether to demonstrate methods of recovering safely from such failures.

There’s probably other ideas you could try4, but this gives you an idea of what you could do for a series of cubesats. Developing the first one might be expensive to get to, especially if you’re establishing a brand new team and its infrastructure, but each one after that would get substantially cheaper. My guess is you could do a half dozen demos, including launch for under $10M. At the end of which you’d be in great shape for scaling the system up and doing a real xGRF.

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Jonathan Goff

Jonathan Goff

President/CEO at Altius Space Machines
Jonathan Goff is a space technologist, inventor, and serial space entrepreneur who created the Selenian Boondocks blog. Jon was a co-founder of Masten Space Systems, and is the founder and CEO of Altius Space Machines, a space robotics startup in Broomfield, CO. His family includes his wife, Tiffany, and five boys: Jarom (deceased), Jonathan, James, Peter, and Andrew. Jon has a BS in Manufacturing Engineering (1999) and an MS in Mechanical Engineering (2007) from Brigham Young University, and served an LDS proselytizing mission in Olongapo, Philippines from 2000-2002.
  1. Hypogravity is levels of gravity between the microgravity and 1G, such as what you’d find on various planetary bodies in the solar system.
  2. [Citation needed]
  3. Just ask PlanetLabs, who tries to do a new iteration of their Dove spacecraft every few weeks
  4. Limited only by “the height of your creativity or the depths of your neurological disorders” as a friend once put it.
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7 Responses to Random Thoughts: µ-xGRF

  1. George Turner says:

    What if you went with something like an extendible boom to regain stiffness and torsion control?

    Northrop Grumman TeleMast (pdf)

    Though not as lightweight as a cable system, it might avoid NASA’s cable aversion.

  2. Paul451 says:

    How would a hollow tube (like the original sticky-boom’s self-forming bi-stable tube) compare to a Hoytether for resistance to MMOD cutting?

  3. George, Paul,
    STEMs and are much, much heavier and bulkier per unit length than a tether would be, so you’d be stuck with a much shorter length. Would it be too short to be interesting? I don’t know. Would they be more tolerant of debris? I also don’t know. I know that for the full-scale system, you’re talking about a tether over 1km long, which is much too long to do with either of those types of deployable structures, so I’m not sure if any simplification you got with the demo would be outweighed by not being representative of what you were trying to study.


  4. George Turner says:

    My thought is that if you’re studying partial gravity, minus the human inner ear, a short length such as 100 feet is dandy. Cable torsion is a real problem unless you terminate an end with a freely rotating bearing, otherwise your control system has to try to damp the rotation at a possibly varying natural neutral point, because any torsion can’t be countered by a thruster for very long. Running two or three cables would end the problem but add a bit more complexity for deployment, though the weight shouldn’t be much affected because the load would be distributed.

    With that in mind, there might be merit in something like the fixed rigging on a square masted sailing ship, where relatively short booms allow you to control very unbalanced loads in X, Y, and Z while keeping the few compression members in pure compression. It’s all spider silk and soda straws and weighs very little.

  5. George,
    Check out the original xGRF paper. I’m not sure if a short tether length works very well. It might, but they had been looking at lengths >1km, though that may have been so that the variable gravity parts would be at suitably low RPM with the tether reeled in. Not sure.

    For the full-scale system, though, you’re probably going to need something long enough that a STEM boom or other form of deployable tube isn’t likely going to work, so as a subscale demo, I’m not sure what the utility would be.


  6. red says:

    I often think various groups like Golden Spike, Inspiration Mars, Boldly Go Institute, B612 Foundation, and others with very ambitious goals requiring major space hardware projects, but without the required funding, should tackle more modest hardware demonstrations in space first. The organizations I mentioned include people with major accomplishments, but as organizations I think the groups need to show what they can do before they can get more support (funding, assistance, etc). Along these lines, the Space Studies Institute has had some accomplishments, but I think they would have a lot better chance of gaining traction with G-Lab if they first show they can do something on the scale of the uxGRF.

  7. Red,
    You make an interesting point, and one I definitely agree with. With the exception of Inspiration Mars, none of those other groups had a wealthy investor who could put up a significant chunk of the cost. Admittedly, SSI has done a bunch of smaller projects over the years and wasn’t able to leverage that into funding for bigger projects, so there’s no guarantee of success by the incremental route either.

    But I still think it’d be better to retire some risk with a µxGRF than swing for the fences and not get any bits because you’re trying to raise too much money. I like Gary, but we don’t see eye-to-eye on this topic.


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