A Gravity Lab

A concept for doing it on the cheap.

One problem I see in the near term is that NASA plans to use Dragons as lifeboats, so I’m not sure when one would become available on orbit.

[Update a while later]

Actually, I think that a cargo Dragon meets the requirements for this much better than a crew Dragon. It’s an on-orbit mission only, so there’s no need for couches, which just take up room. It can’t be used for a lifeboat, because it has no docking adaptor (at least currently), so NASA wouldn’t miss it. Even a Dragon V2 would need an ECLSS upgrade, so might as well just put it in the cargo version. It would have a lot less value to NASA than a V2, so it would be easier to get it from them. All they’d be giving up is the cargo return (which they could even get when the mission was over, months later, if they wanted).

14 thoughts on “A Gravity Lab”

  1. Rand,

    Yeah there are definitely some logistics questions like that. It might be possible, depending on the duration of the experiment to stagger things so that one frees up for a month or two. I’ve forwarded this to a few friends at SpaceX. If it isn’t crazy, maybe we can get some traction with the idea–I don’t think NASA is hostile to getting this sort of information, they just haven’t had an option they could afford reasonably.


  2. “It would have a lot less value to NASA than a V2, so it would be easier to get it from them.”

    Who owns the Dragon?

  3. Great idea!

    I’ve been concerned over long-duration low-G for a long time – since even before they canceled the gravlab for ISS. At the present time, we know what 0 G does to a body over moderate (less than 2 year) duration. One of the things we don’t know is what low G would do, long term. We can guess, and we can extrapolate, but we flat-out don’t know, and I think it’s something we need to know prior to a Mars trip.

    As for mice… any idea how well the mimic human response to 0 G? If they are a good analog for humans, then IMHO it might be possible to do the grav lab tests even cheaper; omit the human and use a cargo dragon – you’d need far, far less ECLSS that way – perhaps within Dragon’s existing capabilities?

    A further aspect of the Dragon gravlab idea; it’d test the concept of artificial G via a tether, which may well be a essential for long duration missions. (What’s the point of sending men to Mars if they arrive too weak to walk and have to spend weeks of precious time adapting to G after so long in zero G?) IMHO, we’d learn far, far more that’s of use for deep space missions with this dragon/grav concept than we would from the asteroid retrieval mission (and for many orders of magnitude lower cost, too).

    One regrettable note of caution (made necessary by recent journalistic standards in evidence from some.) If the dragon-gravlab idea gains traction, using a cargo dragon (with the one man crew boarding at ISS) will surely be derided by Slate as far too dangerous, due to cargo Dragon not having a launch abort system…

    1. “As for mice… any idea how well the mimic human response to 0 G? If they are a good analog for humans, then IMHO it might be possible to do the grav lab tests even cheaper”

      Was listening to a recent episode of the Space Show the other day and someone brought up the idea of using the Japanese centrifuge for testing mice.

  4. Problems, or at least considerations that have to be dealt with.

    The first is that to get any meaningful gravity the tether will have to be considerably longer than the 20 km tether that was used on the TSS and SEDS missions. The tether will have to be deployed below ISS, giving it a short lifetime. The tether end will have to be cut at the Dragon itself or it will boost the tether to higher altitudes, thus causing a orbital debris collision risk to ISS. Then you have to still be able to deorbit the Dragon that will be tossed to a much higher altitude by the tether (for a non swinging tether the altitude gain is 7x the length of the tether.

    1. Dennis, why would the tether have to be more than 20 km in length? That sounds absurdly long. I would think one to two hundred meters at most to generate a useful gravity field and at a reasonable rate of rotation around the common center between the Dragon and the counter weight.

      Also, at the end of the mission the tether is just wound up so the Dragon and conter weight meet in the middle and the Dragon thrusters used to stop the rotation (which may be pretty fast at that point due to conservation of momentum). No need to dangerously cut the cord during rotation.

      1. The issue is how long it has to be to get the necessary spin rate without having to do controlled thrust from the ends. The idea is to have it long enough to GG stabilize at one rotation per orbit, then to reel it in to get a spin rate that provides one gee. That’s the safest way to do it.

        1. What Dennis describes doesn’t sound safer than using Dragon’s thrusters to get the required rotation going, for example if you’re after 0.38g you can do it with 13.6m/s tangential velocity and a radius of rotation of 50m.

      2. A non rigid tether has restoring forces that tend to make the system go gravity gradient, no matter how it starts out. A good rule of thumb is that the forces are 1 microgee per meter. A spinning non rigid tether would tend to do all kinds of interesting things dynamically and you would have to have have active control of the system

        In the first read I missed that you would rotate the system.

        If you only want to do 100 meters it is far less risky to have a deployable truss.

      3. Looks like there is a reason I missed it being a rotating system as that is not what was proposed.


        A short tether does not give you enough G force to have any meaningful effects on the body. Another issue is that in low orbit debris is going to cut the tether fairly quickly.

        On SEDS 3 the tether was cut in a couple months and that was in a much more favorable orbital debris environment.

        1. Dennis,

          No, it’s meant to be rotating. It starts in a gravity gradient configuration, but then uses winches to pull the tether in, increasing the spin rate.

          As for tether breakage, you’d obviously want to use something like TUI’s Hoytether concept, that theoretically has the ability to survive multiple collisions.

          I haven’t run the calculations using the math provided in Kirk Sorensen’s original paper on the overall concept, but IIRC he had been talking about 10km length for his version that had the counterweight being 1/10th of the system mass. With a heavier counterweight (the F9 US dry weight is probably a lot closer to Dragon’s mass than the upper stage for the larger inflatable station he had been suggesting), that should shorten up quite a bit.



    1. Tom

      There is a plethora of data out there regarding tethers. I would suggest reading up on it as a lot of good work was done in the 1990’s with the TSS-1, 1R, and SEDS (Small Expendable Deployer System) hardware.

      The Shuttle deployed a 20 km tether (well it broke at 19.5 km) and two of the SEDS tethers were 20 km long.

      There is a LOT of orbital dynamics considerations with short rotating tethers that have to be dealt with that need to be addressed before this becomes something of serious interest. For a short distance, a rigid deployable truss would be much less risky.

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