Random Thought: Dragon V2 as an xGRF Platform?

Review: How Much Gravity Do We Need, and Why Do We Care?
One of my hobby horses that I’ve blogged about a few times is the question of how much gravity do humans need to be healthy? As I’ve pointed out in the past, we know microgravity is awful for people long-term, and 1G is fine, but we really don’t know what number between 0 and 1G is the minimum that a typical person needs to avoid unacceptable health degradation.


Which Curve is Right? How Much Gravity Do We Need? Is There a Knee In the Curve? If So, Where?

Why do we care? This topic came again today in the context of a twitter conversation about Mars colonization. Obviously, if the magic gravity level people need is higher than 0.38G (Mars gravity), Mars colonization is going to be harder than if it is lower than 0.38G. If the minimum required gravity level is more than Mars levels, you’ll need to come up with some sort of countermeasure on Mars, or face potentially severe health issues over time. This may involved a mix of biochemical countermeasures (drugs), exercise, and even small centrifuges–all of which have large potential drawbacks. If it turns out that the minimum required gravity level is less than 0.38G though, life becomes a lot easier. You might want to do something about the microgravity on the flight out, but wouldn’t have to deal with countermeasures after you landed. And if you went the artificial gravity route on the way out, you wouldn’t need to provide as much of it, which would make the system size a lot smaller and more manageable–for a given max spin rate, the centrifuge radius is inversely proportional to the gravity level required–for lunar gravity you’d only need 8m radius at a 4rpm rotation rate.

But while many in the space community have pointed out the importance of answering this question, and joked about how nice it would be to have a national space program to answer questions like this, no real progress has been lately. Most of the potential solutions have been too expensive to raise the money for. What is needed is a way to start getting the data as cheaply as possible. Hopefully well less than $50M if we want a realistic chance of getting NASA or private donors to fund it.

Dragon V2 as an xGRF Platform
While the idea came to me based on some of the ISS visiting vehicle post-mission reuse ideas we’ve been looking at at Altius, when doing a little research for this blog post, I found that this idea was originally suggested by A. M. Swallow and googaw in comments to my previous post. I usually blow those two off, but they were right a lot sooner than I this time around. The idea is basically using the pressurized volume of a repurposed Dragon V2 as the habitat for a 1-person (and many mouse) version of Kirk Sorensen’s xGRF concept. While there is a lot more info in the previous blog post on xGRF, the high level version is you start with a habitat connected via a variable length tether to a counterweight. As shown below, when the tether is all the way out, the system will settle into a gravity gradient orientation, completing one rotation per orbit. By winching in the cable in the right manner, conservation of angular momentum causes the rotational rate to spin up, creating higher levels of artificial gravity up to a peak level with the tether at its minimum length.


xGRF in a Nutshell

For my Dragon V2 variant, here are some key points in the concept (which is still only partly baked):

  1. You would use a Dragon V2 after it has launched a crew to the station. All but one member would board the station while the last crew member stayed aboard for the experiment. After successful completion of the mission, the Dragon V2 would return to the station to pick up the crew for return to Earth.
  2. You’d have mice on board as well as the human for two reasons: to give the person something useful to do so they don’t go crazy being by themselves for a few weeks or months at a time, and because you can get a lot more data points in a small volume with mice than you can with humans. While the human data is very useful, the mice might give you a better idea of the variability of the effects.
  3. You’d try to locate the Dragon V2 xGRF experiment as close to the station as you could get while still factoring in the risk of tether breakage–Kirk’s paper shows that at station altitudes a tether break wouldn’t lead to immediate reentry, but you’d want to make sure it also had a negligible probability of hitting the station. But if possible it would be great if you could find a position close enough that you could visibly see the station. Being alone for long periods of time might not be so bad if there are people within easy visual range that you can communicate with with no delay.
  4. You’d probably leave the xGRF kit attached to the Falcon 9 upper stage, but tucked into the trunk volume. This kit would include a dumb docking port, the tether/winch system, any required solar panels (if the solar panels on the trunk of Dragon V2 aren’t enough), and possibly inside the docking port some extra life support equipment/consumables (if you can’t cram enough into Dragon itself). Once the crew going to ISS were offloaded (along with most of the launch couches, and the vehicle configured with the mouse and its one human inhabitant for the experiment), it would leave ISS, re-rendezvous with the upper stage, dock with it, maneuver it to the experiment flight position, and then deploy the xGRF system.
  5. You’d probably want to make sure you had way to do a lot of telepresence, to keep the volunteer from getting too lonely. Two people might be psychologically nicer, but a lot harder to cram into a 10m^3 room for long periods of time. Telepresence coupled with being in visible range of ISS might mitigate the issues with having one person flying solo on a mission like this.
  6. You’d probably want to pick an astronaut who had flown a few times before, so you’d have pre-existing data on how their body responds to microgravity, to use as a comparison point. It’s not perfect–long term you’d want as many human data points as you can get, but at least with someone who has flown a few times already, you wouldn’t be dealing with a completely unknown quantity as far as space physiology reactions. If they weren’t already burned out, having twins like Mark and Scott Kelly do the experiment, with one on ISS and one on the xGRF platform might also be an interesting way of screening-out some potential genetic effects.
  7. The most interesting data is if the minimum required gravity level is less than Martian gravity levels (less than lunar would be even better). If it turns out it’s higher than that, Venus and Earth become the only realistic solar system destinations that you could live at without expensive countermeasures. So it might be worth intentionally designing the system for a maximum of say 0.4G at say a little under the 4RPM max limit that people like to stay under. With the upper stage potentially being a larger chunk of the xGRF mass than Kirk’s original paper suggested, this would greatly reduce the required tether length, making the system lighter and more manageable.

Some potential concerns include:

  1. Is 10m^3 enough volume for one person for long durations? I don’t know for sure, but if I’m not reading it wrong, a quick skim of this reference suggests that 10m^3/person might work for durations up to 3-6 months.
  2. Can the Dragon V2 without structural mods handle the loads in question? I would think so–the docking port is usually resisting a pressure load on the order of 15psi acting on the cross-section of the passageway, which is at lest 30in in diameter–yielding pressure loads of >10,000lbf they’re designed to resist. My guess is the loading would be similar in this situation, so probably something that could be handled without modification.
  3. Can Dragon V2 support 1 person on-board for long durations (>3wks) without expensive modifications? I’m less confident on this one, especially since the life support would be acting in partial gravity instead of zero-gravity. Everything I’ve heard suggests that even a little bit of gravity (for fluid settling and natural convection) can make many things a lot easier, but I’m not sure how much could be done leveraging existing hardware and interfaces, and how much would have to be done semi-custom. The less mods the less development cost. Fortunately, aborts to Earth or aborts to ISS can likely be done quickly enough that if life support stuff starts breaking down, there are plenty of quick rescue options.

The biggest question of all is how much would something like this cost, and would it be cheap enough to cross the line into something that could actually get funded? By reusing a Dragon V2 that’s already going to the ISS, you might only have to pay the delta-cost of operations and of the xGRF module. Could you keep that below $50M? Below $10M?

My guess is you’re almost positively above the $2M-ish limit of what is demonstrably crowd-fundable. But are you too high for a wealthy philantrocapitalist? I’ve heard that part of why Dennis Tito suggested Inspiration Mars was that he wanted a way to invest some of his money into making a lasting difference in the development of humanity in space. Could you get the cost low enough that Dennis, or someone like him, could chip in the money? Could you do this with NASA as a partner without NASA’s safety culture turning this into something so expensive it never flies?

Anyway, it’s some food for thought.

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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 the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. 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.
Jonathan Goff

About Jonathan Goff

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 the founder and CEO of Altius Space Machines, a space robotics startup that he sold to Voyager Space in 2019. Jonathan is currently the Product Strategy Lead for the space station startup Gravitics. 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.
This entry was posted in Commercial Crew, Commercial Space, NASA, Space Development, Space Settlement, Space Tethers, SpaceX, Variable Gravity. Bookmark the permalink.

35 Responses to Random Thought: Dragon V2 as an xGRF Platform?

  1. My understanding is that Dragon v2 will have minimal life support, designed for shuttling to and from the station only. SpaceX haven’t said much about it though, and they obviously have more grand aspirations.

  2. Trent,
    Yeah I’m aware of that, but that doesn’t answer the question of whether you could extend that duration easily, or if it would be very expensive.


  3. George Worthington says:

    While it is obviously logical to wait on dragon D V2, Is using cargo Dragon out of the question?? Isn’t Spacex making life support systems for V2 that could be put in a cargo dragon Now? Add a cushioned reentry couch in case of abort since cargo dragon lands harder than D V2. Could CCtCRE money some how be used?

  4. johnhare johnhare says:

    This would certainly be more useful than sending aDragon around the moon as I suggested It should also be safer, cheaper, and more politically feasible.

  5. ken anthony says:

    How small can a RCRS be? What are the specs. on the unit used on the shuttles?

    About another issue: isolation. I know it can be profound for some people but seems overblown. Communication means not really being isolated. Give ’em a big teddy bear as well.

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  7. Ken, agreed. Most of the complaints I’ve heard from astronauts on the ISS is that they can’t get enough alone time.. it’s like a really noisy share house.

  8. Michael S. Kelly says:

    This is a very good idea, for all of the reasons discussed. A few points, though. First, if spin rate means “angular velocity” in rpm, rad/sec, or any other measure, the centrifuge radius is directly, not inversely proportional to the required acceleration. (a = r*w^2, where w is angular velocity) Second, lunar gravity is not necessarily a good design point. One of the little-known results of a whole lot of variable gravity combustion research is that the flammability of most common materials is highest at 1/6 g. If we hadn’t had the Apollo 1 fire, it is much more probable that we would have had one on the moon.

    I like this, though, especialy after Gary Hudson pointed out that human life might not be possible at other than one G for extended periods.

    On the other side of the spectrum, I’ve wondered for the past 30 years what would happen if we built an enourmous centrifuge, and trained our Olympic atheletes in it at, say, 3 G. They might be able to do phenomenal things. Of course, it would be like taking steroids…uh, on steroids.

  9. Jonathan Goff Jonathan Goff says:

    I had the equation right, I just prosified it wrong. I blame being up past my bed time.

    As for peak flammability at 1/6th G, I’m curious how much more flammable it was. Is it something you could handle just via atmospheric composition? For instance, could you do sea level pressure, but higher amounts of buffer gas and less oxygen (say a partial pressure of O2 close to Denver levels, but sea level overall pressure). But yeah, we really need to find out more about hypogravity biology to see if it’s a problem or not.


  10. Jonathan Goff Jonathan Goff says:

    It may not be so easy as just “dropping an ECLSS” into a cargo Dragon. I wouldn’t be surprised if it was more integrated with the vehicle and vehicle controls than that. Plus without a docking port, connecting to the tether becomes a lot harder. Are there ways of making it work? Sure. But my gut tells me that a cargo dragon would be more expensive than a manned Dragon to modify for this mission. But hopefully I can get some unofficial feedback from some SpaceX guys soon to see if my intuition is right.


  11. George Turner says:

    A major addition to the capsule would be a working toilet/shower, but thankfully those wouldn’t need to be zero-G.

    For a funding angle, I would suggest using a crew of two, with the mission commander being the most straight-laced, geeky man imaginable, and the second-in-command chosen from a list of top actresses:

    Marion Cottilard – she’s French, so the ESA might kick in something.
    Charlize Theron – NASA TV would actually show up in the ratings, although they could just stick her in a simulator and we’d still watch.
    Mila Kunis – She’s Ukrainian, so NASA could perhaps get some political capital out of it
    Sandra Bullock – The go-to girl for spinning in space, round and round.
    Jennifer Aniston – Her calendar is clear and the tabloids would pay a fortune to tell us how she really feels about – whatever.

    You could probably pay for the mission just with the cable shows about training them for the mission, and like the mice, all they’d really have to do is eat, pout, and poop and you’ll get your data.

  12. Michael S. Kelly says:

    Jon – I don’t know the relative magnitude of flammability, but it is something that has bothered me for some time. What (or, rather, who) I do know is the guy who did all of the experiments on this. He was my research partner in grad school I’ll find out for you, and let you know. This is an important matter for the future of human space exploration.

  13. Mike,
    I’m really interested in what you find. As someone who is more interested in the Moon than Mars, I’m curious how severe the problem is, and if there are any obvious mitigations that can be tried, that aren’t too onerous.


  14. red says:

    I wonder if something like this would be of interest to the Space Studies Institute as a lower-cost (and thus perhaps more easily fundable) substitute for the G-Lab?

  15. MBMelcon says:

    Michael Kelly,

    Look at Pista Centrifuga (http://it.wikipedia.org/wiki/Pista_centrifuga)
    for training runners at higher gravity. It also bears on adaptation to spin rates.

    If the BA 330 ever flies an astronaut running around the circumference (see the “Skylab 500”) would experience about 1/4 gee at chest level. I asked Bigelow for comment but never got a response.

    If cyclic gravity is useful, an astronaut could bounce off parallel walls with a half flip in between. This quickly becomes tiring, so add a trampoline at each end for energy recovery. There may be resonance issues, and the safety people will have worries.


  16. Will McLean says:

    A larger problem with a cargo dragon is that it currently isn’t able to unberth without having someone on the other side of the hatch. You don’t want to use one of the crew rotation Dragons for this because NASA will want it as a lifeboat. What you can do is send up a Dragon V.2 unmanned, loaded with cargo, and fitted with one couch and the mouse cages. It docks, you unload the cargo, and the astronaut enters.

    The volume is pretty skimpy for the length mission that would be really useful. If you docked the Dragon to a Cygnus, that should be enough. The docking adapter would be a good place to put the enhanced ECLSS system for a long mission.

    There could be operational issues with using the upper stage as a counterweight. It might be simpler to use another trash filled Cygnus.

  17. Doug Jones says:

    Low oxygen partial pressure is used in many libraries to help protect the collection from fire. Widely used in Europe.

  18. Doug Jones says:

    I found a commercial source for reduced-oxygen atmospheres, popular for library repositories and fabric warehouses. One paper mentions Hugo Boss using it for a warehouse that holds fabric sufficient for several years of clothing manufacturing.


  19. Jonathan Goff Jonathan Goff says:

    Interesting find. It’s always cool to see your intuition confirmed like that. ๐Ÿ™‚

    So it’s definitely a solveable problem, the challenge at 1/6th G would be figuring out how much the gravity increases the flammability, and how much you’d have to shift the oxygen/nitrogen ratio in order to cancel that out. Hopefully you come up with something where you can get a decent pO2 (ie not much worse than Denver or Mexico City equivalence), while keeping the overall atmospheric pressure at sea level or lower. Fun exercise for a PhD dissertation… ๐Ÿ™‚


  20. DougSpace says:

    When do we need to know the gravity prescription?

    Stays at the ISS are short enough we don’t need AG for the crew. Initially, a lunar base could have rotations every 6-12 months which is within the 437 days that Polyakov spent on Mir. Polyakov spent 74% of the time on Mir compared to a 589-day Mars 2021 flyby mission. A DRA 5.0 mission has only 375 days in transit and 539 days in Mars’ 3/8th gravity. A vigorous exercise program, meds, eyeglasses? can mitigate much but not all of the effects of zero gravity.

    So, it seems to me that the first time that we may really need artificial gravity would be during a Phobos-Deimos (PhD) mission in which the crew would have essentially no gravity exposure during the entirety of the mission.

    A ways back, I did a calculation of the percentage of the mission mass needed for tethered spin up and down for a full 1-gee at 3 RPMs using the tangential velocity m/s. IIRC, it came to about 15% of the propellant needed for the delta-Vs for a Mars flyby mission. We can safely assume that 1 gee AG is sufficient for human health.

    My point is that I am skeptical that we will truly need to know the gravity prescription until we start to do long-term settlement especially including gestation and childrearing. I’m not at all against an AG experiment provided that it is very low-cost. But I don’t think that we should delay establishing a permanent lunar base or anything else until after we determine the gravity Rx.

    I propose that an unshielded tetherball centrifuge be constructed at a permanent lunar base and that animal experiments be conducted simultaneously as the base becomes increasingly materially independent of Earth (e.g. life-support from ice, metals, silicon, etc). In relatively short order (2 yrs) the gravity prescription for mammalian (including primate) gestation could be obtained and the crew would have primates a few years older than any human children born. However, for humans, a heavily-shielded indoor centrifuge would have to be constructed first.

  21. Paul451 says:

    Re: When do we need it.

    While we’ve figured out how to do micro-g routinely for up to 6 months with only minimal long-term damage to astronauts, it would be pretty stupid if a 0.1g, 4RPM facility solved 99% of the micro-g issues that the astronauts (and spacecraft designers) deal with, but we didn’t do the most basic research to find out for yet another 30 years “until we need it”.

  22. Paul,
    Exactly. Even more to the point, if we find out the answer is something relatively easy, it can significantly change how we do missions. Frequent crew changeouts are expensive. If we could instead baseline crew rotations of say 5 years instead of 6 months, it would change your architecture greatly.


  23. George Turner says:

    Not to mention the benefits we’d get if we could recycle solid waste through viable food crops, perhaps even growing fish or chickens. (Bivalves, on top of being tasty, also make excellent water filters.) Every pound of food from recycled waste saves roughly its weight in gold (at current pressurized cargo delivery prices), and everything that isn’t food (stalks, stems, and roots) gets recycled into the next crop.

    On top of that, although we are 3-D printing tools, drilling and tapping a simple mounting hole in zero-G while not creating a host of potential problems, from shorts to inhaling metal shards in your sleep, is a nightmare that would probably require hours of planning and rehearsal, whereas in some fraction of a G it’s a two minute job hardly worth mentioning.

  24. Connor says:

    I think this is an interesting (and important) research question. One observation though: Mice in enclosed spaces create a noxious atmosphere, producing far more concentrated urine, and (I think) ammonia waste. Any life support system involving mice would need to cover this too.

  25. Connor,
    Very interesting point, and definitely something that would need to be factored in. I wonder if any of these waste streams could safely be used for growing some plants. It would seem that if you could use a mix of plants and synthetic life support, it might be easier to make things close.


  26. George Turner says:

    Urine is rich in nitrogen, phosphorus, and potassium and makes excellent fertilizer. (Urine#Agriculture wiki)

    Using urine and manure fertilize plants would definitely help close the loop, but aside from using a completely enclosed aquatic ecosystem (which is essentially zero-G anyway), you’re going to need some level of gravity to keep the solids, liquids, and gases acting like plants and animals expect. We can grow plants in zero-G, but I think it would be extremely difficult to grow and process enough plants to count as crops, and animals are going to remain out of the picture.

    I see little point in trying to achieve long-term sustainability in an environment that we will abandon as soon as we can spin up a ship. We may be able to sail the new ocean in zero-G, just as we can cross the Atlantic in an open boat, but that’s really not how we want to be doing it.

  27. George Turner says:

    I’ve been wondering how hard it would be to bend an aluminum rocket into a different shape, namely a tall ring (perhaps as tall as a Falcon 9), a hundred to several hundred feet in diameter, and eight to ten feet thick or more. The idea is to get the ring into orbit and then rotate it, thus providing the long-awaiting rotating space station with artificial gravity without assembling it in space with hundreds of individual launches. SSTO isn’t going to work because of the increased drag and structural thickness, but putting Falcon 9H boosters all around the perimeter and letting the ring itself be a wet stage should do the job.

  28. john hare says:

    If you will think of the ring as a structural stiffener for a saucer wing, it seems quite possible to get it into orbit in one go. Inflatable life raft for the conceptual structure of a vehicle that takes off horizontally. Thin skin over the doughnut hole that houses the drop tanks of the first stage.

    Sled take off with horizontal lifting departure to Mach few above the sensible atmosphere. At staging, the skin that provided lift surface is jettisoned along with the center drop tanks and the excess Merlins. Since the ring tank itself is the payload, it seems quite possible to get near SSTO performance from the ring ship.

    Ten foot diameter tube in a two hundred foot doughnut would carry about 2,250,000 pounds of LOX/RP propellant giving a dry mass in LEO of 450,000 pounds assuming an upper stage mass ratio of 6. Calculate and season to taste.

    This would be one expensive project ass opposed to Jon’s concept. I just wanted to point out that it wasn’t really out of the question if it turns out that a station or vehicle this size solved more problems than it caused.

  29. Paul451 says:

    George Turner,
    “a tall ring (perhaps as tall as a Falcon 9), a hundred to several hundred feet in diameter, and eight to ten feet thick or more.”

    If you ignore the curve, then what you’re proposing is structurally and aerodynamically equivalent to launching a more conventional rocket sideways.

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  31. Andrew Swallow says:

    For a demo how about a combined propellant depot and 1g space station for fuelling cubesats?

    It would weight 2-3 tonnes and have a radius of 5 metres. (Too small for people.) I would use 3 Morpheus lander style tanks.

    I would include a diagram but I cannot see how to upload them.

  32. I put the diagram on my webpage. Please reply here.


  33. Andrew,
    I have actually thought a bit about the idea of combining an xGRF facility with a depot facility. Even without the spin settling, just the gravity gradient settling forces you’d get in the non-spinning orientation would keep the propellants settled both in the depot and in any attached craft–so having an xGRF and a depot attached at opposite ends of a tether isn’t silly. I’m not sure if there are big benefits to having the propellants in a hypogravity state higher than just being settled though. I wouldn’t use the Morpheus tanks for the job though–way too heavy, and there are better options.


  34. It is weird but making the spacestation a depot was a side effect. Three manned modules were too expensive so I replaced two with water tanks. When the habitation module’s mass can change by several tonnes corrective action has to be taken. The third water tank and pumps were added so the liquid could be used to equalise the mass reducing the vibration when the station produces gravity by spinning.

    To become a depot simply increase the size of the spacestation’s tanks and replace water by a propellant.

    I chose 3 modules because wind-turbines went from 2 blades to 3 to reduce wear on the bearings. A spinning stick is therefore likely to have some sort of stability problem, it is easier to design problems out in advance.

    For the demo proposal I shrunk everything from 50 metres to 5 metres. As you say if there are no experimental animals on board then a slow spin speed can be used.

    As for the choice of tanks what ever is available in about 6 months.

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