For those of you who aren’t reading our ASM Blog, I participated in a panel at the SSI Conference last week in San Jose. Dallas Bienhoff presented the paper we are working on coauthoring, Gary Hudson talked about earth-to-orbit transportation, and Joe Carroll talked about several other interesting technologies including: mid-air capture (which I’ve talked about before here and here), combining debris mitigation with harvesting aluminum from spent satellites, rotating tethers, and reduced gravity research hubs). I didn’t get to contribute much myself–mostly just sat in on the short panel discussion, and got in a response to one question. However the panel and conference were a lot of fun, and I look forward to helping Dallas finish that paper.
All that aside, this post is related to Joe Carroll’s last topic–reduced gravity research facilities. His talk reminded me that I needed to dig-up and finish this blog post I had started back in May about the importance of such reduced gravity research facilities, and a clever approach I had seen to providing them.
Reduced Gravity Effects on the Human Body
I first made this point almost five years ago, but it bears repeating: while we have a lot of data on human health at 1g and at 0g, we have almost no data in the middle. I say almost, because we did have a dozen people live on the moon for at least 24 hours each…but that’s pretty much the only data we have on reduced gravity health effects, which is far too little to draw any really useful conclusions.
Most readers of this blog know that the data from microgravity impacts on the human body don’t look too promising–even with lots of exercise, there are apparently biophysical mechanisms that can have large negative health impacts (osteoporosis, neurological and pulmonary issues, etc) that begin to show themselves very quickly. However, as I pointed out in that earlier post, we have no idea which of the curves below really represents human health impacts of reduced gravity:
Does just a little bit of gravity go a long way (my personal guess I explain in the other post)? Or do you need almost full earth gravity? Or is there actually some gravity level less than 1g that’s actually better than earth gravity? While natural selection for humans has obviously been focused on a 1g environment, that doesn’t mean that humans are so hyperoptimized to 1g that nothing else will do. It’s unfortunately possible, but right now we don’t know. Without getting some “center points”, any guess at the shape of the response curve is just that–a guess.
Why This Matters
The reason why this knowledge void matters is that it greatly impacts the future expanse of humanity into space, as well as near-term human exploration. For instance, we don’t know if someone who goes to live on the Moon or Mars can ever really come back to earth, or if they have kids, if their kids can return.
If however it turns out that lunar gravity is already enough to counteract the worst of the effects of microgravity, it might be that the best way to do initial lunar human exploration is something like a One-Way To Stay (for a while) approach. If you knew that you could send someone for long durations while still being able to bring them back later if needed, it would open up some big possibilities. The return portion of a human lunar mission is one of the big performance drivers that make human missions so much more expensive than robotic ones. Even if you couldn’t close the life-support loop, just not having to return the initial explorers right away could allow you really enhance robotic exploration of the Moon by having people there on the spot to help troubleshoot, fix, upgrade, iterate, etc on your robotic systems. I know a lot of people think we can just send robots and have them make a turn-key base. It’s possible, but I expect you’re going to break a lot of robots along the way, and you could avoid that by having people in the loop. But its ethically hard to do a mission like that before you have some data on what long-duration exposure to 1/6g is going to do to your explorers.
Returning to the Joe’s talk, he suggested looking at .06g as well as lunar and martian gravity, as a possible minimal gravity level that people could intuitively adapt to without lots of training. If travelers can get by without large negative health hazards by .06g worth of gravity, that would really simplify the concept of providing artificial gravity for long-duration deep-space trips (like to Mars or NEOs). If there’s a “knee in the curve” above which you can avoid the worst of microgravity effects, that can make it a lot easier to provide artificial gravity for trips like that. If you have to provide a full 1g, and can’t go with high RPMs (which Joe suggested that the terrestrial centrifuge data might be suspect due to the presence of a 1g downward gravity vector), that implies very large structures, which become a much bigger engineering challenge.
The question becomes, what’s the best way to get this data? Most of these effects take timescales on the order of hours, days, or weeks to express themselves. And there’s no way on earth to adequately simulate hypogravity. The only real way of testing this, short of going there and finding out the hard way, is to build some sort of orbital research facility. The ISS was originally going to have a Centrifuge Accommodations Module, but that project got defunded, and the hardware is no longer flightworthy from what I hear. I had suggested the idea of doing a “CAM in a Can” before, but even that would be limited to studying small animals–there’s no way you could fit a human in there. To get the data quickly, you really want some sort of artificial gravity facility that is human-sized. In his presentation, Joe Carroll talked about building a large rotating space station with facilities on different lever arms from the CG of the facility. While this is interesting, and would allow you to have your gravity decoupled from your spin rate, I think that Kirk Sorensen’s xGRF “Variable Gravity Research Facility” concept makes more near-term sense (Joe and I disagree on this point BTW).
I’m not sure if Kirk reads this blog very much anymore (he’s pretty busy at his new job as Chief Nuclear Technologist at Teledyne Brown), but I have to toot his horn a bit. While not all of his ideas are ones I’m sold on, he’s had more than his fair share of clever ideas. The idea behind xGRF is very simple. You have a small facility–something on the scale of a Sundancer or Nautilus module from Bigelow, and you attach it via a long tether to a large counterweight (such as the upper stage that delivered the module to orbit in the first place). In LEO the gravity gradient can be used to force such a system to adapt an orientation with the long axis pointing through the center of the earth. In such a situation, the CG will be somewhere between the two end pieces, and the module will be going slightly slower than the orbital velocity of other components at its altitude, and the counterweight will be going slightly faster. This provides a tiny bit of settling force on each end (acting like a tiny bit of gravity with a vector pointed outward from the center of the system).
Ok, you may be thinking, that’s nice. But where do you get the “Variable” Gravity from? That’s where Kirk’s idea gets really clever.
Basically, something in a gravity gradient orientation is still actually spinning–it just completes one complete rotation per orbit around the earth…What happens if you take a spinning object like this, and decrease it’s moment of inertia by, oh say winching in the tether? By conservation of angular momentum, the object has to start spinning faster!
You can winch the habitat and the counterweight together until you reach the desired level of artificial gravity. Depending on the design details, you can pick any gravity level you want between say microgravity and 1g. How do you dock, say to transfer crews or deliver supplies? Well, it turns out you can despin the system by just reeling out the tether:
Pretty clever. By doing this, not only can you pick any gravity level you want, but you can also do your rendezvous and docking in a simple, non-spinning environment, you can eliminate the need for having rotating and nonrotating parts of the station, or of long elevators or connecting tunnels. I really like this concept, because the system ends up being pretty simple, with everything being able to be launched on a single EELV flight. You don’t have to assemble a huge space facility and then spin it up. This can be a small project that might actually get built. I think the big station Joe might have more capabilities, but I’m worried that detractors would paint it as a second ISS, and it would never get funded. Something on this scale though is within the realm of feasibility.
Flagship Technology Demonstrators, Expansion Options, Future Uses, and other Parting Shots
One particularly interesting way to get something like this funded (and what I was originally writing this blog post back in May as a response to) is as a replacement for the “Inflatable Technology” Flagship Technology Demonstrator. Back in Galveston late last spring, NASA rolled out several proposed FTD missions to flesh out plans suggested in Obama’s FY11 budget proposal. One of the missions was to build an inflatable module and attach it to ISS. To be honest, this seemed a little duplicative–it looked for all intents and purposes as though NASA was going to spend $500M-1B duplicating what Bigelow was doing on his own dime. I think a much better way of both flight demonstrating inflatables while killing multiple birds with one stone would be to build something like xGRF as a Flagship Technology Demonstrator. Leverage either a Bigelow Sundancer module or compete it out and have ILC Dover also bid on it. For the same amount of money, you get a much more useful lab, that doesn’t endanger the ISS, and which allows you to do reduced gravity research that compliments ISS’s microgravity focus.
As Joe pointed out, even after the initial experiments (say at lunar gravity first, then Martian, then at the .06g level), a facility like this would have lots of follow-on utility. You can answer initial questions relatively quickly–ie even a few months at each level would tell you a lot compared to what we know right now, but getting longer-duration data could be very useful for future space settlement efforts. I’ll have to dig up my notes on all the reasons, but there’s a lot of long-term potential for a station like this.
Which means you might also want to upgrade it down the road. If you overbuild the tether, and the docking facilities, you could probably attach additional modules to a station like this pretty readily. To add to the counterweight, you could say have facilities on the original upper stage that could allow it to be outfitted as a depot…but that’s getting a little too crazy for now.
But I think the time for something like this is now. FTDs are getting money, even if it’s greatly reduced from what Obama wanted. The budget for exploration technology development, including flagship missions is currently authorized at over $1.1B over the next three years. At that rate, you could fund most of the work on both the depot approach that was proposed by the joint industry/NASA group I participated in last year, as well as xGRF, and still have money left over for starting another FTD like say an aerobraking or aerocapture one. Even if funding gets further reduced in appropriations, there’s enough money to pursue something like xGRF and depots in parallel.
I think this is an idea whose time has come.
Latest posts by Jonathan Goff (see all)
- Fill ‘er Up: New AIAA Aerospace America Article on Propellant Depots - September 2, 2022
- Independent Perspectives on Cislunar Depotization - August 26, 2022
- Starbright Response to ISAM National Strategy RFC - July 2, 2022