Monday, May 9, 2011

Arificial Gravity and What Roller Coasters Can Teach Us About It

        One of the many problems with long-terms spaceflights is the lack of gravity. Your nose stuffs up more or less permanently, making any food not served with hot sauce taste like cardboard, nausea makes it impossible to eat for the first two or three days, and telling the male members of the crew to keep their dirty thoughts to themselves is pretty much unnecessary, given what the fluid redistribution does to “Le Reflex Gallant”. If that weren't enough, going to the space toilet takes about an hour and as Apollo 9 Lunar Module Pilot Russell Sweickart noted, “There ain't no graceful way”.

         If that were all it was, we probably wouldn't worry about it. However, these little symptoms are only the beginning. For example, because all of the body's fluid sensors are above the waist, Astronauts don't feel thirsty when they should and suffer chronic dehydration. They don't feel the need to urinate when they should either, making bladder rupture a embarrassingly real possibility. Lack of convection currents makes forced ventilation a must, as you could end up choking in a cloud of your own breath without it. The same principle makes dying of heatstroke for want of a breeze a possibility as well.

         The most dangerous effect of free-fall is that it causes muscles to atrophy and bones lose calcium. It is generally a truism in biology that organisms are lazy to a degree. If you don't use it, you lose it. In space, that means that bones, since they no longer need to support weight, start shedding calcium like a long-haired cat in July.
Keep going, it's saving your life!

        While it's true that vigorous exercise will help ameliorate this somewhat, Astronauts that serve the standard six-month tour of duty on the ISS lose on average 5% of their bone density. Not so bad right? It is when you consider where that calcium goes. Most of it is expelled via urination, which makes getting wicked kidney stones not only possible, but likely. As bad as kidney stones are on the ground, in space there is pretty much no way to pass them, as gravity will not move them along. While it hasn't happened yet, if we go without gravity for periods much longer than the current limits we will lose Astronaut to kidney failure. This means if we want to go to Mars, or even a Near Earth Asteroid, we are going to have to take gravity with us.

         We've been theorizing about artificial gravity in space since before we went there. Pictures of the old-school spin habitats hovering like giant bicycle tires in orbit have graced the pages of everything from children's books to Colliers magazine over the years. The problem with spin habitats is that they are, as currently imagined, going to be so maintenance intensive the dang things will probably spend more time out of order than doing anything useful.

        What's wrong with spin-habs, you ask? Oh, dear RocketFans, let's count the ways:
  • If the entire spacecraft spins, it has to be wide enough to make it worthwhile, adding superfluous mass,
  • If that wasn't enough, steering this spinning top is whack-o due to the gyroscopic effect, doing maintenance on the hull will cause vertigo in a corpse, and docking an axillary craft is impossible without stopping this monster.
  • If only part of the rocket spins, you need a flywheel, which is aerospace engineer-ish for “big mass penalty”.
  • If you have two spin-habs that counter spin to cancel out the gyroscopic effect, there is pretty much no known way to engineering science to make an air-tight seal that friction won't destroy in a matter of days. Come to think of it, this holds true for any spin-hab, counter spin pairs or no.
  • If you use an air lock to travel between the spin segment and the rest of the spacecraft, you better leave a crew in the free-fall segment or hope you don't have any emergencies that require rapid response. Having a hab in the free-fall segment for the “on watch” crew just means nearly doubling the habitat space, which is yet another mass penalty.
  • Radiation shielding an entire spin hab is – you guessed it – a major mass penalty. Not shielding the spin hab means that you better have plenty of warning when solar flares happen, due to the whole air-lock thing.

        There are probably more reasons that spin-habs are a pain to use, but I think we've seen enough.

        So there you have it, RocketFans; we have to have spin gravity in order to survive a trip to the planets, but they are so ornery that actually building one is going to be next to impossible. Whatever can we do?

        Obviously, I have a suggestion; there are pictures further down the post that are a dead give-away.

        First of all, I've generally thought that putting the motive force at the hub of a spin-hab would put a lot of stress on the spokes from torque (I think; engineer I'm not). It made more sense to me to put the spin hab in a centrifuge, which could be as small as a stationary ring around the hab and them have wheels on the hab add the spin like a car constantly driving uphill. Unfortunately, I couldn't come up with a design that had a prayer of not being a tangle of supports that make the mass penalty of a flywheel seem like a sweet dream. Still, the idea has stuck for awhile, and ended up getting used in the Iceteroid Outposts article in OpenD6 Magazine. Basically the Conestogas are connected and drive up the walls on opposite ends of a circular vault inside an asteroid. That worked pretty well, but didn't help for spacecraft.

Look kids, it's Physics!
        Finally it hit me: Roller Coasters! Specifically, inverted roller coasters. You know the ones; they have tubular tracks and the cars have wheels mounted both above and below the track that allow the coaster to travel in pretty much any direction the track leads, regardless of gravity. The inverted coasters are my inspiration because when they loop the loop, they are on the outside of the track, hanging from their wheels.

         That's all it took. I opened up GIMP and started kit-bashing with the map elements and plans from all of my other stuff and whipped up possible design for an IPV that has spin gravity via modules traveling on tracks.
The USS Example

         This IPV is not large enough for my tastes, but it demonstrates all of the design principles I've mentioned in the past. It has two of everything, lots of room for propellant, two fusion reactors and associated thrusters, and big, beautiful radiators that turn what looks like a dumb bell made of Tinker Toys into a fairly cool looking spacecraft. Or maybe that's just me.

Thank goodness for sprites!
        The important parts for this discussion are the spin tracks and modules. As you can see to the left, there are airlock nodes mounted on drive trains that have two sets of wheels gripping the tubular track. You can have as many or as few of these modules as you like; as long as there is an equal amount on the other track rolling in the opposite direction at the same speed. This versatility will allow IPVs to interchange modules in short time, both increasing flexibility and making maintenance easier.

         The maintenance advantage is that each module node has its own motors and power supply independent of the others. If one konks out, it can be pushed or pulled by the others on the track like a rail car while the techs repair the motors inside the module and in gravity. This turns a good chunk of the maintenance nightmare into something manageable. Even better, preventive maintenance can be done as often as you like and no one need put on their fancy clothes.

         This design is safer too. You may notice that the interior surfaces of the hab modules are flat; there is actually an Astrobot standing on the “top” of the module on the left. The robot arms in the previous graphic allow access to the habs, the consumables cargo pods on the central truss, and the avionics gear in the hub, all without risking the trapeze act while the modules are spinning. If there is a need to crawl along the “underside”, or outer surfaces of the modules, handrails are provided. Have fun with that.

         My favorite part about this design is the outside surface airlock on the module nodes. In the examples above, you can see that I have an inflatable greenhouse on one side and a Paladin Spaceplane docked on the other. Both the greenhouse and the Paladin are in full gravity - even more gravity than the modules because they are further away from the center of rotation. This set up allows the crews and passengers of small craft being ferried across the black deserts of space to enjoy the benefits of gravity without having to increase the number of habitat modules permanently attached to the IPV. Indeed, an IPV could become a veritable super carrier just by adding more airlock nodes. If it were me, I'd have double the number of airlock nodes anyway and use them to ferry passengers (for a fee) to the other module it's in between. This system also makes it much harder for shady types to get unauthorized access to the control module, which will have the command crew's quarters located on the same node.

         But what I like about this spin-gravity hanger space (no pun intended) is the possibilities it gives me as a role-playing game designer.   I need my future Players to be able to do something during that ten-week trip to Mars, or I'll end up having to Handwave shorter travel times out of desperation and hang my head in shame. With the roller coaster design, Characters in my game can explore the IPV, interact with its crew, the crew of other rockets, explore the other rockets, all while traveling to Mars or the asteroids in their personal, short-range spacecraft. This is why rockets like the Heinlein spacecraft have airlocks in their noses, so they can become part of the spin-hab itself.

          Anyway, the roller coaster design may solve a lot of problems, but seems to make some issues worse. Traveling to the non-spinning sections of the IPV, for example, is now impossible. These modules are completely isolated and cannot even use an Airlock to access the hub because the rails are in the way. While it may seem to be not unnecessary to travel to the other parts of the ship, trying to pilot a spacecraft from a control room that is in constant motion would be difficult, to say the least. This isolation also seems to make the problem of radiation shielding worse as well.

Of course, I wouldn't have brought it up if I hadn't figured out a decent work-around. Just like real railroads, our IPV's rails can have sidetracks and switching stations. It's a little more complicated than tracks on flat ground but if a section of the tubular rail can be made out of flexible segments with expandable areas in between, it would work. I made a handy .gif animation showing the process below:
         With a sidetrack, the Modules can leave the spin track when they need to, without the other modules having to slow down. It may be necessary take a damaged node off the spin track for overhaul; the IPV can simply detach the modules, connect them to the node that pushed the damaged unit off track, and put the modules back under gravity, all in a couple hours or so. In combat or an emergency like loss of sensors, the Command module can side track out of spin, stop, and then take as many bearings off the stars with a coelostat that the Emergency Pilot wants. They can also send robots and possibly live crew via suits to any areas of the hull that are stationary.

         But the best part is that in the even of a radiation event, attack, or other calamity, all of the modules can sidetrack and hide. You'll notice our USS Example has a ring of silvery cylinders to located medially to the big orange balls (those are Hydrogen tanks, BTW). These tubes contain the bulk of the ships volatile propellant stored as nice safe water. Because there isn't any solid that hydrogen can't seep through given enough time (it is, after all, the lightest of all elements), the long-range IPVs electrolyze the water when more LH2 is needed and the oxygen is either burned for rocket power or breathed by the crew, take your pick.

Head for the cellar, Maw!  Twister's comin'!
         Anyway, all those water tanks are racked on a ring wide enough that the habitat modules can slide into the space they create, giving the humans and plants within a nice 10-15 meter wall of water between them and radiation. This means that not only is there no additional mass penalty for radiation shielding, there is no need to cramp up the crew in a tiny storm cellar either. The only luxury the crew need do without during a long radiation storm is gravity; they can sleep in their own bunks and eat at their own tables.

          During combat (if it's that kind of IPV), The modules are still protected. I imagine that all of the modules with the exception of FCR-1 will be under cover; FCR-2 will be in a place where it can pop out immediately and take over if FCR-1 is mission-killed. The most logical set up I could come up with is the one above; the Modules go in opposite directions so that the loss to one side of the spacecraft does not mean the loss of all hands. It may be possible to create another sidetrack that allows modules to travel from one spin rail to the other; that way, if one ring is damaged, the full crew can use the other ring while damaged one is repaired. Repair would involved replacing the damaged section from either spares or fabricated sections. We can do that now, as the rails on roller coasters today are prefabbed in sections and assembled on site.
          Anyway, this is a really long post, so I'm gonna go do something else now. Comments are always welcome. See you tomorrow, RocketFans!


  1. The idea is interesting--I hope to one day be one an aerospace engineer, and I've struggled with the spin-grav concept for a while. For the small scale, one idea I like is to enclose the entire spin section within a pressure volume (like a rotating frame inside a Bigelow module), so all that has to be transferred across the spin line is electricity (slip rings), water (rotary couplings), and data (wifi or another set of slip rings). However, this doesn't work so well on the larger scale of your IPVs, and for that scale, I think your concept here is really nifty.

    This site: has some info on how monorails do switches, some of which offer quick-cycling like you'll need for your rails if you want to be able to take off a life-module but not the ones before and after it. The time margin you'll have would be determined by gravity level you're creating, spin rail diameter, and number of cars on the rails at the time. Also note that you need twin siding rails to be able to take off cars two at a time to keep the rail balanced.

    I may take some time tomorrow, hook the part of me that is still a 5-year-old obsessed with trains and my engineer together and get some idea of the operational needs in terms of rates and track layout. I'll let you know if anything comes of it.

  2. That would be awesome! I'm also going to do a post about a less..."elaborate" system for the IPVs some time this week or next.

  3. Cool! In the meantime, my thoughts on the spin rail concept are up on my blog.


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