Henry Spencer was kind enough to send me some feedback on the first two posts in this series, and it appears that I forgot to highlight what both of us see as the key benefit of such repeating ground track orbits: first orbit rendezvous.
First Orbit Rendezvous
Orbital Mechanics isn’t my strongest suit, but I’ll try to explain. For optimal launches you want the destination plane to cross through your launch site. For typical, non-repeating orbits like that of ISS, a given launch window won’t actually have the station itself passing directly overhead, just the plane. So what you do is you launch into what’s called a “phasing orbit”. Basically orbits at different altitudes will complete a revolution in a different amount of time, with lower orbits being faster. So, basically you go around a bunch of times before things actually line up for the transfer out to the final orbit. That’s probably a painfully oversimplified explanation, but gives the general idea.
The problem is that orbit phasing like this typically requires as much as 3 days between launch and rendezvous. That’s a long time. When you think about it, that’s actually about the same time it would take for a vehicle to travel from LEO to a lunar orbital station! Imagine how much fun that must be in something cramped like Soyuz. Long phasing time requirements like that drive up the design complexity and cost of a transfer vehicle, while also typically making it less comfortable. Basically, instead of being just a simple LEO equivalence of a Toyota Corolla, it ends up being an orbital RV, just more cramped.
On the other hand, if both the plane of the station and the station itself cross over the launch site daily, you can do a “first orbit rendezvous”. Ie you don’t have to do phasing orbits first, and could be done with rendezvous and docking in as little as 90 minutes, possibly less with practice. This allows you to go with a much more spartan, lightweight, simple, and inexpensive transfer vehicle. It also makes the trip to orbit a lot shorter, so you get to spend more of your time at an actual station or transfer vehicle which can afford to have nicer…er facilities. Being able to avoid the need for the “diapers” is probably well worth it.
The other nice thing is that once things have gotten going well enough that you have a steady flow of vehicles up and down from the station, the stay-time of any individual vehicle can be kept relatively short, allowing you to turn things around more frequently. This is at least part of the reason why multi-hundred million dollar airplanes can keep costs low enough that Joe Schmoe can buy a ticket from LA to New York for a price that is actually not much more expensive than driving (and possibly cheaper).
Other Repeating Ground Track Orbits
Another thing worth mentioning is that just about every inclination you can imagine has these repeating ground track orbits. It’s just a case of matching things like the time it takes to complete an orbit, and how fast the earth is rotating underneath you. It’s substantially more complicated then that in detail (as usual in engineering), but this 41 degree orbit isn’t the only resonant orbit out there. There exist repeating ground track orbits in 51.6 degrees for instance. I do believe the one which repeats every 24 hours ends up being not much more than 100km higher than ISS is currently located. A friend of mine actually suggested boosting ISS into such an orbit to make it easier to access for multiple users. If ISS weren’t being run partially by NASA (ie if they say auctioned off their stake in things), that might not be a bad approach to things.
[Note: Hopefully sometime next week I’ll have a global ground track to discuss.]