Back in September of last year, Bigelow announced an ambitious plan, that if successful may become one of the key events in the commercial development of space. On the heels of his Genesis I mission, that performed far better than expected for a first orbital mission, Bigelow announced that they were intending to accelerate their schedules. At the AIAA Space 2006 Symposium, he announced the goal of orbiting his first revenue generating manned space station module, Sundancer, by the end of the decade. This three-person, ~170 cubic meter module would be joined a few years later by his previously announced 6-person 330 cubic meter Nautilus module. Bigelow is taking a bit of a gamble on Sundancer, hoping that if he provides a low-cost, visitor-friendly destination for tourism and microgravity research, that he can spur on the development of lower cost orbital transportation, by creating a large demand for manned orbital flight. His goal of reaching up to 16 flights per year to the combined Sundancer/Nautilus station could have a major impact on this fledgling industry, and his efforts to create what some have called an “international astronaut corps” may also create substantial flight demand even at prices not too much lower than current Soyuz prices.
Many, myself included, have written about various aspects of Bigelow’s plans and their potential impact on the commercial spaceflight market. There is a lot more that can be said on these topics I’ve already mentioned, and I’ll probably go into it some more as time goes on. However, one of my goals with this blog has been to try and provide some original content and ideas, and to discuss the business implications of the physics and technology of spaceflight.
So I would like to start a brief series of posts about something interesting I’ve discovered about Bigelow’s announced plans for Sundancer that I don’t think has been discussed elsewhere.
The Mystery of 40 Degrees
My discovery centers around the orbital trajectory that Bigelow announced for his Sundancer module. Back in September when he announced his Sundancer plans, he mentioned that the module would be in a ~250 nautical mile orbit at a ~40 degree inclination. I didn’t think much about it at the time, other than wondering why he picked those exact numbers. However, A few months later in a conversation about commercial orbital propellant depots, someone brought up the fact that most Russian launch vehicles would be unable to reach Sundancer in its proposed orbital inclinations (Russian vehicles are typically limited to orbits of 51.6 degree inclination or higher due to the latitude of their launch sites, and launch azimuth restrictions). This made me all the more curious, because Bigelow is doing a lot of his prototype launches on Russian vehicles, and I wouldn’t think he would lightly pick an orbit that might shut out potential launch possibilities. So, I really started wondering, why he picked 40 degrees? If you aren’t going to pick something high enough inclination for Russian vehicles to reach it, why not pick 28 degrees so you can get more performance from Cape Canaveral launches?
Just when I figured that I wasn’t going to ever find an answer to why they picked the orbit they did, I stumbled across a post in the L2 section of NASASpaceflight.com about some of the work Lockheed has been doing with Bigelow. One of the things discussed in the post was the answer to my question about why Bigelow had selected a 41 degree orbit for Sundancer.
[Note: Before I go on, L2 does cost a little bit of money, but it is often well worth the price. Chris Bergin has a lot of solid contacts in both NASA and the commercial space launch industry, and often has stories there several days before the show up in public. I can’t go into most of what was related in the document, due to restrictions related to stuff posted on L2, but I did get permission from Chris to discuss the part relating to why they picked the orbit in question.]
It turns out that there were three reasons why Bigelow selected 41 degrees as the inclination for the station: the view, launch safety, and the repeating groundtrack.
One of the important factors identified for providing a good personal spaceflight experience is the view. While looking down at almost any part of the earth from orbit is probably going to be pretty cool, many people have expressed the desire to be able to see where they live from space. Most people who can afford orbital spaceflight come from countries in places like North America, Western Europe, Scandinavia, East Asia, and Australia/New Zealand. Most of those locations are far away from the equator. In order to see most of North America and Western Europe, you actually want to pick a moderately high inclination.
Also, if you can see a place from orbit, you can be tracked from that place on the surface. This allows Bigelow to rely mostly on facilities located in the US for tracking and monitoring.
An important part of designing a launch vehicle for carrying people is designing for safe abort modes. One of the main factors in designing for safe aborts is making sure that the launch trajectory does not take the vehicle over terrain that would be unsafe to land in. It turns out that when launching out of Cape Canaveral into a 51.6 degree orbit, the launch vehicle passes right over the Alps as well as over portions of the North Atlantic that might be dangerous for an emergency landing. On the other hand, as you can see from this trace of the Instantaneous Impact Point for a launch into a 41 degree orbit (reprinted with permission from NASASpaceFlight.com), these potential landing and rescue hazards are avoided.
Repeating Ground Track Orbits
The last reason is the most interesting one to me. It turns out that the orbit selected (a 489km high, 41 degree orbit according to the paper) is what I’ve heard called a “resonant orbit” or a “repeating ground track orbit”. What this means is that the station would pass directly over the exact same point on the surface of the earth once every set amount of time–in this case once every 24 hours. According to the paper, this allows daily launch opportunities from Canaveral, as well as also allowing daily landing opportunities at several US landing sites.
In addition to passing over a given point on the ground at the exact same local time each day (ignoring Daylight Savings Time), there are some other interesting properties of repeating ground track orbits like this. For instance, if I’m understanding this right, if you look at the “ascending” portion of the ground track (ie the portion where the track is going from 41 degrees South heading northeast to 41 degrees North on any given orbital pass), it passes over a given latitude at the exact same local time as well. In other words, if it passes over Cape Canaveral heading northeast at 7am, then on the next pass, when it passes over the part of Mexico that is at the same latitude as Cape Canaveral, it will also be 7am. The same applies for descending portions of the path. An interesting corollary is that as the groundtrack passes over the equator on an ascending path, the local time will be exactly 12 hours different from the local time at the equator when the path is descending. Ie if Sundancer crosses the equator going north at 6am local time, it will always cross it going south at 6pm local time.
[Update 2/2/07: It turns out that according to Henry Spencer this isn’t actually the case. Apparently the orbit is still precessing in such a way that the local time at a given launch site will change by about 25 min per day. Not a showstopper by any stretch, but not 100% convenient either. But those are the cards nature has dealt us.]
Another interesting thing is that there are several locations on the globe where the ground track crosses the same point on the earth on both one ascending and one descending pass. As you get closer to the equator, the time difference between the two daily cross-overs will get closer to 12 hours. But as you get closer to 41 degrees, those cross-overs will happen closer and closer together (with the highest crossover points happening on two consecutive orbits–ie only about ~96 minutes apart).
So, the main reasons Bigelow and Lockheed are pursuing a 41 degree orbit for Sundancer are:
- Providing a good view for customers, and tracking from US mainland sites
- Avoiding unsafe abort locations during launches to it from Cape Canaveral
- Allowing for daily launch opportunities from Cape Canaveral, and multiple daily landing opportunities at US sites, even with low-cross range capsules
However, it turns out that there are far more interesting, and potentially important implications of this orbit…that I’ll cover in Part 2 of this series.
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