Friday, September 5, 2008

Why'd We Put the Rockets There?

It happens every Atlantic hurricane season: news reports on all of the popular spaceflight websites about how Hurricane So-and-So delayed the rollout of the Space Shuttle or damaged this rocket on the pad or that important piece of ground equipment. And every year, right around the Atlantic hurricane season, I find myself facing the question: why did the United States put its space launch facilities right in the path of the typical storm track? So every year, just to remind myself that it wasn't a completely ludicrous decision, I go through the physics of the thing.

And physics is exactly why the United States sited its primary national launch site on the south coast of Florida. It wasn't that the land was cheap or that a Floridian senator was on the committee or such (these things may or may not have been true, but even if so, they were not the overriding reason).

One of the reasons for the birth of the Space Coast down there in Florida is safety. Launching rockets was, and indeed remains, a tricky business. The occasionally go...wrong. Even successful flights shed parts (sometines unintentionally but more often intentionally as spent stages and fairings are jettisoned) And for that reason, it is nice to have a large chunk of empty land that your launches can fly over. And, for reasons that we will shortly discuss, most space launches fly to the East, more or less. That would confine a United States launch facility to the East Coast. Conceivably Hawaii could be used as well, since by the time a vehicle launched from that location reaches any significant land mass, it will be flying high enough as to pose no threat. But flying from Hawaii would raise infrastructure and transportation challenges, particularly in the 1950's when the Space Coast was first evolving. One could argue that the wastes of far northern Canada would be safe to fly over and that Alaska would make a reasonable launch facility -- but in addition to the dangerous politics of arguing that anyone's land mass is insignificant there are some good reasons why far Northern launch sites are not the best to pick.

And so now we enter the physics discussion. Before we can go too far, let's pause and think about what a vehicle in orbit is doing. It is going around the Earth at a rate just fast enough to offset gravity's attraction. Objects in orbit are still attracted by the Earth's gravity. It is an easy misconception to imagine that they are some how "beyond" the force of G, but achieving that feat requires a great deal more distance (theoretically an infinite one) and a great deal more velocity. It is just that their motion is such that as gravity relentlessly tries to pull them down to the surface, their own motion offsets the tug -- just like when you whirl a bucket of water, the water's own momentum (as manifested in that handy engineer's shortcut of centrifugal force) holds it in place. In orbit, your whirling velocity around the planet wants to push you off into deep space -- but the attractive force of the massive planet holds you neatly balanced. It is a beautiful thing, really.

The operative point, in case you don't want to spend too much time on the bucket-is-like-a-satellite analogy, is that putting something in space really is all about getting it to go sideways. Not up. Next time you have occasion to watch a space launch on TV (or in person, if you are so lucky) notice the trajectory. It can be a bit hard to follow since the camera guys always zoom way in, but the Shuttle (or whatever) does not go straight up. After just a few seconds, the vehicle begins to pitch over and fly ever more horizontally.

There is a bit of subtlety in the details of the trajectory design, degree of lofting, etc. But the key issue is that a rocket rises a little bit but goes sideways a lot. The first segment of flight is a gradual transition from the vertical (handy for setting things up) where you are climbing out of the irritatingly thick atmosphere to a horizontal motion where you are building up the speed necessary to get your bucket whirling fast enough to offset the planet's gravitational attraction.



Watch this video of the New Horizons launch and notice how the big Atlas V appears to be pitching over to an increasingly horizontal trajectory. It is hard to notice second-to-second, but over the course of a minute of flight it becomes pretty apparent. For a real dramatic illustration, watch for the jettison of the solid rocket boosters just about 2:20 into the video. Then at about 2:36 the rocket executes a very dramatic pitch-down maneuver to bring the direction of its flight increasingly horizontal. Apparently this pitch down was even more noticeable to observers watching the launch in person -- enough so that it caused some moments of real worry for those who did not know to expect it!

As a rule of thumb, it takes a velocity of around 7,800 meters per second (I'm going Metric on you for this one!) relative to the Earth to get something in the lowest possible sustainable orbit (any lower and you will start bumping into enough of the molecules of ethereal atmosphere at that altitude that you'll slow down...and once you start slowing down you hit more atmosphere...slow down more...and the result is obvious). That's awfully fast, and one of the reasons it takes such gargantuan rockets to loft even small payloads is that building up that much velocity takes a lot of energy. As a brief footnote, I'll mention that with practical considerations taken into place, it takes 9,300 to 9,800 m/s of velocity to actually make it to LEO -- the extra is accounted for by aerodynamic drag (100-200 m/s), control and steering losses (200-250m/s), and the losses spent overcoming gravity (the rest).

In such a situation, engineers will try to take advantage of any asset they can. Rockets are built light, fueled with the most desperately energetic propellants possible (and historically some very, very exotic and toxic combinations have been experimented with), and shed unneeded mass at any chance possible. They are also almost always launched to the East. Why? Because the Earth turns.

Picture a sunrise: in the East. A sunset? In the West. Our planet, in addition to a whole complex series of motions relative to various other bodies in nearby space, rotates around its own axis, turning from West to East at a rate such that it completes one full rotation in 24 hours. At the equator, on the surface, that works out to a speed of about 465 meters per second (just about 1,000 mph). Why don't we feel this? Because everything else around us (air, water, train tracks, laptop computers, coffee cups) shares this motion. Actually, there is an important subtlety at work here: at the poles, we have zero velocity due to rotation, we'd just turn in place. Spin a globe. The equator is blurry fast, the middle latitudes (North or South) move at a moderate pace, and the poles barely seem to move at all. The velocity, at a given latitude, is proportional to the distance around the globe at that latitude. Amongst other things, this causes the swirling interactions of atmosphere responsible for no small part of the global weather patterns.

It also provides a powerful incentive for launching rockets to the East, near the Equator. The Earth gives you a boost equal to the rotation-induced velocity of the surface at the latitude of your launch site. At the equator, that amounts to 465 meters per second. At Kennedy Space center, about 28 degrees latitude (of 28/90ths of the way from the equator to the North Pole) this boost is still 450 meters per second. But were I to build a launch pad here in Seattle, at 49 degrees latitude, the boost is only 305 meters per second. If you are curious, the degree of kick varies with the cosine of the latitude.

Given the skin-of-your-teeth challenge of getting something into orbit at all, it is not remarkable that engineers have sought to site launch facilities to take maximum advantage of this simple bit of physics. Now a word of warning -- and clarification for any real rocket scientists who stumble across this: I am ignoring polar orbits, sun-synchronus orbits, non-due-east launches, and the complexities of plane change maneuvers. I know.

atlantisready.jpgLaunching from Kennedy, at 28 degrees North, provides a boost of 450 meters per second -- about 5% of our total rule-of-thumb velocity increment. For a hypothetical rocket I've been doodling out in the form of a Numbers spreadsheet, this works out to a payload (to low Earth orbit) launching from KSC allows the payload to increase from 7000kg (for a mythical zero-velocity launch site) to 8500kg! This happens for no increased launch vehicle mass, no increased cost, just a willingness to put up with a few hurricanes.

I know that I have a good time ripping NASA a new one in this blog (except Alan Stern, and he's no longer with NASA). But this is one area in which I have to say they chose well. Kennedy is effectively the southernmost point in the continental US that has a clear space to the east. It is interesting, however, to look at some other launch facilities in light of this information and to try and understand the rational behind their selection.

610x.jpgFor starters, look at Russia. Devoid of a "space safe" site to the East (almost any Russian East coast launch site would have to fly over Japan), they are forced to launch from the West side of the nation, taking advantage of the vast reaches of emptiness that fill the middle part of Russia. This approach isn't without very serious drawbacks -- spent 1st stages from Russian Proton launchers litter the steppes of Kazakhstan. The toxic traces of the NTO/UDMH propellant that Proton uses have begun leaching into the groundwater supplies with, well, predictable results.

Their other launch facilities are all in the far (for a launch site) North and get even less help from the Earth than my mythical Spaceport Seattle. To make matters worse, even when launching Zenit or Soyuz boosters (which generally avoid the toxic-waste-dump problem of Proton) decent range safety practice dictates narrow and oddly positioned corridors through which launches can fly -- dramatically restringing the orbital options available to Russian flight planners.

Other launch sites face some even more interesting challenges. Japanese launches often must contend with the fishing season. Plentiful fishing grounds to the East of the launch sites mean that, in an island nation that eats a lot of seafood, space launches must wait until the fishing boats get out of the way rather than imposing an exclusion zone is is done off Florida.

Israel faces perhaps the most challenging geographical launch constraints of anyone. Located around 31 degrees North latitude, things wouldn't seem too bad (not as good as Florida, better than Russia) until the political climate of the region is taken into account. Raining debries from a launch (successful or failed!) down on hostile neighbors to the east poses a grave political risk, and a potential security challenge should any piece fall into the hands of hostile intelligence agencies. There is also the risk of a launch, even announced, over hostile territory being seen as an aggressive act.

What all of this means is that, alone among the space capable states, Israel must launch her satellites DUE EAST -- exactly the wrong direction. Not only do Israili launch vehicles get no assist from the Earth's rotation, but they must actually work to overcome it first! The result is a penalty of about 450 m/s beyond the basic 7,800 m/s required for LEO insertion. Another amusing effect are the unique orbits occupied by satellites launched in this manner.

The European Space Agency launches from French Guiana -- from a point only 310 miles north of the equator. This supplies something like 463 m/s of velocity increment. Compared to the launch site in Plesetsk, a Russian Soyuz rocket launched from the ESA spaceport picks up 1200kg of payload to a geostationary transfer orbit. That is an increase of 80% -- though in all fairness the launch azimuths of Plesetsk are particularly poorly suited to this trajectory and the difference for other orbits range down to only 20% -- but still significant!

Similarly close to the equator is the very clever Sea Launch platform and rocket. This is essentially a Russian Zenit rocket mounted on a converted oil platform that migrates down to sit right on the equator for launch. The result is the full 465 meters per second of possible rotational kick -- and freedom to launch on whatever azimuth or pathway is wanted!

So hurricanes are not, I suppose, such a bad price to pay.

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