# How do launch windows work?November 9, 2010 9:41 AM   Subscribe

How do launch windows work?

I recently read that the launch window for the Cosmo-Skymed 4 was only six seconds long per day. I realized I have no idea on how launch windows are calculated or why they're so tiny. So far wikipedia and googling isn't much help. Anyone care to explain with detail or point to a decent resource?
posted by damn dirty ape to Technology (5 answers total) 2 users marked this as a favorite

I am not an astrophysicist and I am just going to give a simple answer, which perhaps will be further elaborated upon by other contributors. First of all, launch windows depend upon the intended destination of what you are launching. For example, if you are sending a probe to Mars, you launch at a specific time so that when your probe gets to the orbit of Mars, the planet will be there, rather than being in another part of its orbit.

Secondly, the Earth is at this time surrounded by a large number of orbiting objects, some of which are useful objects such as communications satellites, and many others of which are simply orbital debris left over from various launches. Even a small piece of orbital debris can cause enormous damage because it is moving at a very high speed. So anything launched into space must find a clear pathway through the other satellites and the debris. This requires the calculation of a precise launch time (as well as a launch angle and velocity for the location from which you are launching).
posted by grizzled at 9:56 AM on November 9, 2010

This satellite is going into a solar-synchronous orbit (according to Wikipedia) along with its 3 siblings orbiting 90° apart, probably with many other satellites whizzing around crossing the poles. Imagine one of those crazy intersections in India where traffic is crossing in all directions with no stop lights. Now imagine trying to throw a car into place in the pattern. They're trying to get it into a specific orbit where it crosses the same places at the same solar time each pass (like crossing Washington, D.C. at high noon every time). Since there's not much fuel available for course correction, they have to wait until just the right time to lob the satellite from wherever they are launching towards one of the poles. The want it on an orbit exactly 90° from two of the other satellites and 180° from the third that is on the opposite orbit.
posted by zengargoyle at 10:30 AM on November 9, 2010

Also, consider the space shuttle launching for a rendezvous with the space station. The shuttle uses most of its fuel in the ascent, so doesn't have much to "catch up" with the space station. The space station is already going round the earth at a fair old clip, but NOT in line with the equator but at an angle, with the result that as the earth turns, the space stations path lies over different parts of the earth.

The shuttle must therefore launch (a) when the space station's path lies over the launch site; and (c) so that it will arrive 'fully on orbit' within just a short distance of the space station, and doing the same speed.

The 'window' is the period of time that allows you to achieve (a), (b), and (c). The window will be bigger the more spare fuel you have for adjustments at the top of your ride. Less spare fuel results in a smaller window (in terms of time).

The space station can be a bit "either side" of Kennedy Space Center, but the more off it is, more of your spare fuel will be required to maneuver. The space station may be the other side of the world, but because the shuttle starts off slow, and accelerates, it has to set out well before the space station gets there: a bit like leaping onto a moving train. The shuttle may also 'coast' most of the last part of the journey and slow down a bit as it nears the space station. This may take the best part of a day or so: it could be done quicker, given more fuel - which you don't have.

Because the space station's path crosses Kennedy Space Center every few days (say), then the window may recur every few days, but its path won't lie *exactly* across it frequently at all, so you have to compromise, or wait a long while. The more accurate the path, the bigger the window. Less accuracy = smaller window. Even for the same vehicle going to the same 'place', therefore, the launch window may be big sometimes (e.g. 15 minutes), or small at others (e.g. just a minute or two, or mere seconds in your case).

As grizzled has said, other objects in the way also rule out certain windows, but those objects are more often other rockets needing to use the same launch pad or airspace, etc.
posted by blue_wardrobe at 10:33 AM on November 9, 2010

something happened in the editing: "and (b) launch in the same direction" got accidentally left out.
posted by blue_wardrobe at 10:34 AM on November 9, 2010

Good explanations above, some links below:

from NASA

Another

from howstuffworks

one more
posted by clanger at 9:25 PM on November 10, 2010

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