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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: Cel nav in space
From: Frank Reed CT
Date: 2005 Jan 4, 20:45 EST
Sorry. We're off-topic here, but since the list is quiet, I think it's "ok" for a few posts.

cls-employees-org wrote:
"Derrick, where did you get this, much of this seems wrong to me."

I want to say up front that most of what Derrick wrote seems right to me. Now on to the details...
cls wrote:
"all flying things follow a "ballistic" trajectory, plus or minus
lift.  there is no "gravity turn trajectory"."

A 'gravity turn trajectory' is a somewhat weird name for the usual flight path of a rocket on its way to orbit. Remember, getting into space (and staying there) is not about altitude --it's about speed. If you want to orbit the Earth, you need to get your final payload up to a speed of some 18,000mph or so with its velocity vector parallel to the Earth's surface. That's much harder than punching up through the atmosphere and then falling right back. If there were a tall enough mountain (over a hundred miles high), and if acceleration of the payload were not a constraint, you would do best to launch your rocket on a rail horizontally. Then all of the rocket's effort could be put into getting it up to orbital speed. But we don't have that super-mountain and there are practical limits on acceleration of the payload, so we can't do that. Instead, you start out with rocket vertical and its initial thrust is just enough to exceed its weight. That gets it off the ground. But as you go, you can begin to roll the trajectory over so that the rocket's thrust is being used primarily to increase its speed and only secondarily keep it up above the ground. There are all sorts of little details to this. For example, aerodynamics dictates that you want to go more "straight up" near the beginning of the trip than gravitational considerations alone would justify. Basically, you need to get out of the thick air before you waste too much energy plowing through it.

"my model and high power rockets are unguided so mostly fly straight up."

Right. And anything flying straight up, even rockets with apogees thousands of miles up, have no need for this 'gravity turn' business.

and:
"even so, they are on a ballistic trajectory which is pretty easily
calculated a number of different ways.  but not commonly with a Taylor series."

You can do anything with a Taylor series. These days, any trajectory calculation except textbook problems would be done with a detailed numerical integration. These were much less practical in the days before desktop computers.
and wrote:
"the shuttle is steered eastward intentionally, to get an extra 1000 MPH
for "free" from the earth's rotation.  most everything else launched from
Kennedy is sent the same way."

Yes, you turn EAST to get the rotation boost, BUT even if the Earth had no rotation you would still roll over in a 'gravity turn trajectory' if your goal is to get into orbit (whether eastbound or westbound). Consider the case of flying off the surface of the Moon. There's no air and essentially no rotation. What path would you have to follow starting at rest on the lunar surface to end up in a nearly circular path at orbital velocity? If you think it through carefully, you'll find just the same sort of launch trajectory that you've seen in a shuttle launch or any other launch from Earth.

Also, note that Israeli satellite launches are sent west instead of east to avoid flying over their neighbors. The path to orbit closely resembles a standard eastbound launch. It's just somewhat more expensive.

-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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