So what would happen if.......

[Mag40]

I used Google Earth to illustrate:  The line starts at Pad 39A and goes due east on heading 090.



Looking straight down on the line, we can see that it is straight; yet on the globe we can see that it follows the great circle route and goes south of the equator.

Hope this helps.

From what I have read.....from the moment powered flight stops, the craft will start falling around the great circle. It is a very hard subject for a complete layman to understand. The hardest part to grasp I suppose, is that even though it sets off East....that tendency to fall around the centre of mass is what swings it south(even during powered flight)...away from 5 degrees above the Tropic of Cancer. So even if a bizarre trajectory was chosen to force it to hold East(with thrusters or whatever), it would still curve around into a SE trajectory once it started to orbit.

Thanks for the replies

[Not Myself]

So let me try to think this figure-8 orbit thing through here . . .

A circular orbit in the z=0 plane with earth's centre at the origin (axis orientation left deliberately vague at this point) follows

x = r*cos(w*t)
y = r*sin(w*t)
z = 0

Rotate the y and z axes to give it some inclination.  (Unlike my earlier prime symbols, these primes just indicate alternate coordinates.)

x' = x = r*cos(w*t)
y' = y*cos(theta)-z*sin(theta)=r*cos(theta)*sin(w*t)
z' = y*sin(theta)+z*cos(theta)=r*sin(theta)*sin(w*t)

If this was all done so that z'=0 includes the equator, then

lon = atan(y'/x') = atan[tan(w*t)/cos(theta)]
lat = atan(z'/sqrt(x'^2+y'^2))=atan[sin(theta)*sin(w*t)/sqrt(cos^2(w*t)+cos^2(theta)*sin^2(w*t))]

But this is coordinate system of a non-rotating planet.  So we need

lon = atan[tan(w*t)/cos(theta)]-k*t
lat = atan[sin(theta)*sin(w*t)/sqrt(cos^2(w*t)+cos^2(theta)*sin^2(w*t))]

and it will be periodic if k=w.  Looks like some simplification is possible, but I need to go do something else right now.

[DataCable]

It doesn't maintain this due east orientation, however, any more than a ship or aircraft on a long voyage maintains a constant compass heading all the way to the destination even if it does follow a single great circle route.

...unless that great circle happens to be the equator or a meridian.

Very funny but don't give up your day job just yet.  Orbital mechanics humor doesn't have a very large audience. 

That's not even orbital mechanics humor, it's "big honkin' planet in your way" humor.

[Not Myself]

I guess "lithobraking" is orbital mechanics humour.

[Chew]

"It didn't crash. It impacted with the ground prematurely."

"Oh, meltdown. It's one of those annoying buzzwords. We prefer to call it an unrequested fission surplus."

[Count Zero]

I used Google Earth to illustrate:  The line starts at Pad 39A and goes due east on heading 090.



Looking straight down on the line, we can see that it is straight; yet on the globe we can see that it follows the great circle route and goes south of the equator.

Hope this helps.

From what I have read.....from the moment powered flight stops, the craft will start falling around the great circle. It is a very hard subject for a complete layman to understand. The hardest part to grasp I suppose, is that even though it sets off East....that tendency to fall around the centre of mass is what swings it south(even during powered flight)...away from 5 degrees above the Tropic of Cancer. So even if a bizarre trajectory was chosen to force it to hold East(with thrusters or whatever), it would still curve around into a SE trajectory once it started to orbit.

Well, on a mercator-progection map, it may look like its curving to the SE, but as you can see in the above image, it actually goes straight (or rather, the orbit is a circle/ellipse on a flat plane, and (like you said) the plane bisects the Earth right through its center-of-mass).

[Noldi400]

I used Google Earth to illustrate:  The line starts at Pad 39A and goes due east on heading 090.



Looking straight down on the line, we can see that it is straight; yet on the globe we can see that it follows the great circle route and goes south of the equator.

Hope this helps.

From what I have read.....from the moment powered flight stops, the craft will start falling around the great circle. It is a very hard subject for a complete layman to understand. The hardest part to grasp I suppose, is that even though it sets off East....that tendency to fall around the centre of mass is what swings it south(even during powered flight)...away from 5 degrees above the Tropic of Cancer. So even if a bizarre trajectory was chosen to force it to hold East(with thrusters or whatever), it would still curve around into a SE trajectory once it started to orbit.

Thanks for the replies

One layman to another, here's a point that may make it easier to picture:

There's a thought experiment called Newton's Cannonball.  Imagine a very large cannon on top of a very tall mountain, aimed in a direction parallel with Earth's surface. If you fire a cannonball at, say, a velocity of 2000 m/s it will travel a certain distance in a path that curves down to strike the ground (assume no air resistance). If you fire another one at 4000 m/s it will follow a similar path, striking ground further away.

Now. If you fire one with enough velocity - about 7300 m/s - the downward curve of its trajectory is equal to or less than the curvature of the Earth and although the cannonball continues to fall it will never strike the ground; it continues to fall around the Earth.

And this is the key to basic orbital mechanics for the layman. The plane of any free orbit by necessity passes through the center of the planet because the satellite is always falling toward the planet with only its tangential velocity keeping it from ever striking the ground. Hence the term "free-fall".

If that was confusing, just ignore it. We now return you to the Astrodynamics Channel.

 

[ka9q]

Would these be omnidirectional?  Or is the system for keeping them oriented separate from the system for keeping them in a particular orbit?

The satellite antennas or Omnitracs?

The satellite antennas are always directional. Usually it's a highly complex array with a custom-designed pattern shaped to cover the service area from the assigned longitude. One pattern is used with the continental USA, another for Europe, another for Japan, etc. The perspective of earth that the satellite sees from an inclined orbit usually doesn't change so much that the service area changes significantly, but east-west drift might be a problem depending on the assigned longitude.

Omnitracs uses an asymmetric "taco-shell" Ku-band horn with azimuth tracking only, necessary on a moving truck. The antenna pattern is narrow in azimuth but wide in elevation. This renders elevation tracking unnecessary and makes an inclined satellite orbit almost irrelevant. The only requirement is that the user links (both up and down) be vertically polarized, so only half the transponders on any one satellite can be used.

So you used their limited bandwidth on a ping   Pretty cool though.  But maybe not quite as cool as the other end of the ping.

Nobody seemed to mind...

[ka9q]

Bugger, I can't remember what satellite radio service I had in my car when I lived in North America (and when I had a car).  I seem to recall that there were two main services - was this one of them?  (Maybe there are more than two now?)

Sirius and XM. Sirius uses three satellites in tundra orbits, with two usually in service at any given time. There's a 4-second delay between the two, giving receivers a better chance of filling in gaps when driving under overpasses (yet it often didn't work with my receiver, for some reason.)

XM uses several conventional geostationary satellites named "Rock", "Roll", "Rhythm" and "Blues" or something like that. They may have more now.

Both services operate in a large chunk stolen from the 2300-2450 MHz (13 cm) amateur radio band (grrr...) Not much is left of it now since WiFi has taken over 2400-2450.

When the spectrum was first assigned to direct satellite radio broadcasting, the FCC insisted on at least two competing services. Sirius and XM accepted this condition. Just a few years later they were back, begging permission to merge and claiming they would both go bankrupt soon if they weren't allowed to do so.

So now we have "Sirius XM". The infrastructure of the two services remains much the same, with two independent constellations of satellites operating on different portions of the band with incompatible modulation methods. They now duplicate the same set of program channels, wasting half the spectrum and doubling the cost of their infrastructure without any compensating redundancy advantages since each receiver can only receive one system.

Grrr.

[ka9q]

"It didn't crash. It impacted with the ground prematurely."

I've always liked "spontaneous disassembly".

"Oh, meltdown. It's one of those annoying buzzwords. We prefer to call it an unrequested fission surplus."

Which is technically inaccurate, at least in western reactors like TMI and Fukushima. A so-called meltdown is caused by the residual decay heat of the fission products after shutdown, not by a "fission surplus".

Chernobyl, on the other hand, was a true unrequested fission surplus. The power spiked to hundreds of gigawatts due to criminally incompetent mishandling by the operators.

[raven]

And I suppose a fire could be called an 'exothermic oxidization event'.
I am rather partial to 'lithobreaking' myself. It has a nice ring to it.

[ka9q]

From what I have read.....from the moment powered flight stops, the craft will start falling around the great circle.

Not from the moment powered flight stops, but all during flight unless the rocket deliberately changes course by yawing (turning) left or right. That's sometimes necessary to avoid flying over land or dropping spent stages on peoples' heads, but it's always inefficient.

Ideally a rocket flies on a perfectly straight course from the moment it begins its roll and pitch maneuver after leaving the launch pad. Once on that course it may roll to point antennas at various ground stations, and it will gradually pitch down to the horizontal (or even below) as it climbs above the atmosphere, but yaw (left-right) changes are avoided whenever possible.
 

[ka9q]

I am rather partial to 'lithobreaking' myself. It has a nice ring to it.

Agreed. Though the canonical spelling should probably be lithobraking, I suppose lithobreaking is an acceptable variation.

But this word is suitable only for terrestrial moons and planets, i.e., those with rocky surfaces. Can we come up with a variant applicable to gas giants? How about surfaces made of ice rather than silicate rock?

And space launchers never actually fail, they simply achieve geostationary orbits with subterranean perigees. After the second Ariane launch dropped our satellite into the Atlantic, I suggested that the next launch should carry a communications satellite in its payload fairing and have an undersea fiber optic cable tied to its tail. That way, one of the two communications systems will work.

[Echnaton]

lithobraking
lithobreaking
subterranean perigees


Orbital mechanics humor is richer than I imagined. 

[ka9q]

Orbital mechanics humor is richer than I imagined.

Yes. Don't forget the "trench" in the Apollo Mission Control Center and all the colorful characters who worked there. Their mascot was Captain REFSMMAT, the ideal flight controller, and according to Gene Kranz (who admitted he knew much more about spacecraft systems than orbital mechanics) everybody who worked in it had a characteristic, uh, "attitude".

My favorite story from him concerns the controller who hurriedly parked his car on the sidewalk when late for work, resulting in the suspension of his parking privileges. Until he got them restored, he parked across from the entrance to MSFC, opened up a trailer, and rode his horse up to the building, tying it off while he worked. Apparently nobody in NASA security thought to require permits for horses.