Re-entry

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lux
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Re: Re-entry

Unread post by lux » Tue Oct 09, 2012 7:25 pm

^ OK, so the geostationary satellite over my head is also not moving relative to the atmosphere too, correct? (Because the atmosphere is revolving with the planet just as I am.)

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Re: Re-entry

Unread post by fbenario » Wed Oct 10, 2012 1:16 am

As a matter of basic, common-sense human logic, producing something visually understandable in each of our brains, this thread may already be the single most confusing one we've produced so far. I'm certainly all at sea!

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Re: Re-entry

Unread post by simonshack » Wed Oct 10, 2012 1:27 am

*
Lux, I was also curious to find out a little more about these so-called "geostationary satellites" - which, we are told, need to attain the "magic" altitude of 35,786 km (roughly one tenth of the distance between the Earth and the Moon) in order to circle over a given point of our planet's equator.
Geosynchronous satellite http://en.wikipedia.org/wiki/Geosynchronous_satellite
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"According to Kepler's Third Law, the orbital period of a satellite in a circular orbit increases with increasing altitude. Space stations and Shuttles in Low Earth orbit (LEO), typically two to four hundred miles (320 to 640 km) above the Earth's surface, make between fifteen and sixteen revolutions per day. The Moon, at an altitude of about 238,900 miles (384,400 km), takes about 27 days 7 hours to make a complete revolution. Between those extremes lies the "magic" altitude of 22,236 miles (35,786 km) at which a satellite's orbital period matches the period at which the Earth rotates: once every sidereal day (23 hours 56 minutes 4 seconds). In that case, the satellite is said to be geosynchronous."
Wickedpedia then tells us about the disadvantages of geostationary satellites - and that's when I started scratching my head sore...:blink:
"Another disadvantage of geostationary satellites is the incomplete geographical coverage, since ground stations at higher than roughly 60 degrees latitude have difficulty reliably receiving signals at low elevations. Satellite dishes at such high latitudes would need to be pointed almost directly towards the horizon [huh?? :huh: Can anyone help me out here? Heiwa? Anyone?]. The signals would have to pass through the largest amount of atmosphere, and could even be blocked by land topography, vegetation or buildings."

I then wondered exactly how anyone had calculated that "magic altitude" of 35,786 km. I found no specific info about that, but I bumped into the purported inventor of the geostationary satellite. So here's introducing HAROLD ROSEN:

HAROLD ROSEN - the inventor of the "geostationary satellite"
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caption: "Photograph by Amanda Friedman"
http://discovermagazine.com/2003/nov/communications

"The idea for a space-based communications satellite network was first suggested in 1945 by writer Arthur C. Clarke. Many scientists had dismissed Clarke’s proposal as far-fetched. But when the Soviet Union launched Sputnik in 1957, the world looked up and saw that the future of communications hung in space. Rosen and a handful of others would put it there by hurling voice, television, facsimile, and data communication around the world."
(...)
"Rosen thought he could design a small satellite with sufficient bandwidth for television transmission or 100 telephone channels and build it in a year for $5 million. He consulted with fellow engineers Tom Hudspeth, John Mendel, and Donald Williams, and they agreed. Rosen thought it would be very profitable. “Being bold, I said we could probably sell an hour a day of television,” he laughs. “The head of the communications division thought an hour a week would be a lot.” He waves at Tom Hanks on the TV screen."


SYNCOM - the world's first geosynchronous satellite:

Image
The young HAROLD ROSEN (at right)
http://www.boeing.com/defense-space/spa ... yncom.html

lux
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Re: Re-entry

Unread post by lux » Wed Oct 10, 2012 2:04 am

fbenario wrote:As a matter of basic, common-sense human logic, producing something visually understandable in each of our brains, this thread may already be the single most confusing one we've produced so far. I'm certainly all at sea!
Yes, so am I. I find this whole subject oddly confusing.

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Re: Re-entry

Unread post by Heiwa » Wed Oct 10, 2012 2:08 am

lux wrote:^ OK, so the geostationary satellite over my head is also not moving relative to the atmosphere too, correct? (Because the atmosphere is revolving with the planet just as I am.)
Yes, but there is no real atmosphere outside the alleged ISS or a geostationary satellite as there is vacuum there = nothing to produce friction braking at those altitudes. The atmosphere is much lower down when used for - topic - braking at re-entry.
Interestingly - off topic - the rotation of the planet causes currents in the oceans and allows Moon gravity to cause tides.
Last edited by Heiwa on Wed Oct 10, 2012 2:12 am, edited 1 time in total.

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Re: Re-entry

Unread post by simonshack » Wed Oct 10, 2012 2:10 am

And here's the wiki page showing the first SYNCOM: http://en.wikipedia.org/wiki/Syncom

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Funny how the above version is 8 "stories" tall - whereas this version presented by HAROLD ROSEN is only 6 "stories" tall:

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Duh - silly me. Its design was probably in constant evolution at the time... <_<

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Heiwa: how did they brake this thing as it reached the "magic" 35,786 km altitude?

lux
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Re: Re-entry

Unread post by lux » Wed Oct 10, 2012 2:28 am

Heiwa wrote:
lux wrote:^ OK, so the geostationary satellite over my head is also not moving relative to the atmosphere too, correct? (Because the atmosphere is revolving with the planet just as I am.)
Yes, but there is no real atmosphere outside the alleged ISS or a geostationary satellite as there is vacuum there = nothing to produce friction braking at those altitudes. The atmosphere is much lower down when used for - topic - braking at re-entry.
Yes, I understand that. My question is: If a geostationary satellite simply descended until it entered the atmosphere it would essentially have zero velocity in relation to the atmosphere, right? In other words, it would be the same as if the object were simply dropped from a balloon at the edge of our atmosphere and would therefore not need a heat shield any more than these alleged high altitude balloon skydivers need one. Assuming it could descend while still maintaining its geostationary status (this may be where my logic is breaking down as this may not be possible).

But, I guess my basic question here is: Why don't these alleged manned space craft simply maneuver into a position and direction where they are more or less matching the spin of the Earth before descending into the atmosphere? Unless I am hopelessly confused I would think this would eliminate the need for a heat shield. :blink:

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Re: Re-entry

Unread post by Heiwa » Wed Oct 10, 2012 2:56 am

The (tangential) velocity of a geostationary satellite is of the order 2 650 m/s at 35 786 km altitude, i.e. it orbits Earth once every 24 hrs. How it gets there is beyond me! My biz is keeping old ships on the oceans afloat. The alleged ISS at 350 km altitude must speed at 7 900 m/s to remain in orbit and is then orbititing Earth every 90 minutes.
Now if you give a geostationary satellite a kick downwards, it will sooner or later reach the outer layer of atmosphere (like Apollo 11) at a high vertical velocity and burn up and the tangential (horizontal) velocity 2 650 m/s slows down in the process due friction, etc. Total velocity of the falling satellite is evidently the vector sum of the horizontal and vertical velocities.
Earth continues to rotate as usual.

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Re: Re-entry

Unread post by lux » Wed Oct 10, 2012 3:12 am

^ OK, in that instance I guess I don't understand why it would burn up if it's moving at the same speed as the Earth and its atmosphere.

I'm assuming the atmosphere spins at the same rate as the Earth. If the object is geostationary then it is also "atmosphere-stationary."
So, when it enters the atmosphere it would have zero airspeed and therefore should behave as a dropped object and dropped objects don't burn up AFAIK. That's my logic anyway.

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Re: Re-entry

Unread post by pov603 » Wed Oct 10, 2012 7:31 am

@Heiwa, can you re-clarify for a layman like me that the speed at which a human on the planet is moving [relative to space] is approximately (6,371km x 2 x pi)/24hrs = 1,669km/hr which would mean that the geo-stationary satellite is moving at ((6,371km +35,786km) x 2 x pi)/24hrs = 11,041km/hr [or 3km/s]?
If so, what is allowing the satellite to maintain that velocity?
Also as the planet is supposed to moving around the sun at the rate of ((150,000,000 km x 2 x pi)/365)/24 = 107,600km/hr [or 30km/s] how do the satellites manage to cope with 'random' objects/asteroids floating on their merry way around space?

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Re: Re-entry

Unread post by Heiwa » Wed Oct 10, 2012 11:37 am

lux wrote:^ OK, in that instance I guess I don't understand why it would burn up if it's moving at the same speed as the Earth and its atmosphere.

I'm assuming the atmosphere spins at the same rate as the Earth. If the object is geostationary then it is also "atmosphere-stationary."
So, when it enters the atmosphere it would have zero airspeed and therefore should behave as a dropped object and dropped objects don't burn up AFAIK. That's my logic anyway.
A person on the Equator and the air around him + everything else there rotates at horizontal velocity only ~463 m/s, so that is the horizontal velocity to aim for when landing or entering atmosphere at Equator. The vertical velocity should also be 0 but gravity will no doubt pull you harder. Have you ever dived from the 10 m tower? At LA the horizontal velocity is less and at North Pole 0.
Anything dropping down from a geostationary satellite needs plenty energy to brake the descent in 3-D and when the absolute velocity and associated friction is too high, it burns up.

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Re: Re-entry

Unread post by Heiwa » Wed Oct 10, 2012 11:51 am

pov603 wrote:@Heiwa, can you re-clarify for a layman like me that the speed at which a human on the planet is moving [relative to space] is approximately (6,371km x 2 x pi)/24hrs = 1,669km/hr which would mean that the geo-stationary satellite is moving at ((6,371km +35,786km) x 2 x pi)/24hrs = 11,041km/hr [or 3km/s]?
If so, what is allowing the satellite to maintain that velocity?
Also as the planet is supposed to moving around the sun at the rate of ((150,000,000 km x 2 x pi)/365)/24 = 107,600km/hr [or 30km/s] how do the satellites manage to cope with 'random' objects/asteroids floating on their merry way around space?
Yes, a geostationary satellite has a tangential velocity about 3 000 m/s or a little less, say 2 650 m/s at 36 000 km altitude, to remain stationary seen from anybody on the rotating Earth.

The satellite has apparently been brought to that altitude by a rocket, which then changes direction and pushes away the satellite at required tangential velocity 3 000 or 2 650 m/s, so it can orbit geostationary around Earth. So far so good.

How the last stage of the rocket having put the satellite up there at the right velocity, drops off and drops down to Earth is beyond me as, logically, it should also remain in geostationary orbit around Earth. There is no friction forces up there.

Repeat: There is no friction slowing down things in space (vacuum) of a geostationary satellite. Only forces due to gravity (down to Earth) and orbiting (away from Earth). And they are equal!

So how does the last rocket stage - empty of fuel - that brought the geostationary satellite up in position, drop down to Earth? :rolleyes:

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Re: Re-entry

Unread post by Farcevalue » Wed Oct 10, 2012 7:04 pm

I have become very curious about the spinning atmosphere, as of late. The earth is in rotation at speed x. The atmosphere is somehow connected to the earth, through pressure, friction or whatever and spins at the same speed, so for practical purposes we do not experience the airflow. But once an item enters the atmosphere from space, wouldn't it experience a violent rush of air flowing in the direction of the earth spinning? I picture a pneumatic tube violently pulling the item in the direction of the airflow. But a pneumatic tube has an enclosure keeping the air inside. What keeps the atmosphere inside? I feel like I am missing something, but what is the transition from vacuum to atmosphere that allows cohesion for the air to spin with the earth, but is relaxed enough for items to escape it?

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Re: Re-entry

Unread post by lux » Wed Oct 10, 2012 7:15 pm

Farcevalue wrote:... once an item enters the atmosphere from space, wouldn't it experience a violent rush of air flowing in the direction of the earth spinning?
I would think that would depend on the speed and direction of the object in relation to the atmosphere. If the object were moving at close to the same speed/direction then the transition should be relatively mild.

Also there isn't a sharp dividing line between atmosphere & no-atmosphere since the atmosphere gradually thins out to nothing with altitude.

But, a meteor, say, entering from space at thousands of mph relative to the atmosphere would cause a sudden, violent friction.

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Re: Re-entry

Unread post by Heiwa » Wed Oct 10, 2012 7:38 pm

Earth atmosphere is just molecules of oxygen and nitrogen (and some other stuff) stacked on top of each other, more close to ground causing a nice pressure there and much less say at 100 000 m altitude (no pressure at all). :)

Assume a meteorite is arriving into this atmosphere at 100 000 m altitude vertically (perpendicular) down at 10 000 m/s speed (relative Earth), it will evidently hit ground 10 seconds later or so … unless it burns up.

If it arrives at an angle, say 45°, it will take a little later, say 14-15 seconds. And hopefully burn up in mean time.

If you happen to stand just below a dropping meteorite, you will not notice being hit. It goes very fast. ^_^

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