Thinking about orbital mechanics makes my head hurt ...

[askold]

Though I have a degree in math.<br /><br />When the astronauts threw the object backwards from the ISS - what is the resultant path of the junk?<br /><br />What if they threw it straight "down"?<br /><br />Thanks

 

[jimfromnsf]

Down doesn't really do anything and the object's orbit still intersects with the ISS.<br /><br />When they threw it opposite of the velocity vector, it put that object in a lower orbit

 

[vogon13]

Early NASA astronauts drove cars on a circular track, at predetermined speeds for the radii they were on so they could develop 'real world' experience with the seemingly contradictory effects of orbital manuvers.<br /><br /><br />An otherwise forgettable movie back in the 70s had a concerned woman (innocent of orbital mechanics, however) attemptimg to dump a largish nuke from a space station simply by shoving it overboard. Her attempt led to some tense moments.<br /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[spacefire]

I got a feel for orbital mechanics by playing Orbiter, the free space simulator.<br />Just take the Delta Glider up and try to achieve a circular orbit. <div class="Discussion_UserSignature"> <p>http://asteroid-invasion.blogspot.com</p><p>http://www.solvengineer.com/asteroid-invasion.html </p><p> </p> </div>

 

[bdewoody]

I too have a degree in math but the physics of a lot of whats true in space eludes me such as. Does the ISS or shuttle naturally keep the same side facing earth like the way a plane flies in the atmosphere or will it show it's opposite side when it is 180 degrees from where it starts. <div class="Discussion_UserSignature"> <em><font size="2">Bob DeWoody</font></em> </div>

 

[thereiwas]

Back in the 1970's (or even earlier) one of the first video games for a computer was called SpaceWar. This was written by some students at MIT when the first graphical displays were being developed. I played it on a PDP-1.<br /><br />It was much simpler than Orbiter, being just two dimensional and running at much faster speeds to make the game playable. A complete orbit of the sun might take only 20 seconds but otherwise the physics was correct. In order to be any good at it you had to develop a "gut" feel for things like Hohmann transfer orbits.<br /><br />The only controls were 4 toggle switches for each player. 1 - rotate counterclockwise, 2 - rotate clockwise, 3 - engine on, 4 - fire gun. <br /><br />Initial conditions had the two player's spaceships on opposite sides of the sun, way out in the solar system, motionless. When the game started you had about 7 seconds to do <i>something</i> or you would fall into the sun (which lost you the round).

 

[jimfromnsf]

"Does the ISS or shuttle naturally keep the same side facing earth like the way a plane flies in the atmosphere or will it show it's opposite side when it is 180 degrees from where it starts."<br /><br />They can do either depending on mission requirements.<br /><br />like the Hubble telescope, the shuttle can point to an object in space, which means it is inertially fixed. This will mean that it looks like it is rotating wrt to earth as it orbits around it<br /><br />Or the ISS, it maintains a LHLV (local horizontal local vertical) attitude, which means the same side faces the earth. This means that the station is actually doing one "flip" per orbit wrt to inertial space. That is why the ISS solar arrays have rotating joints

 

[jimfromnsf]

"Back in the 1970's (or even earlier) one of the first video games for a computer was called SpaceWar. This was written by some students at MIT when the first graphical displays were being developed."<br /><br />I just played it a few years ago at a retro arcade. I would ignore the battle and just play with manuevering my ship, using my orbital mechanic knowledge. <br /><br />My kids berated me as an engineering nerd

 

[thereiwas]

Does ISS have to expend energy (thrusters or flywheel) to accomplish this "flip"? I would think it is too large for magnetic stabilization and too small to use tidal effects. Is the purpose of the flip to make heating more even?

 

[nyarlathotep]

<i>Is the purpose of the flip to make heating more even?</i><br /><br />Nah, the astronauts just like to destroy microgravity experiments so that they can look out the destiny nadir window.

 

[frodo1008]

Perhaps (or perhaps not) some just plain physical explanation would help:<br /><br />In the first place the shuttle is NOT weightless as the force of gravity towards the Earth is only marginally decreased at the altitude of the ISS. The shuttle/ISS system is in "continuous" free fall around the Earth. This is a condition that you would experience in an elevator if you were unfortunate enough to be in it is the cables all broke and all safety systems failed. You too would think that you were weightless (this is different than a parachute jumper's free fall as the jumper uses air resistance to slow down, and therefore is not in total free fall), at least until the elevator crashed into the Earth!<br /><br />If you were to stand on an imaginary mountain top some 200 miles high and fire a rifle horizontally, the bullet would have a certain velocity horizontally and slowly be dragged down towards the Earth by the Earth's gravity. The higher this horizontal muzzle velocity is the further out from your position the bullet would land. IF you could achieve a horizontal velocity of some 17,500 miles per hour your bullet would "continuously fall "around the curvature of the Earth itself. of course, if you stood perfectly still said bullet would eventually (in a little over 90 minutes) kill you by hitting you in the back of your skull! <br /><br />Now, consider the shuttle as your bullet, (the shuttle takes off vertically, but steadily turns to a horizontal position with the doors facing the Earth itself), all the time accelerating to the magicall velocity to "continuously fall" around the curvature of the Earth. So to the people on the shuttle they are "continuously" experiencing this free fall condition. <br /><br />Now, when the shuttle is in orbit, it is like you having a ball on a string, and you whirling the ball around your body as you turn. The string is an analogy to the gravity of the Earth for the shuttle/ISS system. And like your ball the string keeps the same fa

 

[jimfromnsf]

"Does ISS have to expend energy (thrusters or flywheel) to accomplish this "flip"? I would think it is too large for magnetic stabilization and too small to use tidal effects. Is the purpose of the flip to make heating more even?"<br /><br />To keep pointing at a target in space, CMG's keep it stable. <br /><br />It isn't a "flip" it is only from a earth reference. In inertial space, the ISS is maintaining the same attitude, which is good for microgravity experiments<br />

 

[bdewoody]

So if they do "nothing" does it keep the same side facing earth or not. There were 2 replies that both sounded technical but conflicted with each other. <div class="Discussion_UserSignature"> <em><font size="2">Bob DeWoody</font></em> </div>

 

[jimfromnsf]

no, both attitudes need control. otherwise it would tumble or assume a gravity gradient attitude point to who knows where

 

[thereiwas]

If it is stable in inertial space, then why do the alpha joints need to rotate 360 degrees every orbit?

 

[jimfromnsf]

it does many attitudes. LHLV needs the joints

 

[larper]

I think most of the time, the ISS uses Local Vertical Hold, not Inertial Hold.<br /> <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[vulture2]

The Mir generally flew facing the sun, to maximize power from the solar cells. This attitude is sometimes called "solar-inertial". <br /><br />When another spacecraft was docking, Mir was kept in an attitude with one side continuously facing "down" toward earth and one side continuously facing ahead along the orbital path. This attitude (which the ISS normally uses) is often called "LVLH", for "local vertical, local horizontal". <br /><br />I don't know the actual origin of these terms but they sound cool. <br /><br />Interestingly, when garbage is jettisoned "backward" along the orbital path, it then has a slightly slower orbital velocity. This causes it to descend into a slightly lower orbit. But a lower orbit has a shorter period, so the jetsam will soon move ahead of the station again, although it will be slightly lower. As long as it has a higher drag than the station (or the station is reboosted fairly soon) the flotsam and jetsam will descend into a lower orbit and not be back. <br /><br />

 

[larper]

The simplest mnemonic I know to figure out how things work in orbit is from Integral Trees by Larry Niven.<br /><br />East Out<br />Out West<br />West In<br />In East<br />Nort and South return<br /><br />Where:<br />East is your orbital velocity vector<br />Out means you raise your altitude<br />In Means you lower your altitude<br />West is retrograde<br />North and South are out of plane.<br /><br />So, if you thrust East, you move Out.<br />If you thrust Out, you move West<br /><br />etc.<br /><br />It might not be completely accurate, but its a good rule of thumb. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[frodo1008]

Good. as I fully acknowledge you to be the expert, I guess I stand corrected. However, in this slowing down to speed up thing, I am indeed confused. I had somewhat logically understood (and I know that logic does not always work in these areas) that orbital velocity was about 17,500 mph, and escape velocity was 25,000 mph, it would seem to me that between these two the higher your velocity the higher your orbit. 25,000 mph IS a higher velocity than 17,500 mph. <br /><br />But, perhaps orbital velocity is not teamed with orbital altitude? <br /><br />Also, IF your velocity drops below some 17,500 mph, you then fall out of orbit entirely, and (even if you don't burn up from TPS problems) then your velocity eventually becomes 0, as does your altirtude.<br /><br />Then there are those rockets that are taking satellites up to GEO. I know that they take an additional rocket boost once they get to LEO, I would think that additional boost should result in additional velocity. So they must need additional velocity to go from LEO to GEO.<br /><br />At least it seems that way.<br /><br />???

 

[jimfromnsf]

It is not speed that determines orbits, it is energy, kinetic and potential. <br /><br />It is not speeding up and slowing down, it is adding and subtracting energy<br /><br />17,500 is not THE orbital velocity. It is an orbital velocity<br /><br />With no air, an circular orbital velocity of 17550 mph would be at 61 mile altitude. <br /><br />17470 mph is the orbital velocity for 100 mile altitude. At 17380 mph, it would be at 145 miles. <br /><br />the velocity at GSO is 6867 mph

 

[frodo1008]

<font color="yellow"> No, this is not correct. If the Orbiter ( not the Shuttle which is the SRBs/TOrbiter combination) did not change it's payload bay facing attitude toward the earth The Orbiter would continue to point at a point in space and thus it would rotate 360 deg relative to the Earth during each orbit.</font><br /><br />I had thought that satellites at GEO not only were always above the same point on the Earth, but also remained with their antennas continuously pointed towards the earth. So it a satellite does then have to always be using either rocket thrust (or gyroscopes) to make adjustments to continue to point at the Earth?<br /><br />Then, if an object that is thrown backwards ( I do understand that an object thrown straight down from the ISS would just follow close to the ISS, and thus remain a danger to the ISS (at least until the tenuous but still available Earth atmosphere at that altitude steadily brought it down) and then speeds up, what happens to its orbit? <br /><br />I DO understand that it IS only the orbiter that achieves LEO. Sorry about my inexactness there!

 

[larper]

<blockquote><font class="small">In reply to:</font><hr /><p>So it a satellite does then have to always be using either rocket thrust (or gyroscopes) to make adjustments to continue to point at the Earth?<p><hr /></p></p></blockquote><br /><br />Yes and no. <br /><br />Ideally, once you give a satellite a very precise rotation rate, it will maintain that rate (angular momentun is conserved). If that rate JUST SO HAPPENS to have the same period as one revolution around the earth, the satellite will maintain one face to the earth. But, because the satellite does have a non-zero angular momentum, it is NOT inertial. Its internal frame of reference is changing in relation to a truly inertial frame. A truly inertial frame may be closely approximated by assuming the stars are fixed points in space.<br /><br />Now, I said Ideally. In reality, there are always forces acting against a body in motion. To compensate, the satellite needs to either 1) apply an external torque, or 2)exchange momentum internally. In the second instance, angular momentum is conserved, but it is redistributed within the satellite itself, usually with gyroscopes. However, since there are always external forces acting on the satellite, eventually, the gyros will need to be "dumped" using 1). The satellite applies a torque by using fuel to thrust, or by torquing against the Earth's magnetic field. Torgueing against the magfield requires only electricity, acquired from solar cells, so can be used indefinitely. Fuel is, of course, consumed and finite.<br /><br />Large, and more importantly, LONG satellites can use Gravity Gradient to dump the gyros as well. If the Shuttle Orbiter wants to use Gravity Gradient to help maintain attitude, it would do so with its tail, or nose, pointed to the Earth, for maximum effectiveness. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>

 

[frodo1008]

OK, then is the far greater escape velocity (such velocity being measured from the Earth's surface) being then used to increase orbital velocity so much that the object then escapes the pull of the Earth's gravity entirely.<br /><br />As the Earth's gravity does indeed decrease with altitude then it would seem that the farther out you are from the surface of the Earth the less difference you would need between your orbital velocity and escape velocity. So the further out you go BOTH orbital and escape velocities decrease. <br /><br />This seems logical, as the moon's mean orbital velocity is only 1.03 km/sec (or a bit more than 3,600 km/hour). I would then guess that at some millions of miles out into space the orbital velocity affectively becomes zero (and also at that point so does the escape velocity). Just out of my own curiosity, do you happen to know how far out that would be?<br /><br />This IS the kind thread and exchange that I personally think that these message boards was designed for!!!<br /><br />Of course, the mathematics of all this does get a little messy!<br /><br />I do know that a spacecraft must lose its velocity below orbital velocity in order to return from orbit to the Earth (that would then be the higher orbital velocity at the Earth's surface?). So, in losing its orbital velocity the space craft then loses its energy to a point that it can no longer stay in orbit.

 

[larper]

<blockquote><font class="small">In reply to:</font><hr /><p>Just out of my own curiosity, do you happen to know how far out that would be? <p><hr /></p></p></blockquote><br />Escape Velocity is, by definition, the velocity change you need to apply (instantaneously) to reach a point in space which is infinitely far away from the central mass(re, planet), but at which point you are no longer moving away from the central mass.<br /><br />So, you would have to be infinitely far from the earth to achieve zero velocity with no risk of falling back to earth.<br /><br />Another way to think of escape velocity is this. It is the velocity, parallel to the surface of the earth, that results in a parabolic orbit. Below escape velocity, you have an elliptical orbit. Above, you have a hyperbolic orbit. In reality, a parabolic orbit is a hypothetical ideal, and can never be achieved. <div class="Discussion_UserSignature"> <p><strong><font color="#ff0000">Vote </font><font color="#3366ff">Libertarian</font></strong></p> </div>