ApolloHoax.net
Apollo Discussions => The Hoax Theory => Topic started by: Noldi400 on July 03, 2013, 10:30:42 PM
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Just so everyone knows, hunchbacked just posted a video entitled "The Strange Conceptions of Apollohoax.net"
It seems that he's still arguing the case he posted here three years ago (as Inquisitivemind) about the attitude of satellites in orbit.
Link: www.youtube.com/watch?v=QOs_Fsp1bpM (http://www.youtube.com/watch?v=QOs_Fsp1bpM)
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Amazingly enough, I've done it again -- I prompted Hunchbacked to pull down his latest video.
It seems he didn't even consider the effects of varying gravity with altitude on the attitude of a spacecraft. That's rather odd since it's the only thing with an influence on a satellite's attitude.
And I only had to cite the moon itself as evidence.
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Amazingly enough, I've done it again -- I prompted Hunchbacked to pull down his latest video.
It seems he didn't even consider the effects of varying gravity with altitude on the attitude of a spacecraft. That's rather odd since it's the only thing with an influence on a satellite's attitude.
And I only had to cite the moon itself as evidence.
Quick work. I knew I should have D/L'd it.
Varying gravity affects attitude? I don't get it, but at least I'll admit it.
I know gravity varies with altitude, of course; Newton's law, inverse square, got it. But doesn't the satellite's axis of lowest MOI align with the primary's center of mass, regardless of altitude?
I traced his changing-satellite-CoG notion (and diagram) to a student research paper done by a Jacob Bean at Va Tech - truthfully, the paper is over my head, but it seems to be concerned with controlling a satellite's rotation by internally shifting the CoG. It seems to be in the undergrad-student-research-project stage at the moment, not a commonly used method as HB tries to put it across. Bean states, in fact, "we are the first group to obtain experimental data, to the authors' knowledge, involving moving masses as a primary means of satellite attitude control." Experiments have been done on a NASA research plane and a spacecraft simulator, but not actually on an orbiting satellite as yet, AFAIK.
Have you heard anything about this, as an actual working control method?
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Bean states, in fact, "we are the first group to obtain experimental data, to the authors' knowledge, involving moving masses as a primary means of satellite attitude control." Experiments have been done on a NASA research plane and a spacecraft simulator, but not actually on an orbiting satellite as yet, AFAIK.
Have you heard anything about this, as an actual working control method?
Doesn't Hubble use rotating masses like big flywheels for moving the spacecraft. I know it has gyroscopes for stabilisation, but I understood it also has reaction flywheels which are used to change its attitude for targeting stars and other objects in space.
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Bean states, in fact, "we are the first group to obtain experimental data, to the authors' knowledge, involving moving masses as a primary means of satellite attitude control." Experiments have been done on a NASA research plane and a spacecraft simulator, but not actually on an orbiting satellite as yet, AFAIK.
Have you heard anything about this, as an actual working control method?
Doesn't Hubble use rotating masses like big flywheels for moving the spacecraft. I know it has gyroscopes for stabilisation, but I understood it also has reaction flywheels which are used to change its attitude for targeting stars and other objects in space.
Yes. The gyroscopes are an instrument. The reaction wheel assemblies are then used to maintain the desired attitude.
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The ISS also uses Control Moment Gyroscopes for attitude control, like reaction wheels mounted in gimbals.
This is something different - they're not talking about reaction wheels (or momentum wheels) for turning the satellite. Here's a link to the paper:
www.vsgc.odu.edu/awardees/20122013/abstracts/Papers%20-%20Undergrad/Bean,%20Jacob%20-%20paper.pdf (http://www.vsgc.odu.edu/awardees/20122013/abstracts/Papers%20-%20Undergrad/Bean,%20Jacob%20-%20paper.pdf)
My impression is that he's talking about something more like shifting the CoG of a satellite in order to change the speed of rotation, or for steering during reentry, similar to the Apollo CM.
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Varying gravity affects attitude? I don't get it, but at least I'll admit it.
I know gravity varies with altitude, of course; Newton's law, inverse square, got it. But doesn't the satellite's axis of lowest MOI align with the primary's center of mass, regardless of altitude?
Yes, hence the term gravity gradient. A gradient is just a rate of change in a vector quantity, like the force of gravity. Ignoring mass concentrations and centrifugal force the gravity vector always points toward the center of a planet but the magnitude of that vector varies with the inverse square law. Close to the planet there's a greater change in gravitational acceleration (and force per unit mass) over a 1 meter difference in altitude than there is farther away, and it's that gradient in gravity (not its actual amount) that causes the torque we variously call the gravity gradient or tidal torque.
Only the center of mass of the spacecraft is in true free fall; points closer or farther away from the planet are forced to follow the same trajectory by being connected to each other. So they experience a slight acceleration that's the reason NASA uses the term "microgravity" rather than "zero gravity". The direction of those accelerations are such that they tend to orient the spacecraft along the local vertical. In practice the torques are small and difficult to use for stability because there's no friction to damp out oscillations.
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Varying gravity affects attitude? I don't get it, but at least I'll admit it.
I know gravity varies with altitude, of course; Newton's law, inverse square, got it. But doesn't the satellite's axis of lowest MOI align with the primary's center of mass, regardless of altitude?
Yes, hence the term gravity gradient. A gradient is just a rate of change in a vector quantity, like the force of gravity. Ignoring mass concentrations and centrifugal force the gravity vector always points toward the center of a planet but the magnitude of that vector varies with the inverse square law. Close to the planet there's a greater change in gravitational acceleration (and force per unit mass) over a 1 meter difference in altitude than there is farther away, and it's that gradient in gravity (not its actual amount) that causes the torque we variously call the gravity gradient or tidal torque.
Only the center of mass of the spacecraft is in true free fall; points closer or farther away from the planet are forced to follow the same trajectory by being connected to each other. So they experience a slight acceleration that's the reason NASA uses the term "microgravity" rather than "zero gravity". The direction of those accelerations are such that they tend to orient the spacecraft along the local vertical. In practice the torques are small and difficult to use for stability because there's no friction to damp out oscillations.
So, to put it simply they are proposing to use tidal forces as a means of manoeuvring satellites?
It reminds me of a sci-fi short story I read once where tidal forces killed the occupants of a spacecraft orbiting a white dwarf (or something) leaving the (incredibly strong) hull untouched. Can't remember the name of the author or the story.
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It reminds me of a sci-fi short story I read once where tidal forces killed the occupants of a spacecraft orbiting a white dwarf (or something) leaving the (incredibly strong) hull untouched. Can't remember the name of the author or the story.
Let me guess. The craft makes a close pass in an extremely elliptical orbit, so that another passing craft finds this one, drifting through space far from any star, in perfect condition, with the occupants inside crushed.
Am I close?
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I know the story and author, but are we playing guessing games for it instead of naming it outright?
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It reminds me of a sci-fi short story I read once where tidal forces killed the occupants of a spacecraft orbiting a white dwarf (or something) leaving the (incredibly strong) hull untouched. Can't remember the name of the author or the story.
Let me guess. The craft makes a close pass in an extremely elliptical orbit, so that another passing craft finds this one, drifting through space far from any star, in perfect condition, with the occupants inside crushed.
Am I close?
Yes, that sounds familiar. Seems like you have read the same short story.
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I know the story and author, but are we playing guessing games for it instead of naming it outright?
Go on then, put me out of my misery!
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Sounds like Larry Niven's Neutron Star
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Sounds like Larry Niven's Neutron Star
Thats it! Now the memories come back; Crashlanders, Puppeteers, Known Space, General Products, We Made It! etc
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Yes, that sounds familiar. Seems like you have read the same short story.
Never heard of this story before, just speculating about what the plot might have been. So I was in the right area then, I guess.
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I love me some Niven!
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So, to put it simply they are proposing to use tidal forces as a means of manoeuvring satellites?
No, this is something different. They propose to use moving weights within the body of the satellite to shift the CoG and thereby affect the angular velocity of its rotation in a given axis.
As I've already stated, the details are over my head. It doesn't sound very practical, but that may be just my layman's (unreliable) intuition talking.
The link is in my post above for the Physics minded.
And yes, I 2 love me some Niven.
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So, to put it simply they are proposing to use tidal forces as a means of manoeuvring satellites?
It can't be used for maneuvering; only attitude control.
The Transit navigation satellites were GG stabilized. After launch they deployed a long boom with a large weight on the end; even then it took several weeks for the satellite to get torqued into the desired attitude and rotation rate. I pointed this fact out to Hunchbacked but he ignored it, of course.
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Another well-known satellite that used gravity gradient stabilization was LDEF, the Long Duration Exposure Facility.
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So this idea of using a spinning reaction mass to orient satellites (à la mode de Hubble) is how old?
Does anyone know when this idea was first thought of and first put into use?
There is a reason why I'm asking, which I wish to keep to myself for the moment.
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Been around for decades. A lot of early communication satellites were spinners, so the whole spacecraft was a gyroscope; reorienting it involved precessing it much like steering a helicopter involves precessing the rotors.
All the modern ones are 3-axis stabilized, mainly to use their solar arrays more efficiently. They'll use a combination of reaction wheels/control moment gyros, thrusters and magnetic torquing coils (for the low altitude satellites). Some have even used vanes to exploit solar radiation pressure.
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It wouldn't surprise me if Clarke included it in his original proposal for geostationary satellites
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So this idea of using a spinning reaction mass to orient satellites (à la mode de Hubble) is how old?
Does anyone know when this idea was first thought of and first put into use?
There is a reason why I'm asking, which I wish to keep to myself for the moment.
As a concept, it's been around damn near forever. I can remember reading about it in science fiction written in the early 50s.
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It wouldn't surprise me if Clarke included it in his original proposal for geostationary satellites
As a concept, it's been around damn near forever. I can remember reading about it in science fiction written in the early 50s.
Yep, I thought so. The reason I ask is that I came across a reference to it in an old sci-fi story (by Clarke as it happens) called "Hide-and-Seek", about the efforts of the captain (Commander Smith) of a very large, very long spaceship (Doradus) , trying to catch a spacesuited individual (Agent K.15) running around on Phobos.
"For K. 15's plan was a simple one: he must remain as close to the surface of Phobos as possible-and diametrically opposite the cruiser. The Doradus could then fire all her armament against the twenty kilometers of rock, and he wouldn't even feel the concussion. There were only two serious dangers, and one of these did not worry him greatly.
To the layman, knowing nothing of the finer details of astronautics, the plan would have seemed quite suicidal. The Doradus was armed with the latest in ultra-scientific weapons: moreover, the twenty kilometers which separated her from her prey represented less than a second's flight at maximum speed. But Commander Smith knew better, and was already feeling rather unhappy. He realized, only too well, that of all the machines of transport man has ever invented, a cruiser of space is far and away the least manoeuvrable. It was a simple fact that K. 15 could make half a dozen circuits of his little world while her commander was persuading the Doradus to make even one.
There is no need to go into technical details, but those who are still unconvinced might like to consider these elementary facts. A rocket-driven spaceship can, obviously, only accelerate along its major axis-that is, "forward." Any deviation from a straight course demands a physical turning of the ship, so that the motors can blast in another direction. Everyone knows that this is done by internal gyros or tangential steering jets, but very few people know just how long this simple maneuver takes. The average cruiser, fully fueled, has a mass of two or three thousand tons, which does not make for rapid footwork. But things are even worse than this, for it isn't the mass, but the moment of inertia that matters here-and since a cruiser is a long, thin object, its moment of inertia is slightly colossal. The sad fact remains (though it is seldom mentioned by astronautical engineers) that it takes a good ten minutes to rotate a spaceship through 180 degrees, with gyros of any reasonable size. Control jets aren't much quicker, and in any case their use is restricted because the rotation they produce is permanent and they are liable to leave the ship spinning like a slow-motion pinwheel, to the annoyance of all inside."
He's calling them "gyros" but its reasonable to assume that he is using the term rather than "reaction wheel"
This short story was first published in 1949, eight years before Sputnik 1.
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I just re-read that story about two weeks ago and forgot about that! One of my favorite Clarke shorts (love the term "tree rats").
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I just re-read that story about two weeks ago and forgot about that! One of my favorite Clarke shorts (love the term "tree rats").
Hide & Seek
Loophole
Superiority
The Sentinel (which of course formed the basis for the monolith in 2001)
My favourite Clarke shorts
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That's an intriguing story I haven't heard of.
But what about Phobos' low gravity? Its escape velocity is only 11.3 m/s, which means its surface-skimming orbital velocity (assuming a uniform sphere, which it isn't) is 11.3/sqrt(2) = 8 m/s. Were Agent K.15 to reach this velocity along the surface he would literally put himself into orbit, losing all foot traction. Not that it's all that great even when stationary (surface gravity ranges between 2 and 8 mm/s^2 -- that's millimeters per second squared or millinewtons per kg -- so he's not likely to get around very quickly with his feet in any event. He'd have to use some sort of thruster pack with fuel that would be consumed. And he'd have trouble staying close to the surface that's his best protection.
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The obvious solution is to keep his feet off the ground except when stopped, and pull himself along by his hands. On Phobos, of all places, this is made easier by the grooves and striations on the surface (http://mars.jpl.nasa.gov/images/mep/allaboutmars/p1.gif).
Actually, the ultra-low gravity solves another problem I had with this story ever since I saw the LROC images of the Apollo sites: Footprints. Both Heinlein's "The Black Pits of Luna" and Bova's "15 Miles" (to name two) concern finding a lost hiker on the Moon. We now know that it would be a embarrassingly easy task. Since "Black Pits" is set at a time when lunar tourism is common, we could change the story so that the searchers have too many footprint trails to follow (since there's no natural mechanism to cover them in less than a few centuries.
Back to "Hide & Seek", the spy would greatly reduce his disturbed-soil signature by pulling & skimming with his hands; and also he would be in no danger of approaching orbital velocity.
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Control jets aren't much quicker, and in any case their use is restricted because the rotation they produce is permanent and they are liable to leave the ship spinning like a slow-motion pinwheel, to the annoyance of all inside."
This one sentence has long bugged me. What's so hard about firing the "control jets" a second time to counteract the rotation caused by the first firing?
Having said that, I understand the slowness with which the spacecraft would orbit such a small asteroid, but if the captain is willing to use his main engine to change orbits, placing the spacecraft where he wants it shouldn't be quite as hard as Clarke makes it.
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As I've already stated, the details are over my head. It doesn't sound very practical, but that may be just my layman's (unreliable) intuition talking.
It's not currently practical. But I can remember a time when reaction wheels weren't practical either. In addition to the aforementioned problems with oscillation and the miniscule magnitude of the moments, there is the fact that satellite design is already heinously complicated by mass-properties constraints. This would seem to add a lot more.
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That's an intriguing story I haven't heard of.
But what about Phobos' low gravity? Its escape velocity is only 11.3 m/s, which means its surface-skimming orbital velocity (assuming a uniform sphere, which it isn't) is 11.3/sqrt(2) = 8 m/s. Were Agent K.15 to reach this velocity along the surface he would literally put himself into orbit, losing all foot traction. Not that it's all that great even when stationary (surface gravity ranges between 2 and 8 mm/s^2 -- that's millimeters per second squared or millinewtons per kg -- so he's not likely to get around very quickly with his feet in any event. He'd have to use some sort of thruster pack with fuel that would be consumed. And he'd have trouble staying close to the surface that's his best protection.
Clarke did address that in the story.
He stood in the center of an irregular plain about two kilometers across, surrounded by low hills over which he could leap rather easily if he wished. There was a story he remembered reading long ago about a man who had accidentally jumped off Phobos: that wasn't quite possible-though it was on Deimos-as the escape velocity was still about ten meters a second. But unless he was careful, he might easily find himself at such a height that it would take hours to fall back to the surface-and that would be fatal.
At the same moment that resourceful individual was taking stock of the situation, which might very well have been worse. He had reached the hills in three jumps and felt less naked than he had out in the open plain.
He was particularly careful not to overtake her on one of his kilometer-long glides,
That last bit takes care of the "footprints" issue raised by Count Zero. Tracking someone by following footprints a kilometre apart in search area of over 1000 sq km would be no trivial task
You can read the whole story here
http://bookre.org/reader?file=210433&pg=1
Its only two pages long.
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The obvious solution is to keep his feet off the ground except when stopped, and pull himself along by his hands. On Phobos, of all places, this is made easier by the grooves and striations on the surface (http://mars.jpl.nasa.gov/images/mep/allaboutmars/p1.gif).
It doesn't seem likely that there would be natural handholds on Phobos. They'd have to handle a lot of force for our fugitive agent to maneuver quickly across the surface. But since we haven't yet been to Phobos, there's no way to know.
Edited to add: I suppose one way to move across the surface is to push yourself horizontally using boulders big enough for their inertia to keep them more or less stationary even if they're just resting on the surface. Stand on the side of a big boulder (or lay on the ground with your feet against it), slowly crouch down (walking your hands against the surface) and push off horizontally towards another big boulder. Chances are you'll miss it and will have to wait to fall back to the surface so you can slowly make your way to another big boulder and repeat the process.
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All this talk of moving across low-gravity moons reminds me of the Golf Game you can play on Saturn's moons: http://www.ciclops.org/sector6/golf.php?js=1
(I'm not very good.)
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Neat, but it would be nice if the ball was easier to see. It can be really hard to pick out a lot of the time.
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Control jets aren't much quicker, and in any case their use is restricted because the rotation they produce is permanent and they are liable to leave the ship spinning like a slow-motion pinwheel, to the annoyance of all inside."
This one sentence has long bugged me. What's so hard about firing the "control jets" a second time to counteract the rotation caused by the first firing?
Having said that, I understand the slowness with which the spacecraft would orbit such a small asteroid, but if the captain is willing to use his main engine to change orbits, placing the spacecraft where he wants it shouldn't be quite as hard as Clarke makes it.
Precisely, furthermore any rotation initiated by a change in gyro speed will also be permanent, until a countering change is made in the opposite direction. Same problem (or lack thereof) with either method. But it's easy for me to say that after it's been done, whereas Sir Clarke was years ahead of reality :)