Speaking of Hunchbacked...

[AtomicDog]

I love me some Niven!

[Noldi400]

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.

[Chew]

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.

[ka9q]

Another well-known satellite that used gravity gradient stabilization was LDEF, the Long Duration Exposure Facility.

[smartcooky]

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.

[ka9q]

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.

[Al Johnston]

It wouldn't surprise me if Clarke included it in his original proposal for geostationary satellites

[Noldi400]

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.

[smartcooky]

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.
 

[Sus_pilot]

 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").

[smartcooky]

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

[ka9q]

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.

[Count Zero]

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.

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.

[Peter B]

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.

[JayUtah]

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.