Speaking of Hunchbacked...

[Noldi400]

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

[ka9q]

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.

[Noldi400]

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?

[smartcooky]

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. 

[Glom]

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.

[Noldi400]

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

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.

[ka9q]

[
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.

[smartcooky]

[
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.

[qt]

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?

[nomuse]

I know the story and author, but are we playing guessing games for it instead of naming it outright?

[smartcooky]

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.

[smartcooky]

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!

[Al Johnston]

Sounds like Larry Niven's Neutron Star

[smartcooky]

Sounds like Larry Niven's Neutron Star

Thats it! Now the memories come back; Crashlanders, Puppeteers, Known Space, General Products, We Made It! etc

[qt]

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.