[Coco] Re: off-topic, space program

[Gene Heskett]

On Sunday 18 January 2004 08:15, Kenneth Schunk wrote:
>Well, as I understand it the gyro's are used in the Hubble (and on
> the ISS) to MAINTAIN position, not to DETERMINE position. The
> inertia of the moving parts in the gyroscope is the main purpose,
> so removing the moving parts wouldn't be the way to go.
>
>Ken
>
I think Ken is correct, but let me see if I can explain why without 
screwing it up too bad.

>> Certainly the Hubbel won't live till then, due to gyro failures,
>> although I'm still rather puzzled that the gyro's haven't long
>> since been replaced with the laser interferometers that are
>> probably 100x more accurate and no moving parts to wear out, only
>> the laser diodes, or whatever is used to "pump" the real laser. 
>> Maybe they take more power, and thats not in the power budget?

Maintaining position is the thing that the laser interferometers are 
best at, they are capable of detecting very small fractions of a 
degree of motion in reference to their operating wavelength.  
Sub-angstrom accuracy can be achieved on the kitchen table.
They work by using a glass fiber pair, essentially two coils counter 
wound from each other, split a single frequency laser beam in 2, feed 
it through both, and bring them both back to a common phase detector.  
As motion in the plane of the coils is detected, the two beams will 
change their relative phase at the detector.  For maintaining 
something in a motionless state, they excel.  But that does bring up 
the question of what you use for reaction mass since they don't exert 
a precess force like mechanical gyros do.  At least not one thats 
measurable in practice.

If you don't have the force from the precession of the gyros as a 
driver then you'll have to fall back to small jets using expendables 
as reaction mass, which of course there is a limited supply onboard.  

The gyros on the other hand, will if mechanically turned to change the 
axis, cause a precession motion for the whole hubble as its not 
exactly bolted to anything.  These effects can be used to steer it as 
long as the gyros run.

That may have driven the choice to use mechanical gyros as opposed to 
interferometers, all the energy the gyros need is from the solar 
panels.  Constantly refreshed in other words.

Gee, I hope I haven't screwed that up too bad...

-- 
Cheers, Gene
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