Conference 7.286::space

    Gyro stabalization is not new, nor is it used only for small sats. 
    Skylab's primary orientation system was through moment wheel gyros.
    As far as cooling, passive systems are primarily used with heatsinks
    and hard material radiators.  Keep in mind that your sat will be making
    both daylight and night passes during its orbit.  

>>1. How to stabilize orientation in space ?

Gyro have been used, in the case of the MIR station
it is a lot cheaper (power requirement) plus a lot
more stable to use gyro. But they do need power.

If you only want the satellite to point to earth,
then you need one that makes turn over once per
each revolution.  Several Amateur radio satellites
have done this by what is called Gravity stabilization.
    
Think of it as a top, however to get to top to work you
must spin the satellite on its y-axis and have a difference
in mass from top to bottom.  One cleaver way to do this
used on  UoSAT was to put coils on each of the 4 sides
of the satellite.  Think of it as an permanent magnet motor
where the earth is the magnet, thus using a computer controller
one can spin up the satellite to a stable gravity stabilized 
position.  Once in the position it requires very little power.
Another trick for a no power solution is to use a bar magnet
in the center of the satellite but this will cause two
revolutions of the satellite as it passes over the earths
magnetic polls.  I guess if your cleaver you could rotate the
magnet over the equators and keep it always pointed towards
the earth, but in both of these cases the best way to keep
the satellite pointed towards the earth is by rotating it like
a top around its y-axis where one side of the satellite has
more mass then the other.
    
    
    You can also use 'water' for the jets to control the
    satellite postions.  What you do is have a tank of
    water per-heated via the sun.  You take some of that
    watter and quickly heat it using a heating coil (power
    via solar batteries) and open up a pulse steam jet 
    burst.  With with about a gallon of water you can take
    a small sattelite put in very low earth orbit and raise
    it to a useful lifetime oribt.  Why go to this trouble?
    To get around safty regulations on launching a satellite
    form the shuttle.  The only problem will be you can not
    launch it when they are doing optical experiments. It
    is a pulse steam engine.  
    
    
    cheers john
    


    

Re:.0

>1. How to stabilize orientation in space ?
>2. How to let the satellite face to earth ?

	John Biro has the right idea to solve both of these problems: Use
gravity gradient stabilization. If your minimum technology satellite had a 
long, light weight boom on it, the slight difference in the pull of gravity
on the two ends of the satellite will eventually cause the satellite to
orient itself so that its long axis always points towards the empty focus of
its elliptical orbit. In the case of a nearly circular Earth orbit that means 
that one end of the satellite always points straight down towards the Earth.
	To get your satellite oriented in the right direction to begin with
or adjust for minor torques that may arise, again John has the right idea:
Use electromagnetic torquers. The magnetic field they produce can be used
to "push" against the earth's magnetic field and cause the satellite to turn.
A simple arrangement of photodiodes could be used as a rudimentary sensor
to make sure the right end of your satellite is pointed at the Earth.

>3. How to stabilize temperature ?
>   Covering the satellite with aluminium will reflect most sunlight.
>   Devices aboard will produce heat. How to get rid of that ?

	This is a REAL tough and complicated problem that even aerospace
engineers have a tough time with (example: Magellan's problem with 
overheating). Aluminum foil won't be as good at keeping your spacecraft
as cool as you think. True, it's a great reflector of visible light but
it is also a great reflector of infrared. "Terrific" you might think but
it isn't. A good reflector of infrared is a very poor radiator in infrared.
The 1% of sunlight that a piece of aluminum will absorb will cause it
to heat up quite a bit because it can't radiate the heat well enough
in the infrared. Ever touch a shiny piece of metal left out in the sun
on a summer's day? It's hot as hell!
	You'd be better off using gold foil or gold coated mylar. If you
want to go cheap in addition to minimum technology, coat the exterior metal
parts with titanium white paint. Titanium white refelects well in the visible
and radiates well in the infrared (this is the type of paint used on modern
observatory domes to keep the insides cool).
	Unless you want to do some pretty hefty thermal calculations and/or
perform tests in a thermal vaccuum chamber, here are some suggestions:
As a good rule of thumb, I'd make sure that the electronics and other
components can withstand the largest possible range of temperatures. Keep
in mind that virtually every watt of electrical that is generated will 
eventually end up as waste heat. If thermal cycling will be a problem, add
some additional thermal inertia to your system like a small enclosed container
of water. If you have a particular piece of equipment that will produce a
lot of heat (like a TV vidicon tube) or needs to be kept cold (like a CCD
imager), use a heat pipe to thermally connect the item to a radiator located
on the anti-Earth side of the satellite. If the radiator is large enough, I 
can guarantee that it will remain plenty cold.

	I hope all this helps :-)

				Drew

Now all you need is a low tech means of launching...

Burns

    You might want to obtain a copy of the ARRL's Satellite Experimenter's
    Handbook. It has a lot of info the design of some of the amateur
    satellites.
    
    gary

    Bring back The low tech launch system,
     
    The GAS can.... but I do not think NASA will
    bring it back, but if they do -  base on the old
    rates, about $40K.  They actually did launch
    a satellite from one of the GAS Cans,  I think
    it was design by the Universary of Utah. 
    
    The gas can has a big spring, and on command
    pushes out the satellite in to LEO.  The orbit
    is very low and short lived, thus why the idea
    of a steam engine came about.  If you have not
    seen one of  these GAS CAN it is about the size 
    of your typical garbage can and the spring takes 
    up about 1/3 of the space.   
    
    ORBIT BOOKS or publishing company will latter this
    year or early next year be publishing a book
    on control of earth satellite, I will post when
    the the book is in print.
    
    cheers john