Conference 7.286::space

  A condensed version of this appeared in the Worldwide News section of Live-
wire yesterday. The only item not mentioned above is that Franco is a Tele-
communications consultant at DEC.
  As far as I know, he is the first DECcie to be selected as an astronaut.
At least one former astronaut, Eugene Cernan, is a DECcie. 
  This is the first I heard of the Tethered Satellite program. I would like
to know more. What can a tethered satellite do that a regular satellite can't?
Will the satellite fly at a much lower altitude, at which a satellite would
quickly fall out of orbit if it wasn't tethered? How do we know that a 100-
km (62-mile) cable will unroll properly in space?

    Re: .1
    	Tethered satellites have a lot of potential. As you mentioned,
    a tethered satellite can be lowered to an altitude where atmospheric
    drag would make a free orbiting satellite spiral in VERY quickly.
    	Other potential uses for tethers in space include producing
    electricity (by means of a conductive tether passing through the
    Earth's magnetic field lines), as a means of propulsion (by passing
    a current through the tether and using it to push against the Earth's
    magnetic field), and producing artificial gravity (since the satellite
    closer to Earth is traveling at a slightly sub-orbital speed and
    the higher one is traveling at slightly more than orbital speed
    therefore neither satellite would be in a free fall anymore).
    	By far the most promising use of space tethers is for momentum
    transfer. A tethered satellite pair will travel at the orbital speed
    of its center of gravity. The lower satellite will be traveling
    too slow for its altitude (since the lower the orbit the higher
    the orbital speed) and the higher satellite would be traveling too
    fast for its altitude (since the higher the satellite the slower
    its orbital speed). If you had a tethered pair of satellites of
    equal mass on the ends of a 50 mile long tether originally in
    a 225 mile high circular orbit and then the tether was cut, the
    higher satellite would move into a 225 X 400 mile orbit and the
    lower satellite would move into a 50 X 225 mile orbit (and probably
    fall out of orbit on its first perigee). This method could be used
    to boost a space station's orbit or even throw a payload into an
    escape trajectory WITHOUT THE USE OF A ROCKET!
    	Of course there is still the practical matter of seeing how
    a tether works in space, hence this Italian/American experiment.
    Hopefully it can work.
    
    				Drew
    

	Re: 547.2
	HAZEL::LEPAGE

    >   Tethered satellites have a lot of potential. As you mentioned,
    >a tethered satellite can be lowered to an altitude where atmospheric
    >drag would make a free orbiting satellite spiral in VERY quickly.
    
	I would think that the drag created by the lower altitude satellite
	would be transmitted to the higher altitude satellite thereby 
	slowing it down.

    >its orbital speed). If you had a tethered pair of satellites of
    >equal mass on the ends of a 50 mile long tether originally in
    >a 225 mile high circular orbit and then the tether was cut, the
    >higher satellite would move into a 225 X 400 mile orbit and the
    >lower satellite would move into a 50 X 225 mile orbit (and probably
    >fall out of orbit on its first perigee). This method could be used
    >to boost a space station's orbit or even throw a payload into an
    >escape trajectory WITHOUT THE USE OF A ROCKET!
       
	I would think that to get the satellites to the initial condition
	(before cutting the tether), you would have to expend the same 
	amount of energy as boosting the satellite into the new orbit in 
	the first place.

	In order to transfer momentum, you have to get it, first.
    
    	Gregg

    Re: .3
    	It is true that the drag on a tethered satellite in the upper
    atmosphere would also tend to drag down the spacecraft on the upper
    end of the tether. But the tethered satellite would not drop out
    of orbit as quickly than if it were a free orbiting satellite since
    the drag forces would be distributed over TWO satellites.
    	Take the example of the Space Shuttle's tethered satellite
    experiment. The drag on the satellite is related to its frontal
    area while the rate it loses speed is related to the drag force
    divided by the mass of the satellite (i.e. a=F/m). If the mass of
    the satellite were to increase 100 fold (which is essentially what
    happens when you connect the satellite to the shuttle) while the
    frontal area stays constant, the acceleration (and hence the rate
    the satellite falls out of orbit) would decrease 100 fold.
    	In the case of the two tethered satellites moving into different
    orbits after the tether is cut, it may seem anti-intuitive but a
    detailed examination of the physics would show that the total energy
    of the two satellites before and after the tether was cut is the
    same. What is changing is the relative amounts of kinetic and POTENTIAL
    energy in the system. No energy is being added to the system. In
    addition, the momentum being transfered is not linear momentum but
    angular momentum. I really do not have the time right now to go through
    all the details (it has been several years since I read about the
    physics of this system and I forget alot of them anyway) but I will
    try to locate some of the references and post them for those who
    are interested (and if I can find some simple explanation for this
    type of system, I will post it when I can find some more time).
    
    				Drew
    

	Note 547.4
	HAZEL::LEPAGE

    >    In the case of the two tethered satellites moving into different
    >orbits after the tether is cut, it may seem anti-intuitive but a
    >detailed examination of the physics would show that the total energy
    >of the two satellites before and after the tether was cut is the
    >same. What is changing is the relative amounts of kinetic and POTENTIAL
    >energy in the system. No energy is being added to the system. In
  
	I agree that no energy is being added to the system, once it
    has reached equilibrium. But it seems to me that in order to get the 
    two tethered sats in the *inital* 	position, you would have to add 
    the same amount of energy as you would to boost the sat to its higher orbit.


	Gregg

    Re: .5
    	No energy is being added to the system as the satellites are
    reeling out the tether. What is happening is the potential energy
    of the system is being redistributed. Look at the system in this
    way (the following is a first order approximation but in detail
    it works the same):
    	Initially two identical satellites are in contact orbiting at 225
    miles and with a velocity appropriate for that altitude. The satellite
    pair's energy is the sum of the kinetic energy of the pair and of
    the graviational potential energy at that height. As the pair move
    apart connected by the tether, the kinetic energy stays the same
    since both satellites are still traveling at the velocity of a satellite
    in a 225 mile high orbit. The distribution of potential energy is
    changing however. As the upper satellite moves higher, its potential
    energy increases (since the potential energy is equal to the the
    product of the satellite mass, gravitational acceleration, and the
    height). At the same time, the lower satellite moves closer to the
    surface thus losing potential energy. The gain in potential energy
    of the upper satellite exactly equals the loss of potential energy
    of the lower satellite.
    	At the moment the tether is cut, both satellites have the same
    kientic energy BUT the upper satellite has more potential energy
    than the lower satellite. Since the upper satellite has more total
    energy (i.e. kinetic plus potential energy) it moves into a higher
    orbit. The lower satellite with less total energy moves into a lower
    ranging orbit. No energy is gained or lost in the system; the total
    energy of the system remains constant.
    	One may ask about the forces that pull the satellite apart as
    the tether is reeled out. One way of looking at an orbiting body is
    to consider that the inward gravitational force balances the outward
    centrifugal force. As the satellites pull apart, they are traveling
    at identical velocities. As a result, the lower satellite feels
    a downward pull because it is traveling too slowly for that altitude
    (i.e. the gravitational force is greater than the centrifugal force).
    The upper satellite is traveling too quickly for its altitude and
    feels an upward force (i.e. the centrifugal force is greater than
    the gravitaional force). The forces on the pair of satellites are
    in opposite directions and cancel out perfectly. Since there is
    no net force, there is no work being performed, therfore there is
    no energy being added or taken away from the system.
    	In reality, the physics are a little more complicated since
    the acceleration of gravity changes with altitude and the satellite
    pair are actually rotating (with respect to the stars) as the circle
    the Earth. As a result, there is some transfer of kinetic energy
    in the system and the change in potential energy is smaller. This
    and other complications still do not alter the basic conclusion
    of the above analysis. As I said before, I will try to locate some
    references and possibly some other examples of this technique when
    I get a chance.
    				Drew
    

    	I went back to my original reply (i.e. 547.2) and found a minor
    typo/error. In the example I gave, the upper and lower satellites
    should move into 50 X 200 and 250 X 400 mile orbits respectively. Sorry
    for the slight mistake.
    
    				Drew
    

    Here's a use for a moon base (undoubtedly thought of and discarded
    before, but what the heck).
    
    Build yourself a mass driver, one each.  Put a 10 ton mass on it and
    squirt it off so that it will zip down through a 200 mile orbital pass
    on the Earth.  Set things up so that a spacecraft could hitch a ride,
    tethered temporarily to this mass to accelerate to some given speed for
    either increasing their orbit or leaving orbit altogether.  Mass
    returns to impact on the Moon.
    
    I have no idea what the energies involved are, nor whether this is a
    feasible idea, but it sounds good.
    
    Where's the energy come from?  Solar power.  Obviously, the mass comes
    from the Moon.
    
John

    	Here's a method similar in purpose to .8 which would require
    NO lunar mass driver. I do not remember all the specific dimensions
    and other characteristics but it is interesting none the less:
    	Take a long tether and set it into an end over end tumble as
    it orbits the Earth. The length of the tether and rate of tumbling
    is set so that one end periodicly enters the Earth's atmosphere
    and comes quite close to the surface (say a few miles up). Since
    the tether is so long (a few hundred to a couple of thousand miles
    long), the end of the tether as viewed from Earth would seem to
    come almost straight down, come to a momentary stop and then go
    back up again. If a vehicle was right at the end of the tether when
    it came to its momentary stop and clamped on, it would be accelerated
    upwards. The vehicle on the end of the tether would continue to
    accelerate as the tether tumbled and once at its highest point the
    vehicle could unclamp itself escape the Earth.
    	To balance off the whole process (and conserve energy as much
    as possible), while the tether is picking up one vehicle near the
    Earth, it could almost simultaneously pick up another vehicle from
    the high end coming in on a hyperbolic trajectory. If the incoming
    vehicle's trajectory was just right, the end of the tether would
    appear to momentarily stop which is when the incoming vehcle can
    clamp on. In such a way, a "shuttle" system could be set up that
    would not require much rocket power (basicly only for maneuvering)
    and would need only minimal energy input (to make up for atmospheric
    drag and any other minor energy drains in the system)
    	It all sounds a little weird and there would be a relatively
    narrow margin for error but at least on paper it seems to work.
    
    				Drew
    

re back a few:

Drew, I think your physics are sound (I have not analyzed your description
in detail, but I did think hard about the concept a while ago).  The only
issue I have is your description of what it is used for 
" This method could be used
    to boost a space station's orbit or even throw a payload into an
    escape trajectory WITHOUT THE USE OF A ROCKET!"

What you say is true given the initial conditions.  The problem is that in
order to have the mass to drop down to a lower orbit (thus sending something
else higher) you must have gotten the dropping mass up to the initial position
in the first place.  And in order to do this, you must expend exactly the
same amount of energy that you would to fling the "top" mass up higher in
the first place.  More, in fact, since you have to get the falling mass
up through the atmosphere to begin with.

Look at it from the space station point of view.  You want to raise up the
station's orbit.  Ok, you kick out the tethered mass and all is well.  But
next time you need to raise it, you don't have the mass anymore.

This is not to say it is useless.  For example, it might be that this whole
think could work like a momentum wheel if there were alternating up and down
forces (say going with and against solar light pressure). You could
alternatively lower and raise the weight.  At some point, though, you run
out of cable and have to fire some rockets to bring the cable back in w/o 
lowering the orbit.  Probably only useful if you want to keep the orbit
height very stable.

Burns

    Re: .10
    	You are correct in what you have said, i.e. the satellites or
    other mass have to be orbited to begin with but there are still
    advantages to this scheme. Here is one possibility:
    	The Space Station must have its orbit boosted periodicly so
    that it can avoid the fate of Skylab. This boost requires a rocket
    which uses fuel and of course the fuel must be brought up from Earth.
    	On the other hand we have a Space Shuttle making periodic visits
    to exchange crews and drop off supplies such as fuel. After the
    visit is completed, the Shuttle uses even more fuel to deorbit and
    return to Earth.
    	Now if instead of the Shuttle using fuel to deorbit and the
    Space Station using fuel to raise its orbit, why not lower the Shuttle
    from the Space Station with a tether and when it is the proper
    distance, cut it loose and reel the empty tether back up to the
    Station. In this way the Space Station's orbit is raised and the
    Shuttle is deorbited without the use of any fuel. With this scheme the 
    Space Station would require much less fuel (and more useful cargo could 
    be sent up in its place) and the Shuttle would have increased fuel 
    margins (or conversely load less fuel to begin with and send up more useful
    payload). 
    	Using a tether in this way in the long run could save the
    equivalent of a Shuttle's worth of cargo every year or two. This
    would allow either less frequent visits from the Shuttle (which
    would be nice judging from the backed schedule) or it would allow
    more useful cargo to go into orbit. Overall it simplifies the Space
    Station logistics a lot!
                     
    				Drew
    
    

    Re .8 and .9
    
    For more on this read note 451

    re:.8
    
    I thought that the conclusion of the previous discussions of a 'space
    ladder' was that we don't have any material strong enough to make
    the cable that tethers the satellites.  The force on the cable at
    the center of gravity of the tethered satellite system is tremendous.
    Maybe we could construct one from spider web material.  :-)
    
    			Steve

    Re:.13
    	For some of the more ambitous applications, materials stronger than
    anything presently known would be required. However, using materials
    like Kevlar, certain applications (such as the one described in .11)
    are possible. Around smaller bodies such as the Moon and Mars Kevlar is
    more than adequet for many types of tethered operations since the
    "gravity wells" of these bodies is not as deep as Earth's (hence less
    tension on the tethers).
    	As I promised yesterday, I have a preliminary list of references. I
    can post a more comprehensive listing but much of the material is very
    technical and some of it is in Russian (yes, the Russians did a lot of
    pioneering work with tethers starting with Tsiolkovsky in the 1890's).
    This list is by no mean exhaustive but is a good starting point (besides
    these are the only books in my library that have anything to say about
    tethers).
    
    
    >"The New Race for Space" by James Oberg
    		Chapter 18: "Tethered Space Operations"
    	Gives good non-technical descriptions of tethered operations
    especially those that can be used with the Space Shuttle and Space
    Station.
    
    >"The Omni Book of Space"
    		Part II: "High Wire Act" by Robert L. Forward & Hans P.
    		Moravec
    	Another good non-technical description.
    
    >"Case for Mars II"
    		AAS 84-174 "Tethers for Mars Space Operations" by Paul A.
    		Penzo 
    	This paper gives an EXCELLENT overview of the history,
    applications, and physics of space tethers. It also details a space
    tether system using the Martian moons with tethers to boost payloads
    from low Martian orbit to an escape trajectory and vice versa. This
    system could use tethers made of kevlar. Beware, it is slightly
    technical.
    
    				Enjoy,
    				 Drew
    

>    I thought that the conclusion of the previous discussions of a 'space
>    ladder' was that we don't have any material strong enough to make
>    the cable that tethers the satellites.  The force on the cable at
>    the center of gravity of the tethered satellite system is tremendous.
>    Maybe we could construct one from spider web material.  :-)
    
    Don't latch on permanently.  Imagine deploying a series of 200 nets of
    very strong material in the path of the 20 ton mass.  Spaced out a few
    miles.  Each net is attached to a single cable (of the strongest and
    most stretchable material feasible to use for a very long distance in
    space).
    
    20 ton mass comes rifling into the series of nets.  The nets are
    designed to break as the pull on the cable becomes too much. 
    Basically, get what energy you can from the mass as it comes by, and
    get it in bursts.  Smooth out the bursts through the elastic nature of
    the long cable.  Certainly other techniques could be used to smooth out
    the transfer of energy.  Springs?  Surely there must be some clever
    people to solve little problems like this.
    
    As the materials processing people get up to speed on super tensile
    strength cables, those can be used - but damping the energy transfer
    would still be necessary.
    
John

	Note 547.9
	HAZEL::LEPAGE "Truth travels slowly"                 30 lines  25-JUL-1989 

    >    Take a long tether and set it into an end over end tumble as
    >it orbits the Earth. The length of the tether and rate of tumbling
    >is set so that one end periodicly enters the Earth's atmosphere
    >and comes quite close to the surface (say a few miles up). 


	Once the tether hits the earth's atmosphere (which will be quite 
	thick a "few miles up), atmospheric drag will bend the thing 
	backwards putting the entire system out of balance, and sapping
	energy from it (serious energy). 

    >the end of the tether as viewed from Earth would seem to
    >come almost straight down, come to a momentary stop and then go
    >back up again. 

	Nonsense. the end of the tether would be in CONSANT motion. Only 
	it's VERTICAL component of velocity will (for an infinitessimal
	period of time) be zero.

	Note 547.11
	HAZEL::LEPAGE "Truth travels slowly"

    >   Now if instead of the Shuttle using fuel to deorbit and the
    >Space Station using fuel to raise its orbit, why not lower the Shuttle
    >from the Space Station with a tether and when it is the proper
    >distance, cut it loose and reel the empty tether back up to the
    >Station. 

	How do you propose lowering the shuttle? It is going at the speed
	required for it's initial orbit. This is what I do not understand
	about the whole 2-body tether energy exchange issue.

	In order to get the shuttle down to a lower altitude, you have
	to either "Push" it down (normal to earth's surface), or slow it 
	down (which is what you are trying to avoid.

	Either way, it takes energy, and I submit that it takes the same 
	amount of energy to reposition these things (from their starting 
	orbit) to any other position.

	
	Gregg

    Re:.16
    	Let me just start off by saying that I'm not the one "making" any
    of this stuff up. Very respectable aerospace engineers and physicist
    (among others) who are a bit better at this than I am have spent
    considerable time and imagination inventing these tether applications. As a
    first step take my word that they work; I'm just trying to explain what
    they have done (sometimes it seems not too well). Let me also state
    (quite bluntly and please do not take offense) that without a solid
    understanding of physics or at very least an intuitive feeling for
    orbital mechanics, you have no hope of understanding this. I'm sorry
    but this is rather weird, anti-intuitive physics (the universe is
    filled with this kind of stuff).
    
    	In the example of using the a tumbling tether perhaps its
    "tumbling" needs a little more explanation. The tether is set tumbling
    as if it were a spoke in a wheel and the wheel was rolling over the
    Earth. The rate of tumble is set so that the velocity of one end as it
    approaches the surface IS ZERO for an instant (and close to zero for a
    few seconds either side of this time) relative to the Earth's surface.
    As for the tether's end coming almost straight down, it does SEEM to
    come almost straight down since the observer on the Earth is only
    seeing the last few miles of the tether's thousand-plus mile path. 
    	Imagine a wheel rolling over a surface. Now imagine the motion of
    only one point on that wheel's rim as it moves. The path that point
    appears to take is called a "cycloid" and it looks like a series of
    ellipses cut along their major axes and laid end to end. If you look
    closely at that point as it approches the ground, its final path IS
    almost straight down and the velocity of that point relative to the
    ground IS momentarily zero.
    	As I mentioned in 547.9, yes there will be some drag and yes there
    will be need to pump additional energy into the system, as you stated
    (although they are not as large as you were thinking since the tether's
    tip velocity does go to zero). But these energy loses are orders of
    magnitude less than the energy needed to orbit a comparable payload
    using today's rockets.
    
    	As I tried to explain in 547.6 with the example of two satellites
    on a tether, there is NO energy required to pull the satellites apart.
    In a practical application there would probably be the need for an
    intitial shove to get the satellites moving (though this can be made
    arbitrarily small and is negligable), but the force that continues to
    pull the satellites apart are basicly unbalanced gravitational and
    centrifugal forces on the two satellites. The net result is that no
    energy needs to be put into the system. All that is happening is that
    the POTENTIAL energy is being transfered from one satellite to the
    other as one satellite goes higher (and therefore increases its
    potential energy) and the other goes lower (and loses potential
    energy).
    				Drew
    

    re: -.last several
    
    	Not to be a pain or anything, but how does all this relate to
    a DEC employee going on the Shuttle?  I mean, this is all real
    interesting and everything (yawn), but I, for one, would like to
    hear some more about the DECcie, and even other possible private
    citizens, going on future shuttle trips.
    
    	Am I being out of line here? If so, I'll shut up.
    
    	Many 8=)
    
    	McQ
    

    I have moved my reply in .18 to the SKYHOOK note - 451.
    
    	Gregg

    Re: .19 & .20
    	Actually this discussion should be moved probably to 451 since
    the discussion is wandering away from the original topic. I will post
    any further discussions on tethers there.
    
    Re: .18
    Gregg,
    	I did not know what your background was until now (and now I find
    myself at the threshold of a real "struggle"). I have a BS in Physics
    but I am not about to claim I know EVERYTHING on space tethers or their
    applications. To date my understanding has been confined to the static
    rather than the dynamic behavior of tethers. I'll crawl back into my
    books and hopefully come back with some reasonable explanantions that
    we can both understand and accept. See you in 451.
    
    				Drew
     

	Back to the original topic, the flight STS 46 is scheduled for March
	1992 on Discovery. Franco Malerba, our very own, and Umberto Guidoni
	are the candidates for the prime and backup payload specialist. No
	word from NASA as to when the final descision will be made. 

	Good Luck Franco!