A system for tethering one orbiting space vehicle to another was designed so that a tetherline between the vehicles is controlled by a motorized reel which in turn is controlled to deploy, retrieve, or maintain a constant line length while effecting a stabilizing influence on the line. This is accomplished by applying a tension to the line which takes into account the instantaneous length of the line, rate of change of the length of the line, and certain constants which vary depending upon the mode of operation, deployment, retrieval, or station keeping

Kissel, R. R.

Rupp, C. C.

English

NASA Technical Reports Server

United States Patent [191 [111 4,083,520 
Rupp et al. [451 Apr. 11, 1978 
[54] TETHERLINE SYSTEM FOR ORBITING 
[75] Inventors: Charles C. Rupp, Madison; Ralph R. 
[73] Assignee: The United States of America as 
SATELLITES 
Kissel, Huntsville, both of Ala. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. NO.: 739,908 
[22] Filed: Nov. 8, 1976 
[51] Int. Cl .2  ................................................ B64G 1/00 
[52] U.S. Cl. ..................................... 244/167; 244/161 
[58] Field of Search ............... 244/115, 116, 158, 160, 
244/161, 162, 164, 167; 114/144 B, 230; 
242/45, 75.51; 254/173 R, 173 B; 73/143, 144 
~561  References Cited 
U.S. PATENT DOCUMENTS 
2,842,339 7/1958 Gresham et al. ................. 254/173 B 
3,333,788 SA967 Donovan et al. .................... 244/158 
3,381,527 5/1968 Grubbs ................................... 73/14 
3,532,298 10/1970 Swet ..................................... 244/161 
3,582,016 6/1971 Sherman .............................. 244/167 
3,886,887 6/1975 Cunningham ........................ 114/230 
Primary Examiner-Trygve M. Brix 
Assistant Examiner-Charles E. Frankfort 
Attorney, Agent, or Firm-George J. Porter; J. R. 
Manning; L. D. Wofford, Jr. 
1571 ABSTRACX 
A system for tethering one orbiting space vehicle to 
another in which a tetherline between the vehicles is 
controlled by a motorized reel which in turn is con- 
trolled to deploy, retrieve, or maintain a constant line 
length while effecting a stabilizing influence on the line. 
This is accomplished by applying a tension to the line 
which takes into account the instantaneous length of the 
line, rate of change of the length of the line, and certain 
constants which vary depending upon the mode of 
operation, deployment, retrieval, or station keeping. 
6 Claims, 3 Drawing Figures 
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U.S. Patent April 11 ,  1978 Sheet 1 of 2 4,083,520 
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4,083,520 
TETHERLINE SYSTEM FOR ORBITING 
SATELLITES 
5 
ORIGIN OF THE INVENTION 
The invention described herein was made by employ- 
ees of the United States Government, and may be manu- 
factured and used by or for the Government of the 
United States of America for governmental purposes 10 
without the payment of any royalties thereon or there- 
for. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the tethering of orbiting 
space vehicles, and particularly to a system for control- 
ling the position of one space vehicle, e.g., along a local 
vertical relative to another. 
15 
20 2. General Description of the Prior Art 
Tethered sub-satellites have been proposed for a vari- 
ety of space applications. In a simple case, suggested for 
rescuing stranded astronauts, a buoy on a tetherline 
would simply be deployed with an initial velocity from 
the rescue vehicle to the astronaut, and then the tether- 
line would be reeled back after the astronaut had 
grabbed the buoy. A principal difficulty with this 
method is that slight variations in the initial velocity of 
the buoy with respect to the rescue vehicle, or errors in 3o 
the tetherline tension, cause large errors in the final state 
of deployment or retrieval of the buoy. 
Another proposed method of tethering two orbiting 
space vehicles together suggests that one of the vehicles 
be placed in a somewhat egg-shaped trajectory relative 35 
to the other and maintaining them in such position by 
periodically tugging on a tetherline which connects 
them. This approach was determined to be unattractive 
for flight because of the difficulty of precisely determin- 
ing and controlling the tugging forces to be applied and 40 
25 
ofproviding constant mG-edsupervision wGch would 
be required to insure flight safety. 
A thud method calls for the deployment of a sub- 
satellite in an orbit below a main satellite wherein initial 
deployment would be accomplished with a drag bal- 
loon, which would then be destroyed, and final deploy- 
ment effected by slowly unwinding the tetherline from 
a reel. While deployment in this manner would be oper- 
ative, the realization of such a system would encounter 
the same problems discussed above, notably that of 
effecting sufficient stabilization of the sub-satellite and 
of eliminating deployment errors. 
SUMMARY OF THE INVENTION 
In accordance with this invention, a tetherline con- 
necting a sub-satellite to a main satellite would be con- 
trolled by a motorized reel which provides a line ten- 
sion which is varied, increased or decreased, as a func- 
tion of length of tetherline and rate of change of length 
of tetherline, with the effect of deploying, retrieving, 
and/or station keeping the sub-satellite along the main 
satellite local vertical. 
BRIEF DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a pictorial illustration of a sub-satellite teth- 
FIG. 2 is a diagrammatic illustration of an embodi- 
ered from a main satellite. 
ment of this invention. 
45 
50 
55 
2 
FIG. 3 is a diagrammatic illustration of a modification 
of the system shown in FIG. 2. 
DETAILED DESCRIPTION OF THE 
DRAWINGS 
FIG. 1 generally illustrates an application of the in- 
vention wherein sub-satellite 10 is deployed from a main 
satellite 12. It is assumed that the satellites are in orbit 
and that the orbital speed is one which effects a stabili- 
zation in the radius of orbit of main satellite 12. It will be 
further assumed that it is desired to deploy sub-satellite 
10 along a local vertical upward as illustrated by the 
extension and orientation of mast 14 from satellite 12 by 
mast deployment system 15 (not shown in detail). Ini- 
tially, sub-satellite 10 would be held in docking cradle 
16 at the end of mast 14 by line or tetherline 18, the line 
being held fast on reel 20 (FIG. 2) of reel assembly 22. 
By virtue of the vertical displacement of sub-satellite 10 
on mast 14 (typically 30 to 50 meters) from the orbit of 
the main body of satellite 12, the former would be 
moved at a higher absolute velocity than satellite 12, 
and there would thus occur a force along the vertical 
upward which, upon the release of tetherline 18, would 
enable sub-satellite 10 to achieve an initial velocity up- 
ward away from satellite 12. Similarly, if mast 14 had 
been deployed downward, wherein sub-satellite 10 
would have been in a lower orbit, there would have 
been a downward or lowering force acting on it because 
of the smaller radius of orbit in which it is being moved. 
Thus, in either event, initial velocity would be imparted 
to a sub-satellite upon the release of tension in line 18. 
Referring to FIG. 2, it will be noted that tetherline 18 
is wound on a reel or drum 20 of reel assembly 22 which 
is connected to direct drive, D.C. torque motor 24, in 
turn supported (by means not shown) on fmed support 
25. Tachometer 26 is coupled to the shaft of torque 
motor 24 to provide a negative feedback, speed repre- 
sentative, signal to smooth and stabilize the responsive- 
ness of motor 24, as will be further described. Tether- 
line 18 is fed from reel 20 over pulleys 28, 29, and 32, 
along mast 14, and to sub-satellite 10. Pulleys 28 and 29 
are rotably mounted on fmed supports 30 and 31 (by 
means not shown), and driven pulley 32 is rotably 
mounted on movable arm assembly 36, in turn mounted 
on satellite 12 through spring 38. Thus, pulley 32 is 
biased against line 18, and, upon an increase or decrease 
in tension in line 18, spring 38 would compress or ex- 
tend, accordingly. Movable contact 40 of potentiometer 
42 is mechanically mounted on a movable arm portion 
44 of an arm assembly 36 and thus is moved along resis- 
tance element 46 as a function of tension in line 18. A 
reference voltage is applied between terminals 48 and 
50 of resistance element 46, and thus there is provided 
an electrical signal output from movable arm 40 repre- 
sentative of the tension present in line 18. Mechanical 
resonances in spring 38 and pulley 32 are partially 
damped by damper 55 which may be in the form of a 
dash pot. Rotation counter 54 and tachometer 56 are 
coupled to pulley 32 and provide, respectively, signals 1 
60 and-1' on leads 58 and 60, representative of the len-gth of 
line deployed and rate of deployment, respectively. 
The torque to be applied by torque motor 24 to drum 
20, and thereby tension to line 18, is computed by 
torque computer 62 which functions to solve the fol- 
65 lowing equation wherein the length of line signal 1 and 
change in length of line signal 1' are utilized as principal 
variables; 
4,083,520 
r, = (w: + 3.:) 1 + 2d,wf - W: (&,I + k3). 
The terms other than I and I’ are supplied to torque 
computer 62 by control signal generator 64 as follows: 
w, the orbital angular rate of main satellite 12, repre- 
sented by a fvted level signal; 
w, the control or stretch frequency constant signal, 
representative of a frequency approximately one to four 
times that of w; 
dd, a damping constant signal, represented by a se- 
lected signal value equal to approximately 0.2 to 2; 
kl ,  a signal represented constant, approximately rep- 
resentative of 2 for deployment, 0.93 for retrieval, and 0 
for station keeping; and 
resentative of 7,000 for deployment, 0 for retrieval, and 
of the value of final desired length of tetherline 18 for 
station keeping. 
In a simple case, control signal generator 64 would 
include a source of voltages or a voltage and a series of 2o 
potentiometers connected to it which would either be 
adjusted by hand or be automatic to provide the signals 
referred to in the preceding paragraph. 
The output of torque computer 62, to a tension com- 
mand signal, is fed to one input of summing servo ampli- 
fier 66, a “present tension” feedback signal from poten- 
tiometer 42, t,, representative of the tension in line 18, 
is fed to a second input, and a rate feedback signal r,, 
representative of the instantaneous speed of motor 24, 
is, conventionally, fed to a third input of amplifier 66. 
The latter two signals are of opposite sign to the first of 30 
the signals, and thus there is provided at the output of 
amplifier 66 a signal 1, to motor 24 which will then 
produce a tension in line 18 corresponding to the com- 
manded tension signal te. 
w,, w, d, I, I’, kl, k3, to and t,and displays these utilizing 
coventional selector switches to make selections, the 
display typically being a numerical readout either in the 
form of a calibrated meter or a digital display. 
wherein the line tension, f, measurement is obtained 
from strain gauges 70, 72, and 74, forming a portion of 
supports 76,78, and 80 which support platform 82 on a 
frame portion 84 of satellite 12. Reel 20 is then mounted 
on platform 82 by mount 25, together with pulley 86 45 
mounted on platform 82 by mount $5, over which line 
18 is played. The outputs of strain gauges 70,72, and 74 
sense the tension between platform 82 and satellite 12, 
and thereby the tension in line 18. These outputs are 
then summed in amplifier 88 to provide a total tension 50 
output tm which is applied to servo amplifier 66 as illus- 
trated in FIG. 2. 
For the deployment case, the tension which is com- 
manded is slightly lower than the gravity gradient force 
which would be applied to the sub-satellite mass if the 55 
sub-satellite was in equalibrium at its present distance. A 
tension greater than this amount will cause retrieval. 
Stabilization of the sub-satellite at a desired deployment 
distance is enhanced by adjustment of the constants 
described above. Any residual swing motion is damped 60 
by the effect of tension feedback signal tm. The damping 
action can be explained by considering the tetherline as 
a spring. As the sub-satellite swings, variations in the 
gravity gradient force cause the spring to stretch and 
contract. These oscillations in the spring stretching 65 
motion are sensed by a movement of spring 38 and 
movable arm 40 of potentiometer 42, and a feedback 
signal from the potentiometer is used to counteract the 
5, a signal represented constant, approximately rep- 15 
25 
Readout or monitor 69 receives the pertinent outputs 35 
FIG. 3 illustrates a modification of the invention 40 
4 
variable force caused by the oscillations and thus to 
reduce them. Further, as described, a damping constant 
d,may be selected in accordance with a particular mis- 
sion to make the tetherline appear to have any spring 
constant and degree of damping desired. In operation, 
the occurrence of a swinging motion is effectively con- 
verted to a stretching motion, and then the stretching 
motion is damped by the control system to maintain a 
substantially constant length and stabilized state of the 
tetherline. 
Retrieval of the tethered sub-satellite 10 is similar to 
the deployment of it, except that the control signal 
generator and torque computer command a tension 
greater than equalibrium tension at the present sub- 
satellite distance. This causes the motor and reel to 
retrieve the tetherline. The terminal phase of the re- 
trieval occurs very slowly, and the capture of the sub- 
satellite is accomplished using the deployment boom 
and cradle 16. In this terminal phase, the sub-satellite is 
slowly drawn into cradle 16, and then the boom is typi- 
cally retracted, drawing the sub-satellite back into main 
satellite 12. 
By means of the present invention, the motion of the 
satellite is stabilized, basically limiting its motion to 
movement along a vertical during deployment and re- 
trieval and to a fixed relative position with respect to a 
main satellite during station keeping. 
Having thus disclosed our invention, what is claimed 
is: 
1. A system for tethering a first orbiting satellite to a 
second orbiting satellite comprising: 
a reel and motor coupled to said reel, both being 
supported by said fust satellite; 
a tetherline wound on said reel and connected at one 
end to said second satellite; 
first transducer means responsive to the movement of 
said line for generating a signal I representative of 
the length of tetherline betwween satellites; 
second transducer means coupled to said tetherline 
for generating a signal I’ representative of the rate 
of change of length of line between said satellites; 
third transducer means for generating a signal t ,  
representative of the tension in said tetherline; 
signal means responsive to said first, second, and third 
transducer means and signals I and I’ and t,, respec- 
tively, and to the orbital angular speed w, of said 
first satellite for generating an electrical input to 
said motor; 
means for generating a first signal constant k, and a 
second signal constant k3, selectively, and approxi- 
mately as follows: 
for deployment, k,  = 2 and k3 = 7,000, 
for retrieval, kl = 0.93 and k3 = 0, and 
for station keeping, kl = 0 and k3 = the desired 
said signal means being responsive to said constants 
2. A system as set forth in claim 1 wherein said signal 
computation means for providing an output t, where 
wherein w, has an approximate value between w, 
and 4w,, and d, has an approximate value of be- 
tween 0.5 and 2; and 
servo means responsive to the output of said compu- 
tation means, to and said third transducer means, 
tm for providing an input to said motor. 
length of tetherline; and 
for providing said input to said motor. 
means comprises: 
tc = (w,Z + 3~$) I + 2dCwJ’ - w,Z (kll + k3) 
4,083,520 
5 6 
3. A system as set forth in claim 2 including means for 
initially positioning said second satellite at a fued, dis- 
Crete distance from said first satellite dong a radial line 
with respect to the orbit of said first satellite, whereby 
upon a reduction of tension in said tetherline, said teth- 
erline and said second satellite deploy from said first 
satellite. 
4. A system as set forth in claim 2 wherein said third 6. A system as set forth in claim 5 including damping 
transducer means comprises means for supporting said 10 means interconnecting said pulley and said first satellite. 
reel on said first satellite, and including means for sens- 
ing the tension in said support and providing said signal 
tm to said Servo n ~ a n s .  
5. A system as set forth in claim 2 wherein said third 
transducer comprises a pulley in engagement 
5 with said tetherline, spring bias means supporting said 
pulley On said first satellite, and potentiometer 
coupled to said 
put tm representative of the tension in said tetherline. 
for providing said electrical 
* * * * *  
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Tetherline system for orbiting satellites

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