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AIM GUIDANCE AND CONTROL CONFERENCE AUGUST 12-$4, 1963, MASSACHUSETTS INSTITUTE OF

-c ' TECHNOLOGY CAMBRIDGE, MASSACHUSETTS

REQUIREMENTS FOR A SPACE VEHICLE CLOSURE AND DOCKING SIMULATOR

bY W. L. DeROCHER, JR. and GLEN H. SMITH Martin Company Denver, Colo.

NO. 63-363

First publication rights reserved by American institute of AerOnaUtlCS and AStrOnaUtiCS. 500 Fifth Ave., New York, N.Y.AbStractS may be published without permission if credit is given to author and to AIAA. (Price-AIAA Member 504, Nan-Member 81.00).

Z i c o m

REQUIREMENTS FOR A SPACE Vl<IlICLE CLOSIJRE AND DOCKING SIMULATOR

W . L. De Kochel-, J r . :ind (Glen II. Smith Principal Engineer xnd Section llc::~d, Rcs]jecli\,ely

Martin Company/Denver, Aerospace Division of t he Martin Marietta Corporation

Abstract

While many rendezvous simulators have been re- ported on in tlie past , few of them have concentrated on the region just belort, antl :it contact. Thus n desinition of the requirementsof a space vehicle closure and dock- ing simulator is needed. developed from an analysis of the operational uses of a simulator, space missions, representative docking mechanism concepts, system parameters , and the efSect of man in the docking operation. Such a summary must also recognize the evaluation of sensors , the interaction of the closure and docking systems, possible extensions of the hasic capaiiility, and n dynamics analysis.

A summ:iry of requirements is

Three categories ai possible solutions along with their definitions are presented in this paper . examples a r e given for each category, along with obvious advantages and disndvxntages. and disadvantages of the possible solutions are then cum- pared xnd evaluated. vocated solution Tor a r losure and docking simulator is offered, :ilong with :I useful modification of the basic idea. described in some dettiil. These concepts have been turned into reality by tlie construction of :in actual siniu 1:itor of this type in r!i%nver, Colorado.

Specific

'The relative advantages

Following this discussion, an i u -

The configurati<)n m c l usc of this sirnulator are

Introduction

Various methods of simulating the many [ihascs of space rendezvous havc heen t r ied by nearly all major space vehicle m:mufaclurers ;ind many government agen- c ies . lack of study during the early development period 01 the Sinal phases of rendezvous, the closure and docking phases. Appnrently the attitude prevailed that if one cnulrl get within :I few feet of the orljiting vchiclc, the rendezvous p r d l e m was essentially solved. Ilowever , i n the las t year many org:inix:itions have come to nppre- ciate the many prolilems associated with the closure a n d docking phases and have instituted programs to develop simulatirrn Cxil i t ies to study these prablems. paper , the closure phase is considered to bc thxt perinrl from a few hundred Setit of separation distance until f i r s t contact; the clocliiiig phase is that period Srom f i r s t coli- tact until the vehicles :ire securely fastened trigether.

Despite all this elfort, there has been :I iiotaljle

In this

Ikfare ser ious consideration ciin he given to tle- signing :I closure m c ! docking simulatur, an :in:ilysis must be madc to determine the cri t ical parameters OS the proli- lem trr insure accurate simulation oS F x t o r s that aSlect the final performance. Three things that m u s t be c011-

s idered in the simulation a r e : (1) visual and sensor r e - quirements; (?) manu:il anti :mtomatic control system character is t ics ; inid ( 3 ) vehicle dynamics anti mechnnical forces . These c:ni be summarizcd :IS the sensor I I ~ i n - telligence g:ithering Sunc!tion; the control o r action func- tion; a n d the renct im or resulting motion Sunction. E:wh of these functions h a s very ilifferent simulation prolilems

The sensor simu1:ition must either duplicate thc sensor character is t ics or provide an environment f o r the actual sensors, whetht,r they ire ii man o r equipment, that

__ essentially rluplicatrs tjic space environment. rif the cmtrril function i s the easiest of the three since electronic circuits can lie assembled o r simulated with standard electronic equipment to match the spnceliorne system. Tile vehicle dynamics simulation must include the e f f w t o i all iorces -- including docking Sorces, con- trol forms iind orbital dynamics forces -- on lmth ve-

c:illy defining the geometry and Sorces resulting

Simulation

'The major problem resul ts f rom the difficulty in

from the iittachment mechanism during the docking phase.

Many of the early rendezvous simulations have cnn- centrated on one o r two of these functions and either ig- norcrl o r minimized the remaining functions. the riimplcsity and cost of the simulation are directly r e - lated to the accuracy with which each iunction is simu- lated. Since all three functions -- sensor, action, and resulting motion -- a r e p a r t s of a closed loop system, they :rll are intimately related to each other and cannot Ihc: eiiectivcly separated.

Obviously,

'There a r e two other methods of diviiling the simu- rolilem: one, I)? dividing by time phases and two, ' c s oS rendezvous systems. Ihas proved successful in many previous simula-

tion problems. where simulation is broken down into landing simulation, we:qmns delivery systom simulation, navigational simu- Iatioii, etc. ph:ises for simulation However, care must Ire taken in <letermiiiing allowalile separati vints. The separation must w c u r :it a time when tlie s lox l y st:!te coiiriition. ste:ii!y state r:ondition is usually a Sunctioii of the r e n d e ~ v ~ ~ u s logic lieing used, i t is difficult to dcSine il fixcri sap:Lr:ition point. However, nearly all proposed rendczvwis s> s tems have an acceptable b r c : ~ k point he- twcen the approach and closure phases. Most simulators tli:it lh:~ve Ixen ilevelopcd have included it discontinuity o r sepxrxt im nt this point!

The use of t imephase

One example i s airplane experience,

Space rendezvous can also lie divided into

em i s in it quasi- Since the existence of ii quasi-

A c,;ireSul study 01 terminal conditions that occur at the enti of the xppruach phase must he made tu determine the coi,i-ect initial crrnditions for the closure antl docking simiil;ition. 'There does not appear to lie any la ter t ime w i i e r ~ thc nccessary quasi-steady s ta te conditions exist . Y o n w stn<lies hxve been based on a break in the priihlem :it 01' ne:ir the Ihysical contact point. 'This approach ap- pmrs ti, lic acoeptalile only i f the attachment mech:~nisms vpcrxte i w r y rapidlv and if their action is not affected hy coiitrol Sorces or relative vehicle motions aster initial 1:rint:ict. Nearly :ill proposed docliinl: meclianisms do not meet these requirements. For a general stud>, oS closure :ind dwiiing, the entire period must lie simulated together withrint :in? brea!;s o r interruptions. true i f ~ i i i i n u i i l < ~ , n t r o l i s used or iS iioisc and e r r o r :mal- yses iii'c consideved.

This i s espec:ially

In ii study < i f the attachment period or period after iniii;il contact, tlie resulting relative motion from both the initi;il velricities antl the forces generated in the dock- ing mei,h:inism niiist be considered. 'The docking mecha- nisni iorcxs cause the vehicles to rotate and translate tow:rrd their fin;il xttachment position or away from the rlcsirril posi t im, <lependiiig on the initial positions,

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velbcities, and orientations of the two vehicles. The in- herent non-linear relationships and large c ros s coupling between all the degrees of freedom make i t almost im- possible to accurately simulate this motion without the use of physical hardware, either full-size o r scaled. The capability of control must be provided during the attach- nent period since the attachment time may well he suffi-

The second method of dividing .-cient fo r effective control. the simulation problem by c lasses is almost a prerequi- si te. The numerous c lasses of rendezvous -- coopera- tive, uncooperative, manual and automatic -- all have different simulation problems. The main considerationis to develop simulation facility requirements that include as many of the different c lasses a s possible without requiring major revisions.

This general discussion of the problems associated with rendezvous simulation is intended to provide an ap- preciation of the closure and docking simulation problem. A detailed analysis of the closure and docking simulation requirements is included in the following sections of the paper.

Development of Requirements

An outline of detailed requirements for the simula- tion of closure and docking phase of rendezvous must be preceded by an analysis of the objectives of the simula- tion. The obvious objective is to evaluate closure schemes, docking devices, and their interactions. The extension of this objective is the pr imary subject of this paper.

Operational Uses of the Simulator

Nearly every aspect of the development of rendez- vouz systems for space vehicles requires the use of s im- ulation. Some of these major uses are:

(1) &sign Investigations could include the study of closure techniques, evaluation of sensors , evaluation of mechanical devices, and evaluation of guidance and con- trol hardware.

(2) Human Factors Studies could include workload evaluation, display and control choice, effect of tsaining, and effect of s t r e s s .

Training Programs could include both normal and abnormal conditions.

Acceptance Testing of components o r systems could also he performed with a simulator.

Flight Tes t Programs could include the devel- opment of flight programs a s well as post flight analysis.

(3)

(4)

(5)

A t rue general purpose simulator would satisfy all However, experience has shown of these requirements.

in other cases that few truly general purpose simulations have been successful. Usually, the attempt to meet all the requirements becomes too demanding and several special purpose simulation facilities have been developed. However, i t is the purpose of this paper to specify a set of requirements that will meet a s many of these condi- tions a s possible without undue complication of the simu- lation equipment.

Space Missions

, . Future space missions should he considered, a s ._ ., these missions define where closure and docking will be used. hut a s a development program ra ther than a s a method of accomplishing a phase of the mission. In the Apollo program, there a r e two closure and docking operations:

They will he important to the Gemini program,

repositioning of the Lunar Excursion Module (LEM) and re turn of the LEM to the Command Module. The post- Apollo programs axe not a s definite, but the orbiting lab- oratory and the orbiting launch facility seem probable. I t also seems reasonable to assume that the LEM concept will he applied to other planetary and moon operations. Orbital warfare and its implications on a closure and docking simulator will not he considered here .

F rom the above missions, it can he seen that there will he requirements fo r closure and docking. jectives could be summarized as maintenance, crew ro- tation, assembly, refueling, resupply, and salvage. Salvage can he interpreted in the sense of returning something valuable to ear th o r in the sense of changing a vehicle's orbit so that it would burn up in the atmosphere. The LEM mission can he interpreted as one of assembly o r , more properly, reassembly.

The oh-

The above considerations lead to an appreciation of the number of vehicle types and range of conditions that could he involved in closure and docking.

Representative Docking Mechanism Concepts

the available l i terature,2, 3, a r e many concepts of docking mechanisms; certainly, man's ingenuity will provide many more. Gemini and Apollo docking mechanisms a r e but one of many possible fo rms .

From the above consideration of missions and f r o m it is obvious that there

The s imi la r

A way of handling this diversity of mechanisms is Three levels of categorization have been to categorize.

postulated. The first level is cooperative o r uncoopera- tive. (In this paper the question of hostile vehicles is avoided as it introduces a series of questions which are not relevant to this study.) Vehicles belonging to the un- cooperative category a r e those which do not have com- patible docking devices, o r are tumbling out of control, o r in some other way a r e not able to assist in the closure and docking process. The second level of categorization is based on vehicle separation a t the instant of f i r s t con- tact, close o r remote. A close attachment is one where the vehicles a r e essentially flown together; a remote at- tachment is one where the vehicles a r e separated a t some distance and initial contact is made via some extensionof the vehicles such a s an extendable probe OP a controllable auxiliaryvehicle. The third categorizationlevel is based on the size of the capture a rea , small o r large. These seemto be the essentialfactors: cooperative oruncooperative; re- mote o r close attachment: and small o r large capture a rea . I t should be no surpr i se that these a r e a11 relative words with no c lear demarcation lines between the paired t e rms . Fortunately, th i s should not matter a s a similar nncertain- ty will occur when it comes to desiguingthe simulator.

The above categorization is illustrated f o r coopera- tive systems in Figure 1. American Machine and Foundry extensihle probe and drogue, which was disclosed l a s t year and is representa- tive of remote attachment with a small capture area. The sketch to the upper right is representative of remote at- tachment with a la rge capture a rea and uses a la rge cone on one vehicle and a tethered flyable remote attachment device on the other. The Gemini concept shown a t the lower left is representative of the close attachment, small capture a rea class. The sketch a t the lower right i s typ- ical of the close atta.chment, l a rge a rea class; i t is s im- i lar to the Gemini, hutthe cone andprobeare ex t ra la rge .

The upper lef t sketch is the

Figure 2 i l lustrates the four possibilities for the uncooperative case. Note that what is termed close inthe

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uncuogerxtive situation might be termed remote for coop- erative docking. to get close to :I tumbling vehicle hefore initial contact is made. 'The remote-sm;ill situation i s typified Iiy a Martin rrnicept, pictured at the upper lef t , which employs a third vehicle dispntclied : in i l controlleil from the chaser vehicle to sn:irc the unc,ooperative tnrgct. The illustration at up- per right typifies t l i ~ remote-largc c lass whcre the tumlding veliiclc i s "li;irpooned, I ' ulthough actual penetr:i- tion r r f tile vehiclc i s not necessarily implied. 'The use of a Inrge n u t held in the pnth of the uncooperative vehicle, shown :it the 10wer l u l t , irepresents the close-small class. The sketch :it the lowcr right represents the close-large c lass iinrl i1lustr:ites :L trip-wise that, after contact, is rapidly u.i.apperl a r o ~ u d the target vehicle.

This is because it will not be practical

While it i s olivious that some or these approaches a r e not worthy of fur ther consideration, they a r e shown merely to i lhtstrate the cl:issification scheme.

System Pm-ameters

'The system jiar:imeters a r e those character is t ics of a closure and cliicking system, other than the docking mechanism concept, which affect the des ip l of a closure and docking s i m d a t o r . These character is t ics have been grouped into four categories for ease of consideration.

Approach I 'arameters - Those expected and acceptable physical relationships that exist between the two vehicles just liefore mechanical contact of any com- ponent of the mechanism to be evaluated.

the vehicles and a r e pertinent to the closure and docking operations.

Performance Parameters - The essential r e - quirements, plus other desirable objectives, of the clo- s u r e and docking system. Many of these factors will not be parameters in the s t r ic tes t sense since they will not establish magnitude levels. They will essentially ask "yes-or-no" questions.

for and desirable character is t ics of the docking mecha- nism itself.6

(1)

(2) Vehicle Parameters - Those data that define

(3)

(4) Mechanism Parameters - The requirements

To fur ther clarify the nature of these parameters ,

The numbers given a r e representative examples are given in Tables I, 2 , 3 and 4 for each ofthe four categories. of cooperative vehicles using a close contact scheme with a small capture area. Comparable numbers f o r the other five cases will in general be la rger . However, to attempt to s ta te the numbers for all cases would resul t in con- fusion. The phrase, "vehicle dependent, ( ( indicates that the range of possible values is large and even the orderof magnitude of the numbers i s dependent upon the particular vehicle being studied.

Table 1 Approach Parameters

Parameter Level

Closure Rate 0-10 fps Radial Displacement i 1 ft Radial nisplacement Rate * 0.2 fps Attitude Misalignmcnt (anv axis) + 5 deg Attitudc Rate (any uxis) i 0. 5 debTs Intervehicle Thermal Potentid * Intervchiclr. E1ectrio:d Potentinl * 'Theso ritimhers :ire t)pic:d lor ii cooperative, close, sm:ill : iwa closure i int l iIwkit>g system.

*vcliirlc <lepcn<letit

'Table 2 Vehicle Parameters

I':i r :i me te r

L'tbhicle I\.I:iss Vvh i r le Pitch Moment of InerLia \'chicle Yaw Moment of Inertia L't,hiclc lloll Moment of Inertia Location of Docking Mechanism Vehicle cm Location \'chirle Confibwrntinn

\'ehir:lc Size Guici:mce System Characterist ics Alti tude C v n t r a l System Ch;ir:tcteristics Presence of M a n

*vehicle ilcliendent

Level

600 slugs 25,000 slug ft2 28,000 slug ft2

7,000 s iugft2 - - ' * *

Conical Reentry nody - Cylindri- cal Mission Module 10 Et dia x 25 It

* *

*

'Table 3 Performance Parameters

Satisfactory Operation A1tern:ite Modes Simplicity of Design Adaptability to Different Missions Required System Reliability Acceptable Safety Compromise Imposed by System Crash Warning System Abort and Retry Capability Emergency Control Requirements

Yes Yes Y e s

0.95 None

Yes Yes Manual and Override Controls

*

*vehicle dependent

System Environment - Launch and Docking Phases Acceptable Leak Rate of Inter- vehicle Seal (if pressurized) Power Required to Maintain Lock Separation Force Required 'Time to Accomplish Initial Intesvchicle Connection Mech:inism Release Initiation Point

Table 4 Docking Mechanism Parameters

*

0.02 to 0 . 2 cc/ m i d i n . None

Before Rebound Can Occur Either Vehicle

*

Number of Operations Required 5 per Mission Number of Operations Required After :in Emergency Release Mcch;niical Closing Force Required * Aliility to WithstandMinor Off- * center Impacts

tions) Ibcking Mechanism Weight .4llowable * I'rcqt~cncy 01 Mechanism Main- None tcn:inc,c (IS Inspections per Mission

None

l'olumc Allowable (or Size Limita- *

c

Man in the Closure and Docking Operation

The use of man in the docking operation as well as in rendezvous seems likely in all but the most routine cases. Present plans call for the use of man in our first attempts to rendezvous and will undoubtedly call for his use in many of the future attempts to dock. The reason8 for this have been extensively discussed in the past and are headed by the fact that man can use his reasoning powers to evaluate any unexpected situation, decide what corrective action is required, and then initiate that action.

It is assumed that man's primary function in clo- sure and docking is one of control rather than one of direct actuation. That is, man will not be expected to pull the two vehicles together and then bolt them to one another. Rather, he will view the docking operation, evaluate whether it is going according to plan, decide what actions are necessary, and then initiate the proper action through a remote control system. For the more complex closure and docking systems where the opera- tions are relatively discrete, man can check to see that particular operations are complete and then initiate the next phase. Perhaps the most complex operation for man will be the capture of uncooperative vehicles. It is inthis situation that the majority of unexpected events willoccur.

Man introduces certain additional requirements on both the space vehicle and on the closure and docking simulator. The first of these is the need to present man with data on the status of the closure and dooking opera- tion. The primary item of this display will probably be a view of the operation, which could be through a window, periscope, or television system. A performance param- eter of the docking mechanism might then be the ease with which the closure and docking operation can be completely viewed from a single point. While the lack of such a single observation point does not affect the docking mech- anism, it might affect overall design of the space vehicle. Other display data would indicate whether successive op- erations have or have not been completed. The perform- ance parameter would then be the ease with which the docking mechanism can be instrumented to display the proper information.

When man is aboard the space vehicle, one of the prime performance parameters is his safety. This re- quirement will show up in a number of ways. The ap- proach parameters will be at a lower level so that a malfunction will not compromise his safety. The msch- anism must be carefully evaluated to see if there is any possibility of its improper operation or its failure, causing damage to the man's environment or degrading his ability to return safely to earth.

The design of simulators will be complicated by the fact that man must be integrated in the test. This in- creases the likelihood of gross error in the operation of the simulator and thus more cars must be exercised in the design of limit stops and safety devices. The vari- ability of man from person to person and from day to day will increase the number of test runs that must be made to evaluate a particular closure and docking system when man is operating it.

Sensor Evaluation

Many proposed rendezvous systems use sensor equipment either by itself or in conjunction with manual viewing. One typical combination uses pilot viewing for angular data and uses radar to provide the range and range rate information which is difficult for the pilot to observe accurately. For effective simulation, the closure

and docking simulator must provide the capability tause. actual sensor hardware or accurately simulate its out- puts. If simple transfer functions of the sensor are suf- ficient then electronic simulation is quite possible, but if noise inputs, non-linearitisa, accuracy variations with target position, etc. must be included, then direct use of hardware equipment is the only practical method.

To provide this sensor evaluation capability, the simulator must have the weight handling capability to actually rotate and translate the equipment so that the same geometric relationships exist in the simulator as exist in actual space rendezvous. Also, the background conditions must be deaigned so that reflections from the enclosure surfaces do not affect the sensor operation. 8Caling becomes almost impossible for many sensor sim- ulations since in many cases the actual sensor is at the limit of the development capability and scaling would re- quire additional development effort. One example is radar range resolution and short range accuracy.

for rendezvous sensor operation: radar, infra-red and optical. The simulation must be capable of operation at all of these spectrums if general purpose capability is desired.

Closure and Dockina System Interaction

Two general forms of interaotion can occur: inter- action of the closure system with the docking system or vice versa, and interaction of the closure and docking system with other system6 in the space vehicle. An il- lustration of the second of these 1 the disturbances into the vehicle attitude control system from ths docking sya- tem as the two vehiclei come together. An allied point is the interaction of the separate attitude oontrol systems in - each of the two docking vehicles. A solution to this prob- d lem is to arrange to use the aensors of only one vehicle and part or all of the actuator# in both vehicles.

docking aystem can oocur because they will probably use different types of sensors and different methods for ac- celerating the vehicles. The critical period occurs when the docking devices are exerting enough force to disturb the closure system but are not fastened together well enough for the docking system to dominate. The problem is to have the two systems blended together so that they do not fight each other. The simulator must include in- teraction effects between the closure and docking sya- tems. Additionally, it may be highly desirable to be able to include, or simulate, the attitude control systems of the two vehicles.

Extensions of the Basic Capability

The primary study area for a simulator of thistype would be in the simulation of rendezvous between earth satellites. However, there are a number of other prob- lem areas which are similar. Landing of space vehicles on planets or moons can be considered a form of closure and docking. If the planet has a significant atmosphere, then this atmosphere could require a significant change in the simulation. Similarly, rendezvous, closure, and docking In space, far from the gravitational attraction of a planet, are not too different from earth orbital opera- tions. These situations should be kept in mind as pos- sible extensions to the basic capability of a closure and docking simulator.

Lhamics Analyses

docking maneuver involves study of the motion of two

J

Three main frequency spectrums have been proposed

Interactions between the closure system and the

-.J

A dynamics analysis of a closure system or of a

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vehicles in the presence of forces of Interaction andunder certain constraints. Objectives of and techniques for studying the closure system are similar to those for the docking system; thus only the docking system wil l be dis- cussed.

A docking dynamics study in its most general form L is a difficult mathematical exercise. The two rigid bod-

ies possess a total of twelve degrees of freedom before contact, and equations of constraint must be Introduced as successive points of ftxed attachment are made be- tween the docking vehicles. Use of Lagrange equations and generalized coordinates offers the most promising solution because the vehicle interactions will Introduce generalized forces associated with rigid body motion, structural bending, and variables describing motion of the individual docking devices involved. It la then a straight- forward matter to introduce control equations to the re- sulting set gf differential equationa in any given case, if a homing guidance or active vehicle control plays a sig- nificant role.

The value of dynamics analysis in determining the requirements for a closure and docking simulator is that studies of this type can establish the magnitude of peak accelerations, velocities, and displacements of the vari- ous components. These factors, in turn, can be used to size the components of the closure and docking slmula- tor. It should also be noted that the simulator, oncecon- structed, can provide the same information a s does the dynamics analysis. Thus they can be used to complement each other.

Summary of Requirements

Requirements for a closure and docking simulator are headed by the primary objectives: to be able to eval- uate closure schemes, docking devices, and their Inter- action.

Operational uses of design Investigations, human factors studies, training, and flight test planning and analysis become requirements l n part or completely.

and planetary landing is highly desirable, while the cap- ability to slmulate many of the apace missions -- such as maintenance, crew rotation, assembly, refueling, resup- ply, and salvage -- la a Zirm requirement.

Adaptability to the allied problems of rendezvous

A desirable goal Is to be able to evaluate any of the eight categories of docking mechanlsm concepts. The uncooperative concepts are particularly demanding in the design of a simulator in that the number of degrees of freedom and ranges of variables become so large.

The wide variatlon in approach parameters, par- ticularly for the uncooperative case, along with the range of sizes of vehicles and variation in guidance systemsand sensors, also indicates that a great deal of flexibility must be built into the sirnulator.

Performance and mechanlsm parameters affect sirnulator requirements by increasing the sophistication of the simulator, but will not strongly affect the range of magnitude of the variables involved.

Including man in the sirnulator will require a con- %rol station withdisplaysand control handles and could

mean the need for direct vlewing of the closure anddock- ing operation. It also implies the need for safety devices and for repetitive runs.

The simulator should provide for those actual

rendezvous sensors with significant noise inputs, non- linearities, and unproken characteristics.

From dynamics analyses will come definitions of the number of degrees of freedom that will be needed as well as the acceleration, velocity, force, and moment specifications on the simulator.

The remaining requirements are the usual ones of minimum cost, convenience of operation, data gathering capability, easy repetition of runs, low maintenance, and simple calibration.

Possible Simulators

Many forms of simulators have been constructed in the past and even more have been conceived. Since it would require too extensive a study, no attempt has been made to collect data on all of these Simulators. The ap- proach 18 rather to decide on classes of simulators and more particularly on the classes of simulators that have applicability to the closure and docking problem; and then to examine the characteristics of each class. The simu- lator concepts have been grouped as pure computing ma- chine studies, mechanical devices, and combination simulators.

Pure Computing Machine Studies

Employing pure computing machines to simulate closure and docking is perhaps the simplest and most straightforward technique. By pure computing machines are meant analog or digital computers with a minimum of auxiliary equipment. They are capable of simulating the problem from Its simplest aspect to the use of fixed base control stations as ancillary equipment. They are partic- ularly useful in reducing the number of concepts and equipment that would progress to more advanced formsof simulation. Use of fixed base simulators In conjunction with an analog computer is a well respected technique for evaluating pilot operator effectiveness.

Pure computing machines as tools to simulate clo- sure and docking have many advantages. It is possible to revise vehicle or approach parameters by the simple changing of a potentiometer. The computer is very flex- ible in that the system being simulated may be revised drastically or extended or simplified without extensive redesign of the simulator. Many degrees of freedommay be included and thus very complex interactions can be studied. Computers are readily available, especially analog computers. They are amenable to repetitive op- eration, and all of the data is easy to obtain. The asso- ciated ftxed base simulators are relatively inexpensive and can be made so that they are easy to modify.

Pure machine studies for the uncooperative devices will not be as effective as those for cooperative systems. This is primarily due to the difficulty in simulating the random motion and many degrees of freedom associated with the target vehicle. Some aspects of the closure and docking procedure can be evaluated however, e.g., the target motion following its capture by the harpoon-type docking device. It should also be possible to obtain a gross evaluation of the effectiveness of the net-typedock- ing device.

There are two major difficulties associated with the use of pure computing machines as simulators. First is the difficulty in simulating the geometry of the docking device. How to represent the equations of a probe sliding in a conical drogue, or of B net wrapping itself around a target vehicle is a perplexing problem. It is probably

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possible to obtain this simulation of the geometry. but the amount of computing machinery required would be extensive. The second major drawback is the difficulty of introducing actual sensors into the simulation. Some sensors are relatively simple to simulate and it is even possible to introduce something similar to their noise spectrum into the problem; but how. for instance, could the effect of what a man sees as he looks out the window be introduced? An obvious answer might be to use closed circuit television and servo driven models. But this is f a r removed from the idea of pure computing machine simulators.

Mechanical Devices

The value of simple mechanical testing devices is well known and they are widely used. Evaluation methods using such devices are particularly suitable for testing components of a complex system. Furthermore, a little imagination, coupled with suitable instrumentation, can often permit the testing of complex systems without pro- viding a full-scale dynamic simulation. An example of a mechanical test device is shown in Figure 3. The device shown consists of a tube which may be dropped to put it into free fall. Inside the tube, there are a net and an object representing an uncooperative target. After the tube is falling freely, the net is dropped, and then the target is pushed downward so that it catches up with the net before it reaches the bottom of the tube. This scheme could be used to evaluate the capture efficiency of a net, including its folding characteristics. Needless to say, the tube could be evacuated to avoid drag effects and in- strumentation could be provided. It might be necessary to fin the tube to keep it from tumbling.

. .

Figure 3 Net Capture Characteristics Test Device

"he major advantages of mechanical devices are that the geometry problems axe automatically taken care of and that sensors can be included. Disadvantages in- clude the high cost of a specialized simulator, its rela- tive inflexibility in gross and minor design changes, and the difficulty in obtaining a true zero gravity and a true zero viscous friction. It is also somewhat difficult to see how a man could be integrated into the above ex- ample. The time of testing would also tend to be short.

In brief, the pure computing machine and mechan- ical devices have complementary advantages and disad- vantages; where one is weak, the other is strong.

Combination Simulators

The above thmghb lead to consideration of combi- nation simulators, that would combine the advantages of pure machine computation, with its versatility, and the mechanical device, with its inherent solution of the geometry problem and adaptability to sensor inclusion. The approach is to servo power the mechanical device so that the computer can command the mechanical device to desired positions. It thus seems possible to design a simulator that wouid meet all the requirements stated above, with the poasible exception of cost.

This idea is not new and a number of simulators of this type have been conceived7 and constructed in the past. The Link trainer is an early example of this con- cept. Other examples would be the attitude tables used to support the gyros of flight control systems during closed loop development testing in conjunction with an analog computer. More recently the theatre concept has been proposed. where a closed circuit television system projects a scene in front of a pilot. The television cam- era is focused on a servo driven model of the target. The pilot's control actions are fed to an analog computer which solves the appropriate equations of motion and then feeds signals to the servos which position the model. The model's movements are projected in front of the pilot, who then can evaluate the results of his actions and so close the loop.

those of both pure machine computation and mechanical devices. The equations of motion can be readily modi-

The advantages of a combination simulator are

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fied to include revised dynamic characteristicsbr simu- lated vehicle thrust levels. It is possible to handle a large number of degrees of freedom in the computer and a reasonable number in the mechanical device. Many problems can be investigated with as few as three degrees of freedom in the mechanical device, although six is a more reasonable minimum. Complex interactions can be readily studied and the effects of sensor peculiarities can be evaluated as hardware versiona of the sensors can be included in simulation. The simulation is amenable to repeated operation and the accumulation of data is easy and rapid. Different dookhg devices can also be included in the simulation in their hardware forms and thus detailed data on stresses, deflections, etc. can be obtained. Man can readily be given a place in the simulator by using a fixed base control station, or at a somewhat higher price, he can be given a moving base control station.

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There are also disadvantages to this concept. The first of these ie the higher cost that goes with the greater complexity of the simulator. In addition, the mechanical part --once it has been constructed -- is somewhat inflex- ible with regard to major modifications. The mechanical and servo design would be performed for a @van set of requlrements in terms of accelerations, velocities, dis- placements, forces, and momenta. Once the system is built, these quantities are diffloult to change. A major problem is how to overcome the effects of gravity and friction. They can seriously complicata the servo de- sign problem. Fortunately, there is little need for a wide bandpass in the servo systems --the whole process of closure and docking is expected to take place rather slowly.

Discussion

The three classes of oossible simulators are pure computing machines, mechkical devices, and

prPocura% 7

I 'I . combination simulators. The pure computing machine is strong in the areas of flexibility, complexity, use, and ease of modification, while it is weak in geometry simu- lation and inclusion of hardware sensors. The mechani- cal devices are strong in the geometry and their ability b include hardware sensors and docking devices, while

h e y are relatively Inflexible and they offer difficulty in obtaining zero gravity and low friction. The combina- tion simulators are strong in computer flexibility, prob- lem complexity, use, ease of modification of the computer, geometry and ability to include hardware components. They are weak with regard to relative in- flexibility of the mechanical components and cost of the mechanical systems.

Advocated 8olution

If it is important to satisfy most of the require- ments, then the best approach is a combination simulator using an analog or digital computer in conjunction with a servo positioned mechanical system. This Bystem can be designed to meet the primary requirement of evalua- tion of closure schemes, docking devices, and their in- teraction. It c w be used in all operational modes that have been suggested. It fa adaptable to simulation of rendezvous or planetary landing. as well as many of the suggested space missions. If the mechanical design is made sufficiently complex, all eight categories of dock- ing mechanism concepts can be simulated. The approach and vehicle parameters that affect the computer can be readily changed while those that affect the mechanical design must be carefully determined before construction is started. The inclusion of man just increases the sophistication of the mechanical design. The operational problems are not difficult with this form of simulator.

Major disadvantages are the possibly extreme com- plexity of the mechanical system and the high cost of this system. These disadvantages are most important for the uncooperative systems and if the degree of fidelity is to be high. However, they can be minimized if the ob- jectives of a particular simulator are well defined and are not too ambitious. These disadvantages merely point up the impracticability of a universal closure and docking simulator.

what is to be simulated and what this means to the msch- anical design. The large increase in cost and mechani- cal complexity comes with an increase in the number of vehicles being considered. As the problem of closure and docking is a relative one, one of the vehicles can be fixed in the simulator, but the others must be servo driven. Keeping track of where the earth is with respect to the fixed head may be of interest, but this need only be done in the computer. In general, each servo driven head (simulated vehicle) will have six degrees of free- dom. Thus the effect of simulating a closure and dock- ing problem where four vehicles are involved can be readily visualized. In addition, as the number of ve- hicles goes up, so does the size of the simulator, since there must be space for each of the moving heads to op- erate.

One Form of the Advocated Solution

It is very important to have a good definition of

To add substance to the advocated solution, one Lform of it will be discussed in detail. Figure 4 shows

an artist',$ concept of a closure and docking simulator. It consists of a large room, at one end of which is a fixed head representing one of the two rendezvousing ve- hicles. The other vehicle is represented by a moveable head. The moveable head has six degrees of freedom,

three in translation and three in rotation. All of these motions are servo driven to follow up position commands from the analog computer shown just outside the main room.

Two alternate control stations are shown. The one to the left is used for those cases where the pilot is to use displays derived from rendezvous sensors. This capsule is fitted out to look like the inside of a space vehicle so that the pilot will havo a better frame of reference in which to work.8 There are many displays other than the closure and docking displays, which are located at the right and of the capsule. The capsule is also fitted with control handles for the control of vehicle attitude as well as vehicle position. A television monitor, connected to a camera mounted on the moving head, ie shown in this capsule. The alternate control station shown in thefixsd head is fitted out similarly to the capsule control sta- tion, except that it ha6 a window through which the pilot can view the other space vehicle, which is simulated by the moveable head. This atation I s primarily for evalu- ation of "out the window" techniques of closure and dock- ing. Note also that if automatic closure is to be simu- lated, then neither control station need be used except for monitoring. The guidance Bystem can be simulated inthe analog computer or, alternately, hardware nensors a d o r guidance pomputers canbe integratedintothe sim- ulator.

Figure 4 Closure and Docking Simulator

The interior of the test chamber is lined with anechoic material to facilitate testing of radar equip- ment, since radar is one of the possible closure sensors. The room is also painted black to resemble the blackness of epace and to provide a reference condition for sxperi- mentation with various light levels. A possible docking device is shown on the moveable head. The bars are spring mounted to absorb the coupling shock and they mate with a set of overcenter hooks on the fixed head.

It is also possible to evaluate docking devices in this simulator. The outer part of the fixed head is mounted on a series of force detectors (load cells) which measure the forces and moments applied be- tween the vehicles. Bias voltages are used to can- cel out the steady effects of gravity. The forces and moments from the load cells are transmitted to the -

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analog computer, which solves the equations of relative motion between the two vehicles. The relative motion in- was in the process of being checked out. The important u formation is then sent to the electromechanical servos of the moveable head which, in turn, position the moveable head. In this manner, the mechanical simulator 'solves the mechanical aspects of the docking while the analog computer solves the equations of motion.

of this writing, the simulator had been assembled and

capabilities outlined above were designed into the facility, and it can easily be modified to include other capabilities.

Conclusions

The facility is equipped with standard data record- ing equipment to provide time histories of desirable vari- ables. It is also possible to record the critical functions at impact as well as peak forces and accelerations that may occur during a run. The availability of the analog computer permits processing the raw data during a run to form such quantities as variance of the cross range error during the run, total fuel consumed, or the number of times the yaw attitude control switch has commanded a left yaw.

By limiting the command signals to the moveable head, and by the provision of limit stope, the pilot does not have to worry about keeping the moveable head within the confines of the test area. The fixed head is mounted on a set of slide bars and is held in place by magnets; when the force between the moveable head and the fixed head exceeds a definite threshold, the fixed head will move back out of the way, allowing the moveable head to be stbpped by its limit stops. These safety devices are supplemented by a test director, who will have indications of where the moveable head is at all times; thus he will be able to stop a run before limits are entered.

by the addition of a small theatre and the use of closed circuit television. The television camera is fitted with a zoom lens and different sized models are used to give the effect of changing range. The camera would be gimballed to provide the elevation and out of plane motions. If de- sired, the model could also be gimballed to show changes in vehicle attitude. When the capsule control station is used, the television monitor would be used in the usual manner. When the fixed head control station is used, a half silvered mirror could be used so that the pilot. still looking out his window, would see the television monitor instead of the moveable head. One of the models could be the fixed head; or, when the fixed head control station is used, the monitor could be shut off and the man would look directly at the moveable head.

The simulator can also simulate space rendezvous

The problems of closure and docking cannot easily be separated and thus the use of two separate simulators will not be adequate. Rather, the interactions of the closure system with the docking mechanism are impor- tant and thus one simulator must be used to study both areas together. The requirements for a closure and docking simulator are many sided. Operational uses of the simulator, space missions, representative docking mechanism concepts, system parameters, effect of man, sensor evaluation, extensions of the basic capability, and a dynamics analysis should a11 be considered. Of the three general classes of simulators discussed, the com- bination of a computing machine and a mechanical simu- lator is most suitable for closure and docking simula- tion. A description of a combination simulator for cooperative vehicles, using a close contact scheme with a small capturearea,was presented. In addition, the development status of a closure and docking simulator at Denver, Colorado was briefly mentioned.

REFERENCES

1. William H. Phillios. M. J. QueUo. and James J. d - Adams: Langley Research Cent& Simulation Facil- ities For Manned Missions. NASA Langley Reseach Center, Langley Station, Hampton, Virginia, March 1969.

2. Astronautics. September 1962.

3. J. Heilfron and F. H. Kaufman: Rendezvous and Docking Techniaues. ARS Lunar Missions Meeting, July 1962.

4. space Vehicle Attachment and Connection. Technical Documentaw Report, ASD-TDR-62-950, Flight Dynamics L&&tory. Aeronautical Systems Division, Air Systems Command, Wright-Patterson A i r Force Base, Ohio, November 1962.

The closure and docking simulator is amenable to 5. George Alexander: "Marshall Intensifies Rendez- the simulation of landing problems. For this, a closed vouz Studies," Aviation Week and mace Technolow, circuit television system could be used, with the camera March 1962. mounted on the moveable head and the pilot viewing a monitor in the capsule control station. If horizontal 6. J. W. James: %ace Rendezvous Operations, Dock- landing is to be studied, a side wall could be decorated to represent a landing field with proper scaling: if vertical landing is to be evaluated, the end wall could be decorated to represent the landing surface. Obvious extensions are 7. R. 0. Lowery: "space Flight Simulators-Design the use of two or more surfaces to obtain scale changing and the use of a zoom lens on the television camera to ob- tain greater variation in range with one model size.

structed at Denver by the Martin Company. At the time

ing. Mechanisms and Seals. SSPDR-28-1-62, Martin Company, Denver, Colorado, January 1962.

Requirements and Concepts, I' Aerospace Engineer- & October 1960.

Manual and Automatic Rendezvous, " Astronautics, 8. G. H. Smith and C. D. Mc Phail: "Simulating

A simulator of this general type has been con- d April 1962. - -q