0521820022 cambridge university press planetary landers and entry probes jun 2007

It coversengineering aspects specific to such vehicles, such as landing systems,parachutes, planetary protection and entry shields, which are not usually treated in traditional spacecraf

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P L A N E T A R Y L A N D E R S A N D E N T R Y P R O B E S

This book provides a concise but broad overview of the engineering, science andflight history of planetary landers and atmospheric entry probes – vehiclesdesigned to explore the atmospheres and surfaces of other worlds It coversengineering aspects specific to such vehicles, such as landing systems,parachutes, planetary protection and entry shields, which are not usually treated

in traditional spacecraft engineering texts Examples are drawn from over thirtydifferent lander and entry probe designs that have been used for lunar andplanetary missions since the early 1960s The authors provide detailedillustrations of many vehicle designs from space programmes worldwide,and give basic information on their missions and payloads, irrespective of themission’s success or failure Several missions are discussed in more detail,

in order to demonstrate the broad range of the challenges involved and thesolutions implemented Planetary Landers and Entry Probes will form animportant reference for professionals, academic researchers and graduate studentsinvolved in planetary science, aerospace engineering and space missiondevelopment

Andrew Ball is a Postdoctoral Research Fellow at the Planetary and SpaceSciences Research Institute at The Open University, Milton Keynes, UK He is aFellow of the Royal Astronomical Society and the British Interplanetary Society

He has twelve years of experience on European planetary missions includingRosetta and Huygens

James Garry is a Postdoctoral Research Fellow in the School of EngineeringSciences at the University of Southampton, UK, and a Fellow of the RoyalAstronomical Society He has worked on ESA planetary missions for over tenyears and has illustrated several space-related books

Ralph Lorenz is a Scientist at the Johns Hopkins University Applied PhysicsLaboratory, USA He is a fellow of the Royal Astronomical Society and theBritish Interplanetary Society He has fifteen years of experience in NASA andESA spaceflight projects and has authored several space books.

Viktor Kerzhanovich is a Principal Member of Technical Staff of theMobility and Robotic Systems Section of the Autonomous Systems Division,NASA Jet Propulsion Laboratory, USA He was a participant in all Sovietplanetary Venus and Mars entry probe programmes

P L A N E T A R Y L A N D E R S A N D

E N T R Y P R O B E S

AN DREW J BAL LThe Open University

J A M E S R C G A R R YLeiden University

R A L P H D LO R E N ZJohns Hopkins University Applied Physics Laboratory

VIK TOR V K ERZ HAN OVICH

NASA Jet Propulsion Laboratory

CAMBRIDGE UNIVERSITY PRESS

Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo

Cambridge University Press

The Edinburgh Building, Cambridge CB2 8RU, UK

First published in print format

ISBN-13 978-0-521-82002-8

ISBN-13 978-0-511-28461-8

© A Ball, J Garry, R Lorenz and V Kerzhanovich 2007

2007

Information on this title: www.cambridge.org/9780521820028

This publication is in copyright Subject to statutory exception and to the provision of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press

ISBN-10 0-511-28461-6

ISBN-10 0-521-82002-2

Cambridge University Press has no responsibility for the persistence or accuracy of urls for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate

Published in the United States of America by Cambridge University Press, New York

www.cambridge.org

hardback

eBook (EBL)eBook (EBL)hardback

v

4.6 Additional components of a descent control system 45

9.7 Power and thermal control 103

17.6 Beagle 2 191

This book is intended as a concise but broad overview of the engineering, scienceand flight history of planetary landers and atmospheric probes Such vehiclesare subject to a wide range of design and operational issues that are notexperienced by ‘ordinary’ spacecraft such as Earth-orbiting satellites, or even byinterplanetary flyby or orbital craft Such issues deserve special attention, and wehave attempted to bring together in one place brief discussions of many of theseaspects, providing pointers to more detailed (but dispersed) coverage in the widerpublished literature This volume also draws heavily on real examples of landersand probes launched (or, at least, where the launch vehicle’s engines were startedwith that intention!)

More than 45 years have passed since the first vehicles of this type weredesigned To a certain extent some past missions, of which there are over onehundred, may now be considered irrelevant from a scientific point of view,outdated from an engineering point of view and perhaps mere footnotes in thebroader history of planetary exploration achievements However, we believe theyall have a place in the cultural and technical history of such endeavours, serving

to illustrate the evolving technical approaches and requirements as well as lessonslearned along the way They stand as testament to the efforts of those involved intheir conception and implementation

Part one of the book addresses the major engineering issues that are specific tothe vehicles considered, namely atmospheric entry probes, landers andpenetrators for other worlds For material common to spacecraft in general wewould refer the reader to other, existing sources Part II aims to collect together inone place some key information on previous vehicles and their missions, withreference to the main sources of more detailed information Part III covers some

of these missions in further detail as ‘case studies’

January, 2006

xi

The authors wish to thank Susan Francis and her colleagues at CambridgeUniversity Press for their encouragement and patience Many colleagues andcontacts have helped with specific queries, including: Aleksandr T Basilevsky,Jens Biele, Jacques Blamont, Peter Bond, Jim Burke, Alex Ellery, Bernard Foing,Aleksandr Gurshtein, Leonid Gurvits, Ari-Matti Harri, Mat Irvine, OlegKhavroshkin, Vladimir Kurt, Bernard Laub, Mikhail Ya Marov, SergueiMatrossov, Michel Menvielle, Don P Mitchell, Dave Northey, Colin T Pillinger,Sergei Pogrebenko, Jean-Pierre Pommereau, Lutz Richter, Andy Salmon, MarkSims, Oleg A Sorokhtin, Yuri A Surkov, Fred W Taylor, Stephan Ulamec,Paolo Ulivi, David Williams, Andrew Wilson, Ian P Wright, Hajime Yano, andOlga Zhdanovich We would also like to thank Professor John Zarnecki and thestaff at the Open University Library The diagrams that populate Part II weredrawn using information gleaned from a variety of sources While researchingspecific details for spacecraft, the authors were glad to receive help from thefollowing people: Charles Sobeck, Bernard Bienstock, Corby Waste, MartyTomasko, Marcie Smith, Dan Maas, Doug Lombardi, Debra Lueb, MartinTowner, Mark Leese, Steve Lingard and John Underwood

xii

List of acronyms and abbreviations

spectrometer

Spectrometer

xiii

CD Compact Disk

Transmission

Aerospace Centre)

xiv List of acronyms and abbreviations

EADS European Aeronautic, Defence and Space Company

List of acronyms and abbreviations xv

JPL Jet Propulsion Laboratory

xvi List of acronyms and abbreviations

MFEX Microrover Flight EXperiment

ð

Þ

List of acronyms and abbreviations xvii

NTS NEC Toshiba Space Systems

sur Mars

xviii List of acronyms and abbreviations

RAM Random Access Memory

Engineering

Loading

Second-ary Polymer Layer-Impregnated Technique

List of acronyms and abbreviations xix

SNR Signal-to-Noise Ratio

Machine-Building

xx List of acronyms and abbreviations

Part I Engineering issues specific to entry probes,

landers or penetrators

This part of the book is intended to act as a guide to the basic technologicalprinciples that are specific to landers, penetrators and atmospheric-entry probes,and to act as a pointer towards more detailed technical works The chapters of thispart aim to give the reader an overview of the problems and solutions associatedwith each sub-system/flight phase, without going into the minutiae

1 Mission goals and system engineering

Before journeying through the various specific engineering aspects, it is worthexamining two important subjects that have a bearing on many more specificactivities later on First we consider systems engineering as the means to integratethe diverse constraints on a project into a functioning whole We then look at thechoice of landing site for a mission, a decision often based on a combination ofscientific and technical criteria, and one that usually has a bearing on the design

of several sub-systems including thermal, power and communications

1.1 Systems engineeringEngineering has been frivolously but not inaptly defined as ‘the art of building forone dollar that which any damn fool can build for two’ Most technical problemshave solutions, if adequate resources are available Invariably, they are not, andthus skill and ingenuity are required to meet the goals of a project within theimposed constraints, or to achieve some optimum in performance

Systems engineering may be defined as

the art and science of developing an operable system capable of meeting mission ments within imposed constraints including (but not limited to) mass, cost and schedule.

require-The modern discipline of systems engineering owes itself to the development

of large projects, primarily in the USA, in the 1950s and 1960s when projects ofgrowing scale and complexity were undertaken Many of the tools and approa-ches derive from operational research, the quantitative analysis of performancedeveloped in the UK during World War II

Engineering up to that epoch had been confined to projects of sufficientlylimited complexity that a single individual or a team of engineers in a dominantdiscipline could develop and implement the vision of a project As systems

3

became more sophisticated, involving hundreds of subcontractors, the moreabstract art of managing the interfaces of many components became crucial initself.

A general feature is that of satisfying some set of requirements, usually in someoptimal manner To attain this optimal solution, or at least to satisfy as many aspossible of the imposed requirements, usually requires tradeoffs between indi-vidual elements or systems To mediate these tradeoffs requires an engineeringfamiliarity and literacy, if not outright talent, with all of the systems and engi-neering disciplines involved Spacecraft represent particularly broad challenges,

in that a wide range of disciplines is involved – communications, power, thermalcontrol, propulsion and so on Arguably, planetary probes are even more broad, inthat all the usual spacecraft disciplines are involved, plus several aspects related

to delivery to and operation in planetary environments, such as thermodynamics, soil mechanics and so on

aero-While engineers usually like to plough into technical detail as soon as their task

is defined, it is important to examine a broad range of options to meet the goals

As a simple example, a requirement might be to destroy a certain type of missilesilo This in turn requires the delivery of a certain overpressure onto the target.This could be achieved, for example, by the use of a massive nuclear warhead on

a big, dumb missile Or one might attain the same result with a much smallerwarhead, but delivered with precision, requiring a much more sophisti-cated guidance system Clearly, these are two very different, but equally valid,solutions

It is crucial that the requirements be articulated in a manner that adequatelycaptures the intent of the ‘customer’ To this end, it is usual that early designstudies are performed to scope out what is feasible These usually take the form

of an assessment study, followed by a Phase A study and, if selected, the missionproceeds to Phases B and C/D for development, launch and operation During theearly study phases a mission-analysis approach is used prior to the more detailedsystems engineering activity Mission analysis examines quantitatively the top-levelparameters of launch options, transfer trajectories and overall mass budget(propellant, platform and payload), without regard to the details of subsystems

In the case of a planetary probe, the usual mission is to deliver and service aninstrument payload for some particular length of time, where the services mayinclude the provision of power, a benign thermal environment, pointing andcommunications back to Earth

The details of the payload itself are likely to be simply assumed at the earlieststages, by similarity with previous missions Such broad resource requirements asdata rate/volume, power and mass will be defined for the payload as a whole.These allow the design of the engineering system to proceed, from selecting

4 Mission goals and system engineering

among a broad choice of architectures (e.g multiple small probes, or a singlemobile one) through the basic specification of the various subsystems.

The design and construction of the system then proceeds, usually in parallelwith the scientific payload (which is often, but not always, developed in insti-tutions other than that which leads the system development), perhaps requiringadaptation in response to revised mission objectives, cost constraints, etc.Changes to a design become progressively more difficult and expensive toimplement

1.1.1 The project teamThe development team will include a number of specialists dedicated to variousaspects of the project, throughout its development In many organizations,additional expertise will additionally be co-opted on particular occasions (e.g fordesign reviews, or particularly tight schedules)

The project will be led by a project manager, who must maintain the vision ofthe project throughout The project manager is the single individual whose effortsare identified with the success or otherwise of the project The job entails wide(rather than deep) technical expertise, in order to gauge the weight or validity ofthe opinions or reports of various subsystem engineers or others and to makeinterdisciplinary tradeoffs The job requires management skills, in that it is theefforts of the team and contractors that ultimately make things happen – areaswhere members of the team may variously need to be motivated, supported withadditional manpower, or fired Meetings may need to be held, or prevented fromdigressing too far And this demanding job requires political skill, to tread thecompromise path between constraints imposed on the project, and the capabilitiesrequired or desired of it

A broadly similar array of abilities, weighted somewhat towards the technicalexpertise, is required of the systems engineer, usually a nominal deputy to theproject manager A major job for the systems engineer is the resolution oftechnical tradeoffs as the project progresses Mass growth, for example, is atypical feature of a project development – mass can often be saved by usinglighter materials (e.g beryllium rather than aluminium), but at the cost of a longerconstruction schedule or higher development cost

A team of engineers devoted to various aspects of the project, from a handful tohundreds, will perform the detailed design, construction and testing The latter taskmay involve individuals dedicated to arranging the test facilities and the properverification of system performance Where industrial teams are involved, variousstaff may be needed to administer the contractual aspects Usually the amount ofdocumentation generated is such as to require staff dedicated to the maintenance of

records, especially once the project proceeds to a level termed ‘configurationcontrol’, wherein interfaces between various parts of the project are frozen andshould not be changed without an intensive, formal review process.

In addition to the hardware and software engineers involved in the probesystem itself, several other technical areas may be represented to a greater orlesser extent Operations engineers may be involved in the specification, design,build and operation of ground equipment needed to monitor or command thespacecraft, and handle the data it transmits There may be specialists in astro-dynamics or navigation Finally, usually held somewhat independently from therest of the team, are quality-assurance experts to verify that appropriate levels ofreliability and safety are built into the project, and that standards are beingfollowed

In scientific projects there will be a project scientist, a position not applicablefor applications such as communications satellites This individual is the liaisonbetween the scientific community and the project In addition to mediatingthe interface between providers of the scientific payload and the engineeringside of the project, the project scientist will also coordinate, for example, thegeneration or revision of environmental models that may drive the spacecraftdesign

The scientific community usually provides the instruments to a probe The leadscientist behind an instrument, the principal investigator (PI), will be the individualwho is responsible for the success of the investigation Usually this means pro-curing adequate equipment and support to analyse and interpret the data, as well asproviding the actual hardware and software An instrument essentially acts as amini project-within-a-project, with its own engineering team, project manager, etc.For the last decade or so, NASA has embraced so-called PI-led missions, underthe Discovery programme Here a scientist is the originator and authority(in theory) for the whole mission, guiding a team including agency and industrypartners, not just one experiment This PI-led approach has led to some highlyefficient missions (Discovery missions have typically cost around $300M, com-parable with the ESA’s ‘Medium’ missions) although there have also been somenotable failures, as with any programme The PI-led mission concept hasbeen extended to more expensive missions in the New Frontiers line, and forDiscovery-class missions in the Mars programme, called ‘Mars Scout’

A further class of mission deserves mention, namely the opment or technology-validation mission These are intended primarily todemonstrate and gain experience with a new technology, and as such may involve

technology-devel-a higher level of technictechnology-devel-al risk thtechnology-devel-an one might tolertechnology-devel-ate on technology-devel-a science-drivenmission Some missions (such as those under NASA’s New Millenium pro-gramme, notably the DS-2 penetrators) are exclusively driven by technology

6 Mission goals and system engineering

goals, with a minimal science payload (although often substantial science can beaccomplished even with only engineering sensors) In some other cases, thescience/technology borderline is very blurred – one example is the JapaneseHayabusa asteroid sample return: this mission offers a formidable scientificreturn, yet was originally termed MUSES-C (Mu-launched space-engineeringsatellite).

Whatever the political definitions and the origin of the mission requirements, itmust be recognized that there is both engineering challenge and science value inany spacecraft measurement performed in a planetary environment

A dynamic tension usually exists in a project, somewhat mediated by theproject scientist Principal investigators generally care only about their instru-ment, and realizing its maximum scientific return, regardless of the cost of thesystem needed to support it The project manager is usually confronted with analready overconstrained problem – a budget or schedule that may be inadequateand contractors who would prefer to deliver hardware as late as possible whileextorting as much money out of the project as possible One tempting way out is

to descope the mission, to reduce the requirements on, or expectations of, thescientific return Taken to the extreme, however, there is no point in building thesystem at all Or a project that runs too late may miss a launch window andtherefore never happen; a project that threatens to overrun its cost target by toofar may be cancelled So the project must steer a middle path, aided by judgementand experience as well as purely technical analysis – hence the definition ofsystems engineering as an art

1.2 Choice of landing siteTechnical constraints are likely to exist on both the delivery of the probe orlander, and on its long-term operation First we consider the more usual casewhere the probe is delivered from a hyperbolic approach trajectory, rather than aclosed orbit around the target

The astrodynamic aspects of arrival usually specify an arrival direction, whichcannot be changed without involving a large delivered-mass penalty The arrival

unperturbed by the target’s gravity, is usually considered) are hence fixed.Usually the arrival time can be adjusted somewhat, which may allow the long-itude of the asymptote to be selected for sites of particular interest, or to ensurethe landing site is visible from a specific ground station Occasionally this is fixedtoo, as in the case of Luna 9 where the descent systems would not permit anyhorizontal velocity component – the arrival asymptote would only be vertical at

The target body is often viewed in the planning process from this incoming

vector is often called the ‘B-plane’ The target point may be specified by twoparameters The most important is often called the ‘impact parameter’, the dis-tance in the B-plane between the centre and the target point For a given targetbody radius (either the surface radius, or sometimes an arbitrary ‘entry interface’above which aerodynamic effects can be ignored) a given impact parameter willcorrespond to a flight-path angle, the angle between the spacecraft trajectory andthe local horizon at that altitude This may often be termed an entry angle.The entry angle is usually limited to a narrow range because of the aero-thermodynamics of entry Too high an angle (too steep) – corresponding to asmall impact parameter, an entry point close to the centre of the target body – andthe peak heating rate, or the peak deceleration loads, may be too high Too

z

x

yθ

0

–1

-1

3 σ error ellipse from state uncertainty

3 σ error ellipse with ejection uncertainty

3 σ error ellipse with wind

Tilt in attitude (deg.)

11.4 11.2

90.4

3 σ error ellipse from state uncertainty

3 σ error ellipse with ejection uncertainty

3 σ error ellipse with wind

Figure 1.1 Top: cartoon illustrating lander-delivery uncertainty arising from uncertainties in the state vector at deployment Bottom: attitude and landing- error ellipses for Beagle 2 (adapted from Bauske, 2004 ).

8 Mission goals and system engineering

shallow an angle may result in a large total heat load; in the limiting case of alarge impact parameter, the vehicle may not be adequately decelerated or maymiss the target altogether.

The entry protection performance may also introduce constraints other thansimple entry angle For the extremely challenging case of entry into Jupiter’s

aiming at the receding edge of Jupiter (i.e the evening terminator, if coming from

most significant amelioration

The second parameter is the angle relative to the target body equator fically where the equatorial plane crosses the B-plane) of the impact parameter A

central meridian as seen from the incoming vector

Figure 1.2 View of Titan from the arrival asymptote of the Huygens probe, with overlapping annuli reflecting the constraints on entry angle (light grey) and solar elevation (darker grey) Of the choice of two target locations where the regions overlap (A and B), only A accommodates the probe’s delivery ellipse.

Other constraints include the communication geometry – if a delivery vehicle

is being used as a relay spacecraft, it may be that there are external constraints onthe relay’s subsequent trajectory (such as a tour around the Saturnian system)which specify its target point in the B-plane Targeting the flyby spacecraft on theopposite side of the body from the entry probe may limit the duration of thecommunication window Current NASA missions after the Mars mission failures

in 1999 now require mission-critical events to occur while in communicationwith the Earth: thus entry and landing must occur on the Earth-facing hemisphere

of their target body

Another constraint is solar The entry may need solar illumination for attitudedetermination by a Sun sensor (or no illumination to allow determination by starsensor!), or a certain amount of illumination at the landing site for the hoursfollowing landing to recharge batteries These aspects may influence the arrivaltime and/or the B-plane angle

So far, the considerations invoked have been purely technical Scientificconsiderations may also apply Optical sensing, either of atmospheric properties,

or surface imaging, may place constraints on the Sun angle during entry anddescent Altitude goals for science measurements may also drive the entry angle(since this determines the altitude at which the incoming vehicle has beendecelerated to parachute-deployment altitude where entry protection – whichusually interferes with scientific measurements – can be jettisoned)

The entry location (and therefore ‘landing site’) for the Huygens probe waslargely determined by the considerations described so far (at the time the missionwas designed there was no information on the surface anyway) The combination

of the incoming asymptote direction and the entry angle defined an annulus oflocations admissible to the entry system The Sun angles required for scientificmeasurements of light scattering in the atmosphere, and desired shadowing ofsurface features defined another annulus These two annuli intersected in tworegions, with the choice between them being made partly on communicationsgrounds

There may be scientific desires and technical constraints on latitude Latitudemay be directly associated with communication geometry and/or (e.g in the case

of Jupiter), entry speed For Mars landers in particular, the insolation as afunction of latitude and season is a crucial consideration, both for temperaturecontrol and for solar power Many Mars lander missions are restricted to ‘tropical’landing sites in order to secure enough power

So far, the planet has been considered only as a featureless geometric sphere.There may be scientific grounds for selecting a particular landing point, on thebasis of geological features of interest (or sites with particular geochemistry such

as polar ice or hydrated minerals), and, depending on the project specification,

10 Mission goals and system engineering

these may be the overriding factor (driving even the interplanetary deliverytrajectory).

A subtle geographical effect applies on Mars, where there is extreme graphical variation – of order one atmospheric scale height Thus selecting ahigh-altitude landing site would require either a larger parachute (to limit descentrate in the thinner atmosphere), or require that the landing system tolerate higherimpact speed

topo-The landing sites for the Mars Exploration Rover missions (MER-A and -B)

scientific interest and technical risk came to the fore As with the Viking landingsite selection, the most scientifically interesting regions are not the featurelessplains preferred by spacecraft engineers The situation is complicated by theincomplete and imperfect knowledge of the landing environment

One constraint was that the area must have 20% coverage or less by rocks0.5 m across or larger that could tear the airbags at landing Rock distributionscan be estimated from radar techniques, together with geological context fromMars orbit (while rocks cannot be seen directly, geological structures can – rocksare unlikely to be present on sand dunes, for example), and thermal inertia data.Although there are no direct wind measurements near the surface in theseareas, models of Martian winds are reaching reasonable levels of fidelity, andthese models are being used to predict the windspeeds at the candidate landingsites Winds of course vary with season (e.g the Martian dust-storm season,

The Pathfinder lander, for example, landed before dawn, at 3 a.m local solartime, when the atmosphere was at its most stable The MER had an imperative(following on in turn from the Mars Polar Lander failure) that it must be incommunication with the Earth during its descent and landing This requires that itland in the afternoon instead – when winds are strongest! Here, perversely, apolitically driven engineering uncertainty introduces a deterministic (i.e certain!)increase in risk

On Earth, a handful of landing sites are used The US manned missions in the1960s and 1970s relied upon water landing; the mechanical properties of theocean are well understood and uniform over some 60% of the globe, with the onlyvariable being uncertain winds and sea state Other missions (unmanned capsulesand Russian manned missions) have landed on large flat areas, notably theKazakhstan steppe, and Utah was used for the Genesis solar wind and Stardustcomet-sample-return missions A significant factor in the choice of landing is theaccuracy with which the capsule can be targeted (oceans may be less desirablelanding sites, but they are hard to miss) and whether a particularly rapid retrieval(e.g for frozen comet samples) is required

A final possibility, and a good example of systems engineering in action, is toavoid the landing problem altogether by retrieving the payload during its para-chute descent This approach was used for the film capsules in US Discovererreconnaissance satellites, which were recovered by snaring the parachute lineswith a frame suspended from a transport aircraft The choice of this system mayhave been dictated partly by strategic concerns, rather than an EDL optimizationfrom purely mass–performance considerations, but it nevertheless remains anoption.

One way of reducing the importance of the landing-site selection problem is toprovide mobility This may pertain both to the landing itself, and operations afterlanding

In terms of landing, the scale of feature that poses a hazard is comparable withthat of the vehicle itself – landing on a half-metre sharp rock could dent a structure,puncture an airbag, or cause a tilt on a lander that might cripple its ability togenerate power or communicate with Earth

However, such small features cannot currently be imaged from orbit, nor can

an unguided entry and descent system be assured of missing it Such precisionlanding requires closed-loop control during the descent Such guidance mayrequire imaging or other sensing (a simple form of on-board image analysis wasperformed on the Mars Exploration Rovers, in order to determine the sidewaysdrift due to winds, and apply a rocket impulse just prior to landing in order tosuppress the sideways motion and the resultant loads on the airbag landingsystem) A technique that has been explored for Mars precision landing, andlanding on small bodies such as asteroids, is LIDAR or laser ranging This is able

to produce a local high-resolution topographic map around the immediate landingarea The actuation involved in such precision landing may involve smallthrusters, or conceivably steerable parachutes

Clearly, if the goal is to analyse a rock with some instrument, the designer mayequip a lander with a long, powerful arm that can bring the rock to the instrument

Or, the instrument may be brought to the rock, perhaps on a mobile vehicle (see

Whether an arm is used, or a rover, their positions need to be controlled, andtheir positions (and that of the rock) need to be known In general, ground-basedanalysis of image data is used for these tasks However, goniometry (the mea-surement of arm position by recording joint rotations) and dead-reckoning(measuring the number of turns of a rover wheel) can permit some on-boardautonomy The latter suffers, especially on steep slopes of loose material, fromwheel slippage – the wheel may turn without moving the vehicle forward.Closed-loop navigation using on-board analysis of image data is beginning tofind a role here Additionally, crude hazard identification can be performed with

12 Mission goals and system engineering

structured light – such as a pattern of laser lines on the scene, which allows theready identification of rocks or holes.

are offset from a fixed inertial frame, and known to varying levels of accuracy.The landing site of an entry craft will vary as a result of uncertainties in thelocation of the combined spacecraft prior to release, and the path taken bythe landing craft after ejection This is illustrated in the lower two charts of

entry craft at a nominal altitude of 100 km varies as a result of different factors Inthe right-hand plot, the landing footprint of the craft takes on an elliptical form,with the major axis of this error ellipse being dictated by uncertainties in ejectionspeed, cruise time from ejection to impact, and variability in aeroshell drag,amongst other effects

A similar (although numerically different) problem confronted the Huygens

shows Titan’s globe as seen from an incoming asymptote – in this case centred at

this locus is denoted by the intermediate grey ring The intersection of the two isshown by the dark grey areas – thus there is a choice of two target regions.The delivery ellipse is shown centred on the two target locations In general, theellipse is narrow, one direction (often that associated with time of arrival) beingtypically larger than the other This corresponds somewhat to the uncertainty of thespacecraft or target ephemeris and thus here the long axis of the ellipse is E–W

It can be seen that only one of the two sites (A) is acceptable At (B) thedelivery uncertainty is such as to allow an unacceptable probability that the entryangle corridor would be violated At (A) the long axis of the delivery ellipse isaligned with the long axis of the acceptable entry region and thus success isassured

2 Accommodation, launch, cruise and arrival from

orbit or interplanetary trajectory

The challenges involved in designing optimal trajectories for planetary landers oratmospheric probes are shared by many other types of spacecraft projects.Spacecraft, at least for the foreseeable future, have to be launched from theEarth’s surface and then placed on a path that intersects the orbit of the targetbody How this is achieved depends on the mission of the spacecraft and itsassociated cost and design details

2.1 The launch environmentSpacecraft have been delivered to space on a wide variety of launchers, all of whichsubject their payloads to different acoustic, dynamic and thermal regimes Theseparameters vary with the size and nature of the launcher, yet the complex launchvehicle industry often makes it difficult to isolate a preferred launcher type for a

with data taken from their user manuals; the launcher market currently has over adozen vehicles capable of lifting interplanetary payloads Costs are not listed asmany of the vehicles offer dual manifest capability, or other partial-occupancyaccommodation (such as Ariane’s ASAP) which can make heavy launchers andtheir capability available to even modestly funded missions

Of particular interest are the mass values shown for the parameter C3 Thisquantity is the square of the hyperbolic escape speed; the speed an object wouldhave upon leaving the influence of a gravitating body Paths with a C3 greater

transfers to the outer planets Realistic missions to such distant targets wouldendeavour to use more energy-efficient routes by the use of gravitational assists,

14

Missions to the Moon generally, by definition, have negative C3 values

that for greater speed changes, and larger C3 values, heavier and more expensivelaunchers are needed to deliver a given mass; this is shown concretely in therocket equation described below This is the first major tradeoff in a mission’sdesign process as money is often the key finite resource in a mission, and so it isnecessary to consider how a spacecraft or its mission could be resized so that acheaper, and usually less flexible or less powerful, launcher can be used

2.2 Transfer-trajectory choiceRocket propulsion is the sole present means of producing the large speed changesassociated with interplanetary travel Although the technologies used in gen-

depends on the fractional amount of mass that is ejected and the rocket’s

so the preceding definition has its value divided by the gravitational acceleration,

Table 2.1 Parameters of some current launch vehicles, a ‘/’ is used where the

value is not known from official sources

Launcher

Peak axial

occupancy via ASAP Delta IV

Heavy

launch capable Long

1V ¼ g0Ispln mi



ð2:1Þ

spacecraft into interplanetary trajectories range from 300 s to 340 s for liquid

systems such as those used on the Deep Space 1 and Hayabusa craft These twoclasses of rocket engine, chemical and electrical, have very different operatingprofiles Chemical motors and engines are easily scaled to give very high thrustswith little impact on other spacecraft systems such as power generation To give acertain impulse a chemical engine therefore needs to burn for a relatively briefperiod, unlike an electric propulsion system Drives in this category, broadly,

high thrusts without the need for commensurately large and heavy power-raising

operated for much of the journey to the target body

2.2.1 Transfer trajectories: impulsiveThe high thrust levels delivered by chemical propulsion systems result inmanoeuvres that last for a short fraction of the total transfer-trajectory duration.The burns needed at the start and end of the transfer path can be treated asbeing impulsive and of infinitesimal duration With the exception of aerocapture

or impact missions, the spacecraft executes at least two manoeuvres after beinglaunched To make best use of the Earth’s orbital speed around the Sun, thefirst burn results in the craft leaving the Earth’s orbit at a tangent, and movingalong a trajectory that is part of a conic section That trajectory is chosen so

velocity vector and the target planet’s orbit This arrangement is shown in

The hodograph for the arrival point of trajectory A is shown to the right of

with the transfer duration and required velocity changes fixed by the major axis

2

Such as mono-methyl hydrazine and nitrogen tetroxide, as used on the restartable Fregat transfer stage.

16 Accommodation, launch, cruise and arrival from interplanetary trajectory