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New frontiers for space tribology
New frontiers for space tribology D. Wyn-Roberts* The development of space tribology over the last 25 years has brought difficult challenges to the European Space Agency (ESA). These will be covered in this article as well as an examination of the long-term space programme envisaged.
Keywords: lubrication, space tribology, molybdenum disulphide, extreme
environments, European Space Agency Introduction Space tribology refers to that specialized discipline dealing with friction and wear phenomena related to spacecraft components. The problems arising are due, in particular, to the vacuum environment and lack of gravity. In fact, a foretaste of the problems to be encountered occurred before the advent of the space programmes when motor failures started to occur in high altitude aircraft. These turned out to be due to rapid wear of graphite motor brushes, since graphite loses its lubricating properties at low pressure, specifically due to loss of water vapour content ~, 2 The importance of space tribology has been recognized and supported by the European Space Agency (ESA) since its inception 25 years ago. As a result the subject has reached a high degree of maturity in Europe. Nevertheless Europe is embarking on a new and vigorous space programme which is bringing about new challenges to space tribology. This paper will examine those challenges and the ways in which they can be met.
ESA long-term space programme
have complex sampling devices which will have to operate in extreme environments. This aspect will be discussed again later. Next we have the European Space Station programme known as Columbus. It presently consists of three elements. These will be firstly an attached module which will be a manned laboratory fixed to the main United States Space Station 'Freedom' structure, secondly a free-flying unmanned laboratory called the FFL and thirdly an unmanned polar platform, which, as its name implies, will be in polar orbit. Of critical importance to tribology is the impact of the so-called 'micro-gravity' environment that has to be achieved, particularly within the FFL. This is an extremely low vibration environment required to allow the growth of fault-free crystals for material sciences and requires very smooth vibration-free operation of mechanical devices. The FFL can be seen in the foreground of Fig. 1. In the area of telecommunications, feasibility studies are presently being performed on the Data Relay Satellite (DRS) to be used for communicating with the space station, and Sat-2, which is part of a
There are many current and future projects under investigation by ESA and it would take too long to describe them all in detail. Instead, the core items will be mentioned here and especially those that will impact tribological development. A listing of the currently planned key projects is given in Table 1. Generally the Science Programme tends to present the most technological challenges. This programme is organized into four areas of scientific discipline known as 'cornerstones'. At the time of writing all are in an early phase of development. The earliest planned launch date is 1995 for the Soho and Cluster missions. Specific tribological problems arise with the mechanically complex instrumentation of the Soho spacecraft which will additionally have strict limits on the disturbances that are allowed to propagate into the spacecraft structure. Also the planetary explorers will * ESTEC, Noordwijk, The Netherlands.
TRIBOLOGY INTERNATIONAL
Fig1 Artist's impression of the Columbus element 'MTFF' in orbit 0301-679X/90/040149-7 (~) 1990 Butterworth-Heinemann Ltd
149
D. Wyn-Robertsmnew frontiers for space tribology development and experimentation programme. Both of these are large spacecraft with complex mechanical systems foreseen for launch by the mid-1990s. A tribological driver in these programmes will again be vibration aspects since it is intended to utilize interorbital communication using laser beams. The laserpointing systems will require very smooth drives in order to avoid jitter and indeed any mechanical systems on board will have to be virtually vibration free in operation, over a long lifetime of up to ten years. Regarding the earth observation programme, the Earth Resources Satellite (ERS) is now nearing completion. Also, a second generation of meteorological satellite, Meteosat-2, is being studied, with a mechanically complex payload which promises to pose further tribological problems. Finally the Ariane V launcher and the Hermes Space Plane are being developed as transportation support to the above programmes. The bearings and seals for the launcher turbo-pumps are critical items which, although operating for only a short time, do so at very high revolution rate and at extremely low temperature. In the case of Hermes the high temperatures developed during re-entry will necessitate careful attention to tribological aspects of the control surface actuators. More details of all the above programmes can be found in Refs 3 and 4.
Fig 2 Computer-generated view of the 1SO spacecraft in orbit
The challenge to space tribology There will be a definite impact on the direction of space tribology development arising from the programmes just discussed, and their requirements are being taken into account in planning future research and development work. In general the major new requirements can be summarized as follows: • • • • •
extended lubricant lifetimes lubrication under intermittent operating conditions increased reliability lower torque and torque noise characteristics extreme temperature operation.
Some specific examples will now be considered. A particularly interesting area arising from the requirements of the science programme is that of cryotribology, which is tribology at cryogenic temperatures. This problem has been encountered for the scientific instruments of the Infra-red Space Observatory (ISO), a spacecraft which is now in an advanced state of development (see Fig 2). In the future similar problems will be encountered for the Far Infra-Red and Submillimetre Telescope (FIRST, see Table 1). Instruments situated at the focal planes of these space telescopes utilize focusing and shutter mechanisms and have to operate at temperatures down to 4 K. In the case of ISO the focal plane camera incorporates a filter change mechanism with gear drive and shaftmounted ball bearings. Lubrication is by means of an MoSz film and the gears are made from titanium alloy with titanium carbide surfaces. A view of this mechanism is given in Fig 3. In order to qualify the lubricant, special chambers for vibration at cryogenic temperature were developed at the Aerospatiale company in France 5. 150
Table 1 ESA long-term project programme Science Solar, terrestrial physics
Soho and Cluster spacecraft
High throughput X-ray spectroscopy
XMM spacecraft
Sub-millimetre astronomy
FIRST mission
Planetary exploration
Cassini and Rosetta
Space station and transportation European space station Columbus Advanced launcher
Ariane V
Spaceplane
Hermes
Hermes robotic arm
HERA
Telecommunications Data relay satellite
DRS
Payload and spacecraft development and experimentation programme Earth observations Earth resources satellite Second generation meteorology
Sat-2
ERS
Meteosat 2 April 90 Vol 23 No 2
D. Wyn-Roberts--new frontiers for space tribology for performing servicing and other tasks in space. Particularly challenging is the lubrication of the joint gearboxes and end effectors. These devices will have to operate reliably and smoothly and yet will be fully exposed to the space environment. In the case of the telecommunications programme the DRS spacecraft under development will have large Table 2 European antenna pointing mechanisms (APMs)
Fig3 Cryogenic mechanisms of the ISO camera (courtesy Aerospatiale/Cannes-mechanisms and CEA/ Saclay, France-overaU development) Turning now to the Columbus programme we can see major departures from the usual spacecraft with which we have been familiar in Europe up till now. The first difference is a required lifetime of 30 years compared with a maximum of ten years for automatic spacecraft. Secondly we have requirements for reusability and servicing. It will be possible to access the Columbus elements and they can, therefore, be refurbished or serviced for subsequent reuse. Parts of the spacecraft could be exposed to space for long periods, for example the docking mechanisms on the FFL, where they will not only be fully exposed to vacuum and weightlessness but also to radiation and atomic oxygen bombardment. These long-term effects will need to be more closely studied in the future. For example, the exposure of MoS2 lubricant to atomic oxygen can lead to the formation of Mo203 which, far from being a lubricant, is an abrasive 6'7. During the docking process, electrical and fluid connectors will be mated and during the vehicle lifetime many mating/de-mating cycles will occur with relatively long space exposure in between. The tribology of reusable electrical pin connectors, fluid connectors and seals therefore needs to be investigated under these conditions. Work has already started in Europe on this aspect. Another area which will prove to be tribologically challenging is that of the solar arrays for the FFL and polar platform. These will be the largest arrays yet built in Europe with nominal power capacities of around 15 kW and 12 kW respectively. In the case of the MTFF the arrays will be extended and retracted at six-monthly intervals with the associated problem of extension mechanism lubrication. For the polar platform the arrays will be sun-following with a rotation rate of 6 revs day -1. This implies the use of a rotating power transfer assembly which may lead to the requirements for special large-diameter bearings and high-power slip rings. In the longer term the development and utilization of robotics will require careful attention regarding tribological aspects. A robotic manipulator arm will be eventually utilized in the Columbus programme TRIBOLOGY INTERNATIONAL
Acronym
Company responsible
Lubrication/ comments
Olympus APM
BAe
Vapour-deposited MoS2 film for gear, BP135 oil for encoder tracks. Now operating in orbit
IOC APM
Matra
MoS2 dry-lubricated ball bearings. Will fly on an inter-orbit communication experiment
IOC APM
Marconi
Braycote 601 for needle bearings. Chosen as back-up. Not scheduled for flight
Syracuse APM
Alcatel
MoS2 film for ball bearings. Steel versus debris gears. Is due to fly on spacecraft
SOFA
Aerospatiale
Not necessary. Flex pivots used. Development model.
Medium range APM
Aerospatiale
Braycote 601 low vapour pressure grease for needle bearings. MoS2 film for screw jack. Development model
HPM
Teldix
Ball bearings with TiC surfacehardened balls and Fomblin Z25 low vapour pressure oil. Chosen for flight model experiment
Q-model APM
Dornier
Ball bearings with TiC surface balls and MoS=-coated races and cages. Development model
MPA-PA
Matra
TiC-hardened and MoS2-1ubricated parts. Development model 151
D. Wyn-Roberts--new frontiers for space tribology deployable antennas which need to point and track. The pointing mechanisms may need to be of special design and will have their own tribological problems. Several designs have already been studied in Europe together with their lubrication systems 9. Later developments will be based on the experience gained from these. Table 2 lists them, together with their lubrication systems. For Sat-2 a special development of a 5 m diameter unfurlable antenna is planned. Such large structures lead, in particular, to problems of qualification testing of the lubrication system under realistic operating conditions. Regarding the earth observation programme a particular problem is the use of large synthetic aperture radar (SAR) antennas. The SAR antenna of ERS, to be launched in 1990, is 11 m long. This can be seen in Fig 4. Second generation versions, eg to be used on the polar platform, are likely to be even larger. Although the SAR is presently a one-shot device the tribology aspects are, nevertheless, challenging. Low friction hinges are required and ground-testing loads have also to be catered for. In the present case, ball bearings were chosen with dry film lubricant (because of the low deployment speed). These bearings are, however, susceptible to vibration damage during launch. Furthermore dry film lubricants have poor performance in air and extreme care in handling must therefore be exercised during the ground-test phase, where, in addition, much higher loads can be encountered than in orbit. The optimization of these various aspects including the approach required for qualifying these large structures which can no longer be fully tested on the ground (eg due to size restriction of available vacuum chambers) is not yet fully solved.
Meeting the challenge Having seen the types of problem that are approaching we will now consider means of solving them. First a discussion of the generic approach is given, with some specific examples at the end. There are essentially three ways to approach tribology problems arising from projects. These are illustrated in the flow diagram of Fig 5. The 'thick arrow' route commencing at path (1) illustrates the most common way in which these problems are solved. Existing techniques should always be considered first. This is not a very glamorous situation and it is often very tempting to develop new techniques for new applications, whereas the existing techniques may be perfectly adequate. They will, in general, also be the most reliable. The thin, solid lead-film lubricant, for example, has been used for many applications in space, virtually without change, over at least the past 15 years. It is a very successful space lubricant and its use is expected to continue for the foreseeable future. In fact the origin of the use of lead-film lubricant occurred before the advent of the space programme since it was used for the bearings of X-ray tubes, which operated in a vacuum. The early interest of space engineers in such a dry lubricant was due to the fact that oils and greases would evaporate in a vacuum and also cause contamination of, for example, sensitive optical surfaces. However, nowadays low vapour pressure oils and greases are available and can be used for certain space applications. The major space lubrication techniques currently in use can be listed as follows: • dry-film lubrication with lead or MoS2, mainly for ball bearings, though also can be used for gears • titanium carbide surface-hardening of bali bearing balls • plastic composite material cages (especially containing PTFE) for low life/load duty ball bearings • plastic/metal composite liners for plain (journal or ball joint) bearings _
Project requirements
Existing tribological techniques (ETT)
Pro ect generated techniques (PGT)
New tribological
developments (NTD)
Modified '! NTD I
Modified
ETT
V
_]
I
Testing
Final choice
Qualification
Fig 4 Artist's view of ERS-I in orbit (courtesy Dornier System/ Friedrichslafen, FRG ) 152
Fig 5 Pathways to solving tribological problems April 90 Vol 23 No 2
D. Wyn-Roberts--new frontiers for space tribology • low vapour pressure oils and greases for ball bearings and gears • use of oil reservoirs and anti-creep barriers for extending the useful life of bearings • use of plastic/metal combinations for gears (eg titanium/'Delrin' or steel/'Vespel') • use of silver + MoS2 composite brushes for slip rings • thermal vacuum testing techniques for demonstrating lubricant behaviour and performance under simulated space conditions. The most common route in Fig 5 is shown going through modification to existing techniques or lubricants in order to achieve satisfactory tribological performance. This is, to some extent, again well illustrated by the cryogenic investigations. The lack of data of tribologicai properties at low temperatures under space conditions for the new generation of observatory spacecraft is causing research to be carried out on: • friction levels and wear rates of plastic materials (such as those used in space-operating instrument gears) • tribological behaviour of actual instrument components • tribological behaviour of thin solid-film lubricants • thermal conductivity of point contacts. All at low temperatures down to 4 K. Lack of tribological data for space-operating mechanisms was noted initially on the Giotto mission, where the camera had to operate down to -70°C. The scanning mirror system is actuated by an eccentric cam, driven by a motor via a worm gear system. The wheel is of plastic and the worm, metal (see Fig 6). Several tests had to be performed under simulated space conditions before an optimum gear wheel material could be found. Finally polyimide (trade name 'Vespel') was chosen. It is too early to say whether new lubricants or techniques will emerge, but the investigations are certainly new for the space tribologist and are leading to new knowledge, which can be applied to evolving project requirements 1°. Route 3 illustrates the case where new developments occur on a pure research basis independent o f a
Fig 6 Mechanism of Giotto camera (courtesy MaxPlanck-lnstitut/ Katlenburg-Luidan, FRG ) TRIBOLOGY INTERNATIONAL
particular project, but are nonetheless capable of being used later. These developments can occur, for example, when research is carried out to overcome known limitations in existing techniques or lubricants. This can lead to the development or discovery of a new lubricant 'waiting for an application'. There are examples of this in space tribology: the development of vapour-deposited MoS2 film and the use of ceramic coatings in ball bearings. Since projects are conservative by nature this is the route they are most reluctant to take. Any newly discovered tribological technique must be well proven over many different types of test before being accepted for actual application. In Fig 5 all pathways pass through testing. This is essential in tribology even for relatively well-established lubricants since it is still largely an empirical science, and apparent minor changes to configuration or environment can have a large effect on performance and wear life, for example. In addition, life testing at component level in a vacuum is relatively economical to perform. As well as providing data relevant to meeting the specific spacecraft requirements, this type of test can also provide valuable back-up data for inorbit performance. For example the bearings equivalent to those used in the Giotto antenna de-spin mechanism were run in the laboratory for many times their required lifetime, in terms of revolutions. This data provided confidence in the spacecraft performance after it was decided to extend the mission well beyond the point of encounter with Halley's comet. Of course, it is also essential to perform qualification testing of the complete mechanism under environmental conditions. Of particular importance to bearing performance is the simulation of thermal gradients. These approaches just described are being and will continue to be applied to the exciting new phase of space exploration which we are entering in Europe. The most important lines of investigation to be pursued are well illustrated by the new four-year programme of work to be undertaken by the European Space Tribology Laboratory (ESTL) under contract to the European Space Agency. These areas are expected to be crucial for the next decade and are listed in Table 3. Many investigations are extensions of existing work into new areas. For example, solid film lubrication has been under investigation at ESTL since its inception. Fig 6 summarizes the results of torque/life tests on lead and MoS2 films in ball bearings. The torque is highest for the lead film, but so is the life. Up to now no failure has occurred on lead-lubricated ball bearings, but no tests have been continued far beyond 108 revs. For MoS2 it is seen that a considerable life extension occurs when run with 'Duroid' cages (containing PTFE). The combination of MoS2 and PTFE seems to be tribologically advantageous, though the reasons are not presently known, however, torque increases. This life improvement encourages us to investigate further lubricant combinations. As a second example we consider bearing analysis. A relatively recent breakthrough by ESTL has been to develop an analytical model which can predict torque characteristics of oscillating bearings ~'. An example is given in Fig 7 showing the correlation between experiment and analysis. The full potential of this 153
D. Wyn-Roberts--new frontiers for space tribology
Table 3 New space tribology investigations to be performed at ESTL Investigation High speed bearing lubrication
Solid lubricant films
Oils and greases
Ceramics
Aims/applications Lubrication of bearings at speeds of around 25 000 rev/ min for long life. Potential use in energy-storage wheels for future spacecraft Gain fundamental understanding of solid lubricant films. Establish performance of MoS2 and combination films. Potential use in low torque noise bearings in pointing mechanisms Data base compilation and tests on new lubricants. General application Establish performance and tribological properties of ceramic and ceramic/lubricant combinations. General applicability
Bearing analysis
Continue to develop analytical techniques applicable to space tribology problems
Electric contacts
Develop improved slip ring/ brush combinations and motor brushes. Application example: improved lifetime power transfer assemblies for Columbus
Gears
Lubrication of spur, worm gear, cycloid and harmonic drive gearboxes. For use in space-operating instruments and robotics
Cryo-tribology
(See text). Applications in scientific instruments and deep-space probes
High temperature tribology
Tests on new lubricants up to 800°C. Application to re-entry vehicles such as Hermes
development has not yet been realized but, since oscillating bearings are due to become more extensively used in spaceborne instrumentation, these techniques will certainly prove useful in the next decade. In the 'new' areas we can consider some other examples. Returning to the Columbus programme, there is a need to develop large reaction wheels causing a reassessment and extension of the tribological aspects to cover the stringent long life and low torque noise requirements. New investigations will, therefore, commence in the near future covering various tribological aspects including the optimization of lubricant reservoirs and bearing retainers. Additionally the use 154
Head film + lead bronze cage
Z
,= 20 X
o E
"E
// 7/ //
MoS 2 f i l m
+ Duro~d cage
10
/ i / / / /
"//i > <
MoS2 film + steel cage
/ / / / / /
/ / / / / /
/ / / / / /
/ / /
Xz 1.0xl06
/ / z
Beyond 1.0x108
1.0×10 0
Lifetime, revs
Fig 7 Torque~life data for solid film lubricant of titanium carbide coated balls in the wheel bearings, in combination with the 'standard' wheel oils (SRG60 and KG80) will be investigated. This type of ceramic coating is supplied by CSEM in Switzerland, who have pioneered their use for ball bearings operating in the space environment. They have shown excellent performance in achieving long life and very smooth running. Work in the USA la is taking the requirements further to include active reservoirs and in-orbit replaceable bearings. Another area which will challenge tribology beyond the next decade is that of planetary exploration-type spacecraft. An example is the cometary lander mission Rosetta. This is presently in an early stage of definition but will certainly be challenging to the space tribologist. There will be many mechanical parts including anchoring devices for the spacecraft and automatic drill and shovelling mechanisms for taking surface samples from a comet. There will be long periods when the devices are dormant, since comet encounter may take several years. Additionally the devices will have to operate at extremely low temperatures and in 'soil' material whose properties are largely unknown.
Further into the future It is often fashionable to speculate on the eventual discovery of a 'perfect' space lubricant with a set of universal properties able to solve all future tribological problems. However, experience and evolution teaches us that a condensation into one all-embracing solution does not normally occur. Instead it is more likely that lubricants will continue to become even more specialized in their tailoring to individual tasks. Magnetic bearings, for instance, are often thought of as being tribologically ideal since the surfaces are noncontacting during operations. However, magnetic fields continue to be problematic for sensitive spacecraft experiments and, in addition, somewhat complex electronics need to be used. However further dramatic April 90 Vol 23 No 2
D. Wyn-Roberts--new frontiers for space tribology improvements in terms of performance, mass and ease of realization would occur with the advent of high temperature superconductors. Furthermore there would be a good chance to use the low ambient temperatures in space for such superconducting bearings without having to incorporate complex thermal control equipment. Of course, in the area of new materials dramatic improvements could occur at any time, as has been pointed out by Tabor in his fascinating review ~4. Not all of these fulfil their apparent early promise, with respect to space tribology applications. As an example, carbon-fibre-filled plastics have still not been used in this role, although early investigations indicated some promising potential 15. Presently ceramics are raising exciting prospects. Since they are very hard materials they offer the possibility of producing bearings having low Hertzian contact area, leading to low friction. However, there are disadvantages for space use, including their brittleness and thermal compatibility problems. Nonetheless, successful application for specific cases on spacecraft has been achieved in Europe using bearings with titanium carbide coated b a l l s 16, as already mentioned. Recently development of diamondlike layers has occurred which could be of future interest as a tribological surface coating. Another future development we can expect is the development of tribological materials in space, onboard space stations. High purity materials can be produced in the space environment and the production of in situ lubricants for space use can also lead to improved ground-based industrial lubricants, for example. Finally, my own personal prediction is that 'combination lubricants' will grow in importance and will provide solutions to many of the new space tribology problems to be expected. It has already been noted that an apparent symbiotic relationship exists between PTFE and MoS2 used as a thin-film bearing lubricant ~7. The performance of this combination is much better than that of M o S 2 film alone. Also, the addition of platelets of solid oleophilic materials such as lead added to oil have been found to give improved lubricating properties TM. In the future the possibility exists to improve the air-running performance of lead and M o S 2 films by combining with other lubricants, eg small quantities of low vapour pressure oil. These dry-film lubricants perform very well in a space environment but still have the drawback of rapid deterioration when run in air, which severely limits ground-testing possibilities. Another exciting development taking place in Europe, at CSEM, is a technique of incorporating dispersed oil
TRIBOLOGY INTERNATIONAL
into solid materials. This opens up the prospect of selfrepairing bearings for long-term space use. From the foregoing we see that there is a large variety of problems and developments present in space tribology, and the next decade promises to be an exciting one for this discipline.
References 1. Bowden F.P. and Tabor D. The friction and lubrication of solids.
Part L Chapwr VII, Oxford University Press, 1958 2. Atkins and Grifliths Electrical sliding contacts and their behaviour at high altitudes. Inst Mech. Eng., Conf. on Lubrication
and Wear, October 1957 3. European space science. Horizon 2000, ESA SP-I070, December
1984 4. Router, K.E. The European long term space plan. ESA Bull.
54, May 1988 5. Lueiano, G. Cryogenic mechanisms of the ISO camera. Proc.
3rd European Space Mechanisms and Tribology Syrup. ESA SP279, October 1987 6. Bowden F.P. and Tabor D. The friction and lubrication of solids.
Part II, Chapter XI1, Oxford University Press, 1964 7. Arita M. et al Investigations of tribological characteristics of MoS2 film in space environment. Fourth European Mechanisms
and Tribology Syrup. 20-22 September 1989 8. Roberts E.W., Porteus D. and Cable N. Separable electrical contacts for space. A tribological and electrical assessment. 3rd
European Mechanisms and Tribology Syrup. ESA SP-279, October 1987 9. Paratte, L. Antenna pointing mechanisms handbook. ESTEC
Internal working paper, EWPISI7 10. Roberts E.W. et a! A test facility for in-vacuum assessment of dry lubricants and small mechanisms at cryogenic temperatures.
3rd European Mechanisms and Tribology Syrup. ESA SP-279, 1987 11. Todd, M.J. Models for steady and non-steady coulomb torque in ball bearings. 3rd European Mechanisms and Tribology Syrup.
ESA SP-279, 1987 12. Cook L. et ai Design, fabrication and test of a 4750 N-m-s double gimbal control moment gyroscope. Proc. 23rd Aerospace
Mechanisms Syrup. NASA Conf. Publication 3032, May 1989 comet nucleus sample return, ESA Publication SCI(87)3, December 1987
13. Rosetta
14. Tabor, D. Status and direction of tribology as a science in the 1980s-understanding and predictions. Proc. Int. Conf.
Tribology, NASA, Lewis, April 1983 15. Harris, C.L. and Wyn-Roberts, D. Wear of carbon fibre reinforced polymers in a high vacuum environment. Nature 217
(5132) March 9, 1968 16. Boring H.J., Hintermann H.E. and Haenni W. Ball bearings with CVD-TiC coated components. 3rd European Space Mechan-
isms and Tribology Syrup. ESA SP-279, 1987 17. Roberts, E.W. The lubricating properties of magnetron sputtered MoS 2. ESA(ESTL)76 October 1987. 27-29 18. Groszek A.J. et al Development of new space lubricants. Space
Tribology, ESA SPlll, April 1975
155
Keywords: lubrication, space tribology, molybdenum disulphide, extreme
environments, European Space Agency Introduction Space tribology refers to that specialized discipline dealing with friction and wear phenomena related to spacecraft components. The problems arising are due, in particular, to the vacuum environment and lack of gravity. In fact, a foretaste of the problems to be encountered occurred before the advent of the space programmes when motor failures started to occur in high altitude aircraft. These turned out to be due to rapid wear of graphite motor brushes, since graphite loses its lubricating properties at low pressure, specifically due to loss of water vapour content ~, 2 The importance of space tribology has been recognized and supported by the European Space Agency (ESA) since its inception 25 years ago. As a result the subject has reached a high degree of maturity in Europe. Nevertheless Europe is embarking on a new and vigorous space programme which is bringing about new challenges to space tribology. This paper will examine those challenges and the ways in which they can be met.
ESA long-term space programme
have complex sampling devices which will have to operate in extreme environments. This aspect will be discussed again later. Next we have the European Space Station programme known as Columbus. It presently consists of three elements. These will be firstly an attached module which will be a manned laboratory fixed to the main United States Space Station 'Freedom' structure, secondly a free-flying unmanned laboratory called the FFL and thirdly an unmanned polar platform, which, as its name implies, will be in polar orbit. Of critical importance to tribology is the impact of the so-called 'micro-gravity' environment that has to be achieved, particularly within the FFL. This is an extremely low vibration environment required to allow the growth of fault-free crystals for material sciences and requires very smooth vibration-free operation of mechanical devices. The FFL can be seen in the foreground of Fig. 1. In the area of telecommunications, feasibility studies are presently being performed on the Data Relay Satellite (DRS) to be used for communicating with the space station, and Sat-2, which is part of a
There are many current and future projects under investigation by ESA and it would take too long to describe them all in detail. Instead, the core items will be mentioned here and especially those that will impact tribological development. A listing of the currently planned key projects is given in Table 1. Generally the Science Programme tends to present the most technological challenges. This programme is organized into four areas of scientific discipline known as 'cornerstones'. At the time of writing all are in an early phase of development. The earliest planned launch date is 1995 for the Soho and Cluster missions. Specific tribological problems arise with the mechanically complex instrumentation of the Soho spacecraft which will additionally have strict limits on the disturbances that are allowed to propagate into the spacecraft structure. Also the planetary explorers will * ESTEC, Noordwijk, The Netherlands.
TRIBOLOGY INTERNATIONAL
Fig1 Artist's impression of the Columbus element 'MTFF' in orbit 0301-679X/90/040149-7 (~) 1990 Butterworth-Heinemann Ltd
149
D. Wyn-Robertsmnew frontiers for space tribology development and experimentation programme. Both of these are large spacecraft with complex mechanical systems foreseen for launch by the mid-1990s. A tribological driver in these programmes will again be vibration aspects since it is intended to utilize interorbital communication using laser beams. The laserpointing systems will require very smooth drives in order to avoid jitter and indeed any mechanical systems on board will have to be virtually vibration free in operation, over a long lifetime of up to ten years. Regarding the earth observation programme, the Earth Resources Satellite (ERS) is now nearing completion. Also, a second generation of meteorological satellite, Meteosat-2, is being studied, with a mechanically complex payload which promises to pose further tribological problems. Finally the Ariane V launcher and the Hermes Space Plane are being developed as transportation support to the above programmes. The bearings and seals for the launcher turbo-pumps are critical items which, although operating for only a short time, do so at very high revolution rate and at extremely low temperature. In the case of Hermes the high temperatures developed during re-entry will necessitate careful attention to tribological aspects of the control surface actuators. More details of all the above programmes can be found in Refs 3 and 4.
Fig 2 Computer-generated view of the 1SO spacecraft in orbit
The challenge to space tribology There will be a definite impact on the direction of space tribology development arising from the programmes just discussed, and their requirements are being taken into account in planning future research and development work. In general the major new requirements can be summarized as follows: • • • • •
extended lubricant lifetimes lubrication under intermittent operating conditions increased reliability lower torque and torque noise characteristics extreme temperature operation.
Some specific examples will now be considered. A particularly interesting area arising from the requirements of the science programme is that of cryotribology, which is tribology at cryogenic temperatures. This problem has been encountered for the scientific instruments of the Infra-red Space Observatory (ISO), a spacecraft which is now in an advanced state of development (see Fig 2). In the future similar problems will be encountered for the Far Infra-Red and Submillimetre Telescope (FIRST, see Table 1). Instruments situated at the focal planes of these space telescopes utilize focusing and shutter mechanisms and have to operate at temperatures down to 4 K. In the case of ISO the focal plane camera incorporates a filter change mechanism with gear drive and shaftmounted ball bearings. Lubrication is by means of an MoSz film and the gears are made from titanium alloy with titanium carbide surfaces. A view of this mechanism is given in Fig 3. In order to qualify the lubricant, special chambers for vibration at cryogenic temperature were developed at the Aerospatiale company in France 5. 150
Table 1 ESA long-term project programme Science Solar, terrestrial physics
Soho and Cluster spacecraft
High throughput X-ray spectroscopy
XMM spacecraft
Sub-millimetre astronomy
FIRST mission
Planetary exploration
Cassini and Rosetta
Space station and transportation European space station Columbus Advanced launcher
Ariane V
Spaceplane
Hermes
Hermes robotic arm
HERA
Telecommunications Data relay satellite
DRS
Payload and spacecraft development and experimentation programme Earth observations Earth resources satellite Second generation meteorology
Sat-2
ERS
Meteosat 2 April 90 Vol 23 No 2
D. Wyn-Roberts--new frontiers for space tribology for performing servicing and other tasks in space. Particularly challenging is the lubrication of the joint gearboxes and end effectors. These devices will have to operate reliably and smoothly and yet will be fully exposed to the space environment. In the case of the telecommunications programme the DRS spacecraft under development will have large Table 2 European antenna pointing mechanisms (APMs)
Fig3 Cryogenic mechanisms of the ISO camera (courtesy Aerospatiale/Cannes-mechanisms and CEA/ Saclay, France-overaU development) Turning now to the Columbus programme we can see major departures from the usual spacecraft with which we have been familiar in Europe up till now. The first difference is a required lifetime of 30 years compared with a maximum of ten years for automatic spacecraft. Secondly we have requirements for reusability and servicing. It will be possible to access the Columbus elements and they can, therefore, be refurbished or serviced for subsequent reuse. Parts of the spacecraft could be exposed to space for long periods, for example the docking mechanisms on the FFL, where they will not only be fully exposed to vacuum and weightlessness but also to radiation and atomic oxygen bombardment. These long-term effects will need to be more closely studied in the future. For example, the exposure of MoS2 lubricant to atomic oxygen can lead to the formation of Mo203 which, far from being a lubricant, is an abrasive 6'7. During the docking process, electrical and fluid connectors will be mated and during the vehicle lifetime many mating/de-mating cycles will occur with relatively long space exposure in between. The tribology of reusable electrical pin connectors, fluid connectors and seals therefore needs to be investigated under these conditions. Work has already started in Europe on this aspect. Another area which will prove to be tribologically challenging is that of the solar arrays for the FFL and polar platform. These will be the largest arrays yet built in Europe with nominal power capacities of around 15 kW and 12 kW respectively. In the case of the MTFF the arrays will be extended and retracted at six-monthly intervals with the associated problem of extension mechanism lubrication. For the polar platform the arrays will be sun-following with a rotation rate of 6 revs day -1. This implies the use of a rotating power transfer assembly which may lead to the requirements for special large-diameter bearings and high-power slip rings. In the longer term the development and utilization of robotics will require careful attention regarding tribological aspects. A robotic manipulator arm will be eventually utilized in the Columbus programme TRIBOLOGY INTERNATIONAL
Acronym
Company responsible
Lubrication/ comments
Olympus APM
BAe
Vapour-deposited MoS2 film for gear, BP135 oil for encoder tracks. Now operating in orbit
IOC APM
Matra
MoS2 dry-lubricated ball bearings. Will fly on an inter-orbit communication experiment
IOC APM
Marconi
Braycote 601 for needle bearings. Chosen as back-up. Not scheduled for flight
Syracuse APM
Alcatel
MoS2 film for ball bearings. Steel versus debris gears. Is due to fly on spacecraft
SOFA
Aerospatiale
Not necessary. Flex pivots used. Development model.
Medium range APM
Aerospatiale
Braycote 601 low vapour pressure grease for needle bearings. MoS2 film for screw jack. Development model
HPM
Teldix
Ball bearings with TiC surfacehardened balls and Fomblin Z25 low vapour pressure oil. Chosen for flight model experiment
Q-model APM
Dornier
Ball bearings with TiC surface balls and MoS=-coated races and cages. Development model
MPA-PA
Matra
TiC-hardened and MoS2-1ubricated parts. Development model 151
D. Wyn-Roberts--new frontiers for space tribology deployable antennas which need to point and track. The pointing mechanisms may need to be of special design and will have their own tribological problems. Several designs have already been studied in Europe together with their lubrication systems 9. Later developments will be based on the experience gained from these. Table 2 lists them, together with their lubrication systems. For Sat-2 a special development of a 5 m diameter unfurlable antenna is planned. Such large structures lead, in particular, to problems of qualification testing of the lubrication system under realistic operating conditions. Regarding the earth observation programme a particular problem is the use of large synthetic aperture radar (SAR) antennas. The SAR antenna of ERS, to be launched in 1990, is 11 m long. This can be seen in Fig 4. Second generation versions, eg to be used on the polar platform, are likely to be even larger. Although the SAR is presently a one-shot device the tribology aspects are, nevertheless, challenging. Low friction hinges are required and ground-testing loads have also to be catered for. In the present case, ball bearings were chosen with dry film lubricant (because of the low deployment speed). These bearings are, however, susceptible to vibration damage during launch. Furthermore dry film lubricants have poor performance in air and extreme care in handling must therefore be exercised during the ground-test phase, where, in addition, much higher loads can be encountered than in orbit. The optimization of these various aspects including the approach required for qualifying these large structures which can no longer be fully tested on the ground (eg due to size restriction of available vacuum chambers) is not yet fully solved.
Meeting the challenge Having seen the types of problem that are approaching we will now consider means of solving them. First a discussion of the generic approach is given, with some specific examples at the end. There are essentially three ways to approach tribology problems arising from projects. These are illustrated in the flow diagram of Fig 5. The 'thick arrow' route commencing at path (1) illustrates the most common way in which these problems are solved. Existing techniques should always be considered first. This is not a very glamorous situation and it is often very tempting to develop new techniques for new applications, whereas the existing techniques may be perfectly adequate. They will, in general, also be the most reliable. The thin, solid lead-film lubricant, for example, has been used for many applications in space, virtually without change, over at least the past 15 years. It is a very successful space lubricant and its use is expected to continue for the foreseeable future. In fact the origin of the use of lead-film lubricant occurred before the advent of the space programme since it was used for the bearings of X-ray tubes, which operated in a vacuum. The early interest of space engineers in such a dry lubricant was due to the fact that oils and greases would evaporate in a vacuum and also cause contamination of, for example, sensitive optical surfaces. However, nowadays low vapour pressure oils and greases are available and can be used for certain space applications. The major space lubrication techniques currently in use can be listed as follows: • dry-film lubrication with lead or MoS2, mainly for ball bearings, though also can be used for gears • titanium carbide surface-hardening of bali bearing balls • plastic composite material cages (especially containing PTFE) for low life/load duty ball bearings • plastic/metal composite liners for plain (journal or ball joint) bearings _
Project requirements
Existing tribological techniques (ETT)
Pro ect generated techniques (PGT)
New tribological
developments (NTD)
Modified '! NTD I
Modified
ETT
V
_]
I
Testing
Final choice
Qualification
Fig 4 Artist's view of ERS-I in orbit (courtesy Dornier System/ Friedrichslafen, FRG ) 152
Fig 5 Pathways to solving tribological problems April 90 Vol 23 No 2
D. Wyn-Roberts--new frontiers for space tribology • low vapour pressure oils and greases for ball bearings and gears • use of oil reservoirs and anti-creep barriers for extending the useful life of bearings • use of plastic/metal combinations for gears (eg titanium/'Delrin' or steel/'Vespel') • use of silver + MoS2 composite brushes for slip rings • thermal vacuum testing techniques for demonstrating lubricant behaviour and performance under simulated space conditions. The most common route in Fig 5 is shown going through modification to existing techniques or lubricants in order to achieve satisfactory tribological performance. This is, to some extent, again well illustrated by the cryogenic investigations. The lack of data of tribologicai properties at low temperatures under space conditions for the new generation of observatory spacecraft is causing research to be carried out on: • friction levels and wear rates of plastic materials (such as those used in space-operating instrument gears) • tribological behaviour of actual instrument components • tribological behaviour of thin solid-film lubricants • thermal conductivity of point contacts. All at low temperatures down to 4 K. Lack of tribological data for space-operating mechanisms was noted initially on the Giotto mission, where the camera had to operate down to -70°C. The scanning mirror system is actuated by an eccentric cam, driven by a motor via a worm gear system. The wheel is of plastic and the worm, metal (see Fig 6). Several tests had to be performed under simulated space conditions before an optimum gear wheel material could be found. Finally polyimide (trade name 'Vespel') was chosen. It is too early to say whether new lubricants or techniques will emerge, but the investigations are certainly new for the space tribologist and are leading to new knowledge, which can be applied to evolving project requirements 1°. Route 3 illustrates the case where new developments occur on a pure research basis independent o f a
Fig 6 Mechanism of Giotto camera (courtesy MaxPlanck-lnstitut/ Katlenburg-Luidan, FRG ) TRIBOLOGY INTERNATIONAL
particular project, but are nonetheless capable of being used later. These developments can occur, for example, when research is carried out to overcome known limitations in existing techniques or lubricants. This can lead to the development or discovery of a new lubricant 'waiting for an application'. There are examples of this in space tribology: the development of vapour-deposited MoS2 film and the use of ceramic coatings in ball bearings. Since projects are conservative by nature this is the route they are most reluctant to take. Any newly discovered tribological technique must be well proven over many different types of test before being accepted for actual application. In Fig 5 all pathways pass through testing. This is essential in tribology even for relatively well-established lubricants since it is still largely an empirical science, and apparent minor changes to configuration or environment can have a large effect on performance and wear life, for example. In addition, life testing at component level in a vacuum is relatively economical to perform. As well as providing data relevant to meeting the specific spacecraft requirements, this type of test can also provide valuable back-up data for inorbit performance. For example the bearings equivalent to those used in the Giotto antenna de-spin mechanism were run in the laboratory for many times their required lifetime, in terms of revolutions. This data provided confidence in the spacecraft performance after it was decided to extend the mission well beyond the point of encounter with Halley's comet. Of course, it is also essential to perform qualification testing of the complete mechanism under environmental conditions. Of particular importance to bearing performance is the simulation of thermal gradients. These approaches just described are being and will continue to be applied to the exciting new phase of space exploration which we are entering in Europe. The most important lines of investigation to be pursued are well illustrated by the new four-year programme of work to be undertaken by the European Space Tribology Laboratory (ESTL) under contract to the European Space Agency. These areas are expected to be crucial for the next decade and are listed in Table 3. Many investigations are extensions of existing work into new areas. For example, solid film lubrication has been under investigation at ESTL since its inception. Fig 6 summarizes the results of torque/life tests on lead and MoS2 films in ball bearings. The torque is highest for the lead film, but so is the life. Up to now no failure has occurred on lead-lubricated ball bearings, but no tests have been continued far beyond 108 revs. For MoS2 it is seen that a considerable life extension occurs when run with 'Duroid' cages (containing PTFE). The combination of MoS2 and PTFE seems to be tribologically advantageous, though the reasons are not presently known, however, torque increases. This life improvement encourages us to investigate further lubricant combinations. As a second example we consider bearing analysis. A relatively recent breakthrough by ESTL has been to develop an analytical model which can predict torque characteristics of oscillating bearings ~'. An example is given in Fig 7 showing the correlation between experiment and analysis. The full potential of this 153
D. Wyn-Roberts--new frontiers for space tribology
Table 3 New space tribology investigations to be performed at ESTL Investigation High speed bearing lubrication
Solid lubricant films
Oils and greases
Ceramics
Aims/applications Lubrication of bearings at speeds of around 25 000 rev/ min for long life. Potential use in energy-storage wheels for future spacecraft Gain fundamental understanding of solid lubricant films. Establish performance of MoS2 and combination films. Potential use in low torque noise bearings in pointing mechanisms Data base compilation and tests on new lubricants. General application Establish performance and tribological properties of ceramic and ceramic/lubricant combinations. General applicability
Bearing analysis
Continue to develop analytical techniques applicable to space tribology problems
Electric contacts
Develop improved slip ring/ brush combinations and motor brushes. Application example: improved lifetime power transfer assemblies for Columbus
Gears
Lubrication of spur, worm gear, cycloid and harmonic drive gearboxes. For use in space-operating instruments and robotics
Cryo-tribology
(See text). Applications in scientific instruments and deep-space probes
High temperature tribology
Tests on new lubricants up to 800°C. Application to re-entry vehicles such as Hermes
development has not yet been realized but, since oscillating bearings are due to become more extensively used in spaceborne instrumentation, these techniques will certainly prove useful in the next decade. In the 'new' areas we can consider some other examples. Returning to the Columbus programme, there is a need to develop large reaction wheels causing a reassessment and extension of the tribological aspects to cover the stringent long life and low torque noise requirements. New investigations will, therefore, commence in the near future covering various tribological aspects including the optimization of lubricant reservoirs and bearing retainers. Additionally the use 154
Head film + lead bronze cage
Z
,= 20 X
o E
"E
// 7/ //
MoS 2 f i l m
+ Duro~d cage
10
/ i / / / /
"//i > <
MoS2 film + steel cage
/ / / / / /
/ / / / / /
/ / / / / /
/ / /
Xz 1.0xl06
/ / z
Beyond 1.0x108
1.0×10 0
Lifetime, revs
Fig 7 Torque~life data for solid film lubricant of titanium carbide coated balls in the wheel bearings, in combination with the 'standard' wheel oils (SRG60 and KG80) will be investigated. This type of ceramic coating is supplied by CSEM in Switzerland, who have pioneered their use for ball bearings operating in the space environment. They have shown excellent performance in achieving long life and very smooth running. Work in the USA la is taking the requirements further to include active reservoirs and in-orbit replaceable bearings. Another area which will challenge tribology beyond the next decade is that of planetary exploration-type spacecraft. An example is the cometary lander mission Rosetta. This is presently in an early stage of definition but will certainly be challenging to the space tribologist. There will be many mechanical parts including anchoring devices for the spacecraft and automatic drill and shovelling mechanisms for taking surface samples from a comet. There will be long periods when the devices are dormant, since comet encounter may take several years. Additionally the devices will have to operate at extremely low temperatures and in 'soil' material whose properties are largely unknown.
Further into the future It is often fashionable to speculate on the eventual discovery of a 'perfect' space lubricant with a set of universal properties able to solve all future tribological problems. However, experience and evolution teaches us that a condensation into one all-embracing solution does not normally occur. Instead it is more likely that lubricants will continue to become even more specialized in their tailoring to individual tasks. Magnetic bearings, for instance, are often thought of as being tribologically ideal since the surfaces are noncontacting during operations. However, magnetic fields continue to be problematic for sensitive spacecraft experiments and, in addition, somewhat complex electronics need to be used. However further dramatic April 90 Vol 23 No 2
D. Wyn-Roberts--new frontiers for space tribology improvements in terms of performance, mass and ease of realization would occur with the advent of high temperature superconductors. Furthermore there would be a good chance to use the low ambient temperatures in space for such superconducting bearings without having to incorporate complex thermal control equipment. Of course, in the area of new materials dramatic improvements could occur at any time, as has been pointed out by Tabor in his fascinating review ~4. Not all of these fulfil their apparent early promise, with respect to space tribology applications. As an example, carbon-fibre-filled plastics have still not been used in this role, although early investigations indicated some promising potential 15. Presently ceramics are raising exciting prospects. Since they are very hard materials they offer the possibility of producing bearings having low Hertzian contact area, leading to low friction. However, there are disadvantages for space use, including their brittleness and thermal compatibility problems. Nonetheless, successful application for specific cases on spacecraft has been achieved in Europe using bearings with titanium carbide coated b a l l s 16, as already mentioned. Recently development of diamondlike layers has occurred which could be of future interest as a tribological surface coating. Another future development we can expect is the development of tribological materials in space, onboard space stations. High purity materials can be produced in the space environment and the production of in situ lubricants for space use can also lead to improved ground-based industrial lubricants, for example. Finally, my own personal prediction is that 'combination lubricants' will grow in importance and will provide solutions to many of the new space tribology problems to be expected. It has already been noted that an apparent symbiotic relationship exists between PTFE and MoS2 used as a thin-film bearing lubricant ~7. The performance of this combination is much better than that of M o S 2 film alone. Also, the addition of platelets of solid oleophilic materials such as lead added to oil have been found to give improved lubricating properties TM. In the future the possibility exists to improve the air-running performance of lead and M o S 2 films by combining with other lubricants, eg small quantities of low vapour pressure oil. These dry-film lubricants perform very well in a space environment but still have the drawback of rapid deterioration when run in air, which severely limits ground-testing possibilities. Another exciting development taking place in Europe, at CSEM, is a technique of incorporating dispersed oil
TRIBOLOGY INTERNATIONAL
into solid materials. This opens up the prospect of selfrepairing bearings for long-term space use. From the foregoing we see that there is a large variety of problems and developments present in space tribology, and the next decade promises to be an exciting one for this discipline.
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Part L Chapwr VII, Oxford University Press, 1958 2. Atkins and Grifliths Electrical sliding contacts and their behaviour at high altitudes. Inst Mech. Eng., Conf. on Lubrication
and Wear, October 1957 3. European space science. Horizon 2000, ESA SP-I070, December
1984 4. Router, K.E. The European long term space plan. ESA Bull.
54, May 1988 5. Lueiano, G. Cryogenic mechanisms of the ISO camera. Proc.
3rd European Space Mechanisms and Tribology Syrup. ESA SP279, October 1987 6. Bowden F.P. and Tabor D. The friction and lubrication of solids.
Part II, Chapter XI1, Oxford University Press, 1964 7. Arita M. et al Investigations of tribological characteristics of MoS2 film in space environment. Fourth European Mechanisms
and Tribology Syrup. 20-22 September 1989 8. Roberts E.W., Porteus D. and Cable N. Separable electrical contacts for space. A tribological and electrical assessment. 3rd
European Mechanisms and Tribology Syrup. ESA SP-279, October 1987 9. Paratte, L. Antenna pointing mechanisms handbook. ESTEC
Internal working paper, EWPISI7 10. Roberts E.W. et a! A test facility for in-vacuum assessment of dry lubricants and small mechanisms at cryogenic temperatures.
3rd European Mechanisms and Tribology Syrup. ESA SP-279, 1987 11. Todd, M.J. Models for steady and non-steady coulomb torque in ball bearings. 3rd European Mechanisms and Tribology Syrup.
ESA SP-279, 1987 12. Cook L. et ai Design, fabrication and test of a 4750 N-m-s double gimbal control moment gyroscope. Proc. 23rd Aerospace
Mechanisms Syrup. NASA Conf. Publication 3032, May 1989 comet nucleus sample return, ESA Publication SCI(87)3, December 1987
13. Rosetta
14. Tabor, D. Status and direction of tribology as a science in the 1980s-understanding and predictions. Proc. Int. Conf.
Tribology, NASA, Lewis, April 1983 15. Harris, C.L. and Wyn-Roberts, D. Wear of carbon fibre reinforced polymers in a high vacuum environment. Nature 217
(5132) March 9, 1968 16. Boring H.J., Hintermann H.E. and Haenni W. Ball bearings with CVD-TiC coated components. 3rd European Space Mechan-
isms and Tribology Syrup. ESA SP-279, 1987 17. Roberts, E.W. The lubricating properties of magnetron sputtered MoS 2. ESA(ESTL)76 October 1987. 27-29 18. Groszek A.J. et al Development of new space lubricants. Space
Tribology, ESA SPlll, April 1975
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