Aspects of Sunser main structure design and manufacturing fI.R. Schusterl and A.H. Basson2 (Received: January 1995; Final version May 1995) Abstract SUNSAT micro satellite project in 1989. Studies of the Department's capabilities and other programs at the uni- This paper describes sorne ouerall mechanical design 0,s- versities of Surrey and Berlin led to the January 1992 base- pects of the Stellenbosch Uniaersity satellite,'SUNSAT'. An line design compatible with Anlarup launch requirements. introduction to lhe SUNSAT project is giuen. The mechan- In 1994, NASA expressed interest in launching Sut{sar in ical design requirements are described. The design of the the same mission a^s the Danish Oersted magnetic research module trays, which conslitutes a major part of the main satellite,[2] both as secondary payloads on the Argos/Pgl- stracture, is discussed. The design and rnanufacturing of 1 Delta ll mission in October 1996. In exchange, SUNSAT the horizon sensor housing and the imager bearing hous- will provide data gathered by a precision GPS receiver ings are described to illustrate the infl,uence of the design (that NASA will supply) and the mountitrg of a set of requirements, the use of solid modelling CAD software and laser retro-reflectors on SUNSAT. The proposed orbit is the application of design fo, assembly principles in the near-polar with an inclination of 96o and an altitude vary- satellite's mechanical design. The resulting design satis- ing from 450 to 850 km.[3] firt the functional and financial constrainls. Review of previous work Introduction Thorough surveys of other international activities involv- This paper describes the mechanical design and manufac- ittg small spacecraft have been given in previous papers.[4; ture of the trays that form the main structure of Sut{sAT, 5] Only a brief summary of relevant aspects will be given as well as the horizon sensor housings and the imager bear- here. ittg mountings. The particular requirements that the me- The University of Surrey has pioneered micro-satellite chanical design has to satisfy, and the ways in which these technologies, beginning with its Uoset program in 1979. requirements were met, are outlined. A companion pa- Surrey's first experimental micro-satellites (Uosar- 1 & per describes the design of the base plate and launcher 2) were launched free-of-charge as 'piggy-back' payloads mountings in detail.[l] The detail design for other mechan- through a collaborative arrangement with NASA on Delta ical components, e.B. the imager optics and the reaction rockets in 1981 and 1984, respectively. Since then, a wheels, is still in progress. further eight low-cost , yet fairly sophisticated, micro- SUNSAT is an acronym for Stellenbosch University satellites have been placed in low Earth orbit for a va- Satellite. The primary motivation behind SuusAT is to riety of customers using the Antaun Auxiliary Payload increase engineering design opportunities for graduate stu- Adapter. dents, promote interest in technology through school inter- Uosar- 1 and 2 both used a rather conventional struc- action prograrns, and increase industrial and international ture - a framework 'skeleton'onto which modules contain- interaction. The current SuttsAT development team con- ing the various electronic subsystems and payloads were sists of about 30 post-graduate students (rnostly in elec- mounted. However, the need to accommodate a variety of tronic engineering) and academic and technical staff mem- payload customers within a standard launcher envelope, bers of the University of Stellenbosch. SutlsAT is therefore coupled with increased demands on packing density, econ- a typical educational micro-satellite, with a weight of 60 omy of manufacture and ease of fabrication, led to the kg and a cubic size of 450 mm. It is designed to carry an development of a modular design of a multi-mission mi- amateur radio transponder, a store-and-forward commu- cro satellite platform. This structure is based around a nication system and a three-colour imaging system with a series of module trays that house the electronic circuits resolution of 15 meter. Its planned functional lifetime in and themselves form the mechanical structure, onto which orbit is 4 years. solar arrays are mounted as seen in Figure 1.[6] The Department of Electrical and Electronic En- gineering at the University of Stellenbosch started the Design requirements The design requirements can be classified as either finan- l Graduate student, Department of Mecha.nical Engineering, Uni- versity of Stellenbosch, Stellenbosch, 7600 Republic of South Africa cial or functional requirements. The functional require- ments can be further grouped accordittg to the environ- 2Associate Professor, Member SAIMechE ment that i-poses the particular requirement, i.e. before R & D Journal, 1995, 1I(2) 2I launch, during launch, and operation in space. Griffin & not exceed (i" magnitude and duration) those during French [7] described the general requirements in some de- launch. tail. Those that have an appreciable effect on SUNSAT's The design of the satellite must be amenable to be- mechanical design are outlined below. ittg assembled and disassembled a number of times during testing. Requirements related to the launch environment NASA i-posed tight constraints on the total mass (60 kg maximum) and the location of the centre of mass (not fur- ther than 280 mm from the separation plane and within a cylinder of 20 mm diameter centred on the longitudinal axis).[8] This limits the moment that the payload imposes on the satellite-launcher interface and ensures a safe sepa- ration after launch. If the centre of gravity is too far from the longitudinal axis, the separation springs would force the satellite into an undesirable tumbling movement. Dimensional limitations are i-posed on SUNSAT by the space available in the launch vehicle. The marimum dimensions for the body are 450 mm x 450 mm x 500 mm. One of the most stringent design requirements im- posed by launch is the stiffness requirement. NASA re- quires that the first natural frequency of SUNSAT mounted on the launcher must be at least 70 Hz. This requirement Figu re 1 E"p lod ed view of th e S u rrey micro satellite dominated the design of the base plate.[1] structure t6] Launch is characterised by u highly stressful environ- ment for the spacecraft for a relatively brief period (typi- Financial requirements cally a few minutes). The spacecraft is subjected to signif- icant axial loads by the accelerating launch vehicle, as well As an educational project, SUNSAT is subject to severe as lateral loads induced by steering and wind gusts. A 10S financial restraints. The tnain impact in the mechanical steady acceleration, acting simultaneously in the axial and design is that as many as possible of the structural com- both lateral directions, was used as steady load criterion ponents had to be designed to be manufactured in the uni- for large components.[l] Finite element analysis methods versity's workshop. Conventional lathes, milling machines are typically used for the strength design of these com- and a 3-axis NC machine are available in the workshop. ponents. Manual stress calculations were used for design- Basic sheet metal working and welding facilities are also ing brackets, sensor housings, support beams, etc. These available, but aluminium welding and manufacturing in small components were designed to sustain 20g, because composite rnaterials could not be accommodated in this of greater uncertainties in their load conditions and the workshop. sirnple analysis methods used in their design.[9] In lieu of A further impact of the financial restraints is that structural static load testing to provide flight qualification, readily available, moderately priced materials and compo- a 'no flight'factor of 2.0 times the maximum flight stress nents had to be used, as far as possible. levels (limit load factors) was used in the structural analy- Although it was not a primary requirement, the satel- sis.[8] This safety factor of 2.0 was required for both main lite had to be modular to allow future changes to be in- structural components and smaller components, because corporated as easily, and therefore as cheaply, &s possible, neither will be statically tested.[9] whilst not substantially increasing the cost of the first ver- Shock loads during satellite-launcher separation, sub- sion. stantial mechanical vibration and severe acoustic energy input, particularly while the rocket engine noise reflects Requirernents related to the before-launch envi- from the ground, have to be sustained without structural ronment damage. Dynamic loads of typically 13g rms acceleration Materials used, such as lightweight structural alloys, must at frequencies in the 10 to 2 000 Hz band must be sus- not be susceptible to undue atmospheric corrosion, or must tained. These requirements are discussed in more detail in be protected against it. trl In SUNSAT's mechanical design, it was assumed that Enclosed volurnes have to be adequately vented to the satellite will be protected during transport sufficiently avoid the build-up of pressure differentials due to the rapid that mechanical vibration and shock loads i-posed will reduction in arnbient pressure during the launch.[7] 22 R & D Journal, 1995, II(2) Requirernents related to the Space environment insulated portions of a satellite can typically experience ternperature variations from 200 K during darkness to 350 The space environment is characterised by a very hard I( in direct sunlight. vacuum, very low gravitational acceleration, ionising radi- The satellite structure must therefore be able to sus- ation, extremes of thermal radiation source and sink tem- tain substantial thermal stresses and fatigue, and the de- peratures, and micro-meteoroids. sign must be such that thermal deformation must not af- fect the accurate pointing of sensors. Vacuum Material selection is crucially affected by the extent to Design of general mechanical configuration which materials outgas (emit vapour) in a vacuum envi- ronment.