AIAA’07 MAA-8 7- 0583

Get Away Special Class Satellites (CANSATS): Design Constraints, Applications, and Recent Technology Advances at Utah State University

L. Rex Megill, Frank J. Redd, and Rex W. Ridenoure

Utah State University Logan, Utah

A M 25th Aerospace Sciences Meeting January 12-15, 1987/Reno, Nevada

GET AWAY SPECIAL CLASS SATELLITES (CANSATS): DESIGN CONSTRAINTS, APPLICATIONS, AND RECENT TECHNOLOGY ADVANCES

AT UI'AB STATE UNIVERSITY

L. Rex MegiU Pmfeaaor of Physics and Electrical Engineering

and

Franlr J. Redd h i a t a n t Director for Program Development

Center for Space Engineering and

Pmfeaeor of Mechanical Ehgineering

and

Rex W. Ridenoure Rcaeareh Amociate, Center for Atmorpharic and Space Sciencw

Utah State University

January, 1987

Loghn, Utah

INTRODUCTION

Over the past few yeam there has been a rising awareneaa for a need to make access to space lea e x p d v e thau har hktorically been the cam. Thh is not to ray that i n g r p h acuw to apace will raplace ament p m g r a q rather, it wil l allow for additional uldl of space, many of which we are not currently awara Recent naaga of the Get Amy Special (GAS) fsdlitier on the Shuttle and, more recently, concepk for the we of some of the newer commercial b c h vehiclaa make it p d b b to plan for experiments not premntly enviaimned in aiding programe.

The NASA GAS program hao been inahmen tal in developing mod of the current intend in low cost appaa experiments. There am many philosophid differencea between the GAS program and 'normal' space activities. In its original concept the program waa denigned to take advantage of the nooks and crannies d n t in any large cargo q&m. To date, 77 GAS d e n have flown, several with multiple aperiments aboard. At Utah

State University (USU) we have developed a philosophy with respect to these experiments which is much like that uned in a ground baed laboratory. The d t has been that the average experiment in a canister which is not vented to the atmcsphere costa about

transportation costa to the Cape. $2,000 when flown under our pmgram. Thie is appmximately doubled by added fight and W

One of the developmental wed of the GAS canister is the ejection of free flying eatellit- (CANSATS), which d t e d in the launch of the NUSAT & GLOMI2 satellites in the month prior to the Challenger tragedy. "him pmgram allows the launch of a satellite which is txsmtially a 1P diameter sphere from the GAS canister. The shape of the satellite viil largely driven by an early requirement that the satellite be able to rotate within the canister without sticking. The ahape dictated by thb requirement (which has eubsequently been relaxed) b shown in Figure 1.

NUSAT and GLOMR resulted in the establishment of the concept. It in the purpose of thin paper to explore Borne of the conatraints characteristic of thia program and to describe Borne of the capabilitiw available within these constraints.

The restriction on length of the satellite has been relaxed to allow a PAR (Payload Accommodation €&quat) requesting a longer length. It io believed that the current plana will allow for satellites aa long ar 36 incha although thia haa not bean announced aa a formal policy. Thb additional length allows for a substantial increase in the payload capabilities.

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The principle constraint for many individuab is the relatively l m number of launches which can be anticipated in the near futun. GAS experiments are flown on a %pace available', noninterference basis. In the part thh h a resulted in the flight of an aver- age of two GAS experimente per launch. Within the confunion currently extant around the Shuttle program it is extremely difficult to fomast future launch raks. Although there has been amurance that the GAS program will continue to &at , one thing seuns certain-there will be fewer aghb in the future than one could exped prior to Jan. 1986 which m e w there will be added competition for available slob. F'riority in the GAS program in tied to the date at which the reaervation is originally made and to the dote on &h the hunch SeroicU Agreement b aqncd

An additional consideration to many who are intensted in small satellites io the avail- ability of high latitude launchea. For many of the a p p h t b M desired for mall sakllitea, high inclinations are q u i d . One of the mat intewting is the 570 launch a m b l e from the Cape. These were infrequent at beat in the older schedule and may be equally infrquent in the future. In any case these are the options currently available to the d sakllite community.

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Figure 1. NUSAT satellite. Octahedron: 19 in. dia.; base: 1 in. deep x 9 in. dia

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h r many applicatioM, but certainly nr G all, it ie desirable to obtain launch to as high an altitude ag e b l e . In m a t casea ,he conetraining factor in the orbital Hetime of the satate. Because the Earth's atmw,here decreases exponentially with height, the lifetime is a etrong function of the altitude In addition to the deerease in satellite lifetime attributed to atmospheric density, there 1s also a variation with sunapot activity. The response of the atmosphere to thermal inputs from sunspot activity is very faat, with reaponse t i e s that can be leaa than one hour. If the heat input is eubseqnently removed, it can r e k to the old values as rapidly An an example, during some large sunspot activity in Feb. 1986, both NUSAT and GLC dR dropped on the o d e of 500 metem in one day. Under normal decay conditions, thb should have occurred in about a week. In Figure 2 we show a plot of the expected lifet iue as a function of initial altitude for &&tea of the general eiPe which can be launchec from the GAS canietere. Three curvea are shown, one for high sunspot activity , one for average and the other for low activity. Ab0 plotted is a curve for NUSAT together with the point for the life of the satellite aa obwrved. The atmosphere during the life of NUSAT has been annsnally cool, which dowed NUSAT to have a life at least twice that to be expected during the maximum of the solar cycle.

A persistent constraint with sakllitea of this small she is the relatively limited amount of paver available. For example, NUSAT had only about 5 watb or lem available when averaged over an orbit. The energy storage on board consisted of apprcaimately 50 watt horn. With the satellitea pwible with the new launch configuration, one can expect to do somewhat better with q e c t to energy collection. In Figure 3 we show plots of the expected power for orbital indinationa of 28.5O, 5 7 O and 900. Them plotr show conaiderable Variation throughout the year. Thin ir the d t of variations of the orbit p h e with reaped to the sun vector. The calculations aaaume gravity gradient stabfiation, and about so% population of all eight sides of a right octagonal cylinder. The solar cell &ciency is acmumed to be 10%.

STABILIZATION TECHNIQUES

NUSAT & GLOh4R were unstabilised satellites. For many applications it ici desirable to have some type of stabiliration. Methoda which men to be currently feasible indude gravity gradient stabiliation, rpin stabiiation and perhapa magnetic stabfiation. Active methodn inciuding momentum rhcek and/or reaction jet, have not beaa developed for satellites of this sire, although it would appear that such tcchniquca are poaible with d c i e n t developmental dort.

Gravity gradient stabiiation is our choice for a number of applications. It has the advantage that it ir passive and uea ahmat no resoma ona the gravity gradient boom ia deployed. Booms made of a variety of materiala am available in the asroepace market, but are perhaps somewhat more expensive than is viable for many applications. Review of the literature and some computations indicate that it should be possible to get stabiiation with boom lengths of three m e h or so and tip mamed of a few kilogramr. With these length one might expect to obtain libration and orbital p e r i d of the m e order with

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102

10

2 00

CAS SATELLITE PARAMETERS

Right oc tagona l c y l i n d e r Diameter 1 9 " Length 35" Mass 90 kg.

NUSAT PARAMETERS

Octahedron Diameter 19" Mass 65 kg.

ALTITUDE ( K M )

Figure 2. Lifetime of a GAS satellite Venus initial altitiide. .4) High solar activity; B) average conditions; C) low solar activity. The broken line IS co~pured usiug the octahedral geometry of NUSAT.

A

averape wells

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I5

10

5

0 0 23 46 68 92 114 137 160 183 205 228 251 274 297 319 342 365

day of the year

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C

average watt!

0 2 3 45 68 92 114 137 160 183 205 228 251 274 297 319 342 365 d a y O f me "ea-

Figure 3. -4verage so!.r wwer col!ected 3t different times of the year. The variation over the year is due to the chdge in relative angle between the orbital plane and the sun vector. Orbital inclination: A) 283'; B) 5 7 O ; C) 900.

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amplitudes of 100 to lSo. Suitable damping techniques may improve these values, but no systems for tbia sise satellite are, to our knowledge, developed at pnsent. One of the advantages of the gravity gradient technique b that if the momenta of inertia are properly dected not only k the vertical diredion known, but the direction of the velocity vector as well. Figure 4 shows an appropriate arrangement of the momenb of inertia The proper conditions for stability are

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Spin stabfiation haa historically been popular for many satellites. Spinning up d e l - lita of this eira with reaction jets would be poaaible with relatively little fuel. It should be noted, however, that if the satellite h u approximately codant density, the momenta of inertia are improper for stable spin around the axb of cylindrical symmetry. It will therefore be necessary to arrange the satellite so that spin occura about an axis perpen- dicular to the cylindrical d unlem considerable effort b taken to tailor the momenta of inertia to make spin about the cylindrical ;uds atable. For most payloadr thb attempt pnta intolerable constrainta on the distribution of components and subaystemr in the satellite. Figure 5 shows the etable and Mstable contlguratiom of a cylindrical satellite.

SIZE & WEIGHT

The aise of thew dellites is limited by the GAS facilities which accommodate a cylinder lP in diameter and 3P long. Any daired suayskmr mud be included within this envelope. The weight allowance haa been limited to the 200 lba previowly allocated for the GAS payloadr. This value includes the ejection mechanian. There is diecueaion concerning the possibility of increasing this allowance, but no definite guide lines have been published. It should be noted that, contrary to standard pactice for satatellib, it ia not neceudly d*le to make the satellite a light Y pcxaible. Under p m t regalatiom, the cod is constant for weight# up to 200 Iba Since the lifethe of a satellite placed into an orbit of a given height is proportional to the mass, thue is a premium on getting additional maw into orbit. The# condrahb, in particular the conrtraint on volume, make the pbilorophy amrounding the 1w of them gtsllitsr amowhat diftcrant than thore conventionally d

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POSSIBLE USES

G m that satellites in this weight category are available, what are the possible una? The optiom are aa varied aa the potential nsvr with the obdou examplea we wiU urn almwt certainly incomplete. The low power available and the relatively low c& probably dictate that these satellites are bed suitad to one or two applieatiom per dellite. If one needa additional aynterrm in orbit, the obvious anawer ia to jwt put up more!

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Figure 4. Geometry of a gravity-gradient stabilized satellite. "he gravity gradient boom (yaw axis) is oriented along the earth-satellite line, within f 10" to 15", at all points on the orbital path.

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Figure 5. Stable and unstable apin configurations of a cylindrical satellite.

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Store & Forward Communicatiom

One obvious use of these satellites is for store and forward communications. In this application, data are read from one station (most probably remote and unattended), stored on board and later transmitted back to the ground at the appropriate place. The wide range of poseible applications are, if not obvious, apparent after a little thought. They include remote geophysical stations, oil well data, oceanographic data, vocation data and probably a large number of others.

h m o b Obaervationr

Although these satellitea are small, it appears that relatively good resolution can be attained using modern detectora and electronics with a camera viewing the ground from space. Reliminay indication8 are that the principle challenges are the indnaion of adequate aise in the optical system. Our calculation8 show that with modern commercial CCDs, reeolution of 100 meters is readily available in this aise satellite. Higher reeolution may be available, but only at considerable added expense.

Materiala Testing

In the near future, one of the critical needs for the utiliration of space will be the testing of materials which will be used for space construction for the effects of the atmosphere of space, in particular the atomic oxygen and ionic species. In the past such etudia have been attempted with the NASA LDEF facility. Given the current U c u l t y with the LDEF, it is important to perform studies by any means available. Small satellites offer an opportunity .to do thb. Small samples of materiab mounted on the oubide of the satellite can be monitored by a variety of techniques to determine dects of atmospheric constituents. Instrumentation available which could be applied to this problem include quarts microbalances to monitor materiab loas, elastic coefscient measurements to look for changw in the bulk propertia of materiala and perham with some development, a video camera equipped with a miamope to obtain photomicrographs of the d a c e s . A variety of atmospheric instruments can be used to determine the flux of partides and their energies arriving at the surface.

Sciantitte 9.tamtea

A large variety of mienti& meaammaia can potentially be made from theae d satellita. The podbilities indude atmorpheric meatmremntr, magnetic field measure- ments and a variety of plyma experiments. The principal limitation is that relatively few snch experime.de can be installed on a single satellite, although with modern miniatnrisa- tion techniques, reasonable nnmbers can be accommodated.

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AL!L"ERNA!CE UmCH !IZCHNIQUES

As described a h , it d a s not appear that the Shuttle program wiU be able to satisfy the needs of the community for launches. It is therefore of intereut to explore alternate - meana of obtaining launch capability. Among the optiona are the uae of pi--back rid- on large satellites, the development of amail expendable vehides, and the dedication of a single larger rocket to place wed satellites into orbit, thereby reducing the cod per unit.

The cost atrudure of launches is critical to the amall aatellite field. It is ow esti- mate that, when built in mme quantity, good, reliable satellite %d can be produced commercially for about SW0,OOO. This would include the structure, the poner W e m , a command and control computer and a telemetry system, but not the experiment. When one comidexa launch coots, it would sam that an equal amount invested into the launch would be a m n a b l e level. Thh traaslata, for a 200 pound satellik, to a launch coat of about $2,500 per pound. What options would accomplish this?

The Brat option is, O ~ V ~ O U I I ~ Y , the Shuttle, using the GAS canisters, which is eamtially a snbddised launch. A second option is the ude of basically the same hardware on the Shuttle, but wing the Hitchhilrer program. T b program ir intended to m v e r a larger fraction of the launch cosb than d m the GAS program. It is not at preaent clear that the Hitchhiker program will be available for mch launches, although personnel in chaqe of the program do not rule out the pomibility.

The we of piggy-back r ida on larger llakllita is not at all unprecedented. The amateur radio coqmmity h u launched m d aatellita in thb mode with conaideable - mccesu It d o a have the disadvantage that many of the flight# are to geoaynchronoar transfer orbit4 which may not be the orbits of choice for the anall aatelkitea. The Costs of snch launches am not well established and will vary with the deal which can be made with lamcher of the laqe satellite.

We have left the possibility of relathly d rockets to get satdike into orbit. "here seem to be none in the Amuican Stable other than the Scout rocket. "hiE rocket is expensive (of the order of $8,ooO,OOO) and has relatively low launch capability. It would wem that it would be Mcult to get oatellikr of the CANSAT ah into orbit for under $2,MW,tHM per d t e . Some of the propored commamid hcha prodm to have launch cork of tbb oder, but all have thmhold cortr which make it necemary to launch many mail satellike at a time in o d e to make the launches feauible. Is t h e a way to broker a& launcha in order to reduce the coot p a item?

!l"E USU DEMONSTRATION SATELLITE

Utah State University is currently funded to build and launch a demonstration satellite of the CANSAT variety. "him project aiU be a cooperative venture betaan USU and Globerat, a d spin-off company in the USU &aearch Park. The project b, in part,

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spo~lsored by the Speny (now UniSys) Corporation.

The satellite will be a store and forward communications satellite and will be d, among other things, to collect geophyaical data from remote locations for the USU Water Reeearch Laboratories. A conceptnal deaign drawing of the satellite b ahown in Figure 6 and a block diagram of the satellite ia shown in Figure 7. Thia satellite will probably be used also for NSF communications to the Antarctic.

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A anall experiment ntilisig a few quare inches on the outer surface of the satellite will be used to test variations of electrical conductivity of several d a c e materiab in the space environment. "his b a preeruaor experiment for a system to be uaed later on the Shuttle.

Systemn which will be uaed and tested on thia satellite are the on board computer and memory system, an orientation sensor system developed at Globeaat and a gravity gradient stabfiation system, also developed by Globeaat.

This satellite is being developed ao that it can be either ejected from the Shuttle or be launched ad a piggy-back experiment from an expendable &et. It currently appeara that the earliest launch will be aboard a test rocket of the American Rocket Company.

WHAT FOB THE FUTURE?

One can think of a number of developmenta which would be nsefnl for satellites of this general capability. Them include LASEB gym for orientation, libration damping systcmc and mnm general purpow inahmentation for the monitoring of the apace envirommt. One can imagine as well that there will eventually be applications which call for active stabhation, added extendable power pan& and perhapa men some propnlsion to increase the orbital altitude.

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Inevitably there will be a demand for growth in sire of the satellites. An attempt b being made to prepam for thia eventuality by the d 6 g n of the FATCAN, an alternate Shuttle system which would be capable of ejecting aatellited of 36 inch diameter and 28 inch length.

We at Utah State Univemity think that an appropriate role for the UniVVaity is the continuation of the development of s u l m y b for uw by thorn needing satellib of thin general capability. We wiah to acknoarledge support in these area by Unisya Corporation and by Morton Thiokol.

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Figure 6. Configuration of the CANSAT satellite, depicted with its boom extended for gravity-gradient dabiliaation.

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Store and Forward Satellite System

I I Ratteries SOlW

cells Grabber Extension I

Orientation Sensors ?I i