ORS, Vol. 6, No. 3 - Air Force Space Command

ue is much less likely to experience dynamic coupling with itslaunch vehicle (or just about any launch vehicle in the world’sinventory today). Put another way, spacecraft that achievethis minimum stiffness are more easily shifted from launcherto launcher with minimum changes to the systems engineeringintegration that is required. From the graph, the size (andassociated mass) of the spacecraft that consistently attains thisminimum are spacecraft weighing 200 kg or less. 10Small spacecraft yield other engineering benefits that scalepositively with decreasing size and mass. While a small spacecraftultimately has less electrical power available to it comparedto traditional large spacecraft (due to reduced surfacearea or deployed area for solar arrays), the challenges associatedwith thermal load dissipation are also remarkably easier.Operating in vacuum, all satellites eventually rely on radiativecooling to maintain operating temperature. On large-volumesatellites, complicated subsystems such as heat pipes and preferentialplacement of hot, energy-consuming components arerequired to maintain spacecraft thermal balance. The associatedcomputer modeling required to analyze on-orbit behavioris an intensive engineering activity on most large spacecraftprograms. By comparison, the distances involved in heat transferon small satellites are short and relatively straightforwardconductive paths. Other spacecraft subsystems benefit fromsimilar decreases in complexity at smaller scales, althoughthis result is not yet universally true (some subsystems have aminimum-size “form factor” given the current state of technology).As always, the cost and complexity of individual satellitesscales with their mass and volume.ESPA-class Satellite Standards and ImplicationsOver the past decade, a new class of satellites has emergedto take advantage of an emerging set of standards establishedfor the evolved expendable launch vehicle (EELV) secondarypayload adapter, or ESPA. ESPA began as a small businessinnovative research contract with CSA Engineering, Inc., as ajointly sponsored effort of the DoD Space Test Program (STP)and Air Force Research Laboratory’s Space Vehicles Directorateto accommodate secondary payloads on EELV. The ESPAring as designed can hold up to six 180 kg (maximum mass)satellites inserted underneath the primary payload (figure 2). 11ESPA was demonstratedsuccessfullyon its maiden flightin March 2007 duringthe STP-1 missionflown on an Atlas 401(figure 2a) and hasalso been flown successfullyas part of theNational Aeronauticsand Space Administration(NASA) LunarFigure 2 and 2a. Generic EELV ESPA configuration;ESPA first flight configurationon STP-1Launch, 9 March 2007.Crater Observationand Sensing Satellitemission in 2009. InFebruary 2008 the secretary of the Air Force directed that ES-PA-hosted satellite operations be normalized to support responsivespacelift; currently, the EELV budget supports one ESPAflight per year beginning in fiscal year 2012. 12Designing satellites to fly on ESPA is not a “natural” activityas any aerospace engineer can attest. Because the originalEELV specification left out any requirement for secondarysatellites, the ESPA design is a deliberate attempt to minimizeimpacts to the primary payload by simply raising the satellite24 inches inside the launch vehicle fairing. 13 This requires theESPA satellites to hang cantilevered off the ESPA ring, so theprimary launch loads (up to 8.5 g’s) are transmitted throughtheir transverse axes as compared to traditional satellites whichexperience their greatest loads in the axial direction. Additionally,the ESPA satellites experience a relatively severe shockenvironment (up to 400 g’s instantaneous at 1500 Hz) due tothe stiffness of the ESPA ring and the transmission of the primaryspacecraft separation loads. Despite these challenges,the ability to build ESPA-class satellites capable of survivingthese launch environments yields substantial benefits. TheBall Aerospace-built STP standard interface vehicle (SIV) isdesigned to ESPA standards and is compatible not only withESPA itself, but also readily transfers to launches from OrbitalSciences Corporation’s Pegasus, Minotaur I and Minotaur IVlaunch vehicles (and possibly SpaceX Corporation’s Falcon1) with minimum impact and no structural design changes. Infact, the first launch of SIV is occurring in summer 2010 (alongwith three other microsatellites designed to ESPA standards)on the STP-S26 small launch vehicle mission using a MinotaurIV launch vehicle with a STP-sponsored multi-payload adapter.This adapter holds the satellites in a traditional (i.e., axially oriented)configuration.CubeSats Gone WildAt the extreme low-end of satellite weight classes, a revolutionhas occurred in the past decade for satellites in the 0.5-10kg range. This revolution is enabled by technical standards forboth the satellites themselves and their launch vehicle dispensers.These socalledCubeSats(named for theirbasic one-unit[U] design, a 10centimeter [cm]x 10 cm x 10cm cube weighingno more thanFigure 3. Cal Poly CubeSat CP-4 photographedby AeroCube-2 (launched 17 April 2007 as secondarypayloads on a Russian Dnepr launcher).an equivalentliter of water,or 1 kg) werefirst proposed atthe turn of thecentury to fostereducational outreach for high school and college studentsvia hands-on satellite engineering (figure 3). 14 Given the processingpower of today’s commercial-off-the-shelf electronics,The Aerospace Corporation35 High Frontier