Since the last edition of this report there has been further expansion of offerings for off-the-shelf structures and likewise an expansion of custom machined, composite, and even printed structures used, or proposed for use, on small spacecraft missions. This chapter will refer to small spacecraft structures with a focus on 1U – 12U platforms and specifically those components designed to transmit loads through the spacecraft to the interface of the launch and deployment system and provide attachment points for payloads and associated components. These structures are typically classified as the primary structure. For contrast, secondary structures are all other structures, like solar panels, thermal blankets etc., that only need to support themselves. When a primary structure fails it is almost always catastrophic, while a failure of a secondary structure typically does not affect the integrity of the spacecraft but can have a significant impact on the overall mission. These structural categories serve as a good reference but the lines between them can be hard to distinguish for small spacecraft since they are particularly constrained by volume. This is especially true for cubesats, as the capabilities of these spacecraft have expanded but the volume afforded by the standard dispensers (by definition) have not. Therefore, it is often essential that the structural components be as volume efficient as possible. To achieve this volume efficiency, the primary structural components must not only carry mechanical loads, but may also serve as the primary component for thermal management, provide primary means for radiation shielding, serve as a pressure containment vessel, and even behave as a strain actuation component- features that are often assigned to secondary structural components in larger spacecraft.
Important to any discussion of small spacecraft structure is the material of the structure itself. Typically a spacecraft’s structure is made up of both metallic and non-metallic materials. Metals are commonly homogeneous and isotropic, meaning they have the same properties at every point and in every direction. Non-metals, such as composites, are normally neither homogeneous nor isotropic. Material choice is driven by the operational environment of the spacecraft and must ensure adequate margin for launch and operational loads, thermal balance and thermal stress management, and by the sensitivities of the instrumentation and payload to outgassing and thermal displacements.
The design of the structure is not only affected by the different subsystems and launch environments, but also the application and operations of the spacecraft, such as the configuration differences for a spin versus a 3-axis stabilized system. Instrumentation also places requirements on the structure and can require mechanisms, such as deployable boom to create some distance between a magnetometer and the spacecraft to mitigate its effect on the measurement.
Also included in this chapter is an overview of radiation effects and some mitigation strategies as it impacts structural design considerations for small spacecraft.
State of the Art
Two general approaches are common for primary structures in the small spacecraft market: off-the-shelf structures and custom machined or printed components. Maybe unsurprisingly, most off-the-shelf offerings are for the cubesat market. Often the off-the-shelf structures can simplify the development of a small spacecraft, but only as the complexity of the mission, subsystems, and payload requirements fall within the design intent of the off-the-shelf structure offered.
There are now several companies that provide cubesat primary structures (often called frames or chassis). Most are machined from 6061-T6 or 7071 aluminum and are designed with several mounting locations for components in an attempt to offer configuration flexibility for spacecraft designers. This section will highlight several approaches taken by various vendors in the cubesat market. Of the offerings included in the survey, 1U to 3U formats are most prevalent, but there are a few frames that are designed for 6U and even 12U spacecraft. To date, however, the author is not aware of any mission that has flown, or even gone beyond the proposal phase for a 12U spacecraft. Also of note, the exact format of the 12U cubesat seems to be evolving. This trend is similar to the development of the 6U and is typical until a dispenser is qualified, which tends to set the standard for the exact dimensional constraints of the spacecraft within.
PUMPKIN, INC. CUBESAT KIT
The structural approach taken by Pumpkin for their 1U – 3U spacecraft is of a monocoque approach, where loads are carried by the external skin in an attempt to maximize internal volume. Pumpkin, Inc. provides several off-the-shelf cubesat structures intended as components of their CubeSat Kit solutions in 0.5U, 1U, 1.5U, 2U and 3U size .
All structures strictly adhere to the cubesat standard (1U = 10 x 10 x 10 cm), and consist of riveted sheet-metal construction, offered as skeletonized in Figure 6.1 and solid-wall configurations.
Modular Frame Designs
NANOAVIONICS MODULAR FRAME
NanoAvionics has developed what it calls “standardized frames and structural element” that when assembled form the primary structure for 1U to 12U spacecraft. A modular 3U structure from NanoAvionics is shown in Figure 6.2. These components are intended to be modular, made from 7075 aluminum, and like many off-the-shelf cubesat structures, compliant with the PC/104 form factor .
RADIUS CUBESAT STRUCTURES
Radius Space has also chosen a highly modular approach to develop a family of cubeSat structures that range from the 1U to 12U sizes. Figure 6.3 shows this modular approach for 1U to 3U sizes. PCB integration is typically accomplished through a stacked configuration, although Radius Space asserts the structures allow for different PCB orientations for all but the 1U frame .
Card Slot System
COMPLEX SYSTEMS & SMALL SATELLITES (C3S)
C3S has developed a 3U cubesat structure that utilizes a card slot system, as shown in Figure 6.4, which is intended to provide several stated benefits over the more common PC/104 stack solution. These benefits include access to individual cards during integration and testing (PC/104 solutions require de-integration of an entire stack to isolate a single card), improved stack-up tolerance, and better thermal management of individual cards compared to a traditional PC/104 stack, where all cards are connected series and are thermally interdependent .
There are several companies offering mechanisms for the small spacecraft and smaller markets. Although not exhaustive, this section will highlight a few devices for release actuation, component pointing, and boom extension, that represent the state of the art for the cubesat market. Deployable mechanisms used for deorbit devices, please refer to the Deorbit chapter.
CDT: Deployable Booms
Composite Technology Development (CTD) has developed a composite boom called the Stable Tubular Extendable Lock-Out Composite (STELOC), that is rolled up or folded for stowage and deploys using stored strain energy. The slit-tube boom, shown in Figure 6.5, employs an innovative interlocking edge feature along the tube slit that greatly enhances stability. The boom can be fabricated in many custom diameters and lengths, offers a small stowed volume, and has a near-zero coefficient of thermal expansion (CTE) .
Tethers Unlimited: 3 DOF Gimbal Mechanism
Tethers Unlimited offers a 3 DOF gimbal mechanism called the Compact On-Board Robotic Articulator (COBRA). This mechanism provides accurate pointing for sensors and thrusters. The COBRA packages down to 100 x 100 x 33.25 mm and weighs 155 grams . An image of the COBRA is shown in Figure 6.6.
Honeybee: Solar Panel Drive Actuator
Honeybee in cooperation with MMA has developed a Solar Array Drive Actuator (SADA), shown in Figure 6.7, that accommodates ±180° single axis rotation for solar array pointing. Honeybee also offers the unit in a slip-ring configuration for continuous rotation . Table 6.1 highlights a few key specifications for this actuator .
|Mass (slip ring option)180 g|
|Operating Temperature Range-30 to +85°C|
|Size100 x 100 x 6.5 mm|
|Radiation Tolerance10 krad|
|Wire Wrap7 channels per wing @ 1.4 A per channel|
|Slip Ring10 channels per wing @ 0.5 A per channel|
TiNi Aerospace: Frangibolt Release Actuator
TiNi Aerospace has several release mechanisms available for the spacecraft market, but perhaps the most relevant to the cubesat market is the Frangibolt Actuator (particularly the FD04 model), due to its small size and power specifications. The Frangibolt operates by applying power to a Copper-Aluminum-Nickel memory shape alloy cylinder which generates force to fracture a custom notched fastener in tension. The Frangibolt is intended to be reusable by re-compressing the actuator using a custom tool and replacing the notched fastener . Figure 6.7 shows a model of the FD04 Frangibolt actuator and Table 6.2 describes a few key specifications.
|Power15 W @ 9 VDC|
|Operating Temperature Range-50 to +80°C|
|Size13.72 x 10.16 mm|
|Holding Capacity667 N|
|Function TimeTypically 20 sec @ 9 VDC|
|Life50 cycles MIN|
On the Horizon
6U and 12U off-the-shelf structures
Off-the-shelf 6U and 12U structural components are products that could be considered “on the horizon,” though several companies offer 6U and 12U chassis or frames for purchase. However, it appears that at least the 6U offerings will likely have flight heritage within the year 2016.
Pumpkin Supernova 6U
Pumpkin has developed what it has named the “Supernova,” a 6U structure that features a machined aluminum modular architecture. The structure, shown in Figure 6.8 , is designed to integrate with the Planetary Systems Corporation (PSC) Canisterized Satellite Dispenser, and includes accommodation for the PSC Separation Connector for power and data while integrated .
Innovative Solutions In Space 6U
Innovative Solutions In Space (ISIS) has also adopted a modular approach to a 6U structure to maximize payload flexibility. The 6U structure shown in Figure 6.9 is designed for integration within the ISIS Launch and Deployment System, and although not yet flown, is currently used for multiple spacecraft slated for launch in the near future .
Additive Manufacturing Materials
The use of additive manufacturing for spacecraft primary structures has been proposed for several years, but only now has this process been adopted by flight missions (it is important to note however, that additive manufacturing has been quite common for small spacecraft secondary structural elements for many years). Typically, the advantage of additive manufacturing is to free the designer from manufacturing constraints imposed by standard manufacturing processes and allow monolithic structural elements with complex geometry. In practice however, additive manufacturing has its own set of geometric constraints, but when these constraints are understood and respected, the designer can approach a design challenge with a larger tool set that has not been available in the recent past.
CRP Technology is using selective laser sintering (SLS) technology for their carbon filled polyamide based material, called Windform XT 2.0. Table 6.3 shows a summary of material properties published by CRP .
|Tensile Strength83.84 MPa|
|Tensile Modulus8928.20 MPa|
|Resistivity, surface< 108 Ohm|
The Montana State PrintSat mission is a technology demonstrator spacecraft for the effectiveness of additive manufacturing using the Windform XT material. Figure 6.10 shows the complete spacecraft  and Figure 6.11 shows the primary printed structure. The spacecraft is equipped with several sensors to investigate the properties of the material during its mission .
The Morehead State University’s Rapid Prototyped MEMS Propulsion and Radiation Test (RAMPART) spacecraft will also be demonstrating the rapidly prototyped Windform material during its mission. The entire structure is made of high phosphorus, electroless nickel plated material to provide radar reflectivity for tracking purposes. Benefits of the RAMPART propulsion system are the lightweight and specialized cell structures of the propellant tank made from Windform XT. The spacecraft was scheduled for launch in June 2013, but was delayed.
3D Systems Corporation has developed a a stereolithographically fabricated composite material that shows promise for spacecraft structural applications. This material is currently being used as the main structural component for nozzles, tubing, and storage of a cold-gas propulsion system shown in Figure 6.12, originally developed at the University of Texas Austin and now being developed for several missions at Georgia Tech University. Table 6.4 shows a summary of material properties published by 3D Systems .
|Glass Transition (Tg)78-81°|
|Tensile Strength66-68 MPa|
|Tensile Modulus7600-11700 MPa|
|Flexural Strength124-154 MPa|
|Outgassing, TMLlow (To be measured in 2016)|
Radiation Effects and Mitigation Strategies
Shielding from the Space Environment
Shielding the spacecraft is often the simplest method to reduce both a spacecraft’s ratio of total ionizing dose to displacement damage dose (TID/DDD) accumulation and the rate at which SEEs occur if used appropriately, and involves two basic methods: shielding with the spacecraft’s pre-existing mass (including the external skin or chassis, and exists in every case whether desired or not), and spot/sector shielding. This type of shielding, known as passive shielding, is only very effective against lower energy radiation, and is best used against high particle flux environments including the densest portions of the Van Allen belts, the Jovian magnetosphere and short lived solar particle events. In some cases increased shielding is more detrimental than if none was used, owing to the secondaries generated by highly penetrating energetic particles; therefore it is important to analyze both the thickness and type of materials used to shield all critical parts of the spacecraft. The final design consideration is due to the strong omni-directionality of most forms of particle radiation, where spacecraft need to be shielded from the full 4π steradian celestial sphere. This brings the notion of shielding per unit solid angle into the design space, where small holes or gaps in shielding are often only detrimental proportionally to the hole’s solid angle as viewed by the concerned EEE component. Essentially, completely enclosing critical components should not be considered a firm design constraint when other structural considerations exist.
Inherent Mass Shielding
Inherent mass shielding consists of utilizing the entirety of the pre-existing spacecraft’s mass to shield sensitive electronic components that are not heavily dependent on their location within the spacecraft. This often includes the main spacecraft bus processors, power switches, etc. Again the notion of shielding per unit solid angle is invoked here, where a component could be well shielded from its “backside” (2π steradian hemisphere) and weakly shielded from the “front” due to its location near the spacecraft surface. It would only then require additional shielding from its front to meet operational requirements. The classic method employed here is to increase the spacecraft’s structural skin thickness to account for this additional shielding required. This is the classic method largely due to its simplicity, where merely a thicker extrusion of material is used for construction. The disadvantage to this method is the material used, very often aluminum, is mass optimized for structural and surface charging concerns and not for shielding either protons/ions or electrons. Recent research has gone into optimizing structural materials for both structural and shielding concerns and is currently an active area of NASA’s small business innovation research and small business technology transfer investment.
The process to determine exactly how much inherent shielding exists involves using a reverse ray tracing program on the spacecraft solid model from the specific point(s) of interest. After generating the shielding-per-unit-solid-angle map of the critical area(s) of the spacecraft, a trade study can be performed on what and where best to involve further additional shielding.
Ad Hoc Shielding
There are two types of ad hoc shielding utilized on spacecraft: spot shielding, where a single board or component is covered in shield material (often conformally); and sector shielding, where only critical areas of the spacecraft have shielding enhancement. These two methods are often used in concert as necessary to further insulate particularly sensitive components without unnecessarily increasing the overall shield mass and/or volume. Ad hoc shielding is more efficient per unit mass than inherent mass shielding because it can be optimized for the spacecraft’s intended radiation environment while loosening the structural constraints. The most recent methods include: multiple layer shields with layer-unique elemental atomic numbers which are layered advantageously (often in a low-high-low Z scheme), known as “graded-Z” shielding, and advanced low-Z polymer or composite mixtures doped with high-Z metallic microparticle powders. Low-Z elements are particularly capable at shielding protons and ions while generating little secondary radiation, where high Z elements scatter electrons and photons much more efficiently. Neutron shielding is a unique problem, where optimal shield materials often depend on the particle energies involved. Commercial options include most notably Tethers Unlimited’s VSRS system for small spacecraft, which was specifically designed to be manufactured under a 3D printed fused filament fabrication process for conformal coating (a method which optimizes volume and minimizes shield gaps) applications.
The landscape for small spacecraft structural design as well as the firms developing and offering solutions for spacecraft designers are expanding. There are now at least a few different approaches to off-the-shelf frames or chassis, each one with its own set of merits, as well as new vendors offering small-sat specific radiation shielding solutions. Most of the developments have been in the 3U cubesat class and there are now at least a few examples of mature structural designs for 6U class cubesats, with 12U designs being presented for future standardization. There have already been some very interesting uses of 3D printed materials, and it appears that the application of these materials for space flight missions is on the very near horizon, including exploiting the ad hoc nature of its manufacture for purpose-built radiation shielding. Whether or not the promised benefits of these materials outweigh those of more conventional materials in the near future remains to be seen.