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 7075 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, 3U and 6U frames are more prevalent, however 12U frames are becoming more available as well. As there are now dispensers for the 12U CubeSat structure, it is a new standard for CubeSat configuration. 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, and ranging in size from sub-1U to the larger 6U – 12U SUPERNOVA structures. Pumpkin offerings are machined from Al 5052-H32 and can be either solid-wall or skeletonized 1. Pumpkin offerings are machined from Al 5052-H32 and can be either solid-wall or skeletonized; see Figure 6-1 for their skeletonized 1U construction.
Pumpkin has developed the SUPERNOVA, a 6U and 12U structure that features a machined aluminum modular architecture. The structure, shown in Figure 6.2, 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 1.
CLYDE SPACE CS CUBESAT STRUCTURE
Clyde Space also offers a monocoque CubeSat structure from 1U – 3U. The 1U chassis has a total mass of 0.155 kg and dimensions of 100mm x 100mm x 113.5mm. The 2U structure has a mass of 0.275 kg and dimensions of 100mm x 100mm x 227mm. The 3U structure has a mass of 0.155 kg and dimensions of 100mm x 100mm x 113.5mm. Clyde Space design sought to standardize their components to make the spacecraft easier to reconfigure than other off-the-shelf structures as both 1U and 3U structure interfaces with all standard deployment PODs, including NanoRacks 2.
Modular Frame Designs
EnduroSat provides 1U/1.5U/3U/6U CubeSat structures that range in dimensions: 100x100x113.5 to 100×226.3×366 mm (1U – 6U), and the material for all Endursat structures are made of Aluminum 6061-T651 (see Table 6-1 for complete list). While the 1U structure has gone through all the qualified testing and was deployed as EnduroSat-1 in July 2018, the 3U and 6U still have Thermal Cycling and Vacuum, as well as Radiation analysis 3.
|Table 6‑1: Endurosat CubeSat Structures|
|Structure||Dimensions (mm)||Primary Structure Mass (kg)||Material||TRL Status|
|1U||100x100x114||0.1||Al 6061 or 7075||9|
|1.5U||100x100x170.2||0.11||Al 6061 or 7075||Unkn.|
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. The 1 – 3U form factors have masses from 0.090 kg 0.172 kg, and 0.254 kg for 1U, 2U and 3U respectively. 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 4.
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 5.
INNOVATIVE SOLUTIONS IN SPACE (ISIS) STRUCTURES
ISIS offers a wide array of CubeSat structures, with the largest being a 16U structure coming in 2018. Several of their 1U, 2U, 3U and 6U structures have been flown in LEO, see Table 6-2 for more information on these structures.
|Table 6-2: ISIS CubeSat Structures|
|Structure||Dimensions (mm)||Primary Structure Mass (kg)||Primary + Secondary Structure Mass (kg)||TRL Status|
|2U||100 x 100 x 227||0.16||0.2||9|
|3U||100 x 100 x 341||0.24||0.3||9|
|6U||100 x 226 x 340.5||0.9||1.1||9|
|8U||226 x 226 x 227||1.3||1.9||Unkn.|
|12U||226.3 x 226 x 341||1.5||2.0||7|
With the design by ISIS, the CubeSat creator can consider multiple mounting configurations, allowing a high degree of creative flexibility. Detachable shear panels allow for access to all of the parts of the spacecraft’s electronics and avionics, so accessibility is still possible after final integration (ISIS, 2018). See Figure X for a 6U modular approach by ISIS.
GOMSPACE NANOSATELLITE STRUCTURE
GOMspace provides full turn-key solutions for small satellite systems. They offer modular nanosatellite structures from 1 – 6U with a strong flight heritage. The 6U has a 4U payload allocation, mass of 8 kg, and propulsive configuration capabilities. The 3U structure was first deployed from the ISS in 2015 and two 6U systems were deployed early 2018. The 7075 aluminum structure weighs 1.06 kg 6.
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 6‑7, 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 7.
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. For 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 6‑8 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) 8.
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 9. An image of the COBRA is shown in Figure 6.6.
The KRACKEN Robotic Arm is a modular, high-dexterity (up to 11 DOF) robotic arm that will provide capabilities for the CubeSat form factor to perform challenging missions: on-orbit assembly satellite servicing, and debris capture. The arm has a mass of 4.2 kg and can be stowed in a 3U volume with a 2 m diameter hemispherical workspace per arm 10. The TRL for this system is 5.
AlSat-1N: AstroTube Deployable Boom
Oxford Space Systems collaborated with Algeria to develop the AstroTube deployable boom that was recently demonstrated on a 3u CubeSat called AlSat-1N in LEO. It is the longest retractable boom that has been deployed and retracted on the 3U CubeSat platform. It incorporates a flexible, composite structure for the 1.5 m-long boom element and a novel deployment mechanism for actuation. When retracted, the boom is housed within a 1U volume and has a total mass of 0.61 kg 11.
ROCCOR: Deployable Booms
ROCCOR has developed several different deployable booms that have a wide range of applications on a small spacecraft. The ROC (Roll Out Composite) Boom can be deployed with antennas and instruments. This boom has a 1 – 5 m length option and is made out of carbon fiber composite shells that uses a passive spring to unroll the device. The TRAC (Triangle Rollable And Collapsible) Boom, originally developed for AFRL, can be as long as 23 ft.
The CubeSat ROC Boom Deployer is awaiting launch at the end of 2018 to reach TRL 7. The volume of this deployer is 1x1x1.5U, has a length up to 1.5 m, and a total mass of <1 kg. The power this boom deployer uses is x W.
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 12. Table 6.1 highlights a few key specifications for this actuator 13.
|Table 6‑3: Honeybee CubeSat SADA|
|Mass (slip ring option)||0.18 kg|
|Operating Temperature Range||-30 to +85°C|
|Size||100 x 100 x 6.5 mm|
|Radiation Tolerance||10 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 14. Figure 6‑11shows a model of the FD04 Frangibolt actuator and Table 6.2 describes a few key specifications.
|Table 6‑4: TiNi FD04 Frangibolt Actuator|
|Power C||15 W @ 9 VD|
|Operating Temperature Range||-50 to +80°C|
|Size||13.72 x 10.16 mm|
|Holding Capacity||667 N|
|Function Time Typically||20 sec @ 9 VDC|
|Life||50 cycles MIN|
Other offerings from TiNi Aerospace include the Ejector Release Mechanism (ERM), a device which provides high load holding capability and fast actuation time; the Micro Latch, which was developed specifically for new space applications and can release preloads up to 50 lbf; the Pinpuller, a trigger mechanism that retracts with a force of 5 – 1000 lbf; and the Optical Shutter, a simple and effective solution to an actuating aperture for light sensitive instruments.
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
3D Systems Corporation has developed 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 6‑14, originally developed at the University of Texas Austin and now being developed for several missions at Georgia Institute of Technology. Table 6.6 shows a summary of material properties published by 3D Systems 15. The 3D printed attitude thruster designed for BioSentinel, a 6U interplanetary spacecraft that will be launched with EM-1 in 2019, is made from Accura Bluestone 16.
|Table 6-5: Accura Bluestone|
|Glass Transition (Tg)||78-81°|
|Tensile Strength||66-68 MPa|
|Tensile Modulus||7600-11700 MPa|
|Flexural Strength||124-154 MPa|
CRP Technology is using selective laser sintering (SLS) technology for their carbon filled polyamide based material, called Windform XT 2.0. The Windform material has been tested under VUV radiation exposure and did not show any signs of degradation 17. Table 6.5 shows a summary of material properties published by CRP.
|Table 6‑6: Windform XT2.0|
|Tensile Strength||83.84 MPa|
|Tensile Modulus||8928.20 MPa|
|Resistivity, surface||< 108 Ohm|
TuPOD is a nanosatellite that was launched in September 2016 and was constructed using the Windform XT 2.0 from CRP. The successful operation of TuPOD is exciting to the small satellite world because its innovative 3D structure is one of few structures of its kind.
The Montana State PrintSat mission is a technology demonstrator spacecraft for the effectiveness of additive manufacturing using the Windform XT material. Figure 6‑12 shows the complete spacecraft 18and Figure 6-14 shows the primary printed structure. The spacecraft is equipped with several sensors to investigate the properties of the material during its mission 19. PrintSat was unfortunately lost during launch failure in November 2015 and it is unknown whether or not the mission will return.
The Morehead State University’s Rapid Prototyped MEMS Propulsion and Radiation Test (RAMPART) spacecraft will also demonstrate 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.
Made in Space
In 2016, Made in Space introduced a permanent manufacturing facility, the Additive Manufacturing Facility (AMF), aboard the ISS that provides hardware manufacturing services to NASA and the U.S. National Laboratory onboard. The AMF is the first commercially available manufacturing service in space, enabling several on-orbit manufacturing capabilities and providing research opportunities for terrestrial and space-based 3D printing applications, such as CubeSats 20. The MakerSat mission is a proof-of-concept that will utilize the AMF and demonstrate microgravity additive manufacturing, assembly, and deployment of a CubeSat from the ISS. MakerSat-0 will monitor characteristics of different plastics in the vacuum of space in preparation for MakerSat-1 – a CubeSat to be manufactured entirely on the ISS 21.
On the Horizon
In 2017, Tethers Unlimited was awarded a grant through the SBIR (Small Business Innovation Research) to develop the COBRA-Bee carpal-wrist mechanism for NASA’S Astrobee robot. The Astrobee is a small free-flying robot that will assist astronauts aboard the International Space Station (ISS), and the COBRA-Bee gimbal will enable the Astrobee to precisely point and position sensors, grippers, and other tools 22. COBRA-Bee will provide this precise multi-purpose pointing and positioning capability in a small-scale tightly integrated COTS product, with an interface to support third-party sensors, end-effectors, and tools. The Phase I effort will define requirements for a detailed design, based upon a crew safety analysis and a survey of candidate Astrobee end-effectors. A demonstration will be performed with existing COBRA hardware, maturing the COBRA-Bee TRL to 4 22.
RSat-P: Robotic Arms
RSat-P (Repair Satellite-Prototype) is a 3U CubeSat that is part of the Autonomous On-orbit Diagnostic System (AMODS) built by the US Naval Academy Satellite lab to demonstrate capabilities for in-orbit repair systems. RSat-P uses two 60 cm extendable robotic arms with the ability to maneuver around a satellite providing images and other diagnostic information to a ground team. The first robotic arm prototype was scheduled for a launch in early 2017, but has since been postponed for some time in 2018 23.
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 4? 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” (2? 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.