The CubeSat Kit Approach
Given the relatively small size of the CubeSat Kit and the inherently poor volume-to-surface-area ratio endemic to the CubeSat form factor, Pumpkin's now space-proven approach to the design of CubeSat structures focused on several interdependent aspects of structural design:
- The need for maximal internal volume
- The need for lightness wherever possible
- The need for strength
- The ability to plate contact surfaces to meet the CubeSat Design Specification (CDS)
- The ability to create arbitrary surface features
- The desire to avoid a preponderance of fasteners, as they consume the available mass when used in large numbers
- The need to use non-magnetic fasteners
- The desire to use captive fasteners wherever possible
- The desire to present good electromagnetic shielding
- The need to satisfy thermal concerns
- The desire to be affordable, with off-the-shelf availability
The result of Pumpkin's trade-space analysis is aunique design based primarily on precision sheet-metal fabrication, with machined elements as required. Pumpkin's family of CubeSat Kit structures (in 0.5U, 1U, 1.5U, 2U & 3U overall lengths) all share common parts, with the only difference between sizes being the Chassis Walls Assembly. Additionally, only ten M3x5mm stainless-steel flathead screws are required to hold the entire structure together, regardless of size. Pumpkin's CubeSat Kit structures are the lightest and strongest CubeSat structures available, in large part because of the strength of each CubeSat Kit's monolithic Chassis Walls Assembly and its ability to resist forces in compression and in twist.
Sheet-metal CubeSat Kit chassis components are fabricated from 5052-H32 Aluminum sheet metal, hard-anodized and alodyned as required to meet the CDS. Machined CubeSat Kit Pro chassis components are fabricated from 7075-T6 billet Aluminum, hard-anodized and alodyned as required to meet the CDS. Individually machined Aluminum components (e.g. feet and Midplane Stadoffs) are made from 7075-T6 Aluminum, and are hard-anodized. The plating specifications for Aluminum components of the CubeSat Kit are as follows:
- (HARD) ANODIZE, MIL-A-8625F, TYPE III, CLASS 1 (.0005" MIN to .0008" MAX)
- (CLEAR) NON-HEXAVALENT CHROMATE, MIL-DTL-5541F Type II, CLASS 1A (RoHS COMPLIANT)
All fasteners (captive and loose) are made from nonmagnetic stainless steel. Additional hardware (e.g., inter-module spacers) are generally made from stainless steel or aluminum, with conductive platings.
Modularity through Expansion
While the standard CubeSat Kit structure satisfies the needs of many CubeSat missions, some missions require more flexibility in accommodating different user structures. Therefore the CubeSat Kit can easily accommodate external payloads -- i.e. user structures that are attached to CubeSat Kit structural elements via a simple CubeSat Kit External Payload Specification. This scheme has been proven in Pumpkin's MISC 2 nanosatellite bus concept, delivered to the U.S. government and other organizations as part of the NRO's Colony I program.
The CubeSat Kit System Chart illustrates how the various structural components can be combined to form CubeSats of different sizes. Standard CubeSat Kit structural elements (shown in orange) can be used standalone, or in conjunction with user payloads (shown in yellow).