Mechanical Engineering

The Structures and Mechanisms team is responsible for maintaining the CAD model of ELFIN as well as assisting with any integration, assembly, and testing of the spacecraft.

 

An expanded view of the ELFIN spacecraft showcasing all subassemblies within the chassis.

 
mech_alexf.jpg

Structure and Design

The ELFIN spacecraft are 3U+ CubeSats, measuring 10cm x 10cm x 30cm with a mass of about 3.5kg. The satellite chassis is built from 6061 aluminum rails and “top hats” which are fastened at the corners with non-magnetic steel screws. The sensitivity of the magnetic instruments onboard the spacecraft necessitate a magnetically clean design; while most of the fasteners used for ELFIN are brass, non-magnetic A286 and 316 SS screws were used where brass was deemed structurally insignificant.

There are four major subassemblies housed within the chassis. The first is the Energetic Particle Detectors (EPD) and their respective PCBs. The EPD is responsible for measuring high-energy electrons and ions. This instrument was developed in-house at UCLA.

The next subassembly is the Fluxgate Electronics (FGE). This sensitive “gold brick” is reinforced with an aluminum H-brace and a flexible Delrin strap to ensure it is fully constrained during the Delta II launch.

The stacer can houses the fluxgate magnetometer (FGM) as well as the stacer boom. The boom is 75cm long and allows the instrument to be completely clear of the spacecraft bus for data collection.

Lastly, the avionics stack is mounted at the opposite end of the chassis. Composed of the Flight Computer, batteries, radio, and more, the avionics stack oversees sending and receiving commands and controlling the spacecraft. PEEK braces are used to compress the stack and mount it to the chassis rails.

Deployable Mechanisms

There are two main deployable systems on the ELFIN spacecraft: the “tuna can” antenna and the FGM boom. The tuna can antenna system makes use of the additional volume allowed in the 3U+ CubeSat design. The antenna elements are rolled and stowed prior to launch. Once the spacecraft is successfully deployed from the P-POD, resistors will burn through the lines constraining the elements, allowing them to unfurl. The FGM is housed inside the stacer can, and will likewise be deployed once ELFIN is out of the P-POD. To induce a separation between the two ELFIN satellites, one of the satellites will deploy its stacer boom before the other in order to experience greater atmospheric drag. Both mechanisms underwent a series of deployment tests, including stowing the tuna can antennas for over 6 months and testing their deployment in our thermal vacuum.

 FMA and FMB stacer booms and FGM instruments seen during stacer deployment testing.

FMA and FMB stacer booms and FGM instruments seen during stacer deployment testing.

 Due to the small form factor of CubeSats, precise machining was needed to ensure proper clearances and alignment. The SMAR on the stacer can nests perfectly into the cutout on the Y torquer coil.

Due to the small form factor of CubeSats, precise machining was needed to ensure proper clearances and alignment. The SMAR on the stacer can nests perfectly into the cutout on the Y torquer coil.

 The CNC machined ELFIN chassis, tophats and rails, coated with gold Alodine 1201 to prevent corrosion in space.

The CNC machined ELFIN chassis, tophats and rails, coated with gold Alodine 1201 to prevent corrosion in space.

Fabrication

The ELFIN Fabrication team is part of the Mechanical and Structures subsystem.  Fab is responsible for manufacturing the entire ELFIN chassis, instrument components, and customized brackets and fasteners, using materials such as Aluminum, copper, phosphor bronze, brass, ABS, Delrin, and PEEK (an expensive space-grade thermoplastic).  Nearly all ELFIN parts were fabricated and modified in-house using the EPSS Prototyping Lab in the basement of the Geology building, which allowed for quick turnaround time for  iterative design changes, last-minute modifications, and ultimately lower cost than using commercial manufacturing shops.  Over the past 4 years there have been ~25 undergrad machinists trained by their staff mentor, Emmanuel Masongsong, in conjunction with the Physics and Astronomy Student Shop.  In total, Fab produced 4 ELFIN Development Model units containing mass blocks for early assembly and vibration/shock testing, 3 Engineering Model units that contained functioning sensors, 2 Flight Model units and 1 spare, as well as many other support structures and testing platforms used during the mission.

Fab was required to first attain a 0.005" tolerance on all development model parts, with a final tolerance for all flight parts of 0.002" (less than the thickness of a piece of paper) throughout the entire spacecraft.  After receiving a CAD Solidworks model of each part from the Mech team, Fab members reviewed it thoroughly to confirm that the materials and features could be practically manufactured given our 2.5-axis Haas CNC machine capabilities.  Next, Fab had to painstakingly program the machining operations for the CNC to efficiently carry out.  This involved careful deliberation about the best order of operations to allow for holding the piece in the vise, drilling holes and removing large amounts of material while minimizing flex, shifting, and other errors that could lead to a failed part. Many parts such as the chassis rails and magnetorquer coils required specially made vise jaws and other novel fixtures to hold the parts safely and securely, which allowed them to maintain precision throughout each operation.  After completing parts, Fab was responsible for assessing Quality Control, measuring each part carefully to ensure compliance and proper fit.  Last, final sanding, buffing, and plating was performed to protect the materials against the harsh conditions of space, including alodine, anodization, and installing special non-magnetic threaded inserts.

 
 The twin assembled Flight Model units, ELFIN and ELFIN-STAR.

The twin assembled Flight Model units, ELFIN and ELFIN-STAR.