On this beautiful spring day, team members are concurrently working on testing our previous car and constructing our new car. Tests of solar cell efficiency, stress and strain in suspension members, and pull-out strength for our carbon fiber chassis panels have been running smoothly.
Suspension testing is focusing on finding stress and strain in the a-arms, which connect the upright to the chassis. The strain experienced by the members can be used to find the forces acting on the tire at the contact patch with the ground. The team currently designs its vehicles for a worst case senario of a 4G bump, 1G turn, and 1G braking load. These tests will give the team knowledge of the actual loading seen by a vehicle driving down the road.
In addition to suspension testing, Aero team was in the shop today to complete a layup for a removable array panel. The panels are made of fiberglass to avoid shorting the solar cells. Carbon fiber is electrically conductive, so the team is careful to ensure that all electrical components do not have direct contact with the carbon fiber shell and chassis.
The team laid up sharp offsets into the shell, so that panels, doors, windows, and other openings can be filled with separate panels. Those separate panels have a better chance of having good seam lines than if the team just cut out the doors from the shell. This is important for aerodynamics!
Our molds have endured the trip to and from the Delta composites shop, and are now being put to more use for laying up our panels.
Last week our team began the process of building the Aerodynamic shell of our vehicle. We had the opportunity to us the Delta Tech Ops Composite shop at the Minneapolis/St.Paul Airport. The week started off rough, with both molds cracking during transport due to thermal shock. We will be sure to find a functioning heater for our trailer, next time. However, we got to try out our new winch and it worked great!
Once at the Composites shop, we unloaded our molds and repaired them. There was a well-ventilated sanding area and the Delta employees supplied us with extra tools as we needed them. We are very grateful for the use of their shop. They are allowing us to come back for another week, because we were unable to finish all of our lay-ups due to the cracking of our molds.
This week has been a great learning experience for a young team. The completion of our first wet lay-up took patience and trial and error. Along the way, everyone got a little sticky, and everyone had fun!
The composites shop has a large Autoclave that we were allowed to use to cure our part in. Just for reference, our mold base is over 6′ across and 16′ long.
Once the wet fabric is laid on the mold, the part is vacuum bagged and all of the air is sucked out of it to push the fabric against the surface of the mold. The mold was then baked in the autoclave for 12 hours to cure the epoxy resin and hardener, which formed a hard plastic interwoven with carbon fiber fabric. Composites like these are often used on aircraft and spacecraft, and are extremely strong for their weight.
This year, the team is using 18650B type battery cells. The batteries will be held together with ABS plastic brackets. The team chose to manufacture the brackets by milling them out using the CNC machine in the University of Minnesota mechanical engineering student shop. This involved using featureCAM to create tool paths and G-code to run the machine.
A jig was also created so that the blanks could be bolted to a plate and then milled out, four at a time. Despite streamlining the process by creating a fixture, the parts will take the team over two weeks to fully machine because the student shop is only open during business hours, and most team members are full-time students.
The battery pack will be assembled by welding nickel shim onto each of the batteries through the bracket. The nickel is highly conductive and easy to weld. Soldering the batteries could damage the cells if they got too hot. The team built a spot-welder which was used on Daedalus and will be used again on Eos. Below is an image of the cutting of the nickel pieces, donated by Diamond Metal Products in Ham Lake.
There were some difficulties in cutting out the nickel with the laser, but they were overcome by using a backing material when cutting out the fragile pieces.
Look for another post in about two weeks regarding the rest of the manufacturing process for our battery! Below is a closer image of a completed bracket.
Aerodynamics team has been hard at work finishing the mold – milled by Cirrus Aircraft in Duluth – by filling in cracks with drywall plaster and painting.
We received a donation of 10lb foam from Dyplast, and then the foam was milled square up at Enberg Logging Supply in motley, MN. The foam was glued together using a foaming compound that was intended to fill all of the gaps between the blocks. However, the glue did not foam as much as expected and there were larger cracks to fill in the mold.
We are very grateful that Enberg Lumber Supply was able to mill down our foam blocks into square shapes. During the Daedalus build cycle, the team milled down the raw foam buns into square shapes by using a home-made jig and routers. This resulted in much more dust than for Eos. However, there is still foam dust on most surfaces in the shop.
The mold started off with just foam, and Aero team was careful not to sand it too much. Once the mold was smooth, alternating coats of dark and light paint were applied and sanded down so that Aero team could see if they sanded through a layer of paint.
Aero team set up a paint booth that takes up about half of our shop. The booth has ventilation so that the sanders and painters can get fresh air. All team members working on the molds use respirators to avoid breathing paint dust. Despite having a booth, paint dust got everywhere.