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.
Today Aero team had a design review for the doors, dynamic wheel fairings, removable solar array, battery ducts, and driver ducts for our next vehicle, Eos. Many of the Aerodynamics team parts and features will be laid-up with the carbon fiber body.
For Eos, we are using 18650B battery cells like the ones used in most common laptops. Battery cell performance goes down if the temperature gets too hot. Heat can even make the batteries unsafe to charge and discharge. Our solar vehicle will have more than 1200 batteries – compared to 3 to 9 in conventional laptops – so it is very important to ensure that they do not overheat. To to this, there will be three ducts on the front of the vehicle to draw in air as the vehicle moves. This will allow the batteries to be passively ventilated. Aero team uses NACA style ducts to minimize the drag caused by the inlets. The ducts are going to be laid-up with the carbon fiber body of the vehicle.
Dynamic fairings are another aerodynamics team system. They are simply doors that open when the wheels of the vehicle turn. They allow the front area of the car to be thinner – because the body of the car does not have to contain the wheels when they are turned at all angles. This decreases aerodynamic drag but increases the complexity of the design. Erik has a concept that will have the doors opening in conjunction with the motion of the steering rack – so that the wheels will never rub on the doors.