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Prototype

The prototype dragster is a great opportunity for learning about design that also integrates some math and science into the dragster unit. We use polystyrene foam insulation to make our prototype dragster. This serves as practice in using the power tools to drill and cut material. We then use the polystyrene prototype, PSP, to take measurements. We put the PSP into a wind tunnel to measure drag and we also dunk the PSP into water to measure volume of the design using water displacement. With this information, we do a bunch of math to calculate the weight, drag, time and speed of the dragster before we cut the dragster out of basswood. After the actual dragster is made and raced, we will compare the results to what we predicted. While the dragsters are raced two at a time, they are not racing each other, each student's car is racing the math from the prototype to see how closely we come to the prediction.

Below is the text from the Prototype Worksheet and additional comments and guidance in blue.

Step 1 • Setup Blank - You need to transfer your design from your working drawing in your packet to the Polystyrene Blank (PSB). The polystyrene blank is the uncut, pink foam. You need to be sure to mark out the axel line and power plant, the hole drill in the back of the actual dragster blank for the C02 cartridge. You can use a pencil, fine tip marker, or a fid. A fid is the dowel that is somewhat sharpened to act like a carving tool. There are gauges you can use to mark the axel line and power plant on your blank. Whatever you use, DON'T PRESS DOWN HARD while you are drawing. Try to use a light touch to gently score the foam blank with your design. It only needs to be marked enough for you to see it to cut.

Step 2 • Make Prototype - With the design on your PSB, you can drill the axle holes and cut your design out of the blank. One of the big reasons why we are doing this is so that students gain some experience cutting with the band saw. Better to goof up pennies worth of foam then dollars worth of wood when using the band saw the first time. If you mess up, start over, just don't be wasteful and do the best you can. After they are cut, the prototypes are pretty fragile, they may break. If that happens, you can try to tape the prototype back together and it should be fine for what we are doing.

Step 3 • PSP Drag • When you are finished shaping your polystyrene prototype (PSP), you can put it in the wind tunnel. Don’t forget to add wheels onto the prototype for the test! You will get a set of wheels and axels for the wind tunnel test only. The car in the wind tunnel has a set of wheels, and the car on deck has wheels, that's it. You need to be sure to document the drag at all the speeds on the wind tunnel for your prototype and later, the actual dragster. You will need to jam the foam prototype onto the peg that is attached to the test stand in the tunnel.

Step 4 • PSP Volume • The density of polystyrene is significantly lower then basswood, so we can’t weigh it to predict the final mass of your car. However, we can measure the volume of the design by using water displacement. If we know the volume of your design, you can calculate the future mass of your dragster. You may already know how to calculate volume of regular objects, however, it is not possible to take measurements of your prototype with a ruler and arrive at a volume. So to get a volume we simply put the prototype in water and measure the how much the water level rises. You need to be certain that you are reading the water level correctly. A lot of students botch the read on this step and the error causes problems later on.

Once you are certain you have good DRAG and VOLUME, and it doesn't hurt to double check this kind of thing to be certain you have correct readings, you may take home or other wise dispose of the polystyrene prototype. Just be sure you have good numbers for mass and volume, if you destroy your blank and then later discover that you botched the volume reading, you can't go back!

Step 5 • Wood blank volume, use the figure below to calculate the volume of a Wood Body Blank. Use blank area to make you calculations. For greater precision, get a wood block and measure it! SHOW YOUR WORK IN THE OPEN SPACE.

Step 5.5 • Subtract the volume of the hole that is, or will be, drilled in the back of the wood block, 30.427 ml.

Step 6 • 3B Density - Get a basswood blank, write your initials in the whole in the back for it is now your blank. Weigh the blank on the digital scale to find the mass. Write the mass of your blank below. Divide the Mass of your 3B by the Volume of the 3B. The result is the average density of the material you will use for your car.

      Mass          The Mass of my block is ______
Density = -------------            -------------------------------------------- = ________________ g/mL
     Volume                      Volume of 3B is ________

At this point you should have the first page done and you can get a actual basswood dragster blank. Put you initials in the hole, weigh the blank on the scale and WRITE THE MASS from the scale on your worksheet. This is very important. Say for example that you miss a few days around this time and you don't do the calculations. If you write the mass of your blank in here, you can always go in later and do the math. However, if you don't document the mass of the blank here, you'll have to guess the mass and your prediction will likely not come close to your actual dragster results.

Step 7 • Estimating your dragsters final weight. Now that you know the density of the material you are using, you can multiply your prototype volume (your design) by the basswood blank density. The result should be your dragsters approximate mass before painting. This stuff begins to become a little difficult to follow, if you don't understand what's going on, you need to ask someone else in class or Mr. E to help you through it. It comes down to this, your design is represented by the volume of the prototype you got when you put it in the water. The density is specific to the material you will use to build your car. You can multiply your design volume by the density of the material you will use to predict what your dragster's mass will be after you cut it.

Basswood Density ________ TIMES Prototype Volume _________ = ________ grams, shaped weight.

This isn’t the final weight of your car yet. You can add 2g for paint and add the mass of the hardware, 15g.

17g of hardware and paint + the Shaped Weight _______g =_______g Estimated Final Mass

The shaped weight isn't the final mass, you will paint your dragster a couple of times, hence the 2 grams of paint. You dragster will also have metal axels, wheels, washers and hooks, these components add 15 grams of mass to the dragster.

Step 8 • Estimate Race Time • Now that you have estimated the final mass of your car and you measured the drag on the first page, you can look up cars in the database with a similar mass and drag as yours. CLICK HERE to go to the dragster database and find a couple of cars similar to yours in mass and drag. I maintain a record of past dragster races and the results, you use that data to find a expected time/speed for your dragster. Simple sort the data according to mass by click the mass link in the table header and scroll down to the mass that matches your dragster prediction. Find at least three dragsters with mass and drag that most closely matches your prediction. Document the past data on your worksheet and then AVERAGE the times. In the end, your actual dragster time and speed should be very close to the AVERAGE you calculate here.

Step 9 • Now that you have an estimated time for your design you can calculate the expected speed. Speed is calculated by dividing distance by time.
                     Distance          65 ft (distance of track)
SPEED = --------------------       Estimated Speed of your design = ---------------------------------- =__________
Time          Estimated time ______

This is not the end of these calculations. I want you to calculate the speed, then convert that speed to Miles per Hour.

Step 10 • After the race is over the time data for your car, convert the time of the race into speed in miles per hour. Show your work.

Step 11 • Comparing your results to the real thing. The object here is the see how close we can come to anticipating your actual cars properties and performance. After the race, write your estimates and actual data in the table below and calculate the differences to see how close you have come. We would like to see you come within 10 grams of your prediction. The closer you come to your prediction on mass, the closer the time and speed should be as well.
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