Water Rocket Constants

Worksheet Constants

We are now beginning to accumulate many numbers. We need to keep track of them and have them available for use. As we proceed we will do much arithmetic. A LOT of arithmetic. Now it is time to organize our numbers.

We will use a computer utility called Microsoft Excel to keep all of our numbers and do our calculations for us. First, we will do the calculations ourselves to learn what we are doing. Then we will configure Excel to do the work. We start by loading Excel with our constants and our initial conditions.

Constants

Our constants are things that do not change. This includes items such as conversion factors to change from psi to KPa and the standard mass and pressure of air. These do not change.

Notice that the values are in column B and the units are in column C.

Each cell has a first and last name. The first name is the letter at the top of each column. The last name is the number of the row that contains that cell. The cell that contains our first number is: B3. Its value is 101300.00. Find that cell now and be sure you understand the naming convention. When we mention a cell, we must be polite and always use its full name.

With a few exceptions, Excel has been configured to show all numbers with two decimal places. That keeps the numbers all lined up and easy to read. As you can see, cell B6 is an exception. It is a small number and without the exception it would show the value 0.00. That would be confusing as we look at all the values.

Excel cannot process the units of measure. Remember that I was careful to point out how we must always keep track of our units of measure and include them in our arithmetic. We will have to give Excel just the numbers while we keep track of our units. When needed we can put more comments in the worksheet.

excel_constants.jpg

Initial Mass

The next section are our initial mass values. I measured the volume of our first water rocket at 1250 cc. You should be able to recognize the other numbers from the previous lesson.

initial_mass.jpg

Initial Pressures

These are the initial pressures when we launch our rocket. They will change from flight to flight. They can be different for each rocket.

initial_pressure.jpg

This will not be a tutorial on Excel, but some things should be described. You can do a Google search on tutorial Excel to learn more about Excel.

Look in the Excel sheet, and find cell B16. It has the pressure from the bicycle pump, in psi. We are using scientific notation for our calculations so psi is converted to Pa. When you have the Excel sheet on your computer you can click on cell B17 and you will be able to find this:

= B16 * B7

The = symbol tells Excel that the value of this cell will be calculated. Excel then goes to cell B16 and gets that value, the pressure, that we know is in psi. Excel does not know it is psi, it just gets the number. Then it goes to cell B7 and gets the conversion factor. It multiplies the two values and displays the product in the current cell, B17. So Excel does the multiply and shows the result in cell B17. Pretty neat.

Initial Air Mass

For cell B23, the formula is: = B10 - B12
Put in words, that means start with the rocket gross volume (already put in cell B10) and subtract the water volume (already in cell B12). All the cubic centimeters not filled by water are filled by air. Look at all the cells already shown and see if you understand these two sentences.
For cell B24 I have calculated the mass of the air unpressurized. B25 has the mass after the rocket is pressurized. Look back up in B19 to see the pressure in newtons per centimeter squared.

initial_air.jpg

Nozzle

This is the information we measured and calculated for the nozzle.

nozzle_constants.jpg

Entire Worksheet

The below image is the entire constants and initial values worksheet. The cells in yellow name the sections. They are there just to help us read the worksheet. Those in green are the cells that we can change. They will change for each rocket, that is to say, for each different bottle that we use. They will be changed to show the differing amounts of water and pressure for each launch.

All the remaining cells are calculated by the worksheet. We will not be changing them by hand. We hired Excel to do those calculations.

Reminder: The current goal is to learn about water rockets, not to learn about Excel. We will talk about Excel in class. If there is suficient interest in Excel was can have several lessons dedicated to it. For now, it is just a tool used to keep track of our data. If you have a parent or older sibling, you may ask them for a lesson on Excel.

entire_constants_worksheet.jpg

Pay close attention to information contains in this worksheet. The Worksheet is named Initial Conditions 01 When we build another rocket or launch with different conditions, we will copy this worksheet to Initial Conditions 02. By creating a new worksheet for each launch we can can re-visit our previous launches. If you have Microsoft Excel, or any worksheet program, you can enter the data shown here. There is a copy of this workbook on the web page here:
Excel Workbook 01
If you do not have Excel, write all this information on a sheet of paper. Use the format I have provided and put all your numbers and descriptions in the same columns shown here. You might print it out, but your best advantage is served by writing this down yourself. Doing that will help you remember all the things we discuss.

Now its time to continue on and discuss the thrust.

Dec 2014
Bryan Kelly
send comments to: on line at bkelly dot ws