QUICK START- RatCalc is a utility to help determine the proper Chamber to Barrel Ratio (C:B) for your launcher. RatCalc provides flexibility when entering data and three ways to enter the chamber volume. First select a chamber diameter from the pulldown menu in the upper left. Those accustomed to measuring chamber size with a ruler should enter a value in 'combustion chamber length'. Those who measure the chamber volume with water should enter a value in 'fluid volume measurement'. Select a barrel diameter from the pulldown menu in the upper right. Adjust barrel and chamber -/+ 'spinners' to achieve the desired C:B ratio as shown in the lower right.
First determine the proper combustion chamber diameter and length. Chamber volume can be determined using a ruler to measure the chamber length. This can be a bit of a guessing game as the ruler does not take into consideration the irregularities of the fittings, reducers and endcaps that comprise the combustion chamber. If you have experience at estimating the appropriate length using a ruler, you can measure the chamber and enter the length using the 'combustion chamber length' -/+ spinners or by entering a value directly in the 'inch' field. Use the pulldown menu to enter the 'combustion chamber diameter' from a list of common PVC pipe sizes. The resulting chamber volume is displayed in the 'calculated chamber volume' fields.
RatCalc offers an alternate way to measure chamber volume called SmartGuess. Use a ruler to measure the cut length of the main chamber pipe excluding fittings. Enter this value into the 'combustion chamber length' 'inch' field or use the spinners. The 'SmartGuess chamber volume' fields will display the volume of the cut length plus the additional volume of common 3" and 4" sch40 fittings and endcap. It is important to have the 'chamber diameter' pulldown set to either 3" sch40 or 4" sch40 for the corresponding SmartGuess volumes to display properly.
The third and most accurate way to enter chamber volume for RatCalc is to use fluid volume measurement. Measure the combustion chamber volume with water and enter the volume (in milliliters or CC's) directly into the yellow 'fluid volume measurement' field.
Enter the barrel diameter and length using the pulldown menu and spinners in the upper right quarter of RatCalc. The 'calculated barrel volume' is displayed below the barrel length.
The lower right quarter of RatCalc displays the final chamber to barrel ratio. The chamber to barrel ratio is displayed 'chamber volume : barrel volume.' The barrel volume is always '1' in this equation. For example a 2:1 ratio would indicate the chamber's volume is twice as large as the barrel's. Adjust chamber and barrel volume to achieve desired C:B ratio. The four output fields in this section correspond to the chamber measuring type selected in the left side of RatCalc. Only one of the SmartGuess outputs in this section will have a valid ratio. The SmartGuess output that corresponds to the 'chamber diameter' pulldown is correct.
DESIGN CONSIDERATIONS- The optimum C:B ratio is a matter of preference. Shorter barrels will waste some available energy which then turns into noise. A shorter barrel will shoot slower and have a louder report. A longer barrel is capable of harnessing all of the available energy which results in higher speeds and can make a barrel nearly silent as no wasted energy escapes the muzzle. Tests posted here at BurntLatke have shown a ratio of .6:1 to shoot fastest with a 1.5" barrel and spuds as ammo. A large chamber with a .6:1 ratio means the 1.5" barrel would need to be quite long, maybe too long to handhold. A ratio of .7:1 shot fastest when using a 2.5" barrel with tennis balls. A .7:1 with the relatively large 2.5" barrel produces a barrel length that is easily hand holdable for most combustion launchers.
It is a good idea to build your launcher with a detachable barrel so you can experiment with different ratios. This way you can build a variety of barrels to suit your shooting conditions.