Frequently Asked Questions (FAQ):
- Is this simulator reliable? The simulator engine is based on the set of thrust and flight equations described in this website. The equations involve relatively few simplifying assumptions and I believe this simulator can more reliably handle some exotic rocket designs than other simulators I have seen (even commercial water-rocket software). However, like any model--even a good model--the output depends on the input. Probably the most uncertain parameter that makes a huge difference in performance is the coefficient of drag. The simulator provides a conservative estimate you can use, based on the shape of your rocket and assuming relatively straight and stable flight, but this should not be taken as gospel truth. Fins, nosecones, and other external ornamentation, as well as orientation of the rocket to external airflow, all have an effect. I recommend a "fitting" experiment, in which you launch a rocket and assess its apogee height and then adjust the drag coefficient so that the simulator and experiment agree. Then the simulator can be used to explore changes in other variables. As a final note, I would add that the simulator solves different equations for different phases of flight (launch tube, water thrust, and gas thrust). It must figure out when to transition between these equations and sometimes, for certain unpredictable combinations of parameters, it gets stuck in an iteration loop. If the simulator seems to stall or freeze, try a slightly different set of parameters (even just by changing one parameter by a small amount) and hopefully it should work. Did I tell you that this is beta software?
- What is a "launch tube"? It is a smooth hollow tube that is mounted to the launcher and inserted into the rocket nozzle. Upon launch, the rocket slides up the tube. If there is a reasonably close fit between the outer surface of the tube and the inside of the rocket nozzle, then the tube/rocket combination acts like a piston, providing very efficient acceleration to the rocket while it is traveling up the tube. Another benefit of launch tubes is that they guide the initial motion of the rocket so that is flies straight. Wobbly flight of the rocket will increase air drag and make the trajectory less preditable. Some launchers use launch tubes (like the home-built Clark cable design); some do not (like the Pitsco AquaPort launcher used by Science Olympiad). The simulator can predict the principal effect of the launch tube (a boost in total thrust), but makes no attempt to predict flight stability issues.
- What is "launcher volume"? It is the volume of pressurized gas stored inside the launcher apparatus (excluding the launch tube) that is accessible to the rocket. By "accessible" I mean there is no valve or significant constriction between this volume and the volume of gas inside the rocket itself. Why is this quantity needed by the simulator? Because this gas volume works to maintain pressure inside the rocket while the rocket is traveling up the launch tube. So if a launcher does not use a launch tube then there is no effect. For launch-tube-type launchers, creating a large launcher volume provides a modest improvement in performance, depending on the length of the launch tube. But it does waste pressurized gas, so if you are building a launcher that will be used to launch many rockets in succession it is probably a bad idea to give it a large volume.
- What is a "T-nozzle"? In the simulator if you make the launch tube diameter greater than the rocket nozzle diameter, a warning statement is given under the Results tab indicating a T-nozzle is in use. A T-nozzle is a small tube-shaped device that fits inside the rocket. It is shaped to sit in the throat area of the nozzle (without being ejected) and to force the outlet flow of water/gas through a smaller diameter, allowing for a longer duration thrust and more dramatic launch. The T-nozzle initially sits loosely on top of the launch tube and gets seated in place after the rocket flies off of the launch tube. With the T-nozzle in use, the rocket nozzle diameter used in the simulator should be the inside diameter of the T-nozzle. Do a web search on "T-nozzle" if you want more information.
- What is "Nozzle Adjuster" A, B, and C? These three are variables I made up that control the shape of the bottle near its tail-end or nozzle. If you change these variables in the applet you will see the cartoon on the right change as well. Make the cartoon look like the bottle shape you are using for your rocket. Because the shape of the tail changes the external aerodynamics of the rocket, these adjustments will change the appropriate value of drag coefficient. The default values of A, B, and C are set to reproduce the typical shape of 2-liter soda bottles in the United States.
- What is the optimal amount of water to add to a rocket? That's what simulators like this are used for--change the value and see the predicted effect on performance. The fact that there is an optimal amount of water can be explained by the fact that there is a trade-off between competing effects. The water in the rocket is a reaction mass. It allows for efficient conversion of pressure energy (stored in the gas) into kinetic energy (motion) of the rocket. On the other hand, any water you add to the rocket takes away gas volume, which is where the energy is stored. So you are trading off the amount of energy stored in the rocket with the efficiency with which that energy can be used to propel the rocket upward. There is an optimal point where thrust and apogee height are maximized. Understanding and dealing with trade-offs like this is the essence of engineering.
- What do you mean by "total volume" of the rocket? Total volume is the total internal volume used to store water and pressurized gas, i.e. the size of the bottle. Note that under pressure every bottle elastically stretches a bit, increasing its volume above the nominal stated value. This can be observed by doing a pressure test on a bottle in which it is filled 100% with water and then pressurized gas is added. I have found that 2-liter soda bottles, under 100 psi (7 bar) pressure have a total volume closer to 2.1 liters, which is the default value on the simulator.
- What do you mean by "empty mass" of the rocket? Empty mass is the mass of the rocket vehicle (bottle+fins+nosecone+payload) at the conclusion of the thrust phase, when all the liquid and pressurized gas have been expelled.
- What coefficient of drag should I use? See my response to the first question above.
- What are water and gas "thrust efficiency"? This is the fraction of available energy that is NOT lost to friction when trying to expel the water and gas, respectively, from the rocket. As long as the inside of your rocket/bottle is smoothly tapering and does not have additional plumbing, I recommend you leave these two quantities at their default value (0.97).
Sim Water Rocket usage notes:
- This is a beta version of the applet; a few features are not yet implemented (it's been in beta for about 7 years--seems about right). The applet has been tested on IE 5.0 and Mozilla/Netscape 5.0 (and later) browsers.
- This applet requires Java version 1.1 or later on your computer. A Java Virtual Machine (JVM) ordinarily comes pre-installed with most web browsers. If needed, you can download a Java browser plugin from Sun.
- When you click on the “Results” or “Plot” tabs, the applet must recalculate the rocket trajectory. This could take a few seconds, depending on the speed of your computer. With Mozilla/Netscape, I have not yet figured out a bug where you have to toggle between the “Plot” and “Results” tabs once to get the “Plot” tab to update properly.