Piston valves are by far the most common homemade valves built for the purpose of launching things today. They are relatively cheap to build (when compared to valves of similar porting), simple, and can provide very high performance when built correctly.
This document will attempt to explain the two main variations, and other performance additions.
These are the most commonly built valves, and are the simplest. They can be built without any specialized tools. However, if the build is too sloppy, you will need a larger pilot valve to get the thing to open.
We'll start with the basic materials for a valve. The piston, the piston track (pipe which contains the piston), the pilot area, and the port which the barrel seals against.
Most people build barrel sealing piston valves in a tee. The piston track is in the side opposite the rear of the barrel (sealing surface). Take a look at the simple barrel sealing valve below. This is a cross section (1:1 scale, 100 DPI) of a very common beginner's valve, employing standard materials, and can be built with nothing other than a hacksaw (optionally a miter box) and a file or sandpaper.
As mentioned, this is a cross section view. It may look complicated at first, but let's just take a look at the basics:
So what happens?
The launcher is filled from the rear. Due to the slow flow rate around the piston, the piston is pressed forward and seals against the barrel port. Air leaks around the piston and fills the chamber, equalizing the pressure behind and in front of the piston.
So how does the piston seal if the pressure at the front and back are the same? Recall the piston is seated against the 1-1/2" pipe. The exposed surface area on the front of the piston is much less than the surface area exposed at the rear of the piston. Since force equals pressure times area, the net force on the piston is very large and towards the barrel port, keeping the piston sealed.
Now, it's time to fire. The pilot valve vents the volume behind the piston to the atmosphere. The pilot valve must have more flow than the leakage rate of air around the piston for this to work. Once the pressure in the pilot area is low enough (the pilot pressure times the rear piston area equals the chamber pressure times the frontal piston area), the piston unseats, and the sudden increase in exposed surface area at the front of the piston forces the piston backwards, into the bumper, allowing the chamber to exhaust out of the barrel.
This is how outlet sealing valves work. There are some improvements to the above model, though.
While most builders start off with an imperfect sealing piston (the piston allows air to leak past), performance, and ease of piloting, can be increased by sealing the piston and controlling equalization by other means.
For a 2" piston, in PVC, you have it easy. 1-1/4" couplings have an outside diameter of almost exactly 2". By trimming 1-1/4" spigot plugs to your required length, and then hacking up one or more couplings to sleeve your plugs in, you can leave spaces in the coupling sleeves for o-rings to sit in. For 1-1/4" pipe and fittings in 2" pipe, the o-ring size required is 3/16" width x 2" OD, which can usually be found at a good hardware store.
But creating o-rings is the easy part. Now, you have to control equalization. One easy way to do this is to mimic an air pump piston:
In this design, the o-ring grooves are made "sloppy", which is to say they are wider than the o-ring. The o-ring will therefore move to the front or back of the groove at the influence of air pressure.
During filling, the o-ring is pushed to the front of the groove, uncovered a hole which allows air to flow through the piston and into the chamber. For a solid piston, a blind hole can be drilled in the same position, and air will flow past the o-ring and around the rest of the piston. Upon piloting, the pressure at the chamber side of the groove increases, pushing the o-ring against the complete back of the groove, sealing the pilot volume from the chamber completely.
Simple check valves can be made and installed in the piston internally. I would suggest that your check valve not be "perfect", however. If your check seals completely (like a store-bought check valve will), and your piston's o-rings seal completely, a tiny leak in your pilot and fill configuration will have the potential to fire the launcher.
These have seen a definite decline in use, as a scenario in which they would be more efficient than outlet sealing valves is pretty uncommon (explained down the page).
However, a commercial variant is still in production... the infamous "Supah Valve" by the SGTC (see here for internal pictures). The Supah Valve, despite its mysticism, is simply an extremely well made chamber sealing piston valve.
Let's take a look at a cross section of a simple chamber sealing valve:
...and again, let's list the basics:
The operating principle here is different. As evidenced by the name, the piston does not seal off the barrel port, but rather seals off the chamber from the barrel. The internals of the tee, which are pressurized in a barrel sealing valve, are connected straight to the barrel. So it is very important that the piston seals completely in the 2" pipe.
When pressurized, there is still a surface area difference between the front and back of the piston, but in an inlet sealer, the surface area on the front is equal the inside diameter of the chamber port. The effects this has on performance and the reason why the above example uses a 1" port will be explained down the page.
Other than that, the piloting method is the same. The pilot valve vents the pilot area faster than the small equalization hole through the piston can supply air from the chamber, and the pressure drops to a point where the surface area of the rear of the piston times the pilot pressure equals the cross-sectional area of the chamber port times the chamber pressure, at which point the valve moves rearward, unblocking the chamber and allowing air to flow into the barrel.
The "jump" in surface area is less pronounced in a similar sized chamber sealer than a barrel sealer. This is all explained below.
There are two main differences between the two types of valves. One is performance, and one is ease of construction:
The important thing to take away from the two lists above is that, 90% of the time, an outlet sealing valve will provide better performance and, in general, be less of a headache. There are certain situations in which and inlet sealing valve will be better. If you prefer the configuration an inlet sealer provides, then by all means build an inlet sealer. Also, if, for some reason, you must use a much larger piston than your valve porting (which isn't recommended in the first place because of the increase in pilot volume), then an inlet sealing valve is preferred.
Other than that, it's simply a matter of preference, usually. Some people like to build inlet sealing valves just to prove to themselves they can manage it. It's up to you.
One thing common to all valves is the effect of pilot volume on opening times. With a large pilot volume, your valve will take longer to pilot, and have more residual air behind the piston to slow it down when opening.
Every effort you can make to reduce pilot volume should be made. This will increase convenience and power.
Hopefully, you now understand how the two types of piston valves work, and have decided on a type to build if you were looking into making one.