Sunday, May 4, 2008

Valves

Actuated valves are a tricky thing for rocket motors. Their requirements usually exceed what is available from the McMaster catalog. Of course, if you are willing/able to pay big aerospace money, you can have pretty much what ever you like. Unfortunately, most amateur and "new-space" organizations don't find themselves in that situation.

A common solution is to use ball valves. They have several advantages including reliability, availability, and some throttle-ability. However, the packing around the ball generates considerable amounts of friction, especially when the valve hasn't been moved for some time (sticktion). As a result, ball valves require rather significantly sized actuators which are often heavier and larger than the valve itself.

While browsing through the Swagelok catalog some time ago, I discovered a valve they call a "plug valve". It is similar to a ball valve but with a cylindrical rotating plug that is sealed with o-rings. The result is a valve that has very low friction and thus can be actuated with correspondingly smaller actuators. The primary downside is that the o-rings become a consumable item and prohibit their use with cryogenic fluids such as lox.

Here is a picture of a plug valve actuated with a Hitec servo.

This is a movie of some early testing I did with a garden hose and a standard Futaba servo.

video

If you look closely in the video in this post, you can see the valve moving in the lower right-hand corner.

As I alluded to in the previous post, I've had several o-rings tear when being closed after use at part throttle with N2O. The valves come from Swagelok with PTFE coated FKM o-rings. I assumed they were not rated for lower temperatures so I ordered some PTFE and urethane o-rings. Below is a disassembled valve with a urethane o-ring.

Last night I did some simple testing with the urethane o-rings and to see how cold the N2O is getting downstream of the valve. The picture of the setup is below showing the valve, thermocouple, and injector orifice.


As you can see, it gets pretty cold. At minimum throttle, the temperature reached a steady state value of ~ -84F after 20 seconds or so. The urethane o-rings are rated to -65F and the PTFE rings are rated to -100F. I'm going to try the urethane rings first because the PTFE rings are very hard.

Last up is a neat picture showing the formation of solid N2O at the orifice exit. Visible are both ice-sickles and snow.

In summary, with the right o-ring material, these valves should work for all storeable propellants such as ethanol, kerosene, and peroxide. They should also work with saturated liquids stored under their own vapor pressure, like ethane, propane, and N2O. The odds of working with LOX are slim to none.