For the last few years I have been developing my 1000lbf (4448N) LOX/Ethanol rocket and engine, this rocket is based on the construction of rockets as by the SDSU Rocket Project. Consisting of aluminum bulkheads with square box stringers, I saw this design as a good starting point as it enables easy placement of components within the airframe when I am not entirely sure how it will work. Being my first liquid-fueled rocket, it gives me room to move and improve.
As of late though I have been thinking about the future, given the cost and my workshop capability, a smaller diameter rocket could be worth pursuing. Going smaller also means trying to fit all those valves and plumbing into a set diameter airframe.
Lucky for us, Tom Mueller, VP of propulsion at SpaceX has wrote a small article about pyro actuated valves that he has built in the past for his own experimental rocket projects. An example from the article is shown below.
These valves enable the builder to employ them in a smaller diameter airframe and in one case, were used in an airframe of only 1.5″ diameter for a liquid fueled rocket. The downside is that they can only be activated once and therefore you require a rocket/system that you know will work every time you hit fire. The plus side is that you can use them everywhere, from your main propellant valves to vent valves to your helium actuation valve as the document states and outlines.
Paul Breed of Unreasonable Rocket has also explored these valves, there is a nice blog post detailing his approach to the concept.
Although this is nothing new to the world of rocketry is does offer food for thought when going through the design phase and enables another option to be looked into when trying to downsize and save money.