In the past year, our oxidizer and fuel system has moved through six major revisions. Driving forces for change have included updated engine requirements, changes in fuel selection, safety concerns, as well as improvements compiled from team suggestions.
Humble beginnings: Concept Sketch
Our initial engineering sketch of the fuel system was used extensively to explain the project early on. It helped us visualize flow of fluids and where different components might be used, and how they would interact. During a consultation with engineers from Blue Origin, it was pointed out that the purge lines was a dangerous path for high pressure oxygen to mix with fuel. However simple and naive this sketch may be, it certainly got us moving in the right direction.
A Giant Leap: Rev 1 and Rev 2
The the second revision is identical to the first, with the exception that I used industry standard symbols, so only Rev 2 is shown. Two major changes were incorporated at this stage: first, we had changed from using gaseous oxygen to liquid oxygen. This change came from a need for smaller tanks (the ten pounds of O2 would have required ten inch diameter tanks, nearly 80 inches long) and more notably for safety. Also, the high flow velocity of the oxygen was quite likely to cause a fire within our pipes due to the heat generated by the molecules impinging on the metal surfaces (think atmospheric heating of a supersonic aircraft). It was a tough sell convincing some people that cryogenic oxygen was the safer option!
Finer Flight Refinements: Rev 3 and Rev 4
Rev 3 added a set of normally-open solenoid valves to to the fuel and nitrogen tanks. These are intended to provide a measure of “hands-off” safety. In the event that the system becomes unsafe to approach while pressurized, power will be cut from the whole system, the valves open freely, allowing the tanks to vent to the atmosphere.
Rev 4 added a second solenoid valve and a filling manifold to the liquid oxygen tank for the purposes of remote filling.
The Costs of Complexity: Rev 5
In an attempt to simplify, while still maintaining safety, the remote-fill manifold was replaced by a manual valve and a pneumatically actuated vent valve. The switch to air powered valves from electric was made for cost savings: solenoid valves capable of operating with liquid oxygen were prohibitively expensive and required currents much higher than our flight rocket could handle (and our remote test stand, for that matter). The added to the capped lines were safety heads with burst discs.
Slash, Shave, and Save: Rev 6
The time had come to actually shop for specific components, getting specs and pricing. As the items were ticked of the list, the costs added up quickly: The price of the major components in Rev 5 was over $9300! Even with donated components the costs and complexity would be too much for our budget and timeline. Rev 6 axed the two safety heads, as I was informed they were an unnecessary redundancy next to the more effective relief valves. This dropped over $300 from the shopping list. The LOX and butanol fill valves were replaced by capped fill lines; $1400 more gone! In a throwback to my initial sketch, low pressure purge valves were added to each fuel subsystem allowing us to blow out leftover fuel from the lines and start each new test from a known state. Additionally, a second solenoid valve was added to the flow valves so that the fuel and oxidizer can be controlled independently.