Feed System (9/28/20)

This past month was spent in various design challenges including learning about plumbing, fluid dynamics and something very interesting called cavitation venturis. There are a few different kinds of bi-propellant rockets and I am referencing the amount of kinds of feed systems (how you supply the injector with the fuel and oxidizer to make the big boom) and we will be using a pressure-fed blowdown system. The thing that this system offers most is just the absolute simplicity of engineering and designing it. You can't wait to test some things on the test stands which would normally require an absurd amount of calculations, like to what pressure to pressurize the tanks. This was something we decided to wait to see the pressure differential that is inherently introduced in our system by means of the various valves and tubing present in the plumbing system. There are two main methodologies for designing the feed systems which are pressure-fed, where you rely on an amount of pressure normally provided by a largely inert gas mostly helium or nitrogen which serve to force the liquid towards the inlet of the propellant lines and there is the turbo-pump driven system. These systems are highly highly complex and I haven’t bothered studying because they are like when cars moved from natural aspiration to turbo driven engines. It is pretty much the same kind of thing but as you know from anybody who’s blown a turbo on a silly Subaru WRX, turbos make your vehicle inherently more unreliable and the same for turbopumps. They are not necessary for small rocket engines like ours and so we went with pressure fed. There is a whole bunch of types of pressure fed engines but the simplest of them is the blowdown system which relies on ullage within the tank (normally about 30-60% of the tanks volume, depends on propellant chemistries) and is normally either nitrogen or helium gas which is pressurized to a very high degree, which push the propellant towards an outlet in the bottom of the tanks that in our case that is going to be somewhere in the 200-500psi range. That is because we have a very low chamber pressure of 200psi and the range takes into account the pressure drop involved in using tubes and all of our systems pressure drops in addition to the fact that the pressure in the tanks will be slowly dropping over the duration of our 15 second burn because the liquids are being expelled at a very fast rate (about 4lbs of liquid oxygen per second) and that means the pressure drop within the tank will drop significantly and we will need to account for that. As for our lovely new discovery as told to us by our new mentor is a cavitation venturi. So a venturi in general is something that increases the flow of a fluid at a point by withdrawing to a thinner diameter kind of like squeezing a pipe and constricting the flow. Leaving that thought, let’s hop into the phenomenon of cavitation. Cavitation is what happens when a liquid hits its vapor pressure and starts to vaporize forming bubbles in the tube. In most cases this means something is about to explode, normally the pipe. However, scientists in the 1950s wrote about cavitation venturis and it’s really only been used in rockets and on test stands for bipropellant engines and for some reason not expanded to other fields. But pretty much it is a special kind of venturi constructed so that the pressure at the thinnest point would be matched to the vapor pressure of the liquid being worked with and that would allow for us to fix the flow rate of the fluid exiting the venturi regardless of the downstream conditions which is extremely valuable in our application. It works by vaporising the fluid if it goes beyond a certain point and is a function of Bernoulli’s equation which is just the conservation of energy within a fluid and is the core of fluid dynamics. Stating that as the fluid’s velocity increases the pressure decreases and vice versa as to conserve energy. This impacted me because it made it easier for me to design the plumbing system to be as streamlined as possible and if we use a ball valve then we can get a pressure drop in the single digits from single components, which would be phenomenal in terms of weight/pressure savings. In terms of safety factor, we are well within a safety factor of 3-4 and so we aren’t pressurizing the aluminum tanks to the point of tensile yield but to about ⅓ to ¼ of that.

Chen. L.L Navicks. J., 1994. 14.2 The behavior of small cavitating venturis. [online] Springer.link. Available at: <https://link.springer.com/chapter/10.1007%2F978-1-4615-2522-6_129> [Accessed 27 September 2020].