>>10575946>What kind of rocket are you designing?A pressure fed rocket propelled by nitrous oxide and gasoline. My goal is just get as high as possible, and my estimates put it to 8km, but I am assuming some pretty pessimistic values.
>Thanks for you input, maybe I'm underestimating the weight of the engine, pipes, and structure around it?Perhaps, what I do with estimating dry mass is to sum up all the dry mass that I know of and then multiply it by 1.35.
I also use this equation to estimate the mass of the engine...
m_engine = 0.725*F_vacuum*Isp_vacuum*g0*10^-6
Where... m_engine is the mass of the engine in kg, F_vacuum is the thrust of the engine in a vacuum in newtons, Isp_vacuum is the specific impulse of the engine in a vacuum in seconds, g0 is 9.807 m/s2. The equation is empirically derived so it's not perfect, but it works well enough for our purposes.
>What problems could designing injectors for an engine with gaseous propellants compressed at more than 30 bar have?I'm not sure as I don't have alot of injector experience, but one problem I can forsee is that the properties of your gaseous propellants (pressure, density, temperature) may change significantly across the injector in accordance to the Ideal Gas Law. This can really mess with your mas flow rates if you can't properly predict this. Another problem with injectors in pressure fed engines in general is that if the combustion pressure may change dramatically due to random uneven propellant mixing resulting in pressure waves being sent back to your tanks which then affect feeding. I've heard pintle injectors are most resistant to this.
Here's a general overview of some injectors.
https://engineering.purdue.edu/AAECourses/aae539/2007/spring/handouts/InjectorLecture2.pptNote that most injectors are designed with liquid/liquid or liquid/gas propellants in mind so the details may be incorrect for gas/gas.
