u/Sph3k

Hi everyone,

I've been designing a ethanol nitrous unlike doublet injector, and have been struggling to understand how to deal with the dual phase stuff with sizing nitrous orifices. I posted earlier about some confusions I had about the NHNE model and using it to find area.

I was doing some more research and came across this page on the MIT Rocket Team Wiki that talked about how if you lower your Cd to around 0.4-0.5 you can get away with just using the SPI equation.

I was curious, so I looked up some papers and found 2, "A Two-Phase Mass Flow Rate Model for Nitrous Oxide Based on Void Fraction" and "Experimental Study on the Mass Flow Rate of the Self-Pressurizing Propellants in the Rocket Injector."

I'm kind of stupid, so the only thing I understood from the first paper is that by doing a bunch of formulas I don't understand you can find a correction factor Y to attach to the SPI equation that can make it more accurate, but it's only recommended for single-phase nitrous injectors, and not two phase.

SPC equation with Y formula

SPC with Y'

The paper also comes up with their own model that they call FML, but it has the same problems I have with the NHNE (Dyer edition, as they also label their equation NHNE) where the area is a variable in the SPC and in the HEM model. I also did not find any way of calculating the value of x2 in the paper, so even if I wanted to I couldn't find it.

FML equation

Downstream void fraction

The other paper was a little bit different and found that using the normal SPI formula and changing the Cd to 0.45, they could use the SPI formula to accurately model multiple different injector length/diameter ratios up until flow reached "critical flow," where flashing nitrous vapor chokes the flow and limits mass flow rate. In the figure you can see the critical flow happens at a certain pressure drop. My chamber pressure is going to be 350psi, and with a 20% stiffness my pressure drop is 70psi, or about 4.82 bar, which is well before the critical flow range.

Pressure drop to mass flow rate

Now to where I'm stuck. Both models, bot the Y correction factor and the Cd change to my understanding (which isn't a whole lot) model before the critical flow, and with my pressure drop being so low, I can use either one.

For the Y model, I'm confused whether I should use the Y or Y' version, as one is far more complicated than the other and I would much rather use the Y version.

For just changing the Cd, I'm hesitant about the fact that in their data, all of their mass flow rates were in grams per second, while mine are in kilograms per second. Would the difference in mass flow rate my the effect pressure drop has on critical flow be different? Would my pressure drop still be below the critical flow point? If the mass flow rate unit change doesn't matter, I would much prefer using the modified Cd SPI equation.

If I'm missing some obvious explanation in either paper or totally overthinking this, please let me know.

I'm attempting to design a unlike doublet injector, and I'm a little confused about manifold design.

My injector is inspired by the design the MIT Rocket Team used for their Helios engine. Here is an image of their design, which uses a integral injector with only two manifolds.

https://preview.redd.it/scd751vluhjh1.png?width=1036&format=png&auto=webp&s=33131fbb7b25d8020f7e12a72f433425b185c9b6

According to Huzal and Huang, your manifold area should be 4 times the total flow area of all the orifices that are fed by that manifold. I'm confused on the definition of flow area. Is it the area you get when looking from above the engine directly down, where you would only see a annulus or a circle, or is it the rectangular area that is highlighted in red and green on the injector part of the image?

If it's the rectangular area, how do you decide on what your width and height is? Is it arbitrary, or does it depend on other factors? Is there a specific ratio that is commonly used?

Hi,

I'm a freshman mechanical engineering student who is really interested in being a propulsion engineer, specifically working on combustion devices in rocket engines.

For people who have this role, what does your day to day look like? What skills do you wish you had learned earlier, and what do you think the skill most combustion devices engineers use the most?

What kind of programing do you do? Do you mainly use python, or is MATLAB used as well? What do you use the languages for?

How did you develop your skills in CFD software? What resources would you recommend for people trying to learn suites like Ansys?

How do you develop test requirements, and what gave you the experience you needed to develop them?

What roles in a rocket engine design team would you recommend?

Is there any general advice you have?

Sorry if this reads kind of jumbled.

Hi everyone!

I'm trying to design a unlike doublet injector that uses Nitrous Oxide and ethanol. Sizing the orifices for ethanol is easy enough, I'm just using the single phase incompressible flow equation. I already have my pressure drop based on 20% of chamber pressure, and I know my mass flow rate from RPA.

SPI mass flow rate equation

What I don't understand is how do I find the orifice area for the nitrous? I know based on the paper "Mass Flow Rate and Isolation Characteristics of Injectors for Use with Self-Pressurizing Oxidizers in Hybrid Rockets" that you can't just use the SPI equation, because of the weird physics of nitrous. From what I understand you have to use the NHME model, which combines both SPI and HEM models.

HEM equation

NHME equation

What I don't understand is how am I supposed to get the area if the NHME model uses two equations that require you to already have a value of A chosen? Am I just being stupid and missing something? I'd really appreciate some help.

I also don't quite understand how you choose the enthalpy values for the HEM equation.