Measuring the Discharge Coefficients of a Rocket Injector

In summary, the speaker built a liquid rocket injector and conducted hydro-static tests using a program to record data. They were informed by a grad student that the pressure drop across the injector can be measured by the difference between injection pressure and atmospheric pressure. The speaker observed spikes in the pressure transducers and noted that one was defective. They performed a total of 10 tests and varied the injection pressures and flow rates to calculate the discharge coefficients. However, they found that the coefficients varied with injection pressure rather than being consistent. They also mentioned that a discharge coefficient of 0.33 for the oxidizer line would be considered low and that the actual determination of the coefficient would require a nozzle configuration.
  • #1
Roy S Ramirez
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4
TL;DR Summary
I build a feed system for a liquid rocket injector, ran water through it, measures pressures and flow rates, and I'm not sure how to calculate the pressure drops and discharge coeffs.
Hello everybody!

I hope you are all doing well. I built a liquid rocket injector and the following feed system:
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I started to do hydro-static tests (cold flows) and a program I wrote records all the data for me (except for the flow rates which I measured experimentally by collecting the water with a bucket and measuring the volume*). I know the grad students at my school use this kind of data to measure discharge coefficients, and one of them told me that for the pressure drop across the injector you just use the difference between the injection pressure (which my program records), and the atmospheric pressure. I'm not sure if this true... This a a data sample, where green is the ox line and blue is the fuel line. Both lines were tested individually, and the values plotted are in PSIA vs time, where the pressure belongs to pressure transducers PT-21 and PT-31. It is interesting to note that the PT's spike up some seconds after opening the main valves (FC-21/FC-31) at t=4secs. Also note that PT-31 looks noisy, and this is because the device is defective but it still works fairly enough (it will be replaced).
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(A total of 10 individual tests were performed)
For the calculations we have the following:
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Here I vary the values of the injection pressures and the flow rates to get the Cd's. After plugging numbers I got:
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As you can see, the Cd's are varying with the injection pressure, and I thought they were supposed to be nearly fixed numbers. Also for the oxidizer line I think a Cd of 0.33 ish would be very low, wouldn't it?

Thank you very much,

Roy S.
 
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  • #2
The real determination of the Cd of the final flow nozzles is, assuming P21 and P31 are very near their nozzle inlets, are respectively P21 and P31 vs flow. Nozzles have a consistent Cd for uniform fluids; but, flow resistance through tubing/piping increases with flow rate.
Without seeing the nozzle configuration it is not possible to comment on the low Cd; but, yes a Cd = .33 would be considered very low for any reasonably formed nozzle.
 
  • #3
Hello @JBA. I don't have a nozzle attached right now. I only have the injector, and yes, PT21 and PT31 are relatively close to it.
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(Cold flow at Zucrow Propulsion Lab two months ago)

PS: I manged to fix the response time issue.
 

FAQ: Measuring the Discharge Coefficients of a Rocket Injector

1. What is the purpose of measuring the discharge coefficients of a rocket injector?

The purpose of measuring the discharge coefficients of a rocket injector is to understand how efficiently the fuel is being injected into the combustion chamber of a rocket engine. This information is crucial for optimizing the performance and efficiency of the rocket.

2. How is the discharge coefficient of a rocket injector measured?

The discharge coefficient of a rocket injector is typically measured using experimental methods, such as flow rate measurements and pressure measurements. These measurements are then used to calculate the discharge coefficient using mathematical equations.

3. What factors can affect the discharge coefficients of a rocket injector?

Several factors can affect the discharge coefficients of a rocket injector, including the design and geometry of the injector, the properties of the propellant and oxidizer, and the operating conditions of the rocket engine. Changes in these factors can lead to variations in the discharge coefficients.

4. Why is it important to accurately measure the discharge coefficients of a rocket injector?

Accurately measuring the discharge coefficients of a rocket injector is important because it allows for the optimization of the rocket engine's performance and efficiency. It also helps in predicting the thrust and overall performance of the rocket, which is crucial for successful space missions.

5. Can the discharge coefficients of a rocket injector be improved?

Yes, the discharge coefficients of a rocket injector can be improved through design modifications and optimization of the operating conditions. Continuous research and development in this area can lead to more efficient and reliable rocket engines for future space exploration.

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