LRE Project - 07/05/2021

Ok, now that the holiday has passed its time to start digging into this project a little more. The best thing to start off with is some basic requirements of what this project will be tackling and what it will not.

Project Goal

• Build an LRE and associated ground hardware for learning purposes and for applying what I know from industry/collegiate work in a smaller scale and not die or burn down my house in the process.

"Requirements" of the Engine

• 500-1000 lbf thrust @ SL
• Liquid Propellants
• Must survive repeated firings
• Aim for 8 seconds as "full duration"

High Level Sizing

So now what is my current baseline looking like? Well after going back and forth on propellants and availability, etc I think I've narrowed down to 95% Ethanol/Water (Basically 190 proof Everclear) and Liquid Oxygen. This gives me a similar fuel/ox velocities in the main valves to that of kerosene/LOX with slightly higher Cf and lower adiabatic chamber temps.

So for the high level sizing I'm using Rocket Propulsion Analysis by Alexander Ponomarenko (http://www.propulsion-analysis.com), specifically version 1.2.9.0. 

Inputs

• 300 psia chamber pressure
• 95% Ethanol/Water for fuel, LOX for Oxidizer
• O/F = 1.5
• using infinite area combustor assumption for right now
• eps,c = 5
• 15° conical nozzle
• 80% c* efficiency

This leads to the current outputs:

Outputs

• Isp, SL = 183.58 sec
• Cf, SL = 1.3169
• c_eff = 5906.50 ft/sec

First Cut Chamber Sizing and Design

So the plan is for a thrust in the 500-1000 lbf range. Using that and Cf/Pc from above I can back out At needed using the below equation.

Using this with 500 and 1000 lbf thrust values gives a throat diameter between 1.269" and 1.795", for sake of simplicity I'm going to choose a value of around 1.75" which gives around 950 lbf @ SL.

From there we get a throat area and then using an L* of ~2.5-3m or so (I'm going to use 108" right in the middle) we get the following chamber geometry

• ε,c = 6.968 per empirical function from Huzel and Huang
• ε,e = 5 as per above
• Dt = 1.75"
• De = 3.9131"
• Dc = 4.6196"
• θ,c = 30°
• θ,e = 15°
• Total Chamber Length = 16.7612"
• Lc/Dc = 3.628

This isn't a perfect sizing but its what I'll accept as a baseline as I work to do some reading and narrow down some of the parameters. Currently the chamber is a little long for what I'd like. The L* for Ethanol/LOX is rather large leading to a large Lc/Dc if I keep the contraction ratio low. From reading (Space Propulsion Analysis and Design by Humble et al) this value is historically between 0.5 and 2.5. I'll need to do some more reading on the L* as well as any info on other engines in the same scale to determine if this adequate. 

Next Steps

Next is some more literature review for the chamber sizing and injector design as well as some more work to baseline the test setup (size tank volumes, and draft a Plumbing and Instrumentation Diagram (P&ID, etc) for the test setup and engine.