BB - Build

Project Overview

The budget bi-prop is a liquid bi-propellant rocket engine that uses isopropyl alcohol and gaseous oxygen as its primary propellants. This project, a collaboration with a friend of mine, was designed with the goal of making a rocket engine that produced 7lbf of thrust and could be built and tested for under $500. We were also finished all the design work in less than two weeks and began production and testing that same month. This project did not include the cost of some of the tools used to make this engine, but it did include all components for the engine and test stand. 

Design

A predesigned phenolic nozzle was the primary driver of our design. Using the expansion ratio and throat area of the nozzle, the ideal chamber pressure, thrust, and ISP were found. Additionally, a NASA CEA simulation was run to determine optimal O/F ratios and there for propellant flow rates and orifice sizes. Characteristics of the engine are below:

Expansion ratio: 1.9865        O/F: 1.25        ISP: 234.26        Chamber Pressure: 100psi

Thrust: 6.5lbf        Chamber Temperature: ~3200K

Build 

The budget biprop (BB) is made up of 5 separate components: chamber and chamber liner, Nozzle, nozzle cap, and injector. 

The combustion chamber is a thick aluminum pipe with a garolite ablative press-fit into it. The nozzle was then pressed into the end of this assembly and thermal sealant was added to hold it in place. A graphoil gasket was then placed on the end of this assembly and a cap was bolted onto the end. The bolt holes were drilled using a drill press and tapped by hand. 

To simplify design, this engine used a standard off-the-shelf phenolic/graphite nozzle. All parts used for the combustion chamber were found on McMaster or at the local hardware store. 

The injector was an unlike doublet with a pressure tap in the middle. It was machined in two pieces. First, a bottom plate where each orifice was drilled at an angle, and a top plate, where holes directing propellant to the orifices were (this was done because the small McMaster drill bits wouldn't drill through the aluminum stock we already had, and we didn't have time to wait for another disc to arrive). To seal between the interfaces, we used graphoil gaskets like at the nozzle cap and a 3D printed jig was made to hold the bottom plate in place for drilling the orifices. 

3D printed jig for bottom plate. Orifices machined with a mini-mill.

Top and bottom view of the bottom plate. 

Assembled injector. Top plate can be seen with two fittings coming in from each side. 

To verify flow rates through the orifices, a water flow test was conducted, putting a quantity of water in a known volume over a known period of time.