This team developed a universal test stand and basic hybrid rocket motor to help teach students about combustion, instrumentation, aerospace propulsion, and data analysis.

Members: William Hess, William Kim, John Smalley


Motivation and Planning

Mind Map of Project Subsystems

The educational problem that the Propulsion team aims to tackle is the relative lack of propulsion or rocket-related educational material in the Duke University MEMS undergrad curriculum. The construction of a suitable test stand, more robust and modular than those that already exist on Duke campus, would be a step in the right direction to allow student rocket designs to be qualified for thrust readings and potentially other important physical characteristics, such as chamber pressure or engine temperature. 

 
The research problem is the relative lack of published quantifiable information about the characteristics and chamber pressures of acrylic hybrid motors. If time allows, more advanced experimental propellants could be tested on our test stand in order to further the breadth of experimental data of hybrid propellants. 
This project consists of two primary deliverables: a lab scale propulsion test stand, and a low-cost hybrid rocket motor. The test stand shall allow for the safe and effective testing of lab-scale solid, hybrid, liquid, and non-rocket aerospace propulsion systems. Likewise, the hybrid motor is to be a low-cost demonstrator of flame propagation, the effects of nozzle expansion ratio, and other first principles of rocket propulsion. 

Test Stand Design

CAD model of test stand structure (Acrylic Hybrid Installed).

The subsystems of the project are first divided into the hybrid motor and the test stand. Those are the two independent mechanical systems that will be coupled for the final product. The test stand can be further subdivided into the mechanical frame design, the sensor module mechanical and electrical hardware design, and data acquisition software. The test stand is to allow for data acquisition synchronized with ignition and extinguishing of the hybrid rocket motor. 

The test stand’s primary structure is a welded 1″ steel frame, creating a rigid frame onto which any number of components can be added. In the current acrylic hybrid configuration, two pieces of 80/20 extrusion are attached to the steel frame via L-brackets. A low-friction rail system is attached to the 80/20 as well.

Hybrid Rocket Motor and Plumbing

"Mallard" - Acrylic Hybrid Motor Assembly

The hybrid motor is composed of the physical fuel grain (made from acrylic), motor enclosure (made from aluminum plates and threaded rods) and nozzle body (made from silica-based phenolic), and then the plumbing and valve design for the gaseous oxygen feed required for oxidation for combustion.  This system will be modular, such that it can easily be scaled as needed for larger or smaller propulsion systems in the future.  The current acrylic fuel grain is 1.5″ in diameter with a 0.375″ core, although this can be exchanged for any 1.5″ acrylic tube within reason. Two O-rings form a face seal on either side of the fuel grain to prevent oxygen leakage from the system, leading to potential failures. Note the flashback arrestor and check valve, installed prevent a dangerous progression of the flame front towards the oxygen bottle in case of emergency.

Plumbing Assembly

The plumbing system is entirely composed of brass and oxygen-rated, nylon reinforced nylon tubing. This is to avoid the introduction of sparks or other debris in the oxygen flow, which could potentially lead to a failure. The pluming system allows the user to tune the inlet pressure between 20 and 60 PSIg, using the gauge and manual dump valve (the component with the small ring) to adjust the pressure after turning the valve on the small oxygen tank. The dump valve also serves as a safety blowoff valve for the solenoid, which are both rated to 10 bar. The oxygen tank currently designed into this system are disposable, single-use tanks sold by Bernzomatic at hardware stores. These tanks are affordable, portable, and low-pressure. The system, however, is rated to 10 bar of pressure, so larger tanks and regulators may be used if they are under this pressure.

Electrical System

The electrical system that controls motor ignition and data acquisition is based around an Arduino MEGA as the central microcontroller unit. Additional modules connected to the MEGA are the HX711 load cell amplifier that connects to the thrust load cell, a quad relay module that switches on voltage to the solenoid valve and igniter, and the 7 segment display to show the countdown. The system currently must be powered by a triple output 24v power supply, on account of the solenoid valve requiring 24VDC and both positive and negative voltage input.

Hot Fire Test and Results

The team conducted a successful hot fire test on December 6th, 2023. The slow-motion video of the flame propagation is at the top of this page. 

Educational Levels

Schematic of the current system, indicating where it can be improved.

Beginner

This project allows younger students, or students new to engineering, to engage with it meaningfully regardless of experience. From data acquisition using an Arduino to basic manufacturing skills, a large variety of entry-level skills can be explored in this project, listed below:

 

  •  Electronics, Arduino basics, and DAQ: From wiring to programming, the Arduino MEGA installed into this system can expose students to the fundamentals of wiring, power delivery, and useful software in an open-source environment. The Arduino serves as an excellent stepping stone to more advanced controllers like the Raspberry Pi and beyond. As a bonus, data acquisition is an integral part of this software package, giving students an introduction to that technology as well.
  • Fundamentals of CAD and Design: As this project continues, it is inevitable that more instrumentation and features will be added. Creating the structures for that telemetry, and structural components for future propulsion systems, serve as an excellent stepping stone to more advanced components.
  • Fabrication Fundamentals: As new propulsion systems or other attachments are designed, students will actually need to make them. This project gives students exposure to FDM 3D printing for non load-bearing components, and many of the parts can be fabricated on a manual milling machine and lathe.

Intermediate

At the intermediate level, topics become more intertwined with concepts explored in class, and begin to explore experimental design and high-fidelity fabrication in more detail.

  • Instrumentation: Selecting the correct sensors and accompanying systems is critically important in engineering. This project allows students to expand the detection capabilities of the test stand by adding thermocouples, more force sensors, and other instrumentation to better qualify propulsion systems.
  • Improved Manufacturing: As design skill improves, having the hard skills to act on a design is critical. CAM programming and running a CNC program are incredibly useful skills when it comes to manufacturing complex parts. CNC was used extensively in the acrylic hybrid “Mallard”, and inevitably future designs will as well. Welding adds another layer to one’s technical repertoire as well, and this project provides plenty of opportunities to learn this valuable skill.
  • Structural Analysis and Materials Selection: As the propulsion systems tested become more powerful, students will need to ensure that the materials used on those systems are suitable for the temperatures and stresses involved, including materials used on the test stand. A great way to act on this is via FEA and other simulation methodologies, which are critical skills for any engineer to understand.

Advanced

After mastering the fundamentals, more advanced topics can be explored, especially with regards to the experiment itself rather than just the stand and its associated instrumentation.

  • Command and Control: Future hybrid or liquid (and in some cases, solid) propulsion systems can be throttled using PID and gimballed to a variety of angles to achieve even more useful, practical results for propulsion. This stand can be adapted by future students for propulsion systems that they themselves will develop.
  • Performance Simulation and Experimental Propellant: Students can use the data from this test stand to inform internal ballistics calculations on solid propellant, or to inform the accuracy of hybrid or liquid propulsion simulation models. This can help expand this project from just a test stand, to a small part of a larger experimental propellant program.
  • Data Visualization: Future versions of the user interface can include a live data readout from the variety of sensors that can be installed. Real-time data readouts can inform of potential catastrophic failures during testing.
  • Flight Validation: In the future, the data taken from this test stand can be validated on an actual rocket launch or other flight test, just as is done in the industry.

 

Referenced Resources

Publications:

Costa, Fernando Et. Al. (2017) “Building and Testing of a Lab Scale Hybrid Rocket Motor for Acquisition of Regression Rate Equations”

Sutton, G.P. and Biblarz, O. (2001) Rocket Propulsion Elements. 7th Edition, John Wiley, Hoboken. 

Software:

S. Gordon and B. J. McBride, “Computer Program for Calculation of Complex Chemical Equilibrium Compositions and Applications,” NASA Reference Publication 1311 (1996).

Other Referenced Media:

Cusick, Ben. “Building a DIY Transparent Hybrid Rocket Engine – Nighthawkinlight.” YouTube, NightHawkInLight, 10 Nov. 2016, www.youtube.com/watch?v=VI8gdmjEwKc.

NIST Office of Data and Informatics. Poly(Methyl Methacrylate), National Institute of Standards and Technology, webbook.nist.gov/cgi/formula?ID=C87210320&Mask=2#Thermo-Condensed. Accessed 14 Nov. 2023.

Krasnow, Ben. “Hybrid Rocket Engine with Acrylic and Gaseous Oxygen.” YouTube, Applied Science, 24 Sept. 2012, www.youtube.com/watch?v=TLPWqCMb7DE.