Liquid Rocket Engine Test Stand

The most time consuming and expensive part of building rocket engines is the system of tanks, valves, and sensors called the test stand. A test stand's main function is to deliver the cryogenic liquid oxygen oxidizer and kerosene fuel at the correct pressure and flow rate to the engine. The stand also has to withstand the thrust of the engine, remotely execute commands, record data, and mechanically shut down and vent pressure in the event of an anomaly or loss of power.

The test stand consists of the following subsystems:

High pressure panel to regulate and distribute the nitrogen pressurant gas to the tanks, fuel/ox purge system, and low pressure pneumatics
Insulated tanks with relief, vent, fill, and drain valves.
Liquid feedlines with valves modified for cryogenic oxygen service
Purge system to clear the engine and propellant lines of residual propellant after shutdown
Low pressure pneumatics system to provide gas to pneumatically-actuated valves
Control system programmed to actuate valves at the precise timing needed to fire a liquid engine

All of these components (even the tube fittings) are typically very expensive or not sold to consumers. I therefore had to buy most components from Ebay or make them myself. After 4 months of work, I was confident that the test stand was a safe system capable of firing liquid rocket engines. During the test of my first engine, the stand worked flawlessly from startup to shutdown. I expect this stand to fire countless more engines in the future.

Static Fire Video

Part I. Fluid System Development

Screen Shot 2022-06-27 at 12.35.22 AM1.png

After defining the test stand's flow rate and pressure requirements, as set by the engine, the fluid system architecture was developed. This diagram represents the network of tubes, valves, and tanks that make up the test stand.

Screen Shot 2022-06-30 at 10.23.37 PM.png

Several Python codes were written for calculating parameters in the fluid system.

Part II. Test Stand Model

Screen Shot 2022-06-11 at 12.06.02 PM.png

After the fluid system P&ID was finalized, components were selected (or custom built). These components were assembled into a mechanical test stand design matching the P&ID

Screen Shot 2022-06-11 at 12.09.38 PM.png

Part III. Manufacturing and Integration

III.i. Fluid System Components

The test stand I designed required lots of very expensive valves and tube fittings. Building the test stand with new components would have cost >>$100k which is an order of magnitude above my budget. Even if I had infinite money, some components I needed have long (>6 month) lead times that would not fit my timeline. I therefore turned to my good friend Ebay for 99% of valves and fittings on the stand. It is difficult to find the exact component I want on demand on Ebay so I had to buy what was available and change the design if the component didn't meet my original requirements.

Testing the 4x Swagelok pneumatic actuated ball valves to be used as press and propellant valves. These cost >>$1k new but I got them for $150 each.

III.ii. Frame Weldment

The test stand frame was a weldment of 1" square tube - my favorite method of building a strong and cheap frame. I used the TIG welding process for the frame.

Screen Shot 2022-07-12 at 8.28.07 PM.png

ANSYS static structual analysis at 10ksi engine thrust.

III.iii. Propellant Tanks

Screen Shot 2022-06-30 at 11.13.45 PM.png

Both tanks were built from aluminum 80 cf scuba tanks. An AS5202-08 port was drilled in the bottom of the tank as a liquid outlet. The liquid oxygen tank was wrapped in polyethylene foam insulation.

Drilling the liquid outlet port.

III.v. High Pressure Panel

The high pressure panel manifolds the high pressure nitrogen pressurant to several test stand systems. Regulators reduce the pressure for use in the tanks, purge system, and pneumatics system. The 2000 psi pressurant gas is very dangerous and could easily flatten a small building. Care was taken to ensure that there were zero possible failure modes.

III.iv. Tank Press System

The press system takes the regulated nitrogen pressurant and distributes it to the tanks. Press valves isolate the tanks from the regulators. Other components in this system include pneumatic vent valves, manual vent needle valves, relief valves, pressure transducers, and tank check valves.

Starting to integrate the press system.

III.vi. Main Propellant Feedlines

The propellant lines are designed to be as simple and robust as possible to minimize explosion risk. Pneumatically-actuated propellant valves isolate the propellant from the engine. Fill and drain valves are used for loading/unloading propellant safely. The LOX lines are insulated and cleaned for oxygen service using an ultrasonic cleaner.

Completed liquid feed system.

III.vii. Propellant Purge and Pneumatics Systems

The purge system is used to eject residual propellant from the engine before startup and after shutdown. It also extinguishes any post shutdown fires in the engine. Inert nitrogen gas is fed directly into the propallent lines through dedicated solenoids and check valves.

Part IV. Control and Instrumentation

IV.i. Avionics

The avionics system controlled and monitored the valves throughout the system. Tank and engine pressure data was also collected. The electronics were configured as follows. A Teensy 4.1 handled all digital/analog signals including sensor data. Valve actuation signals were fed into a relay board that powered the 24V solenoid valves. The Teensy was connected to a raspberry pi located on the stand that maintained an SSH connection via ethernet cable with the laptop control console. The test stand could be powered down remotely by unplugging a battery connected to a relay on the stand isolating the main 24V battery.

Avionics boxes (no labview in sight!).

IV.ii. Control Software

The control software allows the user to actuate valves, see sensor data, light the igniter, and execute the engine firing sequence. The code was written in Python and ran on the Teensy using a library called Pyfirmata. Valves can be actuated by using the following syntax in the console: ["open"/"close"] ["valve name"] ["toggle time" (optional parameter)]. Sensor and valve state data can be dumped to the console at any time. A firing sequence mode runs automated system checks and guides the user through the pre test valve actuation sequence. Once ready to fire, the software lights the igniter, and on visual confirmation, the user presses "Enter" and the system automatically starts up and shuts down the engine. All the user has to do is watch!

View from mission control.

Part V. System Characterization

V.i. Relief Valve Setting

The relief valves were calibrated to 950 psi using this makeshift setup.

V.ii. Fluid System Characterization

Screen Shot 2022-06-29 at 11.55.55 PM.png

Water flow tests were preformed to evaluate the test stand's (and the engine's) performance. Valve timing was determined using data from these tests.

Screen Shot 2022-06-30 at 12.08.56 AM.png

V.iii. LOX Dewar

Having my own 50 liter liquid oxygen dewar (Cryofab CL50) was necessary for transporting the oxidizer to the test site. It was thoroughly LOX cleaned before use.

Filling the dewar with LN2 before converting it for LOX service.

Part VI. Hot Fire Testing

VI.i. Prop Load

After the test stand was set up at the test site, the propellant was loaded into the tanks. First, the kerosene was loaded. Then, the liquid oxygen was slowly loaded into the tank. It took around an hour to chill the LOX tank and fill it with liquid.

Loading LOX

VI.ii. Hot Fire

Screen Shot 2022-07-09 at 3.39.52 PM.png

The test stand successfully fired my first liquid rocket engine! I'm looking forward to upgrading the stand with more instrumentation for future tests!

Screen Shot 2022-07-09 at 3.36.14 PM.png

Liked this? See how I developed the engine