cislunar explorers

thrust characterization

2021   ·   thermal-fluid-modeling,   ansys-fluent,   circuit   design   ·   research

Overview

As part of the Attitude Control System (ACS) subteam on Cislunar Explorers, I worked on modeling and experimentally characterizing a CO₂-based propulsion system. The goal was to understand both the thermal behavior of the gas flow and the force output of the thruster to estimate reorientation capability.

Thermal Characterization of CO₂ Flow

I first analyzed the temperature change of CO₂ as it expands from the canister into a solenoid valve using the Joule–Thomson effect.

  • Assumed inlet conditions: 850 psi, 15°C, 50 mL, 0.038 kg
  • Computed thermodynamic properties using tabulated data
  • Derived temperature change using the Joule–Thomson formulation

This analysis revealed that the temperature drop could not be determined without accurate exit pressure measurements, which were unavailable at the time.

To further investigate, I modeled the system in ANSYS FLUENT:

  • Built geometry for canister, pipe, and solenoid
  • Approximated bent pipe as straight with equivalent length
  • Simulated laminar flow and examined velocity fields

However, the simulation lacked sufficient boundary conditions for the energy equation, making results inconclusive. We decided to defer this analysis until physical measurements become available.

Force Characterization and Electronics

I then shifted focus to measuring the thrust generated by the CO₂ canister.

Circuit Design

  • Analyzed a spike-and-hold solenoid driver circuit
  • Tuned parameters:
    • ( C_1 = 0.047 \mu F )
    • ( R_1 = 500k\Omega )
  • Proposed modifying the circuit to accept a 3.3V control signal instead of 5V
  • Identified required transistor changes (Q1, Q4)

This reduced system complexity by removing the need for a voltage booster.

Experimental Fixture Design

I designed a test setup to measure thrust using a force/torque sensor:

  • Created a full ACS CAD assembly from scratch
  • Designed a modular fixture:
    • Nozzle mounted via standoffs to 3D-printed structure
    • Structure attached to force/torque sensor
    • Sensor mounted to rigid 80/20 frame

Key design decisions:

  • Simplified bent pipe → straight pipe (4.75 in)
  • Ensured structural rigidity using multi-length 80/20 supports

This setup enables direct measurement of thrust, which can be used to estimate reorientation capability of the system :contentReference[oaicite:2]{index=2}.

Key Takeaways

  • Analytical modeling alone was insufficient without experimental boundary conditions
  • Simulation can become a bottleneck without well-defined parameters
  • Hardware design and testing are essential for validating propulsion performance
  • System-level thinking (thermal + electrical + mechanical) is critical