Engineering Comparison and Multi-Criteria Selection of Micropropulsion Systems for Small Spacecraft

Technical Summary: Engineering Comparison and Multi-Criteria Selection of Micropropulsion Systems for Small Spacecraft

Problem Statement
As small spacecraft (e.g., CubeSats) transition from educational demonstrations to operational platforms for Earth observation, communications, and deep-space precursors, onboard propulsion has shifted from an optional enhancement to a core subsystem. However, the selection of a micropropulsion architecture is not a matter of identifying a single "best" technology. Instead, engineers face a complex trade-off problem involving severe constraints in mass, volume, power, total impulse, safety, and mission duration. While specific impulse ($I_{sp}$) is a common metric, relying on it alone is insufficient. A thruster with high efficiency may be unsuitable if it requires a heavy power-processing unit (PPU) or complex thermal management, while a less efficient system may be optimal if it offers rapid response, simplicity, or safer handling. The current literature often classifies technologies descriptively but lacks a structured framework to compare them against simultaneous engineering criteria.

Methodology
This paper presents a systematic review of micropropulsion literature published between 2020 and 2026, reorganized into an engineering comparison framework. The study introduces a universal, traceable decision-making methodology based on a hybrid Analytic Hierarchy Process (AHP), Entropy weighting, and TOPSIS (Technique for Order of Preference by Similarity to Ideal Solution) framework.

1. Classification: Micropropulsion systems are categorized into three primary groups: Cold-gas, Chemical (monopropellant, bipropellant, solid), and Electric (subdivided into electrothermal, electrostatic, and electromagnetic).
2. Criteria Definition: The selection process evaluates seven key criteria: Thrust capability ($F$), Specific impulse ($I_{sp}$), Electrical power demand ($P$), System mass ($m_{sys}$), System volume ($V$), Technical maturity/flight heritage (TRL), and Safety/operational simplicity ($S_f$).
3. Hybrid Weighting:
   • AHP: Captures subjective engineering judgment through pairwise comparisons of criteria.
   • Entropy Weighting: Derives objective weights based on the data variability across competing alternatives.
   • Combination: A balance factor ($\alpha$) combines subjective and objective weights ($w_j = \alpha w^{AHP}_j + (1-\alpha) w^{E}_j$).
   • TOPSIS: Ranks alternatives based on their proximity to an ideal solution and remoteness from a non-ideal solution.

This approach allows the decision structure to remain consistent while adapting to specific mission priorities by adjusting the pairwise judgments or the input data matrix.

Key Results and Comparative Analysis
The paper provides a comparative assessment of representative thruster types, highlighting distinct performance envelopes rather than universal rankings:

• Cold-Gas Systems: Characterized by simplicity, high reliability, and low contamination risk (TRL 8–9). However, they suffer from low specific impulse (~40–100 s) and low propellant efficiency. They are best suited for attitude control and safe missions where thrust requirements are low.
• Electrothermal Systems (Resistojet/Water): Offer improved efficiency over cold-gas (~70–200 s) and utilize safer propellants like water. They introduce penalties related to heater power and thermal management but are attractive for missions with strict safety or storability constraints.
• Chemical Systems (Monopropellant/Bipropellant/Solid): Provide the highest thrust levels (mN to N range) and fast response times. However, they often involve higher system complexity, catalyst handling risks, and thermal control challenges. Bipropellant systems offer higher performance (~250–350 s) but require complex feed systems and ignition mechanisms.
• Electrostatic Systems (Ion/Electrospray/FEEP): Dominate in specific impulse (~800–5000 s) and precision control. They are ideal for long-duration efficiency and precise station-keeping. Limitations include low absolute thrust, high voltage requirements, and concerns regarding emitter lifetime and contamination.
• Electromagnetic Systems (Hall/Pulsed Plasma): Offer a compromise between power demand, thrust density, and efficiency. Hall thrusters (~1000–2000 s) are suitable for orbit raising and station-keeping but face challenges with cathode erosion and power limits. Pulsed plasma thrusters provide compact solid-propellant solutions but suffer from lower efficiency and electrode erosion.

The analysis confirms that no single technology dominates all missions. Electric systems generally prevail when propellant economy or high-precision control are paramount, whereas cold-gas, electrothermal, water-resistojet, and monopropellant systems remain attractive when simplicity, rapid response, safety, or higher thrust are the primary drivers.

Significance and Contributions
The primary contribution of this paper is methodological. It reframes the micropropulsion selection process from a descriptive taxonomy into a structured engineering comparison supported by a universal AHP-Entropy-TOPSIS weighting logic. This framework makes the reasoning behind propulsion selection traceable, repeatable, and adaptable to different mission types.

The paper argues that the "modern design problem" is not whether to use propulsion, but how to select the most suitable architecture under conflicting constraints. By integrating expert judgment with objective data variability, the proposed hybrid weighting system improves the robustness of early-stage propulsion selection. The authors conclude that future progress depends on the publication of complete system-level data (including wet/dry mass, PPU mass, and volume) and more on-orbit results, particularly for emerging technologies like water-based systems, green monopropellants, and low-power Hall thrusters. The study serves as a tool for spacecraft system engineers to conduct early-stage trade studies and select appropriate micropropulsion technologies for future small-satellite missions.