1. The chemistry that pushes back

Andy Weir’s The Martian returns as the recalled work because Mark Watney’s survival depends on chemical reactions he can control: burning hydrazine to make water, combusting rocket fuel to modify the MAV trajectory. A spacecraft’s chemical propulsion is the same kind of controlled reaction, just directed through a nozzle.

This entry reads about chemical and green propulsion for small satellites.

2. Monopropellant systems

A monopropellant versus bipropellant overview explains that monopropellant systems use a single propellant decomposed by a catalyst. Hydrazine is the traditional choice, with specific impulse around 220 to 230 seconds. The system is simpler than bipropellant because it has one tank, one feed line, and one set of valves.

Monopropellant thrusters are widely used for attitude control and small maneuvers. Their drawback is that hydrazine is highly toxic, requiring careful ground handling and safety procedures.

3. Bipropellant systems

Bipropellant systems combine a fuel and an oxidizer. They achieve higher specific impulse, around 310 to 320 seconds, and higher thrust than monopropellant systems. They are more complex, with two tanks, two feed systems, and more valves. They are typically used for main propulsion and larger orbit maneuvers.

For small satellites, bipropellant systems are less common than on large spacecraft because of mass, volume, and complexity constraints.

4. Green propellants

Green propellants are developed as less-toxic alternatives to hydrazine. A NASA SST overview mentions green bipropellant combinations such as hydrogen peroxide and nitrous oxide. An MDPI paper on a modular green monopropellant system describes MIMPS-G, a system designed for CubeSats in LEO and lunar missions using HAN or ADN-based propellants.

Green propellants reduce ground operations cost and risk, but they have less flight heritage than hydrazine and may require different materials and catalysts.

5. Cold gas

Cold gas propulsion expands an inert gas, such as nitrogen or R-236fa, through a nozzle. It is the simplest propulsion option: no combustion, no catalyst, no high temperatures. Thrust and specific impulse are low, but reliability is high. Cold gas is often used for attitude control and small maneuvers on CubeSats.

6. Multimode propulsion

An MIT Space Propulsion Laboratory page on bimodal systems notes that having both chemical and electric modes on one spacecraft is useful for a wide range of applications. The challenge is that conventional propellants for chemical and electric thrusters are incompatible. Research into ionic liquid monopropellants aims to enable a single propellant to serve both modes.

7. What this changes

  • Chemical propulsion provides the thrust that electric propulsion lacks.
  • Green propellants may reduce operations cost and risk compared to hydrazine.
  • Cold gas is simple but limited in performance.
  • Multimode propulsion is attractive for the desktop if a single propellant can serve both chemical and electric modes.
  • The next entry will synthesize the propulsion reading arc into requirements for the desktop.