Propellant Selection

Types

Cryogenic bipropellants:

• $\mathrm{LOX}/\mathrm{LH}{}_2,\mathrm{LOX}/\mathrm{CH}{}_4,\mathrm{LOX}/\mathrm{RP}\text{-}1$
• Pros:
  • high $I_{sp}$ (esp. $\mathrm{LH}{}_2$)
  • clean combustion
  • high exhaust velocity
• Cons:
  • cryo handling
  • insulation
  • boil‑off
  • $\mathrm{LH}{}_2$ very low density $⟹$ large tanks

Storable bipropellants:

• $\mathrm{N}{}_2\mathrm{O}{}_4/\mathrm{UDMH},\mathrm{N}{}_2\mathrm{O}{}_4/\mathrm{MMH}$ (hypergolic)
• Pros:
  • long‑term storage
  • restartable
  • hypergolic reliability
  • simpler start
• Cons:
  • toxicity/corrosion
  • lower $I_{sp}$
  • thermal management for tanks in space

Monopropellants:

• $\mathrm{N}{}_2\mathrm{H}{}_4$, “green” $\mathrm{HAN}$ or $\mathrm{ADN}$ blends
• Classic:
  • $\mathrm{N}{}_2\mathrm{H}{}_4$ over Ir catalyst bed
  • good throttling and cold‑start behavior
• Green blends:
  • Hydroxylammonium nitrate ($\mathrm{HAN}$) and ammonium dinitramide ($\mathrm{ADN}$) aqueous fuels aim to cut toxicity
  • higher density
  • different thermal management
  • typically higher catalyst light‑off temperatures

Gelled propellants

• thickened fuel/oxidizer for reduced leakage and tailored atomization

Performance metrics

• Specific impulse: $$I_{sp}=\frac{F}{\dot{m}{g}_0}$$
• Characteristic velocity: $$c^{\ast }=\frac{p_0{A}_t}{\dot{m}}$$
• Mixture ratio: $$\text{O/F}=\frac{{\dot{m}}_{\text{ox}}}{{\dot{m}}_{\text{fuel}}}$$
• Density‑$I_{sp}$ figure of merit for tanks & stages.

Trade‑offs

• Cryogenics: high $I_{sp}$, low temperature storage, boil-off/ground ops complexity
• Storables: lower $I_{sp}$, excellent storability, hypergolic ignition, toxicity/handling
• Monoprops: simplest feed/valving, lowest $I_{sp}$ among chemical options
• Gels: leakage mitigation, throttling/shaping benefits, feed/atomization complexity

Materials & safety

• Compatibility (aluminum, stainless, elastomers)
• Ignition Sensitivity (hypergols)
• Toxicity
• Environmental Considerations