Fuels and oxidisers — kerolox, hydrolox, methalox, hypergolics, solids

Kerolox (kerosene + liquid oxygen) is the workhorse of orbital launch. The kerosene is a tightly specified grade called RP-1 (Rocket Propellant 1) — ordinary jet fuel, refined to remove the wax and sulphur that would coke a rocket engine and stripped of the alkenes that polymerise under heat. LOX is loaded chilled. The combination gives moderate Isp (~300 s sea level, ~340 vacuum), high density (so the tanks stay small), and clean ground handling. Saturn V first stage, Falcon 9, Atlas V, Soyuz, Long March 2/3/4, India's PSLV upper stages are all kerolox. The downside is soot: kerosene burns dirty, coking the engine internally and limiting reusability — Merlin works around this with sea-level chamber pressures kept well below the staged-combustion sweet spot.

Hydrolox (liquid hydrogen + liquid oxygen) is the highest-Isp chemical pair humanity flies (~450 s vacuum on RS-25). Hydrogen burns to water, no soot. The penalty is density: liquid H₂ is about 70 kg/m³ versus kerosene's 800 kg/m³, so the fuel tank is enormous and most of the rocket becomes insulation. Hydrolox is therefore an *upper-stage* propellant for almost all launchers (Centaur, DCSS, ICPS, the upper stages of Ariane 5/6, Long March 5, H-IIA) and only used at booster scale where extreme Isp is paid for in vehicle size — Saturn V S-II + S-IVB, Space Shuttle external tank + RS-25, SLS core stage, Ariane 5 Vulcain core. Boil-off is the operational headache: liquid H₂ at 20 K cannot sit on the pad for long.

Methalox (liquid methane + LOX) is the new arrival, operational with Raptor on Starship (2023) and Zhuque-2 (2023, first to orbit), and coming on Vulcan (BE-4), Neutron, and Terran R. Three properties make it interesting. First, methane density is ~420 kg/m³ — about 6× hydrogen, so the tank is sane. Second, it burns clean enough to be reusable (no coking like RP-1). Third, methane and oxygen can both be made on Mars from atmospheric CO₂ and ice via the Sabatier reaction — the entire SpaceX Mars architecture rests on this. Isp is between kerolox and hydrolox (~330 s sea level, ~380 vacuum), which is a sweet-spot value for a single-stage-to-orbit-class vehicle.

Hypergolic propellants (the storable family) ignite spontaneously on contact — no spark, no preburner. The canonical pair is UDMH (unsymmetrical dimethylhydrazine) or MMH (monomethylhydrazine) as fuel with N₂O₄ (dinitrogen tetroxide) as oxidiser. They are filthy: UDMH is a carcinogen, N₂O₄ is acutely toxic, both can sit in a sealed tank for years. The combination ran every Apollo Service Module, every Apollo LM ascent + descent engine, every Space Shuttle OMS burn, Soyuz / Progress orbital manoeuvring, and almost every Chinese / Russian / Indian deep-space probe's main engine. The reason: instant ignition every time, decade-long shelf life, no cryogenic plumbing. Modern Western missions have tried to phase them out (Dragon's Draco runs on safer monopropellant blends, JWST cold-gas thrusters use hydrazine alone) but the bulk of deep-space propulsion is still hypergolic.

Solid propellants are a single cast slug of rubbery binder (HTPB, polybutadiene-based) loaded with ammonium perchlorate (the oxidiser) and aluminium powder (a high-energy fuel additive). The grain is cast around a hollow core whose cross-section sets the burn-rate profile — a star-shaped core gives a long even thrust, a cylindrical core gives a regressive (declining) profile. Once lit, you cannot stop or throttle it; you can only choose how the grain was originally cast. Isp is low (~250 s) but thrust is enormous and the hardware is simple: SLS boosters produce 16 MN each, more than four Merlin Vacuums. The Space Shuttle SRBs, Ariane 6 P120C, every ICBM, India's PSLV first stage, every air-to-air missile, and most CubeSat kick stages are solid.