# Propellant
A propellant is a substance or combination of substances that a rocket expends to produce thrust; it may include a fuel and oxidizer, a monopropellant, or a gas expelled for propulsion. NASA's safety standard defines it as "a solid, liquid, or hybrid chemical substance used in the production of energy or pressurized gas" for propulsion[1](https://standards.nasa.gov/sites/default/files/standards/NASA/A/2/nasa-std-871912a_with_change_2.pdf). The word covers more than fuel: rocket propellants generally consist of oxidizers and fuels, and the term "rocket fuel" is deliberately narrower because the oxidizer is counted as part of the propellant[2](https://www.degruyterbrill.com/document/doi/10.1515/9783110751390-202/pdf).
| Key fact | Detail |
|---|---|
| Definition | A solid, liquid, or hybrid chemical substance used to produce energy or pressurized gas for propulsion; solid propellants contain both fuel and oxidizer in one composition[1](https://standards.nasa.gov/sites/default/files/standards/NASA/A/2/nasa-std-871912a_with_change_2.pdf) |
| Ideal vacuum Isp (fuel with LOX) | Liquid hydrogen 432.9 s, methane 350.0 s, propane 343.7 s, RP-1 337.0 s[3](https://ntrs.nasa.gov/api/citations/19900019426/downloads/19900019426.pdf) |
| Density specific impulse | About 2,900 s·kg/m³ for oxygen-kerosene, 1,620 for oxygen-hydrogen, 3,550 for NTO/MMH[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html) |
| Throttling | Liquid engines throttle across 2:1 to 10:1 and hybrids 2:1 to 5:1; solid motors cannot throttle, shut down, or restart after ignition[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html) |
| Green monopropellants | ASCENT delivers 190–250 s at 1.460 g/cc and LMP-103S 226–255 s at 1.240 g/cc, against hydrazine's 228–236 s at 1.004 g/cc[5](https://ntrs.nasa.gov/api/citations/20240004298/downloads/SP_2024_00239_20240410_Final.pdf) |
| Methalox engines | SpaceX's Raptor and Blue Origin's BE-4 each generate more than half a million pounds of thrust; Starship's booster uses 39 Raptors[6](https://arstechnica.com/space/2026/03/nasa-is-blowing-stuff-up-to-study-the-explosive-potential-of-methalox-rockets/) |
## What a propellant is
The NASA standard splits the category by physical form. Liquid propellants are "liquid and gaseous substances (fuels, oxidizers, or monopropellants)"; solid propellants are compositions that "contain both fuels and oxidizers, and various other chemicals used as burning rate controllers and binders"[1](https://standards.nasa.gov/sites/default/files/standards/NASA/A/2/nasa-std-871912a_with_change_2.pdf). A monopropellant is stored in a single tank and decomposes from its storage form with a catalyst or other ignition method, without a separately stored second propellant[7](https://www.mdpi.com/2226-4310/8/1/20).
The mass budget explains why propellant choice dominates launch vehicle design.
## Classification and chemistry
**Solid propellants.** The fuel and oxidizer are premixed in the grain, which makes the motor simple but carries explosion handling precautions because the solid-state premixed propellant is chemically similar to [trinitrotoluene](https://www.edgepedia.org/trinitrotoluene) (TNT)[8](https://www.mdpi.com/2226-4310/11/9/739).
**Liquid propellants.** A DLR assessment of reusable launch stages selects liquid oxygen paired with liquid hydrogen, liquid methane, liquid propane, or RP-1 kerosene as the four most promising fuel combinations for modern launchers, citing low toxicity, cost, and handling burden[9](https://elib.dlr.de/206596/1/s12567-024-00564-w.pdf). RP-1 is a kerosene-base fuel used with LOX in rockets such as the [Saturn V](https://www.edgepedia.org/saturn-v), with density 0.81 to 1.02 g/mL, an oxidizer-to-fuel ratio of 2.56, and a combustion temperature of 3,670 K[10](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=903675).
**Hybrids.** The most common hybrid configuration uses a solid fuel with a liquid oxidizer; the concept dates to 1937 but saw practical development only much later[11](https://propulsion-skrishnan.com/pdf/Review%20of%20Hybrid%20Rocket%20Technology.pdf).
**Cold and warm gas.** The cold/warm gas thruster is perhaps the simplest spacecraft propulsion type: thrust comes from ejecting a gas, possibly moderately heated, through a nozzle. Non-toxic, non-explosive, and non-hypergolic propellants can be used, removing or reducing handling, loading, ignition, and thermal management issues, at the cost of lower performance[12](https://unsworks.unsw.edu.au/server/api/core/bitstreams/4cbf96e6-f8e5-4271-bfdc-88e51b326907/content).
## Performance measures
[Specific impulse](https://www.edgepedia.org/specific-impulse) (Isp) measures propellant efficiency as thrust divided by propellant mass-flow rate and standard gravity, and is reported in seconds. Values may be stated for sea-level atmospheric pressure or vacuum, with an optimized nozzle area ratio[2](https://www.degruyterbrill.com/document/doi/10.1515/9783110751390-202/pdf).
Density specific impulse multiplies Isp by bulk propellant density, capturing the tankage and volume penalty of light propellants. It is approximately 2,900 s·kg/m³ for oxygen-kerosene, 1,620 for oxygen-hydrogen, and 3,550 for nitrogen tetroxide with [monomethylhydrazine](https://www.edgepedia.org/monomethylhydrazine)[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html).
Typical Isp ranges by family (solid-composite range at sea level): cryogenic liquids 300 to 460 s; storables 270 to 340 s; monopropellants 150 to 250 s; solid composites 240 to 270 s at sea level; hybrids 230 to 350 s[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html). A NASA study reports ideal Isp of 432.9 s for liquid hydrogen, 350.0 s for methane, 343.7 s for propane, and 337.0 s for RP-1[3](https://ntrs.nasa.gov/api/citations/19900019426/downloads/19900019426.pdf). Modern composite solids deliver 240 to 270 s at sea level and 260 to 290 s in vacuum; the [Space Shuttle](https://www.edgepedia.org/space-shuttle) booster propellant delivered 237 s sea level and 268 s vacuum, and the theoretical maximum for a solid, about 310 s vacuum, is reached by no known formulation[13](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/02/rocket_propellant_chemistry_solid_propellants.pdf).
## By the numbers
At 1 bar sea-level tank conditions, the DLR study lists these densities and temperatures[9](https://elib.dlr.de/206596/1/s12567-024-00564-w.pdf):
| Propellant | Density (kg/m³) | Temperature (K) |
|---|---|---|
| LOX | 1,141.8 | 90.1 |
| LH2 | 70.899 | 20.324 |
| LCH4 | 422.6 | 111.66 |
| Liquid propane | 582 | 231.08 |
| RP-1 | 807 | 298.1 |
Hydrogen's 0.071 g/ml density and cryogenic storage make it awkward: cryogenic propellants are difficult to store over long periods, making them less desirable for military rockets that must stay launch-ready for months[14](http://www.braeunig.us/space/propel.htm?xid=PS_smithsonian). Kerolox mixture density is about 1,020 kg/m³ versus about 360 kg/m³ for oxygen-hydrogen at O/F 6.0[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html).
## Solid versus liquid versus hybrid trade-offs
**Control.** Liquid engines throttle across 2:1 to 10:1 depending on injector and pump design; hybrids throttle 2:1 to 5:1 through oxidizer flow control; solid motors cannot throttle, shut down, or restart after ignition, though a thrust schedule can be designed into the grain[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html).
**Cost.** Solid propellants are the least expensive per unit specific impulse for boost applications because the delivery system is mechanically simple; cryogenic liquids are the most expensive per unit vehicle mass because ground infrastructure, tank insulation, and pumping systems are elaborate[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html). For hybrids, sources disagree: one review puts hybrid production cost between solid motors and liquid engines but judges handling safest and cheapest, so that a properly designed hybrid's overall cost is generally lower than both[8](https://www.mdpi.com/2226-4310/11/9/739), while another analysis holds that hybrids remain more expensive per unit performance than solids or storables at current production volumes[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html).
## Monopropellants and green propulsion
Hydrazine has served as the standard satellite monopropellant for spacecraft attitude control; a hydrazine thruster delivers a theoretical specific impulse of about 230 s, but its toxicity is its main disadvantage and drives extra ground-handling costs[15](https://doi.org/10.18321/ectj1491). KSC stocks it to MIL-PRF-26536 in Monopropellant and High Purity grades, supplied by the Defense Logistics Agency[16](https://public.ksc.nasa.gov/propellants/controlledstoragefunction/).
**Green propulsion** means higher-performing, lower-toxicity, safer-to-handle formulations, a direction the propulsion community has pursued since the 1990s[5](https://ntrs.nasa.gov/api/citations/20240004298/downloads/SP_2024_00239_20240410_Final.pdf). Green monopropellants fall into three classes: Energetic Ionic Liquids (EILs), liquid NOx monopropellants, and hydrogen peroxide aqueous solutions; NOx fuel blends have the highest performance and peroxide solutions the lowest, even below hydrazine[7](https://www.mdpi.com/2226-4310/8/1/20). The in-space demonstrated EILs are AF-M315E (ASCENT) and LMP-103S[7](https://www.mdpi.com/2226-4310/8/1/20). A NASA Goddard assessment tabulates hydrazine at 228 to 236 s and 1.004 g/cc, ASCENT at 190 to 250 s and 1.460 g/cc, and LMP-103S at 226 to 255 s and 1.240 g/cc[5](https://ntrs.nasa.gov/api/citations/20240004298/downloads/SP_2024_00239_20240410_Final.pdf); a separate review computes ADN-based propellant at 251 s (NASA-CEA, 10 bar, 40:1 expansion), outperforming hydrazine, but notes higher cost and a supply chain constrained to a single European provider[15](https://doi.org/10.18321/ectj1491). [Nitrous oxide](https://www.edgepedia.org/nitrous-oxide) offers a theoretical 198 s with a 1,640 °C decomposition temperature that complicates catalyst design; the NOx blend NOFBX claims 325 s with no flight history[15](https://doi.org/10.18321/ectj1491).
## Safety, handling, and regulation
Three hazard classes govern propellant choice. Explosive sensitivity applies to solids, whose premixed grains resemble TNT and demand explosion precautions[8](https://www.mdpi.com/2226-4310/11/9/739). Cryogens such as liquid hydrogen require dedicated infrastructure: a DOE handbook based on NASA-STD-8719.16 gives guidelines for safely storing, handling, and using hydrogen in gaseous, liquid, or slush form, covering properties and hazards, facility design, materials compatibility, detection, and transportation[17](https://www.energy.gov/sites/prod/files/2014/03/f11/871916.pdf). Internationally, propellants move under the UN Recommendations on the Transport of Dangerous Goods and the GHS classification of chemicals presenting physical hazards[18](https://unece.org/sites/default/files/2023-08/ST-SG-AC10-11-Rev7e_final_0.pdf).
## Methane engines and the post-2023 shift
Methalox has become the propellant of the new heavy-lift generation. SpaceX's Raptor and [Blue Origin](https://www.edgepedia.org/blue-origin)'s BE-4 each generate more than half a million pounds of thrust, and Starship's booster is powered by 39 Raptors[6](https://arstechnica.com/space/2026/03/nasa-is-blowing-stuff-up-to-study-the-explosive-potential-of-methalox-rockets/). A performance card published August 3, 2024 put Raptor 3 at 280 metric tons-force of sea-level thrust, 350 s specific impulse, and 1,525 kg engine mass, up from Raptor 2's 230 tons; Starship is designed for up to 150 metric tons fully reusable and 250 metric tons expendable[19](https://newspaceeconomy.ca/2026/04/16/spacex-raptor-3-engines-and-the-next-step-in-starship-propulsion/).
The economics rest on three properties. Methane leaves less sooty residue than kerosene, aiding engine reuse; it stores at −162 to −183 °C, far warmer than liquid hydrogen's −253 °C, so cooling it is much less expensive than the roughly −240 °C or below needed for hydrogen[6](https://arstechnica.com/space/2026/03/nasa-is-blowing-stuff-up-to-study-the-explosive-potential-of-methalox-rockets/) • [20](https://theconversation.com/methane-rocket-fuel-is-easier-to-handle-and-convenient-but-as-blue-origin-saw-it-can-be-very-explosive-284528). China has joined the shift: the [Long March 10B](https://www.edgepedia.org/long-march-10b), its first commercial reusable LOX-methane rocket, flew with 98.7 percent purity LNG-derived methane produced domestically by Sinopec, which states the propellant reduces storage and transportation costs by 40 percent compared with liquid hydrogen[21](https://www.chinadaily.com.cn/a/202607/11/WS6a51e90da310986e2b464c37.html), and the 220-tonne-class "Lanyan" full-flow staged-combustion methalox engine completed a long-duration full-system test in early 2026[22](https://news.cgtn.com/news/2026-03-06/China-makes-breakthrough-in-methane-engine-for-heavy-lift-rockets-1LhSbLtck6I/p.html).
## Alternatives and performance ceilings
Chemical rockets span roughly 220 s for solids to 450 s for hydrogen-oxygen. Nuclear thermal rockets can exceed 900 s, and electric propulsion reaches 2,000 to more than 10,000 s[4](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html). The QinetiQ T6 ion thruster achieves 4,120 s at 143 mN of thrust but demands 4.5 kW of electrical power and long burns, making electric systems unsuitable for time-critical maneuvers[15](https://doi.org/10.18321/ectj1491). At the low end, cold and warm gas thrusters trade performance for simplicity and non-toxic propellants[12](https://unsworks.unsw.edu.au/server/api/core/bitstreams/4cbf96e6-f8e5-4271-bfdc-88e51b326907/content).
## History
Robert Goddard, the American physicist, built and flew the world's first liquid propellant rocket on March 16, 1926; it climbed only 12.5 meters[23](https://science.nasa.gov/wp-content/uploads/2026/02/153410main-rockets-history.pdf). The German V-2 (Vergeltungswaffe 2) stands as a later milestone in rocket history[23](https://science.nasa.gov/wp-content/uploads/2026/02/153410main-rockets-history.pdf). Wartime German practice also used hydrogen peroxide both as a monopropellant and as an oxidizer: in the Walter system the peroxide was designated T-Stoff and the hydrazine-hydrate-based fuel C-Stoff[24](https://archive.org/stream/ignition_201612/ignition_djvu.txt).
## References
1. [NASA Safety Standard for Explosives, Propellants, and Pyrotechnics (NASA-STD-8719.12A with Change 2)](https://standards.nasa.gov/sites/default/files/standards/NASA/A/2/nasa-std-871912a_with_change_2.pdf)
2. [Rocket propellants chapter, De Gruyter textbook](https://www.degruyterbrill.com/document/doi/10.1515/9783110751390-202/pdf)
3. [High Energy-Density Liquid Rocket Fuel Performance, NASA NTRS](https://ntrs.nasa.gov/api/citations/19900019426/downloads/19900019426.pdf)
4. [Rocket Propellant Chemistry, A Design-Tradeoff Space](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/01/rocket_propellant_chemistry_a_design_tradeoff_space.html)
5. [Green Propulsion: A NASA GSFC Assessment (2024), NASA NTRS](https://ntrs.nasa.gov/api/citations/20240004298/downloads/SP_2024_00239_20240410_Final.pdf)
6. [NASA wants to know how the launch industry's chic new rocket fuel explodes, Ars Technica](https://arstechnica.com/space/2026/03/nasa-is-blowing-stuff-up-to-study-the-explosive-potential-of-methalox-rockets/)
7. [Review of State-of-the-Art Green Monopropellants, Aerospace (2021)](https://www.mdpi.com/2226-4310/8/1/20)
8. [A Review of Recent Developments in Hybrid Rocket Propulsion and Its Applications, Aerospace (2024)](https://www.mdpi.com/2226-4310/11/9/739)
9. [Selection of propulsion characteristics for future European RLV options, DLR / CEAS Space Journal (2024)](https://elib.dlr.de/206596/1/s12567-024-00564-w.pdf)
10. [RP-1 thermophysical properties, NIST](https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=903675)
11. [Hybrid Rocket Technology: An Overview, S. Krishnan](https://propulsion-skrishnan.com/pdf/Review%20of%20Hybrid%20Rocket%20Technology.pdf)
12. [On the selection of propellants for cold/warm gas propulsion systems, UNSW](https://unsworks.unsw.edu.au/server/api/core/bitstreams/4cbf96e6-f8e5-4271-bfdc-88e51b326907/content)
13. [Rocket Propellant Chemistry, Solid Propellants](https://sgeos.github.io/aerospace/propulsion/chemistry/2026/02/02/rocket_propellant_chemistry_solid_propellants.pdf)
14. [Basics of Space Flight: Rocket Propellants](http://www.braeunig.us/space/propel.htm?xid=PS_smithsonian)
15. [Propulsion Systems, Propellants, Green Propulsion Subsystems and their Applications: A Review](https://doi.org/10.18321/ectj1491)
16. [NASA Kennedy Space Center Controlled Storage Function – Propellants](https://public.ksc.nasa.gov/propellants/controlledstoragefunction/)
17. [DOE Hydrogen Safety Handbook (based on NASA-STD-8719.16)](https://www.energy.gov/sites/prod/files/2014/03/f11/871916.pdf)
18. [UN Recommendations on the Transport of Dangerous Goods, Model Regulations (Rev.7, 2023), UNECE](https://unece.org/sites/default/files/2023-08/ST-SG-AC10-11-Rev7e_final_0.pdf)
19. [SpaceX Raptor 3 Engines and the Next Step in Starship Propulsion, New Space Economy](https://newspaceeconomy.ca/2026/04/16/spacex-raptor-3-engines-and-the-next-step-in-starship-propulsion/)
20. [Methane rocket fuel is easier to handle and convenient but can be very explosive, The Conversation](https://theconversation.com/methane-rocket-fuel-is-easier-to-handle-and-convenient-but-as-blue-origin-saw-it-can-be-very-explosive-284528)
21. [Chinese Long March rocket uses new LNG-derived fuel, China Daily](https://www.chinadaily.com.cn/a/202607/11/WS6a51e90da310986e2b464c37.html)
22. [China makes breakthrough in methane engine for heavy-lift rockets, CGTN](https://news.cgtn.com/news/2026-03-06/China-makes-breakthrough-in-methane-engine-for-heavy-lift-rockets-1LhSbLtck6I/p.html)
23. [Rockets Educator Guide, NASA](https://science.nasa.gov/wp-content/uploads/2026/02/153410main-rockets-history.pdf)
24. [Ignition! An Informal History of Liquid Rocket Propellants, John Drury Clark](https://archive.org/stream/ignition_201612/ignition_djvu.txt)
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*Topic: Encyclopedia › Technology and the built world › Transport and spaceflight › Spaceflight › Launch systems and rocketry › Rocket propulsion › Propellants, stages, and boosters*
*Initially written Sep 17, 2026 · Reviewed: Sep 30, 2026; Oct 15, 2026 · Edited: Oct 15, 2026; Oct 25, 2026 · Last review: Oct 15, 2026*
*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*
License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license