Material Insights · Understanding the Material World

Explosives and propellants: Science and technology

Why explosives hold little energy but release it instantly

What this tab covers The physics and chemistry: how the energy is stored, which materials it needs (scarcity, toxicity, recyclability) and what makes it age.

Explosives and rocket propellants are chemical energy stores designed for very fast, controlled release. Per kilogram they hold less energy than ordinary fuels; their value lies in power, not energy.

Mechanism

Mixed

Explosives and solid rocket propellants carry their own oxidiser, so they can release energy without air, either by detonation (a reaction front faster than sound, used to break rock in mines and quarries) or by rapid burning (used to push rockets). Liquid rocket propellants are held in separate tanks and only react inside the engine. By convention, one tonne of TNT is defined as 4.184 GJ, so a kilogram is 4.184 MJ or about 1.16 kWh, roughly one twenty-ninth of the 33.3 kWh in a kilogram of hydrogen (calculated). Their value is in how fast the energy comes out, not how much there is.12

TNT energy (conventional definition) 4.184 GJ per tonne, i.e. 4.184 MJ/kg or 1.162 kWh/kg (calculated)1 Hydrogen energy for comparison 33.3 kWh/kg (lower heating value)2 Ratio hydrogen to TNT per kg about 29 times (calculated)12

Material scarcity and toxicity

Unfavourable

Commercial explosives rely on bulk industrial chemicals rather than scarce minerals, so mineral scarcity is not the issue. Toxicity is: hydrazine, the long-standing satellite propellant, is described by ESA as extremely toxic, was put on the EU REACH candidate list of substances of very high concern in 2011, and appears among the named carcinogens in the Seveso III directive. Fuelling teams wear full protective suits. Less toxic alternatives have flown in space, but European industry still relies on hydrazine and has asked for an exemption from any future REACH authorisation requirement. Liquid hydrogen and oxygen are non-toxic but cryogenic.356

Hydrazine on REACH SVHC candidate list since June 201156 Seveso III threshold, named carcinogens incl. hydrazine 0.5 t lower tier; 2 t upper tier3

Degradation and ageing (storage stability)

Mixed

Commercial explosives and propellants are made to stay stable in long storage, but that depends on conditions and housekeeping. In Beirut, 2,750 t of ammonium nitrate was kept for nearly six years in a poorly secured, poorly ventilated port warehouse alongside fuel and fireworks, against international guidance, before it exploded in 2020. Cryogenic propellants lose mass to boil-off: NASA's 1960s liquid hydrogen tanks at Kennedy Space Center lost about 0.0625% a day, and the new 4,700 cubic metre sphere is specified for at most 0.048% a day.47

Apollo-era LH2 tank boil-off 0.0625% per day7 New LC-39B LH2 sphere boil-off (max) 0.048% per day7 Beirut ammonium nitrate 2,750 t, stored from 2014 to 20204

Sources

  1. NIST Guide to the SI (Special Publication 811), Appendix B.9: factors for units listed by kind of quantity US National Institute of Standards and Technology, 2008
  2. DOE Hydrogen Program Record 9013: Energy requirements for hydrogen gas compression and liquefaction US Department of Energy, 2009
  3. Directive 2012/18/EU (Seveso III), Annex I legislation.gov.uk (EU directive text as adopted), 2012
  4. "They Killed Us from the Inside": an investigation into the August 4 Beirut blast Human Rights Watch, 2021
  5. Considering hydrazine-free satellite propulsion European Space Agency, 2013
  6. Revised common position of the European space industry on the exemption of propellant-related use of hydrazine Eurospace, 2020
  7. New LH2 sphere at LC-39B (Fesmire and Swanger, NASA Kennedy Space Center) US DOE / NASA, 2021