| liquid hydrogen | 1.97 | The propellants are liquid oxygen and liquid hydrogen. | ||
| pure hydrogen | 1.18 | When pure hydrogen is used, the only byproduct is water vapor. | ||
| compressed hydrogen | 0.66 | The Ballard cell is simply refueled with compressed hydrogen when it runs low. | ||
| explosive hydrogen | 0.53 | Filled with explosive hydrogen, it was just as deadly. | ||
| metallic hydrogen | 0.53 | This incredibly dense, hot soup of protons and electrons is called fluid metallic hydrogen. | ||
| molecular hydrogen | 0.53 | The data is the first to detect molecular hydrogen in the atmosphere of Mars. | ||
| dangerous hydrogen | 0.39 | The new model is filled with nonflammable helium instead of the dangerous hydrogen that doomed the first generation of airships. | ||
| flammable hydrogen | 0.39 | It had been flying from Germany, kept aloft by highly flammable hydrogen gas. | ||
| ordinary hydrogen | 0.39 | However, ordinary hydrogen is not viewed as a fusion fuel that could be used in the foreseeable future. | ||
| supercold hydrogen | 0.33 | The devices combine supercold liquid hydrogen and liquid oxygen to generate electrical power. | ||
| stored hydrogen | 0.26 | Fuel cells use stored hydrogen and oxygen to create electricity, emitting only water vapor. |