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Solid fuel rockets have lower specific impulse, a measure of propellant efficiency, than liquid fuel rockets. As a result, the overall performance of solid upper stages is less than liquid stages even though the solid mass ratios are usually in the .91 to .93 range, as good as or better than most liquid propellant upper stages.
For rockets and space vehicles, propellants usually take up 2/3 or more of their total mass. Large upper-stage rocket engines generally use a cryogenic fuel like liquid hydrogen and liquid oxygen (LOX) as an oxidizer because of the large specific impulse possible, but must carefully consider a problem called "boil off," or the evaporation of the cryogenic propellant.
Hydrogen's buoyancy. Since hydrogen is a deep cryogen it boils quickly and rises, due to its very low density as a gas. Even when hydrogen burns, the gaseous H 2 O that is formed has a molecular weight of only 18 AMU compared to 29.9 AMU for air, so it also rises quickly. Spilled kerosene fuel, on the other hand, falls to the ground and if ...
The small station, now serving 30 hydrogen fuel-cell trucks, could mark the start of a nationwide network for fuel-cell truck refueling. It could also flop. The challenges are immense.
Helium also has a very low boiling point (-268.9°C or -452°F), allowing it to remain a gas even in super-cold environments, an important feature because many rocket fuels are stored in that ...
Bipropellant liquid rockets use a liquid fuel such as liquid hydrogen or RP-1, and a liquid oxidizer such as liquid oxygen. The engine may be a cryogenic rocket engine , where the fuel and oxidizer, such as hydrogen and oxygen, are gases which have been liquefied at very low temperatures.
Furthermore, these balances in efficiency-vs-power makes methane more suitable for a single-fuel rocket, which have proven more economical than dual-fuel rockets (due to less complexity). As such, methalox has made a resurgence in popularity in 21st century rockets, at the expense of kerolox (better efficiency) and hydrolox (better handling).
Hydrogen-powered electric fuel cells are a promising technology for commercial vehicles including large trucks, according to Georgia Department of Transportation.