3 ms·
The propellant is important for its bulk (some mass to shoot out the back) and it's energy. In a chemical rocket, the propellant provides both the bulk and the
by rich_sasha 3y ago
The propellant is important for its bulk (some mass to shoot out the back) and it's energy.
In a chemical rocket, the propellant provides both the bulk and the energy. As it happens, there isn't that much energy in chemical propellants per kg, but the rest of the engine is light enough that these things can fly to orbit.
In a nuclear rocket, the energy comes from a nuclear reaction and the gaseous fuel only provides the bulk. This is way more efficient. But such rockets likely cannot achieve orbit on their own because the whole set up is too heavy.
- morkalork 3y agoWould it be possible to skim the upper atmosphere while in orbit around a planet and top-up the gas reserves?
- MPSimmons 3y agoAlmost definitely not. If there's enough material to collect in a realistically short timeframe, there's enough drag to bring the craft back into the atmosphere.
- dgoldstein0 3y agoAlso you'd be fighting conservation of momentum - if the material you gather isn't going your speed and direction, you lose some of your speed and direction to pick it up. Given how fast anything has to go to reach orbit (about 17000mph for low Earth orbit) and escape velocity is about 41% higher, that speed difference would be a major problem. For atmospheric flight the closest thing is a jet engine which is "air breathing", i.e. requires air to run and works in part by sucking air in, using some of the Oxygen in it for combustion, and shoving the extra air out the back. This gives them much higher efficiency than chemical rocket engines, as they don't have to carry their own oxidizer or the mass to eject for propulsion - but only works because the air is dense enough where they operate. Which is not true for space flight. Commercial jets typically fly a bit under the speed of sound in air which is 767mph. Of course supersonic air craft exist - I think mach 5 is achievable by some military jets - but that's still a fraction of orbital velocity. Anyhow running in the atmosphere means jets don't "scoop up air" in any sense but rather just use it immediately - so any lost momentum can be immediately countered by the engine. Another direction to think about it - a simple model for air resistance says that air resistance increases with the square of your velocity (and with the density of air, which exponentially decays with height though also depends on temperature. I think the height wins out though. Not sure if the equation works at extremely high altitude. But certainly this favors using air at lower altitudes and probably also lower speeds https://en.wikipedia.org/wiki/Drag_(physics) https://en.wikipedia.org/wiki/Drag_(physics) https://en.wikipedia.org/wiki/Density_of_air https://en.wikipedia.org/wiki/Density_of_air
- adastra22 3y agoYes, there are designs for doing exactly this with outer gas giant atmospheres.
- morcheeba 3y agoGood explanation! For completeness: In an Ion jet, the energy comes from (usually) solar power and the gaseous fuel provides the bulk. These are even weaker because of the limited instantaneous power that can be generated to feed it, but they are very efficient in how much thrust they can provide per propellent weight. Some versions (like arcjet) are a combination chemical rocket/Ion jet.
- rgmerk 3y agoAre you sure about that? Some of the nuclear thermal rockets tested way back in the 1960s were very, very powerful. That said, it’s hard to imagine that they’ll ever be used for launches from the Earth’s surface. Mars, maybe.
- rich_sasha 3y agoHmm, maybe. Actually not sure where I got this from, I could very well be wrong. Either way I'm no expert.