explosives and jet-rocket fuel, they both generate thrust. explosions: all at once, usually all directions, uncontrolled once it's started. fuels: gradually, usually in one direction, controlled by it's oxidizer. they both generate this thrust by igniting exothermic materials. exothermic materials being materials which release a huge amount of gas that expands with the great amount of released heat. my question is: why is gas release important? when the fuel burns and releases heat the air around it can absorb it and expand..why does the gases have to be released from the fuel along the explosion? why do we need exothermic fuels?
The energy does not need to come from oxidation of fuels. There are now ion-propulsion units in which the energy comes from nuclear sources. Nuclear jets were designed and built in the 1950s that used nuclear reactors to heat Hydrogen gas, which was then ejected as the propulsion medium. Even gas is not required as the propulsion medium, but is usually used due to efficiencies. The faster the propulsion medium moves away, the greater the force in the opposite direction. So, usually gases are heat to high temperature to get it to move fast in the opposite direction.
yes but why do gasses have to be a product of ignition? why can't it just use the surrounding air as a medium? why exothermic materials are what are used as fuel usually?
It could. Imagine a chemical reaction taking place inside a closed container. The chemical reaction makes the closed container hot. The closed container then heats up surrounding air, which is expelled out the back side. Presto, you now have a chemical reaction that uses the surrounding air. But it is very inefficient. So, since the chemical reaction usually produces combustion products that are gases, it is advantageous to use those gases as the propulsion medium, rather than trying to extract the heat and then heat up surrounding air. But essentially, that was what the nuclear rocket was doing. It had a closed fuel chamber, but instead of using a chemical reaction, it used a nuclear reactor to produce heat. The heat was then extracted, and used to heat gas that was from an external source and not part of the nuclear reaction. The heated gas then became the propulsion medium.
AHA, so that's the catch, but i'm having problems with why? exactly, why would this be less efficient? why does it have to be A+ignition=gasses and heat and not B+ignition=heat only, when gasses are all around the ignition? i mean, they're going to absorb the heat and expand, giving propulsion, no need to be a chemical product of the ignition. put simply, why aren't chemicals which give great amounts of heat considered efficient as fuel compared to those which give heat AND gasses? so why can't chemical rockets do the same?
Because it requires heat transfer through the container walls and then to the propulsive medium, both of which introduce losses and inefficiencies into the system. Also, using your proposed method, the temperature in the reaction chamber is ultimately limited by the melting point of the material the reaction vessel is made from, yes, you do have the option of cooling it, and using the 'waste heat' collected by the cooling system to heat the surrounding compression chamber (I'm picturing here a reaction vessel suspended inside a larger chamber with hole at either end to let the propulsive medium in and out) and therefore improve the thrust, but doing so also introduces further losses into the system by introducing a fourth medium that must be heated before useful work can be extracted (First you have to heat the reaction medium, then you have to heat the reaction vessel, then you have to heat the propulsive medium, and in the extended case, you aslo have to heat the cooling medium, and the walls of the compression chamber). This opposed to the current design which requires only the heating of reaction medium - the heating of the combustion chamber is incidental, and can be used for useful work - preheating the reaction medium. Also bear in mind that your proposed method requires the vessel to now carry a propulsive medium, as well as the reaction medium (requireing ultimately additional mass to be carried). This is where nuclear propulsion has an advantage, as the fuel load for the reactor is substantially less than the fuel load required for a chemical rocket, eeffectively you're dropping half your fuel load, however your proposed method offers no such advantages. I hope that helps.
Two things: 1) Exothermic means energy is released. This energy is what goes into the work to move the rocket, or heat the water, or keep your heart beating. To make 1 kg (2.2 lbs) go 100 m/s (224 mph) from a dead stop, you need (1/2)*(1 kg)*(100 m/s)^2 = 5000 Joules, which usually comes from something exothermic. 2) When considering energy available to do work moving your mass, you do not want to be restricted to heat. You want to skip the heat step and do direct volume work, and have the heat help. \(|W|=\int PdV\) Where \(dV\) is the change in volume at pressure \(P\). Explosives typically go straight to gas instantaneously. At 1 atmosphere the change in volume is usually on the order of 1000 times the initial volume, before even considering heat. That 5000 Joules above, for instance, is easily obtained by about half a liter of whatever going from liquid to gas... not including the exothermic heat of reaction.
