It’s certainly subject to the same laws, but because lasers have effectively zero etendue, it is possible to concentrate them to a very small point. This is in contrast to the sun’s rays which have a relatively high etendue (since it is a non-point source) and thus have a limit as to how much you can focus to one place.
There are other issue with the optics, but I don’t think that’s one of them.
You pretty much have it. The other thing to add is that not only do you use up fuel really quickly if you are throwing its mass out the back at a high rate, but it also means you needed to take a much higher mass of fuel with you, and that mass also needs to be lifted. So you are doubly in trouble. If you look at the equations for kinetic energy and for momentum, and the mass used, you get to the point where you discover that the critical measure for rockets that carry their own fuel as reaction mass is simply the velocity you can throw that mass out of the back. This measure is the same thing as the specific impulse (Isp) just expressed differently. This is why everyone quotes Isp for rocket motors.
I think what is confusing me is for a rocket to have high specific impulse you must have.
How much chemical energy is available per unit of mass.
related to how much energy is available per kilogram a fuel that releases 1000 joules per kilogram can give more specific impulse than a fuel that released only 1 joule be kilogram
What is the average mass of the molecules leaving the exhaust of the rocket.
Not sure why small mass have higher specific impulse.
I think what is confusing me is the density of fuel does not make it higher specific impulse it must have.
How much chemical energy is available per unit of mass.
What is the average mass of the molecules leaving the exhaust of the rocket.
For it to have higher specific impulse. This is what I’m not understanding.
Francis Vaughan it could be
So for a a rocket to go up, mass has to be toss out the back. The more mass being toss out and really fast the more thrust.
It could be this is where more chemical energy per unit of mass comes into play here. That the more energy per unit of mass more mass being toss out the back.
related to how much energy is available per kilogram a fuel that releases 1000 joules per kilogram can give more specific impulse than a fuel that released only 1 joule be kilogram
More energy is available per kilogram more mass being toss out the back or it being tossed out the back faster.
The reason higher mass molecules don’t have a higher specific impulse is because it takes more energy to accelerate them. It takes twice as much energy to accelerate a two kilogram block as it does to accelerate a one kilogram block. So a two kilogram block moving at 1 meter per second has more energy than a one kg block moving at 1 meter per second. But that doesn’t mean you should use 2 kg blocks as your propellant, because it takes more energy to get that 2 kg block moving at 1 meter per second. You don’t get a free lunch just because your propellant molecules have higher mass.
This isn’t correct. The actual reason is that at at particular temperature (and you can’t have too high of a temperature or your rocket motor will melt), heavier molecules have lower velocity. That’s all there is to it.
(the derivation of temperature is complicated but when 2 species at the same temperature interact, they trade energy back and forth equally. So a gas made of heavier molecules but at the same temperature of the lighter gas must have each molecule moving slower in order for the energy trade to be even whenever, on average, the heavy gas molecules hit the lighter gas)
With ablative laser propulsion, the rocket motor melting is the whole point - you actually want the bottom of the rocket motor to vaporize. So, you can have much higher temperatures.
A chemical reaction is by definition the exchange of electrons between molecules. The energy ultimately comes from the potential that these electrons are able to produce, and the limit is related to the electron’s mass. So hydrogen is a very favorable fuel choice because the mass ratio of electron to nucleon is the highest of all elements. So all your inquiries into “super fuels” can be answered by this fundamental property, which appears to be sort of a universal constant.
I think he is saying that heavier molecules the temperature will increase more than molecules that is lighter.
Also heavier molecules have lower velocity and need more energy to move them.
I’m sorry but I think I’m getting confused when people talk about cars or planes they talk in terms of fuel density, fuel volume, chemical bonds and lots bonds more fuel efficient. And fuel efficient some fuels more fuel efficient than other fuels.
But when people talk about rockets they talk in terms of specific impulse and thrust.
How much chemical energy is available per unit of mass.
What is the average mass of the molecules leaving the exhaust of the rocket.
I’m mixing the two up and getting confused. I need to do memory dumped on every thing I think I know on fuel density, fuel volume, chemical bonds that don’t apply to rockets.
And memorize and understand.
For a rocket to go up, mass has to be toss out the back. The more mass being toss out and really fast the more thrust. And the rocket goes up.
The more mass being toss out the back and really fast the more thrust more fuel needed.
And I think this is where chemical energy per unit of mass comes into play here. That the more energy per unit of mass the more mass being toss out the back.
How much chemical energy is available per unit of mass. Some fuels have more chemical energy per unit of mass thus higher specific impulse than other fuels that have lower chemical energy per unit of mass thus lower specific impulse .
So when talking about rocket performance three things come up.
How much chemical energy is available per unit of mass.
So we want a fuel with higher chemical energy available per unit of mass. The more chemical energy available per unit of mass the higher the specific impulse.Where fuels with lower chemical energy available per unit of mass have lower specific impulse.
What is the average mass of the molecules leaving the exhaust of the rocket.
Yea so we want low mass. A higher mass molecules don’t have a higher specific impulse it takes more energy to accelerate them.
I think I need to memorize and understand this. And never bring up fuel density, fuel volume, chemical bonds that don’t apply to rockets.
It is cars and planes they talk in terms of really dense fuel with lot of bonds.
I’m mixing up my older thread into this one and getting confused. When people talk about rockets they don’t talk in terms of fuel density, fuel volume, chemical bonds like they do when they talk about cars or planes.