4 ms·

Funnily it seems to be the exact opposite of the rocket equation, where you want to eject the limited mass quantity you have as fast as possible. The fuel being

Funnily it seems to be the exact opposite of the rocket equation, where you want to eject the limited mass quantity you have as fast as possible. The fuel being more like a mass reserve than a chemical energy reserve. The fuel in an airplane is also exhausted but I'm not sure how much of momentum is gained due to this rocket fuel effect, in the traditional working regime.

Gravityloss 4y ago
Mostly! Sorry, typical Hacker News "well, actually" comment coming up... For lower stages, you can inject water into the nozzle and ISP (exhaust velocity) goes down, but thrust goes up, more than compensating for the weight of the water tanks. But you wouldn't want to do that for an upper stage, as all the rocket below that would need to carry the water tank to a higher velocity. Saturn V second stage had variable mixture ratio, using a leaner mixture with lower ISP (but better density and thrust) at lower altitude and richer mixture with higher ISP at higher altitudes. https://yarchive.net/space/launchers/saturn_variable_mixture.html https://yarchive.net/space/launchers/saturn_variable_mixture... So, ISP is clearly not everything. At low altitudes, you're looking at basically maximum thrust and thrust to weight. At some level you're also looking at volume and thus density, to minimize cost.
- GistNoesis 4y ago
  Cool, glad to learn something :), I'm more of a AI neurosurgeon than a rocket scientist. My upper point was more about asking about the regime in which we can consider an airplane a constant mass system, and could totally neglect the mass of the consumed fuel compared to the momentum of the air moved. One of the main difference between a jet engine and a rocket engine is that the rocket engine carry its own oxygen, but I'm also not quite sure how much the momentum of the surrounding air is relevant, in helping push the plane forward. There is probably some kind of transition where either when air density get lower, or when speed get higher, than your jet engine behave more like a rocket motor than a blowing-fan (like the turbo-prop vs turbo-fan in the other comments).
  - Gravityloss 4y ago
    The Breguet range equation is quite similar to the rocket equation. The airplane has the dimensionless lift to drag ratio to help. And indeed mass ratios are not as drastic for airplanes as rockets, but still matter over long distances.
    - GistNoesis 4y ago
      I wonder how this equation would apply for electrical planes. And whether it could be useful to carry some water as fuel-mass (like in your saturn motor example) that would be ejected via the use the electrical motor to convert more efficiently the energy that is stored in the batteries into the momentum needed to make the plane go forward. The other day I saw the TechIngredients video about afterburner https://www.youtube.com/watch?v=EP4Hf63zR6w https://www.youtube.com/watch?v=EP4Hf63zR6w that I found interesting. Edit: The wikipedia page about specific impulse https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse and in particular its graph https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Specific-impulse-kk-20090105.png/1050px-Specific-impulse-kk-20090105.png https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Sp... helped me quantify the importance of air in the propulsion efficiency of a jet engine with respect to a rocket engine.
      - Gravityloss 4y ago
        The kerosene plane is lighter towards the end of the flight, while the electric one's mass doesn't change. For rockets, there's Rocket Lab's Electron that uses electric pumps to pump the regular propellants to the combustion chamber. This was much easier to develop than a turbine for the turbopump. It drops some spent batteries along its flight. Could an electric airplane drop batteries at certain points? Like after climbing to target altitude. A lot of operational headaches there, so not likely. There have been some military planes that have used takeoff trolleys or takeoff rockets so there is some precedence there. Rocket engines are extremely inefficient, they just have to be used because of the very wide speed range they support. So it wouldn't really make sense to eject water with an electric pump. Batteries have such low energy density and the pumps are heavy too. It's the worst combination to carry your own reaction mass and propel it with battery stored energy! A rocket engine has tremendous trust to weight and power density, it can be gigawatts in a car sized package. This is enabled by the very energy dense liquid fuel and liquid oxidizer. Even a jet engine pales in comparison, as it has to use gaseous air as oxidizer which is not dense, and which is 80% "fluff" nitrogen. Hence the jet engine compressors have to be large, and the compression ratios are meager 10 to 20 bars. A rocket engine can run at 200 bars main chamber pressure. The compressors (pumps) can operate at hundreds of megawatts (this is actually not far from big jet engines). The thrust to weight ratio can be 100 to 1, while for jets it's 10 to 1.
pfdietz 4y ago
If you want to maximize the energy efficiency of a rocket, you want variable exhaust velocity, and make the exhaust velocity = - the velocity of the rocket. That leaves the exhaust motionless in the initial reference frame, so it will have no kinetic energy (in that reference frame). Therefore, all the kinetic energy being generated by the rocket is being retained by the rocket. (This ignores potential energy from changes in altitude.) This is one reason why it's better to use a lower Isp propellant on the first stage of a multistage launcher (also, those propellants are much denser.)

Gravityloss 4y ago

Mostly! Sorry, typical Hacker News "well, actually" comment coming up... For lower stages, you can inject water into the nozzle and ISP (exhaust velocity) goes down, but thrust goes up, more than compensating for the weight of the water tanks. But you wouldn't want to do that for an upper stage, as all the rocket below that would need to carry the water tank to a higher velocity. Saturn V second stage had variable mixture ratio, using a leaner mixture with lower ISP (but better density and thrust) at lower altitude and richer mixture with higher ISP at higher altitudes. https://yarchive.net/space/launchers/saturn_variable_mixture.html https://yarchive.net/space/launchers/saturn_variable_mixture... So, ISP is clearly not everything. At low altitudes, you're looking at basically maximum thrust and thrust to weight. At some level you're also looking at volume and thus density, to minimize cost.

GistNoesis 4y ago
Cool, glad to learn something :), I'm more of a AI neurosurgeon than a rocket scientist. My upper point was more about asking about the regime in which we can consider an airplane a constant mass system, and could totally neglect the mass of the consumed fuel compared to the momentum of the air moved. One of the main difference between a jet engine and a rocket engine is that the rocket engine carry its own oxygen, but I'm also not quite sure how much the momentum of the surrounding air is relevant, in helping push the plane forward. There is probably some kind of transition where either when air density get lower, or when speed get higher, than your jet engine behave more like a rocket motor than a blowing-fan (like the turbo-prop vs turbo-fan in the other comments).
- Gravityloss 4y ago
  The Breguet range equation is quite similar to the rocket equation. The airplane has the dimensionless lift to drag ratio to help. And indeed mass ratios are not as drastic for airplanes as rockets, but still matter over long distances.
  - GistNoesis 4y ago
    I wonder how this equation would apply for electrical planes. And whether it could be useful to carry some water as fuel-mass (like in your saturn motor example) that would be ejected via the use the electrical motor to convert more efficiently the energy that is stored in the batteries into the momentum needed to make the plane go forward. The other day I saw the TechIngredients video about afterburner https://www.youtube.com/watch?v=EP4Hf63zR6w https://www.youtube.com/watch?v=EP4Hf63zR6w that I found interesting. Edit: The wikipedia page about specific impulse https://en.wikipedia.org/wiki/Specific_impulse https://en.wikipedia.org/wiki/Specific_impulse and in particular its graph https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Specific-impulse-kk-20090105.png/1050px-Specific-impulse-kk-20090105.png https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/Sp... helped me quantify the importance of air in the propulsion efficiency of a jet engine with respect to a rocket engine.
    - Gravityloss 4y ago
      The kerosene plane is lighter towards the end of the flight, while the electric one's mass doesn't change. For rockets, there's Rocket Lab's Electron that uses electric pumps to pump the regular propellants to the combustion chamber. This was much easier to develop than a turbine for the turbopump. It drops some spent batteries along its flight. Could an electric airplane drop batteries at certain points? Like after climbing to target altitude. A lot of operational headaches there, so not likely. There have been some military planes that have used takeoff trolleys or takeoff rockets so there is some precedence there. Rocket engines are extremely inefficient, they just have to be used because of the very wide speed range they support. So it wouldn't really make sense to eject water with an electric pump. Batteries have such low energy density and the pumps are heavy too. It's the worst combination to carry your own reaction mass and propel it with battery stored energy! A rocket engine has tremendous trust to weight and power density, it can be gigawatts in a car sized package. This is enabled by the very energy dense liquid fuel and liquid oxidizer. Even a jet engine pales in comparison, as it has to use gaseous air as oxidizer which is not dense, and which is 80% "fluff" nitrogen. Hence the jet engine compressors have to be large, and the compression ratios are meager 10 to 20 bars. A rocket engine can run at 200 bars main chamber pressure. The compressors (pumps) can operate at hundreds of megawatts (this is actually not far from big jet engines). The thrust to weight ratio can be 100 to 1, while for jets it's 10 to 1.

pfdietz 4y ago

If you want to maximize the energy efficiency of a rocket, you want variable exhaust velocity, and make the exhaust velocity = - the velocity of the rocket. That leaves the exhaust motionless in the initial reference frame, so it will have no kinetic energy (in that reference frame). Therefore, all the kinetic energy being generated by the rocket is being retained by the rocket. (This ignores potential energy from changes in altitude.) This is one reason why it's better to use a lower Isp propellant on the first stage of a multistage launcher (also, those propellants are much denser.)