3 ms·

I wonder how this impacts the reusability of the engine the alumina is fired through.

by CarVac 5y ago

burnte 5y ago
Little to none as it's injected into the nozzle. Engines are designed to extract maximum energy from the propellant, so the expansion curve of the nozzile is made to direct exhaust directly aft with no further expansion. When you see a rocket taking off wth a plume bigger than the nozzle, that's lost energy, but unavoidable due to the changing pressure of the atmosphere. Once you're in orbit, you can use vacuum-designed engines. Youd' inject these into the nozzle further enough down that they go directly down and out, not to the sides. You don't need to inject them with very much force at all, just enough to keep it away from the injector and near wall. You can design it so that virtually none of the material ever hits the nozzle walls.
- lmilcin 5y ago
  > so the expansion curve of the nozzile is made to direct exhaust directly aft with no further expansion. Exactly the opposite. The gasses will INFINITELY expand as the atmospheric pressure on the Moon is for all intents and purposes exactly zero. > Engines are designed to extract maximum energy from the propellant, so the expansion curve of the nozzile is made to direct exhaust directly aft with no further expansion. Engines are NOT designed to extract maximum energy. Engines are designed for COMPROMISE between extracting maximum energy and other parameters like mass of the rocket. For example, the engine could extract more energy from the propellant if the bell was larger. But that would make it also heavier and would reduce the performance of the rocket as a system. Another example, part of the fuel is wasted to create screen to protect the engine from overheating. It is injected on the sides of the engine and does not burn. This waste is a planned compromise because better cooling mechanism would be much heavier and so wasting a little bit of fuel is best way to improve total performance of the rocket. > Youd' inject these into the nozzle further enough down that they go directly down and out, not to the sides. (...) You can design it so that virtually none of the material ever hits the nozzle walls No, you can't. You mister, need to learn some physics. The way engine works is by creating pressure. That pressure acts on the bell (the sides) and transfers to the rocket. The "sides" aren't there to ornament the rocket, they serve a very important purpose as they are the surfaces which direct the exhaust back and on which the pressure of the gasses act to propel the rocket. None of these two important roles can be fulfilled without the gasses actually pressing on them. The vacuum regime isn't any different. There is positive pressure within the bell and this propels the rocket forward as the gasses expand backwards. If you injected (ejected) fuel directly down and out you would get pitiful thrust the likes of your bottle of coke gets when a mint is dropped in it. There are engines that do what you describe (ion engines) but they work in a very different way.
  - dang 5y ago
    > You mister, need to learn some physics. Please edit personal swipes out of your comments here. It's not what this site is for, and it destroys what it is for. If you wouldn't mind reviewing https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
    - lmilcin 5y ago
      This is as a response to absolute bonkers explanation to how a rocket engine works. I tried to keep good ratio of actual factual explanation to "swipes", which I think was fully deserved. Sometimes, somebody needs to at least hint you you are telling complete and utter rubbish (ideally backed by some explanation). Now, I don't want to say everybody has to know physics or how an engine works. But if you are starting to explain to people on HN, be ready to get fact-checked. You need to learn some.
      - dang 5y ago
        You can't take that kind of potshot at people regardless of how bonkers their explanation was or you feel it was. Aggression in your reply is much more significant than inaccuracy in the other comment. This is a big deal, because everyone underestimates the negativity of their own comments by (let's say) 10x and overestimates the negativity in other people's comments by another 10x. That makes for a 100x perception gap. This causes people who aren't being careful to blast bile into the the ecosystem here—something that, long term, leads to its destruction. Therefore we all need to be careful. Maybe you don't owe people who know less physics than you better, or don't feel you do, but you owe this community better if you're participating in it. Here's what to do instead: if someone else is wrong, and you know more than they do, then patiently provide correct information. Otherwise don't post; or, alternatively, chalk it up to someone being wrong on the internet and just don't post. Punishing the other person, or strutting your stuff superciliously, does not engage curiosity, and isn't conducive to learning nor to curious conversation. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
      - 5y ago
rtkwe 5y ago
Even if it's high it'd be good for probes. Larger multilanding vehicles might be too heavy and the thrust too high for this to do enough any ways.

I wonder how this impacts the reusability of the engine the alumina is fired through.

by CarVac 5y ago

I wonder how this impacts the reusability of the engine the alumina is fired through.

Little to none as it's injected into the nozzle. Engines are designed to extract maximum energy from the propellant, so the expansion curve of the nozzile is made to direct exhaust directly aft with no further expansion. When you see a rocket taking off wth a plume bigger than the nozzle, that's lost energy, but unavoidable due to the changing pressure of the atmosphere. Once you're in orbit, you can use vacuum-designed engines. Youd' inject these into the nozzle further enough down that they go directly down and out, not to the sides. You don't need to inject them with very much force at all, just enough to keep it away from the injector and near wall. You can design it so that virtually none of the material ever hits the nozzle walls.

lmilcin 5y ago
> so the expansion curve of the nozzile is made to direct exhaust directly aft with no further expansion. Exactly the opposite. The gasses will INFINITELY expand as the atmospheric pressure on the Moon is for all intents and purposes exactly zero. > Engines are designed to extract maximum energy from the propellant, so the expansion curve of the nozzile is made to direct exhaust directly aft with no further expansion. Engines are NOT designed to extract maximum energy. Engines are designed for COMPROMISE between extracting maximum energy and other parameters like mass of the rocket. For example, the engine could extract more energy from the propellant if the bell was larger. But that would make it also heavier and would reduce the performance of the rocket as a system. Another example, part of the fuel is wasted to create screen to protect the engine from overheating. It is injected on the sides of the engine and does not burn. This waste is a planned compromise because better cooling mechanism would be much heavier and so wasting a little bit of fuel is best way to improve total performance of the rocket. > Youd' inject these into the nozzle further enough down that they go directly down and out, not to the sides. (...) You can design it so that virtually none of the material ever hits the nozzle walls No, you can't. You mister, need to learn some physics. The way engine works is by creating pressure. That pressure acts on the bell (the sides) and transfers to the rocket. The "sides" aren't there to ornament the rocket, they serve a very important purpose as they are the surfaces which direct the exhaust back and on which the pressure of the gasses act to propel the rocket. None of these two important roles can be fulfilled without the gasses actually pressing on them. The vacuum regime isn't any different. There is positive pressure within the bell and this propels the rocket forward as the gasses expand backwards. If you injected (ejected) fuel directly down and out you would get pitiful thrust the likes of your bottle of coke gets when a mint is dropped in it. There are engines that do what you describe (ion engines) but they work in a very different way.
- dang 5y ago
  > You mister, need to learn some physics. Please edit personal swipes out of your comments here. It's not what this site is for, and it destroys what it is for. If you wouldn't mind reviewing https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
  - lmilcin 5y ago
    This is as a response to absolute bonkers explanation to how a rocket engine works. I tried to keep good ratio of actual factual explanation to "swipes", which I think was fully deserved. Sometimes, somebody needs to at least hint you you are telling complete and utter rubbish (ideally backed by some explanation). Now, I don't want to say everybody has to know physics or how an engine works. But if you are starting to explain to people on HN, be ready to get fact-checked. You need to learn some.
    - dang 5y ago
      You can't take that kind of potshot at people regardless of how bonkers their explanation was or you feel it was. Aggression in your reply is much more significant than inaccuracy in the other comment. This is a big deal, because everyone underestimates the negativity of their own comments by (let's say) 10x and overestimates the negativity in other people's comments by another 10x. That makes for a 100x perception gap. This causes people who aren't being careful to blast bile into the the ecosystem here—something that, long term, leads to its destruction. Therefore we all need to be careful. Maybe you don't owe people who know less physics than you better, or don't feel you do, but you owe this community better if you're participating in it. Here's what to do instead: if someone else is wrong, and you know more than they do, then patiently provide correct information. Otherwise don't post; or, alternatively, chalk it up to someone being wrong on the internet and just don't post. Punishing the other person, or strutting your stuff superciliously, does not engage curiosity, and isn't conducive to learning nor to curious conversation. https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html
    - 5y ago

rtkwe 5y ago

Even if it's high it'd be good for probes. Larger multilanding vehicles might be too heavy and the thrust too high for this to do enough any ways.