Timeline for Why do rocket engines have a throat?
Current License: CC BY-SA 3.0
11 events
| when toggle format | what | by | license | comment | |
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| May 25, 2018 at 11:06 | comment | added | marshal craft | Interesting answer. Note the f1 engine is nearly U shaped with expanding nozzle? | |
| Dec 25, 2016 at 7:13 | comment | added | Selene Routley | ... get optimized together (i.e. higher exhaust speed implies better collimation)? If this has a complicated answer, I'll ask as a separate question. | |
| Dec 25, 2016 at 7:12 | comment | added | Selene Routley | "no information can travel upstream from the throat into the chamber" - I don't think I've learnt so much from a few short paragraphs in quite a while. Fantastic answer. I have a question. As well as a achieving a high exhaust velocity, a rocket motor must collimate the flow, as any velocity components of gas sideways "wastes" potential momentum: you could imagine a detonation in a blind ended tube as imagined by the OP would lead to a conical flux of gas outwards, and thus inefficiencies. Does nozzle design also consider this collimation into account, or do exhaust speed and collimation ... | |
| Apr 8, 2014 at 14:47 | comment | added | tpg2114 | @aidan It all depends on the pressure ratio between the chamber and the ambient conditions. It could be over-expanded, under-expanded or just right depending on the outside pressure. This is what causes those shock diamonds in the outflow of afterburners and rockets. You can see examples here and some notes here | |
| Apr 8, 2014 at 5:42 | comment | added | aidan | "once the flow is sonic ... the flow then continues to accelerate ...". At what point does it stop accelerating after that? Where is the force coming from? I don't understand! | |
| Apr 7, 2014 at 17:29 | comment | added | supercat | Another way of looking at things, I would think, would be to say that if a rocket had a straight nozzle, all of the thrust would come from the pressure of the engine pushing against the (small) surface area of the throat; if one uses a cone-shaped throat, then even though the pressure of gas further from the throat would be lower than at the throat, it would still be significant and would apply pressure to the cone; the component of that pressure which was parallel to the thrust axis would constitute additional thrust. Would that seem accurate? | |
| Apr 7, 2014 at 17:19 | comment | added | Ghillie Dhu | Just to pick nits, even straight pipes can have throats due to friction (en.wikipedia.org/wiki/Fanno_flow) or heat transfer (en.wikipedia.org/wiki/Rayleigh_flow). | |
| Apr 7, 2014 at 17:05 | comment | added | Ben Voigt | @Ryan: No, the energy is in the exhaust gas already. You're seeing conversion of potential energy (in the form of pressurization and high temperature) to kinetic energy. Which is good because it increases specific impulse. | |
| Apr 7, 2014 at 15:10 | comment | added | Ryan Reich | So, since the increased energy of the exhaust comes from the resistance of the nozzle against the expansion of the gas, in a way, this technique extracts additional thrust from the binding energy in the material of the rocket itself? | |
| Apr 7, 2014 at 7:59 | vote | accept | Jens | ||
| Apr 6, 2014 at 22:34 | history | answered | tpg2114 | CC BY-SA 3.0 |