Timeline for Why should nozzles be perfectly expanded?
Current License: CC BY-SA 4.0
11 events
| when toggle format | what | by | license | comment | |
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| May 21 at 19:57 | comment | added | John Doty | @CharlesStaats That is correct, but the force on the outside of the nozzle balances the force downstream even though they are not connected through the exhaust stream. | |
| May 21 at 19:28 | comment | added | Charles Staats | @JohnDoty While you are correct that overexpanded exhaust slows down after leaving the nozzle, IIUC the effects of that slowdown cannot propagate back up the exhaust stream to affect the thrust because the exhaust is supersonic. | |
| May 21 at 19:24 | comment | added | Charles Staats | @OrganicMarble You'd think not, but the positive thrust that is captured by vacuum nozzle extensions is strictly less than the negative thrust contribution from sea-level ambient air. If the negative thrust contribution were actually insignificant, no one would bother with vacuum nozzle extensions. (Or overexpanded nozzles, by similar reasoning.) That having been said, I don't deny that in many actual use cases, the flow separation is much more significant than the negative thrust. | |
| May 21 at 18:20 | comment | added | Organic Marble | Agree with all! I just don't think the negative thrust caused by the pressure force on the nozzle walls near the exit is very significant. | |
| May 21 at 17:59 | comment | added | Charles Staats | @OrganicMarble As I understand it, the negative thrust of the converging part of the nozzle is more than balanced by the positive thrust on the "ceiling" (injection plate?) of the combustion chamber. My understanding is that the axial component of pressure is quite small toward the nozzle exit. In both cases, the apparent inefficiency is balanced by other considerations. (E.g., without the converging part, the exhaust velocity would be subsonic and the expanding part of the nozzle wouldn't work at all.) | |
| May 21 at 17:43 | comment | added | Charles Staats | @SaturnV An extension of the nozzle does increase the velocity of the exhaust gases. But it also increases the amount of ambient air that the nozzle has to push out of the way. And for an overexpanded nozzle, the latter effect dominates. | |
| May 21 at 17:38 | comment | added | Charles Staats | @SaturnV The acceleration of exhaust gas is not separate from the exertion of pressure. Just as the rocket is accelerated by the pressure of the gas on the nozzle, so the gas is accelerated by the equal and opposite pressure of the nozzle on the gas. Expressing everything in terms of pressure just makes some calculations easier (and others harder). | |
| May 21 at 16:32 | comment | added | John Doty | @SaturnV Overexpanded exhaust compresses when it leaves the engine, slowing it down. | |
| May 21 at 11:04 | comment | added | Organic Marble | Note that the converging part of the nozzle also produces negative thrust. Also only the axial component of the pressure force matters for thrust, and this is quite small towards the nozzle exit. | |
| May 21 at 6:07 | comment | added | Saturn V | I understand that any increase in nozzle expansion causes the resulting thrust to decrease due to pressure, but can't that still increase thrust as a result because the exhaust gases are accelerated more? | |
| May 21 at 5:59 | history | answered | Charles Staats | CC BY-SA 4.0 |