David.Smith@CMU-CS-IUS.ARPA@sri-unix.UUCP (06/13/84)
Can someone answer the following question, which has bothered me for a long time: Why does a rocket engine produce more thrust in space than in the atmosphere? The stock answer is that thrust is proportional to exhaust velocity, and air drag reduces exhaust velocity. But it seems to me that this drag (back pressure) should cause a rise in chamber pressure, thus increasing thrust. Thrust is produced by chamber pressure, which works over a larger surface area at the front of the chamber than at the back (due to the exhaust hole). The rearwards pressure at the exhaust hole is occupied by accelerating the exhaust gas out through the hole, rather than by pushing on engine structure. If some of this rearward force is countered by backpressure from air not attached to the rocket, it shouldn't drag on the rocket. But in inhibiting escape of gases, the backpressure should increase chamber pressure, and hence, thrust. Suppose we replace the forward chamber wall by the base of a rifle bullet, and the air by a closed breech. This combination results in a higher bullet velocity than if the breech were open to allow maximum exhaust velocity of the burnt powder with respect to the bullet. Where is the flaw in this reasoning? David Smith @ cmu-cs-ius.arpa