The rocket, at each time step, has a certain momentum already. Additional fuel burned in the next time step will increase its momentum and so on until the fuel is depleted. In the absense of any drag, the rocket will keep accelerating as long as thrust is being made. If drag is larger than thrust, the rocket will slow down, just more rapidly with no thrust. My wishalloy is platinum, when can I pick it up?
Dan In a message dated 6/12/04 12:17:12 PM, [EMAIL PROTECTED] writes: > Whilst mulling over the mechanics of rocket tipped rotors I came up with > the following paradox. > > Consider a low ISP rocket, say 20 seconds; mounted on an arm pivoting on > a central spindle. The propellent enters the rocket along a tube up the > central spindle, goes through a frictionless coupling and then follows > the tube along to the rocket tip. > > The rocket is oriented so that the exhaust points at 90 degrees to the > rotation axis. > > Now from momentum considerations you can show that the tip speed should > be equal to the exhaust velocity- in this case ~200 m/s (fuel has to be > accelerated up to the tip and then leaves it at ~200m/s. Clearly the > momentum balances when the tip goes at 200m/s.) > > However this implies that the exhaust leaves the nozzle and stops; and > hence has no energy, and hardly any heat, (rockets are typically 90% > efficient at turning hot gas into moving gas, so the exhaust gas is > relatively cool). > > And yet the rocket clearly isn't accelerating and we have burnt all this > fuel, which has liberated energy. Conservation of energy is the law! > > Assuming there is no air drag or other friction; where has the fuel > energy gone? > > Winner gets 1kg of wishalloy. > > "Lisa, in this house we *obey* the laws of thermodynamics" - H. Simpson > _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
