Robert J. Chassell wrote: > >> I don't see - philosophically - how this can be an advantage. >> "Ramming" air is essentially a collision problem, that >> significantly reduces the speed of the rocket. If you carry the >> oxigen with yourself, it is moving with the speed of the rocket. > > Yes, there are problems with a ram jet. But when you carry the oxygen > with yourself, you have to accelerate it. That takes a great deal of > oxidizer and fuel. > But after the oxygen is accelerated, it stays in high speed :-)
> The best estimates I have seen are that a combined cycle rocket/ram > engine has the equivalent of a specific impulse in the 600s (i.e., the > equivalent of a pure rocket with an exhaust velocity of 6 km/sec, > although its actual exhaust velocity is lower), where a nuclear > thermal engine has a specific impulse of 800 - 900 (8 - 9 km/sec) and > a hydrogen-oxygen engine, like the Space Shuttle main engines, has a > specific impulse in the 400s, (4 km/sec) and its solid fuel rocket > engines -- which enable the shuttle to boost -- are have a lower > specific impulse. > Ok, let's do some simulation. Imagine that you want to cross about 400 km of air. First, let's use a classical O-H engine, and let's take your number of an specific impulse [Isp] of 400s. This is the classical problem of the rocket equation, and I won't get into the details, but we come to: V = gIsp ln(M / (M - Consumed_O_and_H)) With a constant exhaust of mass (delta m / dt = constant = w), the equations of movement are: v(t) = gIsp ln(M / (M - w t)) x(t) = integrate this. ln(M / (M - w t)) = -ln((M - w t) / M) u = (M - w t) / M; du = -w dt / M; so x(t) = gIsp (M/w) u (1 - ln(u)) [I hope this is right] The ramming problem can be written as a collision problem:at each instant, we burn dm = 2 units of fuel with 16 units of rammed oxidant, and we decelerate by colliding with 16 units of rammed oxidant. [there must be some way to ignore the useless nitrogen in the air, or the deceleration will be even worse] dv = gIsp (9 * dm) / m - 8 * dm * v / m The ramming effect will become critical when v becomes of the order of 8 * gIsp, and since 8 * gIsp is about 3x the escape speed, the ramjet will be efficient for a long time. Ok, now I believe :-) Alberto Monteiro _______________________________________________ http://www.mccmedia.com/mailman/listinfo/brin-l
