What trajectory does an ancient Chinese rocket follow?

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Joined: Tue Jan 10, 2012 8:51 am

What trajectory does an ancient Chinese rocket follow?

Post by useasdirected » Tue Jan 10, 2012 9:07 am

Does a primitive rocket whose ignition barrel is mounted away from the center of mass cause the projectile's trajectory to differ from the standard textbook shape? Is it parabolic-concave-down (like a typical textbook) or a parabolic-counter-clockwise-loop? Can it possibly be the second? Any aerospace engineers here?

Context: I watched a student in the history and philosophy of science present about Ming Dynasty rockets. He discussed the chemistry of the ignition barrel, mixing ratio of the fuel, its design, and some other talking points. After the presentation some man approached him and claimed that because the Chinese mounted the ignition barrel at the front, away from the center of gravity, as the tank burned, losing mass, the trajectory the arrow would follow is not the typical parabola. In words -- it curved *up* and up and up and sometime back on itself during burn and in some cases whipped *back*, hitting the Chinese troops further up the ranks, due to torque induced by mass loss off gravity-center, and imperfect spin, which would have stabilized it by rotational inertia.

Can this possibly be right?

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Re: What trajectory does an ancient Chinese rocket follow?

Post by theObeast » Thu Feb 16, 2012 9:33 am

Well I don't really know anything about this, but it seems that ideally the net force vector would point through the center of mass, so as to avoid having any torque on the rocket. If the engine is in the back, this is easy. If the engine is in the front, this presents a problem, since the thrust can't be aimed at the center of mass, since the exhaust would hit the main body of the rocket (ignoring impractical geometries). If you want to have the thrust in the front for some reason, it would seem like your only hope of flying straight would be to balance the torque from the rocket with opposing torque from fins. But if you burn the fuel in the front, then the center of mass will move back, meaning a set of forces which create balanced torque about the old center of mass will tend to now curve the rocket upwards.

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