text: In deriving the equation named for these objects, the second order term consisting of `` dm times dv '' is usually neglected, since it is the product of two differentials.
full_question: In deriving the equation named for these objects, the second order term consisting of `` dm times dv '' is usually neglected, since it is the product of two differentials. Gustav de Laval designed an hourglass-shaped tube that is used to improve the efficiency of them. An equation which states the change in velocity of these objects is proportional to the logarithm of the initial mass over the final mass is ( * ) named for Konstantin Tsiolkovsky. Wernher von Braun studied these devices, whose efficiency depends on the number of stages and shape of the exhaust nozzle. For 10 points, name these engines that achieve thrust by expelling fuel, and which may consist of different ``stages. '
first_sentence: In deriving the equation named for these objects, the second order term consisting of `` dm times dv '' is usually neglected, since it is the product of two differentials.