Would anything special happen as we approach the speed of sound? Would there be a sonic boom?

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    $\begingroup$ You get cavitation way before that. Breaking the speed of sound in water would require going at almost 1500m/s, which only a kinetic penetrator can do for a very short distance. The fastest supercavitating torpedoes are essentially rockets that move trough a gas bubble they create on their own: en.wikipedia.org/wiki/VA-111_Shkval, but even they are short by a factor of four. $\endgroup$ – CuriousOne Aug 3 '16 at 0:41
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    $\begingroup$ It's probably not possible to break the sound barrier under water. The drag forces involved increase with the square of the velocity. At 1500 m/s, the forces would no doubt be large enough to destroy the projectile. $\endgroup$ – David White Aug 3 '16 at 2:10
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    $\begingroup$ @CuriousOne , I took a quick look at the article. Those torpedoes are short by a factor of 15. $\endgroup$ – David White Aug 3 '16 at 3:47
  • $\begingroup$ It may be impossible to actually do it, but it would nevertheless be interesting to determine if it is theoretically possible and, if it is, what it would look like. $\endgroup$ – valerio Aug 3 '16 at 8:27