And some missiles, too. They’re more rounded than pointy, but I would have thought that would mean more air/water resistance. Does it just not matter that much when they’re already traveling at such high speeds (and, for torpedoes, when what you’re aiming for is likely to be slower, too)? Is there something else?
I would imagine points get damaged more easily than the rounded ends, and that a damaged point would be extremely bad for the streamlining and control of the torpedo or missile. That’s my theory anyway.
I believe that the most efficient shape for motion through a fluid is actually one with a rounded front end and a point back end.
Current U.S. Navy torpedoes actually have a somewhat flat front. This link shows the profiles of several torpedoes, including the MK-48 and the air-dropped MK-46 and MK-50 torps.
The reason has to do with Mangetout’s point, as well as helping with the active sonar guidance in the nose section.
A MK-48 ADCAP torpedo goes for about $3.5 million per unit. There is nothing that’s left to chance in the design, and I’m obviously glossing over the details.
[Top Gun] I could tell you, but I’d have to kill you. [/Top Gun] 
–robby (ex-submariner)
I was hoping to find more information on drag and shapes, but I couldn’t find anything useful. As far as I can remember, a rounded front and a pointy tail is good because it minimises turbulence. Sorry, that’s not much more than I said above
I’m not an expert in fluid mechanics, however … The drag of an immersed object comes in two varieties. The form drag and the surface drag. The form drag is what is at issue here and it is more a function of the cross sectional area than it is upon the exact shape. A round nose doesn’t have all that much more form drag than a pointed one for the same cross section and is desirable in many cases for other design considerations.
The most important contrubuter to form drag is the reduction in pressure behind the object. This reduction is the result of flow separation from the shape as the fluid stream leaves. In order to reduce the separation, the back end of the shape should taper gradually to a point so that the fluid will follow the surface rather than separate from it. However, an extreme gradual taper results in a longer object with more surface area. It also begins to cause packaging problems for the internal components and wasted space as the diameter gets small. Likewise a pointed nose, which tends to reduce the area of stagnation pressure at the front has more surface area than a rounded one. And again, internal space is wasted as the diameter gets small and packaging space in missiles is scarce in most missiles.
It appears to be for cavitation purposes. Torpedoes aren’t designed to be silent, and as such they rely almost entirely on speed. The shape of the nose would cause cavitation, which through various means (I’m not a physicist) makes the torpedo go faster than it would otherwise.
I understand how it works, and I somewhat understand why, but I can’t explain it to you.
I don’t think so. Cavitation results when the fluid pressure is reduced below the vapor pressure of the fluid, in this case water. Cavitation is, in effect, a boiling of the water resulting from extreme low pressure. The region around the front end of an object traveling through a fluid is one of high pressure, not low.
Turbulence isn’t all bad. For example the dimples on a golf ball creater turbulence which breaks up the boundry layer as the ball spins. It turns out that as the ball goes faster and faster there comes a critical velocity so that, with the turbulence from the dimples, the form drag suddenly drops to 1/3 to 1/4 of its previous value. Expert golfers hit the ball hard enough to get this speed thus not only getting more distance from the harder hit but also from greatly reduced drag. The bastards.
This was briefly explained in a copy of New Scientist and by a Fluid Mechanics lecturer explaining why Chinese space craft could re-enter the Earth’s atmosphere without the expensive heat shielding the US used.
IIRC, the flat front creates some sort of pocket of recirculating water/air that pushes the water/air aside as it moves through it. Its more effective than trying to design a pointy shape.
This could be wrong, but its what I remember.
Also, ‘stability’ must be considered. I believe the flat front creates a more stable path for guided bits. A razor sharp edge would be more responsive to input/adjustments, but it would be more vulnerable to being upset from a stable path/course.
Can’t give a cite for this, but I recall seeing in a show on the Military Channel (or maybe it was Mail Call?) that modern torpedoes have sonar and whatnot packed into the front and that required the more rounded head. But I also seem to remember that WW II torps had a very similar shape, and they were a ‘point and shoot’ weapon sans sonar.
The Russian VA-111 Shkval (Squall) supercavitating torpedo
CMC fnord!
I’m not sure what is meant here. The Shenzhou capsule, like the Soyuz design on which it is based, has a re-entry module which has a heat shield, and performs reentry in a similar fashion to the Apollo/Gemini/Mercury capsule, i.e. arse-end first, although the Soyuz (and presumably the Shenzhou) eject the heat shield prior to ground landing, unlike the American designs which are intended for a water landing. By virtue of not having the narrow leading edge surfaces such as the STS and the Russian Buran shuttles the stagnation pressure and temperature doesn’t achieve such of an extreme heating, but they definitely see very high temperatures. There are conceptual designs for active skin cooling which involve a metal skin spaced off from the main structure by a thermally nonconductive structure and cooled by an internal loop (often serving double duty as a preheater for hydrogen propellent or somesuch) but these are just concepts.
Regarding torpedos: the aerodynamic models and everyday intuition that work in an aeroelastic medium like air don’t hold very well in an effectively inelastic medium like water. While making a torpedo “pointy” might seem like a logical thing to do–and in fact early topedoes were dual-pointy shapes–going with a blunt or gently rounded nose actually reduces drag underwater. The reason for this counterintuitive fact is that drag underwater is roughly proportional to the wetted surface area. Whereas with tube-fusalge aircraft and rocket boosters longer and thinner (all other things being equal) improve aerodynamic performance, with marine objects they become worse with additional length. (The shape of boat hulls complicates the discussion, as longer length can sometimes reduce effective wetted surface and provide greater stability, et cetera, so we’ll ignore that for the purposes of this discussion.)
As David Simmons noted, the ideal shape for a fully submerged moving object is a fairly blunt fore-end which tapers to the rear, which reduced effective wetted area, as the surface aft of and of equal or lesser diameter than the blunt nose sees reduced drag, while a pointed, conical-like fore-end would see increasing drag along the entire surface. See this page for a brief discussion on the topic. In marine animals that swim fast, you’ll find that this is typically the shape adopted. Look at the whales, dolphins, sea lions, or many fish. (Dolphins and whales have some additional adaptations that allow them to swim very fast without generating parasitic turbulence–essentially, their skin has an adaptive reflect that damps out turbulent nuclei. How cool is that?)
An additional benefit, of course, is that a blunt shape allows better utilization of volume in a shorter overall package, and better control over mass properties. In air, a pointy shape is somewhat more advantageous, but not overwhelmingly so. I believe the reason modern guided missiles are round-nose is to package their sensor package and to better use the limited envelope available. What is lost from additional forward drag isn’t critical. With something like an ICBM, however, the additional drag between a rounded fairing and a pointed one can mean a few tens of seconds of I[sub]sp[/sub] (specific impulse), which can be significant in terms of range and altitude. On SLBMs (which suffer from very restrictive length packaging requirements) an aerospike is deployed immediately after launch to give the booster a pseudo-conicial aeroelastic profile instead of the stock blunt nose.
Back to torpedos, the Russians (and perhaps some other nations) now have torpedoes that can reach speeds estimated to be >200 knots by deliberately inducing cavitation; basically, they use a vibrating point on the nose to generate a boundary cavity of low pressure that sheaths the torpedo, letting it move through the water faster than any natural object could possibly go. There’s some question of how controllable these weapons are–they can’t be self-guiding as the boundary layer would isolate their sonar systems from the rest of the ocean–but they’re definitely a paradigm shift in the type of threat posed to submarines and surface ships.
Stranger
Hmm… a nuclear tipped torpedo with a range of only 7500 yards? That’s a bit close for my liking. It’s not like you can duck down behind a mountain as in By Dawn’s Early Light.
The article did state that launching one with a nuclear warhead was a suicide mission.
Yes, as well as the guidance package, a pointy tip would create more surface area and therefore more friction.
Also, supercavitating projectiles usually have a flat tip in order to create a bubble around the projectile.
Aha, that’s what I remembered from the New Scientist article. It talked about concrete Soviet subs that were intended to sit on the sea bed and fire rocket propelled torpedoes at passing ships and subs.
Stranger On A Train, our lecturer had actually mentioned some sort of wooden heat shielding on Chinese satellites that were intended to re-entry and be collected 
The re-circulating air was his explanation.
This supercavitation is interesting and also puzzling. Cavitation bubbles occur because the pressure in the fluid is less than the vapor pressure. When the fluid pressure again exceeds the vapor pressure the bubbles collapse, often with such violence that pieces of metal are knocked out of the structure. The vapor pressure of water at 20 C is only 2.3% of 1 atmosphere. I wonder how low pressure is maintained so as to sustain a bubble all around the torpedo.
I believe in most supercavitating torpedoes there is a flattened tip which creates the bubble around the body. This creates the bubble and allows vastly faster speeds to be achieved.