Aviation / Current / Warfare

Sub-Surface Stupidity

When I read that the Chinese had announced their intention to build a supersonic submarine, I tried not to burst out laughing:

To hear Chinese military sources tell it, the country is on its way to developing a submarine that can travel 6,100 mph—which is why you should never listen to Chinese military sources

Agreed.  But the PLA isn’t announcing their intent to field a supersonic submarine…they just announced a hypersonic submarine–as in travels faster than Mach 5 (Mach 8 at sea level, to be exact).  Jeffrey Kluger tries to explain how this might be possible in the distant future:

The bigger problem involves a couple of matters Friedman knows a thing or two about: physics and engineering. The technology that has caused all the buzz is something called supercavitation, and there’s nothing fanciful about it—it’s been around since the Cold War, though it’s been used only in torpedoes. Supercavitation involves agitating water in such a way that it forms a bubble of vapor completely surrounding the moving body, dramatically reducing friction, and dramatically increasing speed. Traditional propellors can’t be used to generate that speed, since they have to touch the water and all any part of the sub or torpedo touches is vapor. Instead, rocket engines provide the push, relying on the same action-reaction principle rockets use in space.

“It’s not a friction-free ride,” says Friedman, “but you do get some distance out of it and it can move at high speeds.”

But how much distance and how high a speed? There, it turns out, is the rub. The best-known supercavitating torpedo, the Russian Shkval—or squall—achieves a speed of around 200 knots (230 mph), according to Friedman, but it’s a short-range weapon, able to sprint only about 10,000 yards, since it must be stuffed with enough hardware both to churn water to vapor and run the rocket engines and still have enough room left over for an explosive charge. With all that, it can carry only a limited amount of fuel.

A submarine, Friedman estimates, could possibly stretch the range to 40 mi. (64 km). But as for somehow increasing the speed from 230 mph to 6,100 mph? Even the Chinese spokesfolks who are talking so freely don’t pretend to have an answer for that one.

Finally, there’s the problem of trying to point the sub where you want it to go. For both surface vessels and submersibles, that job is achieved by turning a rudder against the water, but poke a rudder into the water of a supercavitating vessel and you pop the bubble that surrounds the ship—not to mention snapping the rudder completely off when it suddenly encounters resistance. “Steering,” Friedman says, “wouldn’t be any fun.”

None of this is to suggest that these problems won’t be solved some day. But that’s true of almost any technical challenge you can name. Despite what China is saying, the submarine’s some day isn’t a soon day.

I personally see the Chinese pie-in-the-sky submarine development as a desperate attempt to gain some sort of way to try to close the immense gap between the incompetent PLAN and the USN Silent Service’s devastating superiority.  The Chinese believe they have neutered American air power and sea power with the vaunted DF-21D,  a missile with a velocity in the neighborhood of Mach 10.  A Mach 8 submarine would send American submarines scurrying for home, right?

Question: are there any aircraft in the PLAAF that fly at Mach 8?  In the USAF?  RAF?  Russian Air Force?  Answer: no.  Second question: why not?

Total Air Temperature

In aviation, designers have to contend with the intense heat as airspeed increases.  Air compresses before the onrushing airframe, and the mathematical formula looks like this:

The relationship between static and total air temperatures is given by:


\frac{T_\mathrm{total}}{T_{s}}={1+\frac{\gamma -1}{2}M_a^2}

where:

T_{s}= static air temperature, SAT (Kelvin or degree Rankine)
T_\mathrm{total}= total air temperature, TAT (Kelvin or degree Rankine)
M_{a}=Mach number
\gamma\ =\, ratio of specific heats, approx 1.400 for dry air

Above the tropopause (in the stratosphere), the SAT is typically -56.5 degrees Celsius, which leads to a TAT of over 2,700 degrees C at Mach 8.  At sea level, where an average temperature of 15 degrees C is standard, TAT is almost 1,000 degrees higher–i.e.; lots of friction.  The density of the air is nothing compared to seawater.

The Chinese should really be more worried about the escalating effectiveness of USN anti-ballistic missile weapons–perhaps before long the DF-21D will become completely useless.  As to pushing the speed envelope, before humanity can even contemplate hypersonic submarines, we need to perfect hypersonic surface ships…right after we perfect hypersonic aircraft.

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