HVP Speed Lie

As you know, the Navy has, for the moment, shelved the electromagnetic rail gun in favor of firing hyper velocity projectiles (HVP) from 5” guns.  The HVP’s distinguishing characteristic is its speed which is reported to be Mach 3 (3344 ft/sec).  This leads the manufacturer, BAE Systems, to make all sorts of amazing claims which, if they are to be believed, would mean a single 5” gun is the only weapon any ship will ever need!  I’m being a bit facetious, here, but not much.  BAE and the Navy claim the HVP will handle land attack, anti-surface, AAW, and ballistic missile defense!

By the way, hyper velocity is defined as greater than Mach 5-8, depending on what definition you choose.  Mach 3 is not hyper velocity.  Moving on …

Let’s take that distinguishing characteristic, speed, and check it out.

The HVPs from a traditional deck gun will be slower than one launched from a railgun — a little over Mach 3 versus Mach 5 [ed: railgun projectile speed] — but still more than double the speed of an unguided regular shell from the service’s Mk 45 five-inch gun … (1)

Mach 3 = 3344 ft/sec

Wow that’s fast!  Double the speed of a standard round from the Mk45 5” gun!  Wow!  Just Wow!

Just out of curiosity, what is the speed of a round from a 5” gun?  Well, since the HVP is double the speed then the 5” round must be half the HVP, right?  That would put the 5” round at 1672 ft/sec. 

Just for giggles, let’s see what NavWeaps says the 5” round muzzle velocity is.  NavWeaps gives the following data. (2)

5”/62 round types:

Mark 80 HE-PD - 67.6 lbs. (30.7 kg)
Mark 91 Illum-MT - 63.9 lbs. (29.0 kg)
Mark 116 HE-VT - 69.7 lbs. (31.6 kg)
Mark 127 HE-CVT - 68.6 lbs. (31.1 kg)
Mark 156 HE-IR - 69.0 lbs. (31.3 kg)

5”/62 round muzzle velocities:

Mark 80 projectile with Mark 67 cartridge - 2,725 fps (831 mps)
Mark 80 projectile with EX-175 cartridge - 3,450 fps (1,052 mps)
Mark 91 projectile with Mark 67 - 2,750 fps (838 mps)

So, the 5” muzzle velocities range from a low of 2725 ft/sec (Mach 2.4) to 3450 ft/sec (Mach 3.1).  Compare that to the HVP’s speed of 3344 ft/sec (Mach 3).

We see, then, that the slowest 5” round is 81% of the HVP speed.  The HVP isn’t double the speed, it’s only 1.2 times faster which is only 23% faster!

The fastest 5” round is actually faster than the HVP!

What’s going on here?  The HVP is only marginally faster than the slowest 5” round.  Someone at BAE or the Navy is lyingconfused about their facts.  Does the Navy really not know basic arithmetic?  We’re being fed a lie.  As we noted, the hyper velocity projectile isn’t hyper velocity, at all.  Ignoring that definition inaccuracy, the HVP is barely faster – or actually slower – than a standard 5” round.  Again, we’re being fed a story that doesn’t match reality.

Setting aside the lying confusion, does it really seem plausible that a 23% increase in projectile speed (or slower than a 5” round, depending on the projectile!) will grant all the magical capabilities that BAE and the Navy are claiming? 

Projectile guidance is certainly a useful capability, one would think, although, depending on the guidance process, likely only useful in certain circumstances.  As I vaguely understand it – it hasn’t been publicly discussed in any detail, as far as I’m aware - , the guidance will be a very small millimeter wave (MMW) sensor/seeker.  The inherent drawback to a MMW sensor/seeker packaged in a small 5” projectile is that the power output of the sensor is limited and, thus, the detection range is limited.  This means that the projectile must get close to the target to be able to “see” it.

Presumably, the projectile is unguided until it reaches a calculated near-intercept position and then guides to the target in the terminal phase, assuming it sees a target.  Herein lies the problem. 

The short millimeter radar detection range is problematic for use in AAW when the convergence speed of the target and projectile may approach Mach 4-5. By the time the projectile is close enough to detect the target, the target is almost past!  The time available for guidance maneuvering is almost zero. Unless the seeker can detect the target from much farther away, a guided AAW-HVP will not be any more effective than a standard 5" round.  For example, at a combined closing speed of Mach 4, if detection occurs at 1 mile, the target will be past the HVP in 1.2 seconds. That's useless for guidance maneuvering. No significant guidance can occur in 1.2 seconds. 

If the HVP round is an air burst round (there is no indication that an air burst HVP is being actively developed but we’ll speculate for sake of discussion) the HVP fuze can electronically react in 1.2 sec but can the round react, detonate, and disperse an effective burst pattern in 1.2 seconds?  I don’t know. 

The critical question, of course, is what range can a millimeter radar actually detect an incoming missile? Given the extremely small size and low power output of a radar packaged into a sub-5" HVP, I'd guess that 1 mile detection is optimistic but I'm not a radar expert.

-Note that the 5” HVP is saboted and, considering the very narrow shape of the HVP projectile, the “average” projectile diameter is decidedly sub-caliber.  Thus, the room available for packaging an electronic power source, radar sensor/seeker, guidance electronics, guidance mechanics, and fins is very limited.  Any radar seeker will be very small and low powered which means very short detection range.  Trying to fit an explosive and fuzing mechanism into such a small package only compounds the problem.

I’m left wondering about the effectiveness of a HVP (that isn’t really hyper velocity and is only marginally faster than a standard 5” round!) in the AAW role.  The speed is actually a potential drawback for a guided and/or air burst HVP because greater speed minimizes reaction/guidance time.  Counterintuitive, I know, but seemingly true.

We see, then, that the entire HVP concept is factually incorrect – it’s not hyper velocity and is only marginally faster than standard 5” rounds – and conceptually questionable.  The HVP round sacrifices a great deal of size and weight to gain, at best, 23% more speed compared to the slowest 5” standard round and is slower than the fastest 5” standard round.  Do the benefits justify this reduction?  I don’t know but I’m dubious.

The one speed related benefit that I could see is that the extreme aerodynamic shape of the HVP might reduce its drag thereby allowing it to retain more of its speed for a longer period of time.  Again, whether that confers an actual performance benefit is questionable.

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Reference Data:  Speed of sound ~ 760 mph = 1115 ft/sec = Mach 1

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(1)USNI News website, “Updated: Navy Researching Firing Mach 3 Guided Round from Standard Deck Guns”, Sam LaGrone, 1-Jun-2015,

https://news.usni.org/2015/06/01/navy-researching-firing-mach-5-guided-round-from-standard-deck-guns

(2)NavWeaps website,

http://navweaps.com/Weapons/WNUS_5-62_mk45.php

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