27 February, 2015

Profile 97: FINISHED— the Pave Knife pod of the 433rd TFS

Finished—the Pave Knife pod!

In case you haven't been following this story, Pave Knife was the amazingly successful laser-guided weapons system used in the last year of the Vietnam War. Though it wasn't the first* (Pave Way was) Pave Knife was certainly the best.    It's worth its own post because of its game-changing effects on everything from tactics to weapons procurement to, some say, even shortening the duration of the Vietnam War itself.

If you ask me, its enough that Pave Knife was a brilliant case of teamwork between the military and private enterprise.  From concept to in-country, the development of the Pave Knife system set records in terms of production and economy.  In case you're curious, the contract for designing and building the thing was a "fixed price."  Meaning, no chance for budget over-runs or cost obfuscation.   And since the pressure was on to deliver accurate munitions with minimal collateral and "political" damage, time was of the essence; Pave Knife went from idea to working device in approximately nine months and in-country in less than twelve.

Of course, with such short notice, not many of the things were built.  Each one was "hand crafted" and the total number that the USAF used in Vietnam was small.   Although a couple sources say there were seven, the number was most likely only six. But even that paltry number quickly whittled down to three or four due to operational loses and the wear and tear of hard use.  The photo below shows #2, the model that I used to create the illustration.

No idea who took this picture.  It's been retouched but I wonder why?!
In case the mere-months time frame doesn't mean anything to you, it'd be like going from empty lot to unloading the moving truck in...like...a week.

From here on out, I'm going to be concentrating on drawing one of the F-4s that carried the Pave Knife into combat.  However, bear with me for just a few more minutes because the gizmo deserves a bit more explanation on how it works because the rest, as my kids say, gets complicated.

The Pave Knife is actually not just the device above.  "Pave Knife" is actually a weapons system, meaning it requires the co-functioning of separate units to perform its work.    The first part of the Pave Knife system is the bomb.

Have a look at the little animation below.
The main body of the bomb is a standard "Mark 84" 2,000lb General Purpose bomb.  However, the fins have been enlarged in the rear and a "seeker head" installed on the front.  In my animation, the seeker head looks like a small rocket stuck on the bomb nose.

Anyway, the tip of the head swivels and pivots. In the nose, there's an 'eye' that detects the reflection of laser light and sends signals to the set of fins just aft.  Those fins are moving fast because in real life, they acted just as you're seeing them - bang! bang!  The fins didn't steer by subtle degrees but by making full-deflection 'bangs' to keep the eye pointed toward the target at the rate of up to ten per second!

Of course, since the bomb relied on gravity to provide motion through space, (as opposed to a rocket that could be propelled) lateral deviation from the bomb's trajectory was dictated by altitude and airspeed.  A laser-guided bomb (LGB) couldn't "fly" much beyond the flight path.  But having a bomb that could be steered like a radio-control airplane wasn't the point of the Pave Knife system.  Dialing in absolute accuracy was.

Parts for a plastic Pave Knife model.  Over the years I've learned to trust the good folks at Hasegawa models to be accurate. And, I've also learned to trust the good folks who follow my work.  This little guy was given to me by a reader who heard my cry for reference material.
Thank YOU Missouri reader!
And boy was it accurate.  In combat tests, over 50% of LGB bombs were dropped on-target.  Not 'sorta' on-target.  No 'close enough' on-target.  We're talkin' down to the pin-point on target! The remaining 50% (unless something got fubar'd) were within a yardstick's distance.  So, consider that we're talking about a 2,000b bomb, "accuracy" is pretty much academic.

Putting this into perspective, consider a typical WW2 Navy dive bomber would deliver 50% of its bombs within a 100' diameter circle.   So, it's rather easy to see how Pave Knife absolutely changed the rules of bombing.

Ok, have a look at the animation below.  It's my attempt at putting it into practice.
For illustrative purposes only.  In real life, the bombing F-4 would be coming right down the bridge line; I offset it for illustrative purposes.  The F-4 at left has just released its bomb (seek the little black speck?) around 7k-10k feet.  The F-4 at right, at about 10k-12k feet, is projecting a laser beam from the wing-mounted Pod onto a specific point on thebridge.  You can see how the two parts, bomb and Pave Knife pod, worked together.
Well, I have to start drawing an F-4D so this little drawing can do some good.  It's going to take a while because I've got a ton more research to do.  Believe it or not, due to the kindness of a long-time reader, I've managed to acquire a book of BDAs (bomb-damage assessments) of Pave Knife missions from May, June and July of 1972. My idea is to pick a particular mission (I'm thinking against a bridge target)  and analyze it.  In the process, I will confer with Dean Failor (one of the trained Pave Knife operators) and a pilot who took the device "up north."

Dean's been introduced here before.  So, it's a good time to introduce you to "Rick" Hilton, Squadron Commander of the 433rd TFS.

It's time to go bridge hunting.  With 'the Knife.'

*Pave Way and Pave Knife were equally accurate systems but Pave Knife was far better for combat for a number of reasons that will be explained later.