Mark 53 VT Fuze

Ever since I first came across the remarkable tubes inside the Mark 53 VT fuze I've wondered how these devices survive that huge G force on launch. The reason for my interest is that I've had a little practice with more conventional tubes, specifically 7 and 9-pin miniature tubes.

Years ago, I worked at an establishment that still used many vacuum tubes in its electronic equipment—many thousands of them in fact. Well, anyway, there was constant war between the electrical and electronics staff (as there always is) and one way we could get back at the electrical personnel was to use a tube gun (for us, a valve gun) which we used to fire defective tubes pin-first into the main wooden door of the electricians' building from one of the entrances of our building some 20 meters away. As the electricians always worked office hours and we worked 24/7, this nefarious activity was done at odd hours when they weren't present.

These tubes would embed themselves pins-first in the wooden door up to the full length of the pins. That's to say the only thing stopping the pins embedding deeper into the door was when the glass base hit the door—obviously the pins couldn't embed any deeper. That said, rarely did a tube's glass envelope break.

Much to their chagrin and annoyance, the electrician would arrive in the AM to find their door covered with embedded 7 and 9-pin tubes which they'd try to prize out—and that's when the tubes were broken (as it was almost impossible to remove them without them breaking).

The tube gun was the key to our success and it was remarabky simple. It consisted of two lengths of plastic electrical conduit of about 2.5 meters in length, the diameter of one would just clear 7-pin tubes and the other 9-pin (the tubes could slide through the conduit easily but not loosely). The tube was loaded pins-first at the breech end (as one loads bullets in a gun) and it was then coupled to/mated with the conical funnel of a fire extinguisher†, then all one had to do was to aim and squeeze the trigger—and whammo one launched tube—and they launched in one hell of a hurry, you couldn't see them leave the muzzle except perhaps for a slight blur.

Now the interesting bit, whilst most tubes would embed themselves in the door up to the depth of the pins without the glass breaking the interior electrodes would usually collapse in on themselves (for example, the plate and grid electrode wire supports would collapse down so the electrodes were hard against the glass base).

I've often wondered what the instantaneous G forces were when these tubes hit the door but it would have been pretty high. Moreover Mark 53 VT Fuze was never far from our minds and there was always speculation what our G forces were in comparison to the Mark 53 VT.

It had always been clear to me that it would not have been that difficult to reinforce the tube to survive our torture test. I reckon increasing the diameter of the supporting wires by about 50% and tripling up on the thickness of the mica spacers would likely have done the trick (incidentally, rarely did the filaments go open-circuit and those that did was principally because the welds between the filament and the lead-in wires broke).

So I sort of understand how the Mk 53 VT must have worked without destroying itself. After all, depending on the type of 155mm antiaircraft howitzer, the Mk 53 VT is accelerated to full muzzle velocity over a reasonably large distance of between about 3.5 and 6.5 meters or so (and sometimes more). So clearly this is not a heavy de-acceleration from full velocity (whatever that was for our tube gun) to a standing stop in about 1cm.

If anyone has some clues—or better still—some actual figures for these types of experiments then I'd love to hear from you.

† No, they were never ones in active service (we were stupid but not that much), rather they were either old units or those awaiting service.