Nuclear Shaped Charges

From Aerospace Project Review vol 2, no.2 by Scott Lowther Conceptual Design by Scott Lowther Conceptual Design by Scott Lowther Back in the 1960's, rocket scientist came up with the infamous "Orion Drive." This was basically a firecracker under a tin can. Except the tin can is a spacecraft, and the firecracker is a nuclear warhead.

Anyway, they realized that about 99% of the nuclear energy of an unmodified nuclear device would be wasted. The blast is radiated isotropically, only a small amount actually hits the pusher-plate and does useful work. So they tried to figure out how to channel all the blast in the desired direction. A nuclear shaped charge.

Remember that in the vacuum of space, most of the energy of a nuclear warhead is in the form of x-rays. The nuclear device is encased in a radiation case of x-ray opaque material (uranium) with a hole in the top. This forces the x-rays to to exit only from the hole. Whereupon they run full tilt into a large mass of beryllium oxide (channel filler).

The beryllium transforms the nuclear fury of x-rays into a nuclear fury of heat. Perched on top of the beryllium is the propellant: a thick plate of tungsten. The nuclear fury of heat turns the tungsten plate into a star-core-hot spindle-shaped-plume of ionized tungsten plasma. The x-ray opaque material and the beryllium oxide also vaporize a few microseconds later, but that's OK, their job is done.

The tungsten plasma jet hits square on the Orion drive pusher plate, said plate is designed to be large enough to catch all of the plasma. With the reference design of nuclear pulse unit, the plume is confined to a cone of about 22.5 degrees. About 85% of the nuclear device's energy is directed into the desired direction, which I think you'd agree is a vast improvement over 1%.

About this time the representatives of the military (who were funding this project) noticed that if you could make the plume a little faster and with a narrower cone, it would no longer be a propulsion system component. It would be a nuclear directed energy weapon. Thus was born project Casaba-Howitzer.

Details are scarce since the project is still classified after all these years. Tungsten has an atomic number (Z) of 74. When the tungsten plate is vaporized, the resulting plasma jet has a relatively low velocity and diverges at a wide angle (22.5 degrees). Now, if you replace the tungsten with a material with a low Z, the plasma jet will instead have a high velocity at a narrow angle ("high velocity" meaning "a recognizable fraction of the speed of light"). The jet angle also grows narrower as the thickness of the plate is reduced. This is undesirable for a propulsion system component (because it will destroy the pusher plate), but just perfect for a weapon (because it will destroy the enemy ship).

The report below suggests that the practical minimum half angle the jet can be focused to is 5.7° (0.1 radians).

They would also be perfect as an anti-ballistic missile defence. One hit by a Casaba Howitzer and a Soviet ICBM would be instantly vaporized. Which is why project Casaba-Howitzer's name came up a few times in the 1983 Strategic Defense Initiative.

Casaba Howitzers fired from orbit at ground targets on Terra would be inefficient, which is not the same as "does no damage." A nuclear warhead fired at a ground target would do far more damage, but the Casaba Howitzer bolt is instantaneous, non-interceptable, and would still do massive damage to an aircraft carrier.

Scott Lowther has done some research into a 1960's design for an Orion-drive battleship. It was to be armed with naval gun turrets, minuteman missiles with city-killing 20 megatons warheads, and Casaba-Howitzer weapons. It appears that the Casaba-Howitzer charges would be from subkiloton to several kilotons in yield, be launched on pancake booster rockets until they were far enough from the battleship to prevent damage (several hundred yards), whereupon they would explode and skewer the hapless target with a spear of nuclear flame. The battleship would probably carry a stockpile of Casaba-Howitzer weapons in the low hundreds.

Mr. Lowther estimates that each Casaba-Howitzer round would have a yield "up to a few kilotons" and could deliver close to 50% of that energy in the spear of nuclear flame. Three kiltons is 1.256 × 1013 joules, 50% of that is 6.276 × 1012 joules per bolt.

This is thirty-five times as powerful as a GBU-43/B Massive Ordnance Air Blast bomb, the second most powerful non-nuclear weapon ever designed. Per bolt.

Get a copy of the report for more details, including a reconstruction of a Casaba-Howitzer charge.

What is the mass and volume of a Casaba-Howitzer charge? Apparently this also is still classified. An Orion Drive nuclear pulse unit would be about 1,150 kg, have a blast yield of about 29 kilotons, and be a cylinder with a radius of 0.4 meters and a height of 0.87 meters. The volume would therefore be about 0.4 cubic meters. As previously mentioned a Casaba-Howitzer charge would have a yield ranging from sub-kiloton to a few kilotons, so presumably it would be smaller and of lower mass than a pulse unit.