When a nuclear technology has been kept classified since the 60s, you know that it is worth looking into. The Casaba Howitzer is one configuration for a nuclear shaped charge, that can concentrate the power of an atom bomb into a narrow cone.
In this post, we’ll look at its potential configurations, its advantages and limits, and how it can be applied to both propulsion and warfare.
Origins
Before the Casaba Howitzer, there was Project Orion.
Project Orion was the nuclear upgrade to the idea of explosive pulse propulsion, first proposed in the late 19th century by the Russian Nikolai Kibalchich (1881), by German Hermann Ganswindt (1891). Instead of chemical explosives, an Orion spaceship would use the awesome power of nuclear bombs.
As you may already know, the original Orion involved a thick plate of steel, called a ‘pusher plate’, mounted on suspension arms, that would be struck by a nuclear explosion. The plate would endure the heat and radiation, while the suspension transferred the plate’s momentum to the spaceship.
Even if the pusher plate only captured a fraction of the energy of the nuclear bombs being used, it would still be a more effective than any chemical rocket. Theoretical calculations and empirical testing gave engineers an idea of the largest explosion the pusher plate could handle (67000°C and 340MPa), and suspension cycling gave them the maximum frequency at which the individual bombs could be detonated (0.86 seconds in the initial design). Used together, we could design an Orion spaceship…
But it would end up being wasteful and nearly impossible to operate in an atmosphere without generating devastating EMPs.
Like all explosions, an atomic detonation is spherical.
The original proposal (USAF 10m Orion) detonated bombs 25m away from the pusher plate. The pusher plate has a radius of 5m, so with tan, we can calculate a half-angle of 11.25 degrees.
In other words, the pusher plate captures at most 10% of the energy delivered by the nuclear pulse unit.
This is wasteful, as most of the expensive fissile material does not get used properly. It would also require very large individual pulse units to produce sufficient acceleration, likely in the megaton range. Megaton-level nuclear detonations produce a lot of fallout, produce enough thermal radiation to damage the spaceship, and most importantly, generate electromagnetic pulses in the upper atmosphere that would disrupt electronic devices.
The Nuclear Shaped Charge
The solution was to create a device that directed the energy of a nuclear explosion into a one narrow enough for the pusher plate to capture almost all of it.
With a higher percentage of the energy captured, the nuclear pulse units could be made much smaller, cheaper and unlikely to produce harmful EMP.
A nuclear shaped charge, which became the Orion pulse units, worked in three steps.
-The nuclear device detonated, producing 80% of its energy as X-rays, released in all directions. They are blocked by the non-fissionable uranium, except for a hole on top.
-The channel filler (Beryllium Oxide) absorbs the x-rays coming through the hole, and re-radiates them as heat (infrared). It is the most important part of the design.
-The tungsten propellant absorbs the infrared emissions and vaporizes, becoming a fast travelling stream of plasma headed towards the Orion’s pusher plate. The tungsten is plate-shaped so that the plasma produced expands into a thin column.
In effect, the tungsten plasma becomes the Orion’s propellant. Exhaust velocities of up to 120km/s have been proposed for the original Orion designs.
The full deltaV equation for an Orion spaceship is given by:
- DeltaV: Collimation factor * Plasma Velocity * ln(Mass Full/Mass Empty)
The collimation factor (between 0 and 1) is how much of the tungsten plasma reaches the pusher plate. In the original proposal, collimation was 0.85. It can be improved by using a wider pusher plate, detonating the pulse units closer or using a thinner and wider tungsten plate. Plasma velocity is the velocity the tungsten travels at.
Other Propulsion
Through the use of nuclear shaped charges, other nuclear pulse propulsion types have been designed.
The simplest of these are larger Orion spaceships. One space battleship proposal gave a mass of 4000 tons, another interstellar concept masses a thousand times that figure.
There are designs that reverse the pusher plate configuration, detonating instead the pulse units between the spaceship and a retractable sail.
More advanced designs include the Mag-Orion, that replace the Orion’s physical pusher-plate with a magnetic field. A Nuclear Shaped charge allows the use of a smaller field and lighter magnets.
Variants and variables
The original Nuclear Shaped charge design is not the only possible configuration.
The design can be modified to achieve more desirable characteristics.
One such modification is the shape of the tungsten propellant. The thinner and wider it is, the more focused the cone of particles produced.
The choice of tungsten itself is not definite. The propellant contained in each nuclear pulse can be replaced by other materials. Lighter materials, such as water, will reach higher exhaust velocities, but need to be thicker to absorb the thermal radiation from the beryllium oxide. This hinders their ability to be spread into a thin plate to achieve a narrow cone.
For mass savings, it is possible to use beryllium oxide itself as the propellant.
For a Mag-Orion drive, a material that produces a plasma more readily affected by magnetic fields might be more suitable than tungsten.
In a setting where tungsten might not be readily available, iron could be used instead. It will have about three times higher exhaust velocity, and proportionally lower thrust, with the risk of hot Fe+ ions chemically reacting with the the pusher plate.
A further development of the Orion is the Mini-Mag Orion. This design centers around externally starting a fission reaction in the pulse units. This is done by compressing the fissile fuel (uranium) using a Z-pinch device. The main advantages are lighter units, lower minimum yields, increased safety (the pulse units are not self-contained bombs in the wrong hands) as well as all the efficiency and mass savings of electromagnetic capture of the particles produced.
Externally-ignited nuclear reactions brings fusion to mind. Using fusion fuels instead of fission fuels in the nuclear pulse units has several advantages. In terms of propulsion, fusion products can achieve much better velocities and power densities than with fission: exhaust velocity of the particles is proportional to the square root of their temperature.
Fusion pulse propulsion allows for the use of much smaller individual units. This is important, as it means the impulse from each detonation is lower, leading to lower structural requirements and suspension mass. The storage of non-fissile elements is better than handing over several tons of uranium along with their triggers to a captain heading off to deep space… However, the spaceship has to carry along its own power source to trigger each nuclear pulse, and carry protection against neutrons emitted by neutronic fusion. The latter would normally be the job of a physical pusher plate.
The advantages of fusion can still be obtained without external power by using a Teller-Ulam device: it uses a fission reaction to trigger a subsequent fusion reaction. However, the individual pulse units will be larger than either pure fission or fusion devices, and more complex, with the disadvantages of both systems. Considering that propellant is cheap in outer space, and that more than 100km/s is wasteful in our Solar System, there should be no reason to ever use these.
The ultimate nuclear pulse unit uses antimatter. Small quantities can be used to ignite fusion pulses in Antimatter-Catalyzed Fusion, large quantities can be used in pure Antimatter-Matter Annihilation. They would allow the use of very small individual pulse units, with corresponding low structural and suspension requirements, and extreme exhaust velocities that are perfectly suited to an interstellar mission. The main challenges though are actually producing useful quantities of antimatter, storing it safely until it is used, and finding an efficient way to convert gamma rays into infrared radiation.
The Casaba Howitzer
The Casaba Howitzer is the result of research into reducing the spread of the particles produced by a nuclear pulse unit. Make the cone narrow enough and it becomes a destructive beam.
The original nuclear shaped charge design called for the use of a tungsten plate. The particles that resulted from the detonation of a pulse unit would fit inside a cone with a spread of 22.5°. The particles would be relatively slow (between 10 and 100km/s depending on thrust requirements) and rather cool (14000°C in transit, 67000°C after hitting the plate).
As noted before, using lighter elements, such as plastics or even hydrogen, in a thick and narrow instead of wide and flat shape, you can achieve a very narrow cone and very high particle velocities. A Science & Global Security report from 1990 used polystyrene as the propellant material to produce a particle beam with a spread of 5.7° and a velocity of 1000km/s.
Particle velocity is derived from the Root Mean Square equation. It can be written as such:
- Particle velocity = (24939 * Temp / Mass) ^ 0.5
24939 is a constant equal to Boltzmann’s constant (1.38*10^-23) divided by unitary molar mass in kg (1.66*10^-27) times the degrees of freedom of motion (3). Temp is the nuclear detonation’s temperature in Kelvin, and Mass is the mass of the propellant used in kg/mol.
For an atom bomb (10⁸ K), uranium (238) will be ejected at 102km/s.
In a fusion reaction (10⁹ K), deuterium (2) will be ejected at 3530km/s.
The difficulty is in transmitting this thermal energy to the propellant, and keeping the particle cone focused.
In a propulsion pulse unit, it is not known how efficiently a nuclear shaped charge is able to heat the propellant. Most sources cite a 85% of the device’s energy being sent in the desired direction. It is unknown also whether this is before or after some of the propellant is accelerated in the wrong direction, and whether larger pulse units are more efficient (higher propellant mass fraction). This is important as it would allow a thermodynamic estimation of the particle velocity.
It would be reasonable to use a lower figure when calculating the amount of energy delivered to the propellant. Scott Lowther gave a 50% figure for small fission charges. An SDI nuclear weapons study, project Prometheus, experimentally tested Casaba Howitzer weapons using plastic propellants. It achieved 10% efficiency. A Princeton University study from 1990 on third-generation nuclear weapons cited 5% instead, but for fusion devices with ten times better beam focus.
Effectiveness
Despite the reduction in cone spread, the stream of particles produced by by Casaba Howitzer dissipates much more quickly than an electro-magnetically accelerated particle beam or a laser.
It is possible to reduce the beam angle to 0.006 degrees in width, as reported by the third-generation nuclear weapons study. 0.057 degrees has been experimentally achieved by project Prometheus. The trade-off is much lower efficiency than propulsive units (5–10% vs 80–85%).
The theoretical maximal performance of a thermonuclear device is 25TJ/kg. Modern weapons are able to achieve 2.5TJ/kg, but this figure is for large weapons that have better scaling. Smaller warheads such as those tested for project Prometheus are likely to be in the kiloton range, and mass about 100kg. Better understanding of fission ignition has reduced the nuclear material requirement down to a kilogram or less.
A nuclear detonation only lasts a microseconds, so we can assume that the entire energy of the unit is delivered to the target in a single pulse of duration 10^-6 seconds. As the particles produced expand in a cone with an angle 𝛉, we can use the following equation to calculate the destructive potential at various distances:
- Intensity = (Yield * Efficiency * 10⁶) / (3.14 * (tan(𝛉) * Distance) ^2)
- Irradiance = (Yield * Efficiency) / (3.14 * (tan(𝛉) * Distance) ^2)
Intensity is measured in watts per square meter. Irradiance is joules per square meter. Yield is how much energy the nuclear charge delivers, converted to joules. Efficiency ranges from the 0.85 of a propulsion unit to the 0.05 of a Casaba Howitzer. 𝛉 is the cone angle. Distance is between the nuclear detonation and the target, in meters.
Let us calculate some examples:
Small Casaba Howitzer (50kg)
0.01 radian directivity (0.057 degrees)
5kt yield, 10% efficiency: 2.09TJ
Distance 1km: Irradiance = 673GJ/m²
Distance 10km: Irradiance = 6.7GJ/m²
Distance 100km: Irradiance = 67.2MJ/m²
Distance 1000km: Irradiance = 672kJ/m²
Large Casaba Howitzer (1000kg)
0.001 radian directivity (0.0057 degrees)
1Mt yield, 5% efficiency: 209TJ
Distance 1km: Irradiance = 6728TJ/m²
Distance 10km: Irradiance = 67.3GJ/m²
Distance 100km: Irradiance = 672MJ/m²
Distance 1000km: Irradiance = 6.7MJ/m²
Futuristic Megaton Nuclear lance
0.0001 radian directivity (0.00057 degrees)
1Mt yield, 20% efficiency: 836TJ
Distance 1000km: Irradiance = 2691GJ/m²
Distance 100000 km: Irradiance = 269MJ/m²
To determine destructive capability, we can model the Casaba Howitzer as a direct energy weapon. We can recreate the particle strike as a laser weapon firing a single pulse with equal properties.
We will describe the strike as a laser pulse of duration 1 microsecond, containing X energy and with Y spot radius. A 1.63 micrometer wavelength laser focused by a 2cm diameter mirror consistently produces the same spot sizes as a 0.01 radian beam. A 20cm mirror is used for 0.001 radian beams, and 200cm for 0.0001. We test penetration against Aluminium.
Small Casaba Howitzer:
X = 2.09TJ
1km, Y = 0.994m: 734mm penetration
10km, Y = 9.94m: 0.73mm penetration
Large Casaba Howitzer:
X = 209TJ
50km, Y = 4.97m: 586mm penetration
500km, Y = 49.7m: 0.59mm penetration
Futuristic Megaton Nuclear lance:
X = 836TJ
1000km, Y = 9.94m: 293mm penetration
5000km, Y = 49.7m: 2.35mm penetration
The results reveal that the Casaba Howitzer is an extremely destructive weapon, with the larger models able to strike at distances usually reserved for lasers. Even a small Casaba Howitzer is effective at up to several kilometers, using technology tested in the 80s. Larger, more modern devices can strike at greater distances. Futuristic devices will reach particle velocities of about 10000km/s, so time to target is negligible.
However, these distances are lower than those of powerful lasers, so the Casaba Howitzer will need a delivery system such as missile, or be used in defensive roles.
Making use of the Casaba Howitzer
The Casaba Howitzer’s advantages are numerous, and can be exploited in four ways:
-Terminal warhead:
Hard science fiction with a military focus usually boil down to where the author has placed their marker on the sliding scale between missile and laser dominance. Make lasers too powerful, and they make mass missile attacks uneconomical. Make missiles cheap and fast enough, and you can overwhelm any laser defense.
Missiles are hindered by the requirement to track the target and follow until impact. Lasers are increasingly effective as missiles close the distance to their target. Past a certain point, any missile touched by a laser is quickly destroyed. So quickly, that a laser defense’s primary limitation is the time it takes to switch targets. In other words, a laser defense sets up a ‘death zone’ around itself, within which any wave of missiles will quickly be annihilated.
A combination of efficient lasers, multiple turrets and competent target handling can cut through hundreds of missiles.
The counter to this, on the missile side, is to perform randomized high-acceleration maneuvers called ‘jinks’. This tactic is already used today by sea-skimming missiles once they enter the range of CIWS defenses. The problem is, in space this requires the missile to have powerful thrusters, lots of propellant and active, autonomous sensors that survive to the terminal stage of its attack. This means that missiles will end up being heavy, hard to bring up to speed, large (easy to track and hit) and expensive due to on-board electronics. These are all characteristics you want to avoid when trying to make massive waves of missiles economical, or if jinking through the death zone.
Using a Casaba Howitzer warhead solves this conundrum.
It allows missiles to deal damage from outside the death zone. It also removes the requirement of saving propellant for the terminal stage, or even the necessity of accelerating up to a high velocity intercept. At allows missiles to be lighter and smaller. Depending on the price of the nuclear technology, a few Casaba-Howitzer missiles may be cheaper than multitudes of kinetic impactors.
-Point defense
The usefulness of a nuclear shaped charge extends further than just being a warhead. As calculated in the Effectiveness section of this post, the particle cones spread quickly, but remain effective at short ranges.
In a defensive role, a Casaba Howitzer will have to be lightweight and efficient in its use of fissile material. This is because it must be deployed in numbers comparable to the incoming projectiles. Optimizing for efficiency has the consequence of producing a wider cone.
This cone can be used to sweep away missiles in the terminal phase. Close enough, it will outright vaporize kinetics. Further away, it can still damage sensors and shatter propellant tanks through impulse shock. The large angle of the cone is advantageous, as it would reduce prevision requirements against jinking missiles, and might even catch several missiles at once.
Other advantages of using Casaba Howitzers as a point defense is that it can easily be aimed, does not consume power and has infinite firing rate. If you detect missiles coming in, dump your entire payload of defensive drones and have them point at targets. Once they come within range, all can detonate simultaneously.
This might actually be the preferred tactic, to prevent previous nuclear detonations from interfering with the detonation of subsequent charges. This is a concern if the Casaba Howitzers use fusion fuels that are sensitive to external sources of neutron radiation.
Example defensive Casaba Howitzer:
100kg, 10kt yield
85% efficiency: 35.56TJ beam
Beam velocity 1000km/s
Beam angle: 10 degrees
Effective range (penetrates 5mm of aluminium): 16km
This warhead can destroy anything within a 6.15km² circle up to 16km away. It reaches targets in less than 16 milliseconds, and unlike a pin-point laser, it affects the entire surface of the target at once.
-Booster
The awesome power of a nuclear shaped charge does not have to be used directly to damage targets. It can be used in innovative ways.
Instead of being used to generate high velocity particles in a narrow cone, a Casaba Howitzer can be used as a nuclear version of modern shaped charges. A metal cone is put in the way of a nuclear-heated beryllium filler. It is accelerated by the blast, like in an Explosively Formed Projectile. The only requirement is that the energy deposited into the metal lining is not sufficient to vaporize it.
-Particle beam weapon
The ionized particles produced by a Casaba Howitzer can be used to feed a particle accelerator. Unlike a traditional accelerator, its main role is not to accelerate particles closer to the speed of light, but to use magnetic lens to focus the ions into a tightly collimated beam. At the muzzle, the ions are neutralized to reduce bloom using a co-axial electron beam.
The greatest point of concern is pushing the particles into the accelerator without reducing their velocity. A magnetic ‘funnel’, much like that of a mass spectrometer, can perform this role.
A shaped-charge-pumped particle beam looks like this, in reverse.
The second point of concern is preventing the particles from damaging the particle accelerator. This can be remedied by building the accelerator as a series of widely spaced loops of wire acting as electromagnets. The particle beam is focused in stages, narrowing after each loop.
The optimal Casaba Howitzer configuration for this weapon is a fusion device that is built to maximize particle velocity. 10000km/s (3% of the speed of light) may be achieved. This is much slower than an electromagnetically-accelerated particle beam weapon, but it has the advantage of requiring little to no external power (the electromagnets can be fed by the heat they receive from the nuclear detonation), massing much less than a regular particle accelerator and able to extend the range of small nuclear pulse weapons to useful distances (in the thousands of kilometers).
Integrating the Nuclear Lance into your setting
The Casaba Howitzer is best used as an ‘early technology’ science fiction setting. When space exploration is still new, and opponents start out in the same orbit, the short-ranged but powerful nuclear shaped charges available are extremely effective.
It can be mounted on modern-technology missiles to allow them to be effective regardless of the impact velocity, alternatively, missiles will accelerate to low velocities then expend the majority of their dV in evasive maneuvers. It will more likely be used by the most technologically advanced nation to greater effectiveness, as the technology is far from well understood even 58 years after its conception.
When the technology becomes widespread, such as following its development in nuclear pulse propulsion, it will still be the favorite of nations with greater access to fissile materials. While a fusion device allows greater yields, and would be better for propulsion, a Casaba Howitzer weapon does not benefit from the 1000km/s particle velocities. Easy to detonate fission charges are easier to handle and use.
They will however fall out of favor as lasers extend the range of combat beyond even their reach, into the tens of thousands of kilometers. More efficient missile propulsion, or the development of cold stealth technology, might change the battlefield even further.
However, as developments in propulsion continue, newer, simpler methods of detonating thermonuclear devices might become commonplace. Antimatter-catalyzed fusion or supercapacitors powering Z-pinch devices might allow Casaba Howitzers to return to the battlefield as cheap anti-missile defenses, free from the requirement of fissile materials.
Throughout history, however popular or effective they are, Casaba Howitzers will force states to carefully watch who and where to fissile materials are sold. Just 2 kilograms of uranium can be converted into a several kiloton-yield weapon, easily hidden in a civilian cargo-bay or remote satellite, and used to destroy an expensive warship or vital space station in an instant.
