A blog dedicated to helping writers and worldbuilders create consistent, plausible Science Fiction.

Friday, 17 June 2016

The Nuclear Spear: Casaba Howitzer

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


Visualization by Rhys Taylor
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.


Orion 10m Propulsion Module
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 angle AĈB is very important
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 simplified representation of a nuclear shaped charge. Note the lack of explosive lenses. 
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. 


Medusa nuclear pulse propulsion concept.

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.

A simplified configuration for water-propellant nuclear pulse units.
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.



ICF pellet design, ignited by lasers
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.


Tellar-Ulam design.
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.


Concept design by Scott Lowther
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 (10^8 K), uranium (238) will be ejected at 102km/s.
In a fusion reaction (10^9 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 * 10^6) / (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 = 2.09GJ/m^2
Distance 10km: Irradiance = 209MJ/m^2
Distance 100km: Irradiance = 2.09MJ/m^2
Distance 1000km: Irradiance = 2kJ/m^2

Large Casaba Howitzer (1000kg)
0.001 radian directivity (0.0057 degrees)
1Mt yield, 5% efficiency: 0.2PJ
Distance 1km: Irradiance = 0.2TJ/m^2
Distance 10km: Irradiance = 24GJ/m^2
Distance 100km: Irradiance = 200MJ/m^2
Distance 1000km: Irradiance = 2.09MJ/m^2

Futuristic Megaton Nuclear lance
0.0001 radian directivity (0.00057 degrees)
1Mt yield, 20% efficiency: 0.83PJ
Distance 1000km: Irradiance = 0.83GJ/m^2
Distance 100000 km: Irradiance = 83kJ/m^2

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 1630nm 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
1000km, Y = 99.43m: 734mm penetration
10000km, Y = 994.3: 0.73mm penetration

Large Casaba Howitzer:
X = 0.2PJ
10000km, Y = 99.4m: 7342mm penetration
100000km, Y = 994m: 7.34mm penetration


Futuristic Megaton Nuclear lance:
X = 0.83PJ
1 million km, Y = 994.3m: 29.2mm 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 2000km, using technology tested in the 80s. Larger, more modern devices can strike at distances where light lag becomes a concern. Futuristic devices will still be limited to particle velocities of about 10000km/s, meaning that time to target becomes problematic.

Making use of the Casaba Howitzer

The Casaba Howitzer's advantages are numerous, and can be exploited in four ways:

-Terminal warhead:


A MIRV's warheads.
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 6.15km^2 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 HEAT rounds. A metal cone is partially melted and squeezed into a jet by the detonation of a nuclear shaped charge. The only requirement is that the energy deposited into the metal lining is not sufficient to vaporize it. It would achieve higher velocities by allowing it to vaporize, but would expand into uselessness, as any gas would. 

A 'direct' nuclear EFP, where the thermonuclear explosive is in contact with the metal cone to exploit the Munroe effect, produces a projectile with a velocity calculated by the Gurney equation:


  • V = ( (2*Y*E)^0.5 )*(((1+2*M)^3 + 1)/(6*(1+M))^-0.5) 

It is rather complicated.
V is the velocity achieved, in m/s.

Y is the yield of the nuclear device, in joules.
E is the efficiency is the device, about 0.85 for nuclear shaped charges.
M is a ratio: explosive mass divided by projectile mass.

In this case, explosive mass is the mass of the beryllium that absorbs the nuclear device's X-rays and converts them into thermal energy, thereby becoming a working fluid.


1kt NEFP
Y is 0.1kt or 0.42TJ
E is 0.85
M is 0.2, as recommended by the original Orion study
V = 2619km/s
  
-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 in an instant.

24 comments:

  1. Correction: the half angle should be 11.31°, not 11.25°, for a pusher plate interception of 0.97%, not 10%.

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  2. Really good article, great work here...

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  3. I realize that the casaba howitzer in your particle beam form resembles a toned down yamato cannon from StarCraft. This got me wondering if such a weapon could be viable in a turret form, or would it be restricted to a spinal mount for very large warships or an unmoving space fortress? If this form of casaba is able to be made into a turret, then wouldn't this solve an issue with kinetic weapons you mentioned in an earlier post? You said that it could be possible for it to use little to no power from the ship, thus giving the ship an advantage in situations where the ship has little power to spare over kinetic weapons (low velocity).

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    1. Sorry for the late reply!

      The casaba howitzer is a nuclear explosion. Yes, it ejects its energy mostly in one direction instead of a sphere, but the remaining percentage is enough to carve out several tons of material from anything near it.

      If the Casaba Howitzer design being used is 85% efficient, it would require that a turret contain the remaining 15%. When the yield is in the terajoules, 15% is no laughing matter.

      In the near future, you will definitely have to build the spaceship around the weapon. The magnetic focusing loops will be enormous, to the point where it might be easier to make each loop an autonomous drone with its own engines and fuel.

      As technology progresses, you'll be able to detonate smaller and smaller thermonuclear charges, meaning you can use them closer and closer to your spaceship.

      In an advanced technology setting, you might use 'micro-fusion' Casaba Howitzers inside a magnetic shell with a small opening for high velocity particles to rush out of. Enclose the whole thing in armor, make it rotate and you're got your nuclear turrets.

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  4. This has always been an intriguing weapon for me, and I'm glad to actually have some idea of range and application now.
    A question that comes to mind is what would performance be like in atmosphere, either fired in atmosphere or fired into the atmosphere from orbit?

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    1. Imagine you dive into a swimming pool. You drag along a garden hose with you. With a twist, you switch on the nose, and water jets out underwater.

      How far does the water keep moving? 30cm? 10 cm? Less?

      That's how effective a Casaba Howitzer is within an atmosphere. The kinetic energy of the particles is very quickly absorbed by collision with atmospheric particles, until it becomes nothing better than a cloud of hot air. A Casaba Howitzer in an atmosphere is a plasma weapon of the sort hard scifi commentors love to explain why they won't work to soft scifi fans.

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  5. 1 megaton yield equals 4.5 PJ not 45 PJ therefore with %20 efficiency energy of the nuclear spear is 0.9 PJ not 8.56 PJ. other yield to energy conversions are also off by a factor of 10. Great article by the way.

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    1. I think I put the correct figures.

      Thanks!

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  6. https://en.wikipedia.org/wiki/TNT_equivalent?wprov=sfla1

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  7. Hi Matt. Thank you for this resource (am big fan also of Winchell's Atomic Rockets website). Am I the only one here who thinks that 70 METERS of aluminum armor penetration by a 1 MT Casaba Howitzer across over an 200 meter-wide area at a range of 10,000 KM is kind of a serious game-changer? That's some serious reach and area of effect! I'm writing a sci-fi novel (Winchell has mentioned it in his Atomic Novels section), and one of the first problems I ran into was how difficult it is to economically engineer a long-range laser without needing seriously large mirrors and ginormous power generators (to say nothing of the heat radiator system). Thoughts?

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    1. It is!

      Casaba Howitzers have the potential to do what guided missiles did to the naval gun: make them utterly obsolete!

      Imagine your 1000 ton laser-firing behemoth with a kilometer squared of radiators and a multi-gigawatt power station onboard, taken out by a single missile.

      The practicality of the devices is where you, as an author, can choose to make or break them.

      If you want Casaba Howitzers to be the supreme weapon, you have to provide very large quantities of rather rare materials. This supposes a large military-industrial complex. Explosive lenses which trigger the nuclear detonations are some of the most advanced technologies right now. The large amounts of high-grade fissile material consumed per warhead has to come from an even larger supply shared with power stations and nuclear rockets.

      With a few advances in beam tightness, the Casaba Howitzers can reach 10000km+. The warhead will mass a lot, but if it can take out a much more expensive warship, it will be worth it.

      The consequences, however, have to be dealt with. With a vast supply of nuclear material, it makes more and more sense to use some of it for creating electricity where sunlight is unavailable. Mars, for example, would built its colonies anywhere it wanted, without worrying about the football fields of solar panels required per home. Mercury would be a less attractive destination, as sunlight can be replaced with nuclear power.

      Nuclear rockets would open up the solar system like never before, meaning it is possibly cheaper to get your nitrogen from Neptune's moon Triton than excising it from Earth's biosphere.

      Warfare will change is possibly unexpected ways. Nothing can survive the beams, so armor is cast off the wayside. Mobility, stealth and first strikes are paramount, much like modern jet combat. Spaceships are likely to be small, each packing only a dozen warheads, but operating in swarms.

      Non-government actors become disproportionately powerful. Any commercial ship can pack ship-killing weaponry in a closed cargo crate. Nuclear bombs can be made easily from the effusion of nuclear material available. Useless in space, but devastating on the ground. Just the explosives required for triggering the nuclear devices can be scavenged and turned into lethal weapons in pressurised environments.

      If you want you traditional capital-ship navy that slugs it out at close ranges, then there are many ways to stop Casaba Howitzers from interfering.

      The easiest way is to limit the supply of fissile material. Its already rare on Earth, and hard to extract from other planets, and unless Asteroid Uranium flies by, we won't have much more.

      You can also put mechanical restrictions on the beam tightness, reducing Casaba Howitzers to devastating but short-ranged defensive weapons. Alternatively, push the laser effectiveness even further, so much so that 10000km range is insufficient.

      Political restrictions, such as the modern day chemical weapons ban, is a cheesy way to do it, and is the most vulnerable to people just not caring, but it removes you from having to go into technical details about why such or such is not trouncing everyone on the battlefield.

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  8. First off, thank you for an excellent article! As a writer aiming to portray at least reasonably realistic space combat, this is a very thought-provoking and detailed resource.

    Second, I had a question/hypothetical: I see above that you answered a comment saying that a weaponized nuclear shaped charge would be effectively worthless in an atmosphere, but was wondering if the potential might exist to use the weapon instead as a bunker-busting munition against ground targets. Specifically, the potential application of detonating a large yield weapon upon contact with the ground above a deeply buried hardened structure, using the focused plasma jet to drill through rock and concrete, or applying it directly to a HEAT-style explosively formed pennetrator.

    Thanks very much for your excellent work, it's a wonderful resource and a great read!

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    1. Hello and welcome! Sorry,t he blog is in a bit of a down period for now, but I'll keep answering comments.

      A Casaba Howitzer is actually a particle beam weapon, but instead of magnets accelerating a small number of atoms, a nuclear weapon blasts away a large amount of plasma.

      In an atmosphere, this plasma will slam against slow, cold gas molecules and transfer their energy. This rapidly disperses the beam and heats up the surrounding air. The range will be terrible, with an effectiveness of a few hundred meters at most.

      However, within that distance, it is more effective than a nuclear bomb in trying to take out bunkers or buildings.

      Better than the Casaba Howitzer is the Nuclear Explosive Formed Projectile. Like modern anti-tank weapons, a disk of metal is is explosively accelerated into a thin stream of high velocity ejecta. Half molten, it remains dense enough to punch through any amount of armor. The NEFP is the nuclear version of this, utilizing a very thick slab of tungsten instead of a copper plate, and drilling through dozens of meters of concrete instead of a few centimeters. It won't be very effective in creating huge craters like regular nuclear bombs, but it will get through bunkers undergrounds and set anything inside on fire.

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    2. Thanks for your reply!

      Is there any existing equation or resource to calculate the effective penetration of a NEFP? I would be very interested to know what sort of RHAE penetration values could be attained by such a weapon, especially in terms of using it in the context of a spacecraft firing on deeply buried anti-orbital weapons positions.

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    3. The equation's I've got allow us to calculate the velocity of the EFP. They're detailed in the post under the title 'Gurney equations'.

      For a very small 1 kiloton yield device and a 0.85 efficiency, we must use a very high mass ratio to achieve acceptable velocities. The Casaba Howitzer used for propulsion purposes had a mass ratio of 0.2 and reached over 2600km/s... unless capped by a tungsten plate. This is essentially a particle beam! It will blow through just about any vehicle, but cannot travel beyond a few meters in air or a few centimetres in rock.

      However, the equations fail when we use nuclear weapons, as they basically become a ball of hard x-rays in microseconds, instead of the relatively gradual pressure building up a 600 degrees Celsius in milliseconds.

      If we want deep penetration of a target, we must use a very thick plate that can take the energy and carry it through the medium without losing velocity. This is done by increasing the mass ratio to about 20 or more (1 part uranium for 20 parts plate).

      An even more efficient method is to use a beryllium filler between the nuclear device and the plate. It absorbs the x-rays more efficiently and becomes a high-pressure gas the pushes instead of vaporizes the metal plate.

      All in all, I came out with a 1kiloton device using 2kg of uranium pushing a 100 ton plate of steel to about 20km/s at the tip of the stream. It is 10cm thick and 13m wide, and can penetrate about 26000mm RHAe. Total device mass is about 111 tons.... and I don't know how to make it smaller without wasting the vast majority of the uranium's energy.

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  9. so you're talking about firing the equivalent of two main battle tanks at 20,000 metres per second and that would take out 26 metres of armour? the numbers are astonishing

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    1. Sometimes, I worry that they're too good. If casaba howitzers are so good, what's the point of lasers and missiles and railguns?

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    2. I think the good news here is that casaba howitzers fall into the (large!) class of techs that are permissible, so to speak, in a hard-ish SF setting, but are not required.

      On the one hand, the basic concept has been demonstrated. On the other hand, it has not been demonstrated that extremely narrow streams can be achieved in practice. Precision sculpting with exploding nuclear weapons is not easy!

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    3. I've tried modelling space warface where casaba howitzers are widespread weapons... and I've concluded that they are something to avoid.

      Even using laboratory results from two to three decades ago (Project Prometheus), they would have the range and destructive capability to render lasers seem wimpy and kinetics pointless. Missiles could be defeated by wide-cone nuclear blasts, so combat would devolve into missile swarm slugfests.

      Each side launches a swarm of missiles with various low- and wide-angle casaba howitzers. The low-angle missiles fire from fire away at spaceships. The wide-angle missiles attempt to fry the opposing wave of missiles. This is an environment that is very hostile towards equipping spaceships with anything above a bare-minimum missile-bus design. Human crews have no place in the middle of waves of radiation and kiloton plasma spears.

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    4. mainly? the howitzers are too expensive to throw around like you can all those other things

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    5. That might be true today, bob, but it might not be the case in a setting where both sides agree that they are very effective.

      If, for example, a handful of nuclear warheads can take out a spaceship reliably, while saving you the cost of a massive nuclear reactor, electric generator, radiators, laser beam generator and focusing array, and costing much less than their target... then they will be deployed.

      This 'cheaper than the target' logic applies to very expensive missiles such as million-dollar cruise missiles and 10-million-dollar anti-carrier ballistic missiles.

      Also consider that dominant weapons receive inordinate amounts of funding towards their research and development. Casaba Howitzers will certainly become cheaper over time. The biggest factor is the cost of nuclear material. However, there is news of uranium being extracted from seawater, and some asteroid containing large amounts of rare and heavy elements...

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  10. The Small Casaba Howitzer has a irradiance of 2kJ/m² at 1 Mm. Still it's capable of penetrating 734 mm of Aluminum. How is that possible? Besides that awesome post, always loves to see something from Project Rho.

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