How powerful are the legendary space-based weapons? You'd never guess!

RODS FROM GOD, the legendary ultimate weapon, can strike any target location in the world within a dozen minutes. Its lethality is comparable to that of tactical nuclear weapons and can even destroy targets hidden deep in underground bunkers.

What is the "Rod of God"? How far has its research progressed? Is it still just a concept? Is its power really as terrifying as the legend says?

The history of war? The history of stone throwing!

The Rod of God is a space-based kinetic weapon . To understand it, we must start with its "previous life" kinetic weapon. Kinetic weapons , as the name suggests, are weapons that use kinetic energy to kill objects . The high-speed moving warhead directly collides with the target, releasing huge kinetic energy to destroy the target.

Kinetic weapons occupy an important position in human history. From the stones and slings thrown by early primitive people to catch prey, to the bows and arrows and catapults in the cold weapon era, to the firearms after the birth of gunpowder, all belong to the category of kinetic weapons. If the Trisolarans really exist, the water droplets they used to destroy the entire Earth fleet are also a kinetic weapon. It can be said that the development of war weapons is actually changing the way of throwing faster and harder stones.

The core of kinetic weapons is how to provide huge kinetic energy to the "warhead" .

The kinetic energy of a stone thrown by human power is about 100 joules; the kinetic energy of a slingshot can reach 200 joules through acceleration through swinging, which is twice as powerful as throwing a stone directly; a bow can convert the elastic potential energy stored in deformation into the kinetic energy of an arrow . The English longbow used by English longbowmen who made great achievements during the Hundred Years' War between England and France can provide a maximum shooting kinetic energy of 160 joules for the arrow, and because the design of the arrowhead can reduce friction loss, its lethality is no less than that of a slingshot.

English longbowmen in art works | Image source: "Battle of Crécy during the Hundred Years' War between England and France" (detail), illustration from an illuminated manuscript by the 15th-century French historian Froissart, collected by the National Library of France

After the invention of gunpowder, the method of throwing stones evolved. The energy of the gunpowder explosion was converted into the kinetic energy of the bullet, which instantly accelerated the bullet to a speed of 400 meters per second, causing huge damage to the target.

The birth of gunpowder also inspired the development of weapons in another direction - explosive killing. Compared with kinetic weapons that first convert chemical energy into kinetic energy and then kill the target after long-distance air friction loss, explosive weapons that directly explode near the target to release huge energy have gradually become dominant in modern warfare. Tank shells with millions of joules of energy have dominated the battlefield, and nuclear weapons with greater power have become the ultimate weapon in the game between major powers.

Will kinetic weapons gradually fade out of history? No. For example, when people really enter the universe and start the era of interstellar war, due to the lack of medium in the universe, the power of weapons that rely on explosions will be greatly reduced. At this time, kinetic weapons may become the best solution. After all, there is no air resistance in space, which is very suitable for kinetic weapons to play.
Not to mention the distant future, kinetic weapons and electromagnetic gun weapons are now gradually being developed and applied. Using electromagnetic force instead of gunpowder explosion to accelerate the projectile can provide the projectile with higher kinetic energy.

We can see that the energy sources for accelerating the "stone" are varied, including the work energy of the human body, the elastic potential energy of the bow body deformation, the chemical energy of gunpowder explosion, and the electromagnetic energy of the electromagnetic field, but there is also a common energy that can be used for kinetic weapons - gravitational potential energy . Space-based kinetic weapons come from this.

The Past and Present of "Rod of God"

As early as the 1950s, the Rand Corporation proposed installing a long pole on the top of an intercontinental missile; and the method of launching using satellites became popular with the works of science fiction writer Jerry Pournelle, and eventually evolved into today's space-based kinetic weapons.

In 2002, the RAND Corporation produced a report, “Space Weapons, Earth Wars,” which included an entire chapter describing the practicality of space-based kinetic weapons.

In 2003, the US Air Force published a report titled "The US Air Force Transformation Flight Plan", which mentioned the "hypervelocity rod bundle", also known as the "Rod of God", in its overview of future space-based weapons.

In this report, the hypervelocity rod bundle system is a 20-foot-long (6.1-meter) and 1-foot-diameter (0.30-meter) tungsten rod ; it is controlled by 6 to 8 satellites deployed in a specific orbit and can strike any target in the world within 12 to 15 minutes. This tungsten rod has a mass of about 8.2 tons and can hit the target at a speed of 10 Mach (10 times the speed of sound, 3400m/s) after launch; the energy equivalent to 11.5 tons of TNT (48GJ) is released instantly, which can provide destructive power similar to that of a conventional bomb of the same weight.

Satellite orbits for space-based weapons deployment | Image source: Reference 3

What really brought space-based weapons into the public eye was their appearance in a series of film and television entertainment works, as well as their shocking performances like "God's judgment."

Jerry Pournelle describes the existence of space-based weapons in his science fiction novel Footfall, which has a Thor-like performance in the novel.

Footfall | Image source: Reference 3

Space-based kinetic weapons are also present in the Mobile Suit Gundam series - except that the entire satellite is used as a kinetic weapon.

"G.I. Joe: Retaliation", released in 2011, used exquisite special effects to show the audience the tremendous power of astronomical energy weapons. Just a tungsten rod can cause an earthquake powerful enough to destroy the entire London.

Space-based kinetic weapons launch scene | Image source: Stills from "G.I. Joe: Retaliation"

In addition to various film and television works, space-based kinetic weapons have always been a popular candidate for the ultimate weapon in various games. The opening of "Call of Duty: Ghosts" revolves around the struggle for control of space-based weapons on satellites. In the end, terrorists still launched a tungsten rod towards the American mainland and completely destroyed a city.

Space-based weapon Odin | Image source: Stills from "Call of Duty: Ghosts"

In addition, space-based weapons also appear in a series of games such as "Syndicate Wars", "Mass Effect 2", "Halo" and "GTA5", all of which are set as powerful ultimate weapons.

Some readers may have discovered that, up to now, all descriptions of space-based weapons are various film and television settings and theoretical predictions. How powerful are space-based weapons, and has there been any experimental verification?

How powerful are space-based weapons?

In 2018, a Chinese research team published a paper titled "Theoretical and Experimental Study on Rock Penetration and Ground Impact Effects of Hypervelocity Long Rod Projectiles", which simulated in detail the effects of space-based weapons on the ground and described the actual combat results of tungsten rods striking the ground.

Unlike low-speed situations, the impact of a super-high-speed long-rod projectile with the ground can no longer be regarded as a rigid body collision, but will be divided into three modes according to the speed: rigid body penetration, semi-fluid penetration and fluid penetration .

1. When the projectile velocity v ≥ 3.0 Mach (1 Mach = 1 times the speed of sound), fluid penetration occurs and the fluid dynamics model is required to calculate the penetration process. The Bernoulli equation and continuity equation are:

Where v∞ is the velocity limit of the backward jet, K0 is the compression jet coefficient of the broken medium jet, which characterizes the change of the example velocity during the backward jetting process of the medium.

2. When the projectile velocity is in the range of 1.5 Mach < v < 3.0 Mach, semi-fluid penetration occurs and the medium is in the transition zone from internal friction to fluid state. The modified fluid dynamic model is used for simulation.

3. When the projectile velocity v≤1.5 Mach, rigid body penetration occurs and the dynamic deformation of the rock medium can be simulated by the linear elastic internal friction compression model.

The impact speed of space-based kinetic weapons is generally above Mach 10. From the above analysis, it can be seen that as the projectile penetrates the rock, the projectile gradually decelerates, and the penetration mode changes from fluid penetration to semi-fluid penetration and then to rigid body penetration . Therefore, when calculating the crater range, the crater can be divided into three areas according to the figure below and calculated separately.

A simplified diagram of the calculation of the crater range in the demonstration of hypervelocity projectile penetration. From the inside to the outside, there are the pseudo-fluid zone, where the broken rock can be regarded as an inviscid, incompressible ideal fluid; the crack zone, where the particle displacement is very small; and the elastic zone, which still retains the initial physical and mechanical characteristics of the rock. Image source: Reference 6

Of course, the team not only provided a model for analysis, but also conducted analysis based on actual verification data.

Original text | Image source: Reference 6

In other words, China has carried out a prototype experiment of space-based kinetic weapons to strike the ground. The experimental results showed that a 140kg tungsten rod only created a parabolic crater with a depth of 3.0m and a diameter of 4.6m.

Schematic diagram of the pit formation results of the tungsten rod impact experiment on the Gobi Desert | Image source: Reference 6

This destructive power is far from destroying a city. It seems that the quality of the projectile needs to be improved a lot if it is to be used in practice. As for how to control the transportation cost and difficulty after the quality is increased, that is another problem.

The difficulty of throwing objects from high places

By utilizing the conversion of gravitational potential energy and kinetic energy, can we obtain greater kinetic energy by increasing the altitude?

On Earth, any problem of throwing objects from high altitudes has to take into account the influence of air resistance. Consider a general spherical object, which is subject to resistance in the air.

Among them, C is the drag coefficient, ρ is the air density, and v is the speed of the object. It can be seen that as the speed of the object increases, the air resistance it encounters will increase rapidly in a parabolic shape until it reaches the gravity of the object. At this time, the gravity and air resistance are balanced, and the speed of the object no longer increases.
Therefore, objects falling from a high altitude will have a terminal velocity, such as the speed of raindrops is 8~9m/s. (So you don't have to worry about whether a drop of water falling from a high altitude will kill you.)

Similarly, for space-based kinetic weapons, air resistance will limit their speed growth. According to simulation calculations, when air resistance is not taken into account, the entry speed of the "Rod of God" kinetic projectile can reach 8221.3m/s, which is more than twice the speed when air resistance is taken into account. This means that the kinetic energy of the projectile can reach more than four times the original speed when there is no air resistance.

Perhaps only when humans truly enter the universe will kinetic weapons be able to demonstrate their true power, as shown in various science fiction works.

References

[1]Rods from God | Popular Science (popsci.com)

[2] China successfully launches space-based kinetic weapon with astonishing power - Zhihu (zhihu.com)

[3]Kinetic bombardment - Wikipedia

[4] After tens of thousands of years of development, are humans ready to get rid of their obsession with "kinetic weapons"? - Zhihu (zhihu.com)

[5] Popular Science on APFSDS (I) Basic Concepts of High-Speed ​​Penetration of Long Rod Projectiles and the Alekseevskii-Tate Model - Bilibili (bilibili.com)

[6] Wang Mingyang, Qiu Yanyu, Li Jie, et al. Theoretical and experimental study on the penetration and ground impact effects of hypervelocity long-rod projectiles on rocks[J]. Chinese Journal of Rock Mechanics and Engineering, 2018, 37(3):9. DOI:10.13722/j.cnki.jrme.2017.1348.

[7] Li Zheng. Evaluation of the damage effect of the “Rod of God” space-based kinetic weapon[J]. Vibration and Shock, 2016, (18): 159-164,180.

Planning and production

Source: Institute of Physics, Chinese Academy of Sciences

Editor: Zhong Yanping