Orbital Manfucturing Making a Literal Impact

One of the more terrifying science fiction sounding weapons that have been proposed by various engineers and military thinkers is the use of hypervelocity rod bundles, often referred to as “Rods of God”. Where a large cylinder of tungsten is placed in Earth orbit waiting for a call to action. When called up, the rod begins to fall from the sky achieving a maximum velocity of roughly  Mach 10. A group of 100 rods each about 1 ft across and 20 ft long would have the same kinetic energy as 1 kiloton of TNT, effectively a small nuclear attack anywhere on the planet. Fortunately, or unfortunately, depending on who you are, the cost of transporting over 8,000 kg of tungsten into space exceeds the military value of having such a weapon. As orbital manufacturing becomes more mainstream the launch cost barrier may not last as long as many would hope.

As of March 2024, dozens of companies of various sizes, and feasibility, have announced aspirations to perform orbital manufacturing. Currently most realistic firms are focused on small scale manufacture of specialty products ranging from medicinal compounds to fiber-optic glass. As time goes on the mass of materials and variety of what can be built in orbit using automated technologies will only grow. These expansions of capabilities of orbital fabrication will represent a potential watershed moment in the tactical viability of hyper-velocity orbital weapon systems (HOWS). A HOWS based on the use of monolithic munitions represents an incredible expense for an organization to perform a large number of high payload launches to provide sufficient coverage to be tactically effective.

In 2018 tungsten had a cost of roughly $30,000/ton, additionally each kilogram you send to orbit has a cost between $1000-2,500/kg (variation depends on a lot of other factors), this puts the conservative cost of a “classic” “hyper-velocity rod bundles”, would be between $8.6 million and $21 million dollars, with the majority of the cost coming from getting that mass into orbit. That direct capital expenditure must be bundled with the opportunity cost of a narrowly defined rocket launch. Could we do cheaper?

Theoretically yes.

As noted earlier, we are currently at the very start of what orbital manufacturing industry(ies) will become. Bulk metal fabrication on orbit has the potential to drastically reduce the cost of the deployment of HOWS. Instead of launching up a singular payload an organization could build an orbital assembly platform intended to convert relatively raw materials into more purpose-built devices. With an orbital platform serving as both a depot and an assembly point there is tremendous opportunity to reduce the per unit cost of a hyper-velocity rod. At minimum assembly in orbit of sub-sections of tungsten into a larger assembled rod would allow for each device to be built over a period of months/years, meaning that mass could be added when there was unused mass on launch vehicles as opposed to purpose built launches.

Where deeper capabilities will get more significant and also increase the likelihood of some kind of orbital bombardment system will come as larger terrestrial launch vehicles become more available. Analysts have suggested that if SpaceX’s Starship is successful, payloads of 200 tons could be sent into low earth orbit for less than $50/kilogram. At those prices you might imagine that the suggestion of orbital fabrication makes no sense, but even in the world of military spending cost considerations still matter. While tungsten is a “cost saving” for launches that cost hundreds if not thousands of dollars per kilogram, at prices less than $50/kilogram, the ability to use cheaper materials to produce the device is worthwhile.

Depending on other manufacturing technologies outside of the scope of this paper, either bulk raw metals, or small building blocks of metal would be sent up on a bulk transport for orbital assembly. Once those materials are deposited at their assembly point they would be built into the reentry platform.

As an example,

Engineers determined that they wanted to achieve the same ballistic coefficient as the Air Force’s Tungsten Hyper-velocity Rod Bundles with stainless steel.

Stainless steel and thermal insulation modules are sent into orbit along with other routine resupplies from Earth. Upon receiving the modules the orbital assembly system begins compiling them into a large cylinder, starting with a central core of steel that is eventually wrapped in thermal protection materials to reduce how much of the rod breaks down during re-entry. While the original rods were proposed to be 20 feet (6.1 meters), these steel rods will need to be much longer to compensate for their lower density. At about 15 meters in length the rod has sufficient mass to achieve the same ballistic co-efficient of the tungsten rod. When called upon these systems will be able to rain hell.

Authors note, I don’t think we should do this, I just realize that it is something that can be done and I don’t think that anyone will be saved if I didn’t write this blog post.

Additional notes. Assembling a rod geometry is not the only approach, honestly if you are able to build such platforms, it is likely that other shapes and solutions would also be considered, no need to only have a single method of attack if you don’t have to.

It does not escape me that the military industrial complex may use a concept similar to this to increase the motivation for mining the moon and asteroids. While rocks or raw ore from orbit are relatively crude, they still do tremendous damage, the world needs to be cautious with how we develop these resources.

Wikipedia Reference: https://en.wikipedia.org/wiki/Kinetic_bombardment

Launch costs https://spaceimpulse.com/2023/08/16/how-much-does-it-cost-to-launch-a-rocket/

Tungsten price https://www.metalary.com/tungsten-price/

Supposed launch cost of starship: https://www.nextbigfuture.com/2024/01/how-will-spacex-bring-the-cost-to-space-down-to-10-per-kilogram-from-over-1000-per-kilogram.html $10-25/kg