From the deserts of Iraq and Syria to the forests and mountains of Korea to the bogs of Estonia, the US is obligated to defend numerous allies around the world in the event of military conflict. In the 20th century, this task was accomplished with nuclear deterrent. During this time, all of the US’s adversaries were states with recognized boundaries. If Warsaw Pact nations attacked a NATO ally with nuclear weapons, the US could respond with nuclear weapons. It was a simple and effective deterrent which managed to keep the Cold War from ever becoming hot.
Now, US military planners are back to the drawing board. In the 21st century, many of the US’s adversaries are not states but small groups of militants, who are not deterred by nuclear weapons: they are far too destructive to be of any use against such groups, as militants tend to blend in to the background population in small concentrations. Furthermore, the civilian casualties involved in any sort of a nuclear weapons employment are prohibitive in any scenario short of a last-ditch effort. In summary, the king of all deterrents is useless against the kinds of militant groups which the US now finds itself facing.
ICBMs are currently the only way to deliver a payload anywhere in the world in a matter of minutes or hours. But ICBMs are armed with nuclear warheads, which cannot be used against militant groups. As a result, US defense planners are in search of a weapons system that can quickly deliver a conventional (non-nuclear) payload anywhere in the world without risking US lives in invasions, airstrikes, or shorter-range missile launches. There are three main proposals for the program, called Conventional Prompt Global Strike (CPGS), all of which will be examined in this article.
Worth noting is that CPGS has uses apart from strikes against militant groups. One feature of all proposed CPGS systems is their ability to penetrate sophisticated air defense systems. While strike aircraft will always a radar signature and be at risk of interception, CPGS would be virtually impossible to defeat with existing technologies. The ability to rapidly strike a heavily defended target could have a number of uses. For example, if the US had intelligence that a nuclear weapons state such as Iran or North Korea was preparing a nuclear launch, the ability to strike the launch site rapidly could save millions of lives. Or, CPGS could be used to eliminate the assets defending the facility, allowing it to be raided and the materials to be confiscated. If a nuclear state is transferring a weapons shipment to an unpredictable actor, a CPGS strike could stop the transfer cold.
What follows is a brief account of each CPGS proposal:
Rods from God
Perhaps the most intriguing CPGS proposal is kinetic bombardment, referred to colloquially as “rods from God.” The mechanics behind kinetic bombardment are fairly simple: release a payload in outer space and it will accelerate to great speeds. The sheer kinetic energy attained by the time the weapon reaches earth would be enough to obliterate most targets, so no explosive warhead is needed.
In order to deploy such a system, a rocket would lift an orbiting launch vehicle equipped with a number of long rods into space. The rods would need to be long and slender to minimize friction on re-entry and maximize terminal velocity. They would also be heavy, because with increased mass comes increased kinetic energy. To achieve such mass in a relatively small package, the rods would likely be made of a dense metal such as tungsten. Because CPGS aims to minimize collateral damage, they would need to be guided in order to ensure they arrive at the correct location.
This setup is highly theoretical and mostly unfeasible, at least for now. Apart from occasional mention, kinetic bombardment has never gained serious traction. For one, it would be massively expensive. Developing new weapons systems is always costly, but kinetic bombardment would be in a whole new league. Not only would the rods themselves need to be carefully designed, but the launch platforms and guidance methods would be exceedingly costly to field. A kinetic bombardment system would also face significant opposition on the grounds of space weaponization, and the deployment of such a novel and dangerous system could prompt a space arms race.
Re-purposing Ballistic Missiles
A more realistic CPGS proposal involves utilizing Submarine-Launched Ballistic Missiles (SLBMs) or Intercontinental Ballistic Missiles (ICBMs) to deliver a conventional warhead. Compared to kinetic bombardment, such a solution would be simple and easy to field, as it would merely re-purpose existing ballistic missiles instead of requiring a whole new system. A ballistic missile-based CPGS solution would also be the least R&D intensive of all the proposals.
However, there are problems with this solution. For one, ICBMs and SLBMs usually carry nuclear warheads. As a result, Russia and China have no way of discriminating with 100% certainty between a CPGS missile or a real nuclear-armed missile, as they appear the same on early detection instrumentation. This means that a misinterpreted CPGS launch has the potential to unnerve nations who are able to observe it, and could result in tensions, or, at worst, a full-on retaliatory nuclear strike.
Some experts point out that the discrimination issue may not be as pertinent as it seems. Unlike missiles in a nuclear first strike, CPGS missiles would not be launched in large volume. If Russia or China observes a launch of 1-10 US ballistic missiles, they could assume the launch to be CPGS: a 10-missile nuclear first strike would be suicidal as it would leave the adversary’s nuclear weapons intact for retaliation. While the risk of misinterpretation may be relatively low, the consequences could be world annihilation, leading to well warranted hesitation regarding the deployment of conventionally equipped ICBMs and SLBMs. Indeed, the risk of adversary misinterpretation was enough to dissuade Congress from endorsing George W. Bush’s proposal to outfit Trident D5 SLBMs with conventional warheads.
Hypersonic Glide Vehicle
A Hypersonic Glide Vehicle (HGV) is the third and likely the most realistic proposal for the CPGS program. HGVs are streamlined vehicles mated to a propulsion stack, which ferries them to high altitude. Once they reach the threshold, the HGV detaches from the booster and glides at hypersonic (mach 5+) speeds toward its target. Test vehicles have reached mach 20; such speeds ensure that the HGVs would be impossible to intercept, at least for the foreseeable future. Unlike the other CPGS proposals, the HGV may not truly have global range, as the US’s current HGV testbed can only travel 5,000 miles.
The main issues surrounding a HGV are those of cost and developmental time frames. While China, Russia and the US are currently testing HGVs, none have produced a mature HGV, and it appears that significant issues remain. Some wonder whether a HGV is significantly less likely to cause an international incident. Indeed, an HGV would actually much harder to track and detect than a ballistic missile warhead. Combine that with the relatively maneuverable nature of the HGV, and other nations may fear that an HGV could be launched under non-threatening pretenses and then re-directed to strike targets on their soil. However, a CPGS HGV would benefit from the impossibility of misinterpreting its employment as a nuclear first strike, as there will be no HGVs fielded with nuclear weapons (for the foreseeable future).
While three CPGS proposals exist, significant doubts remain. For example, is CPGS worth the price? How often will it actually be needed? Can the US accurately detect CPGS targets prior to engagement? Does CPGS offer a significant improval over current delivery systems? Can CPGS be fit into the already stressed budget? While the concept of CPGS is appealing to US leaders, such questions will need to be answered before any serious work can be undertaken. Indeed, many aspects of the CPGS program are contested, and there are indications that the “Global” aspect of CPGS may be up for debate as well. Challenges notwithstanding, CPGS development will likely plod along for the time being (with funding below $100 million), until some sort of a decision is reached and guidelines are established.
Much of the information contained in this article derives from the following documents: