A 47 page Congressional Research Bureau report Conventional Prompt Global Strike and Long-Range Ballistic Missiles: Background and Issues was written by Amy F. Woolf, Specialist in Nuclear Weapons Policy.
Navy Reentry Vehicle Research
In 2009, there was the Medium Lift Reentry Body program. It would be too large to fit on a Trident missile, but could carry the warhead on the intermediate range submarine-launched ballistic missile described below. It would carry a tungsten-rod (“flechette”) warhead, which would be designed to destroy area targets such as airfields and military bases.
Conventional Trident Modification
The Navy began to speak publicly about its plans for the conventional Trident modification (CTM) in early March 2006. Under this concept, the Navy planned to deploy each of its 12 Trident submarines on patrol (two would be in overhaul at any given time) with two missiles equipped to carry four conventional warheads each. The remaining 22 missiles on each submarine would continue to carry nuclear warheads, and the submarines would continue to patrol in areas that would allow them to reach targets specified in the nuclear war plan, although the patrol areas could be adjusted to accommodate targeting requirements for the CTM.
Kinetic Energy Warheads
The Navy considered two types of warheads for the CTM program in the near term. One warhead would be designed to destroy or disable area targets like airfields or buildings, using a reentry vehicle loaded with tungsten rods—known as flechettes—that would rain down on the target and destroy everything within an area of up to 3,000 square feet. The other might be able to destroy hardened targets, like underground bunkers or reinforced structures, if it were accurate enough to strike very close to the target. Each would be deployed within the reentry body developed and tested under the E2 program. The Navy also explored, for possible future deployment, technologies that might be able to penetrate to destroy hardened, buried targets
The two primary advantages of a kinetic energy rod warhead is that 1) it does not rely on precise navigation as is the case with “hit-to-kill” vehicles and 2) it provides better penetration then blast fragmentation type warheads.
A 6.1 m × 0.3 m tungsten cylinder impacting at Mach 10 has a kinetic energy equivalent to approximately 11.5 tons of TNT (or 7.2 tons of dynamite). The mass of such a cylinder is itself greater than 9 tons, so it is clear that the practical applications of such a system are limited to those situations where its other characteristics provide a decisive advantage—a conventional bomb/warhead of similar weight to the tungsten rod, delivered by conventional means, provides similar destructive capability and is a far more practical method. Some other sources suggest a speed of 36,000 ft/s (11,000 m/s), which for the aforementioned rod would amount to a kinetic energy equivalent to 120 tons of TNT or 0.12 kt. With 6–8 satellites on a given orbit, a target could be hit within 12–15 minutes from any given time, less than half the time taken by an ICBM and without the warning. Such a system could also be equipped with sensors to detect incoming anti-ballistic missile-type threats and relatively light protective measures to use against them (e.g. Hit-To-Kill Missiles or megawatt-class chemical laser)
There is a Raytheon patent Kinetic energy rod warhead with optimal penetrators
Project Thor Orbital Rods from God
Project Thor was an idea for a weapons system that launches kinetic projectiles from Earth's orbit to damage targets on the ground.
At speeds of at least 9 kilometers per second. Smaller weapons can deliver measured amounts of energy as large as a 225 kg conventional bomb. Some systems are quoted as having the yield of a small tactical nuclear bomb. These designs are envisioned as a bunker busters.
SOURCES - FAS Congressional Report, Wikipedia