Terminal Phase Defense
Terminal phase is the last one or two minutes of ICBM flight. The warhead, along with any decoys or chaff, reenters the atmosphere. Aerodynamic drag then produces dramatically differing behavior for light as opposed to heavy objects. Decoys decelerate significantly and may burn up, but the warhead does neither. Thus at re-entry the defense can discriminate the warhead unambiguously.
On the other side of the coin, terminal defense presents severe challenges resulting from the very high speed of the offensive warhead and the very short time in which terminal defense can operate.
- Acceleration: The last pure terminal defense design, the Reagan-era High Endoatmospheric Interceptor (HEDI), required forward acceleration of approximately 200 Gs.
- Heat: A terminal defense interceptor reaches high velocity while still within the atmosphere. It therefore becomes so hot that it is difficult to achieve sufficient sensor function. HEDI used a sapphire window separating the sensor from the external environment, but that was insufficient. The subsequent Exoatmospheric/Endoatmospheric Interceptor (E2I) attempted to keep the window cool with a shroud of liquid nitrogen which would be removed a few seconds before the planned intercept. Though there was some limited success with this approach, it was not sufficient to make the E2I an attractive choice. If the E2I were to be revisited, this technical problem would still need to be addressed
- Footprint: Because of its late launch, a terminal defense can only cover a small area, probably a single U.S. urban complex. Coverage of larger areas would require firing interceptors before the warhead could be distinguished from the decoys, meaning many interceptors would be needed.
- Political: If terminal defenses are built for, for example, 25 urban areas, the 26th largest urban area would presumably become a prime target of whatever counter-population missile threat may exist. The mayor of this city, and of the next ten or so largest cities, could be expected to oppose such a terminal defense, as could the senators for their states. Wherever the line is drawn, this problem cannot be avoided.
The most likely terminal phase defenses are either one based on the Exoatmospheric/Endoatmospheric Interceptor (E2I) or a version of the ground-based mid-course defense redeployed as an early terminal-phase system.
The E2I program was terminated in 1993, though much of it was migrated to the THAAD theatre defense system. It was the successor of the High
Endoatmospheric Interceptor (HEDI) program and competed unsuccessfully against
the Ground Based Interceptor (GBI) as the interceptor for the NMD system. Chief
among its technical challenges was developing an infrared sensor system that
would work in the extremely hot environment of a rocket traveling at very high speed inside the atmosphere.
Deploying the ground-based mid-course NMD (currently under development) as a
terminal-phase system would involve other challenges. Assuming that the
currently used sensor suite, which is not designed for endoatmospheric operation, would be retained, the intercept would have to occur very high in the
atmosphere. However, there would also be an upper limit on the intercept
altitude in order for atmospheric filtering to be effective. This would
constrain the intercept to occur roughly 150-250 km above the earth. At this
altitude only one X-band radar would be able to view the threat complex, and
hence the accuracy to which the radar could locate both the warhead and the
decoys would not be particularly good. It would be difficult to make the target map passed to the interceptor sufficiently accurate to allow a successful intercept. It remains to be studied whether the X-band radar in concert with the SBIRS-Low system would be sufficient for that task.
Several countermeasures are available to combat a terminal-phase defense:
The administration's FY02 Budget Estimate devotes $988 million to terminal defenses, with the majority of the funds going to THAAD.
- Speed: Early re-entry vehicle designs used blunt shapes which caused them to decelerate significantly during re-entry. Modern re-entry vehicles are shaped like ice cream cones to minimize aerodynamic drag. While the primary purpose of high-speed re-entry is to improve accuracy, it carries the collateral benefit of reducing the duration of exposure to terminal missile defense.
- Maneuvers: It is possible to design a re-entry vehicle that will perform simple but unpredictable and intense maneuvers upon re-entry. All that is required is that the re-entry vehicle’s center of gravity and center of drag not line up along its trajectory. This can be done by using a slightly bent nose, a small fin at the rear, or an internal weight that is moved laterally during re-entry. In the 1970s the U.S. developed a maneuvering re-entry vehicle, the Mark 500, for the Trident 1 SLBM. Its tests were successful and included 200G maneuvers that would severely challenge any defense. The Mark 500 was not deployed because the Soviet missile defense system did not warrant it. Maneuvering re-entry vehicles of this type sacrifice some accuracy and payload, but for a rogue state attack these are probably not significant. Whether China or a rogue state could now equal such thirty year old American technology requires further study.
- Ladder down: A nuclear warhead exploding in the upper atmosphere would create a cloud of ionized gas that would be opaque to radar for several minutes. One tactic available to the offense would be to use such a precursor explosion to mask a following re-entry vehicle. The second re-entry vehicle would become visible after passing through the cloud, but the time remaining for the defense would be significantly reduced. Possibly a second, lower, precursor could be used to ensure penetration by the third re-entry vehicle.
Maintained by Michael Levi