Force Structure Studies: The Phoenix Study

The Phoenix Study


Only a few weeks before President Bush in September 1991 announced sweeping reductions in US nuclear forces, Strategic Air Command (SAC) completed the Phoenix Study. It was the last force structure study conducted by SAC, which was disbanded and merged into US Strategic Command (STRATCOM) in June 1992.

The Phoenix Study, named after its classification level of Secret/Phoenix Only, attempted to analyze central issues of nuclear war planning: who should be targeted; what targets should be held at risk; how many aimpoints; what quality of weapons is needed; how many weapons; any special requirements; how many reserve weapons; and is it necessary to “hedge” against an uncertain future? The answers to these questions formed the basis of all the other force structure studies conducted in the 1990s and continue to shape the preparations for the 2001 nuclear posture review by the new Bush administration.

The Phoenix Study established “rules of thumb,” based on historical targeting data, for the calculation of the number of weapons required to defeat a given number of installations (targets). The study had roots deep in the Cold War and essentially summarized SAC’s experience from more than 40 years of nuclear planning. Although the details of these rules of thumbs were deleted from the study before it was released under FOIA, the remaining unclassified sections contain sufficient information to give an idea of how the number of targets in the war plans translate into number of warheads to achieve a certain degree of damage to individual targets. This calculation involves four steps in response to the guidance issued by the president or secretary of defense:

Guidance Target Development Probability of Arrival (PA) Aim Points (Desired Ground Zero) Probability of Damage (PD)

Because some warheads will fail to reach their target because of issues such as technical malfunction, prelaunch survivability, local defenses, and adverse whether conditions, the Phoenix Study prescribed an unclassified rule of thumb of 20 warheads per 8 targets to ensure sufficient Probability of Arrival (PA). The number of warheads per target was different for each type of weapon system, and bombers were considered three times more vulnerable than ballistic missiles and therefore require more launch platforms to do the same job. By combining PA with target characteristics such as hardness and proximity to other targets, the number of aimpoints needed to guarantee destruction of each target or target group was calculated. Although some aimpoints in special cases may be assigned more than one warhead (layered targeting), the assumption was that each aimpoint — called Desired Ground Zero (DGZ) — required one warhead. Each target may have more than on DGZ depending on hardness and geographical size. In the table below these rules of thumbs have been used to “calculate backwards” to estimate the ratio between number of targets and warheads:

Warhead to Target Ratio*
Warheads Aimpoints

START I 6000 2400 2500-3500
START II 3500 1400 1450-2000
START III 2500 1000 1050-1430
START IV 1500 600 630-860
START V 1000 400 420-570

* Based on 1991 Phoenix Study example of 20 warheads per 8 aimpoints. This ratio only reflects probably of arrival, not whether desired damage will be achieved.
**There are more warheads than aimpoints to compensate for the fact that some warheads will fail to reach their targets for various reasons. Others warheads are held in reserve.

*** There are more installations than aimpoints because targeting involves grouping installations in the National Target Base (NTB) into aimpoints where the minimum number of weapons (even a single warhead) will achieve guidance-directed Probability of Damage (PD) against individual installations or groups of installations.

Another important feature of the Phoenix Study is its portrayal of strategic submarines as playing a much more prominent role in the strike plans than their normal image of being mainly a retaliatory second-strike force held in reserve. The study states that the secure reserve force “handles contingencies” and provides only a “limited restrike capability.” Instead the SSBNs are described as one of the two main pillars in the Triad — comparable to that of the ICBM force, which has traditionally been the backbone in the offensive nuclear strike force. In fact, the Phoenix Study only sets aside 25 percent of the SSBNs for the strategic reserve force. This change is a result of the dramatic improvement in the capability of sea-based ballistic missiles, and of the bomber force becoming less prominent in the posture. The result is described in the study as a Twin Triad posture based on the two ballistic missile legs as forming the main thrust of the nation’s deterrent, with the bombers mainly providing back-up. Indeed, the Phoenix Study concludes that the Secure Reserve Force, which is mostly SLBMs, “is not a hedge” and that the Twin Triad concept “places the day-to-day deterrence burden on the two ballistic legs.”
The resulting START I ratio of 6,000 warheads for 2,400 aimpoints roughly fits unofficial estimates of some 2,500-3,000 targets in today’s SIOP. As the number of targets in Russia continue to decline to perhaps fewer than 500 under START III, the number of aimpoints — and therefore required warheads — also decrease for this portion of the SIOP. Because of this trend, the importance of China and other potential enemies on the overall targeting matrix will increasingly influence the composition of the nuclear posture and the war plans. Targets identified by military planners in response to vague presidential guidance will therefore have a proportionally greater impact on defining future limits on nuclear cuts. Conversely, if the guidance becomes more precise and requirements for damage probability and degree of destruction are eased, a “credible” deterrence could be maintained with much fewer weapons.

The Phoenix Study also established many of the arms control and force planning principles that later influenced the START II agreement and the 1994 Nuclear Posture Review and continue to guide force planning even today. To that end the study is an interesting comparison of how much of SAC’s nuclear planning principles were inherited and perpetuated by its successor, STRATCOM. These principles include:

  • The Soviet Union (and later Russia) remains the only nation capable of destroying the US; “Handle the Soviet Union and you can deter all other potential threats;”
  • The Triad will be more important in the future because: (1) fewer warheads on fewer delivery vehicles; and (2) fewer types of both warheads and delivery vehicles;
  • The creation of a Twin Triad, a force structure consisting of ICBMs and SLBMs with bombers acting primarily as a backup to failure of either of the two;
  • The Twin Triad concept uses the bombers to augment attacks by ICBMs and SLBMs and then is sized to be used in a real hedge role for the first time against the failure of one of the ballistic legs;
  • Maintain a “reserve” of inactive nuclear weapons that can relatively quickly be reconstituted onto the operational force;
  • To “hedge” against uncertainty in the developments in the Soviet Union;
  • Maintain nuclear force capability so that allies don’t see a need to deploy nuclear weapons.


PDF Version: “The Matrix of Deterrence”


Linked to the above heading is the summary of all 6 force structure studies, including the Phoenix Report.