U.S. Penetration Aids

References Consulted:
From Polaris to Trident: The Development of US Fleet Ballistic Missile Technology by Graham Spinardi

PX-1 (Polaris A2)

PX-1 consisted of six decoys, chaff to be deployed in the mid-course phase, and electronic jammers for the early-re-entry phase. The PX-1 program began in 1961 with an award to Lockheed for penetration aids to be used with the Polaris Mk 1 re-entry vehicle. Twelve flight tests of PX-1 were carried out in 1962. From July 1963 to July 1964, 221 production sets were delivered to the US Navy.

For several reasons, the US Navy decided against widespread deployment of PX-1, with only a single SSBN loadout going to sea equipped with PX-1. Chief amongst these reasons were:

PX-2 (Polaris A3)

PX-2 consisted of several different types of decoys along with chaff. The earlier electronic jammers of the PX-1 program were deleted. Flight tests were conducted as part of the Polaris A3X tests from August 1962 to July 1964. It was decided not to produce or deploy PX-2.

Topsy (Polaris A3)

Topsy consisted of a new hardened version of the Polaris A3 missile; the A3T, which was developed to provide increased resistance against both direct (radiation) and indirect (EMP) effects from nuclear-tipped ABMs.

Exo-PAC (Polaris A3)

Begun in 1965 to defeat exo-atmospheric interceptors. One of the re-entry vehicles of Polaris A3 was replaced with a Penetration Aid Carrier (PAC). The PAC was jettisoned from the warhead bus along with the other two warheads like a normal warhead. After release, the PAC re-oriented itself and launched several solid-rocket propelled penetration aids into seven areas of space, where they would either become chaff or balloon-type decoys. After firing it's last penetration aid, the PAC would fire yet another solid rocket motor to move it into a different area of space than the re-entry bodies.

Mark-Up (Polaris A3)

This program was begun concurrently with Exo-PAC. It's objective was to harden the re-entry bodies against the effects of radiation.

Hexo (Polaris A3)

This program was created in July 1965 by combining both the Exo-PAC and Mark-Up programs.

Antelope (Polaris A3)

This program was created in October 1965 by combining Project Hexo and Topsy into a single program to produce a unified radiation hardened weapons system. Later, in September 1966, Project Impala was incorporated.

Impala (Polaris A3)

This program consisted of the development of endo-atmospheric penetration aids, or more colloquially, “twisters”. Unlike the balloon decoys of exo-atmospheric decoys, Project Impala's decoys would follow the re-entry vehicles into the atmosphere, long after the lighter decoys had fallen behind or burned up.

Super Antelope (Polaris A3)

Ultimate development of the Exo-PAC concept. Developed by the United Kingdom to extend the life of their Polaris deterrent force instead of buying Poseidon missiles. The basic concepts of the final Antelope configuration were retained and refined further, with the exception of the Impala-type endo-atmospheric decoys. Super Antelope's PAC could deploy 27 “hard” (dummy warheads) and “soft” (chaff) decoys. The system was renamed in 1974 to “Chevaline”, deployed on British SSBNs beginning in 1982, and taken out of service in 1996 with the introduction of Trident in British service.


Pre-Chevaline Range (medium grey) vs Post-Chevaline Range (dark grey) against Moscow.

Unfortunately, “Chevaline”-equipped Polaris A3TK missiles had their range reduced to just 1,950 nautical miles, compared to 2,500 nautical miles for the Polaris A3T, reducing the patrol areas open to British SSBNs from where they could still strike their primary target, Moscow. This was of great concern to the Ministry of Defence, as Soviet SSN capabilities and numbers were increasing with each year; and a reduction in range reduced the space in which their SSBNs could hide in.

MK 4 Vacuum Decoy 203451-501

MK 6 Midcourse Decoy 203449

MK 6 Re-entry Decoy Model 1033P

Avco Mark 6 Mod 2 (LB-1) Decoy -- ATLAS program

Avco/Hughes Mark 6 Mod 2 (LB-1A) Decoy -- ATLAS program