U.S. Penetration Aids

Penetration Aids

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

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:

  • The electronic jammers were unreliable due to persistent
    problems with batteries going “flat” while in missiles.

  • The US Navy did not want to have multiple types of missile
    configurations amongst their SSBN force, complicating training and

  • Missiles loaded with PX-1 suffered from a not-insignificant
    reduction in range. Offloading PX-1 restored the lost range.

  • The decoys, jammers, and chaff had been designed around the
    assumption that the Soviet ABM system(s) would be ZEUS-style
    threats. When hard data became available on the A-135 (ABM-1 GALOSH)
    system, in the words of one of the engineers who worked on the
    program: “they [the chaff] were all cut to the wrong
    frequencies, they [the decoys] were all too small to have been seen
    by these low frequency radars and they [the re-entry vehicles] were
    spaced improperly to accommodate the large yield weapons effects
    ranges of the big warheads. So other than that everything was just

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

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

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.

Range (medium grey) vs Post-Chevaline Range (dark grey) against

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

MK 6 Midcourse Decoy

MK 6 Re-entry Decoy Model

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

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

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