Trident I C-4 FBM / SLBM Features
To achieve a 4000 nm range [ versus the 2500 nm range of the POSEIDON C3] the Trident I (C4) is a three-stage solid-propellant missile with basically the same envelope dimension as a C3 (e.g., 34.1 ft in length and 74 in. in diameter), limited by the space available in a POSEIDON SSBN launch tube. There was a weight increase to approximately 73,000 lb. There was an increase in the C4's Nose Fairing [NF] envelope, compared to C3, to allow introduction of a solid propellant Third Stage [TS] booster in the center of the ES/NF. Each of the three stages has a boost rocket motor with advanced propellants, improved case materials, and a single lightweight movable nozzle with a TVC system of lightweight gas-hydraulic design.
Boost velocity control is achieved by burning all boost propulsion stages to burnout, shaping the trajectory to use all the energy, without thrust termination. This method is termed generalized energy management steering (GEMS). The ES is powered by a solid-propellant PBCS. Miniaturizing and repackaging missile electronic components also contributed to reduced package sizes, weights, and calibration, thereby allowing more volume for propulsion.
In the missile electronics, improved system accuracy was achieved by incorporating a stellar-inertial guidance concept, by improving the Navigation and Fire Control systems, and by more accurate control of reentry vehicle separation. Inert weights were reduced with structures fabricated from composite graphite-epoxy materials which represent 40 percent weight saving compared to similar structures made from aluminum.
The largest contribution to attaining the range increase goal came from incorporating a third boost propulsion stage. To fit within the same cylinder as the POSEIDON this third stage motor was to be mounted in the center of the post-boost vehicle with the reentry vehicles carried around the third stage.
The strategy adopted to achieve the remainder of the range goal was to pursue range gaining technologies in the following general ways all in parallel: decrease inert weight throughout the entire missile, increase the volume available for propulsive energy, and increase the usable energy per unit volume. This strategy resulted in efforts directed to developing a smaller and lighter guidance system, lightweight missile structures, low volume and lightweight electrical and electronic components, smaller or lighter post-boost control system, an aerospike to reduce boost phase aerodynamic drag and, most importantly, higher performance rocket motors. In order to withstand reentry heating at long ranges and higher ballistic coefficients, new protection materials needed to be developed for the reentry vehicles.
The range extension dictates for weight reduction were complicated by the unique reentry vehicle placement around the third stage which made thrust termination difficult to engineer. And in introducing a third stage of boost propulsion and making maximum use of the available launch tube volume, the missile nose shape became so much blunter that aerodynamic drag during boost could have significantly detracted from meeting the range increase goal. It therefore became important to reduce boost phase drag.
A deployable aerospike, extended shortly after launch, was incorporated to reduce the frontal drag of the C4 NF by approximately 50 percent. The aerospike is self-contained and requires no functional interface input from other missile subsystems. A small solid propellant gas generator provides the energy to extend and lock the aerospike into position. Its ignition is triggered by acceleration of the missile on ejection from the submarine. This unique feature, utilized for the first time on a ballistic missile, was adopted to offset the aerodynamic drag and performance degradation of the unusually blunt nose fairing. A concentrated effort to reduce the Mk 4 reentry vehicle weight as much as possible was also conducted.
The remaining major technical challenge to achieving the range increase objective was the development of solid propellant rocket motors incorporating technological advancements in both propellants and inert components. In recognition of the importance in the throat, carbon-carbon entrance and exit segments and either carbon or graphite cloth phenolic in other areas. An omnidirectional flexible joint enables movement required for thrust vector control.
Reentry system design objectives included more than doubling the maximum range at which the reentry vehicle with its high ballistic coefficient (weight-to-drag ratio) could reliably withstand reentry heating without significant weight increase. The major technical issues involved in meeting this objective were those of materials technology. Several alternative design concepts for the nosetip, heatshield, and substrate materials were examined in parallel during the early stages of development. A highly successful supplemental flight test program carried out in 1974 and 1975 with surplus Atlas and Minuteman missiles helped in the early selection of materials and design concepts.
The reentry body has a tape-wrapped carbon phenolic (TWCP) heatshield bonded to a thin-wall aluminum substrate for the shell and a graphite nosetip. The TWCP is similar to material previously used by the Air Force for reentry bodies, but with the carbon particles eliminated. It is made from a carbonized rayon cloth, wrapped on a mandrel, and cured in a female mold. The TWCP ablates during a reentry, leaving at least a minimum amount of cool material intact to impact. The graphite is a fine-grain graphite, especially developed for strong and uniform properties. So critical was graphite quality, and so difficult to inspect the end product, that a separate factory, a computer controlled facility, was built for its exclusive production where processes could be completed controlled.
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