Trident II D-5 Fleet Ballistic Missile
The TRIDENT II Weapon System was to be an evolution of the TRIDENT I. However, going back to be an advanced technology missile capable of 4000 nm range when carrying a similar payload as the POSEIDON (C3) would carry to approximately 2000 nm. It was also constrained to fit in the submarine’s circular cylinder launch tube which contained the C3. Thus, the new C4 missile could be used in then-existing submarines (e.g., approximately 74 in. in diameter and close to 34 ft in length). In addition, the accuracy of the new C4 missile system was to be equivalent at 4000 nm to that of the POSEIDON C3 at 2000 nm. To satisfy this range requirement, a TS boost propulsion stage was added to C4 to increase range along with propellant improvements and reduction in inert missile weights. Developments in the guidance system was the major contributor to maintaining accuracy.
The Trident II’s missile footprint is 3 times smaller than the MX Peacekeeper’s footprint. The footprint is the area in which the Multiple Independently-targetable Reentry Vehicles (MIRVs) can land. This creates limitations on the Trident force when planning targets. The smaller footprint is a result of the solid-fueled Post Boost Vehicle (PBV) in the Trident. MX has a liquid-fueled PBV.
Now that the new bigger TRIDENT submarine was available for the TRIDENT II (D5), the additional space could be considered in the missile design. Moreover, with the possibility of a bigger and associated improved performance, a larger payload could be incorporated. Using the concepts from the IAP, improvements could be developed for the subsystems of the weapon system to provide the desired improved accuracy, all leading to hard-target capability.
Thus, with the larger submarine, the TRIDENT II (D5) Weapon System became an evolution of the TRIDENT I (C4) system with technology improve ments in all subsystems of the weapon system: missile (guidance and reentry system), fire control, navigation, launcher and test instrumentation (non-tactical) subsystems, resulting in a missile with additional range, improved accuracy, and heavier payload.
The TRIDENT 11 (D5) is an evolution of the TRIDENT I (C4). Generically speaking, the D5 looks like the C4, only bigger, to provide for additional thrust and increased payload capability. The D5 is 83 in. in diameter versus 74 in. for C4, and 44.6 ft in length versus 34.1 ft for C4. Both missiles taper to 81 in. and 71 in., respectively, forward of the second stage motor.
The missile consists of a first stage section, an interstage [IS] section, a second stage section, an Equipment Section [ES], a Nose Fairing [NF] section, and a nose cap section with an aerospike. There is no adapter section like there is on C4. The D5 ES, along with containing all the guidance and electronics, performs the same function as the ES-adapter section in C4 (e.g., structural support between the aft end of the NF and the forward end of the second stage motor).
The first stage and second stage motors are also primary structural components of the missile, connected by an Interstage (IS). Forward of the second stage motor, the adapter section structure of the C4 has been eliminated in D5, and the equipment section (ES) has been extended to serve as the adapter section plus ES. The third stage motor is mounted within and to the ES similar to C4. Structural bracketry on the forward part of the ES is modified from C4, in order to accommodate the bigger Mk 5 reentry vehicle or, with added fixtures, a payload of Mk 4 RBs.
The first stage section includes the first stage rocket motor, TVC system, and the components to initiate first stage ignition. The IS section connects the first stage and second stage sections and contains electrical and ordnance equipment. The second stage section includes the second stage rocket motor, TVC system, and components to initiate second stage ignition.
When compared to C4, for the D5 to achieve the longer range with its larger, heavier payload, improvements in rocket motor performance would be required plus reductions in the weight of the missile’s components. To improve rocket motor performance, there was a solid-propellant change. The C4 propellent carried the name of XLDB-70, translated to, cross-link double-base70 percent solid fuels. The solids consisted of HMX (His Majesty’s Explosive), aluminum, and ammonium perchlorate. The binder of these solids was Polyglycol Adipate (PGA), Nitrocellulose (NC), Nitroglycerine (NO), and Hexadiisocryanate (HDI). This propellant could have been called PGA/NG, when we consider that D5 propellant is called Polythylene Glycol (PEG)/NG. D5 is called this because the major innovation was the usage of PEG in place of the PGA in the binder. It was still a cross-link, double-base propellant. The use of PEG made the mixture more flexible, more rheologic than the C4 mixture with PGA. Thus, the D5 mixture being more flexible, an increase could be made in the amount of solid fuels; increased to 75 percent solids resulting in improved performance. Thus, D5 propellant’s is PEG/NG75. The Joint Venture (the propulsion subcontractors, Hercules and Thiokol) have given a trade name to the propellant NEPE-75.
The motor case material on the D5’s first stage and second stage became graphite/epoxy versus the Kevlar epoxy of C4, an inert weight saver. The TS motor was to be Kevlar epoxy but, midway through the development program (1988), it was changed to graphite/epoxy. The change was a range gainer (reduced inert weight) plus eliminated any electrical static potential associated with Kevlar and graphite. There was also a change in all D5 rocket motor nozzles’ throat material from segmented rings of pyrolytic graphite in the entrance and throat of the C4 nozzle to a one-piece integral throat and entrance (ITE) of carbon-carbon on D5. This change was for reliability purposes.
The Equipment Section [ES] houses the major guidance and flight control electronics packages. The TS rocket motor and its TVC system are mounted to an eject cylinder at the center of the ES and extends forward of the ES. A small TS eject motor is recessed in a cavity on the TS motor forward dome. When the TS motor is expended, the eject motor pushes the TS motor aft, out of the ES to effect TS separation. The Equipment Section was integrated with the adapter section, using graphite/epoxy versus the aluminum composite structures on C4. This was a weight saver, providing a range gainer. The IS did not change, conventional aluminum. The ES mounting for the third stage rocket motor is similar for both the C4 and D5 with an explosive zip tube used for separation, and the third stage motor has a similar eject rocket motor on the forward end of the rocket motor.
The NF section covers the reentry subsystem components and the forward portion of the TS motor. The NF section consists of a primary structure with provisions for two jettison rocket motors and a locking mechanism. The nose cap assembly at the forward end of the NF houses an extendable aerodynamic spike.
The D5 missile has the capability of carrying either Mk 4 or Mk 5 reentry vehicles as its payload. The D5 reentry subsystem consists of either Mk 4 or Mk 5 reentry vehicle assemblies attached by four captive bolts to their release assembly and mounted on the ES. STAS and pre arming signals are transferred to each reentry vehicle shortly before deployment through the separation sequencer unit. When released, the reentry vehicle follows a ballistic trajectory to the target where detonation occurs in accordance with the fuze option selected by fire control through the preset subsystem.
The reentry vehicle contains an AF&F assembly, a nuclear assembly, and electronics. The AF&F provides a safeguard to prevent detonation of the warhead during storage and inhibits reentry vehicle detonation until all qualifying arming inputs have been received. The nuclear assembly is a Department of Energy (DoE) supplied physics package.
Both C4 and D5 ES PBCSs are similar except C4 had only two simultaneously burning TVC gas generators, whereas D5 has four TVC gas generators. There are two “A” generators which burn initially and provide thrust to the ES, using integrated valve assemblies (IVAs). When the gas pressure drops in the “A” generators due to burnout, the “B” generators are ignited for the remainder of the ES flight.
The post-boost flight of the C4 and D5 ES and reentry vehicle releases are different. With C4, upon completion of the TS rocket motor burn and separation, the PBCS positions the ES, which is maneuvered in space to permit the guidance system to conduct its stellar sightings. Guidance then determines any flight trajectory errors and issues corrections to the ES flight path in preparation for reentry vehicle deployment. The C4 ES then enters a high-thrust mode, the PBCS driving it to the proper position in space and correct velocity for reentry vehicle deployment. During the high-thrust mode, the ES flies “backwards” (RBs face aft to the trajectory). When the correct velocity for reentry vehicle release is obtained, the C4 ES goes into a vernier mode. (ES is adjusted so that the reentry vehicle will be deployed at the proper altitude, velocity, and attitude.)
Upon completion of each reentry vehicle drop, the ES backs off and moves to another position for subsequent reentry drops. During the backing off, gas plumes from the PBCS will impact on reentry vehicles differently, causing reentry vehicle velocity deltas.
In the case of D5, the ES uses its PBCS to maneuver for stellar sighting; this enables the guidance system to update the original inertial guidance as received from the SSBN. The flight control system responding to guidance reorientates the D5 ES and enters a high-thrust mode. However, in the D5 case, the ES flies forward. (RBs are basically down the line of the trajectory.) As in C4, the D5 ES (when it reaches the proper altitude, velocity and attitude) enters the vernier mode to deploy RVs. However, to eliminate the PBCS plume from impacting the reentry vehicle upon release, the ES undergoes a Plume Avoidance Maneuver (PAM). If the reentry vehicle to be released will be disturbed by a PBCS nozzle’s plume, that nozzle will be shut off until the reentry vehicle is away from the nozzle’s plume area. With a nozzle off, the ES will react to the other three nozzles automatically. This causes the ES to rotate as it backs away from the reentry vehicle just released. In a very short time, the reentry vehicle will be beyond the influence of the plume and the nozzle is returned to normal operation. PAM is used only when a nozzle’s plume will disturb the area around an RV. This PAM was one of the design changes to the D5 to provide improved accuracy.
Another design change to help improve accuracy was to the nosetip of the Mk 5 RV. In the TRIDENT I (C4) missile, an error condition existed in some cases upon reentry into the atmosphere when the nosetip ablated at an uneven rate. This caused the reentry vehicle to drift. As the design of the Mk 5 reentry was developed, the change to a shape stable nosetip (SSNT) was established. The nose of the Mk 4 reentry vehicle was boron carbide-coated graphite material. The Mk 5 nose has a metallated center core with carbon/carbon material, forming the rest of the nosetip (“plug”). The metallated center core will ablate at a faster rate than the carbon/carbon parent material on the outer portion of the nosetip. This will result in a blunt, more-symmetrical shape change with less of a tendency to drift and, consequently, a more-accurate and more-reliable system. Prior testing of SSNTs on some C4 missile flights had verified the design concept.
In TRIDENT I (C4), the flight control subsystem converted data signals from the guidance subsystem into steering and valve commands (TVC commands) moderated by missile response rates fed back from the rate gyro package. In TRIDENT II (D5), the rate gyro package was eliminated. The D5 flight control computer receives these missile response rates from the guidance system inertial measuring unit (IMU) as transmitted through the guidance electronic assembly (EA).
The more-extensive use of composites in D5’s structure provided inert weight savings. Redesign of ordnance system D5-versus-C4, although functionally the same, in particular the separation ordnance to “cut” structure, contributed to weight savings.
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