The UD-4L "Cheyenne" Utility Dropship is a trans-atmospheric combat transport aircraft that is primarily used by the United States Colonial Marine Corps to carry troops and provisions from orbit to planetary surfaces and back.
Considered a versatile dropship and tactical transport, the Cheyenne was employed by the United States Colonial Marine Corps. Derived from an original Department of Defense requirement formulated at the end of the Tientsin conflict for a multi-role light aerospace shuttle capable of lifting heavy payloads up to 16,000 kg.
The Cheyenne's unique flexibility comes from its ability to lift itself into orbit under its own power from unprepared landing sites with the aid of its vertical take-off (VTOL) capability. In addition to carrying its large payload, the Cheyenne can operate in the close-support gunship role by deploying weapons pods and hardpoints for rockets and missiles, as well as using its own internal gun.
The UD-4L Cheyenne's lifting-body is built around it's 103.6 cubic meter internal payload bay flanked to the sides and aft by the the tri-skid undercarriage. Forward of the payload space is the cockpit and the mounts for the lift engines. Aft of the space is a raised tail assembly which mounts the ramrocket engines and control surfaces.
The Cheyenne's principle frame is constructed from superplastic-formed diffusion-bonded metal matrix composites (MMC). These light, oxidation-resistant MMC structural members are formed from high-modulus, high-strength gold and chrome doped zirconium (Zr) oxide reinforced fibers in a titanium aluminide matrix. They form a structural web running the length of the fuselage encompassing the payload volume. Spars and members attached to the main fame are constructed from titanium aluminide MMC and refractive composites with titanium root fittings to transfer bending moments to the structural web. The fuselage skinning comprises of tri-layer plates attached to the titanium aluminide frame. The inside layer is a carbon-carbon (C/C) composite (graphite fibers in a carbon matrix) bonded to a middle layer of single-crystal carbon, as it can effectively conduct excess heat away from the leading edge surfaces, therefore preventing the layer from melting on re-entry. The ceramic outer layer also provides thermal and oxidation protection in the high-altitude high speed realm.
To either side of the forward payload space, the structural web extends outwards to form the static load points for the undercarriage, the fuel tank volume and the mounts for the secondary weapon bays. Aft of the payload space is the huge rear skid strut and the fuel tankage for the ramrockets. The UD-4L undercarriage is a tri-skid arrangement in which the skids retract flush with the underside of the fuselage. The ramrocket engines are mounted above the main fuselage module, their intakes accepting the airflow across the upper fuselage. The aft fuselage assembly occupies the space between the engines and extends rearward to the butterfly control surfaces. This volume contains the Fire Control Jamming Suite towed unit and the oxidant tanks for ramrocket operation during exoatmospheric flight, with their associated cryogenic storage equipment. Just aft of the main engine nozzles are a pair of flush fitted extending airbrake panels which can also be deployed during re-entry for transonic and hypersonic stability. The butterfly control surfaces are designed to provide control authority in all axes at all speed regimes. They also supply tail lift at low speeds when dynamic pressure on the underside of the dropship lifting body drops and shifts the static center of lift forwards.
The turbine engines are mounted above the forward fuselage, either side of the cockpit access corridor. Multi-plate variable compression ramps feed air into the canted engines. Thrust from the plenum chambers is fed aft to the swivelling vertical take-off nozzles and forward to a single lifting nozzle between the nose.
The main fuselage also features the mounting points for the main weapons pods and the secondary weapons bay. The main weapons pods are attached to cross-folded pylons just forward of the ramrocket intakes, which at supersonic speeds, the 4.4 m pylons can be deployed crosswise to secure the ordnance within the pods. The total span of the pods when deployed is 15.3 m. The pods cannot be deployed at speeds above transonic because of the adverse effects of drag and the torsion caused by dynamic pressure on the pylons. The secondary bays also fold flush against the sides of the lifting body, and can be sprung out to expose all the weapon hardpoints and allow exhaust space for weapons launch. Unlike the main weapon pods, the secondary bays can be deployed at super-sonic speeds up to Mach 2.4 without adverse effects on dropship handling.
The Cheyenne's payload bay is a 9.5 l x 4.5 w x 2.4 h meter (102.6 m3) volume with a 3.92 meter wide deck ramp suspended from four dual-hydraulic assemblies. The deck ramp can comfortably carry a fully-crewed M577 APC (with turret stowed) or a HALOS stores pallet, and is able to raise the cargo completely into the payload space from ground level. Within the bay, latches are automatically activated, extending to hold cargoes in place when the deck is raised. A 20 cm cavity to either side of the payload bay separates the cargo volume from the outer skin and contains the main structural members, cable runs and the blower pipes from the forward turbines to the aft lift nozzles. Aft of the payload bay, a step gantry can be lowered to the port side to allow crew access. Forward of the bay, a small volume accommodates three seats for passengers and additional crew.
The spacious pressurized cockpit is accessed from the payload bay and features two crew positions, seated in tandem. Both crew sit in Martin-Siekert R2102 zero-zero ejection seats which are cleared for operation at any altitude below 10,000 m and speeds below Mach 1. In the event of an emergency, explosive cord blows the canopy off and the crew are ejected clear of the ship. Canopy transparencies are made from single-crystal quartz, flash coated with gold, germanium, molybdenum and iridium to provide protection against bright light and short-wavelength lasers. The coatings also act as a radar reflecting surface, preventing the entire cockpit volume from becoming a radar reflecting cavity.
At subsonic speeds the lifting body configuration generates little lift and the pilot becomes increasingly reliant on the flight software and lift from the vectored thrust engines to deep the dropship stable in the air. Stall speed is very high, and as the Cheyenne approaches the stall it tends to fly increasingly nose-high. As transition is made through the stall speed, vertical lift from the nose and stern nozzles are bled in to prevent departure. Though the airframe is nominally stressed to +6 g, maneuvers in conventional flight greater than +3 g are prohibited due to the excessive stall speed, which can cause the Cheyenne to prematurely depart controlled flight. When fully loaded, turns greater than +1 g are prohibited. At very low speeds and at altitudes below 500m, VTOL hovering flight is recommended. The Cheyenne is at it's nimblest in the hover; here, response is crisp in all axes and the dropship is a very steady weapons platform.
The Cheyenne has a crew of two, comprising a Pilot and a Crew Chief / Weapons Officer. Flight control is quadruplex digital fly-by-light with automatic self-monitoring and reversion to back-up modes, all handled through the Herriman-Weston 5 / 480 flight computer. There is no manual reversion since the dropship is too unstable to be flown with direct control inputs. Engine thrust and nozzle settings are automatically moved to their optimum positions depending on speed, altitude, throttle and stick settings. An intelligent autopilot facility allows the automatics to fly all phases of the mission profile, including ingress and egress to the target zone as well as landing and docking cycles.
The instrumentation and control layout is basically conventional, with a right-hand displacement-type control stick and left-hand throttles. About twenty fingertip controls on these handles give HOTAS (hands-on-throttles-and-stick) control of sensors, weapons, defense-aids, etc.
The avionics system is designed to facilitate maximum cockpit efficiency, the semi-intelligent software registering all flight information as required on the pilot's wide-angle, heads-up display (HUD) and the three integrated Lorac multi-function displays (MFDs). These displays provide sensor-fusion presentation, map displays, armament status diagrams, checklists, etc. The pilot's workload is reduced by a direct voice input (DVI) system, which may be employed for data entry, the selection of communication channels and operating modes for the MFDs, as well as weapons selection.
Navigation combines an inertial system with ring laser gyros and strapdown accelerometers, backed up by Global Positioning from reference satellites where available. Dropship communications are handled through a AN / ASC-155 digital datalink offering HF, VHF, UHF and SHF broadcast options. The hardware includes two 12-channel receiver / transmitters with the associated antennae capable of establishing high performance voice, video or computer links in a stressed environment. Antijam features are classified, though they are known to include adaptive HF spectrum techniques to achieve a low probability of intercept and frequency hopping.
The UD-4L Cheyenne variant tactical transport carries a wide array of powerful weaponry that can be used in dedicated support of its troop complement. Two main weapons bays fold out on extended pylons to deploy weapon hardpoints each capable of carrying 16 x 150mm unguided rockets, 6 x 7mm unguided rockets and 4 x 120mm guided rockets. Two secondary weapon bays on the port side and starboard side of the fuselage house a further 14 hardpoints for Air-toAir and Air-toSurface missiles.
The dropship also mounts a dedicated 25mm gatling gun in a powered cuploa beneath the nose, which can be rotated 180 degrees side to side and by an angle of 60 degrees downwards. The GAU-113/B is a six barrel weapon driven by a pneumatic motor turned by the engines at 6000 rpm and geared down to the rear of the gun. Rounds are caseless and do not carry their own propellant. Instead, the GAU-113/B system uses hypergolic liquid fuels, stored and loaded separately, as a binary propellent. When fed into the chamber via the spray nozzles, they react simultaneously to explode and propel the shell. Ammunition comprises a mix of Armour Piercing Incendiary, Armour Piercing Discarding Sabot and High Explosive Incendiary and is fed from a 900 round drum beneath the cockpit. It is standard practice to carry at least two spare ammo drums on board. These can be reloaded manually by a crew member from inside the cockpit.
The Cheyenne's design proved to be successful and led it to become essentially the definitive dropship design, influencing the shape of many derivatives and successors. The UD-4L is the latest production variant of the Cheyenne, it incorporated an improved sensor and fire control system as well as being the first to include a comprehensive defensive suite fit. The dropship's weapons appears to be a missile payload in its wings and vulcan cannon on its nose. They also carry the APCs. Prior to the UD-4L's introduction, there were earlier production models of the Cheyenne dropship:
- UD-4B was the original production variant of the Cheyenne. The UD-4B is powered by Atco Wyoming F23 lift turbines, producing 243 kN thrust each. Shorter by 1.5 m than the later variants and with less payload space, this version was also equipped with the main weapons pods only.
- UD-4C was the first gunship variant of the Cheyenne, employing the dropship with secondary weapon bays and a dedicated Gatling gun system.
- UD-4E was basically a slightly upgraded version of the UD-4B, with its engines refitted with F29-L-13 turbines to give extended atmospheric range.
- UD-4H was the definitive production variant of the Cheyenne, the UD-4H included a major redesign of many systems and components. The fuselage was stretched by 1.5 m to allow an extra 16 m3 of payload space, making it the first variant capable of carrying the M577 APC, and the lift engines were upgraded to a broad-spectrum sensing array and command datalink. Secondary missile bays and the Gatling guns were now fitted as standard. Its design was later upgraded, becoming the basis for the UD-4L variant.
- UD-4J was an upgraded version of the UD-4B as a part of the USCMC's life-extension program, using all existing airframes to fit 'H' standard.
- The D77-TC Pelican Dropship featured prominently in the Halo series is heavily inspired by the Cheyenne's design, among other things from Aliens.
- The Cheyenne also inspired the APOD-33 Dropship from StarCraft; in fact, the dropship pilots' appearance and personality are directly based on Corporal Ferro, who famously piloted the Cheyenne from Aliens.