The UD-4 Cheyenne 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.
The UD-4 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.
The 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.
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.
It must be noted that even light damage can prevent a dropship from lifting into orbit. A breach of the fuselage skin will seriously compromise the ship's high-speed thermal protection, and even a tiny hole can cause oxidation or 'burn through' when atmospheric speeds exceed Mach 5.0. To prevent such accidents, a sensor net is bonded to the inside of the skinning to monitor for breaches, differential hull temperature and ionization. If a breach is detected, a warning is flashed to the cockpit monitors to notify the crew.
The UD-4 is a reliable craft in all realms of it's aerospace operating platform, and is a popular craft with it's pilots. It handles well above the specific purpose for which it was designed, but in the world of war, nothing can be expected to handle as it was designed, here the Cheyenne delivers above and beyond... but not as well as the pilots who push these craft to their limits would prefer.
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.
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 Cheyenne is a very reliable and complex machine, not to mention a deadly weapon if deployed correctly. The UD4L 'Cheyenne' is a versatile dropship and tactical transport employed in a primary role in the United States Colonial Marine Corps. Derived from and 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 10 16,000 kg, the UD-4 has evolved into the definitive dropship design, influencing the shape of many derivatives and successors.
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 takeoff (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 'L' 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 benaeth 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 separtely, 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.
Original production variant 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.
A gunship variant, this was the first to employ the secondary weapon bays and a dedicated Gatling gun system.
This was the UD4B re-engined with F29-L-13 turbines to give extended atmospheric range.
The definitive production variant of this type, the UD4H 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.
A USCMC life-extension program upgraded all existing UD4B airframes to 'H' standard. This was designated the UD4J.
An upgrade of the 'H' version assault transport, the UD4L incorporates 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.
- 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.