The MK4 orbiter uses a large delta wing, wingtip vertical tails, canards, and a retractable nose docking port.
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The MK4 orbiter is the crew, payload, and on-orbit operations core of the system. It has a large delta wing, wingtip vertical tails, forward canards, a pressurized crew cabin, a large cargo bay, OMS/RCS propulsion, two atmospheric jet engines, and five RS-25E main engines fed by the external tank during ascent.
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The MK4 orbiter is the crew, payload, and on-orbit operations core of the system. It has a large delta wing, wingtip vertical tails, forward canards, a pressurized crew cabin, a large cargo bay, OMS/RCS propulsion, two atmospheric jet engines, and five RS-25EX main engines fed by the external tank during ascent.
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The RS-25EX is an evolution of the RS-25DX, with development beginning in 2020. Key improvements over the RS-25DX include extensive use of 3D printing, which reduced the parts count by approximately 40%, and the replacement of selected Inconel components with ceramic-matrix composites (CMC) for weight reduction. Each RS-25EX has a dry mass of 2.88 t and a chamber pressure of 26 MPa. It produces 2,812 kN of thrust at a vacuum specific impulse of 454 s. Sea-level performance is substantially improved over earlier RS-25 variants, delivering 2,353.7 kN at 380 s specific impulse. The nominal maximum continuous burn time is 8 minutes 30 seconds, extendable to 12 minutes when multiple engine-out conditions require extended burning on the remaining engines.
The payload bay can accommodate cargo about 5.4 m long, 7.75 m wide, and 29.5 m high. This makes Vulture especially useful for wide station modules, large radiators, truss sections, and mothership components that are difficult to package inside cylindrical expendable fairings.
External tank
The external tank stores liquid oxygen and liquid hydrogen for the orbiter's main engines. It is 10 m in diameter, carries 1612.2 t of propellant, and has a dry mass of 31.5 t. Unlike the STS external tank, it uses a lower-shedding insulation coating rather than traditional foam in order to reduce debris risk to the orbiter thermal protection system.
Two 5 m liquid boosters provide most liftoff thrust. Each booster carries 19 Tianhuo-12 LOX/kerosene engines, burns for about 135 seconds, then separates and performs RTLS recovery using grid fins, hot-gas RCS, landing legs, and propulsive landing.
Mission profile
After booster cutoff, the reusable liquid boosters separate and return to the launch site while the orbiter and external tank continue upward.
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During ascent, the liquid boosters and five RS-25E main engines operate together. After booster cutoff and separation, the boosters return to the launch site while the orbiter and external tank continue to a high suborbital insertion. After main engine cutoff, the orbiter separates from the external tank and uses OMS burns to enter and raise its orbit.
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Abort modes include pad abort, RTLS, Abort to Orbit, and Abort Once Around. Because the orbiter has five RS-25E engines, not every main-engine shutdown triggers an abort. After booster separation, one failed RS-25E is usually tolerated; after T+6 minutes, two failures may still permit orbit insertion; after T+8 minutes, three shutdowns can still result in a low-margin but normal orbit if other systems remain healthy.
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During ascent, the liquid boosters and five RS-25EX main engines operate together. After booster cutoff and separation, the boosters return to the launch site while the orbiter and external tank continue to a high suborbital insertion. After main engine cutoff, the orbiter separates from the external tank and uses OMS burns to enter and raise its orbit.
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Abort Modes
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The Vulture Shuttle relies on its five RS-25EX engines and generous OMS margins to provide a graduated abort hierarchy that escalates with the severity of the failure. The actual mode triggered depends on how many engines have been lost, when in ascent the failures occur, and the vehicle's instantaneous thrust-to-weight ratio. Six abort modes are defined, ordered from least to most severe:
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Abort to Orbit — The remaining main engines push the orbiter into a near-nominal orbit; the OMS takes over from there to complete or adjust the mission.
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On Orbit Abort — The main engines can no longer deliver the mission, but the OMS alone can reach orbit. The payload is placed into a 400 km × 400 km parking orbit when conditions permit, and the vehicle returns at the earliest opportunity.
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Abort Once Around — The orbiter completes one full revolution and returns, jettisoning the payload in orbit to reduce landing weight.
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Trans-Ocean Abort — Orbital insertion is out of reach. The vehicle uses its remaining energy to attempt a landing at a civilian airfield across the ocean.
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RTLS Abort — The vehicle reverses course immediately and returns to the launch site.
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Hard Abort — Controlled flight is no longer possible (Loss of Vehicle). The crew cabin separates from the orbiter, stabilizes, and descends under drogue and main parachutes. This is the last-resort layer, triggered only when every other abort mode is unavailable.
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Main Engine Engine-Out
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The following analysis uses a representative Block 1 mission: 100 t payload launched from Wenchang, targeting a 650 km × 650 km circular orbit at 28° inclination. All TWR values refer to the vehicle's instantaneous thrust-to-weight ratio at the moment of failure.
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One RS-25EX shutdown immediately after liftoff
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With no further RS-25EX failures, the remaining four engines deliver the stack into a 30 km × 650 km transfer orbit. The mission continues without deviation.
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Before TWR reaches 1: loss of any additional engine → RTLS Abort.
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Before TWR reaches 1.15: loss of an additional engine → On Orbit Abort. The OMS can complete insertion, but the vehicle should return promptly; the payload is typically placed into a 350 km × 650 km orbit beforehand.
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After TWR reaches 1.15: loss of an additional engine → Abort to Orbit.
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After TWR reaches 1.8: the vehicle can absorb two consecutive additional shutdowns (three total failures) with the mission continuing nominally.
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After TWR reaches 1.8: loss of three additional engines (four total failures) → Trans-Ocean Abort or Abort Once Around, based on remaining energy and ground-track geometry.
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Normal ascent timeline
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T+3 min 45 s (or TWR ≥ 1.15): can sustain two consecutive shutdowns → Abort to Orbit.
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T+4 min (or TWR ≥ 1.2): can sustain two shutdowns and still reach a nominal orbit.
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T+5 min 45 s (or TWR ≥ 1.75): can sustain three consecutive shutdowns and still reach a nominal orbit. One further shutdown → Trans-Ocean Abort or Abort Once Around.
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Booster Engine-Out
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Each liquid booster carries 19 Tianhuo-12 (TH-12) LOX/kerosene engines and has its own engine-out tolerance: a single booster can absorb up to two TH-12 shutdowns with no effect on the mission.
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When a single booster loses 2 to 6 TH-12 engines, the affected side forgoes its recovery attempt. The opposite booster matches the shutdown count to preserve thrust symmetry, and both boosters extend their burn to compensate for the lost impulse. The mission proceeds normally; only the booster recovery is scrubbed.
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Beyond 6 TH-12 shutdowns, the flight computer evaluates the vehicle's remaining energy and trajectory to select the appropriate abort mode from the hierarchy above.
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The most severe booster case occurs before T+60 seconds, when one booster loses thrust entirely. If all five RS-25EX engines are healthy, the boosters are commanded to shut down, the vehicle coasts through max-Q, the boosters are jettisoned, and the orbiter flies an RTLS maneuver back to the launch site. If any single RS-25EX has also failed at this point, the vehicle is declared unrecoverable (Loss of Vehicle) and Hard Abort is triggered immediately — the crew cabin separates and descends under parachutes.
Fleet
Ten Vulture orbiters have been built across three blocks. Five Block 1 orbiters have been retired, two Block 1.5 orbiters are currently active, and three Block 2 orbiters are currently active. Flight rates are approximately 12 flights per year per Block 1 orbiter, 24 per year per Block 1.5 orbiter, and 48 per year per Block 2 orbiter.
秃鹫航天飞机被设计为高轨重载和载人运输系统，而不只是近地轨道轻载运输工具。其典型任务能力为从文昌发射，将 100 吨级载荷送入 1000 km x 1000 km、45 度倾角轨道。常规任务中，发射组合体首先将轨道器送入约 650 km x 30 km 的亚轨道，随后由轨道器自身的轨道机动系统完成入轨、爬升、圆化、交会、载荷释放和返航机动。
