The National Security Space Launch (NSSL) program, managed by the U.S. Space Force, is responsible for ensuring reliable and affordable access to space for critical national security payloads. To facilitate the launch procurement process and match payloads with appropriate launch vehicles, the NSSL program defines standard payload envelope categories. These categories specify the maximum payload mass and volume that launch vehicles must be capable of delivering to reference orbits.
Defining the Payload Envelope Categories
The NSSL program currently defines three main payload envelope categories: Category A, Category B, and Category C. These categories are based on the payload’s physical dimensions and the diameter of the payload fairing required to encapsulate it.
Category A Payloads
Category A payloads are the smallest of the three categories. They can fit within a 4-meter diameter payload fairing. The usable volume for a Category A payload is typically around 38 cubic meters. Launch vehicles must be capable of delivering Category A payloads with a mass up to approximately 6,800 kg to low Earth orbit (LEO) and 2,300 kg to geostationary orbit (GEO).
Category B Payloads
Category B payloads are larger than Category A and require a 5-meter diameter payload fairing. The available volume for a Category B payload is around 63 cubic meters. Launch vehicles are required to lift Category B payloads with a mass up to roughly 7,000 kg to LEO and 6,600 kg to GEO.
Category C Payloads
Category C payloads are the largest and heaviest payloads in the NSSL program. They necessitate an extended 5-meter diameter payload fairing to accommodate their size. Category C payloads have an available volume of about 76 cubic meters. To be considered for Category C missions, launch vehicles must demonstrate the ability to deliver payloads weighing up to 17,000 kg to a polar orbit and 6,600 kg directly to geostationary orbit.
Reference Orbits and Mission Profiles
In addition to the payload mass and volume specifications, the NSSL program defines a set of reference orbits that launch vehicles must be capable of reaching with each payload category. These reference orbits cover a range of inclinations and altitudes to ensure that launch vehicles have the flexibility to support various national security space missions.
The reference orbits for NSSL payloads include:
Low Earth orbit (LEO) at an altitude of 926 km and an inclination of 63.4 degrees
Polar orbit at an altitude of 830 km and an inclination of 98.2 degrees
Medium Earth orbit (MEO) at an altitude of 18,200 km and an inclination of 50 degrees
Geosynchronous transfer orbit (GTO) with a perigee of 190 km, an apogee of 35,786 km, and an inclination of 27 degrees
Geostationary orbit (GEO) at an altitude of 35,786 km and an inclination of 0 degrees
Molniya orbit with a perigee of 1,200 km, an apogee of 39,200 km, and an inclination of 63.4 degrees
Launch vehicles must demonstrate the ability to deliver payloads to these reference orbits to be certified for NSSL missions. The specific orbit and payload capacity requirements vary depending on the payload category.
Payload Adapter and Separation Systems
To ensure compatibility between payloads and launch vehicles, the NSSL program has standardized payload adapter and separation systems. These systems provide a mechanical and electrical interface between the payload and the launch vehicle’s upper stage.
The most common payload adapter used in the NSSL program is the EELV Secondary Payload Adapter (ESPA). The ESPA is a ring-shaped structure that can accommodate multiple payloads, allowing for rideshare missions. It has a standard bolt pattern and can support payloads weighing up to 450 kg.
For larger payloads, the NSSL program uses a variety of payload adapters, such as the 1575 mm Payload Attach Fitting (PAF) and the 1666 mm PAF. These adapters are designed to handle the increased mass and size of Category B and Category C payloads.
Payload separation systems are responsible for safely deploying the payload from the launch vehicle once the desired orbit is reached. The most common separation systems used in the NSSL program are the Payload Separation System (PSS) and the Payload Adapter Separation System (PASS). These systems use a combination of mechanical and electrical mechanisms to ensure reliable and precise payload separation.
Launch Vehicle Certification Process
To be eligible to launch NSSL payloads, launch vehicles must undergo a rigorous certification process. This process ensures that the launch vehicle meets the stringent reliability, performance, and safety requirements set forth by the Space Force.
The certification process involves a series of reviews and assessments, including:
Design Certification Review (DCR): Evaluates the launch vehicle’s design, manufacturing processes, and quality control procedures.
Flight Readiness Review (FRR): Assesses the launch vehicle’s readiness for flight, including the results of ground testing and simulation.
Post-Flight Review (PFR): Analyzes the data collected during the launch to verify that the vehicle performed as expected and to identify any areas for improvement.
Launch vehicles must also demonstrate their ability to meet the payload envelope category requirements by successfully launching payloads to the reference orbits. Typically, a launch vehicle must complete three successful flights to be considered for NSSL certification.
Current and Future Launch Vehicles
As of 2024, the primary launch vehicles certified for NSSL missions are the United Launch Alliance’s Atlas V and Delta IV Heavy rockets and SpaceX’s Falcon 9 and Falcon Heavy rockets. These vehicles have a proven track record of successfully delivering national security payloads to orbit.
Looking ahead, the Space Force is investing in the development of new launch vehicles to ensure continued access to space and to promote competition in the launch industry. The NSSL Phase 2 procurement, awarded in 2020, selected United Launch Alliance’s Vulcan Centaur and SpaceX’s Starship and Super Heavy vehicles as the next-generation launch providers for national security missions. Both vehicles are pending NSSL certification.
The Vulcan Centaur is designed to replace the Atlas V and Delta IV rockets, offering increased performance and flexibility. It will be capable of delivering payloads to all NSSL reference orbits and supporting all payload envelope categories.
SpaceX’s Starship and Super Heavy system, currently under development, promises to revolutionize space launch with its fully reusable architecture and massive payload capacity. Once operational, it is expected to support NSSL missions across all payload categories and potentially enable new mission profiles.
Summary
The National Security Space Launch payload envelope categories play a crucial role in ensuring that critical national security payloads can be reliably and efficiently delivered to their intended orbits. By defining standard payload sizes and reference orbits, the NSSL program streamlines the launch procurement process and promotes compatibility between payloads and launch vehicles.
As the space domain becomes increasingly important for national security, the continued evolution of the NSSL program and the development of new, advanced launch vehicles will be essential to maintaining the United States’ advantage in space. The payload envelope categories will continue to serve as a foundation for this effort, ensuring that the right payloads are matched with the right launch vehicles to support the nation’s critical space missions.
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