Understanding the Mercury-Atlas System and SSTO Propulsion

Among the many achievements in space exploration, the Mercury-Atlas spacecraft stands as a significant milestone. However, the question arises: does this configuration qualify as a SSTO (Single Stage to Orbit) system? This article explores the technical aspects and distinctions of the Mercury-Atlas system relative to SSTO principles.

What is SSTO?

SSTO, or Single Stage to Orbit, is a class of launch vehicles designed to achieve orbit using a single stage without requiring jettisoned components or stages. The primary goal of SSTO is to reduce logistical complexity and costs associated with multiple-stage rockets.

The Mercury-Atlas System

The Mercury-Atlas spacecraft, specifically the Atlas LV-3B configuration, did not technically qualify as an SSTO system. The Mercury spacecraft itself was not designed to achieve orbit; instead, it was the Atlas rocket that was responsible for launching the Mercury capsule.

The Atlas LV-3B design includes two powerful XLR-89-5 booster engines providing about 80% of the initial thrust. These powerful engines contribute significantly to the overall mass of the system. The center/sustainer stage engine, the XLR-105-5, provided the remaining thrust needed for the mission.

Technical Analysis: The Atlas LV-3B Configuration

The Atlas LV-3B configuration, based on the Atlas SM-65D, was designed in a way that did not trust the second stage engines to ‘airstart,’ i.e., to start after stage separation. This design choice influenced the inclusion of shared turbopumps and engines to ensure a smooth and reliable launch sequence.

Given the nature of the Atlas LV-3B, it is more accurate to describe the system as a single stage with significant internal components. The lack of internally stored propellant in the second stage and the powerful engines contribute to the “one stage” designation. However, the payload (Mercury capsule) was still dependent on the Atlas booster for achieving orbit.

Retrorockets and Orbit Insertion

Upon reentry, the Mercury capsule utilized its own retrorockets for deorbiting and descent, rather than for orbit insertion. This further highlights the dependency of the Mercury-Atlas system on the booster for its primary mission objective.

Conclusion

In summary, while the Mercury-Atlas system was a significant step in space exploration, it did not qualify as an SSTO vehicle. The powerful Atlas LV-3B booster, with its two engines providing a substantial portion of the thrust, and the shared turbopumps, contribute to the system being classified as a single stage launch vehicle. The Mercury spacecraft was solely designed for missions within the orbit achieved by the Atlas rocket.

Further research on the exact masses of the engines and the discarded booster would provide more clarity on the technical aspects of the system and its classification.