J58 Turbojet vs. SSTO Spacecraft: Evaluating the Feasibility and Pitfalls

The question of whether a J58 turbojet engine, such as that used in the SR71 Blackbird, could be adapted for an SSTO (Single Stage to Orbit) spacecraft has intrigued aerospace enthusiasts for years. Space Plane Lazar recently sparked discussion by mentioning the use of a mix of Methane and LOX (Liquid Oxygen) in Area 51 for its engines. This led to questioning if an SSTO spacecraft could use a similar setup. However, there are significant challenges and limitations to consider.

The J58 Turbojet and Its Limitations

Originally designed for the SR71 Blackbird, the J58 Turbojet engine is renowned for its high-altitude and high-speed capabilities. While impressive, it is fundamentally limited by the need for atmospheric air to operate efficiently. In the vacuum of outer space, where there is no air, the J58 and similar jet engines would cease to function. This raises the immediate question of how such spacecraft could achieve and maintain the necessary altitude and speed to reach orbit.

The SR71 Blackbird as a Model

The SR71 Blackbird, which used the J58 engine, was an incredible aircraft capable of speeds of over Mach 3 and reaching altitudes of up to 85,000 feet. However, for SSTO applications, it falls far short of the requirements. First, the SR71 was exponentially too slow for space travel. The critical velocity required to achieve orbit is approximately 25,000 feet per second, while the SR71 could only reach speeds around 2,100 feet per second. Additionally, the SR71 was designed to operate at low altitudes, making it unsuitable for the extreme altitude requirements of orbital flight.

Cost and Practicality

Moreover, the SR71 Blackbird was not only technologically limited but also incredibly expensive to operate. According to estimates, a single flight of the SR71 could cost over $400,000 in 1974 dollars, which is far more than the current cost of launching a SpaceX Falcon 9 rocket. The SpaceX Falcon 9, with its reusable rocket system, provides a more cost-effective and practical solution for reaching space.

Recent Developments and Potential Solutions

Advancements in hybrid rocket and jet engine technology offer potential solutions to some of the limitations faced by the J58 and other jet engines. Consider, for instance, the proposed use of Methane and LOX in the Space Plane Lazar. Methane and LOX are both highly-efficient propellants that could provide the necessary thrust for both atmospheric and space operations. However, such designs would still need to address the critical issue of turbine blade protection during re-entry.

Another potential approach is to design a hybrid propulsion system that combines jet propulsion for atmospheric flight with rocket propulsion for space. Such a system would allow the use of jet turbines for specific portions of the flight path, such as atmospheric entry and ascent, while switching to rocket engines for the final phase of the journey and re-entry.

Conclusion

While the idea of using a J58 Turbojet in an SSTO spacecraft is intriguing, the fundamental limitations of jet engines in space make it a non-viable solution. The need for air to operate, the speed and altitude requirements for reaching orbit, and the high costs associated with the SR71 serve as clear indicators that alternative technologies are necessary. Nonetheless, ongoing research and development in hybrid propulsion systems could provide promising solutions for the future of SSTO spacecraft.

Keywords:

J58, SSTO, Space Shuttle, Methane LOX