Are Single Stage to Orbit (SSTO) Rockets the Ideal Choice for Reusability in Space Exploration?

The debate over the feasibility and desirability of single stage to orbit (SSTO) rockets as the ideal choice for full reusability has been a topic of much discussion in recent years. While these rockets hold the promise of reducing launch costs and increasing the efficiency of space travel, current technological limitations and design constraints make them less practical compared to two-stage rockets.

Challenges with SSTO Rockets

The primary challenge with SSTO rockets lies in the use of conventional chemical rocket engines. To date, it is barely feasible to build a chemical-fueled SSTO rocket capable of reaching orbit. However, the absence of any weight left over for critical systems such as heat shielding, landing legs, and reentry recovery systems, limits its practical utility. The priority becomes almost entirely focused on weight optimization to ensure a minimal launch mass, leaving little room for additional payload or utility.

In contrast, a two-stage rocket design overcomes these challenges. By breaking the journey into two stages, each with optimized engines and fuel systems, significant improvements in mass margins are achieved. This allows for dedicated engines and tanks for the high-thrust, lower-atmosphere first stage, and the low-thrust, high-efficiency upper stage. These optimizations lead to reduced overall weight, allowing for a more substantial payload and better reusability.

The success of SpaceX’s reusable rockets, particularly the first stage, further emphasizes the viability of this approach. While advancements are being made towards full reusability, the concept of a reusable second stage is anticipated to be less complex and more achievable than a reusable SSTO.

Designing for Different Environments

The fundamental issue with SSTO rockets is the necessity of using different engine technologies for different phases of flight. Rockets require specialized engines to operate effectively in the atmosphere versus those designed for space flight. This complexity necessitates the development of two distinct engine types, each optimized for a specific operating environment.

For SSTO rockets to be effective, they would ideally feature an engine that can perform efficiently both in the atmosphere and in space. Aerospike engines, which show promise in this regard, are often proposed as a solution. However, these engines present their own set of challenges. Essentially, aerospike engines are far heavier and less efficient compared to modern engines like SpaceX’s Raptor. The Raptor engine, for example, offers a thrust-to-weight ratio of 200, while the aerospike engine from the J2 Rockwell model achieves only 73.1, albeit weighing much more at 1,788 kg compared to the 1,500 kg of the Raptor.

Moreover, aerospike engines have not been used in any operational capacity and still have unresolved heating issues. A prototype was designed, but it never flew because the program was cancelled, highlighting the technical and financial challenges associated with developing such engines.

Conclusion

The current technological landscape suggests that, for full reusability in space exploration, two-stage rockets hold a significant advantage over single stage to orbit (SSTO) rockets. The combination of specialized engines, optimized stages, and the potential for reducing dead weight through staging offers a more practical and achievable path to sustainable space flight. Until the development of lightweight, efficient, and reliable engines like aerospike engines can overcome current limitations, the use of two-stage rockets remains the feasible approach for advancing space exploration and reducing the cost of access to space.

Keywords: SSTO rockets, reusable rockets, two-stage rockets