The Future of Reusable SSTO Rockets: Likelihood and Cost Analysis
Introduction
Considering the current state of aerospace technology, the concept of a single-stage-to-orbit (SSTO) reusable rocket seems far-fetched. However, the idea of reusable rockets is highly appealing due to its potential to significantly reduce launch costs and improve environmental sustainability. In this article, we will explore the likelihood of seeing reusable SSTO rockets in our lifetime and the associated costs, including fuel expenses.
Current Challenges and Limitations
From an engineering perspective, a well-designed two-stage system offers several advantages over a SSTO design. For instance, the Space Shuttle approached the SSTO goal by firing its engines from liftoff to orbit and returning to Earth for reuse; however, it compromised with a heavy expendable drop tank and solid boosters.
One proposed route to single-stage reusability involves hybrid engines with air-breathing capability. The idea is to reduce the oxidizer mass and volume necessary for launch. However, this approach introduces new challenges, such as increased drag and structural and heat management issues. Hypersonic flight within the atmosphere exacerbates these issues, making the approach less advantageous.
Advantages of Two-Stage Systems
A two-stage rocket system, exemplified by Starship, can deliver substantial reusability with fewer disadvantages compared to SSTO. Each stage can be independently reusable, allowing for frequent launches and extended missions. Unlike the Space Shuttle, a two-stage reusable system could potentially operate multiple times a day, with some stages potentially spending years in space on interplanetary missions.
Design Considerations and Performance
The ratio of fuel to structural mass in a SSTO is exceptionally high, often requiring nearly 95-97% of the mass to be fuel, leaving only 3-5% for structure. This is a considerable engineering challenge and has not been met by any existing design. With a two-stage system like Starship, the ratio is more manageable. For Starship, the ratio is around 90% fuel and 10% structure and rocket, making it feasible to design and build such a system.
For a SSTO rocket, achieving orbital velocity of about 17,500 mph (7.8 km/s) is necessary, which is roughly 33 times the speed of a commercial jet. The journey to orbit involves significant losses due to gravity and drag, until the rocket exceeds the atmosphere. A two-stage rocket design can mitigate these issues by dropping weight and reducing the total mass to be accelerated to orbital velocity.
Cost Analysis
The cost of building a reusable rocket like Starship is estimated to be around $100 million. Theoretically, it can fly 1,000 times. While some engine changeouts and maintenance might be required over time, this is still a highly cost-effective solution compared to traditional expendable rockets. Upon reentry, both stages can be caught and immediately repacked for another launch, much like an airline operates its flights.
Regarding fuel costs, the price for a launch is approximately $1 million. This includes not only the fuel cost but also labor and technical support. The significant reduction in launch costs compared to traditional rockets is a key advantage of reusable systems.
Conclusion
While the idea of SSTO rockets seems appealing, the engineering challenges and current technological limitations make them difficult to achieve. A more energetic propulsion system, such as fusion, might be required to realize the full potential of SSTO. However, with the advancements in two-stage reusable rockets like Starship, significant progress has been made towards reducing launch costs and improving sustainability in space exploration.