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Introduction
The space industry is experiencing a growing demand for more cost-effective and sustainable launch solutions. Reusable launch vehicles (RLVs) have emerged as a viable technology to address these needs, offering the potential to significantly reduce launch costs and increase access to space. The development of RLVs has progressed significantly, with several successful examples demonstrating the feasibility of routine recovery and reuse of major launch vehicle components. However, the optimal approach for RLV development remains a subject of ongoing research and discussion.
Two primary recovery and landing methods have been employed in existing RLVs: vertical takeoff and vertical landing (VTVL) and vertical takeoff and horizontal landing (VTHL). Each method has its own advantages and disadvantages, and the choice between them depends on various factors, including mission requirements, operational constraints, and economic considerations.
This article presents a comparative analysis of VTVL and VTHL concepts, drawing upon the results of the ENTRAIN (European Next Reusable Ariane) study conducted by the German Aerospace Center (DLR). The analysis focuses on the technical characteristics, performance, and potential applications of each method.
VTVL Concepts
VTVL concepts involve the vertical launch and landing of the reusable first stage. This approach has been successfully demonstrated by SpaceX’s Falcon 9 and Falcon Heavy rockets, which have achieved routine recovery and reuse of their first stages.
Advantages of VTVL
- Reduced ground infrastructure: VTVL vehicles can land on relatively small platforms, either on land or at sea, minimizing the need for extensive ground infrastructure.
- Simplified recovery operations: The vertical landing approach allows for rapid and efficient recovery operations, reducing turnaround time between launches.
- Adaptability to diverse landing environments: VTVL vehicles can land in various environments, including on land, at sea, and potentially on other celestial bodies with minimal atmospheric pressure.
Disadvantages of VTVL
- Higher propellant requirements: VTVL vehicles require significant propellant reserves for the powered descent and landing maneuvers, reducing their overall payload capacity.
- Increased engine complexity: The engines used in VTVL vehicles must be capable of deep throttling and multiple restarts, increasing their complexity and development challenges.
- Sensitivity to atmospheric conditions: VTVL landings can be affected by adverse atmospheric conditions, such as high winds or turbulence, potentially requiring delays or alternative landing sites.
VTHL Concepts
VTHL concepts involve the vertical launch and horizontal landing of the reusable first stage, similar to the Space Shuttle. This approach offers several advantages, particularly for missions requiring high payload capacity and flexibility.
Advantages of VTHL
- Higher payload capacity: VTHL vehicles can achieve higher payload capacities compared to VTVL vehicles, as they do not require propellant reserves for powered descent and landing.
- Increased operational flexibility: The horizontal landing approach allows for greater flexibility in landing site selection and the ability to return to the launch site, potentially reducing recovery costs and turnaround time.
- Reduced engine complexity: VTHL vehicles can use simpler and less complex engines, as they do not require deep throttling or multiple restarts.
Disadvantages of VTHL
- Increased ground infrastructure: VTHL vehicles require runways for landing, necessitating more extensive ground infrastructure compared to VTVL vehicles.
- Higher structural complexity: The wings and other aerodynamic control surfaces required for horizontal landing increase the structural complexity and mass of VTHL vehicles.
- Sensitivity to atmospheric conditions: VTHL landings can also be affected by adverse atmospheric conditions, potentially requiring delays or alternative landing sites.
Comparative Analysis
The choice between VTVL and VTHL concepts depends on various factors, including mission requirements, operational constraints, and economic considerations. The ENTRAIN study conducted by DLR provides valuable insights into the trade-offs involved in each approach.
Performance
VTHL concepts generally offer higher payload capacity compared to VTVL concepts, as they do not require propellant reserves for powered descent and landing. However, the specific payload capacity achievable depends on various factors, including the chosen propellant combination, engine cycle, and staging velocity.
The ENTRAIN study results show that VTHL concepts can achieve significantly higher payload fractions compared to VTVL concepts, particularly when using hydrogen fuel and staged combustion engines. For example, a VTHL concept using hydrogen fuel and staged combustion engines achieves a payload fraction of 2.3%, while the equivalent VTVL configuration achieves a payload fraction of only 1.4%.
However, VTVL concepts can also achieve competitive payload capacities, particularly when using denser hydrocarbon fuels and optimized staging velocities. For example, a VTVL configuration using a combination of methane and hydrogen fuel achieves a payload fraction of 1.9%, which is comparable to some VTHL configurations using gas generator engines.
Technical Complexity
VTVL concepts generally involve higher engine complexity compared to VTHL concepts, as they require deep throttling and multiple restarts for powered descent and landing. This can lead to increased development challenges and potentially higher costs.
On the other hand, VTHL concepts involve higher structural complexity due to the wings and other aerodynamic control surfaces required for horizontal landing. This can lead to increased mass and potentially higher development costs.
The choice between VTVL and VTHL concepts from a technical complexity perspective depends on the specific design choices and the available technology and expertise.
Operational Considerations
VTVL concepts offer advantages in terms of reduced ground infrastructure and simplified recovery operations, while VTHL concepts offer advantages in terms of operational flexibility and the ability to return to the launch site.
The choice between VTVL and VTHL concepts from an operational perspective depends on the specific mission requirements and operational constraints.
Economic Considerations
The economic viability of VTVL and VTHL concepts depends on various factors, including development costs, production costs, and operational costs. The ENTRAIN study results suggest that both VTVL and VTHL concepts can achieve significant cost reductions compared to fully expendable launch vehicles.
However, the specific cost advantages of each approach depend on various factors, including the chosen propellant combination, engine cycle, and staging velocity. The ENTRAIN study results show that hydrogen-fueled configurations generally offer lower costs compared to hydrocarbon-fueled configurations, particularly for VTVL concepts.
The choice between VTVL and VTHL concepts from an economic perspective depends on the specific design choices and the assumed market conditions.
Additional Factors and Considerations
Propellant Selection
The choice of propellant combination has a significant impact on the performance and cost of both VTVL and VTHL concepts. Hydrogen fuel offers the highest specific impulse, leading to higher payload capacities and lower propellant consumption. However, hydrogen also has a lower density and requires more complex storage and handling systems compared to hydrocarbon fuels.
Hydrocarbon fuels, such as methane, propane, and kerosene, offer higher densities and simpler storage and handling systems. However, they also have lower specific impulses, leading to lower payload capacities and higher propellant consumption.
The choice of propellant combination depends on various factors, including mission requirements, operational constraints, and economic considerations.
Engine Cycle Selection
The choice of engine cycle also has a significant impact on the performance and cost of both VTVL and VTHL concepts. Staged combustion engines offer higher specific impulses compared to gas generator engines, leading to higher payload capacities and lower propellant consumption. However, staged combustion engines also have higher complexity and development costs.
Gas generator engines offer lower complexity and development costs compared to staged combustion engines. However, they also have lower specific impulses, leading to lower payload capacities and higher propellant consumption.
The choice of engine cycle depends on various factors, including mission requirements, operational constraints, and economic considerations.
Staging Velocity Optimization
The staging velocity, which is the velocity at which the first stage separates from the second stage, has a significant impact on the performance and cost of both VTVL and VTHL concepts. Optimizing the staging velocity can lead to higher payload capacities and lower propellant consumption.
However, optimizing the staging velocity also requires careful consideration of various factors, including the chosen propellant combination, engine cycle, and recovery method.
Technology Readiness and Development Risks
The technology readiness level (TRL) of various components and systems is an important consideration in the development of both VTVL and VTHL concepts. Higher TRL components and systems generally have lower development risks and costs.
However, lower TRL components and systems may offer significant performance or cost advantages, justifying the additional development risks and costs.
The choice between higher and lower TRL components and systems depends on various factors, including mission requirements, operational constraints, and economic considerations.
Operational Flexibility and Responsiveness
The operational flexibility and responsiveness of both VTVL and VTHL concepts are important considerations for various missions. VTVL concepts generally offer faster turnaround times and the ability to land on smaller platforms, while VTHL concepts offer greater flexibility in landing site selection and the ability to return to the launch site.
The choice between VTVL and VTHL concepts from an operational flexibility and responsiveness perspective depends on the specific mission requirements and operational constraints.
Environmental Considerations
The environmental impact of both VTVL and VTHL concepts is an important consideration for sustainable space transportation. Hydrogen fuel offers cleaner combustion products compared to hydrocarbon fuels, leading to lower emissions and reduced environmental impact.
However, hydrogen also requires more complex storage and handling systems, which may have their own environmental impacts.
The choice between hydrogen and hydrocarbon fuels from an environmental perspective depends on various factors, including mission requirements, operational constraints, and the availability of sustainable hydrogen production methods.
Summary
Reusable launch vehicles (RLVs) offer a promising pathway to reduce launch costs and increase access to space. The choice between VTVL and VTHL concepts depends on various factors, including mission requirements, operational constraints, and economic considerations.
The ENTRAIN study conducted by DLR provides valuable insights into the trade-offs involved in each approach. VTHL concepts generally offer higher payload capacity compared to VTVL concepts, particularly when using hydrogen fuel and staged combustion engines. However, VTVL concepts can also achieve competitive payload capacities, particularly when using denser hydrocarbon fuels and optimized staging velocities.
The technical complexity of VTVL and VTHL concepts varies depending on the specific design choices and the available technology and expertise. VTVL concepts generally involve higher engine complexity, while VTHL concepts involve higher structural complexity.
The operational considerations for VTVL and VTHL concepts depend on the specific mission requirements and operational constraints. VTVL concepts offer advantages in terms of reduced ground infrastructure and simplified recovery operations, while VTHL concepts offer advantages in terms of operational flexibility and the ability to return to the launch site.
The economic viability of VTVL and VTHL concepts depends on various factors, including development costs, production costs, and operational costs. The ENTRAIN study results suggest that both VTVL and VTHL concepts can achieve significant cost reductions compared to fully expendable launch vehicles.
The choice between VTVL and VTHL concepts depends on the specific design choices, the assumed market conditions, and the relative importance of various factors, such as payload capacity, technical complexity, operational flexibility, and economic viability.
Additional factors and considerations, such as propellant selection, engine cycle selection, staging velocity optimization, technology readiness and development risks, operational flexibility and responsiveness, and environmental considerations, also play a significant role in the choice between VTVL and VTHL concepts.
The development of RLVs is a complex and challenging endeavor, requiring careful consideration of various technical, operational, and economic factors. The comparative analysis presented in this article provides valuable insights into the trade-offs involved in the choice between VTVL and VTHL concepts, contributing to the ongoing research and discussion on the optimal approach for RLV development.
