Raptor 2: A Closer Look:
Elon Musk showcased the Raptor 2 engine at Starbase, Texas, highlighting its simplified design compared to Raptor 1. The Raptor 2 features a cleaner appearance, with a significant reduction in “fiddly bits” and complexity. It has a completed engine design with a gimbal mount, unlike the intricate Christmas tree-like structure of Raptor 1.

Increased Thrust and Pressure:
Raptor 2 delivers 230 tons of thrust at a standard operating pressure of 300 bar, setting a new record for operational rocket engines. The previous best was around 267 bar, achieved by engines like the RD-180. Musk revealed that Raptor 1 could potentially reach 250 tons of thrust at a lower pressure of 250 bar.

High Production Rate and Engine Blowing Up:
Musk emphasized the importance of a high production rate in accelerating technology development and innovation. SpaceX has blown up numerous engines during the development process, with an estimated count of over 20 or even 30. This high production rate allows for rapid iterations, learning from failures, and continuous improvements.

Chamber Melting and Robustness:
Musk acknowledged the issue of chamber melting in Raptor engines, leading to the loss of the chamber-nozzle assembly. However, the pumps are often salvageable, and the main injector can sometimes be reused. To address this, SpaceX is prioritizing robustness over performance in the current stage of Raptor 2 development. They are employing excessive foam cooling to prevent chamber melting, even at the cost of a slight reduction in efficiency.

Elimination of Torch Igniters:
Raptor 2 features a significant improvement by eliminating torch igniters in the main chamber. This simplification reduces complexity, failure modes, weight, cost, and improves reliability on ignition. The exact method of ignition remains a secret, but Musk hinted at observable outcomes that can be noticed externally.

Complex Start Sequence:
Unlike Merlin engines with a single shaft, Raptor has separate oxygen and fuel powerheads with different shafts, turbines, and pre-burners. This results in a complex start sequence where the fuel and oxygen powerheads must be precisely synchronized to avoid stoichiometric conditions in the pre-burners, which could lead to melting or explosions.

Understanding Rocket Engine Complexity:
Starting a rocket engine is complex, requiring a precise sequence to avoid engine damage. Once running, the engine reaches a steady state, making full-flow operation advantageous.

Theoretical vs. Practical ISP for Methane Engines:
The ISP (specific impulse) for methane engines in Tim Dodd’s video is incorrect, being too generous. The theoretical ISP is not physically possible, and Musk wishes it were attainable.

The Rafter Architecture:
The Rafter architecture represents the highest efficiency known to physics for rocket engines. To improve beyond the Rafter architecture, new physics would be required.

Achieving Near-Perfect Combustion Efficiency:
With the Rafter architecture, 99% combustion efficiency is achievable, which is remarkable. Divine intervention could theoretically yield a 1% improvement.

Design Features for Efficient Combustion:
The Raptor engine’s compact thrust chamber enables efficient combustion due to the use of pre-mixed gases. Swirl injectors create a mixture of oxygen-rich and fuel-rich gases, resulting in 90% pre-mixing before entering the main chamber.

Hydraulic Actuation:
Currently, the Raptor engine uses hydraulic actuators for engine movement due to the availability of kerosene as a working fluid. Switching to electric servo drives is planned for future versions, offering improved efficiency and eliminating the need for a hydraulic system.

Challenges with Methane as a Hydraulic Fluid:
Using methane as a working fluid in the hydraulic actuators is technically feasible but poses risks due to its tendency to gasify. The potential formation of gas bubbles in the hydraulic system could lead to spongy operation and reduced performance.

Design Optimization Approach:
Elon Musk emphasizes the importance of questioning constraints and requirements to simplify designs. The focus is on deleting unnecessary parts or processes rather than optimizing them, aiming for a 10% reduction in the number of components. Optimization is prioritized after deletion and simplification have been addressed.

Production and Automation:
Production rate acceleration is prioritized before automation to streamline the manufacturing process. Musk stresses the importance of following a specific sequence of steps when optimizing designs, with production rate acceleration preceding automation.

Addressing Concerns and Future Improvements:
Musk aims to reduce the number of small pipes and wiring in the Raptor engine to simplify the design and improve reliability. Ongoing efforts are focused on integrating components and eliminating unnecessary elements to enhance engine performance and efficiency.

Eliminating Shrouds:
Deleting shrouds would reduce mass, cost, and perplexity. Shrouds protect susceptible parts of the engine from heat and high aerodynamic forces (Q). Rafter 2 has made progress but still needs a shroud. With further simplification, shrouds may be eliminated for both ship and booster.

Bolted Interfaces:
Elon Musk wants to eliminate many bolted interfaces. Bolted interfaces add weight, complexity, and potential leak points. Sealing interfaces is challenging, especially for cryogenic liquids, hot gases, and ox-rich hot gases. Every bolted interface has a leak rate greater than zero.

Expander Cycle and Boost Pump Integration:
RD-170 and RD-180 engines utilize an innovative approach by employing an expander cycle with a boost pump. This design harnesses the regen channels to drive a boost pump, thereby reducing pressure on the head side or inlet side of the engine. Such an approach offers the advantage of mass reduction by minimizing the requirement for olive gas.

Benefits of Boost Pump:
A boost pump aids in reducing the outage pressure or inlet pressure to the engine, particularly at high thrust levels. As thrust levels are reduced, the need for high inlet pressure diminishes, making a boost pump or similar pressure-increasing device beneficial.

Streamlined Pump Design:
SpaceX’s pump design is characterized by its simplicity and streamlined nature, featuring a dedicated inducer and a flat propeller blade. This design prioritizes avoiding cavitation or bubble generation, which can erode the propeller blade and lead to pressure loss.

Cavitation and Bubble Generation:
Cavitation, or the formation of bubbles, is a phenomenon that can occur when liquid flow is cut at a shallow angle. Bubbles generated through cavitation can cause erosion of metal surfaces, including the propeller blades, leading to potential engine issues. Even small amounts of cavitation can result in bubble erosion, highlighting the importance of mitigating bubble generation.

Propellant-Specific Cavitation Susceptibility:
The susceptibility to cavitation varies depending on the propellant. While the interview does not delve into specific details, it raises the question of whether cavitation is more prevalent in the oxidizer or fuel side of the engine.

Overview:
Elon Musk and Tim Dodd discuss the design and operation of the Raptor engine, focusing on its unique features and challenges.

Inducer and Impeller Stages:
The Raptor engine employs an inducer and two impeller stages to increase the pressure of the propellants. A shallow angle inducer blade is used to minimize cavitation, which is the formation of bubbles in the propellant. The inducer feeds the first impeller, which generates a significant pressure rise.

Inline Powerhead:
The Raptor engine features an inline powerhead, which is a first for a rocket engine flown in space. This design allows for a straight line transfer of forces through the turbine assembly and chamber, eliminating the need for torque tubes.

Dual Expander Cycle:
Tim Dodd mentions the dual expander cycle, where both fuel and oxidizer are heated up. This cycle offers advantages in terms of exploiting additional heat and surface area, but no one has built a dual expander cycle aerospike engine yet.

Aerospikes:
Elon Musk expresses his fascination with aerospikes, which are nozzles that provide increased efficiency at high altitudes. Aerospikes are not as beneficial for two-stage rockets, where separate nozzles optimized for sea level and vacuum are more suitable.

Chamber Pressure:
SpaceX has always focused on increasing the chamber pressure of its engines, as higher pressure leads to increased thrust. Chamber pressure is a key metric for understanding the performance of a rocket engine.

Area Under the Force Time Curve:
Elon Musk emphasizes the importance of looking at the area under the force time curve as a measure of engine performance. This metric provides a more comprehensive understanding of the engine’s overall performance compared to solely relying on ISP (specific impulse).

Force vs. ISP in Rocket Engine Design:
Elon Musk emphasizes the importance of force over ISP (specific impulse) in rocket engine design. ISP is a simplified approximation that assumes constant conditions and does not account for variations in ambient conditions, mixture ratio, or throttle level. The actual measure of an engine’s performance is the area under the force versus time curve, which represents the total force produced over time.

Measuring Engine Performance:
Musk suggests that measuring the area under the force versus time curve is a more objective and accurate way to assess engine performance. This method is especially important for vacuum engines, where assumptions made during ISP calculations may not accurately reflect real-world conditions.

Need for a Catchy Name:
Musk acknowledges the need for a catchy and memorable name for this measure of engine performance. He suggests “integrated force” or “force field plot” as potential alternatives to the established term “specific impulse.”

Improvements and Future Developments:
Musk highlights the ongoing improvements and innovations in rocket engine design. He mentions the goal of eliminating shrouds in future designs, which would further enhance engine performance.

Upcoming Static Fire Test:
Musk confirms that the discussed improvements are intended for Booster 7 and that a static fire test of the booster is expected to take place soon.

Booster 7 LOX Transfer Tube Collapse:
During a test of Booster 7, a part of the LOX transfer tube collapsed, requiring repairs that will take approximately a week.

Importance of Rapid Iteration and Learning:
Elon Musk emphasizes the importance of rapid iteration and learning from tests, as they provide valuable lessons and insights.

Planned Visit to the High Bay:
Elon Musk and Tim Dodd plan to visit the high bay to inspect the booster and other facilities.

Gratitude for Support and Merchandise:
Tim Dodd expresses gratitude to his Patreon supporters for enabling the creation of content like the Everyday Astronaut videos. He also promotes the Everyday Astronaut merch shop, highlighting items such as shirts, dress wear, and scale model rockets.