Skyroot Aerospace, a trailblazing Indian space tech start-up, has achieved a ground breaking milestone in rocket engine development by successfully testing its Raman-2 engine with a vacuum nozzle for seconds using an innovative water injection technique to mitigate flow separation. This achievement marks a critical step toward advancing India’s private space sector, particularly in orbital launch vehicle technology.

The Raman-2 engine, designed for upper-stage propulsion and orbital adjustments, employs a vacuum-optimised nozzle that faces performance challenges during terrestrial testing due to atmospheric pressure. By strategically injecting water into separation zones, Skyroot engineers simulated vacuum conditions on Earth, enabling full-area-ratio testing and gathering vital data for flight qualification.

1. The Raman-2 Engine: A Hypergolic Powerhouse For Orbital Missions

Skyroot’s Raman series encompasses hypergolic engines designed for attitude control and upper-stage propulsion in the Vikram launch vehicles. While Raman-1 focuses on roll control with rapid-pulsing thrusters, Raman-2 represents a higher-thrust variant tailored for orbital adjustments. Key features of the Raman-2 engine include:

Engine Design And Propulsion

Hypergolic Propellants: Utilises monomethylhydrazine (MMH) and nitrogen tetroxide (N₂O₄), which ignite spontaneously upon contact, eliminating the need for complex ignition systems.

Vacuum Nozzle: Optimised for space environments with an extended nozzle to maximize exhaust velocity and specific impulse (Isp) in the vacuum of space.

Regenerative Cooling: A single higher-thrust engine design with regenerative cooling, enhancing efficiency for prolonged burns.

Technical Specifications

ParameterRaman-1 (Roll Control)Raman-2 (Upper Stage)
Peak Thrust890 N (Vacuum)~3.5 kN (Vacuum)*
Pulse Width60 millisecondsContinuous burn
Total Pulses280N/A
Burn Time104 seconds200 seconds
Test Duration352 seconds200 seconds

*Raman-2 thrust figures inferred from Dhawan-II’s 3.5 kN vacuum thrust and Raman-2’s role as a higher-thrust variant.

2. The Challenge of Flow Separation In Vacuum Nozzles

Extended nozzles designed for vacuum operation face inefficiencies during Earth-based testing due to flow separation, a phenomenon where exhaust gases detach from the nozzle walls, reducing performance. This occurs because:

Causes of Flow Separation

Adverse Pressure Gradients: Atmospheric pressure creates a mismatch between the nozzle’s internal and external pressures, causing the exhaust to separate.

Boundary Layer Thickening: Turbulent flow and friction along the nozzle walls thicken the boundary layer, leading to detachment.

Shock Wave Formation: In over-expanded nozzles, shock waves can disrupt smooth exhaust flow, exacerbating separation.

Consequences For Testing

Flow separation reduces the effective area ratio of the nozzle, limiting the achievable exhaust velocity and specific impulse. For a vacuum nozzle, this results in, lower thrust, reduced mass flow rate due to incomplete expansion. Terrestrial tests fail to replicate the nozzle’s vacuum performance, compromising flight readiness.

3. Skyroot’s Innovative Solution: Water Injection For Flow Control

To overcome flow separation, Skyroot engineers employed a water injection system to mimic vacuum conditions during terrestrial testing. This method addressed the core challenge by:

Mechanism of Action

Filling Separation Zones: Water injected into the nozzle’s separation regions fills the low-pressure zones, effectively creating a "simulated vacuum" environment.

Maintaining Continuous Flow: By preventing exhaust detachment, the nozzle operates at its full area ratio, enabling accurate Isp and thrust measurements.

Advantages Over Conventional Methods

MethodLimitationsSkyroot’s Approach
Nozzle RedesignCompromises vacuum performanceNo structural modifications
Active Flow ControlRequires complex actuatorsSimple water injection system
Reduced Test DurationIncomplete data collectionFull 200-second burn achieved

Method Limitations Skyroot’s Approach

Nozzle Redesign Compromises vacuum performance No structural modifications
Active Flow Control Requires complex actuators Simple water injection system
Reduced Test Duration Incomplete data collection Full 200-second burn achieved

4. The Raman-2 Test: A 200-Second Milestone

The April 2025 test of Raman-2 marked a major achievement, demonstrating the engine’s readiness for orbital missions. Key outcomes included:

Test Configuration

Vacuum Nozzle: A divergent nozzle optimized for space exhaust expansion.
Water Injection System: Strategically placed injectors to counteract separation.
Propellant System: MMH/N₂O₄ hypergolic propellants for reliable ignition.

Performance Metrics

Metric Value Significance

Burn Duration 200 seconds Matches orbital mission requirements
Flow Separation Mitigated Achieved full-area-ratio operation
Thrust Consistency Stable output Validated autopilot control systems

5. Broader Implications For Skyroot’s Rocket Development

The success of Raman-2 aligns with Skyroot’s broader propulsion strategy, which combines hypergolic and cryogenic engines to address diverse mission needs:

Hypergolic vs. Cryogenic Engines
Engine Type Raman Series Dhawan Series
Propellants MMH/N₂O₄ LNG/LOX (Cryogenic)
Application Attitude control, upper-stage adjustments Main upper-stage propulsion
Key Advantage Instant ignition, rapid pulsing High Isp, green propellants
Synergy in Vikram Launch Vehicles
Vikram-1: Utilises Raman-1 for roll control and Dhawan-I for upper-stage propulsion.
Vikram-2: Scheduled to deploy Raman-2 for orbital adjustments and Dhawan-II for main upper-stage thrust.

6. Innovations In Manufacturing And Propulsion

Skyroot’s engineering prowess extends beyond propulsion to advanced manufacturing techniques:

3D Printing And Composite Materials

Raman-1: 3D-printed thrust chamber and injector, metal matrix composite throat, and high-temperature composite thermal protection.

Dhawan-II: Fully 3D-printed cryogenic engine with LNG/LOX compatibility, achieving a 200-second burn in 2023.

Sustainable Propellants

LNG/LOX: Replaces traditional RP-1/kerosene, offering lower costs and reduced carbon emissions.

7. Challenges And Future Directions

While the Raman-2 test is a breakthrough, Skyroot faces ongoing challenges:

Upcoming Milestones

Flight Qualification: Two remaining milestones before Vikram-1’s launch.

Scalability: Adapting water injection for larger engines or repeated use.

Cryogenic-Hypergolic Integration: Ensuring seamless operation between Dhawan-II and Raman-2 in Vikram-2.

Global Competition

Skyroot competes with established players like SpaceX (Raptor engines) and emerging firms like Relativity Space (3D-printed rockets). Its focus on hybrid propulsion systems and green propellants positions it uniquely in the small-satellite launch market.

Conclusion

Skyroot Aerospace’s Raman-2 engine test exemplifies ingenuity in overcoming propulsion challenges through innovative flow control techniques. By addressing flow separation with water injection, the team validated a critical component for orbital missions, reinforcing India’s growing role in global space exploration. The synergy between hypergolic and cryogenic engines, coupled with 3D-printed manufacturing, positions Skyroot to disrupt the launch vehicle market with cost-effective, modular solutions. As the company progresses toward its first orbital launch, these advancements underscore its potential to redefine access to space.

IDN