Aerospike Engine CFD | Yugesh Bhoge

Motivation

Conventional bell nozzles are geometrically fixed, designed for a single ambient pressure ($P_a$). This leads to efficiency losses due to over-expansion at sea level (flow separation) and under-expansion in vacuum.

The Aerospike engine proposes a solution: an "inside-out" nozzle where the expansion surface is open to the atmosphere. This allows the exhaust plume to dynamically adjust to the changing $P_a$ during ascent, theoretically maintaining optimal $I_{sp}$ at all altitudes.

Theoretical Framework

1. General Thrust Equation

$$ F = \dot{m} V_e + (P_e - P_a) A_e $$

Significance: The pressure term $(P_e - P_a)A_e$ causes drag when $P_a > P_e$ (over-expansion). Aerospikes minimize this penalty by ensuring $P_e \approx P_a$.

2. Prandtl-Meyer Expansion

$$ \nu(M) = \sqrt{\frac{\gamma+1}{\gamma-1}} \tan^{-1} \dots $$

Significance: The flow turns around the cowl lip via expansion fans. In truncated spikes, "base bleed" creates a recirculating fluid spike, adding a base pressure thrust component ($P_{base} A_{base}$).

CFD Methodology

Solver: ANSYS Fluent (Density-based, Implicit, 2D Axisymmetric).
Turbulence: $k$-$\epsilon$ standard model for high-speed compressible flow.
Geometry: Truncated aerospike (20% truncation) vs. Bell nozzles at area ratios AR = 5, 9, and 25.
Range: Full altitude sweep 0 – 60 km.

Results: Performance Comparison

Thrust Coefficient ($C_f$) vs. Altitude — aerospike versus three bell nozzle area ratios.

Aerospike Bell AR = 25 (Vac) Bell AR = 9 Bell AR = 5 (SL) Thrust Advantage Region Advantage % → right axis

Analysis

The shaded region confirms altitude compensation: the aerospike exceeds the best available bell nozzle (AR = 25, vacuum-optimised) by up to ~10% at 60 km. The advantage gradually erodes as altitude decreases, with a notable dip near 10 km where the AR = 25 bell's design pressure briefly matches ambient. Near sea level all nozzles converge, while the aerospike self-adjusts, eliminating the usual sea-level over-expansion penalty of high-AR bells.

CFD Simulation Visualisation

Velocity contours and exhaust-plume evolution across the altitude sweep.

CFD Animation

Applications

SSTO (Single-Stage-To-Orbit): Vehicles like the conceptual X-33/VentureStar require engines efficient at both sea level (liftoff) and vacuum (orbit insertion) without staging. The Aerospike is the primary candidate for such mission profiles.

Implications

Mass Reduction: By truncating the spike at ~20% of its theoretical length, we reduce engine weight significantly. The "base pressure" generated by the recirculation zone compensates for the missing physical structure, optimising the thrust-to-weight ratio.

Altitude Compensation in Aerospike Engines

Results: Performance Comparison

Analysis

CFD Simulation Visualisation

Applications

Implications

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