Designing and optimizing an aerospike micro-nozzle for the sub-mN range

Abstract

Micro-propulsion technology is under rapid development and
considerably extends the mission capabilities of small spacecraft. However, the
flow in conventional nozzles on a microscale is relatively viscous, which,
combined with the bounding nozzle walls, leads to low efficiencies. Aerospike
nozzles, on the other hand, offer free-flow expansion and mitigate the boundary
layer effects.  The research objective of
this project is to investigate the losses and output efficiency of
micro-nozzles by optimizing a 3D axisymmetric aerospike in the μN-mN range.
Planar nozzles, typically found in micro- propulsion subsystems, limit the flow
radial expansion and considerably suffer from viscous losses, as well as
momentum loss. In addition, in lower thrust settings, the boundary layer
massively limits the performance of conventional nozzles – conical and bell
layouts –, whereas aerospikes do not bound the flow and, subsequently, inherit
better gas expansion. The design optimization relies on numerical analyses,
with the free and open-source OpenFOAM’s solver, rhoCentralFoam, and
self-developed Python scripts that automate the simulations in various
machines.  This study comprises three
design iterations. The first contour parameters derive from Delft University of
Technology’s Vaporizing Liquid Micro-thruster (VLM) requirements and the
preceding work. However, since this work considers axisymmetric geometries, the
aerospike’s throat width is reduced from 45 μm to 15 μm to preserve the
original thrust magnitude (< 10 mN).  The
initial results show that, at the same throat Reynolds number, the aerospike
outperforms the bell nozzle, especially towards lower Reynolds, where the
specific impulse efficiency variation tops 25%. However, at equal thrust
levels, the conventional design surpasses the aerospike thrust and specific
impulse efficiencies. The Mach contours reveal that the small throat width and
high area ratio ineffectively mitigate the viscous losses and lead to extreme
momentum loss.  The following iterations
with four truncation percentages (20%, 40%, 60%, and 80%) prove the first
hypothesis right: when decreasing the area ratio from ~17 to ~3 and raising the
throat width to 30 μm, the efficiency of an 80% long aerospike reaches ~94% for
the specific impulse and ~89% for the thrust. In addition, the aerospike yields
the best performance when it is the least truncated (highest truncation
percentage, i.e., 80%).  Finally, a ±100
K temperature sensitivity study shows that the aerospike performance oscillates
up to 3%, with a maximum thrust efficiency of 91% and specific impulse
efficiency close to 98%, rivaling macroscale performance. With a small area
ratio and a wide throat, the aerospike nozzle outperforms an equivalent bell
nozzle by more than 10% in terms of specific impulse and thrust efficiencies.