An electrospray propulsion system employs an electric field on a conductive fluid, producing a rapid jet of minuscule droplets capable of launching a spacecraft. These compact engines are particularly well-suited for small satellites known as CubeSats, frequently utilized in scientific investigations.

Since electrospray propulsion systems leverage propellant more effectively than the robust, chemical rockets employed during launch, they are more favorable for accurate maneuvers in orbit. The thrust produced by an electrospray emitter is minimal, prompting the use of multiple emitters that function uniformly in parallel.

Nevertheless, these multiplexed electrospray thrusters are generally fabricated through costly and labor-intensive semiconductor cleanroom processes, restricting who can produce them and how such apparatus can be utilized.

To dismantle obstacles in space research, engineers at MIT have showcased the inaugural fully 3D-printed, droplet-emitting electrospray propulsion system. Their invention, which can be constructed swiftly and at a fraction of the price compared to conventional thrusters, employs commercially available 3D printing materials and methods. The devices could potentially be entirely manufactured in orbit, as 3D printing is suitable for in-space fabrication.

By creating a modular strategy that combines two 3D printing techniques, the researchers tackled the complexities involved in creating a sophisticated apparatus consisting of both macroscale and microscale components that must operate cohesively.

Their proof-of-concept thruster includes 32 electrospray emitters working collaboratively, producing a steady and consistent stream of propellant. The 3D-printed device produced thrust matching or exceeding that of current droplet-emitting electrospray engines. With this innovation, astronauts could swiftly fabricate an engine for a satellite without the need to wait for one to be dispatched from Earth.

“Utilizing semiconductor production doesn’t align with the concept of affordable access to space. Our goal is to democratize space hardware. In this research, we are proposing a method to create high-performance hardware with manufacturing techniques accessible to more participants,” states Luis Fernando Velásquez-García, a principal research scientist at MIT’s Microsystems Technology Laboratories (MTL) and senior author of a paper outlining the thrusters, which is published in Advanced Science.

He is accompanied on the paper by lead author Hyeonseok Kim, an MIT graduate student pursuing mechanical engineering.

A modular strategy

An electrospray propulsion system contains a reservoir of propellant that flows through microfluidic pathways to a set of emitters. An electrostatic field is applied at the tip of each emitter, instigating an electrohydrodynamic phenomenon that molds the free surface of the liquid into a cone-shaped meniscus that ejects a stream of high-velocity charged droplets from its apex, generating thrust.

The emitter tips must be as pointed as possible to achieve the electrohydrodynamic ejection of propellant at a low voltage. The device also requires a sophisticated hydraulic system to store and control the flow of liquid, effectively transporting propellant through microfluidic channels.

The emitter array consists of eight emitter modules. Each module is composed of an arrangement of four individual emitters that need to function collectively, creating a larger system of integrated modules.

“Employing a one-size-fits-all fabrication technique doesn’t yield successful results because these subsystems operate at different scales. Our pivotal insight was to merge additive manufacturing techniques to realize the targeted outcomes and develop an interface strategy so the components collaborate as effectively as possible,” Velásquez-García explains.

To achieve this, the researchers implemented two distinct types of vat photopolymerization printing (VPP). VPP entails projecting light onto a light-sensitive resin, which solidifies to form 3D structures with smooth, high-definition features.

The researchers manufactured the emitter modules employing a VPP technique called two-photon printing. This method utilizes an extremely focused laser beam to solidify resin in a specifically designated area, constructing a 3D structure voxel by voxel. This precision allowed them to create exceptionally sharp emitter tips and narrow, uniform capillaries to channel propellant.

The emitter modules are inserted into a rectangular casing referred to as a manifold block, which secures each module and supplies the emitters with propellant. The manifold block also connects the emitter modules with the extractor electrode that initiates propellant ejection from the emitter tips when an appropriate voltage is applied. Fabricating the larger manifold block using two-photon printing would be impractical due to the method’s low output and restricted printing volume.

Instead, the researchers adopted a technique called digital light processing, which employs a projector-sized chip to shine light into the resin, solidifying one layer of the 3D structure at a time.

“Each technique excels at a specific scale. By combining them to function collectively, we harness the best features of each approach,” Velásquez-García remarks.

Propulsion performance

However, merely 3D printing the electrospray engine components is only part of the challenge. The researchers also performed chemical analyses to verify that the printing materials were compatible with the conductive liquid propellant. If they were not compatible, the propellant could corrode the engine or lead to cracking, which is undesirable for equipment intended for prolonged use with minimal maintenance.

They also devised a method to secure the individual components together to prevent misalignments that could hinder performance and ensure the device remains watertight.

Ultimately, their 3D-printed prototype succeeded in producing thrust more efficiently than larger, more costly chemical rockets and surpassed existing droplet electrospray engines.

The researchers also analyzed how varying the propellant pressure and modulating the voltage supplied to the engine affected the droplet flow. Surprisingly, they discovered a broader range of thrust could be achieved through voltage modulation. This might eliminate the necessity for an intricate system of pipes, valves, or pressure signals to manage the flow of liquid, resulting in a lighter, more economical electrospray thruster that is also more efficient.

“We demonstrated that a simpler thruster can yield superior results,” Velásquez-García claims.

The researchers plan to continue investigating the advantages of voltage modulation in future studies. They also aim to construct denser and larger arrays of emitter modules. Furthermore, they may consider employing multiple electrodes to separate the processes of initiating the electrohydrodynamic ejection of propellant from shaping and accelerating the emitted jet. In the long run, they aspire to showcase a CubeSat that utilizes a fully 3D-printed electrospray engine during its function and deorbiting.

This research is partially funded by a MathWorks fellowship and the NewSat Project and was conducted in part using MIT.nano facilities.