Wesley Helgeson

Performance Test Results for the Laser‐Powered Microthruster

Abstract : Microthrusters are useful for orienting and repositioning small craft above the atmosphere. We report technical results obtained during a successful 5-year program to develop a commercially-viable laser-powered microthruster. Its main advantage is the ability to generate a broad thrust range under programmable electronic control with minimal electrical power. The device applies millisecond-duration diode-laser pulses to a fuel tape to produce an ablation jet. By employing laser-initiated energetic polymers in our ablation fuel tapes, we obtained momentum coupling coefficients as large as 2.0mN/W of incident laser power, giving a continuous thrust range from 50 N to 10mN. With our standard 30m x 8mm fuel tape, fueled thruster mass is 0.4kg and 40N-s lifetime impulse is achieved. With an order-of-magnitude greater fuel mass, the thruster could accomplish re-entry or substantial orbit-raising of a 10-kg microsatellite. In its usual configuration, specific impulse is 200 seconds, and ablation efficiency, the ratio of exhaust kinetic energy to incident laser optical energy is 180%. We compare performance of several laser-initiated micropropellants which we studied, including polyvinyl nitrate (PVN), glycidyl azide polymer (GAP), and nitrocellulose (NC). All were doped with a laser-absorbing component, either carbon nanopearls with 10nm mean diameter or dyes tuned to the 920-nm laser wavelength but transparent at visible wavelengths. Our demonstrated momentum coupling coefficient is sufficient to levitate a 0.1-kg object with a 400-W laser beam having appropriate characteristics.

A ns‐Pulse Laser Microthruster

We have developed a prototype device which demonstrates the feasibility of using ns‐duration laser pulses in a laser microthruster. Relative to the ms‐duration thrusters which we have demonstrated in the past, this change offers the use of any target material, the use of reflection‐mode target illumination, and adjustable specific impulse. Specific impulse is adjusted by varying laser intensity on target. In this way, we were able to vary specific impulse from 200s to 3,200s on gold. We used a Concepts Research, Inc. microchip laser with 170mW average optical power, 8kHz repetition rate and 20μJ pulse energy for many of the measurements. Thrust was in the 100nN – 1μN range for all the work, requiring development of an extremely sensitive, low‐noise thrust stand. We will discuss the design of metallic fuel delivery systems. Ablation efficiency near 100% was observed. Results obtained on metallic fuel systems agreed with simulations. We also report time‐of‐flight measurements on ejected metal ions, which ga...

Giant Momentum Coupling Coefficients from Nanoscale Laser-initiated Exothermic Compounds

*† ‡ § ** †† ‡‡ We report here on results obtained with laser-initiated micro-propellants, such as PVN, PVC, GAP, NC, and mixtures of these. All samples were doped with a laser absorbing component. In some cases, this was carbon nanopearls with 10nm mean diameter, while, in others, it was a carbon-based ink with ∝m-size particles. We also report results of performance tests for absorbers tuned to the 935-nm laser wavelength.

A Low‐Noise Thrust Stand for Microthrusters with 25nN Resolution

Accurate measurement of thrust from microthrusters is a significant technical challenge. We report significantly improved performance from our microthruster thrust stand which has now demonstrated 25nN thrust resolution and can carry a fully operational microthruster and power supply with up to 15kg total mass. Thrust capacity is 500μN with 0.5% accuracy and minimum response with 25% accuracy is 100nN. Because it is a critically‐damped torsion balance with tens‐of‐seconds response time, thrust readout noise is essentially absent for frequencies above 0.1 Hz. Other reported measurement techniques require careful leveling of the apparatus, or have higher readout noise, lower sensitivity and greater sensitivity to ambient mechanical vibrations. A key ingredient for the performance of our system design is an optical readout that responds only to rotation of the torsion bar, but not to ambient vibration. This feature eliminates sensitivity to footsteps, nearby vacuum pumps, etc. We will describe the design, ca...

Liquid-fueled, Laser-powered, N-class thrust Space Engine with Variable Specific Impulse

We discuss the requirements for developing a lightweight laser‐powered space engine with specific impulse range 200<Isp<3,600 seconds and 6N maximum thrust. Operating parameters which can achieve this have been demonstrated separately in the laboratory. We review earlier solid fuel data. Such an engine can put small satellites through demanding maneuvers in short times, while generating the optimum specific impulse for each mission segment.We will address specific problems which have been solved. The first of these is fuel delivery to the laser focus. A pulsed laser format is required for reasons we will discuss. Solid‐fuel configurations (such as the fuel tapes used in the Photonic Associates μLPT microthruster) are not amenable to the range of mass delivery rates (μg/s to mg/s) necessary for such an engine. Liquid fuels are suggested. However, liquids with ordinary viscosities splash under pulsed laser illumination, ruining engine performance by causing the majority of fuel mass to be ejected at low Isp...