United States Patent US6488233B1: Laser Propelled Vehicle (Myrabo Lightcraft)

Patent Number: US6488233B1 Title: Laser Propelled Vehicle Inventor: Leik N. Myrabo, Bennington, VT (US) Assignee: United States Department of the Air Force Filed: April 30, 2001; Granted December 3, 2002 Status: Expired - Fee Related (anticipated expiration 2021-04-30) Jurisdiction: United States (US) Source: patents.google.com/patent/US6488233B1/en Track Directory (Physics_Math): N/A — laser-sustained detonation (LSD) propulsion domain; no current track covers remote laser power delivery; candidate for a future laser propulsion track

Image files:

• patents_intl/tweets/raw_download/1957867627256168673_1.jpg (Google Patents abstract page)
• patents_intl/tweets/raw_download/1957867627256168673_2.jpg (full patent text + Figure 1 diagram)

Abstract

"Provided is a laser propelled craft having a) a forebody or nose, b) a tapering parabolic afterbody optic or mirror, c) a shroud mounted therebetween and extending aft to define an annular space around a portion of the afterbody near its base and d) means to transmit a pulsed laser beam toward the laser craft and afterbody optic and thence to focus into the annular shroud. The laser beam is pulsed to heat and pressurize the air in the annular space to expand same and propel such craft, the afterbody and shroud being so shaped as to self center or remain in the laser beam as the craft is propelled thereby. Such craft, which is spin-stabilized, can also carry a fuel insert ring mounted in the shroud around the afterbody, to be ablated by the laser beam at a desired altitude, so as to transition from an air breathing craft to a rocket craft, when the atmospheric density becomes too low, e.g., at 30 km altitude so that the lasercraft can thereafter be propelled into, e.g., low earth orbit."

Patent Geometry

The Figure 1 diagram shows a conical-nose craft (forebody 14, centered on axis 10) with the parabolic afterbody mirror (12) at the base, forming a retroreflector geometry. The annular shroud (16) extends from the base junction rearward, creating the annular plenum (19) where pulsed laser energy is focused and deposited. The ground-based laser beam enters from below, reflects off the parabolic mirror 12 into the annular plenum, where it is absorbed and causes repetitive air-breakdown plasma pulses — effectively a series of laser-sustained detonation waves that produce net upward thrust.

Physics Mechanism: Laser-Sustained Detonation (LSD) Propulsion

The propulsion physics is laser ablation and laser-sustained detonation (LSD) propulsion. In the LSD regime, an intense laser pulse deposits energy into a gas volume faster than acoustic dissipation can carry it away, creating a supercritical detonation wave that propagates toward the beam. The detonation wave converts laser photon energy to thermodynamic enthalpy with an efficiency that, under optimal pulse parameters, exceeds 50% for atmospheric operation.

The specific impulse of a laser-sustained detonation thruster operating in air-breathing mode (no on-board propellant) is theoretically bounded by:

I_sp = c_s/g₀

where c_s is the detonation wave speed, yielding I_sp values of order 500–2000 s — significantly superior to chemical rockets (I_sp ~ 300–450 s) and competitive with ion drives for mass-specific impulse, without the multi-kilowatt power constraint that limits ion drives at large scales.

Inventor and Experimental Validation

Inventor Leik N. Myrabo (Rensselaer Polytechnic Institute, Department of Mechanical, Aerospace and Nuclear Engineering) is the originator of the "Lightcraft" concept, which was experimentally demonstrated with 100 J pulsed CO₂ laser pulses at the White Sands Missile Range in October 1997, achieving a world altitude record for laser-propelled vehicles. The 2001 patent thus represents a mature concept with demonstrated experimental results behind it, not purely theoretical speculation. The assignment to the US Air Force indicates the technology was considered operationally relevant.

Dual-Mode Operation and Self-Centering

The transition mechanism described — from air-breathing LSD mode to ablative rocket mode using a fuel insert ring at 30 km altitude — solves the single-stage-to-orbit problem for compact payloads without requiring on-board oxidizer. The conical geometry is self-centering in the laser beam: if the craft deviates laterally, the parabolic mirror redirects the beam asymmetrically, producing a restoring force. This passive guidance feature makes precision laser tracking unnecessary for stable flight.

Prior Art and Claims

The patent cites prior art back to Kantrowitz (1974) and Minovitch (1974), confirming that laser propulsion had been theoretically developed for nearly three decades before the 2001 filing. Myrabo's contribution was the specific parabolic-mirror annular-shroud geometry that enables self-centering and dual-mode operation. The 17 claims in the patent include broad claims on "any laser propelled craft" incorporating the parabolic afterbody and annular shroud, covering the ground-based-laser architecture comprehensively.

For classified programs, the significance is that a ground-based or platform-based laser of multi-megawatt pulsed power (achievable with high-energy laser weapons programs such as the Airborne Laser, YAL-1A, operational 2001–2011) could propel unmanned craft to orbit without on-board propellant at vastly reduced cost and complexity per sortie. The Air Force's assignment of this patent is consistent with a classified rapid-response satellite-launch or ASAT delivery program.

Sources

• US6488233B1 on Google Patents
• Kantrowitz, A. (1974) — early laser propulsion theory
• Minovitch, M. (1974) — laser propulsion prior art
• White Sands Missile Range Lightcraft demonstration, October 1997
• Myrabo, L.N., Rensselaer Polytechnic Institute

This information was compiled from Break_thrus.mdx staging file.