Patent US6488233B1 — Laser Propelled Vehicle (Myrabo Lightcraft)

Bibliographic Information

Field	Details
Patent Number	US6488233B1
Title	Laser Propelled Vehicle
Inventor	Leik N. Myrabo, Bennington, VT, USA
Assignee	United States Department of the Air Force
Filing Date	April 30, 2001
Publication Date	December 3, 2002
Status	Expired — Fee Related (expired December 3, 2010)
Application Number	US09/845,574
Classification	B64G1/409 — Unconventional spacecraft propulsion systems
Jurisdiction	United States (US)

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.

Claims

Claim 1: A laser propelled craft comprising: a forebody or nose; a tapering parabolic afterbody optic having a tip at the fore end thereof and a base at the aft end thereof; a shroud extending aft from the base of the parabolic afterbody optic and surrounding the parabolic afterbody optic to define an annular space therebetween; means to transmit a laser beam toward the craft; and the afterbody optic receiving the laser beam and focusing same into the annular space to heat and pressurize fluid in the annular space to propel the craft.

Claim 2: The craft of claim 1 wherein the parabolic afterbody optic has the following shape: y² = 4fDx, where y is the radial distance from the axis, x is the distance from the vertex of the parabola, and fD is the focal distance.

Claim 3: The craft of claim 1 wherein the shroud has a trailing edge angle θ of about 10° to 30°.

Claim 4: The craft of claim 1 wherein the shroud has a trailing edge angle θ of about 15°.

Claim 5: The craft of claim 1 wherein the shroud is configured as a plug nozzle.

Claim 6: The craft of claim 1 further comprising spin-stabilization means for spin stabilizing the craft.

Claim 7: The craft of claim 6 wherein the craft is spin stabilized at 1,000–10,000 rpm.

Claim 8: The craft of claim 6 wherein the craft is spin stabilized at 3,000–5,000 rpm.

Claim 9: The craft of claim 1 wherein the laser beam is annular.

Claim 10: The craft of claim 1 wherein the laser beam is a solid circular cross-section beam.

Claim 11: The craft of claim 1 further comprising a fuel insert ring mounted in the shroud around the afterbody optic to be ablated by the laser beam, so as to transition from an air breathing craft to a rocket craft when atmospheric density becomes too low.

Claim 12: The craft of claim 11 wherein the fuel insert ring is composed of acetal resin.

Claim 13: The craft of claim 11 wherein the fuel insert ring is composed of a formaldehyde polymer.

Claim 14: The craft of claim 11 wherein the fuel insert ring is composed of polymethylmethacrylate (PMMA).

Claim 15: The craft of claim 11 wherein the fuel insert ring is composed of polyimide.

Claim 16: The craft of claim 11 wherein the fuel insert ring is composed of polytetrafluoroethylene (PTFE).

Claim 17: The craft of claim 1 wherein the craft has a mass of 20–1,000 grams.

Description / Specification

Three Main Structural Components

1. Forebody aeroshell (component 14, centered on axis 10) — acts as compression surface and payload bay; conical nose
2. Parabolic afterbody optic/mirror (component 12) — at the base of the forebody; shaped as a retroreflector using the parabolic profile y² = 4fDx; reflects the incoming laser beam into the annular plenum
3. Annular shroud (component 16) — extends aft from the base of the afterbody mirror, creating the annular thrust chamber (annular plenum 19)

Propulsion Physics: Laser-Sustained Detonation (LSD) and Laser-Sustained Combustion (LSC)

The ground-based laser beam (preferably CO₂, pulsed at 10–30 Hz) enters from below, reflects off the parabolic afterbody mirror into the annular plenum, where it is absorbed and causes repetitive surface-induced electrical breakdown (at ~10⁶ W/cm² irradiance). This generates "laser supported detonation (LSD)" or "laser supported combustion (LSC)" waves that propagate toward the afterbody and create pulsed thrust impulses.

LSD wave physics: At sufficient laser irradiance, a supercritical detonation wave propagates toward the beam, converting photon energy to thermodynamic enthalpy. At atmospheric pressure and optimal pulse parameters, efficiency exceeds 50%.

Specific impulse in air-breathing mode: bounded by the Chapman-Jouguet detonation speed, yielding I_sp values of 500–2,000 s — superior to chemical rockets (I_sp ~ 300–450 s).

At ~30 km altitude, atmospheric density drops below the threshold for air-breathing LSD operation; the craft transitions to ablative rocket mode by ablating the fuel insert ring (acetal resin, PMMA, polyimide, PTFE, or other hydrogen-carbon-oxygen polymers) with the laser beam, producing propellant vapor that the laser then heats and expands through the nozzle.

Self-Centering Beam-Riding Geometry

The parabolic afterbody geometry is explicitly designed to be self-centering in the laser beam. If the craft deviates laterally, the parabolic mirror redirects the beam asymmetrically — more laser energy enters one side of the annular plenum than the other, producing a differential thrust that acts as a restoring force driving the craft back toward beam center. This passive guidance feature eliminates the need for precision laser tracking for stable flight.

Spin Stabilization

The craft is spin-stabilized at 1,000–10,000 rpm (optimal: 3,000–5,000 rpm). Spin stabilization provides gyroscopic rigidity, preventing tumbling. The annular shroud geometry allows spin without interrupting the circular symmetry of the laser-plenum interaction.

Material Specifications

Shroud materials (must withstand 300–3,000°C plasma):

• Carbon-carbon composites (C/C)
• Carbon-silicon carbide composites (C/SiC)
• Silicon carbide-silicon carbide composites (SiC/SiC)

Structural materials: Aircraft aluminum or ceramic matrix composites (CMC)

Fuel insert ring materials: Acetal resin (formaldehyde polymer), PMMA, polyimide, polytetrafluoroethylene (PTFE), or any hydrogen-carbon-oxygen polymer

Performance Specifications

Parameter	Value
Vehicle mass (baseline)	20–60 grams
Vehicle mass (range)	20–1,000 grams
Laser average power	10 kW (350 J pulses at 28 Hz, 18 μs pulse width)
Altitude capability (air-breathing)	Up to 30 km
Maximum velocity (air-breathing)	Up to Mach 5
Shroud diameter	10–140 cm (scalable)
Spin rate (optimal)	3,000–5,000 rpm

Laser Specifications

Parameter	Value
Preferred type	Pulsed CO₂ (electric discharge)
Alternatives	CO, iodine, oxygen-iodine, excimer
Preferred repetition rate	10–30 Hz (CO₂)
Scalable range	50–1,000 J/pulse at 10–1,000 Hz

Experimental Validation

Inventor Leik N. Myrabo (Rensselaer Polytechnic Institute, Dept. of Mechanical, Aerospace and Nuclear Engineering) demonstrated this concept experimentally at White Sands Missile Range in October 1997 using 100 J pulsed CO₂ laser pulses, achieving a world altitude record for laser-propelled vehicles. The 2001 patent thus represents a mature concept with demonstrated experimental results.

Performance vs. Chemical Rockets

The patent was filed to address space launch costs of $2,500–$12,000 per pound to LEO. The theoretical cost reduction is 100–1,000× compared to chemical rockets for small payloads (up to 100 kg), with no on-board propellant required in air-breathing mode.

Technical Classifications

• B64G1/409 — Cosmonautic vehicles: unconventional spacecraft propulsion systems

Prior Art Referenced

The patent cites prior art back to Kantrowitz (1974) and Minovitch (1974), establishing that laser propulsion had been theoretically developed for nearly three decades before the 2001 filing. Myrabo's specific contribution was the parabolic-mirror annular-shroud self-centering geometry.

Citations

• Google Patents: US6488233B1
• Kantrowitz, A. (1974) — early laser propulsion theoretical framework
• Minovitch, M. (1974) — laser propulsion prior art
• White Sands Missile Range Lightcraft experimental demonstration, October 1997
• Related patent: RU2266420C2 (Russia, 2005) — omnidirectional laser jet engine, cites US6488233B1 as prototype

Patent text compiled from Google Patents. Full original at the above URL.