Maneuver Planning with Laser Ablative Propulsion (LAP)
What is LAP?
Laser ablative propulsion, a promising technique involving the use of high-powered lasers to ablate material and produce thrust, has garnered increased interest in recent years due to its potential for both terrestrial and space applications. My research evaluates the trade spaces associated with implementing laser ablative propulsion systems on the ground and in space. The parameters investigated include system mass, energy requirements, thrust-to-weight ratio, specific impulse, and efficiency. Ground systems primarily focus on the potential for launch assist and rapid transport, while space systems assess interplanetary travel and satellite station-keeping.
For ground systems, considerations include atmospheric absorption, beam divergence, adaptive optics, and cooling mechanisms. The evaluation demonstrates that, while there are challenges due to atmospheric interactions, there are niche applications where laser propulsion can offer advantages over traditional chemical rockets.
Space systems present unique challenges, such as power generation and thermal management in the vacuum of space. However, the absence of atmospheric losses means that greater propulsion efficiencies can be realized, making it especially attractive for long-duration space missions and rapid orbital transfers.
Comparing Ground Based and Orbital Laser Platforms
Inclination change, especially in low orbit, is a costly maneuver. Because of the proximity to Earth’s surface, both ground-based and orbital laser options are available. For a ground-based laser, the spacecraft is restricted to the cone determined by the laser's position and the maximum illumination angle from zenith. A laser pointed directly above will experience less distortion and loss from the atmosphere compared to one pointed close to the horizon. A ground-based laser is cheap, but slow. Alternatively, a laser placed in solar synchronous orbit can illuminate the target for frequently and longer, at the cost of placing this hardware in space. I coded a numerical orbital simulation evaluating both of these approaches. One simulation of each approach is plotted on the right, where the paths taken by the spacecraft and space laser (if present) are shown.
Establishing the Trade Space
By iterating over many different laser parameters, target orbits, and payload amounts, I was able to establish a trade space through which trajectory planners can find the best approach for missions. Typically, the figure of merit chosen was DeltaV cost, a the ability for spacecraft to affect changes in its orbit, which is heavily budgeted for all space missions. One impressive characteristic of LAP is it’s variable specific impulse modes, which changes the constraint from DeltaV to Time of Flight (ToF). If a maneuver takes a very long time, it may invalidate the advantages brought about by variable specific impulse. These contour plots are generated using two plausible ground-based and orbital lasers with achievable laser technology. With launch cost reductions, improvements in material science, and increases in size of manufactured optics, these plots will continuously change. The scripts can automatically generate new trade spaces to account for these technological advancements.
Ground-based laser performance is plotted as a function of payload mass and laser power. Performance of the lowest power (100 kW) can be compared to the laser shown in the plot to the right, where the orbital laser compeletes the maneuver much quicker at the cost of placing hardware in space.
This figure defines a linear correlation between the specific power of the propulsion system and a laser pulse width parameter, necessary for optimization of trajectories for variable specific impulse modes.
Figure 12, https://arc.aiaa.org/doi/10.2514/1.43733
Red segments are when the target is illuminated. It takes multiple orbits for a ground-based laser to complete it.
The cyan shows a solar synchronous orbital laser that can complete the maneuver in a single orbit.
For an orbital laser of fixed power, the ToF is plotted as a function of degrees of inclination changed and orbital altitude. Despite the lower DeltaV costs of a higher altitude inclination change, an orbital laser can perform one much quicker at lower altitudes.