launches, while the rockets could contain inert propellant. Microwave thermal rocket A microwave thermal rocket is similar to a laser thermal rocket, except that it is powered by a microwave source, for example a ground-based phased array. Relative to lasers, the main advantage of using microwaves is that sources currently cost 1-3 orders of magnitude less per Watt. The main disadvantage is that the microwave beam director needs to have a much larger diameter than a laser beam director due to beam diffraction effects. The microwave thermal rocket was invented by Kevin L.G. Parkin in 2002 and was the subject of his Ph.D. dissertation. Between May 2012 and March 2014, the DARPA/NASA millimeter-wave thermal launch system (MTLS) project continued this work, culminating in the first microwave thermal rocket launch in February 2014. Several launches were attempted but problems with the beam director could not be resolved before funding ran out in March 2014. References Rocket engines