Editing Rocket engine (section)

====Beamed thermal====

{| class="wikitable"
|-
! Type
! Description
! Advantages
! Disadvantages
|-
! [[Laser propulsion|Light-beam-powered rocket]]
| Propellant is heated by light beam (often laser) aimed at vehicle from a distance, either directly or indirectly via heat exchanger
| Simple in principle, in principle very high exhaust speeds can be achieved
| ≈1 MW of power per kg of payload is needed to achieve orbit, relatively high accelerations, lasers are blocked by clouds, fog, reflected laser light may be dangerous, pretty much needs hydrogen monopropellant for good performance which needs heavy tankage, some designs are limited to ≈600 seconds due to reemission of light since propellant/heat exchanger gets white hot
|-
! [[Beam-powered propulsion|Microwave-beam-powered rocket]]
| Propellant is heated by microwave beam aimed at vehicle from a distance
| [[Specific impulse|''I''<sub>sp</sub>]] is comparable to Nuclear Thermal rocket combined with T/W comparable to conventional rocket.  While LH<sub>2</sub> propellant offers the highest I<sub>sp</sub> and rocket payload fraction, ammonia or methane are economically superior for earth-to-orbit rockets due to their particular combination of high density and I<sub>sp</sub>. [[Single-stage-to-orbit|SSTO]] operation is possible with these propellants even for small rockets, so there are no location, trajectory and shock constraints added by the rocket staging process.  Microwaves are 10-100× cheaper in $/watt than lasers and have all-weather operation at frequencies below 10&nbsp;GHz.
| 0.3–3{{nbsp}}MW of power per kg of payload is needed to achieve orbit depending on the propellant,<ref>{{cite web|url=http://parkinresearch.com/microwave-thermal-rockets/|title=Microwave Thermal Rockets|last=Parkin|first=Kevin|access-date=8 December 2016}}</ref> and this incurs infrastructure cost for the beam director plus related R&D costs. Concepts operating in the millimeter-wave region have to contend with weather availability and high altitude beam director sites as well as effective transmitter diameters measuring 30–300 meters to propel a vehicle to LEO.  Concepts operating in X-band or below must have effective transmitter diameters measured in kilometers to achieve a fine enough beam to follow a vehicle to LEO. The transmitters are too large to fit on mobile platforms and so microwave-powered rockets are constrained to launch near fixed beam director sites. 
|}