Editing Rocket engine (section)

===Thermal===

====Preheated====
{| class="wikitable"
|-
! Type
! Description
! Advantages
! Disadvantages
|-
! [[Hot water rocket]]
| Hot water is stored in a tank at high temperature&thinsp;/&thinsp;pressure and turns to steam in nozzle
| Simple, fairly safe
| Low overall performance due to heavy tank; [[specific impulse|''I''<sub>sp</sub>]] under 200 seconds
|}

====Solar thermal====

The [[solar thermal rocket]] would make use of solar power to directly heat [[reaction mass]], and therefore does not require an electrical generator as most other forms of solar-powered propulsion do. A solar thermal rocket only has to carry the means of capturing solar energy, such as [[Concentrating solar power|concentrator]]s and [[mirror]]s. The heated propellant is fed through a conventional rocket nozzle to produce thrust. The engine thrust is directly related to the surface area of the solar collector and to the local intensity of the solar radiation and inversely proportional to the ''I''<sub>sp</sub>.

{| class="wikitable"
|-
! Type
! Description
! Advantages
! Disadvantages
|-
! [[Solar thermal rocket]] 
| Propellant is heated by solar collector
| Simple design. Using hydrogen propellant, 900 seconds of [[specific impulse|''I''<sub>sp</sub>]] is comparable to nuclear thermal rocket, without the problems and complexity of controlling a fission reaction.{{citation needed|date=January 2011}} Ability to [[Solar thermal rocket#Proposed solar-thermal space systems|productively use]] waste gaseous [[hydrogen]]—an inevitable byproduct of long-term [[liquid hydrogen]] storage in the [[Radiative heat transfer|radiative heat]] environment of space—for both [[orbital stationkeeping]] and [[Spacecraft attitude control|attitude control]].<ref name=aiaa20100902>{{cite web|last=Zegler|first=Frank |title=Evolving to a Depot-Based Space Transportation Architecture |url=http://www.ulalaunch.com/site/docs/publications/DepotBasedTransportationArchitecture2010.pdf |archive-url=https://web.archive.org/web/20110717150155/http://www.ulalaunch.com/site/docs/publications/DepotBasedTransportationArchitecture2010.pdf |url-status=dead |archive-date=2011-07-17 |work=AIAA SPACE 2010 Conference & Exposition |publisher=AIAA |access-date=2011-01-25 |author2=Bernard Kutter  |date=2010-09-02 }} See page 3.</ref>
| Only useful in space, as thrust is fairly low, but hydrogen has not been traditionally thought to be easily stored in space,<ref name=aiaa20100902/> otherwise moderate/low [[Specific impulse|''I''<sub>sp</sub>]] if higher–molecular-mass propellants are used.
|}

====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. 
|}

====Nuclear thermal====

{| class="wikitable"
|-
! Type
! Description
! Advantages
! Disadvantages
|-
! [[Radioisotope rocket|Radioisotope rocket/"Poodle thruster"]] (radioactive decay energy)
| Heat from radioactive decay is used to heat hydrogen
| About 700–800 seconds, almost no moving parts
| Low thrust/weight ratio.
|-
! [[Nuclear thermal rocket]] (nuclear fission energy)
| Propellant (typically, hydrogen) is passed through a nuclear reactor to heat to high temperature
| [[Specific impulse|''I''<sub>sp</sub>]] can be high, perhaps 900 seconds or more, above unity thrust/weight ratio with some designs
| Maximum temperature is limited by materials technology, some radioactive particles can be present in exhaust in some designs, nuclear reactor shielding is heavy, unlikely to be permitted from surface of the Earth, thrust/weight ratio is not high.
|}