Reaction engine

From Kerbal Space Program Wiki
Revision as of 03:41, 11 May 2015 by Brendan (talk | contribs) (Xenon works for ion drives *because* of its higher atomic weight. Still vastly more efficient.)
Jump to: navigation, search

A reaction engine is an engine that works via “equal and opposite reaction” as in Newton's third law of motion. Specifically, they generate thrust by expelling reaction mass in the opposite direction as their acceleration. For our purposes, the reaction mass propelled outward is always a form of fuel, though not always one modelled on chemical reactions.

Types

Different sources classify kinds of reaction engine a bit differently, but all contradistinguish those which must collect outside material from those using only onboard material as reaction mass.

Staying relevant to KSP, we separate out air-breathing jet engines which can only operate inside an oxygen-rich atmosphere. All[outdated] use the same resources and are controlled the same way.

All[outdated] KSP's other reaction engines can operate using only resources stored or generated onboard and are collectively called rocket engines. There are many types of rocket engines. They differ by the kind of fuel they use and the controls used to operate them in-game.

Listed below are general descriptions of jet engines and several broad types of rocket engines along with their advantages and disadvantages.

Jet engines


→ Main article: Jet engine

Jet engines use the same fuel as rocket engines, but unlike them jets draw oxygen from the atmosphere using air intakes rather than carrying the weight of oxidizer onboard. This is represented in-game by a much lower rate of fuel consumption. The classical rocket equation doesn't hold valid for them. In flight they face the trade-off between the lower atmosphere where intake air is easily available at lower speeds but air resistance is greater, and the upper atmosphere where there's less air resistance but higher speeds are required to collect sufficient intake air.

Advantages

  • Excellent fuel efficiency within an oxygenated atmosphere
  • Excellent power to weight ratio
  • All current[outdated] jet engines provide thrust vectoring for greater maneuverability

Disadvantages

  • Cannot be used outside of an oxygenated atmosphere.
  • Thrust output changes depending on speed
  • Efficiency changes depending on altitude
  • Engine requires time to “spool up” to maximum thrust potential
[Expand] Available jet engines
[Expand] Available "jet fuel" tanks

Solid fuel rocket engines


→ Main article: Solid rocket booster

The most basic type of rocket engine is the solid fuel rocket. Solid fuel is simply a self-oxidizing compound or mixture within a casing with a nozzle at the end to direct the exhaust gases produced. It offers no real control beyond choosing when to ignite it, though tweakables enable altering the thrust limit and total fuel.

Advantages

  • Very high thrust-to-weight ratio
  • Engine and fuel tank are combined in one part, lowering part count and simplifying design

Disadvantages

  • Cannot be throttled or switched off after ignition
  • Cannot be refueled or transfer fuel stored elsewhere on the craft
  • No thrust vectoring
  • Low efficiency compared to other types of engines
[Expand] Available solid fuel rocket engines

Liquid fuel rocket engines


Liquid fuel engines utilize a mixture of liquid fuel and liquid oxidizer in a 9:11 ratio — generally called “rocket fuel”. The lone[outdated] exception is the O-10 MonoPropellant Engine which uses monopropellant.In the real world, typical liquid fuels are liquid hydrogen or kerosene, and typical oxidizers are liquid oxygen or nitrous-oxide.

All liquid fuel rocket engines can be staged and respond to throttle controls. All engines use Bell nozzles except the Toroidal Aerospike Rocket.

The LV-N Atomic Rocket Motor was inspired by real-world nuclear thermal rockets, such as the NERVA. In these, the propellant (typically liquid hydrogen) is heated by a nuclear reactor, rather than being combusted with an oxidizer, expanding into a high velocity jet of gaseous hydrogen. However, in order to reduce development effort and to simplify gameplay, Squad chose to have the LV-N to use the same rocket fuel mixture as other engines.

Advantages

  • Work both in vacuum and in atmosphere
  • Variable throttle allows different levels of thrust at different times, or for the engines to be shut down entirely and restarted later in the flight
  • Full thrust output on demand without “spool up” time like jets
  • Some have gimbals giving thrust vectoring to help steer the craft
  • Available in a wide range of thrusts and efficiencies, all using the same fuel supply
  • Fuel and oxidizer can be moved between tanks or refuelled from another vessel during missions
  • Engines and fuel need not be mounted in the same location on the ship, expanding design possibilities

Disadvantages

  • Lower thrust-to-weight ratio compared to solid rocket engines
  • Less efficient in atmosphere than jet engines
  • Less efficient in space than ion engines
  • Separation of engine and fuel leads to increased part count for all except the KR-1x2.
[Expand] Available liquid fuel rocket engines
[Expand] Available liquid fuel tanks

RCS thrusters


RCS thrusters make up the Reaction Control System designed primarily for translation maneuvers, especially useful during docking. They cannot be throttled and are controlled with a separate set of keys from the pitch-yaw-roll keys. They use only monopropellant fuel, except for the Vernor Engine which uses rocket fuel.

Advantages

  • Monopropellant is automatically distributed throughout a craft, so neither crossfeeding or fuel lines have to be set up
  • Only engines that respond to translation controls
  • Provide additional thrust for SAS to stabilize a craft

Disadvantages

  • Very low total thrust and thrust-to-weight ratio — too weak to escape from most celestial bodies.
  • Low efficiency
  • No thrust vectoring (however, the most commonly used thruster, the RV-105 RCS Thruster Block, can thrust in 4 directions)
  • The user can only switch engines on or off, while the SAS can run them on different thrust levels in between
[Expand] Available RCS engines
[Expand] Available monopropellant tanks

Ion engines


An ion engine uses electric charge to ionize atoms of xenon gas and accelerate them in an electrostatic or electromagnetic field to propel them as exhaust. Remarkably little xenon gas is needed, and probes equipped with just a few tanks are considered able to operate forever, or until something inevitably goes wrong. However, ion engines are very demanding on electrical generation and storage and very slow to accelerate.

In real-life, they are often not considered true “rocket” engines. But given that they require no outside material or mass to operate, for ease and simplicity they are included with rocket engines on this wiki.

Advantages

  • Extremely high efficiency
  • Cool blue glow

Disadvantages

  • Extremely low thrust; inefficient for Hohmann transfers
  • High electric consumption
  • No thrust vectoring currently[outdated] available
  • Xenon higher mass-to-volume than other fuels (very rarely an issue)

Hall effect thruster on Wikipedia

[Expand] Available ion engines
[Expand] Available xenon tanks

See also