Editing Rocket (section)

===Forces on a rocket in flight===
[[File:Rktfor.gif|thumb|upright|Forces on a rocket in flight]]
The general study of the [[force]]s on a rocket is part of the field of [[ballistics]]. Spacecraft are further studied in the subfield of [[astrodynamics]].

Flying rockets are primarily affected by the following:<ref>{{cite web |url=http://www.grc.nasa.gov/WWW/K-12/VirtualAero/BottleRocket/airplane/rktfor.html |title=Four forces on a model rocket |publisher=NASA |date=2000-09-19 |access-date=2012-12-10 |url-status=dead |archive-url=https://web.archive.org/web/20121129204215/http://www.grc.nasa.gov/WWW/k-12/VirtualAero/BottleRocket/airplane/rktfor.html |archive-date=2012-11-29 }}</ref>
* [[Thrust]] from the engine(s)
* [[Gravity]] from [[celestial bodies]]
* [[Drag (physics)|Drag]] if moving in atmosphere
* [[Lift (force)|Lift]]; usually relatively small effect except for [[rocket-powered aircraft]]

In addition, the [[centrifugal force (fictitious)|inertia and centrifugal pseudo-force]] can be significant due to the path of the rocket around the center of a celestial body; when high enough speeds in the right direction and altitude are achieved a stable [[orbit]] or [[escape velocity]] is obtained.

These forces, with a stabilizing tail (the ''[[empennage]]'') present will, unless deliberate control efforts are made, naturally cause the vehicle to follow a roughly [[parabola|parabolic]] trajectory termed a [[gravity turn]], and this trajectory is often used at least during the initial part of a launch. (This is true even if the rocket engine is mounted at the nose.) Vehicles can thus maintain low or even zero [[angle of attack]], which minimizes transverse [[stress (physics)|stress]] on the [[launch vehicle]], permitting a weaker, and hence lighter, launch vehicle.<ref name=space-sourcebook>{{cite book|first1=Samuel|last1=Glasstone|title=Sourcebook on the Space Sciences|url=https://books.google.com/books?id=K6k0AAAAMAAJ|publisher=D. Van Nostrand Co.|date= 1965|access-date=28 May 2016|page=209|oclc=232378|url-status=live|archive-url=https://web.archive.org/web/20171119163047/https://books.google.com/books?id=K6k0AAAAMAAJ|archive-date=19 November 2017}}</ref><ref name=thesis>{{Cite thesis|first=David W. |last=Callaway |title=Coplanar Air Launch with Gravity-Turn Launch Trajectories |type=Master's thesis |publisher=Air Force Institute of Technology |date=March 2004 |url=https://scholar.afit.edu/etd/3922/ |page = 2 }}</ref>
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====Drag====
{{main|Drag (physics)|Gravity drag|Aerodynamic drag}}
Drag is a force opposite to the direction of the rocket's motion relative to any air it is moving through. This slows the speed of the vehicle and produces structural loads. The deceleration forces for fast-moving rockets are calculated using the [[drag equation]].

Drag can be minimised by an aerodynamic [[nose cone]] and by using a shape with a high [[ballistic coefficient]] (the "classic" rocket shape—long and thin), and by keeping the rocket's [[angle of attack]] as low as possible.

During a launch, as the vehicle speed increases, and the atmosphere thins, there is a point of maximum aerodynamic drag called [[max Q]]. This determines the minimum aerodynamic strength of the vehicle, as the rocket must avoid [[buckling]] under these forces.<ref name=maxq>{{cite web |url=http://www.aerospaceweb.org/question/aerodynamics/q0025.shtml |title=Space Shuttle Max-Q |publisher=Aerospaceweb |date=2001-05-06 |access-date=2012-12-10}}</ref>

====Net thrust====
{{For|a more detailed model of the net thrust of a rocket engine that includes the effect of atmospheric pressure|Rocket engine#Net thrust}}
[[File:Rocket nozzle expansion.svg|thumb|upright|[[Rocket engine#Nozzle|A rocket jet shape]] varies based on external air pressure. From top to bottom:{{unbulleted list|Underexpanded|Ideally expanded|Overexpanded|Grossly overexpanded}}]]

A typical rocket engine can handle a significant fraction of its own mass in propellant each second, with the propellant leaving the nozzle at several kilometres per second. This means that the [[thrust-to-weight ratio]] of a rocket engine, and often the entire vehicle can be very high, in extreme cases over 100. This compares with other jet propulsion engines that can exceed 5 for some of the better<ref>{{cite web |url=http://www.geae.com/engines/military/j85/index.html |title=General Electric J85 |publisher=Geae.com |date=2012-09-07 |access-date=2012-12-10 |url-status=dead |archive-url=https://web.archive.org/web/20110722155949/http://www.geae.com/engines/military/j85/index.html |archive-date=2011-07-22 }}</ref> engines.<ref>{{cite web |url=http://www.thrustssc.com/thrustssc/Club/Secure/Arfons_Last_Stand.html |title=Mach 1 Club |publisher=Thrust SSC |access-date=2016-05-28 |url-status=dead |archive-url=https://web.archive.org/web/20160617103717/http://www.thrustssc.com/thrustssc/Club/Secure/Arfons_Last_Stand.html |archive-date=2016-06-17 }}</ref>

The net thrust of a rocket is

{{block indent|<math>F_n = \dot{m}\;v_{e},</math><ref name="RPE7"/>{{rp|2–14}}}}

where

{{block indent|<math> \dot{m} =\,</math>propellant flow (kg/s or lb/s)}}
{{block indent|<math>v_{e} =\,</math>the [[effective exhaust velocity]] (m/s or ft/s).}}

The effective exhaust velocity <math>v_{e}</math> is more or less the speed the exhaust leaves the vehicle, and in the vacuum of space, the effective exhaust velocity is often equal to the actual average exhaust speed along the thrust axis. However, the effective exhaust velocity allows for various losses, and notably, is reduced when operated within an atmosphere.

The rate of propellant flow through a rocket engine is often deliberately varied over a flight, to provide a way to control the thrust and thus the airspeed of the vehicle. This, for example, allows minimization of aerodynamic losses<ref name=maxq/> and can limit the increase of [[g-force|''g''-forces]] due to the reduction in propellant load.