Editing Atmospheric entry (section)

===Sphere-cone===
The sphere-cone is a spherical section with a [[frustum]] or blunted cone attached. The sphere-cone's dynamic stability is typically better than that of a spherical section. The vehicle enters sphere-first. With a sufficiently small half-angle and properly placed center of mass, a sphere-cone can provide aerodynamic stability from Keplerian entry to surface impact. (The ''half-angle'' is the angle between the cone's axis of rotational symmetry and its outer surface, and thus half the angle made by the cone's surface edges.)

[[File:Mk 2.jpg|thumb|left|Prototype of the Mk-2 Reentry Vehicle (RV), based on blunt body theory]]
The original American sphere-cone aeroshell was the Mk-2 RV (reentry vehicle), which was developed in 1955 by the [[General Electric]] Corp. The Mk-2's design was derived from blunt-body theory and used a radiatively cooled thermal protection system (TPS) based upon a metallic heat shield (the different TPS types are later described in this article). The Mk-2 had significant defects as a weapon delivery system, i.e., it loitered too long in the upper atmosphere due to its lower [[ballistic coefficient]] and also trailed a stream of vaporized metal making it very visible to [[radar]]. These defects made the Mk-2 overly susceptible to anti-ballistic missile (ABM) systems. Consequently, an alternative sphere-cone RV to the Mk-2 was developed by General Electric.{{citation needed|date=November 2013}}

[[File:Mk 6 reentry vehicle on display at National Atomic Museum.jpg|thumb|upright|right|[[LGM-25C Titan II#Development|Mk-6]] RV, [[Cold War]] weapon and ancestor to most of the U.S. missile entry vehicles]]
This new RV was the Mk-6 which used a non-metallic ablative TPS, a nylon phenolic. This new TPS was so effective as a reentry heat shield that significantly reduced bluntness was possible.{{citation needed|date=November 2013}} However, the Mk-6 was a huge RV with an entry mass of 3,360&nbsp;kg, a length of 3.1 m and a half-angle of 12.5°. Subsequent advances in nuclear weapon and ablative TPS design allowed RVs to become significantly smaller with a further reduced bluntness ratio compared to the Mk-6. Since the 1960s, the sphere-cone has become the preferred geometry for modern ICBM RVs with typical half-angles being between 10° and 11°.{{citation needed|date=November 2013}}

[[File:Rv film pod.jpg|thumb|upright|left|"Discoverer" type reconnaissance satellite film Recovery Vehicle (RV)]]
[[File:Galileo probe.jpg|thumb|right|[[Galileo Probe]] during final assembly]]
[[Reconnaissance satellite]] RVs (recovery vehicles) also used a sphere-cone shape and were the first American example of a non-munition entry vehicle ([[Corona (satellite)|Discoverer-I]], launched on 28 February 1959). The sphere-cone was later used for space exploration missions to other celestial bodies or for return from open space; e.g., [[Stardust (spacecraft)|''Stardust'']] probe. Unlike with military RVs, the advantage of the blunt body's lower TPS mass remained with space exploration entry vehicles like the [[Galileo Probe]] with a half-angle of 45° or the [[Viking program|Viking aeroshell]] with a half-angle of 70°. Space exploration sphere-cone entry vehicles have landed on the surface or entered the atmospheres of [[Mars]], [[Venus]], [[Jupiter]], and [[Titan (moon)|Titan]].
{{clear}}