The main advantage of "aero-spike" rocket engines.

[Zoomer30]

Engines with normal nozzles are effected by the atmopheric pressue of the air.  At launch the air-pressure is high enough to "pinch" the gas flow from the rocket and make the effective thrust at that point lower then it could be.  As the ship nears space, the low air pressure allows the plume to "bloom" or spread out (look at old Sat V flights to see this very clearly).  This also lowers the thrust.  Engines like the SSME are desgined to have the best thrust at a certian flight level where it is needed most.  Its a trade off.  the best thrust is at the point where the plume is equal to diameter of the nozzle.  

Aerospikes dont have this issue.  they use the air-pressure as their "nozzle" and the plume adjusts all the way up, but the thrust is always good, no loss like in standard engines.

[Jim]

Why this post?  It is discussed else where in the X-33 Venturestar thread

[James Lowe1]

A good time to note people can use the http://forum.nasaspaceflight.com/search/query.asp facility to check they aren't duplicating.

Thread locked.