Twin-stage to orbit
A twin stage to orbit craft (TSTO), also called two stage to orbit or dual stage to orbit, is a method of spacecraft launch that involves two stages, as opposed to a single stage to orbit or traditional multi stage rockets, which use 3 or more stages. It has been proposed as a means of creating an RLV (Reuasable launch vehicle) while avoiding the constraints which have doomed attempts at single stage to orbit spacecraft to date.
The first stage, given that it generates a lower delta-V and reenters at a much lower velocity, can be built to a lower perfomance than the upper stage, and is much easier to make reusable. Because of this, some ideas propose using a reusable lower stage coupled with an expedeble upper stage. This would be easier to achive on a HOTOL craft, although some designs call for a vertical flight profile instead
Due to the environment the lower stage flies in, and it's requirements, Ramjets become a practical suggestion. While current Ramjets cannot generate the required speeds to reach orbit, they can get the craft to around Mach 6. Coupled with onboard rocket mototrs, required for boosting the ramjet to start speed regardless of whether they are used later, such a craft may be capable of reaching Mach 10 with a modest mass ratio.
The upper stage adds the remaining velocity to the payload to send it into orbit. Depending on the velocity added by the lower stage, the requirements can range from being a near-SSTO to merely existing to circularize the orbit. Due to the lack of air, it must carry all it's fuel and oxidiser onboard. Some have proposed using a Nuclear thermal rocket to generate the required delta-V, as the extreme altitdue to attenuate any radioactive material which escapes from the exhaust.
Existing TSTO designs
The DH-1 - from The Rocket Company Spacecab and Spacebus - Bristol Spaceplanes K1 launch vehicle - Rocketplane Kisler