A two-stage-to-orbit (TSTO or DSTO - Double/Dual-Stage-To-Orbit) launch vehicle is a spacecraft in which two distinct stages provide propulsion consecutively in order to achieve orbital velocity. It is intermediate between a three-stage-to-orbit launcher and a hypothetical single-stage-to-orbit (SSTO) launcher.
At liftoff the first stage is responsible for accelerating the vehicle. At some point the second stage detaches from the first stage and continues to orbit under its own power.
An advantage of such a system over single-stage-to-orbit is that the entire mass of the spacecraft is not carried into orbit. This reduces the difficulty involved in reaching orbital velocity.
An advantage over three or more stages is reduction in complexity and fewer separation events, each of which reduces cost and risk of failure.[1]
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It is not always clear when a vehicle is a DSTO. Many designs which use a very small boost at the beginning of their flight are referred to as single-stage-to-orbit. Some have also coined the expression 1.5STO for 'one-and-a-half-stage-to-orbit', e.g., the Atlas. Also, many launch vehicles have side-mounted booster rockets which are jettisoned early which are called "stage-0".
With reference to a reusable launch system this approach is often proposed as an alternative to single-stage-to-orbit (or SSTO). Its supporters argue that, since each stage may have a lower mass ratio than an SSTO launch system, such a system may be built without approaching as close to the limitations of its structural materials. It therefore should require less maintenance, less testing, experience fewer failures and have a longer working life.
Critics argue that the increased complexity of designing two separate stages that must interact, the logistics involved in returning the first stage to the launch site, and the difficulties of conducting incremental testing on a second stage will outweigh these benefits. In the case of airplane-like lower stages they also argue how difficult and expensive high speed aircraft (like the SR-71) are to develop and operate, and question performance claims. Many 'mini-shuttle' designs that use transport aircraft as first stages also face similar problems with ice/foam as the Space Shuttle due to the requirement they also carry a large external tank for their fuel.
On the other hand, the two-stage approach allows the lower stage to be optimized for operation in the Earth's lower atmosphere, where pressure and drag are high, while the upper stage can be optimized for operation in the near-vacuum conditions of the later part of the launch. This allows an increase in the payload mass fraction of a two-stage vehicle over single-stage or stage-and-a-half vehicles, which have to perform in both environments using the same hardware.
Taking the view that airplane like operations do not translate to airplane-like appearance, many TSTOs have first stages that operate as VTOL or VTOHL aircraft. The DC-X has proven the VTOL option design workable. Other designs like the DH-1 concept take it a step further and use a 'pop-up/pop-down' approach, which delivers the orbiting stage to a point about 60 km above the earth's surface, before dropping down to the launch pad again. In the case of the DH-1, the upper stage is effectively an 'almost SSTO' with a more realistic mass fraction and which was optimised for reliability.
TSTO designs comprise an airplane-like first stage and a rocket-like second stage. The airplane elements can be wings, air-breathing engines, or both. This approach appeals because it transforms Earth's atmosphere from an obstacle into an advantage. Above a certain speed and altitude, wings and scramjets cease being effective, and the rocket is deployed to complete the trip to orbit.
While not an orbital vehicle, the successful private SpaceShipOne suborbital spacecraft developed for the Ansari X Prize demonstrated that the problems of integrating a two-stage system, with a winged aircraft as the "lower half", that can reach the edge of space are not insurmountable. As of 2005[update], the team behind SpaceShipOne is working on a commercial sub-orbital launch system -- SpaceShipTwo -- based on this technology.
It has been suggested by Aviation Week & Space Technology that an XB-70-type first stage, with an X-20 spacecraft, possibly manned, may have reached low earth orbit. However, information on this system remains conjectural.[2]