A single-stage to orbit (or SSTO) vehicle would be a vehicle that could reach orbital velocity without the use of multiple stages. The main problem in constructing such a vehicle is to make the engine efficient and the vehicle structure lightweight enough, to make it unnecessary to drop away parts of the vehicle, whilst still delivering a reasonable payload to orbit. It is believed by some that reusable vehicles, particularly SSTO reusable vehicles, would lead to much reduced costs for access to space and allow aircraft-like operations.