Many different projects of spaceplanes are under the development. From commercial projects focused on space tourism like SpaceShipOne or large projects supported by government like British Skylon. Different approaches have one thing in common: make flights into space more accessible.
Word "accessible" has different meanings. From the point of view of Company specialized in space tourism it means "affordable". Low cost per one commercial astronaut generates income, which is most important for commercial organization. In another aspect of profitability will be interested for Company oriented for cargo flights. In such case, payload capacity, flexibility of payload space and ability to dock into space stations will be significant. But still it is economic aspect. But some organizations are oriented for time, not for money. For them "accessible" means possibility of quick lift payload into orbit. Such organization is the army.
DARPA is research agency under Department of Defense established in 1958. It is responsible for various projects from computer science to biology research. Space technologies are naturally one of the areas being under special attention. At the moment, when possible successor of Space Shuttle is under development, DoD decided to search for solutions more corresponding with military requirements under DARPA. Space Shuttle was utilized for military missions, like during STS-27 mission, but still it was not designed especially for military. It was expensive, preparing for launch was long - everything was organized with civilian procedures which do not reflect the military specifics. DARPA decided to work on strictly military oriented system and resigned from taking into account the civilian successors of Space Shuttle. DARPA was participating in rail launched Super Strypi rocket, project of small launch vehicle, able to be launched from special designed ships and lifting small satellites. ALASA project, another DARPA idea for making space launches more accessible, was rocket deployed from F15E fighter like ordinary rocket missile but with small satellite instead warhead. Every one of these projects has one goal - create launching system possible to utilize fast and without conventional launch site. Both Super Strypi and ALASA are very limited with weight of the lift satellite. They are offering payload up to 45 kg (ALASA) which is still not enough for heavier or larger communication satellites. Of course nano satellites are still developing and maybe in future it will be possible to design nano communication satellite operating just as present larger constructions. But probably it will take couple years and will necessitate replacement communication equipment working along with nano communication satellites. Question should be asked, why DoD is interested in developing launch vehicle, if civilian launch service providers offering reasonable prices and offering proven vehicles with possibility of lifting most enormous sized satellites. Present and future costs of developing projects are high and still ALASA or Super Strypi are offering limited payload. The answer is simple. Civilian launch service providers are offering large payloads with poor $/kg ratio for customer. Naturally if launching one or two satellites is concerned the prices are acceptable. But if DoD would like to launch twenty satellites day after day, with cost for every launch at around $223 million (example of cost of single launch of Atlas V which is not most expensive launch vehicle utilized by DoD), price is making the difference. ALASA is offering price at around $1 million per launch. The problem is that civilian launch service providers are not necessary interested in developing small launch vehicles because lack of their economic sense. New technologies are expensive and from the point of view of the commercial company large rockets are giving ability to launch couple satellites from various customers during one launch. It is more profitable then launching only one satellite which is filling only 30% of payload capacity. This way of combining launches has one main drawback for military. This is time required for waiting for combined launch. It is not very often that two or more satellites are ready in one moment – usually satellite of one company is waiting until couple other companies will be ready with their satellites for launch. It results in a lower price per launch for companies. But time is crucial for military in some conditions. For example, during crisis it could be necessary to rapidly launch additional observation, GPS or communication satellites, or to complement destroyed satellites to maintain communication capability. Of course such actions were not necessary recently, when USA were involved in warfare with countries not possessing antisatellite weapon, but DoD is probably considering that prevention is better than cure.
First American attempts of designing spaceplane were radically different from each other. First was designed in the eighties X30 - manned hypersonic passenger plane with ability of deploying satellites in LEO. Next was designed in nineties - X33 designed by Lockheed as unmanned VTHL spaceplane designed only to put payload into orbit. Last serious attempt was RASCAL project relying on modified Convair F106 jet fighter as a first stage of launch system. All projects were cancelled in various phases of development. Most recent project being still in progress is, announced on 1st November 2013, XS1. Its main objective is possibility of rapid utilizing for lifting payload with weight from range 1400 kg - 2300 kg with at least 1800 kg payload on trajectory orbit. Other requirements were following: possibility of one launch per one day with capability of ten launches in ten days. Resulting from this fact was necessary of adopting durable and long lasting thermal protection system and reusable and reliable propulsion. XS1 is considered as reusable first stage of hypersonic (with speed at least 10 Ma) two-stage-to-orbit launch system built on space frames mainly with composites. Cost per flight should be under $5 million. Jess Sponable, manager of XS1 program clearly expressed objectives of the program in February, 2014:
"The vision here is to break the cycle of escalating space system costs, enable routine space access and hypersonic vehicles."
In the same year, in May, it was given to public that following Companies were granted with contracts for creating technology demonstrators: Boeing and Blue Origin, Masten Space Systems with XCOR Aerospace, and Northrop Grumman with Virgin Galactic. Northrop unveiled some details regarding their project: it will be VTHL vehicle utilizing minimal launch pad for start and regular runway for landing. High mobility of system is provided by capability of launching from transporter. Masten is going to base on their experience with different VTVL vehicles designed for Lunar Lander Challenge. Boeing is most mysterious and yet has not unveiled any details. All three pretenders positively finished phase 1A and reach in August 2015 phase 1B with additional contracts. Scheduled date for the first flight of demonstrators is 2017.
Perhaps most of significant impulse for development of unmanned spacecraft was progress in antisatellite weapon around the globe. Antisatellite weapon is under intensive tests in China - first missile launched from aircraft was tested in 2007. In 2013 DoD announced that Chinese science rocket launched for upper ionosphere studying was actually antisatellite missile launched from ground. Russia, after canceling Almaz battle station program (manned space station equipped with 30 mm Nudelmann-Richter autocannon), also developed own antisatellite missile called Vympiel. At the present is strongly working on Sokol Eshelon laser antisatellite system (It will be laser emitter mounted a Beriev A60 plane). In spite of considering involve in warfare with China or Russia, both countries are well known of their export-oriented defense industries combined with tolerant approach to their customer intentions. It should not be weird that DoD started to put an effort for creating launch system able to put into orbit destroyed satellites fast and cheap as possible.