Spaceplanes are another fascination of mine, possibly stemming from the first STS launch in 1981. I've certainly to thank my parents for a surprise trip to Stansted Airport (possibly the only time you'll hear those words together) when Enterprise was on its promotional world tour in 1983. What makes spaceplanes quite so interesting is the number of cancelled concepts that never got off the drawing board, let alone the ground.
( Buran orbiter )
Or as it might otherwise have been called, "I can't believe it it's not the Space Shuttle". Started in the early 1970s as a response to the US Space Shuttle, the Buran orbiter was developed as a payload for the Energia HLV. The key difference from the Shuttle orbiter is that Buran carries no engines bar those required for on-orbit maneuvering (equivalent to the OMS on the Shuttle); the Buran orbiter is mounted on the main Energia stack in the same way that the Shuttle is mounted on the external tank (see below).
( Shuttle/Buran comparison )
Including Buran in a list of cancelled spaceplanes is cheating slightly, because it has actually flown to orbit. Its maiden (and only) flight took place in 1988, and was an unmanned affair (Buran featured an automated landing system). The fall of the Soviet Union killed the Buran programme, and the production of the Energia launcher. Sadly, the only orbiter to have flown (OK-1K1) was destroyed in a hangar collapse in 2002 (see photo below).
( Crushed Buran orbiter )
( Artist's impression of MUSTARD )
From the workaday Buran to the plainly ridiculous, MUSTARD (or to give it its full name, the Multi-Unit Space Transport and Recovery Device) was a spaceplane concept dreamed up by BAC in the mid-1960s. The concept featured a multistage vehicle, where each stage was a fully-recoverable delta-shaped lifting body; two stages would act as boosters, while the third would be the orbiter. The back-of-a-fag-packet design suggested a payload of two tonnes in the orbiter, with the boosters being flown back either by remote control, or by an on-board pilot.
There were a number of proposed configurations, ranging from a tandem launch with an orbiter back-to-back with a booster, to the triple cluster and stack shown below.
( MUSTARD launch configurations )
I have a vague suspicion that I've read of a related vehicle, perhaps a wind tunnel model, called CRESS. One can but assume that this is another tortuous acronym. However, I've been unable to find any reference to this by name, but it could be the model launched by Black Arrow mentioned here.
( Artist's impression of Dyna-Soar separating from Titan upper stage )
Dyna-Soar was a USAF programme running from the late 1950s to early 1960s to develop a military spaceplane. Roughly contemporary with Mercury, it differed in that it chose a winged vehicle rather than a ballistic capsule. The missions planned for Dyna-Soar (intercontinental bombing runs, reconnaissance flights and the like) assumed a skip-glide profile (Dyna-Soar being short for Dynamic Soarer); the craft would leave the atmosphere on a ballistic trajectory, but instead of reentering, would use its wings to generate lift and bounce back out of the atmosphere. This profile was originally envisaged for Bell's skip-glide bomber Bomi (Bomber-Missile) from the early 1950s; much of this work was subsumed into Dyna-Soar, although Boeing had been chosen as the main contractor.
The launch system for Dyna-Soar consisted of a Titan-IIIC (for orbital missions), or a Titan-II (for suborbital), but earlier proposals considered other launchers (Atlas-Centaur amongst others). Dyna-Soar was effectively killed by politics; while a weaponised version of Dyna-Soar lay at some indeterminate point in the future (Dyna-MOWS, or the Manned Orbital Weapons System), the first flight of Dyna-Soar itself would not be until 1966. In 1963, McNamara pulled the plug on funding, replacing it with the Manned Orbital Laboratory (MOL), which in turn would fall beneath the knife in 1969.
The legacy of Dyna-Soar is a complex one. US spaceplane development would turn to the Space Shuttle, which has a number of critical flaws: off-axis thrust during launch, and placement of an orbiter where it can be damaged by falling debris from the rest of the stack (as happened with Columbia). By placing the orbiter at the top of the stack, Dyna-Soar would have avoided both of these problems.
After the death of Dyna-Soar, there have been two other spaceplane proposals which show a clear heritage to Dyna-Soar: the European Hermes spaceplane, developed between 1987 and 1993 for launch atop an Ariane 5, and the Russian Kliper, developed between 2000 and 2005 (current status unclear) for launch atop either a future heavier Soyuz (the Soyuz-3) or the planned Angara-A3.
( Artist's impression of Hermes and Kliper )
( Artist's impression of X-33 )
Developed as part of the NASA/DoD Space Launch Initiative, the X-33 was a reusable single-stage-to-orbit launch vehicle. Three proposals were submitted to the original tender in 1996 (as shown below, l-r) by Rockwell, McDonnell-Douglas, and Lockheed Martin.
( X-33 proposals )
While the Lockheed design won the competition, it's worth considering the lineage and ultimate disposition of the two other designs. McDonnell-Douglas's design was an evolution of the DC-X Delta Clipper, a proof of concept for a VTOL SSTO developed under the Space Defense Initiative. Meanwhile, the Rockwell design looks not unlike the X-34 later developed by Orbital Sciences for NASA.
Of the three, the Lockheed design had the most novel elements, chiefly the lightweight composite cryogenic tanks (required to achieve the mass fraction necessary for hydrogen-fueled SSTO), and the RS-2200 linear aerospike engine (best described as a line of inside-out nozzle-less rocket engines) which promised better performance across the pressure regime during liftoff (the shape of the nozzle of a conventional rocket engines affects its efficiency at different altitudes - long and narrow nozzles work best at high altitudes, while short and wide nozzles work better at low altitudes). Lockheed's intent was to use the X-33 development as a proof-of-concept for a full-scale SSTO which they dubbed the VentureStar (shown below alongside the Shuttle and the X-33).
( X-33/VentureStar/Shuttle comparison )
The innovations in the X-33 were not without risk, however. The linear aerospike engine was heavier than expected, moving the vehicle's centre of gravity to aft and requiring larger (and heavier) control surfaces. More importantly, there were difficulties in the manufacture of the composite tanks, with the prototype tanks failing during testing with evidence of debonding and delamination. While the engineers on the project campaigned to replace the experimental composite tanks with conventional Al-Li alloy tanks (which ironically would be lighter than the composite tanks, due to the complexity of the joints in the composite tanks), NASA management ruled out such a chance, saying "to fly the vehicle with an aluminium tank makes little sense from a technical point of view". NASA terminated the X-33 (and also the X-34) programme in 2001, before the prototype X-33 was complete.
X-33 promised to significantly reduce launch costs (by an order of magnitude), and was the sort of fully reusable fast-turnaround RLV that the Shuttle claimed to be, but wasn't. Whether it would have actually worked is unclear; the design payload to LEO was roughly that of the Shuttle (26 tonnes, compared to the 28 tonnes of early Shuttles or the 24 tonnes of post-Challenger Shuttles), but weight inflation would have decreased that in short order. To my mind, it remains one of the great what-ifs of the last thirty years of manned spaceflight.
( HOTOL with deployed payload )
Saving the best - or at least the most ambitious - for last, we have HOTOL, the British-designed HOrizontal Take-Off and Landing craft. Development lasted from 1986 to 1989, although aspects of the design dated back to 1982. The key component of HOTOL was the RB545 engine, an airbreathing rocket engine (also referred to as a liquid air cycle engine). At lower altitudes, this engine gathered its oxidiser from the atmosphere, reducing the amount of liquid oxygen that needed to be carried (and so the mass of the oxidiser tanks), and would transition to a pure rocket at higher altitudes. This was achieved by means of a variable geometry air intake to the engine (shown below the vehicle in the next image). This cutaway is particularly instructive.
( HOTOL, showing intakes )
The total funding available to HOTOL was negligible, in the way that only British-funded projects can manage: £2 million. By contrast, X-33 received some $1.5 billion during its lifetime. The blame for the lack of funds, and the eventual cancellation of the project can be placed fairly firmly at the feet of the Thatcher government (boo, hiss, etc).
Post-HOTOL, there have been two developments. The Interim HOTOL study by British Aerospace in 1990 investigated a cut-down vehicle powered by Russian RD-0120 engines (conventional rockets used on the Energia core vehicle) and air-launched from an An-225. Meanwhile, the original designers of HOTOL left BAe to form their own company, Reaction Engines. They've further developed the RB545 concept into the SABRE (Synergic Air-Breathing Engine), and the HOTOL concept into a new craft by the name of Skylon (pictured below), not to be confused with this structure from the Festival of Britain in 1955. While the name is an unfortunate reference to some faded golden age of Dan Dare-style British spaceflight, the finances are most likely to stop Skylon from flying; in 1992, the total cost of development was estimated at £10 billion, and there has been little commercial or industrial interest.
( Skylon )
Anyway, that's probably enough aerospace geekery for the moment - tomorrow's top five is likely to be English cathedrals.