MODERN CONCEPTS

Although the large spaceplane programs were canceled, some related developments did survive. Work on hypersonic scramjet propulsion in the US was continued in NASA’s Hyper-X program, and resulted in two test flights of the small X-43A. This unmanned experimental vehicle was launched from the nose of a Pegasus rocket that was itself dropped from a converted airliner, and set new records for an airbreathing vehicle by achieving Mach 9.6 and a scramjet burn of 10 seconds on its latest flight in November 2004. Scramjet development tests are continuing with the similar X-51, which flew for the first time in May 2010 and reached a speed of ‘only’ Mach 5 but a much longer powered-flight time of 200 seconds. The second flight, in June 2011, was unsuccessful due to a failure of the scramjet engine. Some people insist that the US military already has a highly secret orbital spaceplane in operation (such as the ‘Blackstar’ reported by Aviation Week) but the evidence is unconvincing.

Shortly after HOTOL foundered in 1988, members of the engine design team led by Alan Bond set up a new company (Reaction Engines Ltd) to continue to develop the HOTOL concept, focusing initially on an improved version of the RB545 engine called the SABRE (‘Synergistic Air Breathing Engine’) and in particular the crucial precooler section. The spaceplane concept currently being worked on is the Skylon presented in the Introduction of this book as a “perfect spaceplane” (it is named after a futuristic art structure included in the 1951 Festival of Britain, which the fuselage strongly resembles). The designers reckon they have fixed the flaws in the HOTOL design, in particular the stability problem due to the heavy engines in the aft part of spaceplane. The Skylon solution is to locate the engines in the middle of the vehicle, housing them in nacelles at the tips of the delta wings in the same way envisaged for the Keldysh Bomber in 1947. This prevents the center of gravity from moving aft as the propellant tanks are depleted. Moreover, since the engines are not fully integrated with the fuselage they can be tested separately from the remainder of the vehicle. The engine nacelles have a peculiar banana-shape because their air intakes have to point directly into the airflow, whereas the spaceplane’s wings and body must fly at an angle to create lift. Each engine will give a maximum thrust of

1,350,0 Newton in airbreathing mode, and 1,800,000 Newton in rocket mode.

According to the company their SABRE propulsion would make Skylon very safe and reliable, and enable it to take off without the rocket trolley that would have been necessary for HOTOL. But this meant Skylon would need a sturdy undercarriage as well as strong brakes to stop itself before the end of the runway if a problem were to occur just before take-off. It was decided to cool the brakes by water, which would boil away and dissipate the heat caused by the braking friction. The cooling water would be jettisoned following a successful take-off, thus reducing the weight of the undercarriage by several metric tons. At landing Skylon would be empty and hence fairly Ught, so the brakes would not need water cooling in order to be able to stop the plane without catching fire. Due to its aerodynamic characteristics upon re-entry, the vehicle would slow down at higher altitudes than the Space Shuttle Orbiter, keeping the skin of the vehicle significantly cooler, hence requiring only a durable reinforced ceramic for most of its skin. The turbulent airflow around the wings during re-entry

Cutaway of the Skylon concept [Adrian Mann & Reaction Engines Limited],

would, however, necessitate active cooling of some parts of the vehicle. Skylon is expected to be able to put 12,000 kg of payload into low orbit. Its take-off noise is expected to be acceptable for taking off from regular airports in populated areas but the runways would have to be extended to 5.6 km (3.5 miles) in length, of which the first 4 km (2.5 miles) would require to be stronger than usual to cope with the heavily laden Skylon rolling at high speed.

An independent review by the European Space Agency, which is also funding part of the technology development for Skylon, concluded in 2011 that the overall design “does not demonstrate any areas of implausibility”. Reaction Engines is confident that Skylon will soon reach a technical maturity sufficient to convince investors that it is a valid commercial opportunity which warrants funding to full development. The project’s cost estimates indicate that if a fleet of 90 vehicles were produced it would be possible to buy a Skylon for about $650 million, which is roughly comparable to a large jet airliner. Early customers would pay $30 to $40 million per flight but with more aircraft flying and an increasing total number of flights, the price should fall to around $10 milhon per launch. In comparison, a current Ariane 5 expendable rocket costs around $150 million per launch and puts less mass into a low orbit.

At the time of writing, Reaction Engines is doing tests on an experimental version of the precooler and plans to build a sub-scale version of the SABRE to demonstrate (on the ground) the complete engine’s airbreathing and rocket modes as well as the transition between these. Tests of the nacelle in which the SABRE is to be housed are to be performed using a Nacelle Test Vehicle (NTV). This is planned to be launched from the ground and use rocket engines to get up to Mach 5, at which speed internal ramjet combustion systems will simulate the operation of the air-breathing engine. The remainder of the nacelle test article will be as close as possible to the real thing, including the control systems and internal flow ducts. The NTV is also to get some data on shock interactions between the nacelle and the Skylon’s wing. According to Bond, “we could have a Skylon plane leaving Heathrow airport sometime during this century”.

In 1991 Bristol Spaceplanes, another small aerospace company in the UK, began working on rocket plane concepts. Their ‘Spacecab’ design involves a Concorde-like carrier aircraft that uses four ordinary turbojet engines to take off and accelerate to Mach 2 and two rocket engines to reach Mach 4. At that speed a small, delta-winged rocket propelled orbiter carrying two pilots and either six passengers or cargo would separate and climb into orbit. The company insists this is a very conservative design that does not require any new technology to be developed. A next-generation vehicle called ‘Spacebus’ would fit the carrier aircraft with newly developed turbo-ramjets to achieve Mach 4 and rocket engines for Mach 6. Its enlarged orbiter would have room for fifty passengers. David Ashford, the company’s managing director, has published his ideas in two popular science books: Your Spaceflight Manual – How You Could be a Tourist in Space Within Twenty Years (written with space tourism promoter Patrick Collins) and Spaceflight Revolution.

Private companies like Reaction Engines and Bristol Spaceplanes keep working on spaceplane technology, and the development of hypersonic and scramjet engines for military applications is strongly supported in the US (specifically in the Falcon program). Non-military government space agencies such as NASA and the European Space Agency have not completely given up on reusable launch vehicles either. The Future European Space Transportation Investigation Programme (FESTIP) study by ESA in 1994-1998 for instance, investigated many basic reusable launcher concepts, several of which were spaceplanes. In 2005 ESA also ran a small internal study (in which I participated as System Engineer) for a small rocket plane called Socrates. This was intended to fly at speeds up to Mach 12 and be operated for about 30 flights. It was specifically to investigate spaceplane operations and maintenance, such as how long it takes to replace rocket engines, how an onboard health monitoring system could help speed up maintenance, and how long thermal protection materials could last. A reusable rocket plane’s ability to make many test flights at relatively low cost should result in a higher reliability in comparison to modern expendable launchers which are usually deemed operational after only a single qualification flight but typically have a failure rate of 2 to 3% (meaning that two or three payloads per hundred are lost, in turn pushing up insurance costs).

India, a country that is making great strides in spacecraft and launcher technology, is investigating a concept known as AVATAR (for ‘Aerobic Vehicle for hypersonic Aerospace TrAnspoRtation’). This vehicle would take off using airbreathing turbo-ramjet engines and full tanks of fuel but none of the liquid oxygen that it would later need for its rocket propelled flight phase. Instead, during atmospheric flight separate ram air intakes collect air that is subsequently liquefied using liquid hydrogen-cooled heat exchangers; similar to the ACE principle of the JSSTO spaceplane concept. But unlike JSSTO, AVATAR involves another step in which the liquid oxygen in the air is mechanically extracted and stored in the previously empty oxidizer tanks so that they will be full by the time AVATAR requires to switch over to rocket propulsion. Although this is an extremely ambitious project, India is developing its capabilities at a rapid pace.