The Hotol project, a brief description

The Multi-Unit Space Transport And Recovery Device (MUSTARD) was a concept explored by the British Aircraft Corporation (BAC) around 1964-1965 for launching payloads weighing as much as 5,000 lb into orbit. It was never constructed. The British Government also began development of a SSTO-spaceplane, called HOTOL.

HOTOL, for Horizontal Take-Off and Landing, was a British air-breathing space shuttle effort by Rolls Royce and British Aerospace.
Designed as a single-stage-to-orbit (SSTO) reusable winged launch vehicle, it was to be fitted with a unique air-breathing engine, the RB545 called the Swallow, to be developed by Rolls Royce. The engine was technically a liquid hydrogen/liquid oxygen design, but dramatically reduces the amount of oxidizer needed to be carried on board by utilising atmospheric oxygen as the spacecraft climbed through the lower atmosphere.
Since propellant typically represents the majority of the takeoff weight of a rocket, HOTOL was to be considerably smaller than normal pure-rocket designs, roughly the size of a medium-haul airliner such as the McDonnell Douglas DC-9/MD-80. However, comparison with a rocket vehicle using similar construction techniques failed to show much advantage, and funding for the vehicle ceased.

HOTOL would have been 63 metres long, 12.8 metres high, 7 metres in diameter and with a wingspan of 28.3 metres. The unmanned craft was intended to put a payload of around 7 to 8 tonnes in orbit, at 300 km altitude. It was intended to take off from a runway, mounted on the back of a large rocket-boosted trolley that would help get the craft up to "working speed". The engine was intended to switch from jet propulsion to pure rocket propulsion at 26–32 km high, by which time the craft would be traveling at Mach 5 to 7. After reaching orbit, HOTOL was intended to re-enter the atmosphere and glide down to land on a conventional runway (approx 1,500 metres minimum).

During development, it was found that the comparatively heavy rear-mounted engine moved the center of mass of the vehicle rearwards. This meant that the vehicle had to be designed to push the center of drag as far rearward as possible to ensure stability over the entire flight regime.
Cooling the air from an exterior 1000 degrees Celsius to working temperates for the new engine became a problem too, as condensation and frosting blocked airflow and choked the intake supply.
 Redesign of the vehicle to do this cost a significant proportion of the payload, and made the economics unclear. In particular, some of the analysis seemed to indicate that similar technology applied to a pure rocket approach would give at least as good performance at lower cost.

In 1988 the government withdrew further funding, the project was approaching the end of its design phase but the plans were still speculative and dogged with aerodynamic problems and operational disadvantages.
A cheaper redesign, Interim HOTOL or HOTOL 2, to be launched from the back of a modified Antonov An-225 transport aircraft, was offered by BAe in 1991 but that too was rejected. Interim HOTOL was to have dispensed with an air-breathing engine cycle and was designed to use more conventional LOX and liquid hydrogen. Although the British government canceled the Hotol project Alan bond lead engineer looked towards European space agency for funding. But the Hotol project was deemed classified preventing the patent rights and the progress of a space plane, unless for military purposes.
In 1989 HOTOL co-creator Alan Bond formed Reaction Engines Limited which has since been working on the Reaction Engines Skylon vehicle intended to solve the problems of HOTOL. Much of the early work by Allen bond was to utilize his knowledge in computer aided design to work out the design flaws in Hotol. A fundamental problem of center of gravity was solved by simply placing the engines on the tip of the wings much like the design of a modern jet. The patent restriction for the Hotol project ended in 1993 and was later owed by Rolls Royce (who express no further interest), but Allen circumvented much of the thermodynamic ideas on the patent of which he first wrote. This allowed Reaction Engine to move forward and obtain support and funding.

The design problems of the original HOTOL project was ultimately solved by technology and by the will and determination of the original founders Allen Bond Richard Varvill and John Scott-Scott.
Despite the need for a 250 million pound to fund the next three-year development phase in which it plans to build a small-scale version of the complete engine, there seems to be a sense of inevitability that this company will succeed. Considering that there is no alternative then expensive chemical multi stage rockets.

While competitive space craft such as the US space shuttle retires from action, the scram-jet is the only contender. The future of orbital spacecraft need to embrace new concepts of propulsion. As road vehicles slowly change to economical alternatives, so should the space industry. The sad fact that political will power did not support Hotol during a time in which President Ronald Regan had plans to create a star-wars program. The orbital height of a military base as a tactical advantage would of relied heavily on a craft like HOTOL. Consider the possibilities if HOTOL would of been a success. A working fleet would of been flying in the 90's, and the possibilities of a orbital infrastructure might have transformed air and space travel. But despite the slow progress of this technology and the skeptics that would of mothball the project. HOTOL has reborn into SKYLON and a working prototype might be ready as soon as 2020...

Reaction Engine LTD, the future of space is British

My recent article about space travel and the different engines needed to propel a vehicle in both environments of space and aerospace, seems to have overlooked a recent new technology. The Idea of a vehicle is much needed, as the current rocket systems is vastly inefficient.
Consider a ramjet a simple design of no moving parts which works efficiently at speeds around Mach 3 with a upper limit of Mach 6. Nasa had already tested X-43A, a scramjet at hypersonic speeds which is much lighter then conventional rockets because it does not carry oxidizer. Despite a 8 year program costing 230 million dollars it was a successful pioneering piece of technology of which seems to break record speeds.
it size was 12 feet by 5 feet, it also requires a craft to carry the test jet to workable speeds before launching.
The scramjet offers a possible solution to the requirements of escaping the atmosphere by reaching mach 6. The momentum of a vehicle alone could help escape the earths gravitational pull. But without the additional continued thrust in the vacuum of space, it is likely the craft will be drawn back to back to earth. The best current solution is a multi-engine configuration to power the craft to for each environment. This can be a heavy solution to this problem, as weight can be a limiting factor to a aircraft as well as a space craft.

Only recently has a new piece of technology that could solve the multi-environment problem by modifying current rocket technology to allow the burning of hydrogen fuel. Alan Bond had begun work on rocket engines in 1982 with a view to overcoming the inadequate characteristics of existing rocket-based expendable launch vehicles. Within this work Alan realised that the use of heat exchangers within rocket engine cycles can greatly increase their efficiency by allowing them to use atmospheric air to burn in the combustion chambers when flying in the atmosphere like a jet (rather than using heavy liquid oxygen stored in on-board tanks) and extracting heat where it causes a problem whilst using that heat to power the turbo machinery in the engine.
These new Synergetic Air-Breathing Rocket Engines (SABRE) would be suitable for powering modified aircraft directly into Earth orbit and it was subsequently found that in a reconfigured form they would be able to propel aircraft at cruise speeds of up to five times the speed of sound. The key enabling technology that needed to be developed to make these new engines viable was lightweight heat exchangers.
Inspired by the miniaturisation of the silicon chip, and understanding that heat exchangers were far from reaching their physical limit in terms of miniaturisation, developing high power lightweight heat exchangers and understanding the design of the new engines and vehicles that they enable became the focus of Alan Bond and subsequently REL (Reaction Engines Limited). Reaction Engines Ltd ('REL') was founded in 1989 by Alan Bond and the principal two engineers from Rolls Royce behind the RB545 engine programme, John Scott-Scott and Richard Varvill.

There were many problems to be solved for Reaction Engines Ltd at hypersonic speeds such as cooling the oxygen from 1000 degrees to manageable temperatures as well as water vapor forming and freezing in the heat exchange. All main problems seem to be resolved and currently a prototype is being tested and working.
The new atmosphere breathing rocket engine can reduce the fuel payload for the oxidizer, which will only be required for the vacuum of space. The liquid hydrogen will still be required but allows longer run times and is quite clean and cheap to run, also it is reusable for space travel.
Most of the funding for the skylon project is supplied by private investment, which suggests that the commercial sector will be able to provide cheap and affordable global travel. Flight times across the world would be reduced to 4 hours, also this type of vehicle can make space tourism a real possibility. Imagine that a craft can fly over 200 miles in orbit to a space station using liquid hydrogen and oxygen. The cost of orbiting to space will be reduced to a fraction of the current price from current technologies. The future of space might be the exploration of other planets. But the technology to fly to low orbit has just become a reachable low cost achievement.