HOTOL (HOrizontal Take Off and Landing) was a joint space plane project between British Aerospace (BAe) and Rolls Royce. Funding was also provided by the British Government, via the British National Space Council (BNSC) - since replaced by the UK Space Agency1 - to the tune of £2million from 1986. It was designed to be a reusable delivery vehicle which could deliver payloads of up to 7 tonnes2 to geostationary or low earth orbit. It was to be completely reusable and had no throwaway parts.
It should be noted that HOTOL never passed the initial modelling and design phase and thus the numbers quoted in this article are approximate as they vary slightly depending on which design document or press release is used.
HOTOL was designed to launch from any runway which could handle a Boeing 747 or Concorde sized craft. It was to use a rocket powered sled to bring the craft quickly to its launch speed of 330mph from where the main engine would propel it to a height of around 90km.
Once at this height the main engine would cut out and the craft would then continue to its orbital height of 300km. Once there it would manoeuvre into the required position using a series of onboard thrusters - similar to those used in other space craft.
A full payload of 7 tonnes could be delivered to low earth orbit whilst lighter loads would be launched to geostationary orbit with the aid of a perigee motor3.
Another important part of the journey is re-entry. As the HOTOL design had a relatively large wing area, the re-entry temperatures would not rise above 1,400°C and thus expensive insulation tiles (such as the Shuttle uses) were not required. This would also save on development and manufacturing costs.
Another advantage of the design is that HOTOL could be launched from one equatorial location but return to a maintenance facility in Europe.
HOTOL design can be traced back to the 'Swallow' supersonic swingwing vehicle developed by Sir Neville Barnes Wallis4 which had similar ideas behind it but lacked a suitable rocket propulsion system.
The engine was to be the Rolls Royce RB545 based on a design by Alan Bond5 who licensed it to Rolls Royce on the understanding they would bring it to production. The engine was a liquid oxygen (LOX) / liquid hydrogen (LH2) based design. One problem with such engines is that the LOX fuel required makes up a large proportion of the launch weight and this was addressed in the RB545 by making it extract its own oxygen from the air up to 30km and then switch to the internally carried LOX fuel. This would correspond to switching while travelling at around Mach 6.
It was this innovation and reduced weight which allowed the craft to be single stage and thus had significantly less turnaround and launch costs than more conventional designs. It also meant faster turnaround times. Launch costs were estimated to be around £5million at the time. All of this made HOTOL significantly cheaper than the Space Shuttle and it could be used more frequently.
This still entailed a large amount of LOX to be carried and the design of the landing gear was such that it could not support the vehicle when fully laden. Thus if HOTOL were forced to abort and make an emergency landing the fuel would have to be jettisoned first otherwise the undercarriage would collapse.
Unfortunately the British Government cancelled its funding in 1989 due to government reductions in public spending. This effectively ended the main development of HOTOL as its engine was classified as top secret and foreign investors could not be sought.
Following the government funding cancellation Rolls Royce also pulled out of the partnership and BAe cut the project workforce from 100 to 10.
Instead, BAe approached Russia with a proposal to alter the design to use conventional Russian rocket engines. This meant that the innovative air breathing stage was no longer available and thus the initial launch became a problem again. The proposed solution was to launch interimHOTOL (as it was to be known) from the back of a Russian Antonov airplane. This would have reduced the cost of the project and reduced weight and complexity, although now it could only place 4.5 tonnes into orbit. It also made for a smaller, shorter and fatter design.
Neither the European Space Agency nor the British Government had any interest in the project anymore and it was thus cancelled.
HOTOL still lives on, though, as the main members of the team formed their own company called Reaction Engines Limited. The team includes the designer of the RB545 engine and the company, which operates from the United Kingdom, has worked to improve and refine the engine - solving most of the original problems. This has lead to the current SABRE engine and the SKYLON and LAPCAT A2 skyplane projects.
Launch mass: 196tonnes
Landing mass: 34tonnes
Payload: 7-11 tonnes
Takeoff speed: 330mph
Landing speed: 195mph