Since the oxidizer 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.
While HOTOL's proof-of-concept design study was being carried out, attempts were made by both industry and the British government to establish international cooperation to develop, produce, and deploy the spacecraft.
Additionally, technical issues were encountered, and there were allegations that comparisons with alternative launch systems such as conventional rocket vehicle using similar construction techniques failed to show much advantage to HOTOL.
[3] In December 1984, a Department of Trade and Industry (DTI) memorandum noted that West Germany was interested in the programme, while France had adopted a critical attitude towards HOTOL, which the ministry viewed as potentially due to it being seen as a competitor to French-led projects.
[3] In March 1985, there were claims that Rolls-Royce was in the process of conducting licensing talks for HOTOL engine technology with American propulsion company Rocketdyne.
[3] Early on, there was considerable pressure to demonstrate the project's feasibility and credibility in advance of final decisions being taken by ESA on the Hermes and what would become the Ariane 5 launch system, thus the work concentrated on the validation of critical technologies involved.
[2] By November 1985, DTI and RAE discussions noted that Rolls-Royce were seeking American data on ramjet technology to support their work on the engine, which it referred to by the name Swallow.
[3] According to British government files, neither BAe nor the MoD were enthusiastic for the prospects of American involvement in the programme, expressing reluctance out of a belief that the outcome of such a move could result in the UK becoming a junior member in a project that it once led.
[3] In December 1984, project management consultant David Andrews issued an eight-page critique of the programme, noting that the design was optimised for the ascent while exposing itself to extended thermal loads during descent due to a low level of drag.
Redesign of the vehicle to do this required a large mass of hydraulic systems, which cost a significant proportion of the payload, and made the economics unclear.
By 1989, the outlook for HOTOL had become bleak; from the onset of the project, support between the British government and industrial partners had been uneven, while the United States had emerged as the only foreign nation that showed willingness to contribute to the programme,[3] in part because of the secrecy surrounding it.
The project was almost at the end of its design phase while much of the plans remained in a speculative state; the craft was reportedly still dogged with aerodynamic problems and operational disadvantages at this point.
After reaching low Earth orbit (LEO), HOTOL was intended to re-enter the atmosphere and glide down to land on a conventional runway (approx 1,500 metres minimum).
Only a single payload would have been carried at a time as BAe had judged this to be more economic as it removed any need for satellite interfacing and allowed for missions to be tailored to individual requirements.
[11][5] Within the atmosphere, air would be taken in through two vertically mounted intake ramps, then the flow would be split, passing the correct amount to the pre-coolers, and the excess to spill ducts.
The majority of the remaining hot hydrogen was released from the back of the engine, with a small amount drawn off to reheat the air in the spill ducts in a ramjet arrangement to produce "negative intake momentum drag".
Then liquid oxygen (LOX) would have been injected into the airflow to drop the temperature to −50 °C (−58 °F) flash freezing the water into microscopic ice crystals, sufficiently cold that they wouldn't melt due to kinetic heating if they struck the second pre-cooler elements.