Editing Rotary Rocket (section)

=== Helicopter from orbit ===

The revised and redesigned '''Roton''' concept was a cone-shaped launch vehicle, with a helicopter rotor on top for use only during landing. An internal cargo bay could be used both for carrying payloads to orbit and bringing others back to Earth. The projected price to orbit of this design was given as $1,000 per kg of payload, less than one-tenth of the then-current launch price. Payload capacity was limited to a relatively modest {{convert|6000|lb|kg}}.{{Citation needed|date=March 2009}}

The revised version would have used a unique rotating annular [[aerospike engine]]: the engine and base of the launch vehicle would spin at high speed (720 [[Revolutions per minute|rpm]]) to pump fuel and oxidizer to the rim by the rotation. Unlike the landing rotor, due to the shallow angle of the nozzles in the base rotor, the rotation speed self limited and required no control system. Since the density of the LOX ([[liquid oxygen]]) was higher than that of the kerosene, extra pressure was available with the LOX, so it would have been used to cool the engine's throat and other components, rather than using the kerosene as the coolant as in a [[RP-1#Usage and history|conventional LOX/kerosene rocket.]] However, at the high G levels at the outer edge of the rotating engine block, clarity on how LOX would work as a coolant was both unknown and difficult to validate. That added one layer of risk.{{citation needed|date=April 2023}}

In addition, the rotating exhaust acted as a wall at the outer edge of the engine base, lowering the temperature of the base to below ambient due to ejector pump effect and creating a suction cup at the bottom in atmosphere. This could be alleviated using makeup gas to develop base pressure, requiring an additional rocket engine to fill up the base of the main rocket engine. (Similar problems would have occurred in a [[Aerospike engine#Principles|conventional aerospike engine]], but there, natural recirculation plus use of the [[Gas-generator cycle|turbopump gas-generator's exhaust]] as the makeup gas would have largely alleviated the problem "for free."){{citation needed|date=April 2023}}

At the rim, 96 miniature jets would exhaust the burning propellants (LOX and [[kerosene]]) around the rim of the base of the vehicle, which gained the vehicle extra thrust at high altitude – acting as a zero-length truncated aerospike nozzle.<ref>United States Patent 5842665</ref> A similar system with non-rotating engines was studied for the [[N1 rocket]]. That application had a much smaller base area, and did not create the suction effect a larger peripheral engine induces. The Roton engine had a projected vacuum [[Specific impulse|I<sub>SP</sub> (specific impulse)]] of ~{{convert|355|isp}}, which is very high for a LOX/kerosene engine – and a thrust to weight ratio of 150, which is extremely light.<ref>Anselmo, Joseph C., "Rotarians." ''[[Aviation Week & Space Technology]]'', October 5, 1998, p. 17.</ref>

During reentry, the base also served as a water-cooled [[heatshield]]. This was theoretically a good way to survive reentry, particularly for a lightweight reusable vehicle. However, using water as a coolant would require converting it into superheated steam, at high temperatures and pressures, and there were concerns about micrometeorite damage on orbit puncturing the pressure vessel, causing the reentry shield to fail. These concerns were resolved using a failure-resistant massively redundant flow system, created using thin metal sheets chemically etched with a pattern of micropores forming a channel system that was robust against failure and damage.{{citation needed|date=April 2023}}

In addition, cooling was achieved two different ways; one way was the vaporization of the water, but the second was even more significant, and was due to the creation of a layer of "cool" steam surrounding the base surface, reducing the ability to heat. Further, the water metering system would have to be extremely reliable, giving one drop per second per square inch, and was achieved via a trial/error design approach on real hardware. By the end of the Roton program, some hardware had been built and tested. The reentry trajectory was to be trimmed, similar to the Soyuz, to minimize the G loads on the passengers. And the ballistic coefficient was better for the Roton and could be better tailored. When the Soyuz trim system failed and it went full ballistic, the G levels did rise significantly but without incident to the passengers.{{citation needed|date=April 2023}}

The vehicle was also unique in planning to use its [[helicopter]]-style [[helicopter rotor|rotor]]s for landing, rather than wings or parachutes. This concept allowed controlled landings (unlike parachutes), and it was 1/5 the weight of fixed wings. Another advantage was that a helicopter could land almost anywhere, whereas winged [[spaceplane]]s such as the Space Shuttle had to make it back to the runway. The rotor blades were to be powered by peroxide tip rockets. The rotor blades were to be deployed before reentry; some questions were raised about whether the blades would survive until landing.{{citation needed|date=April 2023}}

The initial plan was to have them almost vertical, but that was found to be unstable as they needed to drop lower and lower and spin faster for stability, the heating rates went up dramatically and the air flow became more head on. The implication of that was that the blades went from a lightly heated piece of hardware to one that either had to be actively cooled or made of SiC or other refractory material. The idea of popping out the blades became much more attractive at this point, and initial studies were made for that option. This rotor design concept was not without precedent. In 1955, one of five [[Soviet space program|Soviet]] designs for planned suborbital piloted missions was to include rocket-tipped rotors as its landing system. On May 1, 1958, these plans were dropped as a decision was made to proceed directly to orbital flights.{{citation needed|date=April 2023}}

Rotary Rocket designed and pressure-tested an exceptionally lightweight but strong [[composite material|composite]] LOX tank. It survived a test program which involved it being pressure cycled and ultimately deliberately shot to test its ignition sensitivity.{{citation needed|date=April 2023}}