The Roton was a space launch vehicle designed by the now defunct Rotary Rocket company.

The design is SSTO, and has a large number of unique features.

First the fuel. Most vehicles such as the Space Shuttle, Ariane use liquid hydrogen and liquid oxygen; but the Roton was to use kerosene and liquid oxygen.

This is unusual, because hydrogen and oxygen gives an exhaust velocity of about 4300m/s, whereas kerosene and oxygen is only up to 3500m/s, and so gives proportionately less thrust for each kilogram of fuel. Still, kerosene has about 5x higher density; so for the same tank size, far more impulse (i.e. total thrust) is obtained from the same physical size of tank. In fact kerosene engines usually give better thrust, because the higher density permits more fuel to flow to the engine with thinner pipes and smaller, lighter pumps.

Secondly, the Roton was designed to weigh very little. The liquid oxygen and associated tank contains most of the takeoff mas of the vehicle, and was constructed using composite materials, it has a tiny mass. However, the hydrocarbons in the composite walls can theoretically combust with the LOX inside it, but testing showed that this risk was fairly small in practice. They successfully tested the tank by shooting it with a gun. Overall, the "Roton ATV" managed a dry mass fraction of about 6%; this is very light for a vehicle, but then it needed to be to reach orbit in a single stage.

The engine bay of the vehicle was unique; was to utilise more than 70 rocket engines distributed around the periphery. These engines were to be angled at about 1 degree to the main axis, and this angle would cause the entire engine to spin. Due to the low angle, the rotation speed is entirely stable, no control is necessary. The spin causes the fuel that is fed along the central axis to be forced by the rotation into the combustion chambers at the rim with a pressure of more than 200 atmospheres. This high pressure gives slightly better ISP and allows the nozzles to be comparatively short, making the engines more compact. Also, this form of self pumping is incredibly efficient, about 3x more efficient than conventional turbopumps. The exhaust of the rocket engines are supposed to work as a 'zero length truncated aerospike'. The exhaust of the engines forms a recirculation zone from the rim under the vehicle and partially fills in the vacuum under the vehicle that would form, however it is unclear how well this would work. A 'classical' aerospike is supposed to have gas injection in the base, yet Rotary had no plans to do this, and this is likely to make the engines much less efficient whilst in the atmosphere.

The reentry system for the vehicle was 'semiballistic' (basically similar to the Apollo reentry system) base first. The vehicle was to use water cooling through small holes in the base to avoid burning up.

The landing system was particularly inspired and incredibly lightweight, after reentering down to about 60m/s (150 mph) the vehicle would spin up an autogyro at the top and use this to slow vehicle further to give a sink rate of about 30 mph. Finally, to land gently the rotor would spin up using hydrogen peroxide rocket tips; this gave the vehicle excellent landing characteristics, it could land on any stable surface.

The Roton ATV (Air Test Vehicle) proved many aspects of the vehicle would work- the design weight of the vehicle, it showed that it would fly and was stable (if a little hairy and unusual to control.)

The payload of the Roton was to be 3 tonnes and additionally would carry two people; and a launch would be about 3 million dollars. This is $1000 per kg, which is very cheap for manned space flight. Inspite of the low launch mass, it could very probably have launched fairly large (3 tonne) satellites to GEO; it just would have launched the satellite on one flight, and the fuel on the next two; and docked them together on orbit.

The company folded due to market uncertainties. But would it have worked? In my opinion the answer is probably yes, although they would very likely have had to add their backup plan of strap on boosters to overcome the aerospike issues, atleast initially.

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