Ramjets differ from pulsejets, which use an intermittent combustion; ramjets employ a continuous combustion process. Other types such as pulsejets, ramjets, scramjets and pulse detonation engines have also flown. Other designs includes ramjets or even a complete turbojet engine. Ramjets work most efficiently at supersonic speeds around . Bristol Aero Engines produced a range of rocket motors and ramjets for missile propulsion. In 1958, an analytical paper discussed the merits and disadvantages of supersonic combustion ramjets. Other proposed interstellar propulsion systems include light sails, ramjets, and beam-powered propulsion. On June 13, the ramjets achieved supersonic speed. Unlike an LFRJ, solid propellant ramjets cannot flame out. The Hiller HJ-1 Hornet was an early attempt to build a jet-powered helicopter using ramjets. These included live ramjets, but still had no guidance system or warhead. It had two rectangular "wings" which doubled as the ramjets. YH-32 Hornet was powered by ramjets mounted on the blade ends. They are primarily used on ramjets, such as the D-21 Tagboard and Lockheed X-7. Solid-fuel ramjets are even simpler.