Scramjet nozzle length
Scramjets are also considered a potential enabling technology for reusable launch vehicles. Scramjets are nothing new to aviation, and have been seen by many in the aerospace world as the future's primary method of rapid transportation at the edge of space. The problem has always been getting that scramjet engine through subsonic speeds where it is ineffective. Scramjets are supersonic combustion engines that use oxygen from the atmosphere to burn onboard fuel. By contrast, conventional rockets carry their own oxygen to burn fuel.
Scramjets are the same as ramjets except that they do not slow the air to subsonic speeds. This allows scramjets to achieve much higher speeds, up to somewhere between mach 12 (9134mph) and mach 24 (18269mph)! Scramjets are special jet engines that work at very high speeds and at high altitudes, and research has continued on them for decades with mixed success. A successful and reliable scramjet design would allow travel between London and Sydney, Australia in as little as two hours. Scramjets are harder to make work in general, and much harder to integrate with a turbojet. You can't just vary the ratio, because the turbojet cannot deal with supersonic flow.
Scramjets are force-fed air engines with no moving or meltable components. Scramjets are air-breathing supersonic combustion ramjet engines. Hypersonic speed cuts in at Mach 5 or 5 times the speed of sound. Scramjets are supersonic combustion engines that use oxygen from the atmosphere for fuel, making them lighter and faster than fuel carrying rockets. Scientists hope that one day a scramjet aircraft fired into space could cut traveling time from Sydney to London to as little as two hours.".
Scramjets are the only engines other than rockets that can reach hypersonic speeds. Their first use would likely be by military forces, as light and adaptable engines for ultra-fast and maneuverable missiles.
Rocket ships can reach Mach 25, but they have to carry tremendous amounts of liquid oxygen to burn their fuel. The space shuttle, for example, weighs only 165,000 pounds empty, but it must carry 226,000 pounds of liquid hydrogen and 1.4 million pounds of liquid oxygen to reach orbit. Rockets can easily achieve these speeds, but they are much more expensive to operate. Rockets achieve their great thrust by carrying the fuel as well as the oxygen needed to burn the fuel. Rocket-based launchers, including SSTO, do most of their accelerating in vacuum, away from these problems. Perhaps the biggest problem of air-breathing engines for spaceflight is that they are heavy.
Rockets must carry their own oxygen to combust the fuel they carry aboard. But in scramjets, oxygen is rammed into a combustion chamber where it mixes with fuel and spontaneously ignites.