Hypersonic speed
By the early 1960s, the rapid progress towards faster and faster aircraft since the end of World War II suggested that within a few years, operational aircraft would be flying at "hypersonic" speeds, five times the speed of sound or more.
It didn't happen. Except for specialized rocket research craft like the US X-15, the speeds of operational aircraft have remained level since that time, generally in the range of Mach 1 to Mach 2. To be sure, modern aircraft are much more capable than those of four decades ago, but they are no faster.
Hypersonic flight concepts haven't gone away, however, with low-level investigations continuing over the past decades. Now the US military and the National Aeronautics & Space Administration (NASA) have formulated a "National Hypersonics Strategy" to investigate a range of options for hypersonic flight.
The different organizations have different agendas, but they have all realized they need to coordinate their activities to make progress. The Army, for example, wants to develop hypersonic missiles that can attack mobile missile launchers before they leave their launch site and disappear. NASA wants to develop new, economical, reusable launch vehicles. The Air Force is interested in a wide range of hypersonic systems, from hypersonic air-launched cruise missiles to orbital spaceplanes, that the service believes could transform it into a true "aerospace force".
Progress has been made on hypersonic technology. The USAF and Pratt & Whitney have cooperated on the "Hypersonic Technology (HyTECH)" supersonic combustion ramjet (scramjet) engine, which has now been demonstrated in a wind-tunnel environment. NASA's Marshall Space Propulsion Center has introduced an "Integrated Systems Test Of An Air-Breathing Rocket (ISTAR)" program, prompting Pratt & Whitney, Aerojet, and Rocketdyne to join forces for development.
To coordinate hypersonic technology development, the various factions interested in hypersonic research have formed two "integrated product teams (IPTs)", one to consolidate Army, Air Force, and Navy hypersonic weapons research, the other to consolidate Air Force and NASA space transportation and hypersonic aircraft work. Current funding levels are relatively low, no more than $85 million USD per year in total, but are expected to rise.
At present, the most advanced US hypersonics program is the $185 million USD NASA Langley "X-43A Hyper-X" effort, which will fly small test vehicles to demonstrate hydrogen-fueled scramjet engines. NASA is working with contractors Boeing, Microcraft, and the General Applied Science Laboratory (GASL) on the project.
Each X-43A test vehicle will be carried to operational speed and altitude on the nose of an Orbital Sciences Pegasus air-launched booster. First flight is scheduled for June 2001, and the X-43A is expected to attain Mach 7. The second flight should also attain Mach 7, and the third is expected to attain Mach 10. The US Army's Aviation & Missile Command is using a wind-tunnel model of the X-43A for investigations into advanced missile concepts.
The X-43A program has been in development for several years and is proceeding more or less on track. However, the new National Hypersonics Strategy has persuaded NASA to change their plans for other hypersonic efforts.
The NASA Langley, Marshall, and Glenn Centers are now all heavily engaged in hypersonic propulsion studies. The Glenn Center is taking leadership on a Mach 4 turbine engine of interest to the USAF. As for the X-43A Hyper-X, three follow-on projects are now under consideration:
- X-43B: A scaled-up version of the X-43A, to be powered by the ISTAR engine. ISTAR will use a hydrocarbon-based liquid-rocket mode for initial boost, a ramjet mode for speeds above Mach 2.5, and a scramjet mode for speeds above Mach 5 to take it to maximum speeds of at least Mach 7. A version intended for space launch could then return to rocket mode for final boost into space. ISTAR is based on a proprietary Aerojet design called a "strutjet", which is currently undergoing wind-tunnel testing.
- X-43C: NASA is in discussions with the Air Force on development of a variant of the X-43A that would use the HyTECH hydrocarbon-fueled scramjet engine.
While most scramjet designs to date have used hydrogen fuel, HyTech runs on conventional kerosene-type hydrocarbon fuels, which are much more practical for support of operational vehicles. A full-scale engine is now being built, which will use its own fuel for cooling. Using fuel for engine cooling is nothing new, but the cooling system will also act as a chemical reactor, breaking long-chain hydrocarbons down into short-chain hydrocarbons that burn more rapidly.
- X-43D: A version of the X-43A with a hydrogen-powered scramjet engine with a maximum speed of Mach 15.
The National Hypersonics Plan is still in definition. A final report is expected in August 2001.
Hypersonic development efforts are also in progress in other nations. The French are now considering their own scramjet test vehicle and are in discussions with the Russians for boosters that would carry it to launch speeds. The approach is very similar to that used with the current NASA X-43A demonstrator.
Several scramjet designs are now under investigation with Russian assistance. One of these options or a combination of them will be selected by ONERA, the French aerospace research agency, with the EADS conglomerate providing technical backup. The notional immediate goal of the study is to produce a hypersonic air-to-surface missile named "Promethee", which would be about 6 meters (10 feet) long and weigh 1,700 kilograms (3,750 pounds).