Đoàn Trang
(Ms_Independent)
Điều hành viên
Hypersonic aviation
An aeroplane intended to be the fastest ever built is about to make its first flight
SINCE the first aeroplane wobbled off the ground a hundred years ago, engineers have dreamed of making their magnificent machines fly faster and faster. But for a long time the world speed record has not changed. The SR-71, an American spy plane, has been the fastest publicly acknowledged aircraft for almost 40 years. It has a top speed of just over Mach 3—in other words, three times the velocity of sound, or about 3,000 kilometres per hour. But if all goes well, this will soon change. On March 27th, NASA, America's aerospace agency, will conduct a test flight of an aircraft called the X-43A at Edwards Air Force Base in California. This vehicle is, admittedly, a disposable, unmanned drone. But its top speed is supposed to be a remarkable Mach 7.
The X-43A is powered by an engine known as a scramjet. Scramjets work on a simple principle—simpler, indeed, than the turbojets used in conventional aircraft. The engine is a tube through which air flows at supersonic speeds. While the air is in the tube, it is mixed with a fuel (in this case, hydrogen) and burned. This causes the mixture to expand. The exhaust thus goes out the back even faster than the air came in the front. Newton's third law of motion (to every action there is an equal and opposite reaction) then propels both engine and aircraft forwards.
The tricky part, according to Joel Sitz, the head of the X-43A flight-test programme at NASA's Dryden research centre, located next door to Edwards, is that the air is in the tube for mere milliseconds. Getting such details as the fuel-air mixture right is thus hard—and is made harder by the fact that what is right is different at different speeds.
At the moment, the “variable geometry” engine design needed to accommodate these differences is beyond the skill of aircraft engineers. So, to simplify the task, Saturday's flight will not require the X-43A to accelerate under its own power. Instead, it will be taken to high altitude by a B-52 bomber, dropped from the bomber, and then accelerated to Mach 7 by a booster rocket. Only then will the booster be jettisoned and the scramjet ignited.
Some readers may think that this plan sounds familiar. It is. But the last time NASA tried it, in June 2001, things went haywire a few seconds after the booster started firing. The rocket span out of control and had to be ordered to blow itself up.
An investigation concluded that data from computer simulations and wind-tunnel tests had been misinterpreted. One problem was that the rocket used as a booster (a type known as Pegasus, that is designed to be used as a satellite launcher) is usually dropped from its launch aircraft at an altitude of 12,200 metres. In 2001, the release was done at 7,000 metres. The difference in air pressure made the rocket behave abnormally, and probably contributed to the crash. Saturday's test will therefore take the sensible step of starting at 12,200 metres.
If all goes well this time, a Mach-10 flight of another X-43A will take place in July. A follow-up craft, the X-43C, is also in development. This will use a hydrocarbon fuel (think exotic petrol) rather than hydrogen. That will allow an X-43C to fly for up to five minutes, rather than a mere ten seconds. But the X-43C will still be disposable. A re-usable version, paradoxically known as the X-43B, is planned, but no money has yet been allocated to build it.
Though the X-43 project is a modest start, it could lead to bigger things. In the long term, scramjets offer the possibility of cheap access to space. That is because, unlike the rockets currently used to get into orbit, they are air-breathing. This means they do not have to carry liquid oxygen (or some other oxidant) to burn their fuel, and could thus use the weight saved to carry a bigger payload. A scramjet-powered booster could thus launch heavy loads to the edge of space. Only there would a rocket be required.
Economist.com
An aeroplane intended to be the fastest ever built is about to make its first flight
SINCE the first aeroplane wobbled off the ground a hundred years ago, engineers have dreamed of making their magnificent machines fly faster and faster. But for a long time the world speed record has not changed. The SR-71, an American spy plane, has been the fastest publicly acknowledged aircraft for almost 40 years. It has a top speed of just over Mach 3—in other words, three times the velocity of sound, or about 3,000 kilometres per hour. But if all goes well, this will soon change. On March 27th, NASA, America's aerospace agency, will conduct a test flight of an aircraft called the X-43A at Edwards Air Force Base in California. This vehicle is, admittedly, a disposable, unmanned drone. But its top speed is supposed to be a remarkable Mach 7.
The X-43A is powered by an engine known as a scramjet. Scramjets work on a simple principle—simpler, indeed, than the turbojets used in conventional aircraft. The engine is a tube through which air flows at supersonic speeds. While the air is in the tube, it is mixed with a fuel (in this case, hydrogen) and burned. This causes the mixture to expand. The exhaust thus goes out the back even faster than the air came in the front. Newton's third law of motion (to every action there is an equal and opposite reaction) then propels both engine and aircraft forwards.
The tricky part, according to Joel Sitz, the head of the X-43A flight-test programme at NASA's Dryden research centre, located next door to Edwards, is that the air is in the tube for mere milliseconds. Getting such details as the fuel-air mixture right is thus hard—and is made harder by the fact that what is right is different at different speeds.
At the moment, the “variable geometry” engine design needed to accommodate these differences is beyond the skill of aircraft engineers. So, to simplify the task, Saturday's flight will not require the X-43A to accelerate under its own power. Instead, it will be taken to high altitude by a B-52 bomber, dropped from the bomber, and then accelerated to Mach 7 by a booster rocket. Only then will the booster be jettisoned and the scramjet ignited.
Some readers may think that this plan sounds familiar. It is. But the last time NASA tried it, in June 2001, things went haywire a few seconds after the booster started firing. The rocket span out of control and had to be ordered to blow itself up.
An investigation concluded that data from computer simulations and wind-tunnel tests had been misinterpreted. One problem was that the rocket used as a booster (a type known as Pegasus, that is designed to be used as a satellite launcher) is usually dropped from its launch aircraft at an altitude of 12,200 metres. In 2001, the release was done at 7,000 metres. The difference in air pressure made the rocket behave abnormally, and probably contributed to the crash. Saturday's test will therefore take the sensible step of starting at 12,200 metres.
If all goes well this time, a Mach-10 flight of another X-43A will take place in July. A follow-up craft, the X-43C, is also in development. This will use a hydrocarbon fuel (think exotic petrol) rather than hydrogen. That will allow an X-43C to fly for up to five minutes, rather than a mere ten seconds. But the X-43C will still be disposable. A re-usable version, paradoxically known as the X-43B, is planned, but no money has yet been allocated to build it.
Though the X-43 project is a modest start, it could lead to bigger things. In the long term, scramjets offer the possibility of cheap access to space. That is because, unlike the rockets currently used to get into orbit, they are air-breathing. This means they do not have to carry liquid oxygen (or some other oxidant) to burn their fuel, and could thus use the weight saved to carry a bigger payload. A scramjet-powered booster could thus launch heavy loads to the edge of space. Only there would a rocket be required.
Economist.com
