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This is the engine that powers this aircraft: http://en.wikipedia.org/wiki/Scramjet http://en.wikipedia.org/wiki/Scramjet Right now, it can only operate at hig

by mustpax 16y ago

This is the engine that powers this aircraft: http://en.wikipedia.org/wiki/Scramjet http://en.wikipedia.org/wiki/Scramjet Right now, it can only operate at high speeds, hence the bomber-based launch.

retube 16y ago
a scramjet will only _ever_ be able to operate at high speeds. The compression of intake air for combustion is achieved by the forward velocity of the engine itself (the engine has no moving parts) unlike a normal jet engine which uses a fan to compress the intake. Any scramjet, current or future, will require some combination of other engines in order to accelerate the aircraft to a velocity at which the scramjet becomes self-sustaining (typically 3000+ mph I think). Edit: needless to say, a stable self-sustaining scramjet is an INSANELY hard fluid dynamics problem.
- rbanffy 16y ago
  You could use a conventional jet engine to pump its exhaust into the scramjet intake. It would have less oxygen, but a turbofan engine could compress enough air through the scramjet fast enough. The trick in this is to transition from subsonic flow in the turbofan to full supersonic engine operation. Looks like an interesting concept.
  - retube 16y ago
    Yeah but then the turbo fan is blocking the air intake at high speeds. plus the exhaust of a turbo fan is subsonic, whereas a scramjet intake needs hypersonic intake. the way to do it is either a) have a short-lived rocket engine to take you to the threshold speed, or b) bootstrap the aircraft using separate turbofan, ramjet and finally scramjet engines.
    - rbanffy 16y ago
      > plus the exhaust of a turbo fan is subsonic You could funnel it to increase its speed. I am not a jet-engine expert, much less a ramjet/scramjet one, but the vestigial engineer in me suspects there is some practical solution hiding in this space.
  - run4yourlives 16y ago
    Putting a conventional jet engine in front of a scramjet defeats any real benefit you derive from having the scramjet there in the first place. The scramjet's main advantages are cost and efficiency; to accomplish these gains it comes with a myriad of technical problems and hurdles. The viability of the technology is actually still very much up for debate.
    - rbanffy 16y ago
      You don't keep the turbofans in front of the scramjet all the time - you use them to give the scramjet enough air to start. As the speed increases, the turbofan engines would eventually cease to operate and could be shut off completely, leaving the scramjet to operate freely. The turbofans are dead weight when you are cruising at Mach 7 in the upper atmosphere, but are useful for take off and landing.
  - andrewf 16y ago
    The SR-71 Blackbird's engines were hybrids; AFAICT they didn't quite use this approach, but instead had a layout where the same engine could function as a conventional jet and a ramjet depending on how fast air was hitting it. http://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58 http://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58
    - rbanffy 16y ago
      Hmmm. The J58 is humongous, but it would be kind of cool to use them to start a scramjet (or, better, use it as the basis of another engine that can function in all three modes). The SJX6 combustion chamber is not cylindrical and would not fit around a jet engine the way the J58 works in ramjet regime, but I bet many people are thinking about how to do it right now. A scramjet engine that can take off and land like a more or less ordinary plane (it would certainly not look ordinary) is something very desirable.

by mustpax 16y ago

This is the engine that powers this aircraft: http://en.wikipedia.org/wiki/Scramjet http://en.wikipedia.org/wiki/Scramjet Right now, it can only operate at high speeds, hence the bomber-based launch.

retube 16y ago

a scramjet will only _ever_ be able to operate at high speeds. The compression of intake air for combustion is achieved by the forward velocity of the engine itself (the engine has no moving parts) unlike a normal jet engine which uses a fan to compress the intake. Any scramjet, current or future, will require some combination of other engines in order to accelerate the aircraft to a velocity at which the scramjet becomes self-sustaining (typically 3000+ mph I think). Edit: needless to say, a stable self-sustaining scramjet is an INSANELY hard fluid dynamics problem.

rbanffy 16y ago
You could use a conventional jet engine to pump its exhaust into the scramjet intake. It would have less oxygen, but a turbofan engine could compress enough air through the scramjet fast enough. The trick in this is to transition from subsonic flow in the turbofan to full supersonic engine operation. Looks like an interesting concept.
- retube 16y ago
  Yeah but then the turbo fan is blocking the air intake at high speeds. plus the exhaust of a turbo fan is subsonic, whereas a scramjet intake needs hypersonic intake. the way to do it is either a) have a short-lived rocket engine to take you to the threshold speed, or b) bootstrap the aircraft using separate turbofan, ramjet and finally scramjet engines.
  - rbanffy 16y ago
    > plus the exhaust of a turbo fan is subsonic You could funnel it to increase its speed. I am not a jet-engine expert, much less a ramjet/scramjet one, but the vestigial engineer in me suspects there is some practical solution hiding in this space.
- run4yourlives 16y ago
  Putting a conventional jet engine in front of a scramjet defeats any real benefit you derive from having the scramjet there in the first place. The scramjet's main advantages are cost and efficiency; to accomplish these gains it comes with a myriad of technical problems and hurdles. The viability of the technology is actually still very much up for debate.
  - rbanffy 16y ago
    You don't keep the turbofans in front of the scramjet all the time - you use them to give the scramjet enough air to start. As the speed increases, the turbofan engines would eventually cease to operate and could be shut off completely, leaving the scramjet to operate freely. The turbofans are dead weight when you are cruising at Mach 7 in the upper atmosphere, but are useful for take off and landing.
- andrewf 16y ago
  The SR-71 Blackbird's engines were hybrids; AFAICT they didn't quite use this approach, but instead had a layout where the same engine could function as a conventional jet and a ramjet depending on how fast air was hitting it. http://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58 http://en.wikipedia.org/wiki/Pratt_%26_Whitney_J58
  - rbanffy 16y ago
    Hmmm. The J58 is humongous, but it would be kind of cool to use them to start a scramjet (or, better, use it as the basis of another engine that can function in all three modes). The SJX6 combustion chamber is not cylindrical and would not fit around a jet engine the way the J58 works in ramjet regime, but I bet many people are thinking about how to do it right now. A scramjet engine that can take off and land like a more or less ordinary plane (it would certainly not look ordinary) is something very desirable.