6 ms·

> The HAWC is exploring air-breathing hypersonic flight > propelling the cruiser at a speed greater than Mach 5 I wonder if they are running a scramjet engine

> The HAWC is exploring air-breathing hypersonic flight > propelling the cruiser at a speed greater than Mach 5 I wonder if they are running a scramjet engine: https://en.wikipedia.org/wiki/Scramjet https://en.wikipedia.org/wiki/Scramjet

heywherelogingo 5y ago
"In order for the scramjet engine to ignite,..."
- HPsquared 5y ago
  These engines (ramjets, including scramjets - the 'sc' means supersonic combustion, which is the hard part) rely on forward airspeed to compress the air as it enters the engine. You can't have a ramjet without some sort of booster stage (or non-ramjet mode of operation). In this case, there's probably a conventional rocket engine to get it up to speed, which is then sustained by the (sc)ramjet engine.
  - varjag 5y ago
    "In order for the scramjet engine to ignite, the vehicle must be moving at hypersonic speed, so a booster is used for that portion of the flight."
MilStdJunkie 5y ago
There's a sort of fuzzy line betwwwn high supersonic and hypersonic, but the secret sauce that definitely separates them - and the hardest part by far - is supersonic combustion in the scramjet. You basically have to figure out how to make something sit still long enough to impart a thrust impulse, while at the same time sitting in an airstream that's starting to act like a cutting laser. Very little tech data from working scramjet shots have been made public, but it's likely the most successful use some kind of incredible heat exchange system to take at least some of the energy out of the flow. Maybe pipe it into the fuel. The Skylon / Sabre engine justa went "fuck it" and liquified the air at lower speeds, then transitioned to liquified oxidizer like a rocket when hypersonic time came around. If that sounds like some kind of miracle heat exchanger that's because it is. It's not been worked out. Pulse Detonation is something I always wondered why we didn't see more of it, but apparently when you make a PDE big enough it hammers the plane apart like an infant with a frying pan. Who knew?
- alexisread 5y ago
  As far as I know, the Sabre heat exchanger doesn't liquefy the air as frost kills the tubing. It cools down to the boundary without going over. I believe they've demonstrated mach 3 conditions but not mach 5, and they're on track https://www.pprune.org/military-aviation/print-619462-reaction-engines-sabre-rocket-engine-demo-core-passes-review.html https://www.pprune.org/military-aviation/print-619462-reacti...
- twic 5y ago
  The SABRE engine won't liquefy incoming air, it just cools it down a lot (to -150 C!). But SABRE is an evolution of a 1980s design, the RB545, designed for the HOTOL spaceplane, which would indeed have liquefied incoming air. The RB545 was in turn derived from a 1950s concept, LACE: https://en.wikipedia.org/wiki/Liquid_air_cycle_engine https://en.wikipedia.org/wiki/Liquid_air_cycle_engine
  - MilStdJunkie 5y ago
    Ha, thanks for the correction! I haven't been following the project all that closely, but I always liked it. It seems to me like a more . . durable . . solution to the hypersonic propulsion problem, almost an extension of the hybrid turbo-rams of the SR71. I mean, the alternative, relying on converging shock fronts to serve as a flame holder . . so, ok, I am not an expert, but that sounds iffy.

heywherelogingo 5y ago

"In order for the scramjet engine to ignite,..."

HPsquared 5y ago
These engines (ramjets, including scramjets - the 'sc' means supersonic combustion, which is the hard part) rely on forward airspeed to compress the air as it enters the engine. You can't have a ramjet without some sort of booster stage (or non-ramjet mode of operation). In this case, there's probably a conventional rocket engine to get it up to speed, which is then sustained by the (sc)ramjet engine.
- varjag 5y ago
  "In order for the scramjet engine to ignite, the vehicle must be moving at hypersonic speed, so a booster is used for that portion of the flight."

MilStdJunkie 5y ago

There's a sort of fuzzy line betwwwn high supersonic and hypersonic, but the secret sauce that definitely separates them - and the hardest part by far - is supersonic combustion in the scramjet. You basically have to figure out how to make something sit still long enough to impart a thrust impulse, while at the same time sitting in an airstream that's starting to act like a cutting laser. Very little tech data from working scramjet shots have been made public, but it's likely the most successful use some kind of incredible heat exchange system to take at least some of the energy out of the flow. Maybe pipe it into the fuel. The Skylon / Sabre engine justa went "fuck it" and liquified the air at lower speeds, then transitioned to liquified oxidizer like a rocket when hypersonic time came around. If that sounds like some kind of miracle heat exchanger that's because it is. It's not been worked out. Pulse Detonation is something I always wondered why we didn't see more of it, but apparently when you make a PDE big enough it hammers the plane apart like an infant with a frying pan. Who knew?

alexisread 5y ago
As far as I know, the Sabre heat exchanger doesn't liquefy the air as frost kills the tubing. It cools down to the boundary without going over. I believe they've demonstrated mach 3 conditions but not mach 5, and they're on track https://www.pprune.org/military-aviation/print-619462-reaction-engines-sabre-rocket-engine-demo-core-passes-review.html https://www.pprune.org/military-aviation/print-619462-reacti...
twic 5y ago
The SABRE engine won't liquefy incoming air, it just cools it down a lot (to -150 C!). But SABRE is an evolution of a 1980s design, the RB545, designed for the HOTOL spaceplane, which would indeed have liquefied incoming air. The RB545 was in turn derived from a 1950s concept, LACE: https://en.wikipedia.org/wiki/Liquid_air_cycle_engine https://en.wikipedia.org/wiki/Liquid_air_cycle_engine
- MilStdJunkie 5y ago
  Ha, thanks for the correction! I haven't been following the project all that closely, but I always liked it. It seems to me like a more . . durable . . solution to the hypersonic propulsion problem, almost an extension of the hybrid turbo-rams of the SR71. I mean, the alternative, relying on converging shock fronts to serve as a flame holder . . so, ok, I am not an expert, but that sounds iffy.