The Minimum Speed Necessary for a Scramjet to Operate

[|Orion's Thought|]

The basic principle of a scramjet is to mix fuel with compressed air going supersonic speeds with relatively few or no moving parts. What I'm wondering is what the minimum speed the scramjet has to be moving to work properly. I've been looking on the web at numerous places, but no place gives me the same answer.

 

[Astronuc]

Maybe one can find information here.

http://www.nasa.gov/centers/glenn/news/pressrel/2002/02-051.html

http://www.nasa.gov/centers/dryden/news/NewsReleases/2004/04-56.html

http://trc.dfrc.nasa.gov/Newsroom/FactSheets/FS-053-DFRC.html

The R&D in hypersonic flight is done at Glenn and Dryden Centers, AFAIK, with maybe some work at Goddard.

http://facilities.grc.nasa.gov/htf/index.html - hypersonic test facility at Plum Brook affiliated with Glenn

 

[Cyrus]

http://www.jhuapl.edu/areas/sciencetech/Propulsion/RamjetsScramjets.asp

http://www.jhuapl.edu/newscenter/aplnews/2002/hyfly.asp

 

[DaveC426913]

I've been looking on the web at numerous places, but no place gives me the same answer.

That's becasue they're still experimental and probably a well-guarded secret.

 

[FredGarvin]

The basic principle of a scramjet is to mix fuel with compressed air going supersonic speeds with relatively few or no moving parts. What I'm wondering is what the minimum speed the scramjet has to be moving to work properly. I've been looking on the web at numerous places, but no place gives me the same answer.

You might be confusing a ramjet with a scramjet. There is a big difference between the two.

 

[DaveC426913]

You might be confusing a ramjet with a scramjet. There is a big difference between the two.

In that case, I stand with him in the "confused" camp. I was my understanding that a scramjet is, literally, a supersonic ramjet.


And ... (hang on) ... it would appear that Wikipedia is in our camp too:

"A scramjet (supersonic combustion ramjet) is a variation of a ramjet where the flow of the air and combustion of the fuel air mixture through the engine is done at supersonic speeds. This allows the scramjet to achieve greater speeds than a conventional ramjet which slows the incoming air to subsonic speeds before entering the combustion chamber."

 

[Astronuc]

Definitely check out the Wikipedia article mentioned by Dave, but it will not answer the original question. - http://en.wikipedia.org/wiki/Scramjet

Secondly, the heating of the gas by combustion causes the speed of sound in the gas to increase (and the Mach number to decrease) even though the gas is still traveling at the same speed. Forcing the speed of air flow in the combustion chamber under Mach one in this way is called "thermal choking". It is clear that a pure scramjet can operate at Mach numbers of 6-8, but in the lower limit, it depends on the definition of a scramjet. Certainly there are designs where a ramjet transforms into a scramjet over the Mach 3-6 range (Dual-mode scramjets). In this range however, the engine is still receiving significant thrust from subsonic combustion of "ramjet" type.

The issue is that the nozzle design optimized for the high Mach range doesn't perform as well in the subsonic or low Mach range. The optimal geometry has to be tailored based on the physics of the shock waves in the inlet. The details may be available from NASA.

See also - http://en.wikipedia.org/wiki/Hyper-X

 

[FredGarvin]

The basic principle of a scramjet is to mix fuel with compressed air going supersonic speeds with relatively few or no moving parts.

When I first read this, I immediately thought that the question was saying "...mix fuel with compressed air WHILE going supersonic speeds." I had to reread the post to see that he meant that the airflow was indeed supersonic. I stand corrected. The only confused one here is me.

 

[|Orion's Thought|]

Sorry Fred, I should have worded that better.

 

[Astronuc]

Found some useful and otherwise cool stuff!

Rocket propulsion - http://exploration.grc.nasa.gov/education/rocket/shortr.html

Low hypersonic - http://www.grc.nasa.gov/WWW/K-12/airplane/lowhyper.html

High hypersonic - http://www.grc.nasa.gov/WWW/K-12/airplane/hihyper.html

Engine inlets - http://www.grc.nasa.gov/WWW/K-12/airplane/inlet.html

Ramjet/scramjet thrust - http://www.grc.nasa.gov/WWW/K-12/airplane/ramth.html

Scramjet - http://www.grc.nasa.gov/WWW/K-12/airplane/scramjet.html

Simulation software - http://www.cfdrc.com/bizareas/aerospace/propulsion/sim_analysis.html

 

[Astronuc]

Revolutionary jet engine tested - Hyshot III

A new jet engine designed to fly at seven times the speed of sound appears to have been successfully tested.

The scramjet engine, the Hyshot III, was launched at Woomera, 500km north of Adelaide in Australia, on the back of a two stage Terrier-Orion rocket.

Once 314km up, the Hyshot III fell back to Earth, reaching speeds analysts hope will have topped Mach 7.6 (9,000km/h).

It is hoped the British-designed Hyshot III will pave the way for ultra fast, intercontinental air travel.

. . .

Rachel Owen, a researcher from UK defence firm QinetiQ, which designed the scramjet, said it looked like everything had gone according to plan.

BBC Science News - http://news.bbc.co.uk/2/hi/science/nature/4832254.stm

http://news.bbc.co.uk/2/hi/science/nature/4832254.stm#graphic

Scramjets do not work until they reach five times the speed of sound

See related thread - Mach 10 - https://www.physicsforums.com/showthread.php?t=52355

Other links -

Hypersonics at University of Queensland - http://www.uq.edu.au/hypersonics/

QinetiQ - http://www.qinetiq.com/

Australia's Defence Scientific and Technology Organisation - http://www.dsto.defence.gov.au/