Air Force Builds Quiet Mach 6 Wind Tunnel

An anonymous reader writes "To help design 'scramjets' -- vehicles that'll travel thousands of miles per hour as they leave the atmosphere and zip around the globe -- the U.S. Air Force has just funded a wind tunnel that operates quietly at Mach 6. To get a quiet flow, the throat of the Mach 6 nozzle must be polished to a near-perfect mirror finish, eliminating roughness that would trip the flow."

Re:whee!

[Anonymous Coward]

Wikipedia on Scramjets [wikipedia.org]. AC to avoid karma whoring..

ooo..

[User 956]

neat scramjet pictures here [bbc.co.uk].

Re:Quiet windtunnel needed?

[User 956]

I do not want to live near the (military) airport where that thing will take off.

The scramjet engine only starts to work at speeds above Mach 5. Average takeoff speed for a regular plane is about 150mph.

Re:I loved the part where...

[green1]

proper lockout/tagout procedures would involve the person doing the work personally putting a padlock on the circuit breaker (in the off position), one to which the only key is in the posession of the person working inside the device, along with a tag stating who he is, what he's working on, and when he expects to be done, after which he would personally test that the equipment is not capable of powering on before climbing inside.

Removing a fuse is no more effective then turning off the switch if some idiot comes along and puts it back together (the same idiot who first tried the switch and found it didn't work) always LOCK it out.

ok... so there's always some moron with bolt-cutters... but I'd love to see him claim THAT was an accident when he goes to trial...

No, it sucks.

[Short Circuit]

This tunnel works in a fashion opposite most wind tunnels. Instead of pressurizing one end, they create a vacuum at the other. That means they only get a run time of 8 seconds, but they use computers to get all the data they need in that short of a time frame.

So, yeah, it really does suck.

Re:18 inches

[bluelip]

No matter how accurate the model resembles the larger craft the data collected won't be 100% accurate for many reasons. One of the main concerns is Reynold's Number [wikipedia.org].

This number, basically, relates the size of air molecules to the size of the object. The size of the air molecules are the same in the airtunnel as in the atmosphere. The model, oviously, differs in size from the actual craft.

Re:Cultural Reference

[afidel]

Here [advertisementave.com] is an example of a line of commercials that Holiday Inn Express did. The best one is a guy who saves a nuclear power plant from a three mile island style nuclear disaster.

Re:ScramJet takeoffs

[MichaelSmith]

(2) give it two types of engines?

Yep.

Current test models use standard rocket boosters to get speed and altitude.

Re:"Quiet"?

[joeljkp]

"And it runs only for 8 seconds."

This is typical for high-speed wind tunnels. The runs are captured on high-speed cameras, then examined frame-by-frame or in slow motion to pick out the details of what actually happened. Supersonic flow in a nozzle develops very quickly, and there's no real benefit to running it for long periods of time.

Re:Some further comments

[Forbman]

Yep. For those that don't know, there hopefully are archives of teh Skunkworks-L mail digest on the net, where several people who were associated with the SR-71 programs (USAF and NASA) have some great stories about this incredible aircraft, and how bad engine unstarts were when zipping along at Mach 3, with the typical reason being that the shock wave entered into the engine inlet faster than the inlet spike system could respond to it. IIRC, more than one SR71 was lost operationally because the restart didn't go well or the plane broke because of the violence of the yaw caused by the unstart.

Re:Some further comments

[Moofie]

If you pitch the aircraft at hypersonic speeds, you will disrupt the shock wave system that is compressing the air going into your engine. You will probably create a normal shock wave in the throat of the engine, and if that happens, everybody aboard will die. Maneuvering just does not happen at those sorts of speeds.

Mechanical control systems on high-performance aircraft are a thing of the past. The system would CERTAINLY be fly-by-wire, and the pilot would be rendered pretty much incapable of direct control of the airplane at speed. Yes, the pilot could absolutely command the airplane to change course, but that would happen mediated by the computer, which will have full and exclusive authority to change the airplane's attitude.

NASA Ames

[HumanCarbonUnit]

As I recall, there are already some very high power and large wind tunnels at the NASA Ames research center in Mountain View California. http://windtunnels.arc.nasa.gov/ [nasa.gov]. For those of you that live in Silicon Valley, I'm sure you are all familiar with the gigantic wind tunnel that is large enouph to handle a complete mid-sized airliner.

Re:ScramJet takeoffs

[ddopson]

(crap, no formatting... reposting) Yes, there is one of the major challenges of both RAMjets and SCRAMjets. There is actually a whole range of technologies designed for different speed and air density regimes

Turbo-Props (propeller driven by jet like turbine power) is good up to a few hundred mph. Then the tips of the prop start going supersonic and cavitating. Highest efficiency

Turbo-Fan (same turbo jet power like a turbo-prop, but with an enclosed fan rather than a prop. Most of thrust still comes from the air driven by the fan. think 747) is capable of working in a faster regime up to somewhere near sonic speed (~780mph). Used for most commercial aviation because it is still fairly efficient, but faster than turboprop.

Turbo-Jet (same turbo jet power as turbo-prop, but little or no "bypass" air. The main purpose of the intake fan is now to pressurize air at intake for combustion with jet fuel. Thrust comes from) can provide substantial power at high velocites. TurboJets are the big muscular loud as hell engines used on fighter planes. They are several times less efficient than the TurboFans used in commercial airliners, but they produce many times more thrust and can run well in super-sonic regimes. More power + less efficiency = burns lots of fuel. Fighters can chew through thousands of gallons of jet fuel each hour just cruising. Temperature (melting point of metal) is a huge limit to the perf of these engines. Afterburners burn even more fuel in a way that isn't as temperature constrained, but is even less efficient. Modern fighters can burn through their fuel in something like a half hour of combat.

RAMjets work by using a constriction at the intake (rather than a fan) to pressurize the air. RAMjets don't work at low speeds, and are better designed to operate at a single design point. They typically run in the low mach numbers, although to operate, the intake air must be slowed to subsonic speeds. They are fast and efficient, but not very flexible. Typically used on missiles (due to their tendancy to operate in a single regime).

SCRAMjets are the same basic idea as a RAMjet, but the intake air remains supersonic.

Rockets do not burn any atmospheric O2 at all. For this reason, they operate equally well (or poor) at all speeds and air densities, providing a consistent predictable thrust. Due to the need to carry O2 around (which is far heavier than most of the fuels), their efficiency is appalling when compared to air-breathing engines. The uber-efficient space shuttle engines (2H2 + O2 -> 2H2O) have a specific impulse of ~440s. Solid rockets are more like ~200s. Kerosene rockets are in between. Air-breathing engines are in the thousands.