First Ever Scramjet Reaches Mach 10

stjobe writes with the news that a group of US and Australian scientists successfully tested a supersonic scramjet engine in the Australian Outback on Friday. The Sydney Morning Herald reports that a rocket carrying the engine reached mach 10, and climbed to an altitude of 330 miles before the apparatus re-entered the Earth's atmosphere. "Australia's Defense Science and Technology Organization (DSTO) said it was believed to be the first time a scramjet had been ignited within the Earth's atmosphere ... Scramjets are supersonic combustion engines that use oxygen from the atmosphere for fuel, making them lighter and faster than fuel carrying rockets. Scientists hope that one day a scramjet aircraft fired into space could cut traveling time from Sydney to London to as little as two hours."

Suborbital trajectories?

[caseih]

This is very interesting to read as I just finished reading Ben Rich's book "Skunk Works" where he talks about the SR71. When president Reagan announced the administration's intention to build a hypersonic airplane, he just shook his head. It's simply not practical, with or without the scramjet engine. The SR71 flew at 85,000 feet at about Mach 3.2, and reaches skin temperatures of 2000-3000 degrees (F I presume) just from moving through the atmosphere. Accelerating to Mach 10 would burn up or otherwise compromise any current building material, except for the carbon-carbon and ceramic materials used on the space shuttle's heat shield, but aren't practical for airplanes. So what good is this scramjet, at least as far as a hypersonic airplane goes? Seems to me all this talk of Sydney to London in 12 hours is a bit fanciful. So the question is, how exactly will this engine be used to accomplish this? The only way to reach hypersonic speeds without burning up is to make the trajectory sub-orbital so that the aircraft is in the thinnest atmosphere possible when it's firing it's engines to go Mach 10. But of course there's not a lot of oxygen at that altitude. And to really achieve sub-orbital trajectory you need a rocket engine, not any kind of air-breathing engine. So my questions are: Is Ben Rich right that hypersonic travel is essentially impossible? Will the scramjet help with a suborbital trajectory? I understand that igniting the scramjet is a breakthrough. Jet turbines at supersonic velocity have always been problematic.

Off-topic, Ben Rich says in his book that the codename Aurora that everyone likes to think refers to some hypersonic aircraft, was actually the codename placed on the B-2 project as Lockheed and Northrop were competing for the contract. It's funny to think that to this day, folks still hang onto this and imagine some mythical hypersonic airplane. Which never existed. Or does it?

Re:Suborbital trajectories?

[richdun]

Er, it's the night for corrections - the speed of sound in a vacuum approaches a theoretical asymptote, not infinity. The speed of sound generally gets lower as the material loses density, higher as the material gains density (think about a wave traveling through a solid block, as opposed to one traveling through water, then one traveling through the air)

Re:Altitude of 330 miles???

[MikShapi]

You seem to be sharing the common misconception that LEO altitudes and above cannot be reached at low speeds.

Dude, you can reach an altitude of 330 miles just fine with a perfectly low speed. There's nothing unphysical about it that requires the invocation of holy cows. It is also true that with the lack of a *horizontal* velocity of about mach 30 (at ~100km, you'd need less if you get as far as 330 miles high), you fall back down (well, not back to the same place, you may have traveled halfway around the world by then, but still, back *down*) like a rock. This is what spaceship-1 did, this is what this experiment did. This is what ICBM's do. This is even what proposed "2-hour-sydney-to-london" flights will do. Speed is only needed to get into low orbit.
They go high, fast enough to stay in space for the duration of cruising their trajectory, without air resistance and at pretty dang fast speed, then they just drop back into the atmpsphere.

Nothing to get overly excited about. The concept was already proven to work, and we haven't reached a point where the technology is generating value yet, so it's all still technological limbo. Not that it wouldn't be nice if it actually got done after 20 years of R&D. Shorter times for less fuel would pro'lly mean many more flights in lifetime of aircraft, less fuel burned, less time-in-air per trip, more in-range accessible destination for carriers and while I haven't the slightest clue as to what operational costs on scramjet-based planes would look like, it would seem to have the potential to cheapen things from where they are today.

Re:Military, not Civilian applications

[moikka]

Also think about their primary selling-point,
capable of using oxygen from air and not having to carry it,
is only an advantage over rocket-engines.
Jet engines already use oxygen from the air.


In civilian travel there is great need for fuel-efficiency.
If their biggest problem is excess heat,
it automatically means they are wasting huge amounts of fuel to create that heat.
Only military can afford this wasted fuel.


Also there is a huge problem in take-off and landing from ground.
Ramjet is not going to work in those cases.
So for civilian aircraft use they are going to need conventional jet engines for that purpose.
Guess how aerodynamically efficient these extra jet engines are going to be at 10 Mach?
Also another problem that does not exist in military use.


So 100% certainty the only application this is going to have is delivering bombs.

Six seconds of flight

[Richard Kirk]

Scramjets look good on paper. The thin air coming in is compressed by a series of standing shock waves. Unfortunately, the geometry of these shock waves can easily be upset by small distortions in the engine, which in turn can lead to changes in the stresses with in the engine, which - to cut a long story short - can mean the engine spectacularly demolishes itself when faced with real bits of atmosphere with unpredictable air currents. I found the flight time in...

http://www.abc.net.au/science/slab/hyshot/default. htm [abc.net.au]

It may not sound like much, but six seconds is very respectable for a scramjet. Yay!

There is a lot of touting about how this would get you from London to Sydney in 40 minutes and stuff. I am not sure how true or economical this is, even if scramjets can be made safe. When you are flying fast, you can either take your oxidant with you (as rockets do) or you can scoop it up as you go along. Scooping it up as you go along means taking in air that was initially at rest and getting to move at the speed the engine is currently going. As only 20% of the air is actually the oxygen you want, this is not necessarily an effective thing to do. It becomes most effective when the oxidant (oxygen) is a lot heavier than the reductant (fuel - and hydrogen is particularly light), so scooping it up as you go takes a lot off the take-off weight.

The other London to Sydney option is to get just beyond the atmosphere using a conventional rocket, then going ballistic and weightless for the main distance, and re-entering and gliding, a lot like the space shuttle. While being weightless is fun, being weightless for 20 minutes makes most people puke, so a large passenger jet might skip the atmosphere and retain a little gravity. A scramjet might be used for this.

Nevertheless, yay!

Re:Suborbital trajectories?

[Anonymous Coward]

There's also the problem that a hypersonic aircraft would sustain heating for a lot longer than the shuttle does. Passive heat shield material probably isn't going to cut it. (Incidentally, there's been some interesting work done on titanium alloy heat shields...)

Almost any serious hypersonic proposal includes active cooling, using cryogenic fuel to cool the leading edge (which is the only part that's problematic; passive materials can handle the rest). It'd certainly make me nervous to ride in a vehicle that requires an active cooling system to work perfectly to avoid burning up, but it could solve the problem.

Re:Scramjets need an atmosphere

[dyslexicbunny]

lommer gave a pretty solid explanation of what is going on with thrust reversers.

Reversal depends on the engine type (turbojet, turbofan, etc..) and the manufacturer. I've seen some information that one manufacturer blocks the bypass nozzle and redirects the flow out to reverse thrust. Pretty much, the fan still operates as usual but the bypass air is used and not the core flow (through the burner). It sounds like you have some mild confusion as to engine classes/terminology so I'll provide some details that might help clear things up plus a couple decent links with flowpaths.

Turbojet describes a simpler turbomachine engine -> inlet, compressor, burner, turbine, nozzle http://www.aerospaceweb.org/question/propulsion/je t/turbojet.jpg [aerospaceweb.org]
Turbofan describes a more complex engine -> inlet, fan, splitter, compressor, burner, hp turbine, lp turbine, nozzle... splitter, bypass duct, bypass nozzle http://www.aerospaceweb.org/question/propulsion/je t/turbofan.jpg [aerospaceweb.org]

Turbofans are used nowadays on commercial aircraft because they have a significantly better fuel consumption. Engine thrust can be calculated using
Thrust = mass flow * (jet velocity - intake velocity)

You have two ways of increasing thrust: you can have a small mass flow and have a large Vjet (turbojet) or you can have a large mass flow and smaller Vjet (turbofan). The only way to produce a large Vjet is to burn more fuel. The term bypass ratio is used to compare the bypass air / core flow. The GE90 sports bypass ratio of 9. Bypass is difficult to deal with since increasing bypass ratio will reduce fuel burn BUT when in cruise, they have a ridiculous profile drag.

Ultimately, engine design is a complicated trade with multiple attributes at different mission segments matching vehicle thrust requirements, vehicle dimension needs, field conditions, maintainability, and noise and emissions regulations just make up a handful of the design concerns. So next time you fly, try getting a seat behind the engine and try to check things out. The design process is really quite amazing.

I'm going to stop here since I've written a lot. I'm just excited about having insightful things to say for once.