How We Might Have Scramjets Sooner than Expected

loralai writes "Recent breakthroughs in scramjet engines could mean two-hour flights from New York to Tokyo. This technology, decades in the making, could redefine our understanding of air travel and military encounters. 'To put things in context, the world's fastest jet, the Air Force's SR-71 Blackbird spy plane, set a speed record of Mach 3.3 in 1990 when it flew from Los Angeles to Washington, D.C., in just over an hour. That's about the limit for jet engines; the fastest fighter planes barely crack Mach 1.6. Scramjets, on the other hand, can theoretically fly as fast as Mach 15--nearly 10,000 mph.'"

SR-71 Blackbird

[wilder_card]

"set a speed record of Mach 3.3 in 1990 when it flew from Los Angeles to Washington, D.C., in just over an hour."

I feel compelled to point out that's the unclassified speed record. Its actual top speed is still speculative.

Re:10000mph!

[Deadstick]

Scramjets have the potential to do their high-speed cruise at 100,000 feet. Until we get birds that can go that high, don't be too worried.

rj

Re:SR-71 Blackbird

[Hamilton Lovecraft]

Hybrid turbojet-ramjet, according to wikipedia [wikipedia.org]:

The J58 was unique in that it was a hybrid jet engine. It could operate as a regular turbojet at low speeds, but at high speeds it became a ramjet. The engine can be thought of as a turbojet engine inside a ramjet engine. At lower speeds, the turbojet provided most of the compression and most of the energy from fuel combustion. At higher speeds, the turbojet throttled back and just sat in the middle of the engine as air bypassed around it, having been compressed by the shock cones and only burning fuel in the afterburner.

Re:Wrong, wrong, wrong

[jdhutchins]

1) Mach 3.3 speed record by SR-71 -> official speed record. NASA's X-15 set an unofficial one of Mach 6.7.
2) So.. 3.3 is NOWHERE NEAR the limit for jet engines.
Neither the SR-71 or the X-15 have conventional jet engines- the X-15 had a rocket and the SR-71 has ramjets

Re:10000mph!

[Cecil]

You pretty much need forcefields to protect you from air particles at that speed. The SR-71 expanded so much during flight due to frictional heating that even the fuel tanks needed to be built with expansion joints (so the fuel would leak out until it reached operating temperature at altitude). The fuselage would be about 300 degrees Celsius by the time it landed. Getting out of the plane was apparently a bit of a challenge.

Re:Wrong, wrong, wrong

[LabRat]

I'll agree for the most part..though I'll respectfully point out that the X-15 was rocket powered, not jet-powered ;)

Everything else is spot-on for the most part...even the venerable F-15 has a "public" top-speed of Mach 2.5 :) Although getting upwards of Mach 4 is a practical limit for turbojets due to the drag issues of slowing down the stream to subsonic via a "tuned" shockwave ala the SR-71 "cones". That's where SCRAMjets come in...they can sustain combustion with a supersonic stream flowing through the engine from inlet to outlet, thus they don't have the same "upper" limit.

Re:Wrong, wrong, wrong

[Anonymous Coward]

Fact-checking is YOUR friend, too, buddy. The X-15 [wikipedia.org] was rocket powered, not jet powered.

Fast fighters barely crack Mach 1.6? Since when?

[DragonWriter]

the fastest fighter planes barely crack Mach 1.6.

Huh?

MiG 29 [fas.org] - Mach 2.3
F-14 [fas.org] - Mach 2.5+
Kfir [fas.org] - Mach 2.3
JAS 39 Gripen [fas.org] - Mach 2.0

Re:SR-71 Blackbird

[Gospodin]

Well, let's think about that for a second. Mach 6 at high altitude is (roughly) 2,000 mph. Orbital velocity at LEO is around 17,500 mph. It's really hard to get into orbit.

Re:SR-71 Blackbird

[Dr_Banzai]

You need about 7 km/s to reach orbit, which is above Mach 25. You could have rockets which would kick in at the maximum altitude of the scramjet to give the final push to orbit.

Re:Mach 1.6 is speed without afterburners

[vought]

The MiG based of the F-15 with larger engines can reach Mach 3

What are you talking about? There's a MiG based on the F-15?

If you're talking about the MiG-25 Foxbat, it was flying well ahead of the F-15 (which itself was a response to the development of the MiG-25), and was designed to intercept bombers like the XB-70, which were never made operational.

The nose melts ...

[AHumbleOpinion]

Yes, infact I knew someone who use to fly those things and they weren't allowed to fully throttle up. He also said that during normal missions the plane would damage itself when going the faster speeds. Now of course this is all at someones word, so I have no written proof.

I heard the same thing from an SR-71 pilot, the damage was melting the nose and other leading edges. So advances in materials, not necessarily thrust, would presumably allow for greater speeds.

mig-31, mig-25 reach Mach 3+

[sdssds]

http://en.wikipedia.org/wiki/MiG-31 [wikipedia.org]
these are interceptors though, not fighters.

Re:SR-71 Blackbird

[Zebra_X]

I though the same thing for years. However it appears that the POH for the Blackbird has become public record. This manual basically describes how to fly the plane. The manual is now online @ http://www.sr-71.org/blackbird/ [sr-71.org]

The manual clearly shows that the planes design speed is mach 3.2 - exceeding this speed requires authorization from command.
The thing that not everyone realizes is that unlike other planes that can go mach 2 or 3, they cannot sustain this speed due to excessive heating and or fuel consumption constraints. The blackbird is different in that it is designed to fly for ~ 3 hours at these speeds. In fact there are several guages dedicated to external heating for the plane. http://www.sr-71.org/blackbird/manual/5/5-9.php [sr-71.org]

So with all that said, the flat out top speed may be higher, but the operating manual usually wins out.

The summary for the article is mostly incorrect regarding the blackbird. The engine design of the blackbird is a hybrid design. The engine is a turbojet but there is a ramjet bypass for higher speeds. Ramjets are also known to work at speeds of up to Mach 5+. Though the scramjet engine is not much different it's just that the characteristics of the shockwaves change so much that the shape of the engine needs to change to achieve the same effect. So the limitation is not its engines, it mostly has to do with heating of the aircraft surfaces. Of the many topics discussed in the manual for the blackbird, external and internal heating was a major area of attention.

So if the Blackbird has issues with heating - you can bet that any other plane operating at that speed or higher will have the same problem. Unfortunately it is difficult to find a place to dump the excess heat. Any surface that comes into contact with the airstream causes friction, and heat buildup. You can use the fuel as a coolant, and the blackbird did. The JP-7 fuel that the blackbird used had an extremely high flashpoint. So it could be used to absorb some of the internal heat before being burned off. The blackbird is also much more like today's aircraft in construction - it was one of the first aircraft to use titanium alloys extensively in its construction.

The bottom line is that you don't just build a scram jet powered plane. It's not just about the engine, but about the entire plane. The challenges run the entire range from thermal to mechanical. To simply throw out a number like mach 15 and think that it's feasible to obtain any lasting operation at that speed using today's technology shows a distinct lack of understanding of the subject matter.

one way ticket straight down.

[meglon]

X-15 Hypersonic Research Program (from http://www.nasa.gov/centers/dryden/news/FactSheets/FS-052-DFRC.html [nasa.gov])

In the joint X-15 hypersonic research program that NASA conducted with the Air Force, the Navy, and North American Aviation, Inc., the aircraft flew over a period of nearly 10 years and set the world's unofficial speed and altitude records of 4,520 mph (Mach 6.7--on Oct. 3, 1967, with Air Force pilot Pete Knight at the controls) and 354,200 feet (on Aug. 22, 1963, with NASA pilot Joseph Walker in the cockpit) in a program to investigate all aspects of piloted hypersonic flight.

Early flights of the aircraft initially flew with two XLR-11 engines, producing a thrust of 16,380 lb. Once the XLR-99 was installed, the thrust became 57,000 lb.

Re:2 seconds of research reveals...

[imsabbel]

ACtually, the concord didnt need the afterburner to reach supercruise.
But it happened that using is was far more efficient (with out afterburner, it spend quite a few minutes in the transition region of the speed of sound which used up way more fuel than a quick boost past the barrier)

Re:Running out of oil a myth ...

[DragonWriter]

We are not going to run out of oil.

No one said we were. So what?

The price of oil will increase and make alternatives feasible.

The increase in the price of oil may contribute to making alternatives feasible, but what that really means is that the number of hours of human labor that need to be exchanged for energy in any form will increase, which increases the cost of, pretty much, everything compared to labor.

The rate of consumption will also peak, it just lags production.

No, it will be in lockstep with production; there aren't substantial stockpiles to draw down, and there isn't substantial use of stockpiled fuel, so consumption is pretty tightly chained to production.

Oil prices skyrocket as demand out paces supply, we switch to alternatives, oil prices crash as supply now out paces demand.

Unlikely. The only reason demand (not consumption which is "quantity demand", a different thing from the demand curve) changes lag behind supply (not production, which is "quantity supplied") changes is that there are transition costs and barriers on the demand side. And that's what drives the price increases. Even as those are overcome, its more likely that demand approximately catches up to supply, dropping prices back from their peak to something like the prior levels with ongoing gradual increase than that things switch over and demand radically plummets.

Someone who always flew Concorde

[MichaelCrawford]

Many of you have raised the reasonable objection that a scramjet wouldn't be economical. But it might be economical for certain people: the very rich.

The mother of a friend of mine was a top executive at Dow Chemical, at the time the company's highest-paid woman. She always flew Concorde when she could because the company was paying her salary during her flight.

Being able to get across the ocean with time left in the work day meant that Dow actually saved money paying for a Concorde ticket.

Re:SR-71 Blackbird

[sconeu]

No. Orbital velocity is roughly Mach 25 (as calculated at sea level) -- 18000 mph. Escape velocity is about 40% more than that (25000 mph or so), or roughly Mach 33.

Re:SR-71 Blackbird

[iminplaya]

Why would you want to build a vehicle with 40+ year old technology?

Oh, I don't know... Because it works? Everything presently flying is 50 year old technology. Even the shuttle is just a complex bottle rocket. And worst of all we still have to burn kerosene. Even the scram jet will burn it in some form. Our knowledge of propulsion and natural forces is extremely limited and progress is very slow. It that department, very little has changed over 100 years. That problem is more due to politics than anything else. Moving fast is nice, But personally I'm more interested in finding alternatives in the area of power plants for the vehicle where progress has been next to nil. That sure isn't coming "sooner than expected". I'd call it way overdue.

Re:SR-71 Blackbird

[kryten_nl]

Much like the bow shock of a boat, a "sonic boom"'s intensity is determined by the speed of the aircraft. GP's statement is flawed, it should read:

A sonic boom is produced when pressure waves compound near Mach 1.

It should be noted that all sound waves are pressure waves (with infinitesimal pressure increase), but not all pressure waves are sound waves. If you want to learn more about the subject, stay away from Wikipedia and read a good book on the subject (anything from John D. Anderson jr. would be good).

Re:SR-71 Blackbird

[Carnildo]

I admittedly don't know what I'm talking about, but I believe that a quieter engine would produce a smaller sonic boom- thus, if the scramjet is quieter, it may have a smaller boom.

A sonic boom is the shockwave generated by an object moving faster than the speed of sound. It doesn't matter if it's a rocket, a scramjet, a ramjet, or something completely unpowered like a machine-gun bullet: the size of the shockwave depends mainly on the size of the object and how fast it's travelling.

Re:SR-71 Blackbird

[Atlantis-Rising]

There are theoretical designs such as Busemann's Biplane that don't appear to create any sonic booms at all, and DARPA was able to reduce the sonic signature of an F-5 by almost a third at one point. [wikipedia.org]

It's possible to eliminate the sonic boom, with a correct airframe shape; apparently people have made working models of the Busemann's Biplane in tests, but the shape itself generates no lift, slightly problematically.

Re:The nose melts ...

[Anonymous Coward]

The thrust problem has been solved long ago. The MiG-25 can exceed Mach 3 using pure turbojets. It can't do it for any length of time due to mechanical stress on the engine intake and thermal stress on the airframe.
Here is a video of the heat damage to the X-15A2 after its Mach 6.7 sprint. [youtube.com]
Note that this aircraft had an ablative coating that gradually burned away during flight.

Re:SR-71 Blackbird

[Gospodin]

If you can get up to 100,000 feet and Mach 6 on scramjet power, wouldn't that dramatically reduce the rocket's fuel requirements for going the rest of the way?

Sure, it helps. But all you're really saving is the weight of oxidizer for the scramjet portion of flight. This weight savings has to be balanced against the need to carry two types of engines (or one type of hybrid engine, if you can design it). This is not to mention the added complexity. I'm not saying this can't be solved, but it isn't trivial. Just making scramjets won't get us there.

Re:SR-71 Blackbird

[Sique]

Mig 31 hits almost the same speed as the Blackbird.

The version of MiG 25, that hit Mach 3.0 even was an early prototype, the E-266. Production MiG 25 or MiG 31 aren't that fast. And this was back in 1967, so really 40 years ago. The MiG 31 is a redesign of the MiG 25, starting from the E-155MP prototype, an improved version of the E-155 prototype of the MiG 25 from 1964. Yeah, we are talking about very old tech here.

Re:SR-71 Blackbird

[SL Baur]

One of the things that was always controversial about Concorde was the sonic noise. I don't see how they intend to address this problem with their new scramjet.

You have it right here, stop.

Not only that, Concorde went out of business.

Because they had so much NIMBY they couldn't get the best flights across the Pacific - no West coast airport allowed them to fly. Give me a choice between 15+ hours flight time to Manila from the West coast of the United States and a couple of hours or so and I'll pay a whole lot more for the shorter flight.

Concorde failed in part because of US West coast NIMBY. LA/SF to Tokyo/Taipei/Singapore/Manila/Hong Kong could have been most profitable, except that LA & SF didn't allow them to land there.

Re:SR-71 Blackbird

[nmg196]

> Concorde would be flying to this day except for one thing: 9/11.

?! Not sure what you're talking about - there were no Concordes involved on 9/11 at all.

The last commercial concorde flight was on 23 October 2003 (source [wikipedia.org]). Therefore it was flying more than two years AFTER 9/11/2001.

Concorde was actually grounded due to a massive crash and nothing to do with 9/11.

Re:SR-71 Blackbird

[wulfhound]

Concorde didn't have the range to go cross the Pacific - to even go trans-Atlantic, it had to be given landing priority at the airports it serviced. The major trans-Pacific routes are a good 50% longer than trans-Atlantic.

Re:Scramjets are air breathers

[icebrain]

I believe the SST fleet only does about 8000mph on re-entry

Eh... you mean STS fleet, and try around 17,000 mph at entry interface.

Aerodynamic heating at super/hypersonic speeds is not due to friction (at least as most people think of it), but rather compressibility effects. Air gets hotter as you slow it down(highly simplified explanation--kinetic energy turns into thermal); the change is dramatic across a shockwave.

I don't think the materials are sufficiently developed to allow a non-ablative shield at Mach 12, say; but I think lower speeds around Mach 6 should be possible in a few years. And around those speeds, you don't necessarily need scramjets; a standard ramjet would work fine, assuming your engine can take the static pressure and temperature inside it (my memory from a design project back in school seems to tell me that Mach 6 gives you a pressure ratio of about 50:1, and temperatures approaching modern limits).

Re:SR-71 Blackbird

[AndersOSU]

The problem with scramjets is that they have a minimum operating speed in excess of current standard jet engine speed. Currently the way they test scramjets is to launch them on a rocket to get up to speed.

Any practicle incarnation will have to be multi-stage as it is, likely turbofan/ramjet/scramjet. I suppose it is within the realm of possibility to add a fourth rocket stage, but you are hauling a lot of engine parts that don't do anything for the entire flight at that point.

Re:SR-71 Blackbird

[Guysmiley777]

Actually the SR-71 and the YF-12 flight manuals have been declassified and are available online.

For the SR-71A, "Mach 3.2 is the maximum design Mach number. Mach 3.17 is the maximum scheduled cruise speed recommended for normal operations. However, when authorized by the Commander, speeds of up to Mach 3.3 may be flown if the limit CIT of 427 degrees C is not exceeded." (CIT is the compressor inlet temperature and was a critical limiting factor in maximum speed)

From Mach 2.6 to Mach 3.2 the SR-71 is limited to -0.1 to +1.5g maneuvering.

Source: http://www.sr-71.org/blackbird/manual/5/5-8.php [sr-71.org]

Which is an amazing read, tons of details.