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.'"

Comments

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:SR-71 Blackbird

[Hamilton Lovecraft]

Hybrid turbojet-ramjet, according to wikipedia:

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: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/

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

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.

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.

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: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: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

Fast fighters barely crack Mach 1.6? Since when?

[DragonWriter]

the fastest fighter planes barely crack Mach 1.6.

Huh?

MiG 29 - Mach 2.3
F-14 - Mach 2.5+
Kfir - Mach 2.3
JAS 39 Gripen - Mach 2.0

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.

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.