Supersonic flight / Supersonic / first supersonic flight

The term supersonic is used to define a speed that is over the speed of sound (Mach 1). In dry air at 20 °C (68 °F), the threshold value required for an object to be traveling at a supersonic speed is approximately 343 m/s, (1,125 ft/s, 768 mph or 1,236 km/h). Speeds greater than 5 times the speed of sound are often referred to as hypersonic. Speeds where only some parts of the air around an object (such as the ends of rotor blades) reach supersonic speeds are labeled transonic (typically somewhere between Mach 0.8 and Mach 1.2).

Sounds are travelling vibrations (pressure waves) in an elastic medium. In gases sound travels longitudinally at different speeds, mostly depending on the molecular mass and temperature of the gas; (pressure has little effect). Since air temperature and composition varies significantly with altitude, Mach numbers for aircraft can change without airspeed varying. In water at room temperature supersonic can be considered as any speed greater than 1,440 m/s (4,724 ft/s). In solids, sound waves can be longitudinal or transverse and have even higher velocities. Supersonic fracture is crack motion faster than the speed of sound in a brittle material.

Supersonic flight
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Supersonic aerodynamics are simpler than subsonic because the airsheets at different points along the plane often can't affect each other. Supersonic jets and rocket vehicles require several times greater thrust to push through the extra drag experienced within the transonic region (around Mach 0.85-1.2). At these speeds Aerospace engineers can gently guide air around the fuselage of the aircraft without producing new shock waves but any change in cross sectional area further down the vehicle leads to shock waves along the body. Designers use the Supersonic area rule and the Whitcomb area rule to minimize sudden changes in size.



It should be kept in mind, however, that the aerodynamic principles behind a supersonic aircraft are often more complex than described above because such an aircraft must be efficient and stable at supersonic, transonic and subsonic flight.



One problem with sustained supersonic flight is the generation of heat in flight. At high speeds aerodynamic heating can occur, so an aircraft must be designed to operate and function under very high temperatures. Duralumin, the traditional aircraft material, starts to lose strength and go into plastic deformation at relatively low temperatures, and is unsuitable for continuous use at speeds above Mach 2.2 to 2.4. Materials such as titanium and stainless steel allow operations at much higher temperatures. For example, the SR-71 Blackbird jet could fly continuously at Mach 3.1 while some parts were above 315°C (600°F).



Another area of concern for continued high-speed operation is the engines. Jet engines create thrust by increasing the temperature of the air they ingest, and as the aircraft speeds up, friction and compression heats this air before it reaches the engines. The maximum temperature of the exhaust is determined by the materials in the turbine at the rear of the engine, so as the aircraft speeds up the difference in intake and exhaust temperature the engine can extract decreases, and the thrust along with it. Air cooling the turbine area to allow operations at higher temperatures was a key solution, one that continued to improve through the 1950s and on to this day.


Intake design was also a major issue. Normal jet engines can only ingest subsonic air, so for supersonic operation the air has to be slowed down. Ramps or cones in the intake are used to create shock waves that slows the airflow before it reaches the engine. Doing so removes energy from the airflow, causing drag. The key to reducing this drag is to use multiple small oblique shock waves, but this was difficult because the angle they make inside the intake changes with Mach number. In order to efficiently operate across a range of speeds, the shock waves have to be "tuned."


An aircraft able to operate for extended periods at supersonic speeds has a potential range advantage over a similar design operating subsonically. Most of the drag an aircraft sees while speeding up to supersonic speeds occurs just below the speed of sound, due to an aerodynamic effect known as wave drag. An aircraft that can accelerate past this speed sees a significant drag decrease, and can fly supersonically with improved fuel economy. However, due to the way lift is generated supersonically, the lift-to-drag ratio of the aircraft as a whole drops, leading to lower range, offsetting or overturning this advantage.


The key to having low supersonic drag is to properly shape the overall aircraft to be long and skinny, as close as possible to a "perfect" shape, the von Karman ogive or Sears-Haack body. This has led to almost every supersonic cruising aircraft looking very similar to every other, with a very long and skinny fuselage and large delta wings, cf. SR-71, Concorde, etc. Although not ideal for passenger aircraft, this shaping is quite adaptable for bomber use.
 

Facebook: 500 Million Friends -- and 'a Few Enemies'

Facebook, the hugely popular and often controversial social networking service, is expected to announce this week that it has reached 500 million users. It's a momentous milestone for CEO Mark Zuckerberg's company, which just over six years ago was still a dorm-room project on a shoestring budget.

Judging by influence and reach, Facebook today is one of the most successful Internet startups in history, even if not the most profitable. But as the Silicon Valley mantra goes: First build an audience, then monetize it.

Thanks to a deft understanding of the "network effect," in which a service becomes more valuable as more people use it, Facebook has accomplished the first task. And with annual revenue nearing $1 billion, it's making progress on the second.

And the timing of Facebook's milestone couldn't be better for a forthcoming movie, The Social Network. That 500 millionth member will likely sign on just days after the release of the full-length trailer for the Aaron Sorkin-penned, David Fincher-directed film about the company's origins (see video below). The movie's tagline? "You don't get to 500 million friends without making a few enemies."

Time to "Celebrate Users"

Facebook has its own strategy for marking the occasion: It's launching a publicity campaign called "Facebook Stories," which is designed to highlight how the service has affected the lives of individuals, according to AllThingsD.

"As we anticipated our 500-million milestone, and we wanted to find a different way to announce and celebrate it," Facebook's Randi Zuckerberg (CEO Mark Zuckerberg's sister), told Kara Swisher. "In the past, it's been all about the numbers and milestones, and we realized we had never taken the opportunity to celebrate users."

"Facebook Stories" will be a collection of posts up to 420 words -- the status update word limit -- grouped around geography and themes like love, grief and natural disaster, Swisher reports. Facebook users can submit their stories here.

But remember, by doing so, you're granting Facebook "an irrevocable, perpetual, royalty-free, worldwide right and license (with right to sublicense) to use, distribute, reproduce, create derivate works of, perform and display Your Content, in whole or part, on or off the Facebook site for any purpose, commercial or otherwise without acknowledgment, consent or monetary or other compensation to you."

Facebook is no doubt keen for a dose of positive press after a rash of bad publicity earlier this year over its privacy policies. But the respite may be somewhat short-lived: Judging from The Social Network's theatrical trailer, the film doesn't portray company CEO Mark Zuckerberg in a particularly flattering light.

Obama Says Republicans Holding Unemployed Hostage to Politics

Obama Says Republicans Holding Unemployed Hostage to Politics

President Barack Obama is urging the U.S. Senate to act this week on extending benefits for 2.5 million unemployed Americans.  Mr. Obama says Americans without jobs are being held "hostage" by Senate Republicans who have blocked legislation to extend unemployment insurance that expired in June.

Three times in recent weeks, Republicans have used parliamentary tactics in the Senate to block the extension of benefits, angering the president at a time when he is trying to move this and other legislation through Congress before the congressional recess.

The White House has declared the unemployment bill an emergency measure, meaning it does not have to be offset by spending cuts in other areas of the federal budget.

Republicans have taken aim at that, as they continue to attack Mr. Obama on several fronts and challenge the White House over deficit spending and rising debt.

Addressing media in the White House Rose Garden, the president said Republicans who had for years supported policies that turned big surpluses into deficits had no place blocking aid for Americans desperately searching for work.

"It is time to stop holding workers laid off in this recession hostage to Washington politics," he said. "It is time to do what is right, not for the next election, but for the middle class."

Joining the president at the podium were three unemployed Americans - a former auto dealership parts manager, a former fitness center employee and a former real estate agent - whose government support has run out or is about to expire.

Mr. Obama said maneuvering by Senate Republicans to block extension of unemployment benefits demonstrates, what he called, a lack of faith in the American people.  He described as mis-guided the notion that emergency relief would discourage people from looking for jobs.

The president's appearance was a final opportunity to fire back at Republicans on the issue before an expected Tuesday vote in which majority Senate Democrats are expected to be able to break the Republican roadblock.

House of Representatives Republican leader John Boehner said Republicans support extending unemployment benefits by cutting spending elsewhere.  He accused Mr. Obama of offering "disingenuous attacks" rather than answers on job growth.

Later this week, Mr. Obama will be able to point to another legislative accomplishment when he signs sweeping financial-system regulatory reform legislation he has battled for since coming to office.

Financial reform, with the unemployment extension and another measure to provide small business tax benefits, are the three major legislative objectives the president and Democratic leaders agreed to focus on before the long summer recess.

The entire legislative schedule is even more compressed as lawmakers and the White House increasingly turn their focus to the November mid-term congressional elections.

President Obama will be hitting the road frequently to campaign for his party's candidates in a number of states important to Democrat's hope of retaining control of the House of Representatives and the U.S. Senate.