When it was first flown in 1969, the Concorde supersonic jet was arguably the apex of aviation. With a maximum airspeed that was twice the speed of sound, the Concorde could travel up to 1,370 mph and carry passengers from London to New York in less than 3.5 hours. Comparatively, with our current sub-supersonic passenger planes it takes more than twice that long to make the trip at a cruising speed of around 550 mph. Which is why ever since the Concorde was retired in 2003, aviation designers have been drawing up new models and next-generation aerospace concepts in the hopes that at least one enterprise will see the value in resurrecting the supersonic passenger plane.

But, as we learned with the Concorde, the costs associated with developing and operating supersonic aircrafts is astronomical. And, once in service, the jets require exceptionally long runways to take off and are restricted to certain flight paths due the loud sonic booms they make when they break the sound barrier. Add to that the enormous fuel costs and increased maintenance needed due to the extreme conditions they’re exposed to, and it becomes a little more clear why it’s taken so long for all of the aeronautical notions listed here to get off the ground.

11. Tupolev Tu-444

Though their early design was never as successful as the Concorde, Russian aerospace and defense firm Tupolev actually pioneered supersonic passenger transport back in the 1960’s with their Tu-444 supersonic airliner. Unfortunately, very little information has been disclosed about the current state of their supersonic business jet and the official details are no longer available. This has caused widespread speculation that the project might have been completely scrapped.

10. Spike Aerospace S-512

In 2013, tech startup Spike Aerospace unveiled their ambitious plans to build a supersonic business jet that will fly from London to New York in less than three hours. Though very little is known about the S-512 other than its estimated cruising speed of Mach 1.6 and its carrying capacity of 18 passengers, the latest design specs show it replacing conventional wings with a modified delta wing shape.

The S-512 is still in the very early stages of development but already it has attracted the interest of experienced engineers from NASA, Boeing, Airbus, and Gulfstream Aerospace.

9. Boeing 2707

The Boeing 2707 was intended to be America’s answer to the Concorde. In the early 1960’s, it was thought that the Concorde was too far ahead in development to bother building a direct competitor, so the U.S. decided to comission the development of a much larger, faster, and more advanced aircraft.

The Boeing 2707 was intended to be 60 feet longer than a 747, carry up to 250 passengers (double the Concorde’s capacity), reach speeds of up to Mach 3, and have a trans-Atlantic range of approximately 4,000 miles. But, as it would turn out, all of these improvements dramatically increased production costs while only providing a marginal benefit to consumers over the existing Concorde. In addition, at the time there was growing public concern surrounding the noise pollution and potential damage to the ozone layer that the 2707 would cause. Consequently, after spending millions of dollars in research that yielded three prototypes, the U.S. government cancelled the program in 1971.

8. SAI Quiet Supersonic Transport

Utilizing a curved gull-wing design, inverted V-tail, and a curvilinear fuselage, the SAI Quiet Supersonic Transport is shooting to be the most silent supersonic aircraft ever created. Unfortunately, a spat of patent disputes with Lockheed Skunkworks led to progress on the QSST stalling in 2010. Since 2013, SAI has scaled up its initial plans and now envisions an aircraft the size of a Boeing 737 that would carry up to 30 passengers in an all-first-class cabin. However, it’s still uncertain whether or not SAI will find the investors necessary to carry on the endeavor.

7. Sukhoi-Gulfstream S-21

In the late 1980’s, Gulfstream Aerospace partnered with Russian aircraft manufacturer Sukhoi Design Bureau to develop a small supersonic business jet. Codenamed the S-21, the aircraft was supposed to be capable of sustaining a cruising speed somewhere between Mach 2 and Mach 3. The design team also conducted a lot of research into finding ways to muffle the deafening noise that accompanies supersonic flight. However, because the market for a supersonic business jet was beginning to look questionable in the early 90’s, the project experienced mounting delays until Gulfstream eventually dissolved their partnership with Sukhoi which led to the cancelled development of the S-21.

6. Next Generation Supersonic Transport

Currently in development by the Japanese Space Agency JAXA, the Next Generation Supersonic Transport is designed to carry three times as many passengers as the Concorde at roughly the same speed, but with twice the range. For Jaxa the ultimate goal is to deliver supersonic travel to the masses and offer affordable ticket prices that compete with standard subsonic air fares.

In 2005, an 11.5-meter scale model of the plane was successfully flown at speeds up to Mach 2. Although Jaxa had originally intended to have a full scale model up and running by the end of 2015, that stage of development has yet to be realized.

5. Reaction Engines LAPCAT A2

As part of a European Union program aimed at developing advanced propulsion systems, British aerospace firm Reaction Engines has created a unique design study that envisions an environmentally friendly, long-range, high-capacity commercial transport that can reach speeds in excess of Mach 5.

The engine concept for the LAPCAT A2 involves the use of a liquid hydrogen fuel that would make it nearly twice as efficient as a typical turbojet engine that’s fueled by kerosene. The hydrogen would also have the added benefit of pre-cooling the highly compressed air entering the engine. And because the cooler air will burn more effectively, it will allow the engine to be constructed out of lighter materials that wouldn’t normally tolerate the extreme heat associated with conventional jet fuel.

While proposals for the LAPCAT 2 are reported to be many and varied, developing all the technology necessary to realize the design is still a long way off.

4. HyperMach SonicStar

The HyperMach SonicStar was a concept first proposed by Sonicblue Aerospace CEO Richard Luggs. Making use of theoretical Jet electric turbine hybrid supersonic engines, the SonicStar is expected to travel at speeds approaching Mach 4.

With common jet engine, a bypass fan at the front is connected to a free-spinning turbine that is spun by hot exhaust gasses exiting the rear. But the SonicStar’s electric hybrid design would use electricity generated from burning fuel to rotate the bypass fan. The fan speed could then be adjusted for optimal speed and maximum efficiency.

To reduce the high temperatures caused by friction between the air and the skin of the aircraft, plans also include the use of carbon composite structural skins and panels with alloy leading edges wrapped around titanium structure wings to reduce weight. And to eliminate the noisy sonic boom, the designers want to implement a new form of futuristic electromagnetic drag reduction technology.

If all goes according to plan, first test flights for the SonicStar are expected to begin in 2021.

3. Aerion AS2

After developing their concept for a high-speed business jet on paper for more than 13 years, the billionaire-backed Aerion Corporation recently got a whole lot closer to making supersonic passenger travel a reality again. In late 2015, the company received its first order for 20 of their soon to be rolled out $120 million AS2 jets.

With the AS2, Aerion seems to have come up with a clever solution for companies and individuals that want to take advantage of supersonic intercontinental flight as well fast subsonic intra-continental travel. The AS2 is designed to cruise efficiently at speeds just below the sound barrier while over land and then increase speeds to roughly Mach 1.5 when crossing oceans. At supersonic speeds, the As2 will be capable of carrying between eight and 12 passengers up to 4,750 nautical miles — shaing three hours off a transatlantic flight and more off longer trans-Pacific routes.

Flexjet, the company that placed the first order for the aircraft, anticipates there will be a large demand among its clientele for single-day intercontinental round-trip travel.

2. Zero Emissions Hypersonic Transport

At the 2011 Paris Air Show, Airbus’s parent company EADS unveiled the Zero Emissions Hypersonic Transport (Zehst) concept plane. The Zehst was meant to be the hypersonic heir to the Concorde but much more advanced. According to the specifications, that aircraft would be constructed from new smart materials that would enable it to fly at altitudes above 100,000 feet and reach a maximum speed of Mach 4. However, in order to achieve those speeds the designers came up with an incredibly complicated three-tier propulsion system. The power required for takeoff would be generated by a standard turbo jet engine that would allow the craft to take off from a normal sized runway and then accelerate it to just below the speed of sound. Rocket boosters would then take over, propelling the aircraft in excess of Mach 2, after which supersonic ramjets (scramjets) would continue the acceleration to Mach 4. At that speed the Zehst could take 100 passengers from London to Tokyo in just two hours.

Furthermore, to tackle the problem of high fuel costs, the plane is said to run on a seaweed-based biofuel, that is comprised of only oxygen and hydrogen, leaving water vapour as the only emission.

It almost sounds too good to be true but a few years ago Airbus actually filed patents that seem to confirm its plans to develop the aircraft. Although you probably shouldn’t hold your breath. Even if the project sees completion it’s estimated that it will be roughly a 40 year endeavor.

1. Skreemr

Similar to Airbus’ design, the Skreemr would make use of a multiple propulsion systems to reach peak altitudes and top speeds. But unlike the Zhest, the Skreemr forgoes the use of conventional turbine jet engines to produce the initial launching thrust. Instead of the plane using its own power for launch, designer Charles Bombardier wants to use a magnetic railgun to fire the jet and achieve the speeds necessary for takeoff.

Following the railgun launch, the Skreemr would ignite liquid-fueled rocket motors that help it gain the speed and altitude required for the scramjet engines to kick in. Then, making use of the highly compressed air being gathered from the aircraft’s forward momentum and combining it with combustable hydrogen gas, it’s believed that the Skreemr should theoretically be able to hit speeds of Mach 10 — which would essentially make it the fastest manned plane ever built.

Of course, there are still massive technological barriers to be cleared before even a prototype can be made. It will be difficult to make a material that can withstand the intense heat produced by the aircraft’s acceleration, and even more difficult to produce it affordably. Another hurdle involves finding a way to make the rail gun launch work without every passenger on board passing out from extreme g-forces exerted on the body. But regardless of those challenges, Bombardier is confident he can make the Skreemr a reality.