Ever pause to wonder why there's such a huge technology gap between a standard jetliner airplane and a spacecraft? Everyone knows that airplanes can't fly into space. But if you were really put on the spot, could you actually explain why? With specifics? In this article, we'll take a look at the science behind why it's not possible, and look to the future, where it may actually be possible.
The dream of passenger space travel has been a longstanding one. Until recently, it was still purely the realm of science fiction. However, new technological innovation has started to blur the boundary between fact and fantasy. We are still quite a ways off from even preliminary test flights. But the tech is in the works, and many people are excited for what the future may hold. This burgeoning new industry is referred to as "space tourism," and it's being led by the companies Blue Origin, Virgin Galactic, SpaceX and the Russian Space Agency.
Until then, we are stuck with our measly 30,000 foot flights. We are bound to follow the rules of physics, which dictate that jet powered airplanes cannot, and probably will not ever, be able to make it to space. Here's why.
Maybe the most obvious limitation on airplanes that disqualifies them from space travel is that they need air in order to fly. "Air" is, after all, in the name.
An airplane generates lift when air passes over its wings, or "airfoils." During takeoff, an airplane builds up speed until the air passing over its wings generates lift that is stronger than the weight of the plane. At this point, the plane becomes airborne. The plane has to maintain an adequate speed, in relation to air density, to stay airborne. Outer space is, of course, completely devoid of air, rendering an airplane's wings useless.
A jetliner would never make it that far anyway. The atmosphere thins as altitude increases. At a certain point, even though the plane is technically surrounded by air, the air density is too low to generate adequate lift. Airplanes stay well clear of this limit. It is not possible for an airplane to surpass this boundary, which is well short of the boundary between space and the atmosphere.
Jet engines require oxygen to work, as well. Jet engines, unlike rockets and other forms of air propulsion, require enough oxygen to burn in order to produce thrust. Not only would the wings stop generating enough lift to keep the airplane in the air, the engines would also stop firing. A bad recipe.
The kind of plane you board when you take a normal flight, unless you're making a short jaunt in a propeller plane, are jetliners. Jetliners rely on jet engines to produce the thrust they need to stay in the air. While these jet engines are massively powerful when compared to your car, they're nowhere near as powerful as the rockets used to launch spacecraft.
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Jet engines, also called gas turbines, suck in air through their fronts with a fan. A compressor inside the engine then greatly increases the pressure of the air. The compressed air is sprayed with fuel and then the mixture is set alight by an electric spark. The ignited mixture expands violently and escapes through a narrowed aperture in the back of the engine called the nozzle, creating thrust. As the air passes to the nozzle, it spins another fan called the turbine, which, in turn, rotates the compressor.
The key ingredient is oxygen. Without an adequate supply of oxygen, a jet engine cannot work. Space travel is made possible by rocket propulsion, which works differently than a jet engine. We will discuss this later in the article. Jet engines have an altitude limitation, but it's definitely much higher than a human being's.
A human being has a standard maximum altitude of 26,000 feet. Beyond this, you cannot survive for very long due to oxygen density in the air being critically low. Mountain climbers call altitudes past 26,000 feet "The Death Zone." They're not just being morbid - high altitude is one of the most serious mortal threats a mountain climber faces.
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Take Mount Everest, for example. The world's tallest mountain has claimed the lives of nearly three hundred people who elected to attempt the climb. Its summit sits at 29,000 feet, well above the Death Zone boundary. Climbers have to carry supplemental oxygen tanks with them, and climb with face masks, if they hope to make the peak without dying.
The air at the top of Mt. Everest is about a third as dense as at sea level. That means you're only getting about 33% as much oxygen per breath as you would normally. If you stay in that environment for too long, you will suffer from a condition called Hypoxia. In Hypoxia, tissues in the body are deprived of oxygen. It's actually the phenomenon that's responsible for altitude sickness. In its extreme form, it can be very fatal indeed.
Most jetliners can fly at a maximum altitude of about 40,000 feet. At that height, air density is only 18% of sea level.
Although air travel is very safe, high altitude does pose a potentially very serious threat to the lives of passengers if the cabin were to be depressurized. Cabin pressure is maintained at a standard level of 11 or 12 psi at cruising altitude. Without this artificially produced air pressure, high altitude air travel would not be possible.
At forty thousand feet, if the cabin were to depressurize, you would only have between five and ten seconds to place an emergency oxygen mask over your nose and mouth before you lost consciousness.
Not to freak you out, but uncontrolled decompression incidents are not as uncommon as you might think. Worldwide, including both military and civilian aircraft, there are about 40-50 incidents every year. With rare exceptions, they do not typically result in any injuries or deaths. There was a recent incident, on Southwest Airlines Flight 1380, when engine failure after takeoff resulted in engine debris hitting the fuselage and cracking open a window. One passenger died and eight were injured. Thankfully, incidents like that are extremely rare.
Just how close can a jet get to space? The jet plane that climbed the highest in history was the Lockheed SR-71 Blackbird. The Blackbird was a highly technologically advanced stealth plane that could climb to a staggering max height of 85,069 feet.
At that height, air is only 2% as dense as the air at sea level. The Blackbird was designed to fly at extreme altitudes and high speeds in order to evade enemy aircraft and anti-aircraft missiles. While many of them were destroyed in accidents, none were ever brought down by hostile fire. They were mostly used for reconnaissance missions. It still holds the record for history's fastest "air-breathing" aircraft.
During the flight that set that record, the Blackbird was traveling at a speed of Mach 3.2, or 2,190 miles per hour. For comparison, most commercial jet airliners travel between 460 and 575 mph. Blackbird pilots had to wear full-body pressure suits that had self-contained supplemental oxygen systems, in the event of an emergency ejection or unplanned decompression of the cockpit.
Unfortunately, that safety measure was put to the test on January 25, 1966. A Blackbird, piloted by Bill Weaver, disintegrated.
During a Blackbird test flight, Bill Weaver's plane disintegrated in mid-air. At the time of the incident, the plane was flying at 78,000 feet and traveling at Mach 3.1. Weaver and a test specialist named Jim Zwayer were taking the Blackbird through a programmed 35 degree bank when the right engine experienced a malfunction. The Blackbird started to roll to the right. Weaver tried to correct with his control stick but nothing happened.
Knowing that the chances of survival if they ejected that high up were slim, the two men decided to try to navigate the aircraft to a lower altitude and slower speed. Weaver lost consciousness just before the aircraft disintegrated. When he woke up, he was in freefall. Or so he gathered after a few moments, unable to see anything through an iced-over pressure suit face plate.
Thankfully, the pressure suit did its job. An oxygen cylinder attached to his parachute harness was still operational. It gave him air to breathe and also kept the suit pressurized, preventing his blood from literally boiling in his body. He was able to deploy his parachute and land, alive, not far from where Zwayer's body also landed. Unfortunately, his neck had been broken in the accident. He likely died instantly.
Not all terrestrial aircraft use jet engines. Are there other systems that might allow an airplane to reach space?
The answer is no. However, the alternatives are worth learning about. One is the SCRAMJET engine, used on the NASA X-43 unmanned experimental hypersonic aircraft. The X-43 is able to reach speeds that far outpace the Blackbird.
The Blackbird has a maximum air speed limit of Mach 3.5. Any faster than that, the engine's compressors stop functioning due to air pressure and temperature spikes. The X-43, in turn, set a speed record in 2004 by flying at Mach 9.6. That's 7,310 mph. Unlike jet engines, the SCRAMJET system does not use turbine compressors. Indeed, the engines have no moving parts at all.
The engine generates internal shockwaves that compress and heat the air, which is used to burn fuel and generate an insane amount of thrust. Nobody knows exactly how fast an X-43 can fly. It is speculated that they may be able to reach up to Mach 20, possibly surpassing even that unimaginable speed. One idiosyncrasy of the X-43 is that they don't work if they're traveling slower than Mach 5. They have to be launched by booster rockets before the SCRAMJET can start working its magic.
So, in terms of space travel, air-breathing aircraft are out. How do spacecraft make it into orbit and beyond, then?
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A spacecraft has to reach what's called "escape velocity," the speed at which it is possible to defeat Earth's gravitational pull. Escape velocity is 25,020 mph. The craft also has to be able to climb past 328,000 feet, which is where the planet's atmosphere ends and space begins. No air-breathing craft has ever come close. 328,000 feet is more than three times the highest altitude reached by the Blackbird.
To date, this has only ever been accomplished by rocket propulsion. There have been 5,038 known rockets launched. Of those, 4,621 actually reached space. The success rate is 92%, a testament to how well we have adapted to space flight's learning curve. Interestingly, Russia actually has a slightly better success rate than America.
In the United States, there have been 833 people launched into space, on 135 space shuttle missions. Some astronauts had the privilege of riding multiple flights. There have been fourteen astronaut fatalities. Obviously sad, but when you consider the danger involved, it's really quite remarkably small. These fatalities occurred during the Challenger and Columbia accidents. The Challenger explosion is still remembered as one of the great tragedies in the history of manned space flight.
As it stands, the only method by which an aircraft can reach sustained speeds adequate to hit escape velocity, and be able to stay airborne in low or no atmosphere, is rocket propulsion. Unlike the SCRAMJET engine, a rocket can operate from a total standstill. It also generates significantly more power, and travels much faster.
A rocket can travel faster than Mach 33, which is the minimum for breaking Earth's gravity. That's 4.9 miles per second, which is over twenty times faster than the speed of sound. No other means of propulsion has ever been discovered that can meet these criteria. Rocket propulsion has been the only means by which we can launch objects into space. Though we do it at great financial and material cost. Rockets also come with a suite of problems and limitations, which we'll get into later.
The first rocket able to break through the atmosphere was the V2, developed by Germany in 1942. The V2 carried no payload, though. The first rocket used to carry something else into space was the R-7 ICBM, the rocket that launched Sputnik. Sputnik was the world's first satellite, developed and launched by the Soviet Union in 1957.
There were a number of intervening decades between the V2 rocket and the launch of the first space shuttle in April of 1981. One important waypoint on the development journey was the North American X-15, one of the first rocket powered "space planes."
The X-15 was a hypersonic rocket-powered aircraft that set both altitude and speed records in 1960. It flew all the way to the edge of space, returning safely Earthside afterwards. The data gathered by the X-15 were critical in the design process that would result in manned space flights.
The X-15 set the still-standing world record for highest speed attained by a manned, powered aircraft in October of 1967. It was piloted by William J. Knight, who flew to 102,100 feet and reached an incredible speed of 4,520 miles per hour. X-15 pilots qualified as astronauts by NASA's standard of having surpassed the 50 mile mark. The X-15 was still a long way from the dream of being able to board an aircraft, fly to space, and return to a landing strip under its own power, though. They had to be carried to 45,000 feet on the underside of a B52 bomber.
NASA started its research and design on potential space shuttle designs before the Project Apollo moon landing in 1969. The earliest designs were drafted in October of 1968. They were originally intended to play a support role for space stations, ferrying crews and cargo back and forth. A group called the Space Task Group proposed a reusable space shuttle, called the Space Transportation System, in 1969. This would eventually become the space shuttle we're familiar with.
The first orbital spaceflight taken by a space shuttle was STS-1, which launched on April 12, 1981. It was the 20th anniversary of the first ever manned spaceflight, taken by Yuri Gagarin. STS-1, or Space Transportation System-1, touched down 54 and a half hours after it launched. In that window, it orbited the Earth an incredible thirty-six times.
Space shuttles are basically gliders with rocket boosters. They are mobile while in orbit and while gliding back to Earth, but they require a massive amount of fuel and rocket assistance to get into space. The space shuttle is iconic, and the thing most people associate with space flight. However, they are a long way off from fulfilling the role of a true "space plane."
Unlike jet engines, rockets carry their own supplies of liquid oxygen to burn rocket fuel. This is how they are able to operate in the upper reaches of the atmosphere and in space, where standard air-breathing aircraft would suffocate.
The oxygen supply, along with the fuel, makes the entire rocket/shuttle system extremely bulky and heavy. In total, the external fuel tank and the two solid rocket boosters to which the space shuttle attaches weigh a total of 1,940 metric tons on the ground. That's not including the weight of the space shuttle itself, which, comparatively, is extremely small. At most, the shuttle can weigh 1.3% of the total weight when the system is on the launch pad.
A space shuttle weighs 27.5 metric tons. Which is not light. But it is minuscule compared to the mass and power needed to get it airborne. There are currently designs to allow higher payloads to be carried to space, though they are still in development. For now, we are constrained to using huge storehouses of oxygen and fuel, encased in rockets, to launch things into space. It works just fine, but is expensive and not especially suited for space tourism.
So what do we have coming down the pipe, in the way of designs that fit the bill for a reusable "space plane?" Right now, all the proposed designs are still speculative. Since this is a new, and very daunting, problem, researchers are laboring to overcome some really major design hurdles that may wind up thwarting the efforts altogether. However, there is hope.
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One major frontrunner in the design race is the Reaction Engines Skylon. The Skylon is an SSTO, which is short for "Single-stage-to-orbit" design. SSTOs are a hypothetical class of aircraft that are able to reach orbit without jettisoning any hardware, only expelling propellants and/or fluids. The Skylon fits that bill. If we ever see a prototype, it will be able to reach space without relying on separate booster rockets that would have to be discarded mid-flight.
The Skylon would run on hydrogen fuel, and would take off from a specially made runway. It would be able to fly at Mach 5.4, and would be able to fly in low Earth orbit or in outer space. It is proposed that the Skylon would be able to reach the International Space Station, and would be able to deliver significantly more cargo than the European Space Agency's current model of Automated Transfer Vehicle, the craft they use to resupply the ISS.
A ceramic composite skin would protect the Skylon upon re-entry into Earth's atmosphere.
The Skylon will use a specially designed new kind of engine called the SABRE, or Synthetic Air Breathing Rocket Engine. The SABRE will allow the Skylon to use a combination of rocket propulsion and air-breathing jet propulsion. Kind of like how hybrid electric cars kick in a gas engine above a certain speed, the SABRE will activate rocket propulsion above a certain altitude.
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The SABRE will be able to use atmospheric oxygen up to 93,000 feet. Past that, it will utilize rocket power to complete the journey into space. The Skylon will then be able to land on a specialized runway, to be checked, refueled and launched again, much like a jetliner airplane.
In its current design, the Skylon's highly fuel efficient engines and aerodynamic design would allow it to burn fuel about 20% more efficiently than a standard rocket. Because it will require less liquid oxygen, it will be able to carry much more weight than a standard rocket launch setup. The Skylon can carry a payload of about 5.5% of total weight, as opposed to a rocket's 1.3%.
The Skylon is very promising. And very expensive. The total estimated cost of the program is projected to be about twelve billion dollars.
The Skylon may be having its first unmanned test flights as early as 2025. Its future is still unknown, though. There are the development costs and possible unforeseen pitfalls in the still-new technology, and there is also competition from other proposed ideas. Namely, from the reusable rocket.
There are two reusable rockets currently reaching the end of their research and development phases. The SpaceX Falcon series of rockets and the Blue Origin New Shepard. The pet projects of Elon Musk and Jeff Bezos, respectively, the reusable rockets are showing to be less costly than the Skylon. They will be used for launching satellites, resupplying the International Space Station, space tourism, and possibly even colonizing Mars.
Blue Origin's aims are seemingly more modest than SpaceX's, with its New Shepard spacecraft intended only for carrying groups of passengers up for suborbital flights and deliver them safely back again. SpaceX's vision is a bit more grandiose. They are hoping to reduce the cost of space travel in general, and also hope to send a manned mission to Mars. Which, if the movies are any indication, may not be a good idea. SpaceX is also more industrially focused, while Blue Origin is purely tourism oriented.
This is the New Shepard, named after Alan Shepard, the first American in space. It is a VTVL, or Vertical-Takeoff, Vertical-Landing suborbital rocket designed expressly and exclusively for space tourism.
The New Shepard has been in the works since 2006. Unmanned testing started in 2015, with the first manned tests planned for later this year. The New Shepard is so close to being launch ready that the company claims it will start selling tickets for flights happening in 2019.
The New Shepard claims the distinction of having the first ever booster rocket to make a successful vertical landing, after being discarded post-launch. Currently, three New Shepards have been built. The last flight, which occurred in July, carried a mannequin that showed no signs of being space-exploded upon landing.
While the first commercial flights are slated for next year, a select few special guests will be participating in the first few manned launches of the New Shepard later this year. While the New Shepard may not look much like a plane, and is more like a hyper-expensive ride than a means of transportation, it may be the closest thing we have to an actual space plane.
SpaceX has been forefront in the news when it comes to private companies entering the space travel / space tourism market. The current darling of the handful of SpaceX rockets is the BFR, or Big Falcon Rocket. It is a reusable launch vehicle and spacecraft, classified as a "super heavy-lift launch vehicle" for its ability to carry 220,000 pounds of cargo. Its first orbital test flight will occur in 2020.
The BFR used to be called the Mars Colonial Transporter, or MCT. The project was shrouded in secrecy until CEO Elon Musk presented its design to the International Astronomical Congress in 2016. It was pitched as part of Musk's proposed Interplanetary Transport System. Previously meant to be twelve meters long, the BFR was shortened to nine meters in 2017. The mission statement was also updated.
The BFR was originally being designed solely as a spacecraft that would be used for a manned mission to Mars. It was updated, however, to serve multiple roles, including Earth orbit, lunar orbit, interplanetary travel and even good old passenger travel on Earth.
When complete, the BFR is meant to totally replace all the other SpaceX vehicles, including the Falcon Heavy, the Falcon 9 and the Dragon spacecraft.
It appears that our "space planes" are almost here. With so much research and development money being poured into these new technologies, we may also enjoy spillover into the terrestrial air travel market. If we can go to space and back, we can also, hopefully, fly between continents in less time.
One promising possibility is the incorporation of the Skylon's SABRE engine into passenger aircraft. These modified SABREs would forego the rocket capability, going fully air-breathing. This would make hypersonic air travel much more feasible. Certainly more feasible than using a SCRAMJET engine, which is another idea that's been floated.
In addition to the obvious discomfort of the first leg of your flight being accelerated from zero to Mach 5 by a booster rocket, the SCRAMJET engine is also much less efficient than the SABRE. If the SABRE performs as expected, it will have a thrust-to-weight ration of fourteen, as opposed to a conventional jet's five and a SCRAMJET's measly two.
If SABRE engines become a commonplace, we can expect air travel to take significantly less time than it does now. Of course, those SABRE planes won't be able to travel to space. But that's a reasonable price to pay for getting from New York to London in a few blinks of the eye (not literally).
So, there you have it. Jetliners as we know them will never be able to reach space. We are, for now, still shackled to the technologies that are familiar to us. There are, of course, worse fates. Air travel is still an incredible convenience, and, considering the stakes, remarkably safe.
Like many new technologies, recreational space travel will probably remain the exclusive domain of the hyper-wealthy for a very long time. But who knows, maybe there will be the equivalent of a coach flight to the edge of space that will be affordable for the common person. It is also unforeseeable how these new technological developments will impact other parts of our lives.
As gratuitously expensive and seemingly pointless of an experiment as flying people to Mars is, it is also conceivable that it might become a more and more common part of human life. Not to mention the moon. India is already planning a manned Moon mission, and there is also talk of it becoming a destination for space tourism. Some day, you may be looking at the surface of the Moon while sipping a complimentary in-flight beverage. It just won't be the window of an airplane.