Rockets are magnificent engineering feats that have allowed us to travel into space and orbit
over the past several decades. But, though rockets are extremely cool, they cost massive amounts of
money and carry substantial risk during each flight. In contrast, we have our other flying
masterpiece: the airplane. Not only can airplanes also fly, but they’re astronomically cheaper and
more reliable than rockets. In fact, the annual risk of an American being killed in an airplane
crash is only 1 in 11 million. To put that into perspective, Nasa estimates that there’s a 1 in
276 chance of a Falcon 9 disaster leading to loss of life. That means that one of the most reliable
rockets of all time is still 40,000 times riskier than your average plane. But let’s be real here,
I’m sure the brainiacs working at space agencies around the world have already considered this. So,
why can’t planes just fly into space? Well, to answer this question, we have to first
understand the physics behind how airplanes fly in the first place. While it may seem like the
powerful engines on planes are doing all the work in keeping us airborne, this is actually not true.
In fact, the engines on airplanes are quite weak in comparison to the maximum capacity of
the plane. The ratio between the force generated by a plane’s engines and the gravitational force
pulling down the airplane is called the thrust to weight ratio. And you’ll be surprised to hear how
low these ratios really are. The Boeing 747-8 only has a thrust to weight ratio of 0.269,
and the largest passenger aircraft, the airbus a380, only has a thrust to weight ratio of 0.227.
The thrust to weight ratios of most commercial aircraft land between 0.2 and 0.35.
Even the concorde, which was a supersonic commercial aircraft, only has a thrust to
weight ratio of 0.372. But, if airplane engines are only generating enough thrust to lift a fifth
to a third of the plane's maximum weight, how is the plane even able to gain altitude? Well,
this is where extremely clever engineering of the wings comes into play. The underside of an
airplane’s wings are flat while the topside of the airplane’s wings are rounded. This means that air
flowing on the top side of the wing has to travel a further distance than the air on the bottom side
of the wing. The total volume of air flowing on either side of the wing, however, is the same. So,
the air on the top side of the wing is spread over a larger area than the air on the bottom side of
the wing. This difference causes the top side of the wing to have a lower pressure than the bottom
side of the wing. Given that particles naturally want to move from high pressure to lower pressure,
the wing naturally wants to move upwards, and this force is called lift. If we’re able to
create a large enough pressure difference between two sides of the wings, the generated lift will
be greater than the gravitational force pulling down the airplane, and that’s the secret behind
how airplanes actually gain altitude. Over the past several decades, engineers have basically
perfected every single component of the plane from bending the tips of wings to utilizing flaps.
All of these efforts have one goal in mind, maximize lift and minimize fuel consumption.
While this has made airplanes insanely efficient, it has also made them extremely dependent on air.
Without air, most of our planes wouldn’t leave the ground. But what if we just put
more powerful engines onto our planes or made the planes themselves smaller. If we could get
the thrust to weight ratio to be greater than 1, airplanes wouldn’t need lift to gain altitude
right? Well, that’s true, and we actually have a bunch of fighter jets that do have a thrust to
weight ratio that’s greater than 1. The F-15, for instance, has a ratio of 1.04 and the F-22 has a
ratio 1.09 which can even be increased to 1.26 with the right combination of fuel and payload.
This is why these jets can do vertical takeoffs, loops, barrel rolls, and who knows what else. But
just because these jets are not dependent on lift doesn’t mean that they aren’t dependent
on air. And that brings me into the fundamental difference between rocket engines and jet engines.
Both engines use combustion and Newton’s third law to create thrust, but if you think about it,
you’d quickly realize that a jet engine has both in intake and an exhaust
while rocket engines only have an exhaust. Jet engines have an intake because they rely on the
oxygen in the air for combustion. Without a steady intake of oxygen, combustion can’t take place,
and the jet engines simply won't work. Now, it’s not like rocket engines are any better. Rocket
engines also need oxygen to work, and this is why we have solid and liquid rocket propellant.
In a solid fuel rocket, the fuel and oxidizer is mixed together into a chemical and engines
simply use the chemical during flight. With liquid fuel rockets, the fuel and the oxidizer
are actually stored separately and the engines combine the two ingredients during combustion.
But either way, both jet and rocket engines need oxygen to operate,
and since jet’s don’t carry oxygen along with them, they can’t just fly into space.
So, the bottom line is basically just a lack of atmosphere in space. Planes need the atmosphere
to generate lift and they need the oxygen in the atmosphere to fire up their engines.
If the atmosphere didn’t thin out as we reach space, planes should theoretically be able to fly
anywhere in the solar system, though it would be a rather slow mode of transport. But, since that’s
not the case, airplanes have a service ceiling which is generally around 42 to 43,000 feet.
What if we just strapped rocket engines to airplanes though, as long as the thrust to weight
ratio is greater than 1, the aircraft would no longer be dependent on the atmosphere.
People have actually had this idea for nearly a hundred years at this point with the first rocket
powered flight dating back to the early 1900s. In 1928 a German plane the Lippisch Ente became
the first aircraft to fly using rocket power. A couple of years later in 1931, an Italian aviator
named Ettore Cattaneo built the first privately built rocket plane. We also saw several rocket
powered aircraft throughout World War 2, but it wasn’t until the cold war that we actually tried
to use this technology to reach space. The most notable spaceplane attempt of all time was likely
the North American X-15. The X-15 was proposed way back in December of 1954 which was before
Nasa even existed, so the plane was to be built by North American Aviation and Reaction Motors. The
plan was to carry the X-15 on a mother ship called the B-52 to an altitude of 8.5 miles at a speed
of 500 miles per hour. At this point, the B-52 would release the X-15, and the X-15 would use two
liquid propellant rocket engines to fly as high as possible. Fortunately, the X-15 actually made
it past the concept board, and the first rocket powered flight would take place on September 17,
1959. Over the next decade the various versions of the X-15 would be flown 199 times.
Fun fact, one of the pilots of the X-15 was Neil Armstrong. Anyway, in 1963, pilot Joe Walker would
successfully fly the plane past 100 km in altitude which meant that the X-15 could officially reach
space. But, aside from technically reaching space, the X-15 couldn’t actually complete anything
meaningful in space. It couldn’t reach orbit and it was nowhere near escape velocity of mach 33.
But, nonetheless, the plane was extremely impressive as it could reach mach 6 or 4000 mph,
and it was a great first step in developing a plane that could actually travel into space.
After the X-15 program came to a close, Nasa decided to basically develop a hybrid space
plane with the space shuttle. The idea was to launch like a rocket but land like a plane,
but this is when Nasa would run into by far the largest obstacle when it comes to space planes
and that’s excessive heat. Though the X-15 was an extremely fast aircraft,
it could only reach 1/5th the speed of escape velocity. Speeding up the aircraft
to escape velocity was extremely hard, but that wasn’t the main obstacle. The main problem was
that it was and still is nearly impossible to handle the generated heat. But wait a minute,
we have rockets that go way faster than the X-15, so how are they able to handle the heat? Well,
that’s the reason that rockets launch vertically at least at the start. The idea is to get the
rocket as high as possible before it reaches extreme velocities. At these higher altitudes,
the atmosphere will be much thinner which heavily reduces the generated heat. The easy solution to
this problem would be to just launch the space plane vertically, and that’s basically what
Nasa settled on with the Space Shuttle. At that point, it’s more of a rocket than a space plane,
but even after doing this, Nasa faced major heat problems when it came to reentry.
The underside of the space shuttle had a total of 20,548 heat shield tiles, and every single
tile had to be individually inspected after every single flight. Even a small amount of damage to
these heat shield tiles could lead to catastrophic results like we saw with Columbia. So, truthfully,
Nasa never figured out an efficient way to refly the Space Shuttle, and this was one of the biggest
downfalls of the space shuttle. Considering this, Nasa should’ve never tried to make a full on space
plane until they figured out a more efficient solution for the Space Shuttle, but nonetheless,
Nasa started to work on a single stage to orbit space plane called the X-30 in the 1980s.
The plane was slated to have top speeds of Mach 25 and it seemed like the plane would be straight out
of movie. That should’ve been a major red flag though as the plane was way too overambitious.
According to Nasa engineer Ivan Bekey, the X-30 was “the biggest swindle ever to be foisted on the
country” and it “was full of dubious aerodynamic claims and engine performance claims and thermal
claims.” Basically, Nasa had nowhere near enough technology to make the plane a reality,
and the $1.7 billion invested into the project was a complete waste. Nasa didn’t give up after
the X-30 though. Later on, they started working on the X-43 which did reach production and it
was able to achieve a top speed of Mach 9.6. But even that isn't nearly enough to reach
orbit using a space plane. In the meantime, the overarching space community has come to a broad
conclusion. What’s even the point of trying to solve this nearly impossible heat problem
with space planes when we can just keep using traditional rockets and continue exploring space.
At the end of the day, who cares if the vehicle launches vertically or horizontally,
the destination is what really matters. Though it would be super cool to fly a plane into space,
it really doesn’t pose any additional benefits. Originally, many scientists thought that space
planes were the key to reusability and cheap launch costs. But, SpaceX is proving that these
attributes can be achieved using traditional rockets, so the excitement around space planes
is more or less dying off. Maybe a hundred years from now after we’ve mastered traditional rockets,
we’ll see space planes make a comeback. But in the meantime,
it makes no sense to try to battle with the laws of physics when you can just launch vertically.
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