The sight of contrails above us in the skies is a common one especially in the
developed world since the advent of the jet engine and at around the same time
we've also had the much less prevalent but equally potent kind of exhaust from
rocket engines but just how dangerous is jet and rocket exhaust for you meet the
environment.
We've become accustomed to increasing controls on cars and vehicles to make
our air cleaner and safer from removing lead in petrol in the 1980s to the
fitting of diesel particulates and NOx or nitrogen oxide absorbers in the last
10 to 15 years or so but when you look up at the contrails left by planes or
rocket launches it's interesting to know that there are no such regulations on
the aerospace industry. You've only got to see a B-52 takeoff to get an idea of
the pollution that can be produced although that's an exceptional case
these days and new jet engines are much more efficient but the jet fuel used has
barely changed since the 1950s and still has a high sulfur content. Avgas the
aviation version of petrol used by smaller piston-engined aircraft still
has lead in it long after it was removed from cars and as for solid rocket
boosters that's a whole other level which we'll look at later.
Nearly all jet engines in use today use a form of kerosene fuel called jet A1
for commercial aircraft or its military equivalent JP8 Jet B or military version
JP4 is used in the extremely low temperatures of northern canada and
alaska and sometimes russia for its low freezing point. Although jet fuel is used
for jets it's actually very similar to diesel, the main difference between the
jet fuel and the diesel you put in your car is that jet fuel is a more highly
refined cleaner "dry" version of diesel without the lubricants that are added to
diesel to keep the fuel pump and injectors from prematurely wearing out.
Jet fuel also has other additives added to allow it to work of lower
temperatures and in general it has a slightly lower viscosity.
Cleaner it may be but jet fuel has a high sulfur content which can combine
with the water vapor in the exhaust to create sulfuric acid which corrodes the
engines and is also a major pollutant. Sulfur is not added to jet fuel it's
just there in the oil but it's refined from but jet fuel can have a Sulfur
content of up to 3,000 per million and on average it's around
600 parts per million compared to a maximum of 10 parts per million for the
ultra-low sulfur diesel we have for cars and trucks. The reason why sulfur is not
removed from jet fuel is that it's not high enough to cause significant engine
damage and more importantly there's no regulatory requirement to do so it's
cheaper to leave it in than it is to take it out. Removing the sulfur from jet
fuel would add about two percent to the price and it's reckoned cost for global
aviation industry around an extra $1bn to $4Bn per year. Modern
diesel engines in cars and trucks have catalytic diesel particulate filters and
nitrogen oxide absorbers these remove around 85% or more of the tiny soot
particles and NOx which can get stuck deep in the lungs and even be absorbed
into bloodstream to affect the heart and it's also now believed could even alter
DNA. It's difficult to give precise figures but it's believed to be
responsible for the deaths of tens of thousands of people each year worldwide.
Diesel fuel which is designed for use in cars and trucks has a low soft content
because a high sulfur content stops the catalytic converters from working. In a
jet engine, there are no catalytic converters or NOx absorbers so soot
particulates from unburnt fuel and sulfate goes straight into the atmosphere.
This is where much of the contrails or the condensation trails you see come
from. The main byproducts of burning jet fuel is carbon dioxide and water vapor.
At the high altitudes where most planes fly around 36,000 feet the outside
temperature is about minus 56 degrees Celsius. It takes a little while for the
hot exhaust to cool which is why you see a gap between the contrail and the plane
as the water vapor cools it turns to ice if it is cold enough and forms a
contrail. This usually happens at above 26,000 feet or 8,000 meters but can
lower altitudes if the air is moist and cold but at the very low temperatures
where jet planes normally cruise the water in the exhaust can exist as a
supercooled water vapor and it needs a trigger to condense and then freeze. The
tiny soot particles in the exhaust act as the trigger for the water vapor to
condense around which then freezes. These contrails are effectively
artificial cirrus clouds and are sometimes called Cirrus aviaticus and
can last from a few seconds to several hours. Contrails vary greatly even from
planes traveling at the same altitude. Different makes and models of engines
will have different exhaust temperatures and create different trials, newer
engines tend to produce more water vapor and hence more contrails than older ones
and wet air will form more contrails than drier. Wet and dry air can also
exist very close to each other making trails appear and then disappear
apparently at random and you can't see from the ground which is which until
clouds have formed that's a form of wet air you can see. Engines also running on
greater thrust when planes are climbing out put more exhaust than those that are
cruising and finally it's very difficult to accurately judge the altitude of
planes from the ground. All of this leads some people into believing that planes
are spraying chemicals into the atmosphere because the trails look
different or stop-and-start in an apparently clear sky and this is
where the chemtrail conspiracy theories come from. Now while some of the soot
particulates are wrapped up in ice from water much of it the unseen part which
is escaped being captured is blown on the high-altitude winds for thousands of
kilometres. So away from the airports where the low altitude flights occur,
areas which have the highest air traffic like in the US and Europe actually have
quite low levels of aviation based pollution.
This is because most of it ends up being blown westwards towards places like
India and Asia where it eventually falls back to earth to be breathed in by the
population there but of course it's not just us humans which are affected the
environment is also affected but it's not quite as clear cut as it would first
seem. Jets like any other fossil fuel engines produce a huge amount of co2
which contributes to climate change and ocean acidification
but the contrails themselves are ice and that reflects sunlight in the daytime
but acts as an insulator to keep the heat in at night. Soot particles absorb
heat to warm up the upper atmosphere but sulfate particles reflect light and heat
the NOx or nitrogen oxide produced and released at the high altitudes is very
effective at creating ozone which adds to the warming effect but it also
reduces levels of methane a very potent greenhouse gas which has about 20 times
the warming effect of co2. The sulfur in the exhaust can combine with water to
create sulfuric acid which depletes ozone but can also form acid rain to
damage plant life, so as you can see it's a very complicated picture. So what about
rockets? with the recent talk by space flight companies of reaching airline
like launch frequency it's surprising to find out that there is still very little
actually known about the effects of rocket pollutants being deposited into
the upper atmosphere where can have the greatest impact. The fuels used during
takeoff fall into two types liquid fuels and solid rocket fuels. The main liquid
fuels are RP-1 or kerosene and liquid hydrogen these are used with liquid
oxygen instead of air like in a jet engine so they can also work in space
where there is no oxygen. Another branch of liquid fuels are hypergolics.
Hypergolic fuels are liquids which spontaneously combust on contact with
each other, one half is the fuel the other half is the oxidizer but unlike
many of the other liquid fuels hypergolics are
usually highly corrosive and toxic before being burned. The solid rocket
fuels which are used in the solid rocket boosters of the space shuttle and in the
Titan and the upcoming SLS consist of ammonium perchlorate and aluminium
powder and a binder to hold it together. Many of the large rocket launches of the
past like Apollo and many of the first stage rocket launches of today use RP-1
which is a highly refined version of kerosene or jet fuel. RP-1 is designed
to leave less residue behind and has an ultra-low sulfur content as
high-temperature sulphur attacks metals and residues can carbonize and create
hot spots which could damage the engines. At launch the main exhaust components of
RP-1 is carbon monoxide, carbon dioxide and water vapor. The amount of soot
produced is less in comparison to a jet engine
because of the use of liquid oxygen, this means that the combustion is more
complete and there is less unburned fuel left
even though rocket engines normally run fuel-rich to help with cooling. Rockets
which burn liquid hydrogen and oxygen are the cleanest of all. Other than some
NOx which is produced when the hot exhaust reacts with the air, the
byproduct is water vapor. The engineers that refurbished the Space Shuttle main
engines said it looked like they've been steam-cleaned which in a way they had.
SpaceX is also currently developing the raptor engine which will burn liquid
methane and liquid oxygen which would eliminate the soot issue which affects
RP-1 fuel engines. But the worst polluters of all are the solid rocket boosters
these produce mostly water carbon dioxide and metel oxide in the form of
aluminium oxide particles which were first though to have a cooling effect on
the upper atmosphere by reflecting sunlight but is now thought that they
have a warming effect because they absorb the outgoing heat radiation from
the earth. SRBs also create hydrogen chloride which can dissolve in water to
create corrosive hydrochloric acid though little is known about what
happens to it once he enters the environment though chlorine does have a
detrimental effect on the ozone layer. The USAF performed Studies on large SRBs
and their effects on the atmosphere and concluded that 9 shuttle launches and 6
Titan launches per year would account for just less than 1 percent of the
annual ozone depletion. The biggest saving grace is that the rocket launches
are a tiny percentage of a flights compared to the aviation industry. The
Saturn V, the biggest rocket ever built carried enough kerosene to fill just
over 4 Boeing 747s. According to the FAA they handled around 42,000 flights per
day in 2016, the number of rocket flights can be counted on two hands per month, so
even if there is big increase in launches it would still be a very small
amount compared to the commercial aviation industry. The biggest issue is
the lack of regulation of the aviation industry to clean up the exhaust
products like the automotive industry has had to do and this could be done by
providing cleaner fuel in the first place. This alone could reduce the
pollution with minimal modifications required to current engine technology
ultimately though the best way forward will be with electrically powered planes
with electricity coming from renewable sources and maybe even hydrogen fuel
cells using sustainably produced hydrogen. A lot of research will be
required into designing not only the engines but also the batteries or some
other way of storing electrical power in a lightweight form over the coming
decades and this needs scientists and engineers with the skills to work on not
only the Aero engineering but also to study the effects of the existing
pollution on the environment whilst we are getting there. Our sponsor for this
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