A few days ago on the 6th of February 2018
SpaceX's first test launch of the Falcon 9 heavy the most powerful rocket since
the Saturn 5 successfully launched a payload of Elon Musk's own cherry red
Tesla roadster complete with a Starman mannequin first into orbit and then on
what was meant to be a journey to Mars but now looks more like an elliptical
orbit extending out to the asteroid belt between Mars and Jupiter. Whilst the
Falcon 9 heavy proved that it could launch a payload into deep space
Starman and the Roasters journey itself is going to be a pretty leisurely one just
like everything else we've launched into deep space fine for mannequins, cars and
robotic probes but not so good for humans. With an average time of nine
months to get to Mars the risk from radiation, weightlessness and
psychological issues is high, not to mention with supplies they need to carry
even if they're going to grow their own food and recycle their own water. But if
you can reduce the time it takes to get there all these things become much more
manageable, however there is a technology using nuclear power that was developed
for about 20 years from the 50s to the 1970s originally by the US and the
Soviets for Mars missions proposed in the 1970s and 80s but was shelved after
the demise of the missions themselves. Now with our revived interest in Mars
missions and trips to deep space nuclear-powered rockets are back on the
agenda as a way to cut transit time and carry greater payloads. But we have to
make a distinction here the nuclear rocket engines are only for use in space
because they have a much lower thrust to weight ratio compared to chemical
rockets and so chemical ones will still be doing the heavy lifting from Earth
into orbit but once they're an away from a gravity of Earth nuclear engines can
be much more efficient. Now as powerful as chemical rockets are
they have a problem they need to carry not only the fuel but also the oxygen
allow it to burn which makes the Rockets much heavier and also reduces the
payload available. The first type of nuclear rocket, the nuclear thermal one
does away with the liquid oxygen altogether and instead passes liquid
hydrogen fuel through a nuclear reactor to heat it up to a superheated gas, this
hot gas has been ejected from the engine in the same way as a chemical rocket and
that's where it creates its thrust. The difference is that when a chemical
rocket engine burns hydrogen & oxygen the byproduct which is water vapour is heavy
and therefore as a given temperature its velocity is lower and therefore the
thrust created is less. Because a nuclear thermal engine doesn't burn the hydrogen
it just heats it up it's still pure hydrogen when it leaves the rocket
engine and as hydrogen is the lightest element its exit velocity is the highest
for a given temperature and therefore the thrust is considerably more. A
rockets efficiency or how well it makes use of its fuel is measured in seconds
of specific impulse sounds complicated but put simply it means how long in
seconds one pound of propellant can deliver one pound of thrust, the best
chemical rockets have a specific impulse around 450 seconds whereas the early
test nuclear Rockets had a specific impulse of around 900 seconds twice as
efficient. Now couple that to a fact when you don't now need to carry a lot of
heavy liquid oxygen around with you and this means you can either go faster or
carry greater payloads. From the mid-1950s this principle was studied
extensively at the Atomic Energy Commission laboratory at Los Alamos in
New Mexico through a program called Project Rover by 1959 an experimental
reactor called Kiwi who was ready for testing it with jackass flats in Nevada
and successfully ran at a power of a 100 megawatts and then up to a full
power rating of 1000 megawatts in 1962 the program to turn Kiwi into a working
engine was called nuclear engines rocket vehicle applications or NERVA
again the tests were successful and in 1967 the NRX-A6
engine fired successfully for an hour as the tests continued the engine
runtime was only really limited by the supplies of liquid hydrogen at the test
site. Whilst these test engines proved the concept there were still issues the
weight of the shielding to protect for crew and the control electronics from
the radiation made the engines very heavy, there was also the problem of what
would happen if a nuclear engine rocket were to fail on a launch pad or were to
fall back to earth from orbit and spread highly radioactive material over
potentially highly populated areas and there was also issues with overheating
if the fuel which acted as a coolant ran out before the engine was shut down
properly. But after years of testing they were deemed ready for use in space and
NRX seemed to be on track to make it there by the late 1970s. On the other
side of the Iron Curtain the Soviet chief rocket designer Sergey Korolyov
expected his N1 super heavy lift rocket to carry a nuclear upper stage giving
the vehicle a formidable interplanetary capability his team considered several
options either to nuclear other stages on a three-stage rocket or a single
nuclear stage on a two or three stage rocket with any of these options the N1
upper stages would have had to cluster large numbers of nuclear engines
together many more than on the American designs. But a nuclear thermal upper
stage was calculated to be able to deliver up to 50% more payload to
Mars than a chemical one. However the nuclear thermal engines weren't the only
options available on the table Korolev favored another way of harnessing atomic
power pairing a nuclear reactor with an electric ion engine although the system
provided much less thrust it was even more efficient and would provide a low
thrust for a very long period of time compared to the quick bursts like the
nuclear thermal ones. In recent times advances in solar panel
design has made nuclear reactor powered iron engines much less likely but for
deep space missions beyond Saturn where there is not enough sunlight to operate
solar panels successfully nuclear power becomes pretty much the only option.
Calculations showed that ion drives will be able to transport 70% more payload
than chemical engines on a mission to Mars. After the N1 rocket quite literally
fell by the wayside the chief designer Vladimir Chelomei was in pole position
to design a rocket for the Soviet manned Mars mission
the enormous MK 700 was to be assembled in orbit with multiple launches for the
modular UR700 rocket the MK 700 would make its interplanetary burn with a
nuclear thermal engine built by Valentin Glushko's design bureau and called the
RD-0410 the engine made it as far as testing at the test site in north
east kazakhstan demonstrating yet again Glushko's mastery of rocket engine
efficiency the RD-0410 showed a capability of 910 seconds of specific
impulse. However by 1972 and due to the loss of
interest in space by the public and the government in the u.s. funding for
nasa's mars mission and NERVA was cancelled by congress as the space
shuttle returned to focus to low-earth orbit where nuclear engines weren't
required and the soviet priority was to match this capability their research
into nuclear-powered spacecraft also stalled. However things picked up again
with President Ronald Reagan's Star Wars initiative in the late 1980s project
Timberwind advanced the engines using a pebble bed reactor design and increased
the specific impulse to nearly 1,000 seconds it also led to the development
of modern carbon composite materials but it also faced technical issues which
would have made it very difficult to use as a space
based engine system and funding was eventually dropped. More recently Russian
engineers at the State Energy Corporation ROSATOM are working on a
form of nuclear electric propulsion called 'TEM' the acronym translates to
transport an energy unit the name indicates a nuclear reactor capable of
bimodal operation that is when it's not powering engines it can be switched to a
low power mode to power the living quarters and other onboard systems.
According to the designers TEM has a goal of testing a ground-based engine
this year and launching a prototype by 2025 if funding can be secured. The
technology could in theory enable a journey from Earth to Mars in just 45
days. NASA is also conducting new research into rapid transit technologies
to mitigate the long-term effects of interplanetary space on the human body
like Russia, NASA sees nuclear thermal propulsion as a solution here the goal
of the current research is to reduce for cost and risk of the nuclear fuel by
developing a suitable form of low enriched uranium with a concentration of
3-4% uranium-235 compared to a 90% concentration
of weapons-grade fuel in the earlier designs. NASA's research may well find
its first use in a spacecraft like this the Copernicus mass transfer vehicle
conceived as part of the now cancelled constellation program Copernicus B is
being considered for an in orbit assembly using NASA's upcoming SLS block
2 launch system. At one end of Copernicus the Orion spacecraft would
dock with a manned payload living space Copernicus would then make its
interplanetary journey using three nuclear thermal engines each capable of
producing 25,000 pounds of thrust the efficiency of the nuclear engines would
enable a trip to Mars in a hundred days not quite as quickly as were proposed
Russian mission but still a significant improvement over traditional chemical
engines and also Copernicus would be capable of
by modal reactor operation. So what do you think of using nuclear Rockets for
future manned missions let me know in the comments below.
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