In 1965, a young Caltech graduate student by the name of Gary Flandro was poring over
his notes in Building 180 of the Jet Propulsion Laboratory, studying the intricacies of using
planets gravitational fields to slingshot spacecraft into the outer reaches of space
with no need for additional propellant. The gravity assist. A method of propulsion that
uses the gravity of a gravitational body to literally pull the spacecraft into a different
trajectory and velocity. As he examined the possible trajectories Gary noticed something
incredible. In the late 70s a window would open up where the alignment of the planets
would allow for a single satellite to pinball its way between Jupiter, Saturn, Uranus and
Neptune before eventually being spit out by the outer reaches of our solar system and
beyond. [1]
With this knowledge a flurry of activity began within NASA and JPL. A once in a lifetime
opportunity had been found that would not come about again for another 175 years, and
it was imperative that the opportunity was not squandered. Planning began and in 1977,
two satellites, Voyager 1 and 2, would launch aboard the 3 staged Titan 3e rocket.
Voyager 2 would utilize a gravity assist with Jupiter, Saturn and Uranus before racing by
Neptune before it eventually exited the solar system. While Voyager 1 completed a more direct
path, whipping by Jupiter and Saturn before venturing into deep space, moving away from
earth faster than any other spacecraft. [2] Voyager 1 is now over 22.8 billion kilometres
away from earth. [3] That’s so far away, that Voyager 1 is now outside the influence
of the sun’s constant stream of solar wind, meaning it is now in interstellar space, officially
reaching the region in August 2012. [4]
We, as a species, have now left our mark in interstellar space after a 35 year long journey.
A ponderously long time in the scale of a human life, and that was with the help of
multiple gravity assists.
While we can build and deploy these technical marvels to investigate other worlds, sending
ourselves into space is another thing entirely, and using gravity assists to visit our closest
neighbors makes little sense.
To date, the furthest humans have ventured into space is to the dark side of the moon,
a mere 400,171 kilometers away. [5] If our ambitions to create colonies on Mars
are to be realised, humans are going to need spacecraft that are faster and more efficient
than the ones we currently have at our disposal.
Mars is on average about 64 million kilometers away. [6] The fastest and most efficient method
we have to reach mars, using the Hohmann transfer method, with launch windows every 26 months,
takes about nine months to complete.
Faster transfer times are possible, but engineers are stuck in a catch twenty two. It takes
fuel not only to accelerate the spacecraft, but to decelerate it too. There are no disk
brakes in space. The more we accelerate the more we have to decelerate too. In order to
carry all that additional propellant engineers will be forced to sacrifice payload weight.
Reducing the space available for food, water and other vital supplies for a crewed mission
to Mars. We can take some solace in the fact faster transfer times will reduce the supplies
needed and will reduce the crews exposure to the high radiation of space.
But, what if we could achieve faster transfer times without sacrificing payload. What if
we could achieve faster transfer times with even more payload?
Let’s first examine current technology to see where things could be improved. The Atlas
V rocket that brought Perseverance to Mars utilized chemical combustion to propel itself.
A method where a fuel and an oxidiser are combined in a combustion chamber and ignited.
The resulting exothermic reaction causes the combustion products to rapidly heat up and
expand. The nozzle design then directs this expanding gas in one direction to achieve
thrust.
Getting the most out of our fuel and oxidiser is the first step to maximising our thrust
per unit weight of fuel. There is a useful quantity engineers use to describe this property
of fuels and oxidisers. Specific Impulse.
We described in detail what this value represented in our last video on the X-15, explaining
how it represents the total energy we can extract from our propellants per unit weight.
In that video we used this equation, where specific impulse is defined by thrust forces
divided by fuel flow rate. [7]
This is an extremely important metric for our Mars Transfer vehicle. The higher we can
push our specific impulse the less fuel we need to bring, which frees up space for payload,
or we can bring the same amount of fuel and increase our velocity to reach Mars sooner,
or even be able to leave outside of that ideal Hohmann transfer window.
The technology with the best specific impulse currently is ion propulsion. Take the NSTAR
ion drive aboard the now retired Dawn spacecraft. [8] This engine used electric power to propel
ions and achieve astronomical specific impulses. The engine releases xenon atoms into an ionization
chamber and then bombards them with high energy electrons, the collision produces a positive
xenon atom and more electrons. These electrons are then collected by the positively charged
chamber walls, while the positive xenon atoms migrate towards the chamber exit which contains
two grids. A positive grid called the screen grid, and a negative grid called the accelerator
grid. The high electrical potential between these grids causes the positive ions to accelerate
and shoot out of the engine at speeds up to 40 kilometres per second, which is vastly
higher than what chemical combustion can provide, which has a typical exhaust velocity of about
3 to 4 kilometres per second.
This exhaust velocity is paramount to achieving higher specific impulses. If we play around
with the specific impulse equation, we can see why. Specific impulse equals the thrust
force divided by fuel flow rate.
Thrust force is equal to the mass flow rate times the velocity. While the fuel flow rate
is the weight of the fuel on earth, and thus this value changes to mass flow rate times
acceleration due to gravity, and as we see the mass flow rates cancel themselves out.
Leaving us with only exhaust velocity divided by gravity. So it’s rather obvious, to maximise
specific impulse, we need to maximise exhaust velocity, and ion propulsion is the best technology
we have to do that right now.
With 10 times the exhaust velocity the ion drive can achieve 10 times the specific impulse.
[9] (ranging from 1,500 to 10,0000 seconds.)
That’s a phenomenal increase. So why aren’t we using this technology for interplanetary
missions?Mass flow rate may not be important to specific impulse, but it is massively important
to thrust, as we saw just now. Thrust force is equal to mass flow rate times the velocity.
Chemical combustion happens extremely quickly, it is after all a controlled explosion. And
so, it is capable of accelerating millions upon millions of molecules out in a very short
space of time. Leading to a high mass flow rate.
High thrust is very important for particular maneuvers, like capture burns, where a rocket
will fire to slow its speed down enough to be captured by a planet's gravity. The window
for this deceleration may only be a few hours, where ion propulsion simply cannot provide
enough thrust in a short enough time to successfully complete the maneuver.
Ion Propulsion simply does not have the mass flow rate necessary to achieve high thrust.
It took the Dawn spacecraft 4 days to change its velocity by just 94 kilometers per hour.
[10]
To increase our ability to change velocity quickly, we need to increase the mass flow
rate. To do this, we need to increase our input power, for ion propulsion that power
comes in the form of electricity. Which provides the energy to ionize our propellant and accelerate
it using an electric or magnetic field. Current generation ion propelled spacecraft use solar
panels to provide that electricity, the dawn spacecraft has panels capable of producing
10 kilowatts of power when orbiting earth, which diminished to about 1.3 kilowatts by
the time it reached its destination in the asteroid belt 3 times further away from the
sun. Scaling that solar power becomes impractical very quickly. NASA estimates that a mars transport
vehicle would need at least 400 to 2000 kilowatts of power to carry astronauts and cargo to
and from Mars. [11]
So, how do we power something like that? Nuclear power is the only thing that can give the
power density needed to make this viable.
This isn’t a new concept. In 1961, the Atomic Energy Commision and NASA launched the Nuclear
Engine for Rocket Vehicle Applications program (NERVA). This program developed and ground
tested 20 reactors before it was disbanded in 1973 due to budget constraints. [12]
But was recently revitalized, when the US Congress approved 125 million dollars in research
funding for nuclear propulsion. [13]
There are two primary types of nuclear space propulsion, nuclear electric, which would
power an ion drive like we saw above, and nuclear thermal, which was the focus of the
NERVA program. So let’s start there.
Nuclear Thermal Propulsion works by harnessing the heat created during nuclear fission to
provide the energy needed to expand and accelerate a propellant through an exhaust nozzle.
Here, the nuclear reactors work in much the same way as a nuclear reactor here on earth
would. Where a chain reaction of neutrons colliding with uranium atoms splits them and
creates more neutrons and a tremendous amount of heat. To capture this heat, a propellant,
typically liquid hydrogen, is pumped through the reactor core, which will cool the reactor
core and pass the heat to liquid hydrogen which rapidly expands and accelerates out
of the thruster nozzle at high speeds. Typically around 8 kilometres per second, twice as fast
as chemical combustion, and thus about twice the specific impulse at around 887 seconds.
[178,000 Newtons of thrust] [14]
However, it’s not all sunshine and rainbows, using hydrogen as a propellant comes with
some issues. It can attack the fuel rods if they are not adequately protected with a material
that is resistant to hydrogens destructive tendencies. Liquid Hydrogen also has to be
stored at extremely low cryogenic temperatures, and if it is allowed to rise in temperature
it needs to be vented to prevent an explosion, and, on top of this, the tiny molecule is
so small it can slip through seemingly solid materials, as it can fit between the spaces
of larger molecules.
This makes it unsuitable for long storage periods, and ideally we want a mars transfer
vehicle that can sit in orbit around the earth or mars for extended periods, waiting for
the crew to arrive and begin its journey between the planets. Then when it arrives the crew
descends in a separate vehicle, leaving the transfer vehicle parked in orbit once again.Liquid
hydrogen is just a pain to use in this application. So why use it? Because, when it comes to maximising
exhaust velocities, and thus specific impulse, low molecular weight exhaust products are
important.
Assume for a moment that all the heat energy we input into the system is converted to kinetic
energy in the exhaust products. Kinetic energy equals a half times the mass times the velocity
squared. [E = ½mv2 ]. To find the velocity we can rearrange this equation, so now we
see that velocity equals the square root of 2 times the energy divided by mass. It’s
clear here that increasing the mass of the exhaust particles will decrease the velocity
of our exhaust. Hydrogen is the lightest element, and thus it maximises specific impulse.
If we were to use another propellant it would be extremely difficult to make a nuclear thermal
propelled spacecraft with a high enough specific impulse to justify its use. The next lightest
gas is helium, which is twice as heavy as hydrogen, and thus will reduce our specific
impulse by the square root of 2 (1.4). Nearly negating all the advantage nuclear thermal
propulsion can provide. The next lightest element, which isn’t a solid at the temperatures
we require, is nitrogen, which is 14 times heavier, and thus would decrease our specific
impulse by the square root of 14, which is 3.7 times worse, making a nitrogen nuclear
thermal engine worse than a traditional combustion engine.
So, we can’t get away from this hydrogen storage problem, and if we hope to use nuclear
thermal propulsion, we are going to need to figure out how to keep the hydrogen cryogenically
stored for extended periods.
If this problem could be solved, the higher specific impulse and high thrust could cut
our transit time to mars by half or potentially open launch windows outside of the ideal hohmann
transfer window. [12]
So, can we get around this hydrogen storage problem, while using nuclear power to achieve
higher specific impulses? This is where ion propulsion becomes really attractive again.
One massive advantage in ion propulsions favour is its ability to use heavier, inert and easily
storable noble gases as propellants, like Xenon or Krypton. This goes against our previous
understanding where low exhaust molecular masses are beneficial to higher exhaust velocities.
This is possible because we are using electric power to launch these atoms at tremendous
speeds.
The ion exhaust velocity is defined by the charge of the ion, the voltage that it is
being accelerated by, and the mass of ion. [15] The charge and mass of the ion are defined
by our propellant choice, but we can scale that voltage very high before we hit a limit
in performance due to material properties or some other physical limit.
For both combustion and nuclear thermal engines we are converting thermal power to kinetic
energy. That thermal power is difficult to scale. Chemical combustion is limited by the
energy we can liberate from the chemical bonds of the propellants, and by the temperature
our engine can operate at before it melts.
This is a problem for nuclear thermal power too, which has to run extremely high reactor
core temperatures of 2500 degrees celsius to achieve exhaust velocities high enough
to justify its uses. Specialized nuclear fuel designs are needed to survive these temperatures
and any higher would destroy the reactor.[16]
For reference, this is an order of magnitude higher than nuclear reactors here on earth
need to achieve, which typically operate at about 300 degrees celsius, as they are in
effect, just boiling high pressure water. [17]
Ion Thrusters do not come close to the operating temperatures that thermally driven engines
do, [18] and we can crank voltage up high enough that the added mass of the ion barely
matters. We are still achieving 10 times the specific impulse of traditional engines. Could
we use a lighter propellant to increase specific impulse, ofcourse, but the advantages of using
propellants like Xenon and Krypton are so good that the drop in exhaust velocity and
specific impulse are worth it.
Being inert they can easily be stored over the long thrust cycles ion propulsion needs.
Making them the ideal propellant for long duration interplanetary missions.
Larger atoms, like Xenon, also hold onto the electrons in their electron cloud much looser
than smaller atoms like hydrogen. So it takes less energy to ionize Xenon than it takes
to ionize hydrogen. So, this reduces the electric power needed for the first step in our ion
propulsion process.
And most critically higher mass exhaust improves thrust. This equation defines the thrust an
ion propulsion engine can generate. Where ion mass forms the denominator of our specific
impulse equation, it forms the numerator for our thrust equation. Meaning an increase in
ion mass will increase our thrust, which is the spec that ion propulsion struggles with
most. A worthy trade off.
For the Nuclear Thermal Engine, the propellant acts as a coolant. For the Nuclear Electric
engine we will need a closed loop coolant system, where we do not expend the coolant,
but keep it in a cycle between the hot engine and a heat exchanger.
The only method we have to dump heat overboard in space is through radiative cooling, so
a nuclear electric propelled spacecraft will need massive radiator fins where this coolant
can pass through. This is feasible, but we have a long way to go with developing nuclear
engines for space.
And even with this added power, nuclear powered ion propulsion would still be on the low end
of thrust. In all likelihood, these ion propelled engines will need to be hybrid engines that
can use chemical combustion for high thrust maneuvers, or if the problem of long term
hydrogen storage can be addressed, a nuclear hybrid engine is extremely attractive, where
our high thrust burns can be provided by the nuclear thermal engine,
and then through neutron absorbing control mechanisms, like these rotating drums where
one side is coated in a neutron reflector and the other is coated in a neutron absorber,
the engine temperature could be lowered and switched to a closed loop coolant system that
would power our electric generator
and provide extremely high specific impulse and a gradual increase in velocity that could
drastically cut our travel times to Mars, or perhaps, allow humans to venture even further
into our solar system and begin our gradual exploration and settlement of our cosmic neighbourhood.
This is an incredibly complex topic, with many nuanced and complicated ideas that I
struggled to grasp until I found the right equations. I struggled to understand how thrust
would require high mass flow rate, yet specific impulse is much higher when your exhaust molecular
weight is low. The eureka moments all came from dimensional analysis of the equations
and their derivations. Being able to understand the language of the universe is a vital tool
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