Senior project for in-space propulsion systems

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Analysis of Nuclear Propulsion in
Spacecraft
Liam Ramsay
Department of Mechanical and Aerospace Engineering
Rutgers University, Piscataway, New Jersey 08854
Nuclear Thermal Propulsion (NTP)
Systems have demonstrated enormous
potential in the space propulsion industry
due to their versatility. NTP utilizing the
scientific processes of radiation, fission,
fusion and Matter-Antimatter-Annihilation
(MAA). Alpha decay propulsion has been
tested to reveal its potential in rapid space
travel over extremely far distances,
performing 909% faster than solar sails.
Lower Enriched Uranium (LEU) has grown in
interest as a power source for NTP by
heating a Rankine cycle to vaporize liquid
hydrogen for efficient, low thrust. NASA’s
Fusion Driven Rocket (FDR) compresses
lithium and aluminum bands around a
deuterium-tritium fuel under great magnetic
forces; this causes fusion, releasing an
immense amount of energy with a specific
power of 2.4 kW/kg. The ability of NTP to
apply to a range of different missions
illustrates the importance of this method of
propulsion in space travel.
INTRODUCTION
Current demand for an optimized
propulsion system is at an all time high in the
space community, especially with proposed
missions such as Mars One. For this reason,
propulsion engineers have shown great
interest in the designs built upon NTP
concepts. Particularly, the various forms of
nuclear energy have shown attractive values
in mass specific energy and power in
comparison to different modes of propulsion
(See Table 1). In order of energy potential,
we have four separate forms of nuclear
energy: Radioactive decay, fission, fusion and
MAA.

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Table 1: Specific Energy and Power from Varied NTP Methods
The concept of nuclear thermal
propulsion stems from the principle of
conservation of momentum where, in this
case, the exhaust velocity from our propellant
and its mass directly influence the forward
velocity of our spacecraft. Our NTP systems
utilize a coolant as a propellant that would
absorb the heat from the nuclear-hot core
and is illustrated below (See Figure 1).
Figure 1: NTP Propulsion System Design
Radioactive decay propulsion, also
known as Radioisotope Heated Thermal
Propulsion (RHTP), utilize the concept of
nuclear decay from isotopes as a heat source.
This decay is a result of too few or too many
neutrons in the nucleus of an isotope, causing
it to be unstable and expel radiation.
Radiation from these isotopes can be
released as alpha particles, beta particles,
gamma energy or neutron particles.
Nuclear Fission Thermal Propulsion
(NFTP) consists of a critical fission reactor
core which supplies the necessary energy
needed to heat the expanding working
material. Fission works by the natural
processes of a destabilized atom core where
free neutrons collide with these atoms. This
process starts with neutrons that are released
immediately once fission begins, known as
prompt neutrons. The impacted atoms
convert into a meta stable isotope which
unleashes more neutrons into the system,
unleashing a significant amount of energy.
The state of this chain reaction is dependent
on the number of neutrons in each
generation of reactions (See Eq. 1):
𝑘 =
𝑁 𝑛+1
𝑁 𝑛
Eq. 1

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In the case that k < 1, each generation
decreases in size and the chain reaction will
eventually stop. If k > 1, each generation
increases and the nuclear core will be
supercritical. Lastly, if k = 1, the chain
reaction will continue in a steady state. The
criterion for nuclear fission space travel only
requires that the prompt neutrons are
restricted from maintaining the chain
reaction on their own.
Nuclear Fusion can produce enough
thrust for launch by rapidly heating up a
working medium. The process of fusion
consists of joining two lighter atoms into a
heavier atom, releasing large amounts of
energy. For this process to occur, the fusing
particles need to overcome the Coulomb wall.
The Coulomb wall is the requirement that a
particle’s kinetic energy must be at least
equal the peak of the Coulomb potential; this
generally extends to an enormous demand in
kinetic energy. Due to the high energy
requirements and inability to supply them
with modern technology, this method of NTP
is mostly conceptual.
Lastly, MAA is a conceptual nuclear
process that would aim to reach an exhaust
velocity of the speed of light. Antiparticles
have the same mass, spin and magnetic
momentum as their normal particle
counterparts. The scientific process of MAA
comprises of antiparticles combining with
their counterparts (e.g. positrons and
electrons) to result in a massive burst of
energy due to the Einsteinian mass equivalent
(E = mc2). However, the issue with this
method is the availability and storage of
antiprotons as they do not appear naturally
and need to be produced in particle
accelerators. Resultingly, the efficiency of
this process is extremely low as 1 GJ in
production would give about 1 J in thrust.
This obstacle makes this method infeasible

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with modern technology, however, it should
be re-explored with future innovation.
This paper will explore the current
technological advances in NTP systems
utilizing these methods. Most of these
advancements utilize the nuclear methods
more obtainable with modern technology,
especially RHTP systems. MAA was briefly
mentioned even though systems based on
this method cannot be developed until
further technologies are developed in the
future: it does however explain the full
potential in NTP.
METHODS AND RESULTS
A concept of using alpha particle
decay proposes that these rapid-moving
particles could result in near-light-speed
particle propulsion (NcPP). This takes into
account the speed of light at which the
particles eject off the material that
contributes to the thrust of the spacecraft. A
sail, similar to a solar sail, would contain
uranium-232 as the fuel (See Figure 2).
Figure 2: Alpha Decay Propulsion Sail and Thrust Diagram
This method works optimally over a
large distance due to its long-time
acceleration, reaching speeds greater than
150 km/s (See Table 2). When looking at the
following table, the data shows the allotted
time per year that it takes a spacecraft to
travel. Our spacecraft methods of propulsion
consist of a spacecraft without a sail while
moving at a constant speed of 16 km/s, with a
solar sail and with the U-232 sail (U-232 sail is
tested with both a load-fuel ratio γ of 1 and
0.2). In the case of traveling to Mars, it is
evident that the solar sail is the better choice

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in propulsion. However, in the case of
traveling to Proxima Centauri, the U-232
performs 909% faster than the solar sail. This
is due to the long half-life of the Uranium: it
takes longer distances to reap into the
benefits of the U-232 sail, making it an
effective system for longer missions.
Table 2: Travel Time with Galaxy of Solar versus Alpha Decay
Next, we have nuclear fission at hand
with the LEU system. The Korea Advanced
NUclear Thermal Engine Rocket (KANUTER)
LEU is a non-proliferative, small NTP engine
that pairs with an Extremely High
Temperature Gas-cooled Reactor (EHTGR).
This system operates by utilizing the heat
from the controlled nuclear fission reaction to
power the high temperature reactor (See
Figure 3). This reactor burns liquid hydrogen
as a fuel for propulsion. The LEU generates
the required energy through a Rankine cycle
that converts the liquid hydrogen into a vapor
which is pumped into the reactor for ignition
(See Figure 4).
Figure 3: Engine Components of the KANUTER-LEU

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Figure 4: The Rankine Cycle Component of the KANUTER-LEU
Commonly, most fission NTP engines propel
by breaking down Highly Enriched Uranium
(HEU) fuel and ejecting out the LEU as
exhaust, resulting in a large amount of thrust.
The advantages of the KANUTER-LEU engine
compared to traditional fission engines is its
ultra-small core (total engine mass of only
390 kg) and ability to recycle LEU as a source
of power while only sacrificing a relatively
small amount of rocket performance. Even
though this rocket is designed for low thrust,
it can generate over 50 kN in thrust with a
thrust-to-engine weight ratio of over 4 (See
Figure 5). This would be achieved by reusing
LEU to build a reactor capable of producing
250 MW of power.
Figure 5: Thrust and Thrust-to-Weight Ratio of Reactor
Lastly, nuclear fusion has contributed
to the design of NASA’s Fusion Driven Rocket
(FDR). This experimental concept introduces
lithium and aluminum bands to high magnetic
forces, compressing them around our fuel of
deuterium-tritium. The magnetic forces are
so great that they compress the solid fuel
enough to cause fusion, releasing a significant
amount of energy. The project assures that
fusion can be achieved at a much smaller
scale while having the engine run off of
200kW of solar panels, same as the
International Space Station. Current
prototypes are being tested in NASA
propulsion labs (See Figure 6):

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Figure 6: NASA’s FDR Prototype
Tests and calculations show us that
the FDR system can provide a jet power of 36
MW and a specific power of 2.4 kW/kg. This
means this system has the potential to send a
manned-missioned to Mars with a transit
time on only 80-90 days, greatly surpassing
traditional methods which would take 180
days.
Table 3: Propulsion Requirements of the FDR
CONCLUSIONS
It is no surprise how necessary nuclear
power is toward future space missions. NTP
can perform better over extreme distances,
with lower enriched energy sources and also
run efficiently with hyper-speed high-energy
engines than traditional methods. Even
though the NTP systems are still conceptual,
it will not be long until they replace
traditional methods of interplanetary
propulsion of our spacecraft.
REFERENCES
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Nam, Seung Hyun, Paolo Venneri, YongHee
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Bruno, C., and C. Dujarric. “In-Space Nuclear
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Zhang, Wenwu, Zhen Liu, Yang Yang, and
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2016. Web.
<https://www.youtube.com/watch?v=eMzdKT0
WdRM>.

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