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<br /><br />Russian Nuclear Rocket Engine Design for Mars Exploration
Vadim Zakirov1,**, Vladimir Pavshook2
Introduction
Recent plans regarding manned missions to Mars include
consideration of propulsion technology options.
The three main types of propulsion technologies under
consideration are: chemical, electrical, and nuclear.
According to the assessments[1] summarized in Fig. 1,
the most suitable envelop for such a mission is limited
to a less than 900-ton initial mass spaceship in earth
orbit and a total mission duration of less than 550 days.
The first requirement is due to the assumption that the
100-ton-payload to low earth orbit (LEO) “Energia”
launchers can take off 9 times per year and the spaceship
should be assembled in one year. Longer assembly
times may result in expiration of the spaceship components’
lifetime before the end of the mission. A more
frequent launch schedule is now seen as impractical in
view of the fact that the international space station (ISS)
construction which started in 1998 has still not yet
been completed. Extension of the total mission duration
to longer than 550 days is undesirable due to several
reasons:
The average radiation dose an astronaut gets per
year from galactic cosmic rays, solar flares, etc.
is ~90 rem (roentgen equivalent man). This is
double the limit safety value set by nuclear industry
standards.
The substantial decalcification of bone that occurs
in a zero gravity environment.
&#1048698; A concern about psychological problems associated
with living in confined quarters for long periods
of time.
As seen in Fig. 1, the nuclear rocket engine (NRE) is
the only option that fits within this envelop at present
1 Design and Developments
The research and development work on the NRE for
manned Mars missions started in the former Soviet
Union in the 1950s[2]. During the beginning phase of
the research, the thermal, solid-core NRE concept was
selected as the most practical option. In 1959-1960 advances
in the development of high-temperature, corrosion-
resistant ternary carbide nuclear fuels finally resulted
in a heterogeneous NRE core design.
In this design the nuclear fuel elements are made in
the shape of a twisted ribbon (Fig. 2a)[3,4] and assembled
in bundles (Fig. 2b)[3,4] with 6 to 8 integrated into each
rod-type fuel assembly (Fig. 2c)[4] that are inserted into
channels in the propellant cooled zirconium hydride
neutron moderator filling the reactor’s core (Fig. 2d)[4].
Such a highly integrated modular structure design
has several advantages:
&#1048698; The twisted ribbon surface-to-volume ratio is 2.6
times higher than that of the NERVA (the US developed
NRE[5-8]) fuel elements, which enhances
the heat transfer between fuel and propellant.
&#1048698; The Russian fuel elements are made of ternary
carbides (UC-ZrC-NbC and UC-ZrC-C) with a
maximum operating temperature of about 3200
K. During reactor tests, gas exit temperatures of
3100 K were demonstrated for one hour and
2000 K for 4000 hours. The lifetime of such fuel
elements at ROVER/NERVA[7,8] demonstrated
temperatures is expected to exceed 25 hours.
&#1048698; The fuel composition along the fuel assembly
axis can be easily changed, making axial physical
profiling possible. In fact, the UC-ZrC-NbC
and UC-ZrC-C fuel elements are placed upstream
while the UC-ZrC-NbC are downstream[
2].
&#1048698; Because the high temperatures are localized to
the fuel element bundles, the rest of the core operates
at much lower temperatures (<800℃). (a)
Common materials can be used in the core’s
structure. In particular, low melting point neutron
moderators such as zirconium and lithium
hydrides are possible. (b) The core support structure
with up- and down-stream support plates is
simpler and lighter than the NERVA design[8].
&#1048698; The locations of the fuel assembly rods inside
the core can be changed in both the radial and
circumferential directions for radial profiling.
&#1048698; The propellant mass flow rate through each fuel
assembly rod can be controlled, making hydraulic
profiling possible.
A power generation loop can be integrated inside
the core to change the NRE to a nuclear power
and propulsion system (NPPS)[9].
&#1048698; Only one fuel assembly rod needs to be tested to
assess the performance of future NRE while the
NERVA design requires tests of the whole new
core. The single rod fuel assembly testing made
it possible for the Russians to accomplish NRE
research and development at about one tenth of
the cost of the ROVER/NERVA project that is
now estimated to be ~US$ 7.71 billion inflated
to the current financial year.
Fig. 2 Russian NRE concept
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MW
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MWt/L[11]. At 3100 K, the
temperature climb rate reached 1000 K/s with up to 12
thermal cycles tested[11]. These results were then used
for performance assessments of prospective NREs. Out
of the dozens of NRE designs considered, the most developed
were the RD-0411 for ~392 kN and the RD-
0410 for ~35 kN thrusts.
A full scale NRE prototype (Fig. 3)[12] has been built
and tested using electric heaters instead of nuclear fuel.
The prototype has the performance characteristics
listed in Table 1. Hexane has been suggested as a propellant
additive to reduce the fuel element erosion by
the hot hydrogen.
2 Recent Plans
Recent plans for Mars exploration suggest modifications
of the baseline RD-0140 design in Table 1 to the
NPPS design integrating a Brayton cycle power
loop[11,13]. Figure 4 shows one recent NPPS design. In
such an NPPS, a mixture of xenon and helium is
suggested as the working fluid for the power mode.
The chemical stability of recently developed carbidenitride
nuclear fuel has been confirmed during 100-
hour tests at ~2800 K so this fuel is expected to replace
the initial carbide design[11].
A Martian spaceship would be equipped with 3 or 4
NPPS units[9].
Besides providing a continuous source of reactor
thermal energy, the NPPS design reduces thermal
stresses in the reactor since the engine is “pre-heated”,
minimizes large thermal cycles since there is no prolonged,
deep “cold soak” of the engine, allows rapid
reactor restart in case of an emergency, minimizes the
“decay heat removal” propellant penalty by rejecting
low power, decay heat through the power system's
space radiator, and supplies hot, gaseous hydrogen for
propellant tank pressurization and possible high specific
impulse attitude control and orbital maneuvering
systems[14].
Although the advantages of NPPS applications are
obvious, there is no practical experience utilizing
power loop integration into the NRE. Development of
such an NPPS will require significant effort and financial
investment.
Most of the problems associated with nuclear pollution
hazards are expected to be solved by operating the
NRE only 1000 km above the earth in orbit. The other
problems will require further studies. Lack of fundingis
currently the main obstacle for NRE program
realization.
3 Conclusions
The nuclear rocket engine is a key technology for
manned Mars exploration because it is currently the
only propulsion option that fits the requirements for
initial spaceship mass and mission duration. This is
possible because NRE technology is capable of producing
high thrust while its specific impulse is significantly
higher than that of the most advanced oxygenhydrogen
LRE.
The Russian heterogeneous reactor core NRE is an
advantageous and the most suitable starting point concept
for manned Mars mission application study comparative
to homogeneous reactor core NRE developed
during the ROVER/NERVA program in the United
States. Because the Russian NRE reactor core, except
for in the fuel element bundles, operates at moderate
temperatures, a power loop integration inside the core
is beneficial since it can convert the generated heat to
electric power required onboard the spaceship and enables
a smoother propulsion system operation. This
modification will transform the NRE to an NPPS.
Although the NRE still needs development for space
application, the problems are solvable with additional
effort and funding.
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推荐→第一投注:倍率高←存取速度快.国内最好的投注平台<br /><br />Russian Nuclear Rocket Engine Design for Mars Exploration
Vadim Zakirov1,**, Vladimir Pavshook2
Introduction
Recent plans regarding manned missions to Mars include
consideration of propulsion technology options.
The three main types of propulsion technologies under
consideration are: chemical, electrical, and nuclear.
According to the assessments[1] summarized in Fig. 1,
the most suitable envelop for such a mission is limited
to a less than 900-ton initial mass spaceship in earth
orbit and a total mission duration of less than 550 days.
The first requirement is due to the assumption that the
100-ton-payload to low earth orbit (LEO) “Energia”
launchers can take off 9 times per year and the spaceship
should be assembled in one year. Longer assembly
times may result in expiration of the spaceship components’
lifetime before the end of the mission. A more
frequent launch schedule is now seen as impractical in
view of the fact that the international space station (ISS)
construction which started in 1998 has still not yet
been completed. Extension of the total mission duration
to longer than 550 days is undesirable due to several
reasons:
The average radiation dose an astronaut gets per
year from galactic cosmic rays, solar flares, etc.
is ~90 rem (roentgen equivalent man). This is
double the limit safety value set by nuclear industry
standards.
The substantial decalcification of bone that occurs
in a zero gravity environment.
&#1048698; A concern about psychological problems associated
with living in confined quarters for long periods
of time.
As seen in Fig. 1, the nuclear rocket engine (NRE) is
the only option that fits within this envelop at present
1 Design and Developments
The research and development work on the NRE for
manned Mars missions started in the former Soviet
Union in the 1950s[2]. During the beginning phase of
the research, the thermal, solid-core NRE concept was
selected as the most practical option. In 1959-1960 advances
in the development of high-temperature, corrosion-
resistant ternary carbide nuclear fuels finally resulted
in a heterogeneous NRE core design.
In this design the nuclear fuel elements are made in
the shape of a twisted ribbon (Fig. 2a)[3,4] and assembled
in bundles (Fig. 2b)[3,4] with 6 to 8 integrated into each
rod-type fuel assembly (Fig. 2c)[4] that are inserted into
channels in the propellant cooled zirconium hydride
neutron moderator filling the reactor’s core (Fig. 2d)[4].
Such a highly integrated modular structure design
has several advantages:
&#1048698; The twisted ribbon surface-to-volume ratio is 2.6
times higher than that of the NERVA (the US developed
NRE[5-8]) fuel elements, which enhances
the heat transfer between fuel and propellant.
&#1048698; The Russian fuel elements are made of ternary
carbides (UC-ZrC-NbC and UC-ZrC-C) with a
maximum operating temperature of about 3200
K. During reactor tests, gas exit temperatures of
3100 K were demonstrated for one hour and
2000 K for 4000 hours. The lifetime of such fuel
elements at ROVER/NERVA[7,8] demonstrated
temperatures is expected to exceed 25 hours.
&#1048698; The fuel composition along the fuel assembly
axis can be easily changed, making axial physical
profiling possible. In fact, the UC-ZrC-NbC
and UC-ZrC-C fuel elements are placed upstream
while the UC-ZrC-NbC are downstream[
2].
&#1048698; Because the high temperatures are localized to
the fuel element bundles, the rest of the core operates
at much lower temperatures (<800℃). (a)
Common materials can be used in the core’s
structure. In particular, low melting point neutron
moderators such as zirconium and lithium
hydrides are possible. (b) The core support structure
with up- and down-stream support plates is
simpler and lighter than the NERVA design[8].
&#1048698; The locations of the fuel assembly rods inside
the core can be changed in both the radial and
circumferential directions for radial profiling.
&#1048698; The propellant mass flow rate through each fuel
assembly rod can be controlled, making hydraulic
profiling possible.
A power generation loop can be integrated inside
the core to change the NRE to a nuclear power
and propulsion system (NPPS)[9].
&#1048698; Only one fuel assembly rod needs to be tested to
assess the performance of future NRE while the
NERVA design requires tests of the whole new
core. The single rod fuel assembly testing made
it possible for the Russians to accomplish NRE
research and development at about one tenth of
the cost of the ROVER/NERVA project that is
now estimated to be ~US$ 7.71 billion inflated
to the current financial year.
Fig. 2 Russian NRE concept
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MW
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MWt/L[11]. At 3100 K, the
temperature climb rate reached 1000 K/s with up to 12
thermal cycles tested[11]. These results were then used
for performance assessments of prospective NREs. Out
of the dozens of NRE designs considered, the most developed
were the RD-0411 for ~392 kN and the RD-
0410 for ~35 kN thrusts.
A full scale NRE prototype (Fig. 3)[12] has been built
and tested using electric heaters instead of nuclear fuel.
The prototype has the performance characteristics
listed in Table 1. Hexane has been suggested as a propellant
additive to reduce the fuel element erosion by
the hot hydrogen.
2 Recent Plans
Recent plans for Mars exploration suggest modifications
of the baseline RD-0140 design in Table 1 to the
NPPS design integrating a Brayton cycle power
loop[11,13]. Figure 4 shows one recent NPPS design. In
such an NPPS, a mixture of xenon and helium is
suggested as the working fluid for the power mode.
The chemical stability of recently developed carbidenitride
nuclear fuel has been confirmed during 100-
hour tests at ~2800 K so this fuel is expected to replace
the initial carbide design[11].
A Martian spaceship would be equipped with 3 or 4
NPPS units[9].
Besides providing a continuous source of reactor
thermal energy, the NPPS design reduces thermal
stresses in the reactor since the engine is “pre-heated”,
minimizes large thermal cycles since there is no prolonged,
deep “cold soak” of the engine, allows rapid
reactor restart in case of an emergency, minimizes the
“decay heat removal” propellant penalty by rejecting
low power, decay heat through the power system's
space radiator, and supplies hot, gaseous hydrogen for
propellant tank pressurization and possible high specific
impulse attitude control and orbital maneuvering
systems[14].
Although the advantages of NPPS applications are
obvious, there is no practical experience utilizing
power loop integration into the NRE. Development of
such an NPPS will require significant effort and financial
investment.
Most of the problems associated with nuclear pollution
hazards are expected to be solved by operating the
NRE only 1000 km above the earth in orbit. The other
problems will require further studies. Lack of fundingis
currently the main obstacle for NRE program
realization.
3 Conclusions
The nuclear rocket engine is a key technology for
manned Mars exploration because it is currently the
only propulsion option that fits the requirements for
initial spaceship mass and mission duration. This is
possible because NRE technology is capable of producing
high thrust while its specific impulse is significantly
higher than that of the most advanced oxygenhydrogen
LRE.
The Russian heterogeneous reactor core NRE is an
advantageous and the most suitable starting point concept
for manned Mars mission application study comparative
to homogeneous reactor core NRE developed
during the ROVER/NERVA program in the United
States. Because the Russian NRE reactor core, except
for in the fuel element bundles, operates at moderate
temperatures, a power loop integration inside the core
is beneficial since it can convert the generated heat to
electric power required onboard the spaceship and enables
a smoother propulsion system operation. This
modification will transform the NRE to an NPPS.
Although the NRE still needs development for space
application, the problems are solvable with additional
effort and funding.
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Vadim Zakirov1,**, Vladimir Pavshook2
Introduction
Recent plans regarding manned missions to Mars include
consideration of propulsion technology options.
The three main types of propulsion technologies under
consideration are: chemical, electrical, and nuclear.
According to the assessments[1] summarized in Fig. 1,
the most suitable envelop for such a mission is limited
to a less than 900-ton initial mass spaceship in earth
orbit and a total mission duration of less than 550 days.
The first requirement is due to the assumption that the
100-ton-payload to low earth orbit (LEO) “Energia”
launchers can take off 9 times per year and the spaceship
should be assembled in one year. Longer assembly
times may result in expiration of the spaceship components’
lifetime before the end of the mission. A more
frequent launch schedule is now seen as impractical in
view of the fact that the international space station (ISS)
construction which started in 1998 has still not yet
been completed. Extension of the total mission duration
to longer than 550 days is undesirable due to several
reasons:
The average radiation dose an astronaut gets per
year from galactic cosmic rays, solar flares, etc.
is ~90 rem (roentgen equivalent man). This is
double the limit safety value set by nuclear industry
standards.
The substantial decalcification of bone that occurs
in a zero gravity environment.
&#1048698; A concern about psychological problems associated
with living in confined quarters for long periods
of time.
As seen in Fig. 1, the nuclear rocket engine (NRE) is
the only option that fits within this envelop at present
1 Design and Developments
The research and development work on the NRE for
manned Mars missions started in the former Soviet
Union in the 1950s[2]. During the beginning phase of
the research, the thermal, solid-core NRE concept was
selected as the most practical option. In 1959-1960 advances
in the development of high-temperature, corrosion-
resistant ternary carbide nuclear fuels finally resulted
in a heterogeneous NRE core design.
In this design the nuclear fuel elements are made in
the shape of a twisted ribbon (Fig. 2a)[3,4] and assembled
in bundles (Fig. 2b)[3,4] with 6 to 8 integrated into each
rod-type fuel assembly (Fig. 2c)[4] that are inserted into
channels in the propellant cooled zirconium hydride
neutron moderator filling the reactor’s core (Fig. 2d)[4].
Such a highly integrated modular structure design
has several advantages:
&#1048698; The twisted ribbon surface-to-volume ratio is 2.6
times higher than that of the NERVA (the US developed
NRE[5-8]) fuel elements, which enhances
the heat transfer between fuel and propellant.
&#1048698; The Russian fuel elements are made of ternary
carbides (UC-ZrC-NbC and UC-ZrC-C) with a
maximum operating temperature of about 3200
K. During reactor tests, gas exit temperatures of
3100 K were demonstrated for one hour and
2000 K for 4000 hours. The lifetime of such fuel
elements at ROVER/NERVA[7,8] demonstrated
temperatures is expected to exceed 25 hours.
&#1048698; The fuel composition along the fuel assembly
axis can be easily changed, making axial physical
profiling possible. In fact, the UC-ZrC-NbC
and UC-ZrC-C fuel elements are placed upstream
while the UC-ZrC-NbC are downstream[
2].
&#1048698; Because the high temperatures are localized to
the fuel element bundles, the rest of the core operates
at much lower temperatures (<800℃). (a)
Common materials can be used in the core’s
structure. In particular, low melting point neutron
moderators such as zirconium and lithium
hydrides are possible. (b) The core support structure
with up- and down-stream support plates is
simpler and lighter than the NERVA design[8].
&#1048698; The locations of the fuel assembly rods inside
the core can be changed in both the radial and
circumferential directions for radial profiling.
&#1048698; The propellant mass flow rate through each fuel
assembly rod can be controlled, making hydraulic
profiling possible.
A power generation loop can be integrated inside
the core to change the NRE to a nuclear power
and propulsion system (NPPS)[9].
&#1048698; Only one fuel assembly rod needs to be tested to
assess the performance of future NRE while the
NERVA design requires tests of the whole new
core. The single rod fuel assembly testing made
it possible for the Russians to accomplish NRE
research and development at about one tenth of
the cost of the ROVER/NERVA project that is
now estimated to be ~US$ 7.71 billion inflated
to the current financial year.
Fig. 2 Russian NRE concept
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MW
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MWt/L[11]. At 3100 K, the
temperature climb rate reached 1000 K/s with up to 12
thermal cycles tested[11]. These results were then used
for performance assessments of prospective NREs. Out
of the dozens of NRE designs considered, the most developed
were the RD-0411 for ~392 kN and the RD-
0410 for ~35 kN thrusts.
A full scale NRE prototype (Fig. 3)[12] has been built
and tested using electric heaters instead of nuclear fuel.
The prototype has the performance characteristics
listed in Table 1. Hexane has been suggested as a propellant
additive to reduce the fuel element erosion by
the hot hydrogen.
2 Recent Plans
Recent plans for Mars exploration suggest modifications
of the baseline RD-0140 design in Table 1 to the
NPPS design integrating a Brayton cycle power
loop[11,13]. Figure 4 shows one recent NPPS design. In
such an NPPS, a mixture of xenon and helium is
suggested as the working fluid for the power mode.
The chemical stability of recently developed carbidenitride
nuclear fuel has been confirmed during 100-
hour tests at ~2800 K so this fuel is expected to replace
the initial carbide design[11].
A Martian spaceship would be equipped with 3 or 4
NPPS units[9].
Besides providing a continuous source of reactor
thermal energy, the NPPS design reduces thermal
stresses in the reactor since the engine is “pre-heated”,
minimizes large thermal cycles since there is no prolonged,
deep “cold soak” of the engine, allows rapid
reactor restart in case of an emergency, minimizes the
“decay heat removal” propellant penalty by rejecting
low power, decay heat through the power system's
space radiator, and supplies hot, gaseous hydrogen for
propellant tank pressurization and possible high specific
impulse attitude control and orbital maneuvering
systems[14].
Although the advantages of NPPS applications are
obvious, there is no practical experience utilizing
power loop integration into the NRE. Development of
such an NPPS will require significant effort and financial
investment.
Most of the problems associated with nuclear pollution
hazards are expected to be solved by operating the
NRE only 1000 km above the earth in orbit. The other
problems will require further studies. Lack of fundingis
currently the main obstacle for NRE program
realization.
3 Conclusions
The nuclear rocket engine is a key technology for
manned Mars exploration because it is currently the
only propulsion option that fits the requirements for
initial spaceship mass and mission duration. This is
possible because NRE technology is capable of producing
high thrust while its specific impulse is significantly
higher than that of the most advanced oxygenhydrogen
LRE.
The Russian heterogeneous reactor core NRE is an
advantageous and the most suitable starting point concept
for manned Mars mission application study comparative
to homogeneous reactor core NRE developed
during the ROVER/NERVA program in the United
States. Because the Russian NRE reactor core, except
for in the fuel element bundles, operates at moderate
temperatures, a power loop integration inside the core
is beneficial since it can convert the generated heat to
electric power required onboard the spaceship and enables
a smoother propulsion system operation. This
modification will transform the NRE to an NPPS.
Although the NRE still needs development for space
application, the problems are solvable with additional
effort and funding.
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推荐→第一投注:倍率高←存取速度快.国内最好的投注平台<br /><br />Russian Nuclear Rocket Engine Design for Mars Exploration
Vadim Zakirov1,**, Vladimir Pavshook2
Introduction
Recent plans regarding manned missions to Mars include
consideration of propulsion technology options.
The three main types of propulsion technologies under
consideration are: chemical, electrical, and nuclear.
According to the assessments[1] summarized in Fig. 1,
the most suitable envelop for such a mission is limited
to a less than 900-ton initial mass spaceship in earth
orbit and a total mission duration of less than 550 days.
The first requirement is due to the assumption that the
100-ton-payload to low earth orbit (LEO) “Energia”
launchers can take off 9 times per year and the spaceship
should be assembled in one year. Longer assembly
times may result in expiration of the spaceship components’
lifetime before the end of the mission. A more
frequent launch schedule is now seen as impractical in
view of the fact that the international space station (ISS)
construction which started in 1998 has still not yet
been completed. Extension of the total mission duration
to longer than 550 days is undesirable due to several
reasons:
The average radiation dose an astronaut gets per
year from galactic cosmic rays, solar flares, etc.
is ~90 rem (roentgen equivalent man). This is
double the limit safety value set by nuclear industry
standards.
The substantial decalcification of bone that occurs
in a zero gravity environment.
&#1048698; A concern about psychological problems associated
with living in confined quarters for long periods
of time.
As seen in Fig. 1, the nuclear rocket engine (NRE) is
the only option that fits within this envelop at present
1 Design and Developments
The research and development work on the NRE for
manned Mars missions started in the former Soviet
Union in the 1950s[2]. During the beginning phase of
the research, the thermal, solid-core NRE concept was
selected as the most practical option. In 1959-1960 advances
in the development of high-temperature, corrosion-
resistant ternary carbide nuclear fuels finally resulted
in a heterogeneous NRE core design.
In this design the nuclear fuel elements are made in
the shape of a twisted ribbon (Fig. 2a)[3,4] and assembled
in bundles (Fig. 2b)[3,4] with 6 to 8 integrated into each
rod-type fuel assembly (Fig. 2c)[4] that are inserted into
channels in the propellant cooled zirconium hydride
neutron moderator filling the reactor’s core (Fig. 2d)[4].
Such a highly integrated modular structure design
has several advantages:
&#1048698; The twisted ribbon surface-to-volume ratio is 2.6
times higher than that of the NERVA (the US developed
NRE[5-8]) fuel elements, which enhances
the heat transfer between fuel and propellant.
&#1048698; The Russian fuel elements are made of ternary
carbides (UC-ZrC-NbC and UC-ZrC-C) with a
maximum operating temperature of about 3200
K. During reactor tests, gas exit temperatures of
3100 K were demonstrated for one hour and
2000 K for 4000 hours. The lifetime of such fuel
elements at ROVER/NERVA[7,8] demonstrated
temperatures is expected to exceed 25 hours.
&#1048698; The fuel composition along the fuel assembly
axis can be easily changed, making axial physical
profiling possible. In fact, the UC-ZrC-NbC
and UC-ZrC-C fuel elements are placed upstream
while the UC-ZrC-NbC are downstream[
2].
&#1048698; Because the high temperatures are localized to
the fuel element bundles, the rest of the core operates
at much lower temperatures (<800℃). (a)
Common materials can be used in the core’s
structure. In particular, low melting point neutron
moderators such as zirconium and lithium
hydrides are possible. (b) The core support structure
with up- and down-stream support plates is
simpler and lighter than the NERVA design[8].
&#1048698; The locations of the fuel assembly rods inside
the core can be changed in both the radial and
circumferential directions for radial profiling.
&#1048698; The propellant mass flow rate through each fuel
assembly rod can be controlled, making hydraulic
profiling possible.
A power generation loop can be integrated inside
the core to change the NRE to a nuclear power
and propulsion system (NPPS)[9].
&#1048698; Only one fuel assembly rod needs to be tested to
assess the performance of future NRE while the
NERVA design requires tests of the whole new
core. The single rod fuel assembly testing made
it possible for the Russians to accomplish NRE
research and development at about one tenth of
the cost of the ROVER/NERVA project that is
now estimated to be ~US$ 7.71 billion inflated
to the current financial year.
Fig. 2 Russian NRE concept
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MW
Since there is no need to test the whole reactor core
to assess the NRE’s performance, such a reactor has
never been built so the performance results quoted are,
indeed, the results for a single rod fuel assembly (Fig.
2c) tested in a research reactor core (Fig. 2d). During
about 15 tests in the EWG-1 reactor, the measurements
gave a maximum hydrogen temperature of 3100 K that
corresponds to a specific impulse of 925 s (this is double
the specific impulse for the most advanced oxygenhydrogen
liquid rocket engine (LRE), a total lifetime
with 10 consecutive restarts of 4000 s, and a thermal
power density of 10 MWt/L[2]. Up to 30 firings were
conducted in research reactors from 1970 to 1988.
During these tests, thermal power levels up to 230
MWt were explored at propellant mass flow rates up to
16.5 kg/s[10], the maximum power density in the fuel
composition reached 25 MWt/L[9], the uranium enrichment
was 90% with the U235 load varied from 6.7
to 15.9 kg, and the radioactive product release from the
reactor core through the exhaust ≤1% (by mass). The
reliability of the carbide fuel elements was proven by
successful testing of 330 fuel element bundles in 1975-
1989[11]. The fuel elements were tested at maximum
power densities up to 35 MWt/L[11]. At 3100 K, the
temperature climb rate reached 1000 K/s with up to 12
thermal cycles tested[11]. These results were then used
for performance assessments of prospective NREs. Out
of the dozens of NRE designs considered, the most developed
were the RD-0411 for ~392 kN and the RD-
0410 for ~35 kN thrusts.
A full scale NRE prototype (Fig. 3)[12] has been built
and tested using electric heaters instead of nuclear fuel.
The prototype has the performance characteristics
listed in Table 1. Hexane has been suggested as a propellant
additive to reduce the fuel element erosion by
the hot hydrogen.
2 Recent Plans
Recent plans for Mars exploration suggest modifications
of the baseline RD-0140 design in Table 1 to the
NPPS design integrating a Brayton cycle power
loop[11,13]. Figure 4 shows one recent NPPS design. In
such an NPPS, a mixture of xenon and helium is
suggested as the working fluid for the power mode.
The chemical stability of recently developed carbidenitride
nuclear fuel has been confirmed during 100-
hour tests at ~2800 K so this fuel is expected to replace
the initial carbide design[11].
A Martian spaceship would be equipped with 3 or 4
NPPS units[9].
Besides providing a continuous source of reactor
thermal energy, the NPPS design reduces thermal
stresses in the reactor since the engine is “pre-heated”,
minimizes large thermal cycles since there is no prolonged,
deep “cold soak” of the engine, allows rapid
reactor restart in case of an emergency, minimizes the
“decay heat removal” propellant penalty by rejecting
low power, decay heat through the power system's
space radiator, and supplies hot, gaseous hydrogen for
propellant tank pressurization and possible high specific
impulse attitude control and orbital maneuvering
systems[14].
Although the advantages of NPPS applications are
obvious, there is no practical experience utilizing
power loop integration into the NRE. Development of
such an NPPS will require significant effort and financial
investment.
Most of the problems associated with nuclear pollution
hazards are expected to be solved by operating the
NRE only 1000 km above the earth in orbit. The other
problems will require further studies. Lack of fundingis
currently the main obstacle for NRE program
realization.
3 Conclusions
The nuclear rocket engine is a key technology for
manned Mars exploration because it is currently the
only propulsion option that fits the requirements for
initial spaceship mass and mission duration. This is
possible because NRE technology is capable of producing
high thrust while its specific impulse is significantly
higher than that of the most advanced oxygenhydrogen
LRE.
The Russian heterogeneous reactor core NRE is an
advantageous and the most suitable starting point concept
for manned Mars mission application study comparative
to homogeneous reactor core NRE developed
during the ROVER/NERVA program in the United
States. Because the Russian NRE reactor core, except
for in the fuel element bundles, operates at moderate
temperatures, a power loop integration inside the core
is beneficial since it can convert the generated heat to
electric power required onboard the spaceship and enables
a smoother propulsion system operation. This
modification will transform the NRE to an NPPS.
Although the NRE still needs development for space
application, the problems are solvable with additional
effort and funding.
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