PROPULSION AND ENERGY

Nuclear and future flight
propulsion
This year, advances in various future flight propulsion methods have been made in industry,
both here and abroad; at universities; and at
NASA installations.
Corporate and university efforts
For the past two years, Andrewspace has conducted research on improving the original Orion
nuclear pulse propulsion concept with funding
through NASA’s SBIR program and the Dept. of
Energy (DOE). The revised approach may hold
promise for future terrestrial power generation,
which is of interest to the DOE. The Mini-Mag
Orion concept is a gigajoule-scale, pulsed nuclear fission device wherein externally initiated
electromagnetic compression of individual fission pellets yields low-mass criticality. The resulting fission reaction produces highly energetic plasma, which is then expanded through a
magnetic nozzle.
To access concept feasibility, experiments
were conducted at Sandia National Laboratory
using the Saturn and Z-pinch pulsed-power
machines. Results verified the possibility of
transmitting the required power discharges
through low-mass Mylar transmission lines. In
addition, imploding pinch experiments proved
the ability to compress cylindrical liners of
high-Z material (gold) to very high densities.
European companies ELV, Avio (Italy), and
Reaction Engines (U.K.) teamed to advance the
development of SKYLON—a single-stage-to-orbit spaceplane. This reusable vehicle employs
the synergetic air-breathing and rocket engine
(SABRE) concept. Transition between the airbreathing and rocket modes is around Mach 5
at an altitude of 26 km. In the air-breathing
Solar sail materials undergo
mechanical stress testing after
exposure to space environment
radiation.

by Ivana Hrbud
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AEROSPACE AMERICA/DECEMBER 2003

mode, precooling and conventional axial flow
compression are combined to deliver air at approximately 140 bar to the combustion chamber of a conventional rocket engine. After the
transition to the rocket mode, the external air
supply is disabled and internal oxygen tanks
supply the oxidizer to the rocket engine.
The fuel of these dual-mode engines during the single-stage ascent is hydrogen, which
also serves as a heat sink for cooling the inlet
air. During the past two years, Reaction Engines
has designed, built, and tested heat exchanger
technologies for the engine at their site in Oxfordshire. Supported by QinetiQ in Farnborough, Reaction Engines conducted preliminary
wind tunnel tests at Mach 12 to explore shockshock interactions during reentry.
Researchers at Innovative Nuclear Space
Power and Propulsion Institute (University of
Florida) are continuing their focus on both
near-term nuclear propulsion concepts and revolutionary power generation systems. Encouraged by results from (U, Zr, Nb)C tricarbide
fuels that showed improved stability at high
temperatures (2,700 K), work was extended to
fuels with tantalum carbide, critical for watersubmerged, subcritical reactors and for operation at high temperatures (3,000 K). Ultra-clean
and efficient glove boxes and vacuum systems
allow for enhanced production at greater purity
with fewer dissolved interstitials such as oxygen
and nitrogen. Future work will focus on gradient coatings of refractory carbides.
A detailed engineering analysis was conducted on a vapor core reactor design with
magnetohydrodynamic energy conversion. This
study indicated that multi-megawatt-class systems are capable of achieving specific masses of
less than 1 kg/kWe. The study focused on the
reactor system, shielding, and radiator. Results
included the analysis and modeling of a condensing, ultra-high-temperature radiator. The
study incorporated advances in molybdenumbased high-temperature material alloys due to
their strength and compatibility with liquid/
vapor UF4 at high temperatures.
For researchers at the University of Michigan, this past year saw another significant milestone for the development of the laser accelerated plasma propulsion system. Theoretical
predictions and experiments confirmed that the
protons received added energy during the expansion phase of the plasma upon leaving the
target. In experiments where laser pulse lengths
are shorter than the ion acceleration time in the
target, the expansion is adiabatic, thus maximizing the velocity of the protons. Furthermore, this velocity scales directly with the dia-

meter-to-thickness ratio of the target. This result is particularly significant since the energy of
the ejected protons increases with increasing
spot size. This relationship will have significant
impact on thrust enhancement.
NASA efforts
NASA-Marshall continues to explore innovative

technologies for solar sail propulsion systems,
where a spacecraft gains momentum from incident photons. This past year, researchers concentrated their efforts on characterizing the mechanical properties of candidate solar sail
materials exposed to space environments. Sails,
when fully deployed, can have biaxial stress
states as low as 1 psi, while stress states are expected to be considerably higher at sail suspension points. The objective of the investigation
was to determine the radiation dose that would
induce material failure for the specific tension
load applied to the candidate materials.
The materials investigated were 2-µm Mylar and Teonex with and without aluminum
coating, which were loaded with uniaxial tensioners up to 5,000 psi. Preliminary results
show that radiation dose has an effect on the integrity and degrades mechanical properties of
these materials. More research is under way to
better understand the underlying failure mechanisms and to develop better models for predicting functional lifetimes.
Marshall pursued a range of efforts aimed
at identifying and developing new technologies
for primary spacecraft propulsion. Research efforts focused on high-power pulsed electromagnetic thruster concepts that use novel methods
of plasma formation to address life-limiting erosion issues and other disadvantages of current
electric propulsion devices. One such project is
the plasmoid thruster experiment, which inductively forms and accelerates compact toroids.
Initial tests have demonstrated the ability to efficiently create plasmas propagating at over 20
km/sec into a neutral backfill of gas. The liquidfed pulsed-plasma thruster is a two-stage electromagnetic device that uses a porous cathode
to inject liquid metal propellant. This approach
refreshes the cathode surface and significantly
alleviates erosion issues.
The center also continues with design, fabrication, and testing of nonnuclear test beds
and innovative ground testing techniques. To
achieve the maximum benefits of a strong nonnuclear testing program, tangible realistic prototypic hardware milestones must be shown on
a yearly basis. In addition, each product must
build on the success of its predecessor and must
be applicable to the next system.

This approach was used for the Safe Affordable Fission Engine (SAFE) test article development program and accomplished via cooperative efforts with DOE labs, industry, universities, and other NASA centers. Through
hardware-based design and testing, the SAFE
program investigates component, subsystem,
and integrated design and performance. The
approach is being adapted for all reactor concepts tested at Marshall. Significant accomplishments of 2003 include demonstrations of an
automated control zone on a 183-pin, 100-kW
heat pipe reactor concept and a direct-drive gas
(DDG) -cooled reactor concept.
This year, NASA’s Nuclear Systems Initiative was renamed Project Prometheus, after
Greek mythology’s famous titan, who gave rise
to human civilization with his gift of fire. The
project’s ultimate goal is to significantly increase available power for spacecraft, which
consequently will open up the solar system for
more ambitious exploration and much greater
scientific return.
The most significant attributes of high sustainable
power levels would permit
agility, longevity, flexibility,
and comprehensive scientific
exploration to be designed
into the new-era missions.
Project Prometheus named
the Jupiter Icy Moons Orbiter (JIMO) mission as its
first endeavor. Nuclear electric power and propulsion
uniquely augments this mission and enables the combined exploration of Jupiter’s
moons Callisto, Ganymede,
and Europa. JIMO’s project office is located
at JPL. Team members include NASA-Glenn,
-Kennedy, and -Marshall, as well as the DOE,
with its affiliates at Los Alamos and Oak Ridge
National Lab.
To date, the program office has awarded
three parallel JIMO mission study contracts to
explore spacecraft conceptual designs and to
conduct technology trade studies. In addition, it
awarded, through competitive selection, 15 R&D
contracts to teams from industry, academia,
and government. The three major contract areas
are electric propulsion, space nuclear fission,
and radioisotope power system development.
The multimission radioisotope thermoelectric
generator and the Stirling radioisotope generator are considered candidates to power the 2009
Mars science lab. Over 60 organizations are participating in Project Prometheus.

SAFE and DDG tests are being
conducted at NASA-Marshall.

AEROSPACE AMERICA/DECEMBER 2003

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