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Determining Mars Mission NTP Thrust Size and Architecture Impact for Mission Options
... Since 2019 other fuel materials have been studied such as CERCER (Ceramic-Ceramic) (e.g., Uranium Nitride-Zirconium Carbide). The design process for HALEU NTP studies has included detailed thermodynamic cycle analysis and has been documented in several published papers [2,3,4,5,6]. Fig. 1 [19] shows a simplified rocket thermal cycle; specifically, an expander cycle configuration suitable for the 25,000 lbf thrust NTP. ...
... Since 2019 other fuel materials have been studied such as CERCER (Ceramic-Ceramic) (e.g., Uranium Nitride-Zirconium Carbide). The design process for HALEU NTP studies has included detailed thermodynamic cycle analysis and has been documented in several published papers [2,3,4,5,6]. Fig. 1 [19] shows a simplified rocket thermal cycle; specifically, an expander cycle configuration suitable for the 25,000 lbf thrust NTP. ...
... The stay times are approximately 600 days in the vicinity of Mars. The EMC and MSC ground rules had all landers (e.g., descent/ascent stage, habitat, power systems, etc.) pre-positioned with the crew descent/ascent lander remaining in Mars orbit awaiting the arrival of the Mars crew vehicle.[13][14] ...
Aerojet Rocketdyne is working with NASA, the Department of Energy, and other industry to define a more affordable path to nuclear propulsion and power use for lunar, Mars and broader solar system exploration missions.
Aerojet Rocketdyne's (AR) recent work builds on the legacy design, analysis, and testing knowledge gained from the Rover/NERVA (Nuclear Engine for Rocket Vehicle Applications) to create a feasible Low Enriched Uranium (LEU) design that has been shown to provide high thrust capability (e.g., 25,000 lbf) for faster trajectories and a higher specific impulse (Isp) (e.g., 850 to 900 seconds) than can be achieved with chemical propulsion (e.g., 460 seconds) systems. Some evolutionary approaches with carbide fuel may be able to provide over 1,000 seconds of specific impulse. Evolving and modernizing Nuclear Thermal Propulsion (NTP) engine designs to use LEU reactor fuel has proven to be feasible, and affordable approaches to manufacturing and testing are being pursued using an organized technology maturity plan (TMP) with NASA oversight. LEU NTP engine and reactor development activity is proceeding in 2019 and architecture analysis is showing NTP will provide enabling benefits for multiple solar system missions. Making LEU NTP practical for use in Lunar and Mars missions is foremost and is the expected first use of the engine system. Later missions can evolve using the NASA Space Launch System (SLS) and LEU NTP stages to send orbiters to the gas giants and to the interstellar medium.
This paper discusses and provides background on the analysis and results from the work that is proceeding on developing a LEU NTP system. It is expected that this propulsion system can be used for lunar tugs, crewed and cargo missions to Mars and as a rapid transfer space transport stage.
... A thrust size of 25,000 pounds was selected to provide margin for "engine out" operation or margin for any growth in the overall vehicle mass to reduce the gravity losses at earth departure. 15,16 The driving requirements of LEU NTP engine system are derived from the mission architecture and the vehicle that satisfies the ground rules and assumptions for that mission architecture. Figure 5 shows the characteristics of the Mars vehicle that has been used evaluate the impact of the vehicle figures of merit (Fom) . ...
Studies of Nuclear Thermal Propulsion (NTP) over the past several decades, and updated most recently with the examination of Low Enriched Uranium (LEU), have shown nuclear propulsion is an enabling technology to reach beyond this planet and establish permanent human outposts at Mars or rapidly travel to any other solar system body. The propulsion needed to propel human spacecraft needs high thrust to operate withinthe deep gravity well of a planet and provide high propulsive efficiency for rapid travel and reduced total spacecraft mass.
NTP can provide the thrust to move a spacecraft between orbits, can operate as a dual-mode system that provides power and propulsion capability, provides a strong architectural benefit to human and robotic exploration missions, and provides a path toward reusable in-space transportation systems. NTP provides smaller vehicle systems due to its specific impulse (ISP) being twice that of the best cryogenic liquid rocket propulsion and can thus provide reduced trip times for round-trip missions from Earth to Mars.
Aerojet Rocketdyne (AR) is working with NASA, other government agencies, and other industry partners to improve the design and reduce the cost of NTP engine systems. Current NTP designs focus on thrust sizes between 15,000-lbf (~67-kN) and 25,000-lbf (~111-kN). AR in 2019 has examined various reactor cores and enhancements to optimize LEU NTP designs. The enhancements improve the mission architecture robustness and provide more design margin for Mars vehicles across many mission opportunities, trip times, and mission types.
The LEU NTP design can offer mission architecture stages or elements that can be used for both Lunar and deep space exploration missions. The NTP designs enable packaging of various NTP stage designs (e.g., crew Mars vehicle stage elements, cargo stage derivatives, a deep space stage with payload, Lunar stage elements) on the NASA SLS Block 2 using the 8.4-meter fairing. The primary LEU core designs studied, for the above-mentioned missions, have relied on liquid hydrogen for the propellant and coolant and use Zirconium Hydride within a structural element as the neutron moderator. Several designs have been examined that use Beryllium Oxide with the fuel elements in the core to eliminate the structural elements.
The LEU NTP engine systems studied have typically been used only for the primary delta-V burns (e.g., earth escape, planetary capture, planetary escape, earth return capture). LEU NTP engine systems have also been examined using a the LEU reactor fuel element and moderator element approach to perform orbital maneuvering system (OMS) burns during the mission simply by permitting the reactor to keep operating at very low power levels during the entire mission.
This paper will discuss the various engine system and mission design trades performed in 2019 for Mars and lunar missions when using a single NTP or a cluster of NTP engines.
... The LEU NTP studies are focused on MTV concepts that use clusters of 25,000 pound thrust engine systems, with a specific impulse (ISP) around 900 seconds. 1 NASA EMC activity was initiated to get clarity for the many different approaches that could be used to put humans on Mars. EMC isn't directly proposing one particular path but seeks to identify how various technologies can be employed to meet the requirements of a long-term goal of getting humans to Mars. ...
Future human exploration missions to Mars are being studied by NASA and industry. Several approaches to the Mars mission are being examined that use various types of propulsion for the different phases of the mission. The choice and implementation of propulsion system options can significantly impact mission performance in terms of trip time, spacecraft mass, and mission abort capability for the crew. Understanding the trajectory requirements relative to the round-trip Earth to Mars mission opportunities in the 2030’s, and beyond, that set the design of the Mars crew vehicle is important in order to determine the impact of key propulsion choices. Additionally some propulsion and propellant choices for the crew vehicle can enable mission abort trajectories while others will not.
Nuclear Thermal propulsion (NTP) with 900 seconds and higher specific impulse (Isp) can permit the capability to abort back to Earth at several opportunities during the outbound or return trajectory that regular low Isp chemical propulsion cannot. Abort scenarios after the Mars crew vehicle has been injected along the path to Mars have been studied as well as timing of fly-by aborts to quickly return crew to Earth.
These trajectory studies are based off missions NASA defined during the Evolvable Mars Campaign (EMC) with crew going to Mars in 2033, 2037, 2043 and 2048. Detailed trajectory analysis was performed with the NASA Copernicus program for the several crew missions that were in the EMC. The goal was to determine how the heliocentric trajectory elements change and determine the “abort trajectory” impulse requirements. The NTP Mars crew vehicles studied had propellant loads sized for either Trans-Mars Injection (TMI), Mars Orbit Capture (MOC), Trans-Earth Injection (TEI), and Earth Orbit Capture (EOC) or just TMI and MOC with TEI and EOC pre-positioned as was done in the EMC chemical crew vehicle architecture.
The impulse requirements were then compared to the remaining impulse capability after TMI for the architecture’s remaining propellant load for the baseline 2033, 2037, 2043, and 2048 crew vehicle concepts. Abort scenarios that were studied included fast returns some number of days after TMI as well as fly-by aborts using available propellants (e.g., main propulsion system (MPS) and reaction control system (RCS)). The NTP system in this study shows great promise for adding mission flexibility via abort trajectories to Mars and at Mars.
This paper describes the current research and development effort s currently underway within the United States on Nuclear Thermal Propulsion (NTP), with a particular focus on the Demonstration Rocket for Agile Cislunar Operations (DRACO) project, a joint effort of the United States Defense Advanced Projects Agency and the National Aeronautics and Space Administration. However, to put the DRACO project into context, the prior United States’ prior effort s on NTR are described and the foundation those efforts pro- vided to enable DRACO. The impact of NTP propulsion on both human and scientific exploration of the Solar System will also be discussed. And finally, the topic of advanced NTP propulsion will be addressed, including liquid fuel NTP engines.
View Video Presentation: https://doi.org/10.2514/6.2022-4373.vid Aerojet Rocketdyne (AR) has long had a vision for providing propulsion that permits exploration and extensive travel capability across the Solar System. AR’s history and current efforts include providing propulsion and power for NASA’s far-reaching exploration goals and science missions. These include propulsion for the return to the moon with the Space Launch System (SLS), battery and power systems for the International Space Station (ISS), propulsion for Mars landers, and power for Mars rovers (e.g., Perseverance) and propulsion to power deep space missions like New Horizon. AR has continued the race in developing advanced propulsion systems that help the USA and NASA advance their goal of getting humans to Mars by working on a High-Assay Low Enriched Uranium (HALEU) Nuclear Thermal Propulsion (NTP) system. Current mission studies are focused on Mars missions in the late 2030’s and beyond. NTP engine requirements (e.g., thrust size, Isp) have been connected to those current studies that have been on-going since 2019. Those studies have shown a wide range of thrust sizes (e.g., 12,500 to 25,000-lbf) which can close the architecture and vehicle design using a nuclear fuel material that is capable of high temperature (e.g., peak temperatures between 2,800 to 3,000-deg K) operation to achieve a specific impulse (Isp) at or above 900 seconds. Although Mars missions dominate where NTP shows high pay-off, cis-lunar missions where NTP can support faster 2-3 day missions and more cargo mass also show payoff versus typical chemical propulsion systems. The HALEU NTP designs continue to use hydrogen propellant as the coolant and can use either a Ceramic-Metallic, Ceramic-Ceramic, or Carbide based fuel. The new approach in 2021 for the NTP fuel arrangement utilizes discrete cylindrical assemblies in the moderator block. The grouping is optimized for achieving criticality, which produces the required power level but is similar to the previous approach of using hexagonal (prismatic form) fuel elements with hydrogen flowing in channels within the fuel assembly. Current work has extended the fuel and core designs beyond NTP fuels initially analyzed between 2017 to 2020 and have started looking at other fuels with Carbide material approaches since 2021. Engine design trades are still on going to identify the optimum core/engine system operating characteristics. The NTP design trades that are continuing rely heavily on thermodynamic cycle modeling that includes the neutronic design attributes of the fuel and how it operates within a reactor core design. This paper presents a discussion on the methods for NTP modeling of the engine system for both steady state and transient operation, examining start, shutdown and post-cool down operations. In addition to steady state and transient modeling architectures, the implications of capturing component design influences on the NTP design is discussed.
A flight demonstrator (FD) High-assay Low Enriched Uranium (HALEU) Nuclear Thermal Propulsion (NTP) system that can advance the technology for a Mars crew transportation system extensively studied in late 2019 and early 2020. This demonstrator development activity would use a nuclear fuel material that is capable of high temperature (e.g., peak temperatures between 2,800 to 3,000-deg K) operation to achieve specific impulse (Isp) at or above 900 seconds. This flight demonstrator NTP would likely use hydrogen propellant as the coolant and use either a Ceramic-Metallic (Cermet), Ceramic-Ceramic (CerCer), or Carbide based fuel in a moderator block design versus moderator elements as used in Rover/NERVA. The NTP demonstrator would employ as much off-the-shelf liquid rocket hardware as determined possible to save cost during the development. The demonstrator nuclear fuel, components, cryo-fluid management (CFM) systems, control systems, and stage integration approaches will have direct lineage to the full-scale development (FSD) NTP system that crewed Mars NTP missions would employ. A flight or ground demonstrator is envisioned that the would be sized between 12,000-lbf and 15,000-lbf thrust and use a fuel assembly that would be used on the larger, 25,000-lbf class, Mars crew NTP. The capability to achieve a NTP demonstrator within the next 6 to 8 years will depend on obtaining adequate NTP data that can reduce the risk of an operational fission reactor system operating within orbital proximity of the Earth. AR has stayed engaged in working nuclear space systems and has worked with NASA recently to perform an extensive study on using LEU NTP engine systems for a demonstrator system. The NTP demonstrator must have a direct connection to a full-size NTP development for future Mars crewed vehicle missions. AR NTP (flight-like) demonstrator studies in 2019 and 2020 have defined several low thrust LEU NTP designs (i.e., less than 25,000-lbf full size Mars NTP) that would reduce the technical risk for the larger thrust NTP when ground tested or flown as a demonstrator stage. This paper presents a discussion on an approach for testing NTP (flight) demonstrator hardware before any larger scale NTP testing or a flight test. The approach builds on advancements in analysis that, when coupled with Digital Engineering methods, will provide a shorter path for reducing NTP engine system risk. This approach uses non-nuclear testing of rocket system components in parallel to reactor development and uses the tested components in cold-flow testing with a reactor sub-system before the NTP engine is an integrated flight system.
Future human exploration missions to Mars are being studied by NASA and industry.
Several approaches to the Mars mission are being examined that use various
types of propulsion for the different phases of the mission. The choice and
implementation of certain propulsion systems can significantly impact mission
performance in terms of trip time, spacecraft mass, and especially mission abort
capability. Understanding the trajectory requirements relative to the round-trip
Earth to Mars mission opportunities in the 2030’s and beyond is important in
order to determine the impact of trajectory abort capability. Additionally, some
propulsion choices for the crew vehicle can enable mission abort trajectories
while others will most likely provide less flexibility and increase mission risk.
This paper focuses on recent modeling of Earth to Mars abort scenarios for human
missions to determine the capability to provide fast returns to Earth. The
modeling assumed that the abort would occur after the Mars crew vehicle has
been injected along the path to Mars (i.e., after the Trans Mars Injection (TMI)
burn). These aborts have been defined as well as the timing of fly-by aborts to
quickly return crew to Earth.
These abort trajectory studies are based on missions NASA defined during the
Evolvable Mars Campaign (EMC) with crew going to Mars in 2033, 2039, 2043
and 2048. Detailed trajectory analysis was performed with the NASA Copernicus
program for the several crew missions that were in the EMC as well as other
new missions being considered using finite-burn low thrust electric propulsion.
The goal was to determine how the heliocentric trajectory elements change and
the “abort trajectory” impulse requirements.
Abort scenarios that were studied included fast returns N-days after TMI as well
as fly-by aborts and multiple revolution cases, using all available propellants
(e.g., main propulsion system and reaction control system (RCS)) to provide the
required abort velocity change. Trajectories were investigated for impulsive maneuvers
and for finite burn cases and the abort timelines for each are examined
and compared.
This paper and presentation will focus on the Copernicus trajectory analysis results
that were performed to determine the abort trajectories that altered the primary
mission to return to Earth as soon as possible.
This paper is concerned with the possibility of implementing Low-Enriched Uranium (LEU) fuels in NERVA (Nuclear Engine for Rocket Vehicle Application) type Nuclear Thermal Rockets (NTR). It presents a detailed study of the requirements for implementing LEU in a NERVA type NTR revolving around the softening of the neutron spectrum. This work has been done to demonstrate the possibility to enhance the proliferation resistance of these nuclear space systems in the hope of fostering the groundwork for their eventual commercial use. The necessity of thermalizing the neutron spectrum is discussed along with a detailed study of various methods by which this can be achieved. The effect of minimizing non fission neutron loss through the selection of appropriate structural materials is then presented along with a cursory study of various options. Finally a reference conceptual reactor configuration is proposed and compared with previous NTR designs where its relative shortcomings and obvious advantages are presented and discussed. It is concluded that an LEU-NTR is not only a definite possibility, but also offers distinct advantages over existing Highly-Enriched Uranium (HEU) NTR designs, particularly the increased proliferation resistance of the system and the reduction of the fuel cost and development schedule. Finally, some suggestions are made for future work that would enable the progression of the current conceptual design to a more finalized reactor design.