John Gyekenyesi

Radioisotope Electric Propulsion Centaur Orbiter Spacecraft Design Overview

Radioisotope electric propulsion (REP) has been shown in past studies to enable missions to outerplanetary bodies including the orbiting of Centaur asteroids. Key to the feasibility for REP missions are long life, low power electric propulsion (EP) devices, low mass radioisotope power systems (RPS) and light spacecraft (S/C) components. In order to determine what are the key parameters for EP devices to perform these REP missions a design study was completed to design an REP S/C to orbit a Centaur in a New Frontiers cost cap. The design shows that an orbiter using several long lived (approximately 200 kg Xenon throughput), low power (approximately 700 W) Hall thrusters teamed with six (150 W each) Advanced Stirling Radioisotope Generators (ASRG) can deliver 60 kg of science instruments to a Centaur in 10 yr within the New Frontiers cost cap. Optimal specific impulses for the Hall thrusters were found to be around 2000 sec with thruster efficiencies over 40%. Not only can the REP S/C enable orbiting a Centaur (when compared to an all chemical mission only capable of flybys) but the additional power from the REP system can be reused to enhance science and simplify communications.

Solar vs. Fission Surface Power for Mars

A multi-discipline team of experts from the National Aeronautics and Space Administration (NASA) developed Mars surface power system point design solutions for two conceptual missions. The primary goal of this study was to compare the relative merits of solar- versus fission-powered versions of each surface mission. First, the team compared three different solar power options against a fission power system concept for a sub-scale, uncrewed demonstration mission. The 4.5 meter (m) diameter pathfinder landers primary mission would be to demonstrate Mars entry, descent, and landing techniques. Once on the Martian surface, the landers In Situ Resource Utilization (ISRU) payload would demonstrate liquid oxygen propellant production using atmospheric resources. For the purpose of this exercise, location was assumed to be at the Martian equator. The three solar concepts considered included a system that only operated during daylight hours (at roughly half the daily propellant production rate of a round-the-clock fission design), a battery-augmented system that operated through the night (matching the fission concepts propellant production rate), and a system that operated only during daylight, but at a higher rate (again, matching the fission concepts propellant production rate). Including 30% mass growth allowance, total payload masses for the three solar concepts ranged from 1,116 to 2,396 kg, versus the 2,686 kg fission power scheme. However, solar power masses are expected to approach or exceed the fission payload mass at landing sites further from the equator, making landing site selection a key driver in the final power system decision. The team also noted that detailed reliability analysis should be performed on daytime-only solar power schemes to assess potential issues with frequent ISRU system on/off cycling. Next, the team developed a solar-powered point design solution for a conceptual four-crew, 500-day surface mission consisting of up to four landers per crewed expedition mission. Unlike the demonstration mission, a lengthy power outage due to the global dust storms that are known to occur on Mars would pose a safety hazard to a crewed mission. A similar fission versus solar power trade study performed by NASA in 2007 concluded that fission power was more reliable-with a much lower mass penalty-than solar power for this application. However, recent advances in solar cell and energy storage technologies and changes in operational assumptions prompted NASA to revisit the analysis. For the purpose of this exercise a particular landing site at Jezero Crater, located at 18o north latitude, was assumed. A fission power system consisting of four each 10 kW Kilopower fission reactors was compared to a distributed network of Orion-derived Ultraflex solar arrays and Lithium ion batteries mounted on every lander. The team found that a solar power system mass of about 9,800 kg would provide the 22 kilowatts (kW) keep-alive power needed to survive a dust storm lasting up to 120-days at average optical depth of 5, and 35 kW peak power for normal operations under clear skies. Although this is less than half the mass estimated during the 2007 work (which assumed latitudes up to 30o) it is still more than the 7,000 kg mass of the fission system which provides full power regardless of dust storm conditions.

Human Exploration Using Real-Time Robotic Operations (HERRO)- Crew Telerobotic Control Vehicle (CTCV) Design

The HERRO concept allows real time investigation of planets and small bodies by sending astronauts to orbit these targets and telerobotically explore them using robotic systems. Several targets have been put forward by past studies including Mars, Venus, and near Earth asteroids. A conceptual design study was funded by the NASA Innovation Fund to explore what the HERRO concept and its vehicles would look like and what technological challenges need to be met. This design study chose Mars as the target destination. In this way the HERRO studies can define the endpoint design concepts for an all-up telerobotic exploration of the number one target of interest Mars. This endpoint design will serve to help planners define combined precursor telerobotics science missions and technology development flights. A suggested set of these technologies and demonstrator missions is shown in Appendix B. The HERRO concept includes a crewed telerobotics orbit vehicle as well three Truck rovers, each supporting two teleoperated geologist robots Rockhounds (each truck/Rockhounds set is landed using a commercially launched aeroshell landing system.) Options include a sample ascent system teamed with an orbital telerobotic sample rendezvous and return spacecraft (S/C) (yet to be designed). Each truck rover would be landed in a science location with the ability to traverse a 100 km diameter area, carrying the Rockhounds to 100 m diameter science areas for several week science activities. The truck is not only responsible for transporting the Rockhounds to science areas, but also for relaying telecontrol and high-res communications to/from the Rockhound and powering/heating the Rockhound during the non-science times (including night-time). The Rockhounds take the place of human geologists by providing an agile robotic platform with real-time telerobotics control to the Rockhound from the crew telerobotics orbiter. The designs of the Truck rovers and Rockhounds will be described in other publications. This document focuses on the CTCV design.

Mars Earth Return Vehicle (MERV) Propulsion Options

The COMPASS Team was tasked with the design of a Mars Sample Return Vehicle. The current Mars sample return mission is a joint National Aeronautics and Space Administration (NASA) and European Space Agency (ESA) mission, with ESA contributing the launch vehicle for the Mars Sample Return Vehicle. The COMPASS Team ran a series of design trades for this Mars sample return vehicle. Four design options were investigated: Chemical Return /solar electric propulsion (SEP) stage outbound, all-SEP, all chemical and chemical with aerobraking. The all-SEP and Chemical with aerobraking were deemed the best choices for comparison. SEP can eliminate both the Earth flyby and the aerobraking maneuver (both considered high risk by the Mars Sample Return Project) required by the chemical propulsion option but also require long low thrust spiral times. However this is offset somewhat by the chemical/aerobrake missions use of an Earth flyby and aerobraking which also take many months. Cost and risk analyses are used to further differentiate the all-SEP and Chemical/Aerobrake options.