Low Thrust Missions

The paper investigates the utility of how nuclear thermal rocket (NTR) upper stage propulsion could substantially reduce the interplanetary transit time while greatly enhancing the economics of performing the manned Mars mission. au ECCV GTO IMLEO Isp kg kg/kw km km/sec kw LEO MEV mw NTR sec !N Nomenclature astronomical unit Earth capture crew vehicle geosynchronous transfer orbit initial mass in low Earth orbit specific impulse kilogram kilograms per kilowatt kilometer kilometers per second kilowatt low Earth orbit Mars excursion vehicle megawatt nuclear thermal rocket seconds delta V 1. Background Initial responses to the launch cost issue have been to suppose the development of heavy lift launch vehicles that are assumed to have fundamental launch cost savings. In general, most studies have isolated launch vehicle systems from the interplanetary vehicle assembled in low Earth orbit. This paper investigates a potential synergism between applying nuclear thermal rockets as both upper stages of the launch vehicle and as departure propulsion for a Mars vehicle departure stage. II. InterPlanetary Vehicle Description The mission to Mars is a propulsion technology dominated mission. The velocity change requirements (llV or delta V) are very large and very sensitive to trip time constraints. Due to this, there has been a heavy emphasis on high specific impulse (Isp) propulsion systems for performing the manned missions to Mars. The fundamental problem of high Isp systems is that they are generally thermally limited by material temperature limits. This issue is eliminated by electric propulsion systems that use electro-magnetic forces for accelerating a working fluid, but are thrust limited due to practical limitations in power supply mass. More exotic propulsion systems tend to use plasma working fluids that can only be contained by such methods as magnetic nozzles. The Space Exploration Initiative (SET) has a long range goal to perform a manned mission to Mars. Preliminary design goals of an interplanetary vehicle for this mission stress extremely high propulsion requirements to minimize transit times while allowing significant stay times on Mars. Related to this key trade between propulsion performance and trip time is the space transportation economics of 20 or more Earth launches required of the current launch vehicle architecture to place adequate vehicle mass in low Earth orbit to perform a single manned mission to Mars. The large initial mass in low Earth orbit (IMLEO) to perform the manned Mars mission is recognized as a dominant mission cost driver using the current launch infrastructure. IMLEO has been used as a preliminary figure of merit to evaluate various design approaches. * Senior Advanced Propulsion Engineer, Member AIAA Most analysis to date has focused on nuclear thermal rockets (Isp ... 1000 sec) and megawatt class electric propulsion systems (Isp ... 5000+ sec). The vehicle strategies have been to split the mission between a one way only cargo mission to Mars and a fast transfer manned vehicle. Mars rendezvous is essential for mission success, and techniques to slow at Earth return have used Earth capture crew vehicles (ECCV) with very high excess velocities (9+ km/sec) that are ballistically braked. Analyzed here is a very simple single vehicle. It uses an NTR driven, high thrust, Earth departure stage that is dropped once the vehicle has entered a fast Mars transfer ellipse. Once the vehicle has exited the deep Earth gravity well, it relies solely on a 5000 second Isp electric propulsion system for capture at Mars, departure from Mars, and final mission capture at Mars with zero excess velocity (a conservative assumption) The payload to Mars is broken into two elements. The ECCV is assumed to weigh 40,000 kg for a crew of four, and is the crew's interplanetary travel habitation module. The Mars excursion vehicle (MEV) is the element used to drop the crew from Mars orbit to the planet surface, be the crew's planetary habitation module, carry all science equipment, and carry the ascent module for returning to Mars orbit. The MEV has a baseline weight of 84,000 kg for a nominal 30 day planetary stay time and is scaled with the squareroot of normalized stay time. The MEV is not returned to Earth. Consumables are estimated at 10 kg per person per mission day. optimized for minimum l1V by adjusting the up leg perihelion and aphelion while maintaining the constraints of Earth rendezvous and travel time. Figure 2 (with the radius in millions of kilometers) shows a transfer ellipse for the up leg for the 320 day travel time, 80 day Mars stay time, 0 degree mean Mars true anomaly case. Figure 3 shows the down leg of that mission. The mission l1V requirement of 29.1 krn/sec is divided into 4.2 krn/sec for Earth departure (in heliocentric space), 6.0 krn/sec for Mars capture, 7.4 krn/sec for Mars departure, and 11.5 krn/sec for Earth capture. Transfer Ellipse Up. III. Interplanetary Mission Modeling The problem of analyzing the interplanetary mission can be driven by very exacting issues such as actual planetary configurations for particular mission opportunities, the requirements for travel time, and the Mars stay time. The issue that needs near term resolution, however, is the general efficacy of various propulsion and/or vehicle configuration options for the Mars mission in general. To answer these type of questions, it was necessary to generalize the Mars transfer problem. To do so, simplifying assumptions were that the Mars and Earth orbits were coplanar, and that the Earth orbit was circular. The nominal planetary configurations for Mars, Earth, and Venus are shown in Figure 1. The radius units are in astronomical units (Earth radius =1 au) and the angular units are the true anomaly of the orbital positions in degrees (Mars perihelion = 0 degrees). tojMJoC:: ~ •• ~: ••• . -

Fast sample return from the outer Solar System would open an entirely new avenue for space science, but the vast distances make this a daunting task. The achievable transit velocity and the need for extra propellant on the return trip limit the feasibility of returning extraterrestrial samples to Earth. To keep the mission duration short enough to be of interest, sample return from objects farther out in the Solar System requires increasingly higher velocities. High specific impulse, electric propulsion reduces the propellant required for the outbound and return trips, but decelerating the spacecraft at the inner Solar System from high velocity still involves a long, inward spiral trajectory. The use of solar sails to rapidly decelerate incoming sample capsules and eliminate propellant is explored in this paper. The sail is essentially a ''solar parachute'' used for braking at the end of the interplanetary return flight, permitting a higher transit speed and truncating the deceleration spiral. In this application the sail is relatively small and manageable since only the sample capsule and its sail are decelerated. A comparison is made between using all-electric propulsion versus combining electric propulsive acceleration with sail deceleration for sample return from the distances of Saturn, Uranus, and Pluto. Solar-sail braking dramatically reduces the return flight time by one-third or more compared to using electric rocket deceleration. To elucidate the technology requirements, wide ranges for both the loaded sail density and electric propulsion specific mass are considered in this initial parametric study.

At the initiative of S.P.Korolev, in 1959, Special Design Bureau No.1 (now RSC Energia) established the High-temperature Power Engineering and Electric Propulsion Center which was tasked with development of nuclear electric propulsion for heavy interplanetary vehicles. Selected as the source of electric power was a nuclear power unit based on a thermionic converter reactor, and selected as the engine was a stationary low-voltage magnetoplasmodynamic (MPD) high-power (0.5–1.0 MW) thruster which had thousands of hours of service life. The paper presents the results of extensive efforts in research, development, design, materials science experiments, and tests on the MPD-thruster, including the results of development and 500-hours life tests of an MPD-thruster with a 500-600 kW electric power input that used lithium propellant. The world’s first lithium 17 kW MPD-thruster was built and successfully tested in space. The paper points out that to this day nobody has surpassed the then achievements of RSC Energia neither in thruster output during long steady-state operation, nor in performance and service life. Key words: Martian expeditionary vehicle, nuclear electric rocket propulsion system, electric rocket thruster, magnetoplasmodynamic thruster, lithium, cathode, anode, barium, electric propulsion tests in space.

Accomplishment and prospect of isas electric propulsion

The advantages of using electric propulsion for propulsion are well‐known in the aerospace community. The high specific impulse and, therefore, lower propellant requirements make it a very attractive propulsion option for the Space Exploration Initiative (SEI). Recent studies have shown that nuclear electric propulsion (NEP) is not only attractive for the transport of cargo but that fast piloted missions to Mars are possible as well, with alphas on the order of 7.5 kg/kW. An advanced NEP system with a pecific power (alpha) of 2.5 kg/kW or less would significantly enhance the manned mission option of NEP by reducing the trip time even further. This paper describes an advanced system that combines the PEGASUS Drive with systems of the Rotating Multimegawatt Boiling Liquid Metal (RMBLR) power system that was developed as part of the DOE multimegawatt program and just recently declassified. In its original configuration, the PEGASUS Drive was a 10‐MWe propulsion system. The RMBLR was a 20‐MW electric power system. By combining the two, a second‐generation PEGASUS Drive can be developed with an alpha less than 2.5 kg/kW. This paper will address the technology advancements incorporated into the PEGASUS Drive, the analysis of a fast piloted mission and an unmanned cargo transport Mars mission, and the integration of laser power beaming to provide surface power.

The significant increase of available electric power for the propulsion unit onboard satellites makes high and ultra-high specific impulse thrusters attractive for different applications and missions. Especially thrusters of the radio frequency type show a great potentiality for high Isp applications. Two different rf-thrusters are described: The RIT15LP engine optimized for a moderate input power consumption and the RIT15S designed for a high specific impulse. The main, but only difference between these engines is the extraction grid system. During the development of RIT15LP, principle improvements of rfthrusters were invented. The given description of RIT15LP is focused on these performance improvements whereas the advantages of the rf-principle for (ultra-) high specific impulse applications were pointed out on basis of the RIT15S thruster. The function principle of gridded ion thrusters can be described as a two step process. In the first step the propellant is ionized and after that, in the second step it is accelerated in the electrostatic fields of an ion extraction and acceleration grid system. Both tasks a nearly independent from each other. The acceleration has no effect on the ionisation process and therefore the beam velocity can simply be controlled by the selection of the voltages applied to the grid system. Amoung different types of electric thrusters this is a unique feature of all gridded ion engines. In contrast, the use of other electric propulsion concepts requires often deep changes in the thruster design for an increased beam velocity. For example, it was necessary to invent a two stage concept for SPTs to reach a beam velocity comparable with that of gridded thrusters. Such efforts are not required in case of the later ones. Presently, the beam velocity of gridded ion thrusthers is merely limited by their electric input power consumption and not in a conceptional way. It is important to be aware that the specific input power consumption (the total consumption of electric power Ptot over the thrust F, usually given in>luWatt per Millinewton') of any kind of thruster is a simple function of the beam velocity Vbeam and the electric efficiency of the thrusters rjel:

A new extended spacing law governing midcourse correction points is given for electrically propelled interplanetary missions. The use of electric propulsion is inappropriate for guidance of flyby missions because little time is available to guide the spacecraft, especially when it is within close range of the target. Therefore, some chemical fuel is inevitably required onboard the spacecraft. The problem treated here is to determine how much chemical fuel is required to cancel out the potential electric and impulsive correction errors. A generalized spacing rule that determines the optimal correction points is derived. In addition, a simple analytical fuel estimate is presented. Numerical illustrations include a Monte Carlo simulation as well as the return flight from the asteroid back to Earth with rigorous ephemerides and time system, demonstrating the newly developed spacing rule. Nomenclature aep = acceleration by the electric propulsion c = coasting E[] = expectation operator Epsxf - specified terminal miss distance allowance j = time node k = correction epochs n = number of correction nodes or the number of time index S = total velocity increment accelerated by the electric propulsion T = flight period to the encounter T' = centroid time or end time of the coasting arc OL - flight time ratio A V = impulsive correction velocity increment A*/ = terminal miss distance S(A V) = impulsive correction mechanization error 8$ = electric correction mechanization error s = velocity navigation error (standard deviation)

The electron cyclotron resonance ion engine, “μ10,” has a long life and high reliability because of electrodeless plasma generation in both the ion generator and the neutralizer. Four μ10, each generating a thrust of 8 mN, specific impulse of 3,200 seconds, and consuming 350 W of electric power, propel the “HAYABUSA” asteroid explorer that was launched on May 2003. After vacuum exposure and several runs of baking to reduce residual gas, the ion engine system established continuous acceleration. Delta-V Earth Gravity Assist, a new orbit change scheme that uses electric propulsion with a high specific impulse was applied to change from a terrestrial orbit to an asteroid-based orbit. In 2005, HAYABUSA, using solar electric propulsion, managed to successfully cover the distance between 0.86 AU and 1.7 AU in the solar system, as well as rendezvous with, land on, and lift off from the asteroid Itokawa. During the 3-year flight, the ion engine system generated a delta-V of 1,400 m/s while consuming 22 kg of xenon propellant and operating for 25,900 hours.

The space nuclear electric propulsion (NEP) system is a revolutionary space propulsion technology that converts nuclear heat energy into electrical energy and drives high-power electric thrusters to generate thrust. Compared with traditional propulsion technology, NEP has the advantages of high specific impulse, high power and long life, which is very suitable for large-scale deep space exploration missions in the future. In this paper, the specific mass model of high power NEP system is established based on NEP system composition and the theory of small thrust orbit to obtain the maximum payload ratio. The complex coupling relation of orbit time, specific mass, power and payload ratio of NEP spacecraft can be decoupled by this model, which provides a computational basis for task optimization. The model was used to evaluate the technical specifications of the NEP system to complete the NASA Juno space mission. The calculation shows that when the specific mass of the NEP system reaches 4.8kg/kWe, it can the ground transfer time of the Juno space mission from 2266 days to 665 days, and the payload from 160kg to 1179kg, which greatly improves the spacecraft's detection capability and providing a useful reference for subsequent design.

Miniaturization perspectives of electrostatic propulsion for small spacecraft platforms

SummaryThis paper researches the optimal opportunities and trajectories for an asteroid‐Earth return mission and compares different options in terms of time length and reentry velocity. High specific impulse and steering capabilities of solar electric propulsion are exploited to perform deep space maneuvers. An indirect method based on the theory of optimal control is applied to find the trajectory that maximizes the final mass. The boundary value problem that arises from the application of the optimal control theory is solved by means of a shooting procedure based on Newton's method. In particular, the best opportunities to reach the Earth are first analyzed by searching a time‐free transfer that intercepts the Earth's orbit at the most favorable point; suitable launch windows are then determined. This technique shows significant benefit to define the initial solution that is required to start the optimization process so that no initial guess of adjoints is required for the optimization. Mission opportunities returning from asteroid Bennu (101955) are presented.

Solar sails have long been known to be an attractive method of propulsion in the inner solar system if the areal density of the overall spacecraft (S/C) could be reduced to ∼10 g/m2. It has also long been recognized that the figure (precise shape) of useful solar sails needs to be reasonably good, so that the reflected light goes mostly in the desired direction. If one could make large reflective surfaces with reasonable figure at an areal density of ∼10 g/m2, then several other attractive options emerge. One is to use such sails as solar concentrators for solar-electric propulsion. Current flight solar arrays have a specific output of ∼100W/kg at 1 Astronomical Unit (AU) from the sun, and near-term advances promise to significantly increase this figure. A S/C with an areal density of 10 g/m2 could accelerate up to 29 km/s per year as a solar sail at 1 AU. Using the same sail as a concentrator at 30 AU, the same spacecraft could have up to ∼45 W of electric power per kg of total S/C mass available for electric propulsion (EP). With an EP system that is 50% power-efficient, exhausting 10% of the initial S/C mass per year as propellant, the exhaust velocity is ∼119 km/s and the acceleration is ∼12 km/s per year. This hybrid thus opens attractive options for missions to the outer solar system, including sample-return missions. If solar-thermal propulsion were perfected, it would offer an attractive intermediate between solar sailing in the inner solar system and solar electric propulsion for the outer solar system. In the example above, both the solar sail and solar electric systems don't have a specific impulse that is near-optimal for the mission. Solar thermal propulsion, with an exhaust velocity of the order of 10 km/s, is better matched to many solar system exploration missions. This paper derives the basic relationships between these three propulsion options and gives examples of missions that might be enabled by such hybrids.

Electric thrusters generate high exhaust velocities and can achieve specific impulses in excess of 1000 s. The low thrust generation and high specific impulse make electric propulsion ideal for interplanetary missions, spacecraft station keeping, and orbit raising maneuvers. Consequently, these devices have been used on a variety of space missions including Deep Space 1, Dawn, and hundreds of commercial spacecraft in Earth orbit. In order to provide the required total impulses, thruster burn time can often exceed 10,000 hours, making thruster lifetime essential. One of the main life-limiting components on ion engines is the hollow cathode, which serves as the electron source for ionization of the xenon propellant gas. Reactive contaminants such as oxygen can modify the cathode surface morphology and degrade the electron emission properties. Hollow cathodes that operate with reactive impurities in the propellant will experience higher operating temperatures, which increase evaporation of the emission materials and reduce cathode life. A deeper understanding of the mechanisms initiating cathode failure will improve thruster operation, increase lifetime, and ultimately reduce cost. A significant amount of work has been done previously to understand the effects of oxygen poisoning on vacuum cathodes; however, the xenon plasma adds complexity, and its role during cathode poisoning is not completely understood. The work presented here represents the first attempt at understanding how oxygen impurities in the xenon discharge plasma alter the emitter surface and affect operation of a 4:1:1 BaO-CaO-Al2O3 hollow cathode. A combination of experimentation and modeling was used to investigate how oxygen impurities in the discharge plasma alter the emitter surface and reduce the electron emission capability. The experimental effort involved operating a 4:1:1 hollow cathode at various conditions with oxygen impurities in the xenon flow. Since direct measurements of the emitter surface state cannot be obtained because of the cathode geometry and high particles fluxes, measurements of the emitter temperature using a two-color pyrometer were used to determine the oxygen surface coverage and characterize the rate processes that occur during poisoning. A model describing the material transport in the plasma discharge was developed and is used to predict the barium and oxygen fluxes to the emitter surface during cathode operation by solving the species continuity and momentum equations. The dominant ionization process for molecular oxygen in the plasma gas is resonant charge exchange with xenon ions. Barium is effectively recycled in the plasma; however, BaO and O2 are not. The model shows that the oxygen flux to the surface is not diffusion limited. Experimental results indicate that the oxygen poisoning rate is slow and that the oxygen poisoning coverage on the emitter surface is less than 3%. A time-dependent model of the reaction kinetics of oxygen and barium at the tungsten surface was developed using the experimental results. The experiments and kinetics model indicate that the dominant processes at the emitter surface are dissociative adsorption of O2, sputtering of the O2 precursor, and desorption of O. Ion sputtering of the weakly bound O2 precursor state limits the poisoning rate and yields low oxygen coverage. Removal of chemisorbed atomic oxygen is dominated by thermal processes. Based on the low oxygen coverage and long poisoning transients, plasma cathodes appear to be able to withstand higher oxygen concentrations than vacuum cathodes.

Optimum sun-alignment of large solar arrays in electric propulsion spacecraft operating in earth orbit requires periodic roll motions around the thrust axis, synchronized with the apparent conical motion of the sun line. This oscillation is sustained effectively with the aid of gravity gradient torques while only a small share of the total torque is being contributed by the attitude control system. Tuning the system for resonance requires an appropriate choice of moment-of-inertia characteristics. To minimize atmospheric drag at low orbital altitudes the solar array is oriented parallel, or nearly parallel, to the flight direction. This can increase the thrust-to-drag ratio by as much as an order of magnitude. Coupled with optimal roll orientation, this feathering technique will permit use of electric propulsion effectively at low altitudes in support of space shuttle or space station activities and in spiral ascent missions.

Full electric propulsion system becomes popular recently and getting more common in the communication satellite industry. Full electric or hybrid propulsion selection causes results in significant reduction in satellite propellant mass. Conventional full chemical satellite propellant mass is roughly 2/3 of launch mass so this huge reduction in propellant mass changes satellite design and preferences. Depending on satellite size and launch vehicle performance, full electric satellite requires 3-4 times less propellant compared to full chemical satellite and 1-2 times less propellant compared to the hybrid system. Satellite launch mass can be reduced by 40% for full electric propulsion and 15% for hybrid propulsion hence launch cost can be decreased by 40% and 15 % with the benefit of reducing propellant mass. It is possible to extend satellite communication capacity average 30 transponders for full electric and average 10 transponders for a hybrid system by using the benefit of propellant mass reduction. It takes 4-8 months to reach geostationary orbit for full electric satellite but in a chemical propulsion satellite, it takes a few days. Satellite subjects to additional radiation due to this long trip and hence additional aging. Expected revenue loss is another issue and conventional insurance policy needs an amendment to be in line with electric propulsion technology. The development of full electric satellite lowers the cost per transponder significantly.