ASE 387P- Mission Analysis and Design
Final Project for Spring 1997
By Justin H. Carter
The following is a plan for an unmanned deep space mission to a distance of 550 Astronomical Units (AU) from the Sun and beyond. The spacecraft, once assembled in orbit, will speed out of the solar system and reach 550 AU in approximately 20 years. The spacecraft has several objectives. Its primary objective will be to search for solar gravitational lensing, believed to focus distant electromagnetic signals at around 550-800 AU from the sun. Gravitational lensing is the bending of light due to gravity. The sun's gravity bends, and at certain positions in space, refocuses distant incoming light. These positions of focused light and other electromagnetic energy due to the sun are known collectively as the solar foci. Gravitational lensing is a potentially revolutionary effect for astronomy. It enables the direct imaging of previously undetectable faint objects throughout the cosmos. In simple terms, a relatively small telescope at the solar foci could see the universe many orders of magnitude better than any other instrument astronomers use today. The spacecraft will also serve as one of two astronomical interferometry observatories. The other matching interferometer being located on or near Earth. A large baseline over 550 AU provided by this mission will also be revolutionary. Finally the spacecraft will fulfill most of the JPL's proposed TAU (Thousand Astronomical Unit) mission objectives. For details on these all of the above objectives see History and Background Information.
Investigate the Sun's gravitational lens, or solar foci, provide a deep space astronomical observatory, complete most of the TAU mission objectives, and provide proof of concept for many unflown technologies. TAU mission objectives include the investigation of the location and properties of the heliopause, the properties of the interstellar medium, and the detection of low energy cosmic rays. Also TAU was to determine the mass of the solar system, collect data to simulate the approach of a starship to another solar system, and determine solar distances by measuring parallaxes.
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The mission calls for a large (400 metric ton) spacecraft to be assembled in low Earth orbit (LEO). The primary option considered here is a spacecraft using nuclear electric propulsion (NEP) to make a slow (~180 day) Earth escape and a burp solid rocket to achieve an precise escape trajectory. The NEP option requires a large degree of radiation hardening of the spacecraft to protect it from the Van Allen Radiation belts. Once in a heliocentric orbit, the spacecraft will continue to use it's nuclear electric propulsion system to accelerate gradually over the period of several years. Then the spacecraft will cruise out of the solar system. Along its solar escape trajectory it will deploy its instruments and make a number of measurements. At around 550 AU (the mean distance from the Earth to the Sun, 1.49597870e11 m) it will begin searching for solar foci events. The events manifest themselves as a peak in observed intensity of distant objects. A secondary option is to use nuclear thermal propulsion to achieve Earth escape. This would increase the size of the spacecraft, but allow a rapid Earth escape and a proof of concept for nuclear thermal propulsion. Gravitational assists could provide a cheap method to change the orbital plane of the spacecraft if orbital plane change was desirable. However only the nuclear electric mission within the solar ecliptic plane is the option considered here.
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Nuclear propulsion and power is highly controversial. This mission requires nuclear reactors in space. Nuclear electric propulsion must be politically permissible to accomplish this mission. The distance and time requirements for this mission make nuclear electric propulsion the only near term option. The permissibility of nuclear thermal propulsion would enable the first flight of this provocative technology. In addition to political requirements, the spacecraft is large and must be assembled in orbit. Therefore an orbital assembly station may be necessary for the mission. Also a heavy launch vehicle will be required to put each of the large components into LEO. Energiya, Saturn V, or large launch vehicles will be required. The uncertainty of the exact location of the solar gravitational lens makes that portion of the mission more of a discovery segment. Also the trajectory itself is a first order analysis that does not take into account perturbed motion.
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Knowledge of the TAU mission, gravitational lensing, and interferometry are necessary to understand this mission. Also nuclear thermal and nuclear electric propulsion are discussed. Finally the Energiya rocket, a choice launch vehicle for this mission, is discussed. Details begin with the TAU mission, seen in the picture below.
TAU Interstellar Precursor Probe
The TAU mission was planned at JPL in the late 1970's. It is a an automated interstellar precursor mission. It was planned as a 20 to 50 year mission to return data from a distance of 400 to 1,000 AU. It is not a science fiction concept, and with the proper financial support it could be launched in a decade. By journeying out far from the Sun, the properties and location of the heliopause (a tenuous boundary between the solar system and interstellar space) can be investigated. The interstellar medium including gases, dust, magnetic fields, and cosmic radiation would be investigated with the TAU mission. Knowledge of the interstellar medium could identify potential flight hazards and environmental fuel sources for interstellar travel. Low energy cosmic rays could be seen (they are currently unobservable through the heliopause). Also by doing a trajectory analysis of the spacecraft's trip, the reverse, and approach to a solar system, could be simulated. The TAU objective are important, however they have not yet won funding for such a mission. However there are additional reasons to send a spacecraft outside the solar system that were not considered in the TAU plan. An important one is gravitational lensing.
The Gravitational Lensing Effect
Gravity Lensing is an effect first theorized by Einstein. It is the apparent bending of light. Due to space/time curvature around a large mass, light does no travel in straight lines. This effect has been proven, and has been used in astronomy since the late 1970's. This mission takes advantage of the fact that the Sun's mass significantly bends light. Light from a distant source is recombined at a single "observer's" position, thereby increasing the observed intensity of the source. In a three dimensional case an "Einstein Ring" can be formed. With a ideal point mass light from the source is infinitely intensified at the observer's position. The sun approximates a point mass. Its lens is thought to begin focusing at about 550 AU. The suns focus is known as the "solar foci". An observatory at the solar foci would have its capabilities greatly enhanced. A modest telescope could see continents on a earth sized worlds that could possibly be circling around the nearest star systems. If an observatory could be positioned correctly in a large number of locations around the sun it could see almost any planet in the galaxy. Due to the current uncertainty of the distance of the solar foci, the lensing portion of the mission is more of a discovery segment. The interferometry segment of the mission may have greater certainty.
Interferometry is a direct imaging technique that uses electromagnetic phase information to create an image. Multiple, widely spaced telescopes are used for astronomical interferometry purposes. A larger lens or dish is simulated by digitally combining the multiple observations. Each telescope is separated by a distance called the "baseline". The longer the baseline the better the resolution. This mission would have a baseline from the Earth to 550 AU and greater. This is considerably longer than any baseline achievable on Earth, or within the solar system. This mission would provide a platform for a baseline of revolutionary length. The spacecraft's counterpart interferometer would be located on or near Earth.
A nuclear thermal rocket is powered by fission. Current designs and prototypes have a solid core of nuclear material. They have been built and tested since the 1950's, but have not been flown. The rocket is a compact nuclear reactor that propellant is passed through and heated. These rockets have a much better specific impulse than chemical rockets. Nuclear rockets have an Isp of 500 to 1,100 seconds, while viable chemical rockets are at best 450 seconds. Like chemical rockets nuclear thermal rockets have a high thrust to weight ratio.
An ion electric rocket engine
Nuclear Electric Propulsion uses a nuclear reactor to produce large amounts of electrical power. The electrical power is used to run low thrust electric propulsion rocket engines. There are a wide variety of electric propulsion thrusters. They include ion thrusters like the one shown above, Hall thrusters, arcjets, resistojets, pulsed plasma thrusters (PPT), and magneto plasma dynamic (MPD) thrusters. An electric propulsion thruster generally accelerates ionized fuel to high velocity (tens of thousands of m/s). Their high exhaust velocity translates into a high fuel efficiency, or specific impulse (Isp). For this mission very high specific impulses are required. An Isp of around 10,000 sec is desirable. Therefore ion and MPD thrusters, each with Isp potentials in this range, are ideal. On this mission, the electric thrusters would fire constantly for a duration of several years. Electric thrusters are rated for time periods on this order. However backup thrusters would be included as there weight is relatively small and some burnouts are expected. MPD thrusters are relatively untested and may require a larger number of backups than the ion thruster option. Extensive development of ion thrusters is currently underway. Ion thrusters are an established form of electric propulsion while MPD thrusters are under laboratory investigation.
A Russian Energiya rocket launching with a cargo pod
The Russian Energiya rocket is the heaviest lift launch vehicle in the world. It can put a 100 metric ton payload into low Earth orbit. That is more lift capacity than America's largest space vehicle ever, the Saturn V, which has been out of production for a number of years. The Energiya is a two stage reusable launch vehicle originally designed to carry the Soviet space shuttle Buran. However, as seen in the picture above, the Energiya is independent of the Buran and can be launched with just a cargo pod. For this mission several Energiya cargo pod launches would be required.
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Initially the mission was analyzed with conventional Hohmann transfers and impulsive fast transfers. It was discovered that a Hohmann transfer to 550 AU would take over 2 millennia. Fast transfers required a tremendous delta-V to accomplish the mission in the lifetime of the mission initiators. Even highly efficient nuclear thermal impulse propulsion was not adequate to generate a delta-V from LEO of around 100,000 m/s. The remaining near term solution was nuclear electric propulsion. This solution is supported by the fact that JPL's TAU mission with similar mission profiles also used nuclear electric propulsion.
Spacecraft Components and Sizing
The payload itself must operate at a distance of over seventy light hours from Earth. Seventy light hours is a significant communications delay. Therefore the spacecraft for this mission must be highly autonomous, capable of handling its own day to day operations. High bandwidth radio communication is difficult from this range and a significant amount of power may be necessary for communication. Tight beam laser communications were proposed for the TAU mission and may suit this mission better than using radio as a primary communication device. Of great importance to the mission was the scientific instruments the spacecraft will carry. This mission is a space astronomical observatory and will carry several astronomy instruments for a wide variety of purposes. It will most likely carry a combination of radio, infrared, and optical telescopes. A radio telescope is a simple, reliable, general purpose device. At the solar foci it could act as a powerful observatory and SETI (Search for Extraterrestrial Life) instrument. Infrared and optical telescopes would allow a broad band of wavelengths to be investigated. Optical telescopes may be especially useful in direct imaging of planets. It could provide spectroscopic analysis of distant planets' atmosphere. The detection of certain ratios of chemicals in a planet's atmosphere could give significant evidence of life on other planets. The mass of the Hubble space telescope (11.2 metric tons) as an estimate for the mass of the payload for this mission. Total payload was estimated at 15 metric tons. It is important to carry a large number of instruments, as this mission's duration is very long and should be done correctly the first time.
Next the nuclear reactor needed to be sized. It's size is primarily driven by the electric propulsion requirements. The reactor will use thermoionic or thermoelectric heat to electricity converters. These heat conversion methods have poor efficiencies, but have no moving parts and are highly reliable, a requirement for this long duration mission. The poor efficiencies of power conversion will require large radiators to dispense of unwanted heat. Initial analysis of the power requirements are approximately 5 MWe, or 5 megawatts electricity, with a total life output of 10 years at full power. Once the initial multi year boost is complete, the reactor would lower power production to preserve nuclear fuel. The reactor will generate a large amount of radiation. Even though the spacecraft will be radiation hardened so that its journey through the Van Allen radiation belts will not be damaging, protection from the long term effects of the reactor is necessary. The simplest method of protection is the separation of the payload from the reactor. This will be done with the layout of the spacecraft. Assembled in 4 cylindrical sections, like a stacked rocket, the payload will be the front nose of the spacecraft. The reactor will be at the tail of the spacecraft, and the fuel will be in between, serving as additional protection during the most radioactive boost phase of the mission. A shadow radiation shield will also be used to intercept some of the radiation from the reactor traveling in the direction of the spacecraft. This radiation shielding is quite heavy, so it will be kept to a minimum. For trajectory analysis ion engines were considered. These engines are relatively small and light. An additional set of these engines will most likely be used for attitude. This will save on the amount of attitude propellant required for the mission, which can be a significant portion of final spacecraft mass in any mission. Internal momentum wheels could be used for stabilization. The mass of the burp solid rocket booster providing a delta-V of 10 m/s will be approximately 14 metric tons. Approximately 300 metric tons of fuel for the electric thrusters would be required for a total delta-V of 130,000 m/s (includes Earth escape and attitude fuel).
This spacecraft follows a constant thrust trajectory the first several years of the mission. However a more simplistic TK solver fast transfer model was used to simulate the entire trajectory. Perturbations were neglected for this analysis, and motion was entirely in the plane of the ecliptic. The TK model gives a flight time of around 20 years for a delta-V of 120,000 m/s . The spacecraft begins in LEO at an altitude of 160 km. The electric thrusters constantly fire for months to escape Earth. Nearing Earth escape, a 10 m/s delta-V solid rocket burp thruster fires to ensure correct departure angle. In heliocentric space the electric thruster continues to fire for years. Once most of the delta-V is spent, the spacecraft spends the following years cruising out of the solar system and making scientific measurements. As long as the power and electronics continue to function it can collect data far out into interstellar space.
Click here to see a similar heliocentric Trajectory
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The above picture is from the Apollo 11 moon mission. Earth is a blue and white ball in the distance over the moon's cratered landscape. Imagine another picture, much like the one above, except without the moon in the foreground and with a blue and white planet slightly different than Earth. It is an alien world not so different than the planet we live on. The image is not science fiction or imagination. It is an actual scientific photo of another planet in our galaxy. The space mission described in this report could possibly generate such and image. Such and image could act as inspiration to explore near space and the infinity beyond our solar system. A space mission like this would revolutionize astronomy and reveal countless secrets of the cosmos. This is a near term mission that could be launched in around ten years. It is a mission with potentially fantastic returns. This probe could locate new worlds with alien life and also serve as the first step in traveling to those worlds. The mission, if successful, could revolutionize the field of astronomy, change our world view, and answer the question "Are we alone?" The cost is modest compared to the potential benefits. For a few billion dollars, a small percentage of the federal government's annual budget, we could change the world.
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to the Outer Solar System and Beyond
Thousand Astronomical Unit Probe
Canonical List of Space Transportation Methods
Antiproton-Catalyzed Microfission/Fusion Prop. for Exploration of the Outer Solar System and Beyond
Overview of Electric Propulsion Devices and Power Systems
Sagan, Carl, Pale Blue Dot, Random House, New York, 1994.
Mallove, E. and Matloff, G. The Starflight Handbook, John Wiley and Sons, Inc., New York, 1989.
Curtis, A. Space Almanac, Gulf Publishing Co., Houston, 1992.
C. Maccone, Mission Design to Get to the Solar Foci, IAF-94-A.6.054, 45th Congress of the International Astronautical Federation, Oct. 9-14, 1994/Jerusalem, Israel.
Griffin, M.D.French, J.R., Space Vehicle Design, AIAA, Washington, D.C., 1991.
Larson, W.J. and Wertz, J.R., Space Mission Analysis and Design, Microcosm Inc. and Kluwer Academic Publishers, Boston, MA, 1992.
Nuclear Propulsion Technical Interchange Meeting Proceedings, NASA, Sandusky, OH, 1992.
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