Masters Theses

Date of Award

8-1994

Degree Type

Thesis

Degree Name

Master of Science

Major

Aerospace Engineering

Major Professor

Gary Flandro

Committee Members

Robert Roach, Roy Schulz

Abstract

With the growing economic, social, and political problems here on earth, it is becoming clearer that many of the answers to the questions concerning mankind's survival may lie in space. However, even with the numerous interplanetary probes that have been sent out to study the planets in search of these answers, their remain one known planet on the edge of the solar system awaiting reconnaissance: Pluto. Because of Pluto's great distance from earth, there has not been an opportunity to visit the tiny world, until now. Pluto is slowly moving away from the rest of the solar system and will soon become virtually unreachable for the next 200 years. As Pluto moves out, its methane atmosphere is expected to freeze and then collapse. Pluto is the only planet in our solar system where this phenomenon occurs. However, current space propulsion may not be sufficient for getting a spacecraft to Pluto for prolonged, intense, scientific study. Advanced propulsion concepts, such as beamed power propulsion, must be developed and used if practical and inexpensive access to the outer solar system, as well as other solar systems near our own, is to be possible.

Beamed power propulsion is a concept in which the reaction mass remains at one location. A beam, such as laser or microwave, is used in conjunction with a lens and lightsail to provide thrust to the spacecraft. This thrust results from the momentum transfer of the photons as they reflect off the sail. This concept overcomes the limitations of solar sailing which confines itself to the inner solar system where the sun's radiation is sufficient to supply the needed photons.

An ANSI C program was modified on Silicon Graphic workstations to model the three-dimensional interplanetary flight between earth and Pluto using laser-pushed lightsail propulsion. An optimization technique which minimizes the flight time for these missions was implemented. The analysis resulted in a two point boundary value problem which was solved using two iterative techniques: the method of Steepest Descent, and Newton's Method.

Analysis in this study determined that a 1 µm laser which has a power output of 940 MW could be used. Also, a total spacecraft mass constraint of 4700 kg was used to insure that current launch vehicle technology can be used to inject the spacecraft into low earth orbit. There, it will be coupled with the lightsail and injected into the heliocentric transfer between earth and Pluto. Minimum flight time considerations were used to determine the allowable approach velocity at Pluto where an orbital maneuvering system would then be used to inject the spacecraft into its proper orbit. Three types of orbital maneuvering systems were considered for these missions; chemical, nuclear thermal, and nuclear electric.

The basis for comparison in this study is the dry spacecraft mass on station, which gives indication of the scientific capability and reliability of the spacecraft, and the flight time to Pluto for the missions in this study and those under investigation by NASA.

Current mission options being studied by NASA concentrate on a Pluto flyby. This obviously limits the time frame for which scientific study can take place. Also, these missions are based on low mass spacecraft designs which may constrain the scientific capabilities of these missions. The Pluto VVS mission baselines an 83 kg dry spacecraft mass and a short 7 year flight time. However, the flyby altitude is 15,000 km and the spacecraft velocity is almost 19 km/s. The PF350 option uses a 316 kg dry spacecraft mass to increase scientific capability and has a 14 year flight time. Its flyby altitude is 400 km with a spacecraft velocity of almost 13 km/s. NASA also has a 'fast flyby' mission under consideration. This PFF option has a flight time of just 6-11 years and uses a dry spacecraft mass of only 69 kg. This mission has a flyby distance of 15,000 km with the spacecraft traveling at almost 13 km/s. NASA also has a rendezvous, or orbiter, mission being studied. This mission uses a dry spacecraft mass of about 100 kg. This mission also uses an OMS to slow the spacecraft from its 6-8 km/s velocity and inject it into Pluto orbit. The flight time for this mission is expected to be 18-22 years.

The missions in this study were all based on a net spacecraft mass of 1000 kg. This mass includes the scientific and operational payloads. The chemical OMS option, which has a launch date of 27 December 2000, resulted in a 1548.5 kg dry spacecraft mass and a flight time of 18.4 years. The nuclear thermal option, which also has a launch date of 27 December 2000, resulted in a dry spacecraft mass of 2165.4 kg and a flight time of 15.9 years. The nuclear electric option, with a launch date of 28 December 2003, resulted in a dry spacecraft mass of 2384.3 kg and a flight time of just 10.3 years. The laser- pushed lightsail was used to slow the spacecraft to between 5-12 km/s in the vicinity of Pluto where the OMS was able to inject the spacecraft into orbit. A final parking orbit of 100 km altitude was assumed for these missions.

This study clearly shows that use of beamed power propulsion greatly increases the mass capability, thus increasing the scientific capability, of a spacecraft. This increase in mass, however, is not at the cost of greatly increased flight times. The advantage of rendezvous over flyby missions is clear. The results of this study show that beamed power propulsion, if developed, can provide practical access to the far reaches of the solar system. Large mass missions, such as manned missions, to the outer solar system, as well as to neighboring solar systems, could become possible. It is the hope that this study encourages new enthusiasm in the study and development of advanced propulsion concepts, such as beamed power propulsion.

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