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Proposed engine is faster.

An advanced nuclear rocket engine could cut the travel time for a manned mission to Mars to 90 days, or about one third the time of previous proposals, according to a Sandia National Laboratories (Albuquerque, N.M.) study. Other benefits of such an engine would be a reduction in spacecraft mass and launch costs. Additionally, reducing travel time would lessen the adverse physical and psychological effects of prolonged space time on the crew.

The study recommends pursuing improvements in tank weight, nuclear engine thrust-to-weight ratio, and specific impulse-a performance parameter of rocket propellant that measures the thrust-producing energy content of the propellant; the higher the specific impulse, the less propellant mass is needed to perform the mission.

"We really feel it is important to get the crew out there in much less than 260 days to prevent levels of physical and mental deterioration that could jeopardize their ability to function on Mars and carry out the mission," said Leonard Connell, a Sandia scientist. The improvements also satisfy some of the primary recommendations made by the Synthesis Group, a committee appointed by President Bush in 1990 to study ways of achieving human exploration of the solar system.

Use of a high-thrust-to-weight-ratio particle bed reactor (PBR), high-temperature nuclear core materials, and lightweight graphite-composite propellant tanks could reduce vehicle mass by more than 300 tons and launch costs by nearly $4 billion per mission, compared to an earlier nuclear propulsion concept. A baseline spacecraft using the older Nuclear Engine for Rocket Vehicle Applications (NERVA) technology and conventional aluminum-alloy propellant tanks was used for making mass comparisons.

In the PBR concept, the fuel is formed into 500-micron-diameter particles, which increases the heat transfer and allows a more compact lightweight engine. The PBR's thrust-to-weight ratio is estimated to be nearly five times that of NERVA, which translates into a PBR engine mass for the Mars mission of 2 tons, compared to 10 tons for NERVA.

Sandia researchers see a two-stage mission to Mars. First, cargo and supplies would be sent via an energy-efficient trajectory that would take about 260 days to reach Mars. The crew would travel in a sprint vehicle, reaching Mars in about 90 days. At that point the crew vehicle would rendezvous the cargo vehicle.

The total mass of the NERVA baseline crew vehicle would be 640 tons, 90 percent of which is represented by the tanks and propellant. Although tank weight is usually about 15 percent of propellant weight for standard aluminum-alloy tanks, Sandia calculated that graphite-composite tanks could reduce this to 8 percent, resulting in a mass reduction of 130 tons.

Using a PBR, with its high thrust-to-weight ratio, could mean an additional savings of 85 tons. Assuming that two engines may be required, one for backup, the mass savings could amount to 150 tons.

By improving all three areas--tank weight, nuclear engine thrust-to-weight ratio, and specific impulse-total spacecraft mass could be reduced from 640 to 300 tons.

Sandia also said that a heat shield on nuclear rocket engines could prevent the release of nuclear and other potentially hazardous materials from accidentaly entering the earth's atmosphere.

A nuclear thermal rocket engine uses a type of nuclear propulsion technology that heats propellant using the energy produced by fission. As the propellant, typically liquid hydrogen, expands through a conventional nozzle, it provides the thrust that accelerates the spacecraft.

Unprotected, solid-core nuclear thermal rockets would fail upon entering the earth's atmosphere, Sandia researchers said. The aluminum pressure vessel housing the nuclear core would melt, releasing and exposing the nuclear core materials to the severe entry environment.

Although heat shield specifications would depend on the type of engine used, the study found that a graphite-composite shield less than 3 centimeters thick could enable an aluminum pressure vessel to maintain its integrity for intact entry. Total mass of the shield would be approximately 375 kilograms.
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Title Annotation:Sandia National Laboratories study finds improved rocket engine would aid flight to Mars
Author:Falcioni, John G.
Publication:Mechanical Engineering-CIME
Date:Apr 1, 1992
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