ALGOR FEA AND THE CASE OF THE MARTIAN SNAKE

This artist's conception depicts the daily mission profile of the Martian Snake probe. During the day it floats above the planet. At night, when falling temperatures reduce the balloon's buoyancy, it flies lower, dragging the Snake across the surface of Mars.

In 1996, a Russian Proton rocket will blast off from central Asia with a unique payload. Destination: Mars. Aboard this spacecraft will be a one-of-a-kind planetary probe that will allow scientists to cover a large area of the Martian surface, gathering a vast amount of data. The probe, carried by a helium balloon, is known as the Martian Snake.

Flight to Mars

MARS 94/96 is a multinational scientific mission to Mars which is being spearheaded by the Russian and French space agencies with significant participation by both Germany and the United States. The mission consists of two flights. The first, in 1994, will place a satellite in orbit around the planet, deploy a small weather station and drop a penetrator which will become embedded several meters into the Martian surface.

The second flight, in 1996, will carry a rover designed to study a limited area of the planet's surface, and the helium balloon probe which will cover a much larger area and gather data in a unique and ingenious manner.

Day and Night Missions

During the day, the balloon will fly high in the Martian sky (altitude 34 kilometers) using cameras and sensing instruments to study the planet's atmosphere and surface. At night, when Mars grows much colder, the balloon will lose some of its buoyancy, dropping low enough to allow a rope-like structure, known as the Snake or Guiderope, to drag along the Martian surface. As the ambient night winds blow the balloon around the planet, the Snake will take radar and gamma ray measurements of the soil, using instruments contained in its core.

The Martian Snake is being designed by the Space Dynamics Lab in Logan, Utah under contract to the French Space Agency and the Planetary Society of Pasadena, California. The primary mechanical engineer for the Snake is Mr. James Cantrell. His main responsibility is to see to it that the Snake is capable of withstanding the punishment of being dragged across the rocky surface of Mars, at speeds up to 35 km/hr (20 mph) and temperatures as low as 120^oC, without damaging the sensors.

James Cantrell, primary engineer for the Martian Snake probe, is shown here in the offices of the French Space Agency in Toulouse, France. He has since returned to the United States to finalize the Martian Snake design.

One Tough Snake

"The Snake is composed of 22 slightly conical segments connected to form a chain seven meters long," says Mr. Cantrell. "In addition to surviving the hellish environment of Mars, the Snake cannot weigh more than 6 kilograms. To achieve these goals, each segment is composed of a lightweight titanium shell, designed to withstand impacts with rocks and an internal tube, made of aluminum alloy, to retain overall structural integrity. A spherical titanium bearing articulation connects the segments. The outer and inner shells were, and will continue to be, the subject of critical finite element analyses using Algor software."

Recently, Mr. Cantrell completed a series of Algor finite element analyses in which he tested a variety of reinforcement methods in an effort to increase the Snake's mechanical strength without significantly adding to its weight. These methods included increasing the thickness of the outer shell, the use of intershell foam and the addition of reinforcing rings between the shells.

Analyzing the Snake

According to Mr. Cantrell, "Because of symmetry, I created a reference model of one-half of one segment of the snake. The inner and outer shells were constructed of plate elements and a point and distributed load was placed on the midpoint of the outer shell. The reference model was a simple internal and external tube. To create the first reinforced model, I added a thicker outer tube. The second reinforced model uses brick elements to represent foam between the shells, while the third model has reinforcing rings on the interior of the outer shell." Mr. Cantrell then performed a series of static stress analyses on each model."

Here a stress contour is shown for one of the reinforced Snake segment models. The reinforcement rings will be added to the final Snake design.

Results Add Strength

"The results showed that for extremely robust reinforcements, rings are much more mass efficient," says Mr. Cantrell. "As a result, the shells will now be changed to include reinforcement rings.

"The Algor analyses permitted a comparison between the classic reinforcement method of a simple thickness increase, where formulas are available, and the more efficient foam or ring reinforcements, for which there are no formulas. The former approach would likely have been taken if it were not for the availability of a fast and simple means of analysis such as Algor. This not only reduced the development time and costs, it also demonstrated to a skeptical project team (my colleagues) that finite element analyses are a valuable part of the design process."

Results Confirmed

To confirm his analysis results, Mr. Cantrell performed an impact test on a simplified version of the Snake segment constructed of aluminum. He also performed manual calculations, based on published formulas for a titanium outer shell without the inner shell. In his words, "The theoretical maximum displacement, according to the Algor results, is 1.06 mm and the measured displacement was 1.12 mm. I consider this difference to be remarkable since there are certainly measurement errors on that order and the displacement level nearly warrants a nonlinear analysis. Compared to the published formulas, the difference is only 0.5%."

More Analyses Planned

Algor FEA continues to be a major tool in the development of the Martian Snake. According to Mr. Cantrell, "The project team has been very impressed with the results." Mr. Cantrell plans to perform a complete stress analysis of the articulation and a more detailed analysis of the shell.

About Algor software, Mr. Cantrell says, "The best features are 3-D modeling and visualization. Normally, I use AutoCad to construct all my drawings and solid models and I consider it to be a very good product. However, I have found the Algor system to be more efficient for finite element solid modeling, since the software seems much more at ease creating meshed 3-D models."

Copyright © 1992 Algor, Inc. All rights reserved.