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Samvel AyrapetyanBoom Structural Design and Analysis

The structural design and analysis was to assess a Remote Manipulator System that handles launch vehicles, which are typically used on space stations and expeditions. Considering optimum design specifications, there were a few constraints and configurations we needed to consider. The booms were to be in a straight configuration and a right angle form and deflect no more than an inch at the tip of the fifty-foot beam with a ten-pound force applied respectively. Because we were considering an environmentally friendly solution that was also going to be sturdy enough, our team chose to use a hollow cylindrical shaft. Therefore the design will be conserving quite a bit of material. Our design will be isotropic with a circular cross section. The outer radius will be 11 inches, while the inner radius will be 10.93 inches. Wall thickness will end up being 0.07 inches. The material that will be used for our beam is a chemical compound known as silicon nitride. It is high in strength, material toughness, and fracture toughness. The young’s modulus of the material is about 310 GPa, which, if compared to structural steel, is much more durable in terms of load. Our team believes that under uniform material the two beams attached with a shoulder would be optimal in deflecting very little. In a 180 degree, full-length cantilever orientation, the tip deflects .913 inches under the 10 lb load constraint (See Appendix). However, in a 90 degree “arm” orientation, the beam is under more torsion and weight so accounting for this, our team calculated the deflection of the first beam in addition to the “arm” would be .519 inches, hence, another reason why we justify using silicon nitride, to carry a tough torsional load (See Appendix).

Approaching our final design configuration, our team rationally believed that a 50ft long beam would easy deflect under normal circumstances due to gravity and other supporting forces, as a result, we wanted a hollow shaft to decrease cross sectional area and in turn decrease our distributed weight load of the beams itself significantly. In the developing stages, our team had gone with choosing a rectangular I-beam, because we had figured that it would be the only beam to withstand a long length and minimum deflection. After realizing that modulus of elasticity makes is an equal counterpart in making the difference of deflection, we thought we would go with a more efficient material to meet our needs and standards. Iron was another consideration but we feared that its self-weight was too heavy therefore we ruled it out as well. Steel is the one that most people look towards when designing. It is reliable and tough, making it a useful material that does not deflect easily. However, our main concern with steel was its density. After comparing its density to other materials, it was clear that steel is extremely heavy and that could increase the max weight of our booms, which we had hoped to minimize. In the end, silicon nitride was a material we found from a chart online (Young’s Modulus vs. Weight) and it fit our team’s criteria. We had first gone about choosing an aluminum alloy and carbon fiber reinforced polymer, but there were some flaws in the design, so we introduced it to the ceramic, silicon nitride. Numerous calculations proved that we needed a material with the right weight to moment of inertia ratio to satisfy our max tip deflection constraint. Because using a hollow shaft reduced weight, we played around with a lot of outer and inner radii and found one pertinent to

03Eric Ni

Samvel Ayrapetyanour design. The moment of inertia calculated was approximately 18.1 with its respective units. In our approach we neglected the weight and existence of the shoulder and the wrists to the actual RMS vehicle.

Our team really focused on making the project as real-life as possible, as if we were actually going to send this design to space. A few things we considered were cost, material efficiency, stability, and weight. First, the material density compared to its mass and young’s modulus was at a great ratio where we could have worked with. The material has great chemical properties, having high resistance to heat and shock. Silicon Nitride is generally a very stable material as well. It has a high fracture toughness, which prevents it from breaking easily. It is also one of the most thermodynamically stable of the ceramics. Although all these are awesome properties of silicon nitride, the key property that stands out is its outstanding strength to density ratio. It can support a lot of load, yet its own weight is quite small. Silicon nitride is a rather expensive material, but it’s performance to cost benefit ratio is excellent in the applications where it can outperform the normally utilized materials with long life and very reliable low maintenance operation. Silicon Nitride is slowly gaining fame in the materials industry and typically being used for cutting tools, engine parts, turbine blades, and precision shafts. Companies are starting to revert to it as its price falls, because it is 68% stronger than steel and 58% lighter. Also, it is used in high-temperature applications today. Silicon Nitride is one of the few of the ceramic material family that can withstand the thermal upbringings of a hydrogen rocket engine. NASA tested it in a 1-inch diameter tubing to survive the temperature and it took on about 5 cycles of 1000+ degrees Celsius material temperature.

Although our design is environmentally friendly and capable of supporting the 10-pound load, there is definitely room for improvement. A key concern in our design is that despite the strength of silicon nitride compared to its low density, it is a brittle material. This means that once it reaches its yield load, it cannot last long before it fractures. Our design can have some potential improvements as well. For one, we stretched our design to the very limit, nearly reaching a deflection of 1 inch making it a design that works but can deflect more than an inch if the load is heavier than 10 pounds. For a company that wants a higher factor of safety, they should increase the wall thickness. By doing this, the deflection due to the point load will decrease resulting in a bigger margin of error. At a cost, however, the weight and cost of building the design will increase significantly. Another improvement can be using carbon fiber reinforced polymers. This material will proves to be an extremely high strength material for its weight. If our beam was made of it, the deflection-weight ratio should be extremely low. However, it is expensive to produce and the endurance limit is hard to predict in modern times, making it a risky material. In the end, we still believe that our design is the most environmentally friendly in terms of satisfying the requirements. We reduced as much material as we could without allowing the design to deflect more than one inch. Although improvements can be made, we strictly enforced our concept on being environmentally friendly and decided on our design.

03Eric Ni

Samvel AyrapetyanAppendix A: Drawing Package

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Samvel AyrapetyanAppendix B: Table and Calculations

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Samvel Ayrapetyan