Colorado Space Grant Consortium



Colorado Space Grant Consortium

RocketSat II

Structures Design Document

Written by:

Jason Farmer

Contributions by:

Brad Brisnehan

Lee Jasper

2-1100

July 9, 2007

Revision: 0

|Revision Level |Author |Date |Changes Made |PM Initial |TL Initial |

|0 |Jason Farmer |7/9/07 | | | |

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Abstract

This Structures Design Document contains all drawings, analysis and assemblies used to model, test and create the RocketSat II payload. It outlines the structural stresses and requirements created by the RocketSat Team in conjunction with UP Aerospace, and the structures Team’s design solution for these constraints.

i. Abbreviations

FOS – Factor of Safety

PSI – Pounds per Square Inch

PTS – Payload Transportation System

Table of Contents

Abstract 3

i. Abbreviations 3

Table of Contents 4

1.0. Objectives 5

2.0. Requirements 5

2.1 RocketSat II General Structural Requirements 5

2.1.1 Up Aerospace Defined Requirements 5

2.1.2 Structures Team Defined Requirements 5

2.1.3 RocketSat Cross-System Requirements 6

2.2 Up Aerospace Defined Launch Environment Requirements 6

2.3 G-Switch Requirements 6

2.4 Requirements on Science 6

2.4.1 UP Restricted Requirements: 6

2.4.2 Structure’s Team Requirements: 7

3.0. System Overview 7

3.1 Chosen Pillar Design Solution 7

3.2 Payload Support System 9

3.3 Construction and Assembly 10

3.3.1 Additional Design Modifications 10

3.3.2 Final Payload Build Characteristics 11

4.0. Design Information 12

4.1 Design Research 12

4.1.1 Materials 12

4.1.2 Configuration Trade Study 13

4.1.2.1 Rack System 13

4.1.2.2 Hollow Pillar Structure Trade Study: 15

4.1.2.3 Four Pillar System 18

4.1.2.4 Central Pillar Structure: 20

4.1.2.5 Adjustable Rack Structure Trade Study: 22

4.2 Design Solution: The Pillar Design 25

4.2.1 3D Solidworks Modeling 25

4.2.2 Assembly of Final Structure 28

4.2.2.1 Machining Process 28

4.2.2.2 Brackets and Fasteners 28

4.2.2.3 G-switch Calibration 29

5.0. Performance Analysis 30

6.0. References 35

7.0. Appendix – Drawings and Assembly Pictures 35

Objectives

The main objective of the RSII structure is to create an accessible, lightweight, strong structure to appropriately house and protect the hardware and experiments designed by the CDH and Payload teams.

The secondary objective of the structures team is to create a structure design that makes best use out of the provided canister volume, while remaining lightweight but strong enough to withstand the launch environment with an appropriate FOS. Also, a g-switch needs to be configured by the structures team to trigger at the right amount of acceleration, with the purpose of providing a passive way of activating the payload upon launch.

The tertiary objective of this system is to correctly machine the provided access panel to accommodate any experiments or electronics that need visual and/or physical access to the outside of the rocket.

Requirements

1 RocketSat II General Structural Requirements

1 Up Aerospace Defined Requirements

• The cylinder shell shall not be altered from original specifications.

• The holding structure shall be independent of the provided cylinder and will only interface with the top and bottom cylinder plates.

• There should be a minimum of four mounting holes, at least one-inch apart, to support the allowed payload weight. Support system bolts shall be no smaller than ¼ inch diameter.

• Only the top and bottom of the cylinder maybe altered and no object shall protrude more than .190 inches above either surface.

• The center of mass of the payload shall be within 1 inch of the center of the cylinder relative to the top and bottom plates (along the z axis). The center shall also be no more than .1 inch radially from center of plate in the x and y axes (relative to cylinder shell.)

• The shell shall have a FOS of at least 2 (experiencing less than 20,000 PSI.)

• The entire payload shall be less than 7.5 lbs (excluding provided cylinder.)

2 Structures Team Defined Requirements

• The structure shall be less than 5 lbs.

• The plates shall interface with the structure with the capability for multiple configurations.

• The diameter of the holding structure shall be no larger than 9.75 inches and the height shall be no larger than 8.5 inches.

3 RocketSat Cross-System Requirements

All cross-system level interaction and requirements defined from these interactions are outlined in the Systems Design Document (DOC #2-0000).

2 Up Aerospace Defined Launch Environment Requirements

The holding structure shall withstand these stresses:

Max g load: 18.9 g during peak acceleration (first 13.5 seconds of flight)

Atmosphere reentry deceleration: 5.25 g peak at 150,000 feet

Max shock: 60 g for .25 seconds at vehicle touchdown

Spin rate: 6 Hz at motor burnout (T +13.5 seconds)

Temperature: 80-120 F typical; 150 F at maximum

3 G-Switch Requirements

• The G-switch shall be able to be triggered with less than 19g’s of acceleration; however, the switch shall also endure smaller shocks. The switch shall not be triggered by less than 3 g’s.

• The G-switch shall endure lateral shocks by not allowing the switch to be triggered by any force of less than 19 g’s in a lateral direction.

• The G-switch shall be the Omron 187733CK SS-5GL2 Subminiature Basic Switch. This switch is rated to a 50g contact force which will be enough to survive flight and not trigger for small vertical forces.

• The G-switch shall not be taller than 1.25 inches, and shall not have a mass of over 30 grams. The G-switch shall be inexpensive costing less than $15.

4 Requirements on Science

1 UP Restricted Requirements:

• Payloads cannot contain any volatile or living organisms (all equipment must be self sustaining while in the hands of UP).

• Plates must not contain an RF Transmission system unless it is previously cleared with UP Aerospace.

• For further rocket environment information not pertaining to structure, please reference the Mission Planner’s Guide (found in appendix). You will also find omitted structural requirements: these govern the requirements given to experiments.

2 Structure’s Team Requirements:

• All plates must meet the safety inspection of the Structure’s Team and not be detrimental to the internal structure: all cuts and pillar attachments must meet Structure’s approval.

• Each plate is allotted approximately 70.6 square inches of mounting space in the form of a 9.5 inch diameter circle.

• Each plate is allotted 1.18 inches in vertical height. Parts can be mounted through plates for more room, but – as stated above – approval is needed.

• Each plate is allowed 1 lb of experimental attachments.

• Macrolon is easily machined and countersunk with patience and heat control. Mounting is simple but, on the other hand, it cannot be laser-cut. Plan experiments accordingly.

• While the center of gravity can meet the above UP aerospace requirements by being balanced across the payload, it is the preference of the structure that mass be evenly distributed across each plate (meaning each plate’s CG is within the x and y axis tolerances) to do so.

System Overview

In picking the final structural design for construction and testing, numerous design factors were considered. There were three major categories of design analysis. First, the method of restraint for the overall structure within the PTS-10 cylinder was analyzed. Next, the system of support for hardware, the plates, was analyzed. Finally, the materials to be used for the structure were analyzed. This whole process spanned four months utilizing computer aided design, calculations, and finite element analysis to determine a practical design. Solidworks was used extensively throughout the design process.

1 Chosen Pillar Design Solution

For restraint, several different configurations were created. The best was considered to be the Four Pillar structure. This structure had four pillars symmetrically placed in a square orientation relative to the center of the PTS-10 cylinder.

Figure 1: Final Structural Design

[pic]

Figure 1 shows the ideal positioning of the pillars from the center of the cylinder is 3.5 inches. This conclusion was reached by positioning the pillars at multiple locations along the plates and using finite element analysis with loading on the plate. The least deflection and stress seen in the plate were the most important factors that showed the strength of a particular position. The closer in the pillars were to the center, the more the plate had a tendency to cantilever. The further apart the pillars were, the more bowing at the center occurred. Also taken into consideration, was the usable area allowed by the pillars. If the pillars were close to the center, then all hardware would have to be placed in a doughnut shape, which could restrict the use of some longer pieces of hardware. The further apart the pillars were, the more usable space would appear.

Chart 1:

|Stand off Positioning | | | |

|*No orientation with a factor of safety under one was considered. | | |

|*The annulus orientations have a factor of safety greater than 2 | | |

|*The puck orientations have a factor of safety greater than 1 | | |

|*These orientations were decided upon as the strongest while allowing best space for equipment |

| | | | | |

|Orientation |Displacement (mm) |Factor of safety (min) |Max Stress (N/m2) | |

|Middle Puck 3 in |1.5 |5.3 |9.40E+07 | |

|Outside Annulus 3 in |0.2 |5.4 |1.00E+07 | |

| | | | | |

|Middle Puck 3.5 in |2.1 |5.3 |9.40E+07 | |

|Outside Annulus 3.5 in |0.095 |5.3 |1.00E+07 | |

| | | | | |

|Middle Puck 4 in |2.8 |5.3 |9.40E+07 | |

|Outside Annulus 4 in |0.04 |5.3 |1.00E+07 | |

The four pillars were assembled out of male-female stand offs. These stand offs thread through the plates and into each other. Because the PTS-10 cylinder is only about 10 inches tall, the height of each stand off needed to be the perfect height to allow equal distances between each plate. Six stand offs per pillar were lathed to 1.27 inches tall. The bottom stand off of each pillar was lathed to .5 inches to allow for some deformation space above the PTS-10 end cap. The entire canister contains 28 stand offs when fully assembled. Each stand off is 3/8 inch in diameter with #8-32 threading and made of aluminum. Aluminum was chosen for its high tensile and compressive strengths along with its relatively light density for a metal.

2 Payload Support System

The plates were the next major components of the structure to be analyzed. Most of the material research occurred in conjunction with the plate designs. Because the PTS-10 container is a cylinder, all plates were designed to be circular. Each plate is 9.5 inches in diameter and 3/16 inches thick. The PTS-10 cylinder is 10 inches in diameter. The gap between the plate and the cylinder wall was created to allow some vibration room during flight. The plate thickness necessary to withstand all the stresses was found using COSMOS Works and a design load scenario This thickness was highly dependent upon the material used. A more rigid material like a metal could be fairly thin to see the same amount of deflection a plastic plate would see that was much thicker. Material choices are discussed later.

To allow a relatively large amount of space between plates while still maximizing the number of plates, multiple configurations were considered. Based off of RocketSat I which had a vertical operating space of about one inch, a similar height was desired for RocketSat II payloads. Because the plates deflected during flight, space had to be allowed for the deflection along with the hardware space. The optimal height was 1.27 inches using six plates. The plate thicknesses, no matter how thin or thick within reason for a given material, did not alter the heights enough to allow for extra plates to be added. Because of this fact, a thicker plate could be used to ensure a more stable platform for all hardware.

To restrain payload components, a system of aluminum “L” brackets, brass braces, nylon locking nuts, zinc plated screws and nylon spacers was implemented. All the restraint components and fasteners were purchased from a local hardware store. The brackets were all custom created in the machine shop in the ITLL, and bolted down to the plates. All components were either bolted to an aluminum bracket, or bolted down with the locking nuts, spacers and screws. This locking fastener system resists forces in all 3 principle directions, was cheap to purchase, and easy to use.

During the process of design for RocketSat II, several different types of materials were researched and studied to be used as plates for the RocketSat II structure. These materials had to have the desired qualities that both Up Aerospace and RocketSat II required be met. One of the major restrictions included having only an allowable weight of 10 pounds. This restricted the use of some strong but heavy materials. Following that requirement, the structure was only allowed to be 5 pounds while making as many payload plates as possible. Another requirement was that the structure as a whole would need to be able to be assembled in various orientations. This requirement causes each plate to be similar in construction and securing mechanism. Finally, each plate would need to keep its’ structural integrity at loads of up to 60g’s and more constant loads of 19g’s.

With these restrictions, a general picture of the plate material was developed. The material had to be strong to withstand flight while being light enough to not make the structure become the entire weight of the payload. Also, a material that would be relatively cheep was also desired so that the ease of obtaining it would be high. The material also had to be moderately easy to work with, so that it could be machined the same way every time to ensure that various orientations could be obtained.

3 Construction and Assembly

The Structure of RSII was constructed in the ITLL machine shop and the Space grant assembly lab. Construction did not require a large amount of technical skill, and mostly involved hands-on work with general manufacturing equipment and tools. The CNC mill was used to create the hole patterns in all 6 plates, this removed the possibility for a large amount of human error to occur during the drilling process, and created very accurate hole configurations for the components. All bracket creation was done by hand using a vertical band saw and a drill press. The 4 shorter standoffs that connected the payload to the lower canister lid had to be lathed down to the appropriate length and re-drilled and tapped (re-threaded). The plates also had to have wire routing holes milled out by hand to accommodate the CDH subsystem’s PCB’s.

The payload was assembled first by bolting all components to their respective plate. Then the plates were integrated together using aluminum standoffs, starting with the middle plates first (plates 3&4) and connecting outwards to the PTS-10 canister lids.

1 Additional Design Modifications

Several Design Modifications were made during the assembly process, out of necessity, but were not necessarily fully developed modifications, and therefore more liable to fail than well planned design attributes.

The key-switch design for the flight switch was changed to a rocker switch. This required customizing a bracket for the flight switch, adding a small LED near the flight switch, and a small reset button for the payload. The bracket created had very thin edges near the rocker switch, and also had a large hole below this drilled (~1/4” diameter) for the reset switch and LED.

A second modification made close to deadline was a series of weight saving holes cut into each plate. Close to launch deadline, the launch company (UP Aerospace) informed RocketSat that they were going to drop the weight requirement from 10lbs to 7.5lbs. This caused the team to cut an entire experimental plate from the structure (plate 6), along with cutting the sixth plate into an X shaped plate. RocketSat (the team) still didn’t meet the requirement, so holes were cut into the remaining plates, to further remove weight from the structure. COSMOS Works was used check the FOS on the new plates, as a final assurance of integrity.

2 Final Payload Build Characteristics

The final payload weighed 7.26 lbs, contained 5 plates with experiments and 1 plate without (the ‘X’ plate). The rocker switch also was integrated onto the payload instead of the key switch.

|Device |Pound |ounce |

|All plates |2.46875 |0 |

|All standoffs |0.325 |0 |

|Camera |0.1875 |0 |

|RSX (on FBD) |0.978125 |0 |

|RS2 batts (x2) |0 |2.4 |

|GPS batt |1 |0.1 |

|RS2 board |0 |2.1 |

|Serial Adaptor |0 |2.1 |

|Memory stick (x2) |0 |0 |

|GPS computer |1 |1.6 |

|Antenna |0 |1.5 |

|G-switch RS2 |0 |0.2 |

|Flight Pin Switch |0 |0.1 |

|Brackets |0 |5.5 |

|Wires |0 |1 |

|Wire Connectors |0 |0.61272 |

|CLD (x1) |0 |0.7 |

|Bolts/Nuts |0 |3.5274 |

|Battery Straps |  |1.41 |

|Subtotal (oz) |95.35 |22.85012 |

|Sub-Total (lb) |5.959375 |1.428133 |

|Sub-Total (g) |2703.113 |647.8009 |

|Total (lb) |7.3875075 |

|Plates with 2 oz reductions |7.2625075 |

|(lb) | |

| | |

|Desired (lb) |7.25 |

|Difference (lb) |0.0125075 |

|Allowable (lb) |7.33125 |

|Difference (lb) |0.0687425 |

|Total w/canister (lb) |12.9812575 |

|UP Weight Allotted (lb) |13.05 |

|Difference (lb) |-0.0687425 |

Design Information

1 Design Research

1 Materials

As the standard in the Aerospace industry, Aluminum was the first material to be considered. Aluminum 6061 is light weight having a density of only 2.7 g/cm3 and it has about a 276 MPa tensile yield strength with a shear strength of 207 MPa.



Another strong metal was considered. Titanium is strong and can make great structural material. Titanium Ti-5 Al-2.5 Sn has a density of 4.48 g/cm3 and a tensile yield strength of 827 MPa and a shear strength of 520 MPa.



For a lighter material acrylic was considered. Extruded Acrylic has a density of 1.19 g/cm3 and a tensile yield strength of about 73.8 MPa.



The final type of material considered was polycarbonate. Makrolon polycarbonate has a density of 1.2 g/cm3 with a tensile yield strength of around 62.4 MPa and a shear strength of 68.9 MPa.



Due to the nature of the canister that the structure must conform to, while optimizing space, each plate was going to be 9.5inches in diameter. There was no defined thickness for each plate or material, and there was no set number of plates that had to be flown. All plates were modeled so that they could withstand the entire mass of the payload. This would ensure that each plate would be more than sufficient to support only one-sixth of the payload mass.

After initial review, certain properties of two materials were unwanted for the structure. Titanium was quickly found to be undesirable. Titanium’s density was much larger than the other materials. Being so massive, Titanium plates could have a mass of almost twice that of the other materials that were being considered. If four plates were flown then the Titanium plates would be almost 8 times the mass. If six plates were flown then the plates would be about 12 times the mass as the other materials. This was unacceptable due to the mass restrictions put in place. Acrylic was another material to be undesirable. Acrylic is known to shatter when large forces are applied. If the plates were acrylic and they did not survive the stresses of flight, or there was an unexpectedly harsh landing, the acrylic had a larger chance of catastrophically failing. If the plates could not survive landing, then any instrumentation and data attached to the plates would be harder, if not impossible, to retrieve.

Acrylic did have some desirable properties however. Acrylic was light and reasonably strong. While the strength of Titanium would have been exceptional, it would have also been over engineering. Since the structure was only going to be 10 pounds, each plate would only need to support no more than about 600 pounds of force due to the 60g impact. Acrylic would have enough strength but something like it would need to be used that would not shatter.

After some further research, polycarbonate was found to be a similar candidate as acrylic. Polycarbonate is light and strong enough to handle the loads applied during flight. Polycarbonate, unlike acrylic, will not shatter. This has been verified by the fact that polycarbonate sheets are used in some instances as bullet proof glass that will survive large impacts and not fail. Lexan Polycarbonate was the first polycarbonate to be researched but, Makrolon was chosen. Many types of Lexan (including the types that were being considered) are very similar in density and strength to the chosen Makrolon. Makrolon is on average, slightly stronger than Lexans of similar density by about .7 MPa to about 3.4 MPa.

Based off of the structure and loads each plate had similar values that were compared to find the best material. These values were mass per plate, thickness of plate, distance between plates, deflection, overall mass of structure, and mass allotted per plate for each experiment.

Mass per plate and overall mass of the structure needed to be as small as possible. The mass of the plate was dependent on the thickness but the thickness also prevented some deflection of the plate. Therefore the plates needed to be thick enough to not deflect large distances and possibly come in contact with the upper and lower plates. The plates had to be thin enough to also be as light as possible. Also, based off of the thicknesses of the plates the vertical distances between the plates would vary. The maximum distance possible was desired. Finally, based off the entire mass of the structure, the mass allotted per plate would be determined.

2 Configuration Trade Study

Five Different configurations were studied before the final choice of the Four-Pillar assembly was chosen. Each different configuration was researched and the final choice was made based on weight and cost.

1 Rack System

Pros:

Simple / accessible assembly

Minimal space requirement on the edge of plates

Cost effective – few materials required

Variable height without purchase or alteration

Cons:

Possible difficulty creating assembly

Weakness on inner slots (connection points)

Very weak rack-to-shell support

Rack length = 9.60 inches

74.25 inches squared of area on each plate.

[pic]

Figure 2: Rack Assembly

Aluminum:

Density 2.7 g/cm^3

Single Rack = .18 lbs.

.54 lb structure without plates and 3 Rack system.

3.39 lbs total structure assuming 6 polycarbonate plates used (Within early requirements)

1.10 lbs/ plate for actual equipment.

8.73 lbs total structure assuming 6 aluminum plates used (Violates early requirements)

.21 lbs/ plate for actual equipment

Aluminum 6061 T6: 40,000 psi Tensile Yield Strength



27,000 psi Shear Yield Strength (or 19ksi??)

_____ Compressive Strength

GE Lexan Polycarbonate:

Density 1.19 g/cm^3

Single Rack = .0799 lbs.

.240 lb structure without plates and 3 Rack system.

3.09 lbs total structure assuming 6 polycarbonate plates used (Within early requirements)

1.15 lbs/ plate for actual equipment.

8.432 lbs total structure assuming 6 aluminum plates used (Violates early requirements)

.26 lbs/ plate for actual equipment

Tensile Yield Strength = 9,000 psi

Flexural Yield Strength = 13,500 psi

Compressive Yield Strength = 12,500 psi



2 Hollow Pillar Structure Trade Study:

Pro’s:

Plate-to-plate electronic interlink age

Simple assembly

Comparable weight with other possible designs

Centralized supports

Interchangeable components between levels

Low precision required in plate creation

Con’s:

Cantilevered stress applied to experimental plates

Out-of-house machining of pillars

Limitation of any optical experimentation with center support

Center support inhibits proper weight distribution

Statistics:

Hollow Pillar Height = 1.38 inches

70.35 inches squared of area on each plate.

9.5 inches diameter plates

[pic]

Figure 3: Hollow Pillar Assembly

Aluminum:

Density 2.7 g/cm^3

Hollow Pillar mass = .067 lbs

.469 lbs structure with 6 teers without plates (7 pillars)

.536 lbs structure with 7 teers without plates (8 pillars)

.38 lbs a polycarbonate plate

2.75 lbs total structure assuming 6 plates used (Within early requirements)

1.21 lbs/ plate for actual equipment

3.20 lbs total structure assuming 7 plates used (Within early requirements)

.972 lbs/ plate for actual equipment.

.86 lbs an aluminum plate

5.63 lbs. total structure assuming 6 plates used (Violates requirements)

.730 lbs. /plate for actual equipment

6.49 lbs. total structure assuming 7 plates used (Violates requirements)

.492 lbs. /plate for actual equipment

Aluminum 6061 T6: 40,000 psi Tensile Yield Strength



Zelux Polycarbonate:

Density 1.19 g/cm^3

Hollow Pillar mass = .029 lbs

.203 lbs structure with 6 teers without plates (7 pillars)

.232 lbs structure with 7 teers without plates (8 pillars)

.38 lbs a polycarbonate plate

2.48 lbs total structure assuming 6 plates used (Within early requirements)

1.25 lbs/ plate for actual equipment

2.89 lbs total structure assuming 7 plates used (Within early requirements)

1.02 lbs/ plate for actual equipment.

.86 lbs an aluminum plate

5.36 lbs. total structure assuming 6 plates used (Violates early requirements)

.773 lbs/ plate for actual equipment

6.25 lbs total structure assuming 7 plates used (Violates early requirements)

.535 lbs/ plate for actual equipment.

Tensile strength = 8,400 psi

Compressive strength = 12,500 psi

GE Lexan Polycarbonate:

Density 1.19 g/cm^3

Hollow Pillar mass = .029 lbs

.203 lbs structure with 6 teers without plates (7 pillars)

.232 lbs structure with 7 teers without plates (8 pillars)

.38 lbs a plate

2.48 lbs total structure assuming 6 polycarbonate plates used (Within early requirements)

1.25 lbs/ plate for actual equipment

2.89 lbs total structure assuming 7 polycarbonate plates used (Within early requirements)

1.02 lbs/ plate for actual equipment.

.86 lbs a plate

5.36 lbs. total structure assuming 6 aluminum plates used (Violates early requirements)

.773 lbs/ plate for actual equipment

6.25 lbs total structure assuming 7 polycarbonate plates used (Within early requirements)

.535 lbs/ plate for actual equipment.

Tensile strength = 8,500 psi

Titanium:

Density g/cm^3

Hollow Pillar mass = lbs

lbs structure with 6 teers without plates (7 pillars)

lbs structure with 7 teers without plates (8 pillars)

lbs a polycarbonate plate

lbs total structure assuming 6 plates used (Within early requirements)

lbs/ plate for actual equipment

lbs total structure assuming 7 lates used (Within early requirements)

lbs/ plate for actual equipment.

lbs an aluminum plate

lbs. total structure assuming 6 plates used (Violates requirements)

lbs. /plate for actual equipment

lbs. total structure assuming 7 plates used (Violates requirements)

lbs. /plate for actual equipment

3 Four Pillar System

Pros:

Interchangeable parts between levels

Possible differential in pillar heights

Minimal footprint on plates

Use of standoff provides convenient actualization of design

Cons:

Complexity of assembly stack

All weight in tension at top pillars / all weight in compression at bottom pillars

Without standoff use, variability achieved through manufacture only

Analysis:

See Section 3.*, 4.2, 4.3, 5.* for further analysis.

[pic]

Figure 4: Four Pillar Assembly

Aluminum Standoffs:

Density: 2.7 kg/dm^3

Mass of plate: 0.4 lbs

Mass of standoff: .01 lbs

Mass of system: 2.14 lbs

Mass allotted for experiments: 7.42 lbs

Mass per plate: 1.23667 lbs

Plate surface area: 74.58 in^2

The tensile yield stress of 6061-T6 Aluminum is 37 ksi

The compressive yield stress of 6061-T6 Aluminum is 37 ksi

The shear yield stress of 6061-T6 Aluminum is 19 ksi

Lexan Standoffs:

Density: 0.94 kg/dm^3

Mass of plate: 0.4 lbs

Mass of standoff: 0.003481 lbs

Mass of system: 2.08 lbs

Mass allotted for experiments: 7.92 lbs

Mass per plate: 1.32 lbs

Plate surface area: 74.58 in^2

The tensile yield stress of Lexan EXL9330 is 8.5 ksi

The compressive yield stress of Lexan EXL9330 is non-specified

The shear yield stress of Lexan EXL9330 is non-specified

Steel Standoffs:

Density: 7.8 kg/dm^3

Mass of plate: 0.4 lbs

Mass of standoff: 0.028889 lbs

Mass of system: 2.35 lbs

Mass allotted for experiments: 7.65 lbs

Mass per plate: 1.275 lbs

Plate surface area: 74.58 in^2

The tensile yield stress of Structural A36 Steel is 36 ksi

The compressive yield stress of Structural A36 Steel is 36 ksi

The shear yield stress of Structural A36 Steel is non-specified

4 Central Pillar Structure:

Pro’s:

Freedom of movement with plates in total range of PTS-10 cylinder.

Simple Design

Easy to assemble

Con’s:

Two specialized nuts required for each plate to hold it in place.

Requires a top and bottom stabilizing plate inside of PTS-10 cylinder, adding mass and taking away usable volume.

Plates may spin out of alignment.

Statistics:

Central pillar length = 8.5 inches

71.52 inches squared of area on each plate.

[pic]

Figure 5: Central Pillar Assembly

Aluminum:

Density 2.7 g/cm^3

Central pillar mass = .55lbs

Bottom nut = .03 lbs

Top nut = .029 lbs

.904 lb structure without plates

3.75 lbs total structure assuming 6 polycarbonate plates used (Within early requirements)

1.04 lbs/ plate for actual equipment.

9.09 lbs. total structure assuming 6 aluminum plates used (Violates requirements)

.15 lbs. /plate for actual equipment

Aluminum 6061 T6: 40,000 psi Tensile Yield Strength



Zelux Polycarbonate:

Density 1.19 g/cm^3

Central pillar mass = .240 568 42 lbs

Bottom nut = .0153 lbs

Top nut = .0129 lbs

.268 lb structure without plates

2.17 lbs total structure assuming 6 polycarbonate plates used (Within early requirements)

1.31 lbs/ plate for actual equipment

8.46 lbs. total structure assuming 6 aluminum plates used (Violates early requirements)

.256 lbs/ plate for actual equipment

Tensile strength = 8,400 psi

Compressive strength = 12,500 psi

GE Lexan Polycarbonate:

Density 1.19 g/cm^3

Central pillar mass = .240 568 42 lbs

Bottom nut = .0153 lbs

Top nut = .0129 lbs

.268 lb structure without plates

2.17 lbs total structure assuming 6 polycarbonate plates used (Within early requirements)

1.31 lbs/ plate for actual equipment.

8.46 lbs. total structure assuming 6 aluminum plates used (Violates early requirements)

.256 lbs/ plate for actual equipment

Tensile strength = 8,500 psi

5 Adjustable Rack Structure Trade Study:

Pro’s:

Freedom of movement within entire PTS-10 cylinder

Easy to assemble and use.

Only moving parts are threaded, less chance of failure

Disk supported on edges to prevent warping.

Con’s:

Possible mounting difficulty with PTS-10 cylinder

Overall large footprint on the plate

Possible failure points with the tall pillars.

Possible failure points on pillar clamps

Statistics:

Central pillar length = 8.5 inches

70.72 inches squared of area on each plate.

[pic]

Figure 6: Adjustable Rack Assembly

Aluminum:

Density 2.7 g/cm^3

Pillar mass = .041 lbs

Pillar Clamp = .02 lbs

.644 lb structure without plates assuming 6 plates

.56 lbs a plate assuming 3/16 inch polycarbonate

4.00 lbs total structure assuming 6 plates used (Within requirements)

.999 lbs/ plate for actual equipment.

1.27 lbs a plate assuming 3/16 inch aluminum

8.26 lbs. total structure assuming 6 plates used (Violates requirements)

.289 lbs. /plate for actual equipment

Aluminum 6061 T6: 40,000 psi Tensile Yield Strength



Zelux Polycarbonate:

Density 1.19 g/cm^3

Pillar mass = .018 lbs

Pillar Clamp = .01 lbs

.112 lb structure without plates

.38 lbs a plate

2.39 lbs total structure assuming 6 polycarbonate plates used (Within requirements)

1.27 lbs/ plate for actual equipment

.87 lbs a plate

5.33 lbs. total structure assuming 6 aluminum plates used (Violates requirements)

.778 lbs/ plate for actual equipment

Tensile strength = 8,400 psi

Compressive strength = 12,500 psi

GE Lexan Polycarbonate:

Density 1.19 g/cm^3

Pillar mass = .018 lbs

Pillar Clamp = .01 lbs

.112 lb structure without plates

.38 lbs a plate

2.39 lbs total structure assuming 6 polycarbonate plates used (Within requirements)

1.27 lbs/ plate for actual equipment

.87 lbs a plate

5.33 lbs. total structure assuming 6 aluminum plates used (Violates requirements)

.778 lbs/ plate for actual equipment

Tensile strength = 8,500 psi

2 Design Solution: The Pillar Design

[pic]

Figure 6: The Final Payload

The RocketSat II final housing structure was made up of six plates (seen in figure 6), constructed of Macrolon. Each plate was ten inches in diameter and separated by a distance of one inch. Standoffs were used to separate and support the plates. The standoffs were connected through four .125 inch diameter holes, symmetrically drilled through the plate at a diameter of 3.75in. The configuration contained a total of twenty four male-female standoffs and four female-female connections. The standoffs were attached to one another through the four holes, oriented perpendicular to the plates, with the male side of the male-female standoffs facing the center of the payload, and the female side facing the ends of the payload. The female-female standoffs were located at the center of the payload to accept the male ends of the male-female standoffs.

1 3D Solidworks Modeling

The Solidworks 3D design suite was used to create a model of the payload. The main purposes that this model served were to design various component brackets, create a hole pattern for the access panel, verify that the CG of the payload was within the requirements, to serve as a visual aid, and to use the COSMOS Works add-on program to conduct FEM analysis on the payload structure.

Shown in the appendix are the final assembly model, looking at the access panel with the PTS-10 canister removed, and all necessary drawings and pictures from the program. Most parts were modeled without much detailing on the parts themselves. This is because the small details of the parts don’t matter; just the total dimensions and mass of the part were needed to adequately create the assembly (with exception of the structural elements, and in that case the correct material also had to be included in the model).

Plate and Component Position Breakdown

Plate 1 (RSX):

▪ RSX AVR Board

▪ Geiger Counter

▪ 2 9V Batteries

▪ Accelerometer Board

▪ G-switch

Plate 2 (Battery Plate):

▪ GPS Battery

▪ 2 9V Batteries (for RSII on plate 3)

Plate 3 (RSII):

▪ COS Digital Camera

▪ RS2 AVR Board

▪ Strain Gauges

▪ G-Switch

Plate 4:

▪ GPS Antenna

▪ Flight Switch Bracket, reset switch

▪ Large cutout for GPS computer on plate 5

Plate 5 (GPS Plate):

▪ GPS Computer

▪ 2x 4GB USB Memory Sticks

▪ Antenna Wire to Serial connection converter

Plate 6 (X-Plate Cutout)

Rocket Sat II Solidworks Assembly Tree

|Rocket Sat II Solidworks Assembly Tree | | |

|** All Solidworks Part/Assembly Names are RSII-STR-2##---------something | |

|** Part Numbers are followed by the Part Title | | |

|** Drawing names and Numbers are RSII-STR-3##---something | |

| | | | | | |

| | | | | |# occuring |

|200.0 |Final Assembly | | |1 |

| |201.0 |RSII Assembly | |1 |

| | |207.0 |Plate And Pillars |  |4 |

| | |  |208.0 |Aluminum Standoff |4 |

| | |  |209.0 |Makrolon Plate |1 |

| | |210.0 |RSX Plate |  |1 |

| | |211.0 |Camera |  |1 |

| | |212.0 |GPS Computer |  |1 |

| | |213.0 |GPS Battery |  |1 |

| | |214.0 |GPS Serial Converter |  |1 |

| | |215.0 |GPS Antenna |  |1 |

| | |216.0 |Half Inch Standoff |  |4 |

| | |217.0 |One Inch Standoff |  |8 |

| | |218.0 |Tall Nylon Spacer |  |16 |

| | |219.0 |Z-Axis Acclerometer Board |  |1 |

| | |220.0 |RSII PCB |  |1 |

| | |221.0 |Flight Switch |  |1 |

| | |222.0 |Flight Switch Bracket |  |1 |

| | |223.0 |G-Switch |  |2 |

| | |224.0 |G-Switch Mount |  |2 |

| | |225.0 |Z-Axis Acclerometer Bracket |  |1 |

| | |226.0 |RSX Geiger Counter |  |1 |

| | |227.0 |Battery Restraint 2x9V |  |2 |

| | |228.0 |9V Battery |  |4 |

| | |229.0 |Microwave Sensor |  |1 |

| | |230.0 |GPS Battery Mount - Large |  |2 |

| | |231.0 |GPS Battery Mount - Small |  |2 |

| | |232.0 |GPS Antenna Bracket |  |2 |

| | |233.0 |GPS Antenna Lexan |  |1 |

| | |234.0 |GPS Serial Converter Mount 1 |  |1 |

| | |235.0 |GPS Serial Converter Mount 2 |  |1 |

| | |236.0 |RSX AVR Board |  |1 |

| | |237.0 |9V Battery Strap |  |2 |

| | |238.0 |9V Battery Back Bracket |  |1 |

| | |239.0 |9V Battery Side Bracket |  |1 |

| | |240.0 |X-Plate |  |1 |

| | |241.0 |Memory Stick Assembly |  |1 |

| | | |242.0 |Memory Stick Bracket 1 |1 |

| | | |243.0 |Memory Stick Bracket 2 |1 |

| | | |244.0 |Memory Stick - 4GB |2 |

| |202.0 |PTS-10 Closure Bottom |  |  |1 |

| |203.0 |PTS-10 Closure Top |  |  |1 |

| |204.0 |PTS-10 Cylinder and Access Panel |  |  |1 |

| | |205.0 |PTS-10 Cylinder |  |1 |

| | |206.0 |PTS-10 Access Panel |  |1 |

2 Assembly of Final Structure

1 Machining Process

The plates were cut from larger rectangle of Lexan. The shape of the plate, and the four holes, were machined using a CNC mill. The CAD design of the plate was converted to CNC machine code using Mastercam 9.1 software and a file converter. The CNC then had to be calibrated for the design, involving installing the required drills and adjusting the machines coordinate system relative to the center of the part.

The first step in calibrating the machine to cut the lexan plates was to clamp the Lexan rectangles on the machining bed, using clamps located on the four corners of the rectangle. The drills were then installed into the CNC machine. A ¼ inch end mill drill was used to cut the shape of the plate, and a ¼ inch drill was used to cut the four holes. The length of each drill bit was then zeroed so that the machine knew where the tip was. A CNC tool was used to determine the locations of a point on two perpendicular sides of the Lexan rectangle. With those two points, and the dimensions of the rectangle, the machine was able to determine an origin position.

Two layers of Lexan were cut at a time, and two circles were cut from the material to form the plate. That translates into a total of four plates that were machined at a time. A wood board, of equal dimensions to the Lexan rectangles, was placed under the two layers of material to provide a buffer between the material and the machine bed.

The CNC was also used to drill the holes in their proper locations for each plate individually, according to the components that were placed on that plate. Hole locations and diameters varied from component to component.

2 Brackets and Fasteners

3 G-switch Calibration

Prepared by Lee Jasper

The Omron subminiature basic switch was used as the main component in the g-switch. It is a snap action switch which means that it creates an electrical connection when the lever arm is depressed. The specific switch was the 187733CK SS-5GL2 which has a roller ball at the end of the lever arm.

This switch was intended to trigger as a certain specified g-force. This g-force was decided upon in conjunction with an Up Aerospace recommendation and the desired performance of equipment connecting to the g-switch. In the catalog (Jameco Electronics, Catalog 262 May 2006, p. 159) the Omron switch had a contact force of 50g’s. As of the RocketSat II Conceptual Design Review, a g-force of about 7 g’s was desired. A maximum of 19 g’s was expected during launch, therefore 7 g’s would assuredly be attained. The hope is that 7 g’s will occur extremely quickly, thus turning on equipment which will give useful data early in the flight.

To attain a 7 g-force triggering point from a 50 g-force triggering point, mass had to be added to the lever arm.

First the roller ball was removed by breaking the pin that holds the ball in place. Once this piece was removed, the ball was no longer restrained and fell out.

A new pin was inserted through the lever arm holes. Attached to this pin was a string. Mass was added to this string in increments until the switch was depressed. (Experimental apparatus shown below in figure XX).

Note: The mass was added directly below the pin so that no bending moments occurred on the lever arm which could have thrown the ‘triggering mass’ off. In order to accomplish this, the switch was restrained and held over a ledge so that the mass could be hanging freely to allow a real weight force to be measured.

[pic]

Figure 7- Experimental Appartus

It was found that 18 grams was needed to trigger the g-switch in a one g-force environment. Based off of this value, the relation to mass and ‘triggering g-force’ was assumed to be linear (this could be a very bad assumption). Therefore the equation that related g-force and mass was related to be:

[pic]

[pic]

This mass was added to the end of the lever arm using a screw with nuts on it. The mass was all added at the same distance on the lever arm so that no changes to the moment on the lever arm were created.

[pic]

Figure 8- Modified G-Switch

Performance Analysis

The launch environment experienced by the payload on April 28th was not as expected. The landing was not nominal and a great deal of stress was experienced by the payload during the landing sequence. Unfortunately the direction of shock and stress was not aligned with the central axis of the payload (perpendicular to the plate surfaces) but was focused at an orthogonal angle to the central axis (aligned from the access panel inwards). Landing was absorbed by the rocket skin, access panel, and finally the structure. Upon recovery and de-integration, many structural failures were found.

Upon removing the access panel cover from the cylinder, it was evident that the landing was off-nominal. As seen in figure #9 on the following page, the structure suffered severe damage upon landing from an impact centered on the access panel. Initial inspection shows complete stress fracture and failure in the GPS Antenna Lexan (1). The GPS Antenna cover did what it was meant to do, mainly break before the GPS antenna. The other components and items near the access panel all suffered heavy structural damage. Callouts 2 and 3 call attention to the camera and the third plate itself, which both took the brunt of the impact. The camera case was shattered, and the camera itself shoved back into the payload, but the SD card was completely unharmed, and the structure once again did its job, and prevented data loss from the SD video. The third plate shows some interesting stress though, and is bent downwards approximately ½ inches. The plate is permanently bent downwards in this position, and this is evidence of heat stress from re-entry, and this deformation was not caused by impact. The Makrolon material is a plastic material, and is not elastic. This means that the material tends to break before stretching or bending. Comparable is the viewable section of plate 2 (right above callout #2) where the plate has broken from impact. The deformation here is smaller than that of the deformation on plate 3, showing that Makrolon breaks before the larger deformation is reached.

[pic]

Figure 9 – Looking At the Payload through the Access Window.

The next figure (# 10), shows the RSX AVR. This AVR sheared out sideways out of the socket. The direction that the pins sheared is consistent with the direction of impact, i.e. the pins point towards the access panel. The nylon string that was used to tie down all the IC’s in the payload was not sufficient and all IC’s sheared out of socket upon impact. In this case, the structural support for the IC’s was not adequate and a better way of securing IC’s must be used for future missions to ensure CDH system integrity.

[pic]

Figure 10 –Plate 1 - The RSX AVR – sheared sideways out of its socket.

[pic]

Figure 11 – Plate 2 – the remains of the GPS Battery.

The figure above shows the remains of the GPS battery on plate 2. This component was completely disintegrated when the payload was recovered. The brass straps used to secure this item are obviously not adequate as seen by their complete flattening and failure above. Callout 1 points to where one of the straps used to be bolted down with a nylon locking nut. The lock nut was obviously sheared off the screw it was screwed to, and this brings to light the inability of locking nuts to resist a (estimated from pressure data) 141 G impact. Locking nuts are still a good structural choice besides this point, as a 141 G impact is highly unexpected for this launch system.

[pic]

Figure 12 – Plate 2 – the crack in the plate near the GPS battery’s location.

Figure 12 shows a crack in plate 2, near the location of the previous picture. The plate was cracked straight towards the hole that the screw and locking nut utilized for a bracket. The lesson here is that the hole placement with respect to the edge of a Makrolon plate does matter when off-nominal impacts occur. A material will fragment and will split and head towards the areas of weakest strength in the material, in this case the holes for fasteners and brackets in the plates. The bracket still held in place though, and the crack didn’t cause the destruction of the GPS battery on plate 2, the large G force of the impact did.

[pic]

Figure 13 – Plate 5 –The standoff from plate 6 that was shoved through.

Figure 13 above shows that a standoff was shoved through a plate upon impact. Also note the absence of the standoff threads and the connection to plate 4. Almost every single standoff sheared apart at the threads, where the standoff was thinnest. The aluminum standoffs that were chosen were not appropriate to hold the structure together. New standoffs or an alternative means of holding the layered structure together must be found before another payload is built. Also, a possible new material choice besides Makrolon would also be prudent, as this stress failure is in the axial direction, the direction that the structure was designed to handle stress.

Overall the payload structure performed moderately well, with some successes and some failures. Some design modifications obviously need to be made before the next payload, but the structure design itself (the four pillar setup) is a good idea. New standoffs, thicker brass straps, and possibly a different material choice would be necessary modifications to be made.

References

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Appendix – Drawings and Assembly Pictures

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1

3

2

1

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