Master Mechanical Interface Document

University of LeicesterPLUMERef: PLM-SYS-MasterMechanical-607-0

Date: 13/08/2009

Master mechanical interface document

Philip Peterson

Date / Updated Reference Number / change
13/08/2009 / PLM-SYS-MasterMechanical-607-0 / Preliminaryversion
[dd/mm/yyyy] / [PLM-XXX-XXX-xxx-x] / [eg. first draft]

Introduction

This document describes how PLUME’s physical structure should conform to the CubeSat standard, as well as the satellite’s structural layout. The master document references the cubesat design specification (CDS) given in reference [1].

Definitions

The co-ordinate system for PLUME is defined according to figure 1:

Figure 1: Co-ordinate designation.

This co-ordinate system is specific to PLUME, and is compatible with the CDS.Figure 1 also shows the location of the major external components – the interface connector (including the USB port), the camera and the two detectors. It also shows the internal PC-104 style connector stack, which carries the electrical connections between the various subsystems. A dotted line indicates that a structure is obscured by the body of the satellite.

For this interface document, the origin of the co-ordinate system is defined at the geometric centre of the satellite unless otherwise stated.

PC-104 connectors ‘stack through’; an individual pin forms a continuous electrical connection through all the subsystem boards.

The PC-104 connector is actually composed of two long sockets side-by-side, each with 52 pins. The one closest to the edge of the satellite is called ‘H2’, and the one closest the centre of the satellite is ‘H1’. Pins are labelled H1.xx or H2.xx, where ‘xx’ is a number from 1 to 52. For the purposes of this mechanical interface document,references to the ‘PC-104 connector’ mean both H1 and H2 connectors together. References to H1 and H2 will be made individually.

CubeSat mechanical specifications

PLUME’s shape is determined by the CubeSat design specifications [1]. These specifications include checklists for making sure PLUME is fit to be launched from the CubeSatPPOD (poly-picosatellite orbital deployer). The CubeSat kit supplied to us by Pumpkin fulfils many of these requirements [2], but there are some specific to PLUME that must be met.

A diagram of the satellite’s frame, including the space for externally mounted components, is shown in figure 2:

Figure 2: Diagram of the satellite’s physical size constraints. The four rails on which the satellite rests in the PPOD launcher are in dark grey. The coloured boxes illustrate the external area on the satellite to which solar panels, antennae and detectors must be confined.

The remaining constraints on the mechanical design, along with their references from the CDS, are:

• 2.2.15: Total satellite mass must be less than 1.33kg.
• 2.2.4: The satellite frame is built around a 100mm-sided cube (excluding the rails), but there is a space allowance on all sides of 6.5mm for fitting such things as solar panels. Externally mounted equipment must not interfere with the p-pod rails.It must be confined to a volume 100mm by 83mm by 6.5mm on the X and Y sides (shown as red and blue areas in figure 1), and 83mm by 83mm by 6.5mm on the Z sides (shown in green).
• 2.2.6: The rails on the corners of the satellite chassis are used to guide it out of the PPOD. Obviously, these must be unobstructed. Figure 3 shows the rail locations.

Figure 3: Cross section of PPOD. Note the spring at the back which ejects the CubeSats, and the rails at the corners hold the rails on PLUME [1].

• 2.3.3There are access panels on the PPOD through which power cables to charge the satellite battery and run diagnostic software can be connected after integration. These panels are on both X sides of the launcher.
• 2.2.7: The only part of the satellite that can come into contact with the PPOD should be the rails and the feet.
• 2.2.17: The centre of gravity for the satellite must be within a 20mm sphere around the craft’s geometric centre.
• 2.1.7: Mass loss due to outgassing must be less than 1.0% of the initial satellite mass. ‘Collected volatile condensable material’, which is the portion of outgassedsubstances capable of condensing onto cold surfaces of the satellite must be less than 0.1% of the satellite’s initial mass. (All materials emit gas when placed in a vacuum – this is called ‘outgassing’. Most plastics, especially polyvinylchloride or ‘PVC’, are insuitable for spaceflight, as they outgas quite severely.)

There are additional electronic constraints that affect the mechanical configuration:

• 2.1.5: Stored chemical energy (ie. in batteries) should be less than 100WHrs.
• 2.3.1: No electronics should be running during launch.

Internal layout

Based on the clearances required by the different subsystems, the boards inside the satellite will be laid out as shown in figure 5. The bottom (closes to –Z) of the stack is occupied by the MCU (15mm clearance), on top of which is the COMS modem and transceiver (25mm clearance), then the PSU board (15mm clearance) and the top slot contains the payload/adcs and camera boards (27.5mm clearance).

The flight OBDH board is supplied fixed to the interior of the –Z face of the satellite. The other subsystem boards stack on top of the OBDH board supported by four steel M3-threaded posts, one in each corner of the board, around which are 6mm diameter aluminium standoffs that hold the boards in position. Boards are interfaced electrically through the PC-104 connector.Standard PC-104 connectors have plastic sockets10mm high, but the pins underneath are at least as long again. The 15mm spacing between subsystems is accommodated by pressing the pins of the upper connector 5mm into the lower connector socket. To assemble the bus, four 10mm connectors are required plus a single non-stackthrough connector.

The steel posts fit onto the OBDH board through threaded holes that are currently occupied by screws. The posts must be inserted onto the OBDH board one at a time, to avoid introducing new stresses into it [3]. At the top of the stack, four midplane standoffs [4] are used to attach the steel rod to the external casing. Figure 4 is an exploded view of a CubeSatkit satellite, and gives a good illustration of the different structural components of a typical CubeSat.

Figure 4: CAD drawing of a skeletizedCubeSat kit. The yellow circle zooms in on a midplane standoff [4]. Note the position of the PCB cutouts and the aluminium standoffs holding the stack together.

The order in which PLUME’s internal circuit boards are stacked is given in figure 5.

Figure 5: Board stacking order and mass estimates. Blue lines show distances between boards, purple lines show distances from the Z-axis origin. Note that the measurements on the right of the diagram are from the top of the PC-104 connector.

Assembly will require two sets of four 15mm aluminium standoffs (for OBDH and PSU), one set of four 25mm standoffs (for COMS), and one set of four 20mm aluminium standoffs (for PAY). The latter set of standoffs will also mount the midplane standoffs at their top, which will brace the steel posts to the side of the satellite. Four stackthrough PC-104 connectors are required – one for each board except the payload and COMS boards. The former board should have a lighter non-stacking connector that does not require a socket; the latter requires two connectors to make up its height.

Board mass distribution

This distribution assumes an even distribution of components on the sub-boards, with the exception of the PC-104 connector along one edge. Each individual 10mm-high PC-104stackthrough connector weighs 16g.

Figure 5 also shows the mass estimates of each subsystem, along with the mass of the PC-104 connector.

To comply with constraint 2.2.17 [1], the satellite’s centre of mass must be within 20mm of the geometric centre of the satellite. The subsystem circuit board layouts are incomplete, but rough calculations have been performed to validate the mechanical design on this point. We assume for now that the centre of mass for each board is located in the geometric centre in the X and Y axis. The centre of mass in the Z-axis is unknown but can be estimated as somewhere between the surface of the PCB and half way up the board’s clearance allocation (so for the COMS board this is 12.5mm above the PCB).The centre of mass () of any system of objects is given by:

Where is the position of the centre of mass of object , and is the object’s mass.

The centre of mass on the Z axis was determined as above to be located between -0.39mm and -6.79mm. This is within the required specifications.

For the X and Y axes, the assumption about the centre of mass of each board being at the geometric centre will only be sound if the board layout is fairly homogenous. For OBDH and PSU, the prefabricated boards appear to have a roughly even layout (although the centre of mass has not been accurately measured for either). The COMS system’s transceiver weighs 120g, and so it should be placed as close to the centre of the board as possible. The payload board - which incorporates the ADCS circuitry and the camera – has the most complex design, but unlike COMS there is no single large, heavy unit to accommodate and it will be easier to design with an even weight distribution.

Board and connector shapes

The rods that hold the boards in position are not in a perfectly square formation, and the subsystem PCBs must have cutouts as shown in figure 6. The location of these must be common throughout all the subsystem boards.

Figure 6: Positions of centres of structure rods, PCB dimensions and CubeSat kit bus connector, with respect to the centre of the lower right rod. All dimensions are in millimetres, with the origin at the lower right post cutout (shaded red). Blue numbers indicate bus positioning. Red numbers indicate rod positions. Green numbers indicate x axis positions. Orange numbers indicate y axis positions [3].

References

[1] CubeSat design specification: (retrieved 19th August 2009) – California Polytechnic University.

[2] CubeSatkit specs: (retrieved 19th August 2009) – Pumpkin inc.

[3] PLM-PSU-MechInter-308-1 (David Gray)

[4] (retrieved 18th august 2009) – Pumpkin inc.

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