# Presentation: 033958

## SNAP Strawman Spacecraft Bus Structure

**11 March 2005**

## Structural Functions

**Houses and supports spacecraft functions including:**
**Avionics.**
**Electrical power.**
**Communications.**
**Guidance.**
**Propulsion, including fuel.**
**Current estimated mass of above items is 465kg.**
**Provides mechanical support between launch vehicle and:**
**Optical telescope assembly (OTA)/instrument (~550kg).**
**Stray light baffle/cover/solar array/radiator (~479kg).**

## Structural Requirements

**Launch environment:**
**Minimum observatory natural frequencies:**
**10Hz lateral (current design > 18Hz).**
**27Hz axial (current design > 31Hz).**
**Minimum secondary structure natural frequency = 35Hz (OTA natural frequency > 39Hz).**
**Current limit design loads:**
**12.5g axial.**
**8g lateral.**
**Current design has safety factors >5 at limit loads.**
**On orbit**
**Observatory free-free natural frequencies high enough for ACS req’ts (current design > 15Hz).**
**Thermal/dimensional stability to minimize influence on OTA.**
**For integration & test, and on orbit, spacecraft must have minimal distortion influence on OTA.**
**Tight tolerance on interface between spacecraft and OTA.**
**Instrument base ring stiffness dominates spacecraft stiffness.**
**Thermal isolators between spacecraft and OTA/Baffle.**

## Baseline Design Description

**Central thrust tube:**
**4mm thick aluminum hollow cylinder.**
**Diameter approximately 1666mm to match EELV payload adapter.**
**Approximately 500mm tall.**
**Spacecraft equipment deck:**
**~2.6m outside diameter, ~1.7m thru hole.**
**Aluminum honeycomb construction:  1.25mm Al skins, 32mm thick core.**
**Supports spacecraft equipment on ‘bottom’ side between gussets.**
**12 gussets between thrust tube and spacecraft equipment deck:**
**Equispaced in a polar array around thrust tube.**
**Aligned with mounting points for OTA and baffle supports.**
**4mm thick aluminum plates.**
**12 load spreaders:**
**Spread concentrated loads at OTA and baffle mounts and distribute them more evenly at the level of the EELV payload mount ring.**
**Essentially in the form of a truncated cone, with 12 cutouts that allow access and cooling to deck-mounted equipment.**
**4mm thick aluminum plates.**
**Internal fuel deck:**
**Support up to 8 420mm dia tanks plus plumbing and rocket motor.**
**Aluminum honeycomb with 0.5mm Al skins.**
**Spacecraft structure mass ~158kg.**

## Telescope/Baffle/Array/Radiator Config.

- Bipod Mounts for Telescope Structure

- Solar Array & Thermal Radiator
- Mount on Baffle (adiabatic)

- Pedestal Mount for Light Baffle Structure

**Telescope Ring & Deck**
-  Ring is Primary Load Path
-  Stiff enough for OTA tests
-  Modest Light Closeout Deck

**Telescope/Baffle/Array/Radiator Config.**

## Strawman Spacecraft Config.

- Aluminum Honeycomb Equipment Deck

- Internal Fuel Deck
-  8 – 16.5”** Tanks will Fit (257 kg)
-  w/ Pressurant Tanks for Slosh

- Aluminum Central Thrust Tube (1666 PAF)

- Aluminum Load Bearing & Spreading Gussets

- Avionics Mount as Desired
-  Radiant Box Cooling is Baselined

**Strawman Spacecraft Config.**

## Optional Expandability

-  2.5 m Diameter Primary Light Baffle
- Significant radial clearance from baseline S/C to EELV shroud.
-  IMDC / Team X Avionics Comfortably Fit 0.5m Tall Bus
-  Box Details are in the Solid Model
-  Deck & Boxes can Grow Outward to ~3.5 m dia. , If Needed

**Optional Expandability**