SYSTEM DESIGN NOTE
SDN0007.04 - Baseline Thermal Design
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Prepared by |
Date |
Approved by |
Date |
Rev. |
Rev Date |
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J Elias |
6/21/99 |
N. Gaughan |
6/22/99 |
A |
5/12/00 |
1. Introduction
This document is intended to summarize the thermal design of the instrument resulting from the discussions in the preceding design notes (SDN007.xx). This document has been updated (5/00) by removing discussions of design choices no longer under consideration (for example, use of castings for the bench structure).
The design described in this document supersedes (where different) anything described in SDN007.
2. Instrument Thermal
Design
2.1 Number of Cryocoolers
The instrument will have 4 cryocoolers. The baseline permits use of either the Leybold 130/150 heads or the RGD 5/100 heads. The larger number of heads is needed primarily to support the final stages of cool-down (SDN007.02), although “stiffness” with respect to varying ambient temperature is also an issue (SDN007.01).
It is unlikely that the instrument can operate successfully if a cryocooler fails, as an inoperative head becomes a heat leak. No analysis of this situation has been done, however.
2.2 Active Thermal
Control
There will be active thermal control of the cold mass, for a number of reasons. The OIWFS ICD strongly recommends such control. In addition, analysis of the effects of temperature variations (SDN007.01) shows that spectrograph refocus is required in the absence of such control. Thus, although operation of the spectrograph does not require temperature control, such control is likely to be operationally simpler than adjusting focus for temperature variation.
The baseline assumes it is possible to operate using a single control loop. That is, a single control point is adequate to maintain temperature stability for both OIWFS and spectrograph channels.
2.3 Separate Active
Shield
An “active” radiation shield should be provided that is thermally decoupled from the optical bench (except to the extent that they are cooled by the same cryocooler heads). This is done in order to minimize gradients and variations in gradients within the bench (SDN007.01).
Parameters for the active shield are discussed in SDN007.03.
2.4 Floating Shields
The passive or floating shields will be 2 thin polished sheet-metal shields, rather than multi-layer insulation. As shown in SDN007.03, the performance of the former is sufficient for the instrument requirements. MLI might reduce heat input to the point where only two cryocoolers were required, but its performance is more difficult to model, so the 2 head/MLI approach must be considered one with higher risk. MLI will also increase the time required to evacuate the dewar.
This decision can be revisited if the sheet metal shields lead to weight or space problems. MLI is also available as a means of risk mitigation (shield performance worse than model predictions) since it can be added to the sheet metal shields.
2.5 Liquid Nitrogen
Pre-Cool
A simple liquid nitrogen pre-cool system will be included. Its design should be kept simple, as calculations show (SDN007.02) that such a design suffices to reduce the initial cooling (to 80 K) to less than 24 hours. Operational considerations may lengthen this somewhat, for example if running the pre-cool at full efficiency overnight is impractical. The pre-cool system is expected to cut cool-down time by 2-3 days, which is desirable for integration and test provided operation of the pre-cool system does not become a major expense in terms of manpower.
The design should permit cooling of the instrument with the cryocoolers alone, although it is uncertain whether they can do so in under 4 days (the Gemini requirement).
There is no reason to remove the pre-cool system even if Gemini prefers not to use it, as its weight should be modest.
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