Design of a Heat Pipe for a Lunar Lander

Diplomarbeit von Carina Buck

Introduction:

At the Milwaukee School of Engineering, senior students are required to take part in a Senior Design Project during their final year for 2 to 3 quarters. The project is a group project related to a field in mechanical engineering. Students participating in the exchange program between MSOE and Fachhochschule Lübeck have to be enrolled in the Senior Design Project for 3 quarters. During this time the student has to write his or her diploma thesis as an individual work within the topic of the project.

This Senior Design Project was in the section ‘Energy systems’. The task as a group was to design a thermal control system for a Lunar Lander (see Figure 1.1) in cooperation with NASA´s Exploration System Mission Directorate. ‘A Lunar Lander will be exposed to extreme temperature differences. There is a need to control the thermal environment within the lander in order to provide functionality for both personnel and equipment. Previous lunar missions utilized consumable materials for cooling. Future lunar missions will require a more robust thermal control approach, one that allows longer duration missions while minimizing resources. Compared to the previous Lunar Lander, the new lander will be larger to include an additional astronaut as well as additional equipment. The thermal control system must be capable of handling this increase in thermal energy. After the evaluation of a number of possible systems based on research and in depth feasibility in the fall quarter the three most promising systems were chosen by the group to be examined in greater detail. The aim of this project was then to produce a design for each of the remaining thermal control systems until the end of the winter quarter’.. The first two quarters ended with a presentation of our accomplishments to a committee of professors at MSOE and an invitation to the Marshall Flight Center in Huntsville, Alabama by NASA to present our designs to a committee of scientists. For the spring quarter we chose two experiments to be performed. One was the building of a vacuum chamber in order to test the thermal properties of the lunar regolith simulant. The other one was the building and testing of the heat pipe design.

Table of Contents:

	List of Figures	5
	List of Tables	6
1.	Introduction	7
1.1	The Senior Design Project at MSOE	7
1.2	The Specifications and Requirements given by NASA	8
1.3	The Focus of my Thesis	10
1.4	The Schedule for the Completion of the Thesis	11
2.	Background Research	12
2.1	The History of the Heat Pipe	12
2.2	The Principle of a Heat Pipe	13
2.3	The Theory of the Preliminary Design of a Heat Pipe	16
2.4	Theory of the Design of the Heat Pipe	26
2.5	Theory of Building a Heat Pipe	28
3.	Preliminary Design of the Heat Pipe for the Lunar Lander	29
3.1	The Working Fluid	29
3.2	The Material of the Heat Pipe	33
3.3	The Limitations for Ammonia at 240 K	35
3.4	The Thermal Resistances	41
3.5	The Preliminary Design Outcome	48
4.	Design of the Heat Pipe for the Physical Model	49
4.1	The Viscous Limit	49
4.2	The Boiling Limitation	50
4.3	The Capillary Limitation	51
4.4	The Pressure Limitation	52
4.5	The Thermal Resistances	53
4.6	The Calculations for the Tube	60
4.7	Calculations for the End Caps	65
4.8	Drawings	71
4.9	The Design of the Wick	73
4.10	The Input/ Output Devices	73
4.11	The Fill Tube/ Valve	73
4.12	The Pressure Gauge	74
4.13	The Temperature Device	74
4.14	The Working Fluid	74
4.15	The Weld	74
5.	The Building of the Heat Pipe	75
6.	The Experiment	77
6.1	The Objective	77
6.2	The Theory	77
6.3	The First Experiment	79
6.4	The Second Experiment	82
6.5	The Third Experiment	85
7.	Conclusion	88
8.	Works Cited	89

Text Sample:

Chapter 2.4, Theory of the Design of the Heat Pipe:

In the preliminary design several methods are used to choose the container material, the working fluid, the wick material and the wick structure and assumptions considering physical properties are made. In the actual design of the model of the heat pipe this outcome is used as the base for the design calculations. A heat pipe consists of different components that have to be designed, machined and assembled in the process of constructing a physical model. The components are the container, the wick, the input and output devices, the fill tube, the instrumentation and the working fluid.

The Design of the Container:

The container consists of three components, a tube and an end cap for each side.

One end cap has to have an opening for the fill tube for the working fluid. In designing the container, the pressure and stress development have to be taken in consideration in order to provide the necessary safety when operating the heat pipe. To obtain optimal conditions for a leak-proof welding, the wall thickness of the tube and the end caps should be as close as possible.

The Design of the Wick:

The wick structure is important for the capability of the heat pipe to return the liquid.

There are several different wick structures with varying mesh numbers and pore sizes available that are made of several different materials ranging from cotton to metal. The right wick structure has to be chosen based on the requirements for the heat pipe such as the capability, the working fluid, the material – fluid compatibility, the working temperature, the size and the costs.

If using a metal material a wick made of the same material as the container is preferable to ensure a sufficient thermal conductivity.

The Input and Output Devices:

The input and output devices provide the temperature difference needed for the heat transfer in the heat pipe. Depending on the type of heat pipe, different ways and different fluids have to be used to reach the desired temperatures.

The Fill Tube:

The fill tube provides the only access for charging the heat pipe with the working fluid. For safety reasons it should only be as wide as absolutely necessary. After charging, the inlet must be closed and sealed, for example by using a crimp seal followed by welding. For a good crimp, the tube should not be wider than ¼ inch. For certain materials a minimum wall thickness is also required.

The Instrumentation:

In order to measure and record the results of the experiments, several instruments such as multipoint thermocouples, manometers and else, depending on the focus of the testing, can be added to the physical model.

The Working Fluid:

The heat pipe container needs to be charged with the working fluid through the fill tube. Especially for toxic fluids special caution is necessary and the heat pipe should not be overfilled or under filled.