Thermal Control Flashcards
Why is ammonia used in 2 phase heat pipes?
What is usually chosen to be the wick material?
- Ammonia has a low mass compared to other coolants
- low boiling temperature
-Al is usually the wick material
What are the advantages of two-phase heat transport?
passive device is the liquid cycles through the system
very reliable and insensitive to microgravity
large heat transport at low temperature gradients
What do the acronyms LHP and CPL stand for?
Loop heat pipes and capillary-pumped loops
What is an ablative heat shield and what is the concept behind how it works?
ablative heat shield is a layer of material on a spacecraft that protects it during re-entry into the Earth’s atmosphere. The heat shield is made of ablative materials that burn away during re-entry, absorbing the heat and protecting the craft. The heat shield consists of a layer of plastic resin, the outer surface of which is heated to a gas, which then carries the heat away by convection. The technology was originally used during the early space missions.
Why are ablative heat shields blunt shaped?
To prevent the hottest air from the shockwave due to re-entry from being incident on the spacecraft
What are the 5 types of active control used in a spacecraft?
Heaters
Mechanically pumped two phase systems
mechanically pumped liquid loops
louvres and shutters
refrigerators and heat pumps
under what conditions are active thermal control deployed>?
- For temperature-sensitive equipment where environmental conditions or heat dissapation is otherwise variable
How does a spacesuit regulate the astronauts body temp?
- uses MLI to isolate from the extreme hot and cold of space
- Pumps chilled water within the suit to keep the astronaut cool within the suit
- Heaters in the gloves to keep fingers warm
Describe how a 2 phase heat transport system like a heat pipe works
- When heat is added to a liquid it will eventually vaporise
- Heat pipes take advantage of this by using an evaporator to transfer heat to a suitable liquid causing it to transition
- The resulting vapour then flows down to a condenser where the vapour is condensed and can be removed from the spacecraft
- the process is said to transfer heat in the form of latent heat of vaporisation
What are the four parts of the two phase heat transport system
- Evaporator
- Porous wick material
- Adiabatic Section
- Condenser
What is meant by pyrolysis?
pyrolysis, the chemical decomposition of organic (carbon-based) materials through the application of heat. Pyrolysis, which is also the first step in gasification and combustion, occurs in the absence or near absence of oxygen, and it is thus distinct from combustion (burning), which can take place only if sufficient oxygen is present.
What technology takes advantage of pyrolysis
ablative heat shields which usually have an ablative material such as phenolic epoxy resin reinforced by SS honeycomb panel
What surface finishes yield the coldest a/e
white paints and second surface mirrors
What are the advantages of using a second surface mirror and what is the basic setup?
kapton or teflon with thickness from 12 to 125 microns (thickness proportional to emittance) with a metal deposit underneath.
Altogether this has a longer life span than just a paint coated surface and provides solar and IR protection
What does a typical radiator look like?
honeycomb panel coated in silver-glass (Optical solar reflector single surface mirror tiles)
What materials are used to make MLI blankets?
- doubly aluminized plastic films like mylar or kapton (low e_IR and therefore low a_IR)
- each layer is seperated by a low-conductance spacer like dacron netting
- each layer 10 um thick
What must be considered when testing a mechanical joint on the ground that wont be present in vacuum
thin air gaps exist which modify the thermal conductivity at the joint. To combat this, a filler is used such as soft metals and loaded polymers
Why are thermal doublers disadvantagous?
They are metal blocks that are used to improve conductivity but may incur a large mass cost. A better alternative is usually a heat pipe
What are the advantages and disadvantages of using heaters on a spacecraft?
Advantages: The heater is easy to implement, and can be powered continuously, or, be switched on/off according to the temperature of the controlled element.
It can have local control by thermostats or central control via a dedicated switching unit.
Disadvantages:The need for electrical energy.
The complexity of its control unit, or the reduced reliability if thermostats are used
What form do heaters usually take?
metal-mounted resistors or metal coax cable with the heating element at the cable’s core.
Sometimes radioisotope heater units are used
What are the two dfifferent types of cooler used?
Thermoelectric peltier devices and Stirling coolers
How does a louvre function?
- Mounted on the outside of a radiator panel
- uses an array of metal blades to control the radiator’s view of space i.e. its ability to radiate
- when the array is closed the radiator sees a low emittance surface and its radiated heat is reflected back
what are the advantages and disadvantages of a louvre?
reduced power requirement
mass and presence of mechanisms which may fail
what are the advantages and disadvantages of mechanically-pumped fluid systems?
Flexibility and lack of sensitivity to their orientation and mechanical environment. The fluid flow rate can be regulated (via a variable-speed pump), allowing different levels of temperature accuracy, stability and uniformity
Disadvantages: The power needed to drive the pump and possible vibrations induced by the pump and fluid flows.
