Thermal Management System Questions

[barrykirk]

I've got a couple of questions about thermal management systems in general.<br /><br />They seem to come in two varieties.<br /><br />1) Re-usable tiles like on the shuttle.<br />2) Ablative Heat shields that are one shot use.<br /><br />Somebody correct me if i'm wrong, but my general impression is that tiles although re-usable are delicate and easily damaged.<br /><br />Ablative heat shields are more robust or put in a more protected location?<br /><br />Ablative heat shields can handle higher re-entry velocities? Earth re-entry direct from the moon as in Apollo.<br /><br />How expensive are ablative heat shields? How expensive are tile systems?<br /><br />The area protected by tile systems like on the shuttle would be much larger than the areas protected on a capsule on re-entry.<br /><br />Is their a difference in weight per area protected for the two systems?<br /><br />It seems that these and other issues of thermal management that I just don't know enough about the subject to bring up, would seriously affect a lot of other launch vehicle considerations.... <br /><br />I brought this up because most other discussions have centered on the relative merits of rocket types and configurations.

 

[vogon13]

IIRC, some of the early Soviet era capsules used plywood (!) for the ablative heat shield. US used carbon phenolic material (it was lighter, I think).<br /><br />I think weight is a greater driver than cost of materials for heat shields. Huygens used a silca tile heat shield. I don't recall what the Galileo Jupiter probe used, but I do remember it was <b>THICK</b>. <img src="/images/icons/wink.gif" /><br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[tap_sa]

Dunno about soviets using wood but at least the Chinese used oak in FSW spysat's reentry capsule.<br /><br />Galileo heat shield was carbon-phenolic.

 

[propforce]

<i>"...They seem to come in two varieties. <br /><br />1) Re-usable tiles like on the shuttle. <br />2) Ablative Heat shields that are one shot use...."</i><br /><br />There are also other choices, e.g.,<br /><br />3) Heat Pipes<br />4) Active Coolings<br /><br />However the other 2 methods have very specialized uses. Heatpipes are used on a small area such as a lead edge, and active cooling methods require you carry coolants (= weight) that's to be consumed.<br /><br />Weight comparison between reusable tiles and ablative heat shields depends on its temperature capability as well as durations. <br /><br />Shuttle has heavier TPS tiles because it has a higher acreage (wings & belly) requiring protection. A capsule would have a much smaller acreage for protection. <br /><br />A capsule also will have a lower peak heating rate than compared to Shuttle. Capsule will weight less than Shuttle Orbiter, therefore will subject to a lower aerodynamic heating. Capsule will also have a shorter time in reentry without Shuttle's cross range capability, therefore it's TPS will absorb less heat. <div class="Discussion_UserSignature"> </div>

 

[barrykirk]

OK, Lets see.<br /><br />The total amount of heat energy generated depends on the change in velocity squared and the mass of the object being de-accelerated.<br /><br />That energy has to be carried off in some fashion.<br /><br />1) Radiation - The heat shield glows.<br /><br />2) Convection - The air moving past the heat shield gets really hot.<br /><br />3) Conduction doesn't apply.<br /><br />4) For an ablative heat shield, some of that energy goes into heating and then vaporizing the heat shield. Although some solid or liquid chunks may fall off during the process.<br /><br />The slower / longer it takes to de-accelerate, the more time their is for radiation and convection to do their magic. Is that why people like lifting bodies? To keep the re-entry in the upper atmosphere for longer so that the re-entry time is extended?<br /><br />These are just random musings on my part.<br /><br />I know it's sci-fi at this point, but correct me if i'm wrong.<br /><br />As the vehicle passes through air it ionizes that air. Can a magnetic field be used to "capture" some of that ionized air and use it as a protective blanket, to absorb or reject some of that generated heat?<br /><br />Has any research been done on this?<br /><br />Just a thought.

 

[vogon13]

Captured air would be very hot from instantaneous accel to craft velocity, you would not want to keep it around, let it rather absorb the kinetic energy of the vehicle and then be dispersed (with its heat) to the void.<br /><br />Your idea is not without merit, however, perhaps it could be used to modulate the flow field and sustain the lift of the craft (and therefore it's altitude) for a longer deceleration period in the rarefied upper atmosphere, reducing peak heating, and since you are lengthening the time to dissipate the enormous kinetic energy, you would also be lowering the average heating.<br /><br />Of course, your idea would be practical for any craft at any body with an atmosphere.<br /><br /><br />Excellent!<br /> Montgomery Burns<br /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[propforce]

Oh no, you forgot about conduction.<br /><br />Conduction is very important. In fact, some says it's all about conduction baby !! <img src="/images/icons/laugh.gif" /><br /><br />Think about it, the heat comes into the <i> surface </i> of TPS (whether reusable tiles or ablatives) via convection, where does it go? If it stays in the 1/8 ich surface, then it will quickly build up to a point of exceeding the heat capacity (Cp) of the TPS material. At this point, it will undergoes a "phase change" to liquid (melt) or gas (ablate).<br /><br />The heat will be "drawn" into the inner layer of material via conduction. The sizing of how thick of a TPS depending on how fast the heat is coming in (convection) and how long of exposure (conduction of heat within the TPS material). The problem with the wing leading edge (WLE) is that it has a very high heating rate but can only afford a very thin layer of TPS material to absorb the heat. One of the challenge of Shuttle TPS design is how to avoid this heat being conducted into the vehicle structure, which is aluminum, as well as to handle the high temperature difference between the TPS and the aluminum structures. <br /><br />If the conductivity of TPS is too slow relative to the heating rate (conduction), then the TPS will char, melt, and/or outgas. If the conductivity of TPS is very fast relative to the heating rate, then its surface will be relatively cool and temperature distribution is relatively uniform inside.<br /><br />When you think about it, it's not that much different from BBQ a hamburger or a steak, really <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> </div>

 

[propforce]

<i>"... As the vehicle passes through air it ionizes that air. Can a magnetic field be used to "capture" some of that ionized air and use it as a protective blanket, to absorb or reject some of that generated heat? ..."</i><br /><br />I guess you could stick a turbine into the air stream and try to recoup some energy as electricity.<br /><br />But the turbine would melt.... only if we could solve that material problem.... hmmm....<br /> <div class="Discussion_UserSignature"> </div>

 

[barrykirk]

I was thinking more along the lines of a fusion magnetic bottle, where the plasma is at millions of degrees, but it doesn't touch the container walls.<br /><br />Now if we can obtain such a state where we still get a drag force causing de-acceleration, then all of the heat energy goes into the air passing by the re-entry vehicle and none of it goes into the heat shield. That of course would be the ideal situation.

 

[propforce]

That would be nice... but one must understand the underlying physics first before we can devise an engineering solution.... <div class="Discussion_UserSignature"> </div>

 

[magick58]

a little off topic, but if you just slow the object (Shuttle, capsule, lifting body) down more, befor it hits the atmoshere there is a lot less heat to be disipated. But Balistic re-entry seems like something we like to do.

 

[vogon13]

Look how much fuel it took to get the orbiter up to 17000 + MPH. Have to take a great deal of fuel to slow it in orbit drastically.<br /><br />And you still have to dissipate the potential energy as the shuttle drops 250 miles from orbit.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>