At 04:00 PM 2/3/2003 -0800, David Weinshenker wrote:
Pierce Nichols wrote:
> Another thing that comes out of my reading of the SERV document is
> how much the heating will vary across the base heatshield and across the
> re-entry profile. At peak heating, the coolest part of the base heat shield
> has a heat transfer rate of 244 kJ/sec-m^2. Therefore, relatively fine
> control of the amount of water being delivered to each portion of the heat
> shield will need to be readily controllable in order to prevent the waste
> of a great deal of water.
Or an ablative shield could be thickness-contoured to suit the variation of
total heat load over its surface.
How does overall mass-effectiveness of a solid ablator compare with that of
a fluid-cooled "wet" shield? The solid one has the advantage that it's basically
a static structural panel - it just has to be there, it doesn't require plumbing
and accurate control of liquid flow.
I think that the water-transpiration might be slightly more mass efficient, because water has a far better heat capaity per kilo then any ablator material. However, the ablator also, as Henry pointed out, doubles as insulation, so transpiration loses some there. Water transpiration also has the support gear and tankage, which cuts into its mass advantage. I think that it's very hard to say a priori which is more mass efficient, and it may depend on the specifics of the application. However, the (apparent) big win of a fluid cooled shield is not in mass, it's in operations. Beyond the obvious labor savings of filling a tank v. changing out panels, the water system is also much easier to test as part of the pre-flight, or even on orbit. You can just turn on the water and observe the flow out of the heat shield pores. With a sectional ablator, you have to test each panel's connections to the underlying hull and its neighbors, which is a bit trickier to validate.
-p
Mars or Bust!
www.marssociety.com
_______________________________________________
ERPS-list mailing list
[EMAIL PROTECTED]
http://lists.erps.org/mailman/listinfo/erps-list
