[ERPS] more on transpiration cooled TPS

[Pierce Nichols]

One of the things that Mike left behind when he had to go back to Canada was a bound report on the Chrysler SERV project. For those not familiar with it, it was a paper SSTO entered in the initial shuttle competition in the late 1960s/early 1970s. It's basically a giant Apollo capsule (90 foot diameter) with an aerospike engine, and therefore is a close approximation of what ERPS and others have contemplated as the ideal form of an SSTO. The study includes an extensive discussion of TPS and re-entry heat loads, including replaceable ablators and re-radiative heat shielding.
As it turns out, the heat loads for this sort of vehicle aren't all that bad. Peak radiative equilibrium temps 2300F max for the maximum heating case and 2600F for the peak heat loads case, assuming emissivity of 0.85. The lower one is within the upper service range of certain high temp alloys, btw. Peak heat transfer for the peak loads case is 35.8 BTU/sec-ft^2, or about 406 kJ/sec-m^2. That latent heat of vaporization of water is 2360 kJ/kg. Therefore, you only need to transpire 0.17 kg/sec-m^2 of water at the point of maximum heat transfer, even if you completely ignore the heat absorbed by the water as it is heated from feed temperature and the additional insulation provided by film boiling (if you use a wet heatshield).
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.

-p


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