Comparing Fig.2.7 with Fig.2.5, one can easily spot out the effect of
different electrical conductivities (graphite and tungsten). Current
flows through tungsten up to midway of the heat shield before actually
flowing through the heat shield material. The part of the heat shield in
contact with tungsten is cold because it is easier for current (which
produces Joule heating) to flow through tungsten and not graphite. On
the other hand tungsten has a lower specific heat capacity \(\left(132JKg^{-1}K^{-1}\right)\) compared to graphite \(\left(710Kg^{-1}K^{_{-1}}\right)\). This implies that heat holding
capacity of graphite is 5.4 times higher than that of tungsten. Again
compared to graphite, tungsten has higher thermal conductivity making it
act as a heat sink. Hence Fig.2.5 and 2.7 are not desirable and not what
is intended to be achieved. Hence further improvement through prototype
optimisation will result in better hypersonic re-entry temperature
fields.