Why couldn't the space shuttle slow down in space?

[Filip Larsen]

Latent Heat of water is pretty good but would it be enough?

Considering that the lowest possible amount of total orbital mechanical energy that needs to be dissipated on reentry is enough to vaporize any vehicle "made of water" around 13 times over, you would indeed need a way to avoid handling most of this energy internally, for instance by radiating the energy away "at once" by a heat shield. By the way, the total specific mechanical energy to dissipate from a circular orbit of altitude $h$ is $$\varepsilon = \left ( \frac{1}{2} + \frac{h}{R} \right ) \frac{\mu}{R+h}, $$ where $R$ is radius of Earth and $\mu = GM$ is the gravitational parameter for Earth.

For the Space Shuttle the above energy amount corresponds to around 2.2 MW dissipated when averaged over 1500 sec (peak power is then obviously much higher), so that is kind of an upper limit on the cooling capacity if the vehicle where to soak up all orbital energy internally. Finding the exact internal heating profile for an actual vehicle in an actual reentry is of course a bit more involved.

I found an old heat analysis paper that goes into some details on the calculated heating profiles at various points on the Shuttle during reentry, including internal points, where it can be seen that even the internal heating load is significant enough to be a technical non-trivial problem in a vehicle that already have significant design constraints originating from other concerns.

 

[sophiecentaur]

vaporize any vehicle "made of water" around 13 times over

Right, thank you. That's the sort of useful figure that's needed. It puts the mockers on such a simple approach. The present system means that you basically need to delay the heat getting to the core until the atmosphere provides the cooling.