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

[mfb]

The additional radiation would be negligible and you would need much more material that can withstand high temperatures, vacuum, the atmosphere, and much more. Once the heat is in the cooling system you are good, you don't need that much cooling fluid.

 

[russ_watters]

Better insulation is pretty much always better than a better cooling system for protection against the effects of outside heat getting in.

 

[sophiecentaur]

Once the heat is in the cooling system you are good, you don't need that much cooling fluid.

Do you mean somewhere to dump the heat that gets through? How would that not imply a large mass would be required?
I understood that a slow re-entry would result in the core getting too hot - that the heat in the shields has to be dissipated, soon after the shields get to max temperature, by the cool atmosphere. I have read that a multiple dipping in the upper atmosphere would cause a dangerous rise in core temperature unless heat could be dissipated during the frictionless portions of the multiple orbits. Any cooling system could only shed heat by radiation (from the back end, I imagine).

 

[Filip Larsen]

Any cooling system could only shed heat by radiation (from the back end, I imagine).

For the Shuttle, https://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/wsp/index.html was used both at lift-off and at reentry for normal cooling of internal systems (e.g. APU's and hydraulics), and these vent water steam overboard. While it would be strange to use this cooling mechanism in orbit where radiative cooling is a reliable option (the Shuttle used radiators on the inside of the bay doors when in orbit) I don't see any reason why it could not be used. Of course, due to expenditure of water the amount of heat it can remove will be limited by the amount of water that is accessible.

 

[sophiecentaur]

or the Shuttle, https://spaceflight.nasa.gov/shuttle/reference/shutref/orbiter/wsp/index.html was used both at lift-off and at reentry for normal cooling of internal systems (e.g. APU's and hydraulics), and these vent water steam overboard

That's interesting but the amount of heat to be dealt with would be much more if water spray were used for re-entry. Latent Heat of water is pretty good but would it be enough? It's like other ablative systems in that you have to carry it with you all the way. A heat pump feeding a radiator would also require fuel mass. Would . could that be comparable with the mass of water needed for the same effect? I think the sums are beyond me and appropriate heat pump technology is probably not there yet, in any case.
But the present system seems to work OK and it's a real commercial issue so we can rely on the industry to be considering the situation; it's very much in their interests not to waste any payload mass.

 

[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.