Self-accelerating projectile for a space gun. How realistic is this?

Question

Space

After reading an article about the Slingatron - a variant of a space gun for launching a small load into a near-Earth orbit, I made 3 conclusions:
1. It is possible to accelerate the projectile to the first space velocity (or close to it), and, in principle, it is possible to shoot it.
2. Such a projectile will lose most of its speed, passing through the dense layers of the atmosphere.
3. The projectile will get very hot.
4. It still needs an engine to enter Earth orbit.
Hence, the question is: if, when a projectile is launched, kinetic energy is converted into thermal energy, then is it really impossible to come up with a way to convert it back into kinetic energy?
For example, it is possible to place a payload in an outer case made of a material that is strong, heat-resistant and has good thermal conductivity, i.e. some kind of metal. Fill the space between the hull and the payload with some substance that accumulates the received heat and allows it to be converted into kinetic energy, for example, it can be the same water that, under the influence of heat, will evaporate through the nozzle, giving the projectile jet propulsion, or twist some then a turbine, which in turn can turn a propeller, or feed some kind of electric motor.

Of course, it will not be possible to achieve an energy balance between the energy received during heating and the energy necessary for overclocking due to losses during heating of the surrounding air, but still I want to understand how realistic this is?
The advantage of such a system is that something environmentally friendly and cheap can be used as fuel, again, combustion as such does not occur.
Are there experts here who can make estimates?

Answer 1

There is no point in this. More water - more mass - more power to launch - less usable mass. No calculations are needed here. You are going down the path of trying to reuse wasted energy when it is much more efficient to go down the path of reducing energy waste.
At supersonic speeds, the aerodynamics are very different than at subsonic speeds. Have you seen the sharp nose of the concorde and other supersonic aircraft? And at subsonic speeds, the nose can be made round and blunt - this has practically no effect on drag. Air resistance at supersonic speed is highly dependent on the cross-sectional area of ​​the aircraft - that's why the same concrod is long and narrow.
And yet, for example, there is an idea with a plasma "fur coat" (yes, since Soviet times), which reduces friction to almost zero.

Answer 2

Pretty interesting idea by the way. Only it seems to me that it would be more interesting not to accelerate the projectile, but to decelerate the descent vehicle upon entry into the atmosphere. Instead of water, it is better to take dry ice, for example (CO2 in solid state). It easily passes into the gas phase.