I watched Star Trek and Star Wars series and movies for many years. I have a question for you. I noticed that spaceships do not burn through atmospheric entry. One episode in Mandalorian series showed that his broken ship attempted to enter atomsphere but did burn through. I believe that spaceships uses anti-gravity brakes to slow down before enter atomsphere.
Does anyone have know any formula to minimum velocity for avoiding atimospheric entry burn? I think that they have to reduce velocity to a few hundred miles per hour prior entrry to avoid burning.
Well, what they mean is powered descent. More or less what Falcon9 does at a lower altitude. I guess there is no set speed limit, as 'burn' doesn't really happen, it's just ionized gases. More like a speed at which ionization doesn't happen, but that might still mean it gets hot. If you have unlimited engines etc, you can absolutely plop it down at 300m/s or something and no burn would happen. Like, it may not even be in a classical orbit, but just hovering in space. It may not even have much energy to bleed
In real life, it's always a trade-off between heat and G force and time. The centrifugal force keeping you up decreases in theory gradually, but in reality this doesn't help you much below, say, 6500m/s. Gravity works as a vector pointing down, as if it were just the core of the planet as a source. Technically speaking, you start decreasing your perigee slowly , a few dozens km is nothing compared to the planet radius, but suddenly you have the issue of now having the planet in your way, so to speak. And density variation means the lower you go, the more you decelerate and heat up etc, and so you need lift and/or proper thermal protection to survive. And even if you have movie-grade thermal, a bad trajectory might mean you end up too fast in the lower atmosphere and then the G loads when it decelerates there are not human-compatible, even if the ship holds, or you don't decelerate enough and then you sort of 'decelerate by using the ground'
Then again, if they used warp drive, the ship wouldn't technically be moving so there'd be no energy to lose. Even if , say , they fell from a hover above the atmosphere , we'd be talking of something like 1000m/s vs at least 7200. It would heat up lower down, but it's mostly Mach3-4 coming straight down and decelerating hard in the troposphere.
The only way to avoid (much) heating IRL would be to have a huge surface to weight ratio, either by design or by having a huge parabolic heat shield/airbrake structure deployed. Perhaps a big empty SSTO might need less thermal shielding, but anything that goes for a re-entry like that must be unmanned, since it would be pulling hundreds of Gs