I remember this from Apollo 13, they had to hit the atmosphere at an angle, if they came in too directly they’d burn up. My stupid layman thought is that I’d want to come in directly because if the atmosphere is making me burn up I’d want to take the directest and shortest route to landing so that there’s less atmosphere to burn me up. Obviously that’s not how it works, why not
In: Planetary Science
A heat shield on a re-entering spacecraft has to contend with heat in two different forms: peak heating and total heating. Coming straight back into the atmosphere decreases total heating, but increases peak heating. Taking a long gentle angled path across the edge of the atmosphere increases total heating, but decreases peak heating. You need a happy medium for both.
As a re-entering object rips through the atmosphere, it compresses the air to the point that the air starts to heat to the point of glowing (this is what you see when you see a shooting star), and the glowing air radiates heat into the object. That heat has to go somewhere, or the spacecraft will melt. Most of the human space capsules that have re-entered use ablative heat shields — cork or some other carbon-based shield with a surface that burns off gradually to carry away the heat. If your path through the atmosphere is too long, you don’t slow down particularly fast and so you remain at ultrasonic velocities for a long time, which means that your heat shield might burn away completely. That would be bad. But if you dive straight down into the atmosphere, you hit the very dense atmosphere much more quickly, and so the air gets MUCH hotter, hot enough that your heat shield cannot carry away heat quickly enough and so it fails. That would also be bad.
The Space Shuttle and the SpaceX Starship (and the X-37) use radiative tiles instead of ablative heat shields. Radiative tiles are able to get rid of waste heat by simply radiating it away, which works very well; they don’t get “used up” at all. But radiative tiles can’t handle the same level of peak heating that an ablative heat shield can manage, so the Shuttle and Starship have to use a much more gradual trajectory. Spending more time on re-entry isn’t a problem for these vehicles since the heat shield isn’t burning away and thus won’t get used up.
The Shuttle also had the additional problem of aerodynamics; it had to be able to maintain aerodynamic control during re-entry while also being able to fly well enough to land on a runway. So it had to do a bunch of additional things to maintain the correct trajectory.
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