What exactly are re-entry “windows” for astronauts to re-enter through?
In: Physics
The “window” means the acceptable range of angles at which a spacecraft trajectory can reenter the atmosphere. Spacecraft in Earth’s orbit are going really fast. Like 28,000km/h fast. When a spacecraft enters the atmosphere, the air gets thicker as you descend, and this slows down the spacecraft, but it generates a tremendous amount of heat and high G forces.
If the entry angle is too steep, G forces (the braking effect due to atmosphere) will become too large and the spacecraft can break up. Additionally, the steeper the entry angle, the higher the heat flux. This is a measure for the amount of heat absorbed by the heat shield every second. If the heat flux is higher than what the heat shield material can take, the heat shield will fail – most likely it will burn through, and melt and then vaporize the spacecraft.
Conversely, if the entry angle is a bit too shallow, other unpleasant things can happen. Firstly, the deceleration then will be too low, so the spacecraft will travel much farther than it is supposed to. It might end up landing on land or even in rugged terrain (which is disastrous if it was designed to land only in water), on inhabited regions or in busy shipping lanes. Also, although the heat flux – like the deceleration – would be lower than expected, heat is still a problem because the heat shield will be exposed to the flux for a much longer time, so the total heat load may be a lot larger. At some point, all of the protective insulation will have been burnt away, or heat might begin to seep through the shield and temperatures inside the spacecraft might become too high, at which point the spacecraft will melt and vaporize.
The “window” is the safe angle between these areas where the spacecraft can safely reenter.
Here is an experiment for ya, take a golf ball and drill a hole through the center. Then spin the golf ball on the drill bit to see how you did. Odds are you will be off center, you hole will not be “square” to the round surface. If you get it perfect the golf ball will not wobble on the axis. Re entry angles are important, and with the golf ball experiment we can see that it can be difficult to calculate the appropriate place on the round surface.
Re entry is a calculation of angle of entry, to shallow they will bounce off the atmosphere and back into space. Too steep and they will drop too quickly and the heat generated by air resistance will ignite the ship. Location and timing combine to create an entry point that allows a landing point somewhere on the surface of the earth they can be received.
When an object enters the atmosphere the path it’s following is changed due to friction, aerodynamics (shape of the object), heating, etc. It is slowed down for example. Depending on the object and how it dissipates heat, and how it “flies” it needs to enter at a specific point so that it can successfully land (runway, splashdown, etc) at a specific point. The control team backtracks the desired landing area, and uses these characteristics to figure out where it should leave orbit and enter the atmosphere to get to that point in the best way possible.
Most of our space vehicles don’t have the fuel or aero dynamics to enter and fly around like an airplane to have a very big entry window. They get one shot. If they need a different entry window, or enter at the wrong place, that changes the landing site.
To come out of orbit you have to slow down. The ‘window’ is the time you have to slow down with the thrusters you have and still land where you’re aiming.
If you re-enter the atmosphere at the wrong point you’re likely to hit land, rather than sea. Or sea, if you’re aiming for land. It depends what you’re aiming for.
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