I’ve got an uncle that constantly posts conspiracy theories all the time, latest one is
‘The shuttle heats up during reentry at speeds of 16,700 mph’
‘But while leaving earth at speeds of 25,000 mph there’s no heat?’
In: Physics
When spacecraft launch, they start with 0 speed and accelerate, but that takes time, By the time they’re going fast enough for atmospheric heating to matter, they’re already clear of the densest part of the atmosphere that causes heating in the first place.
On the way back, they’re already starting with a speed of 27,500 km/h. They hit the atmosphere at mach 25 and the atmosphere is the only thing to slow them down. That’s a tremendous amount of energy that has to be lost.
It has to do with air friction and the planets atmospheric pressure in relation to the mass of the craft
The shuttle does most of the accelerating high enough that the atmospheric heating is much less severe (but still present). On the way down the fuel tanks are empty so the only option is to slam into the thicker part of the atmosphere to shed speed.
When travelling at mach+ speeds even normal planes heat up. The two most famous are the SR71 Blackbird which leaks fuel on the ground because the seals were made with the thermally expanded plane in mind and the Concorde whose cockpit expanded enough in flight from the heating that on the last flight before decommissioning the pilots could put their hats between two consoles and when the plane landed the hat got permanently stuck as a memento.
When it’s leaving the atmosphere, it’s trying to accelerate – so it’s making itself as narrow as possible in relation to the air it’s moving through, ie its pointed straight up. When coming back, the opposite is true, because crashing is bad – so it makes itself as wide and un-aerodynamic as possible, which means a lot more air is hitting it than before. (That’s slightly simplified, because using the rotation of the earth to increase your relative velocity is a thing, as is coming back in too shallow and bouncing back off the atmosphere, but the general premise is accurate)
Ignoring that the numbers aren’t exactly right…the entire problem boils down to the shuttle traveling through air; the air exerts forces on the shuttle’s body that result in heating.
When the shuttle exits Earth’s atmosphere to get into orbit, it first takes straight up going relatively slowly. As soon as it gets high enough that the air is thin and the resultant forces (and thus, heat) from moving through the air goes down, then it basically tips over and throttles up to maximum power to get up to it’s top speed (you can typically hear this in the old shuttle launch videos as the “Go at Throttle Up” command, where the shuttle goes from 70% (iirc) throttle to full throttle). Note that there *is* heating during this, just not enough to matter.
When the shuttle reenters the atmosphere, it has to pass back through that air, but this time it can’t really avoid traveling through that thicker cushion of air at high speed (as it uses that same air to slow down). Thus, you get lots of heat generation.
It’s actually due to air compression. This is the same principle that diesel engines use to ignite fuel. https://youtu.be/Bjy6m6MR-PQ
Because a craft entering the atmosphere is moving up to 25x the speed of sound, air molecules simply can’t move out of the way of the spacecraft, and instead tend to build up in front, resulting in heavy compression. This produces a powerful shock wave or “Sonic boom” that acts to slow the spacecraft.
Some heating does occur at launch, for the same reason. However rockets do the vast majority of their acceleration outside of the Earth’s atmosphere.
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