why is atmosphere reentry such an issue? If it is just because of speed hitting the atmosphere why can’t the spacecraft just slow down before and synchronise with earth rotation?
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Sidequest: you can have less trouble with speed matching if you landed on the rotation poles center coming from afar and aligned with the pole normal vector, although not very practical afterwards 😆
If your spacecraft has an incredibly powerful motor, then it could be possible to cancel orbital speed (25 000 km/h at ISS altitude) down to about 1600 km/h which would match the rotational speed of the Earth. However, I don’t believe a motor exists that could reduce the speed to such a degree faster than the amount of time it takes for a spaceship to break orbit and start the re-entry. Also, if a spacecraft comes in from another place, or a higher orbit, the speed would be even greater.
I’m willing to bet, without having calculated anything, that a human would not survive the massive deceleration from 25k+ km/h to 1600 km/h in such a short time.
Even if a motor that could perform this feat existed, it would also have to carry enough fuel to complete the operation. We’re not talking about a measly Dragon for that.
When you are in orbit you are going around the earth, in outer space (where there is basically no air or friction), fast enough that you move sideways as fast as you fall downwards. To go that fast takes a lot of energy (generally the entire amount of energy in the rocket). Since rocketry requires that you take all the fuel for everything you’ll be doing, that would mean we’d need something like (this is fairly inaccurate but run with it) the amount of fuel in 2 rockets, which would need even more fuel to get that much fuel into space. It is a problem that gets worse as it grows.
Then, if we did stop ourselves in space, we would still be falling from space to earth, which is fairly dangerous and hard anyways. It is far easier and efficient to take advantage of the atmosphere and it’s friction to slow us down. We essentially get to get rid of all that extra speed and energy we have for almost free!
The reason reentry is so hard is because the craft goes from spaceflight (no friction) to air flight (friction), which means it must be piloted in a way that accounts for both. If you hit the atmosphere too deeply, you just burn up. Too shallowly, and you will skip off like a skipping stone on a pond.
In addition, because of the forces and temperatures involved, there is little room for error and virtually no chance to do anything if something goes wrong.
The SpaceShip1 that won the first X-prize found away around all that. The creator looked at a badminton shuttle and thought “If my ship could do that it would do re-entry without protective tiles!” The trailing wings turn up to create fantastic air resistance even in thin atmosphere.
NASA told them they would die on re-entry and were totally blown away when it landed perfectly on its maiden flight.
As folks noted, it would take tons of fuel to slow down. The enxt problem is most low orbit craft are around 350 miles up. If you slow them to zero mph over the ground/air mass, they then go into true frefall. Frefall in a vacuum gets you going really fast, really quickly. From 350, you accelerate to over 7400mph, so you are not much better off than before.
So now you can’t really slow down until 50k feet or so, and are traveling at 11,000 feet per second. So not much time for everything to go right, insane energy to bleed off without break equipment, or killing people with g forces.
Even from a simple first principles perspective, the combined kinetic and potential energy of being in orbit is insane. You have to bleed off that excess energy off somehow to return back to the state of “stationary” at ground level. It just so happens that converting all of that energy into thermal energy that you then deal with via heat transfer is the most cost effective alternative.
It’s all basically the rocket equation. To speed up or slow down costs fuel, but fuel itself adds weight. So you wind up carrying more fuel, just to offset the fuel, which adds more weight…
Hitting the atmosphere at a speed that we can design heat resistance for simplifies things as you are no longer having to add more fuel to slow down all the way. Everything in space is about weight because it requires more fuel to move.
To get to orbital speed a rocket has to use all its fuel, so to get the speed back down to zero it would need to use basically the same amount of fuel.
So for example you would need a fully fueled Falcon 9 in orbit to get a Dragon capsule back down without atmospheric heating.
You can’t just make it have twice as much fuel either, because you need more fuel to lift the extra fuel, then even more fuel to lift that extra fuel, and so on.
For example a fully fueled Falcon 9 weighs 500T. Even a rocket the size of a Saturn 5 can only lift a 140T payload.
So basically you could need an absolutely enormous rocket (the tyranny of the rocket equation), so it’s actually much easier to use the atmosphere to provide breaking for free.
The problem is how you slow down before you hit the atmosphere.
On Earth we mostly slow down due to friction against the earth itself in some way or the other… we kinda dig in our heels against the planet directly or indirectly (our car brakes apply friction against the tires which then apply friction against the road).
But… what if you are in space? NOTHING to brake against? How do you slow down???
You do it by throwing stuff away from you as hard as you can.
Think about two ice skaters that push off from each other. They’ll move in opposite directions.
Now imagine that the two skaters are moving QUICKLY in the same direction. If they push off each other one of them will be moving faster in the original direction, but the other will be moving more slowly in that direction.
That’s basically the only way you can change velocity in space (not going into slingshot stuff here).
So… we can’t just throw equal masses in the opposite direction to reduce our velocity since that means shedding a lot of our total spacecraft all the time. Instead, we burn fuel to throw much smaller masses out at large velocities.
To slow down a lot we need either a BUNCH of mass or a way to accelerate a smaller mass a LOT. Right now that means using small masses that interact explosively so that the small mass can slow us down a lot.
With enough fuel you can slow down as much as you want… but our problem is that you have to get all that fuel (mass) to space first… and that means spending a LOT of fuel/mass/energy to get it up there to being with.
Maybe in the future we’ll discover some better way to travel to and from space, but right now it’s all about mass and momentum… and that’s some very costly physics!
The speed to remain in orbit is something like 7,000 miles per hour. Slowing down from that speed would take an immense amount of fuel – the mass of that fuel and the size of storing it would require even more fuel to slow it down. Re-entry relies on the density of the atmosphere to bleed off most of the speed due to friction, which creates a lot of heat – the heat is either bled off with ablative surfaces, or insulated (like the Shuttle tiles) with a very complex series of dips and turns to control how hot the exterior gets.
Apollo 10 gets the re-entry speed record, at (IIRC) 24,000-ish MPH.
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