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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To be fair I’m no expert but, if you slow down you fall down, because you lose orbital speed. You can’t slow down without losing height. If you synchronize with earth orbit its like standing still, which means you’re falling straight down
Orbit isn’t just go up until the gravity goes to zero and you float, astronauts in low orbit experience ~90% the same force of gravity as we do on the ground. They don’t appear to “fall” because they move sideways so fast, that the Earth curves away from them at the same rate they fall – thus they are in constant free fall which feels like zero-g.
The velocities to do this are ludicrous, on the order of 8-9 kilometers per second. For context that means the ISS, a 400 thousand kilogram station, travels 10 times faster than a .50 bullet.
It takes so much fuel to get up to speed that if you wanted to slow down with retro-rockets you’d need the same amount of fuel you originally started with at launch, with you in orbit. However in order to get this fuel up to orbit, it requires even more fuel. The math works out that no spacecraft in history has ever even tried to bring fuel when theres this convenient atmosphere you can slam into to slow down. Rather that costing millions of pounds in fuel, it costs a few hundred of heat shield
Your velocity in orbit is the thing keeping your craft in orbit. By slowing down you begin a change of your orbital parameters that leads to a lower and lower orbit, to the point that you begin reentry.
The entire idea of orbit is imagining a cannon capable of firing a ball so fast that its horizontal motion is so much more than its vertical descent that the curvature of the planet drops away faster (or at the same speed) as its ballistic arc.
You could, in theory halt your velocity suddenly and sharply, but at the speeds we’re talking about any tech aboard, or humans, would be crushed to paste by the acceleration.
Not an expert but this was my logic. The thing that maintains u in orbit is speed. If u slow down u loose orbital velocity which means u loose altitude. So slowing down to do away with re- entry burn will not work.
The only way it will work is in sci-fi where orbital mechanics don’t apply
I have a suggestion: spend some money on kerbal space program, and see for yourself.
This game is a gem. You will learn tons with it.
Basically being in orbit means falling forward so fast that you do not lose altitude. If you want to go lower, you need to slow down, in order to fall toward the ground.
The issue is how you could transition from one to the other.
Using atmosphere to brake is “free braking”. The ship “only” needs to convert its kinetic energy into heat, which is what it does, the issue being … that is a LOT of energy, so that also is a lot of heat to shed.
The alternative is to burn fuel, which means bringing more fuel, and making sure the engines can still work. And it is much better not to do that, because that is added weight, complexity and hazard.
This will change when we develop fusion power for spacecraft, then we can use fusion to create thrust from heating captured air on decent. We already understand the engineering, maths, gimballing, and control systems to land from orbital speeds. We just lack a fuel with high enough specific impulse / mass ratio to make it viable.
Of course, a fusion rocket is also a weapon, so I hope that humanity is not all dickheads in that possible future.
You know how when you get a jetpack in a video game or whatever, you could be falling at terminal velocity then just tap the jetpack and suddenly you’re not falling anymore? For the space craft to slow down it’s fighting the same gravitational force it to take off because that force doesn’t care about speed, just acceleration. Acceleration from gravity is a constant, all of that to say you’d have to bring all that fuel you had taking off doubled to slow down the descent the same amount. The fuel on a space ship is many factors heavier than everything else combined, so the goal is Always to bring the least amount of it possible. The heavier the space craft is, the harder the engines have to work up and down to manage the speed.
TL;DR the fuel on a spaceship is really heavy, and using the engines as the main way to slow it down would double the amount of fuel needed. Bringing That fuel would be making the vessel heavier, defeating the purpose.
To launch you need to accelerate to some particular speed if you want to get into orbit and even faster if you want to go to the moon.
After your trip you need to reverse that acceleration back down to a stop.
You *could* let the ground do that for you but then you’d need a new spacecraft and fresh astronauts for every mission.
The alternative you mentioned would be to use a rocket to slow you down just like you used a rocket to speed you up. Intuitively this makes sense. Use half your tank to speed up and half your tank to slow down. Done, right?
The problem with this analogy is that cars are mostly car by weight (fuel is about 1-2%) while a rocket is mostly fuel by weight (the usefuly stuff is about 1-2%) so “a tank twice as big” actually means “a rocket twice as big”. So, ok then, let’s do that, right?
Not quite. The problem with rockets is that twice the amount of fuel does not get you twice as far. Because a rocket is mostly fuel you spend most of your fuel, speeding up the rest of the fuel. To get to the moon you need about 100x as much fuel than spaceship. So a spaceship that lands back on Earth with rockets would not be twice as big, it would be 100x as big.
So using the atmosphere to slow down is less of “an issue” and more of a hugely convenient cheat. Landing on Earth is so much easier than landing on Mars, precisely *because* Earth has a dense atmosphere.
To slow down in a vacuum, you need to spend as much energy (fuel) as you used speeding up. More fuel=more weight=more complexity to the mission. It’s much easier and lighter to add a heat shield and some parachutes and let the atmosphere turn velocity into heat for free.
It doesn’t carry enough fuel to land slow enough all the way down. Fuel weighs a lot so you take as little as absolutely necessary on the way up
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