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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It’s difficult to understand orbital mechanics jist by reading comments here. It’s way to complex to ELI5. I recommend trying out KSP, where you can learn by doing and experimenting with stuff. Try out what you describe in the question and you’ll see it’s a terrible (and terribly expensive) way to reenter the atmosphere.
Okay, gonna be honest, I have no clue how any of this works. But I do have a question. Can we power a spacecraft with solar panels? Maybe use those to slow the craft down?
Because that takes an immense amount of fuel. Spacecraft decelerate substantially during re-entry without expending any fuel. It’s much cheaper and easier to engineer a spacecraft that can handle re-entry than it would be to haul a huge amount of excess fuel, throughout the entire course of a spacecraft’s mission, just so it can get a smoother re-entry.
Optimizing fuel consumption is critical to making spaceflight economical. Adding fuel to a spacecraft quickly leads to diminishing returns because additional fuel means additional weight, which means you need even more fuel than you first thought to bring the extra fuel into orbit.
That means each additional ounce of fuel adds less “delta-v” (the ability of a spacecraft to change its velocity) than the previous ounce.
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
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.
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.
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.
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.
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
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