Eli5: why can spacecrafts not be slowed down enough to red-enter the atmosphere without burning up?
In: 228
In principle, they can, but it would require nearly as much fuel as it took to get them into orbit in the first place. And because they’d have to carry that fuel, the requirement to get them into orbit would be higher still (by a *lot*, because fuel requirements grow exponentially with the amount of total thrust [technically: delta-v] you need).
It’s easier both from an engineering and cost perspective to just brake them against the atmosphere, rather than try to slow them down via rocket, because aerobraking only requires a heat shield (which doesn’t add that much weight).
If you want to use rockets to slow down, then you need to carry that extra fuel to run the rockets up with you during blast-off. That’s gonna make the rocket WAY heavier. I’d estimate that it would more than ~~double~~ 15x the weight of the rocket.
One thing you have to realize is that a space ship in orbit is going over 10,000 miles per hour. That’s needed for orbit, and a lot of the fuel you launch with is just to accelerate to this speed. Any slower and you don’t orbit – you’ll fall back down. If you want a rocket to also slow you back down from 10,000 MPH as well… it’s.. it’s too much.
Air drag slowdown is just way better from an engineering standpoint.
They could, but the problem is weight. Getting them out of the atmosphere requires a *bunch* of fuel. If you add weight, the needed fuel increases as well.
Slowing it down would require working against gravity, aka more fuel. And that would be even more weight to carry that fuel out of the atmosphere as well.
So at the end of the day, it’s just a lot cheaper and easier to just plan for it to come in fast than it is to try and slow it down.
Weight. Slowing down requires fuel. Every kilogram of fuel to slow down has to first be launched with the craft into orbit. Which means even more fuel to launch that extra weight. And more fuel to launch the weight of that extra fuel.
Much more efficient to use the atmosphere to slow down.
If a spacecraft is in orbit, it has a very high tangential velocity relative to Earth. If it didn’t, gravity would pull it in instead of it being flung around. And this is exactly what happens should a spacecraft start slowing down. And as soon as you slow down enough to hit the outer layer of the atmosphere, air resistance starts to slow you even more, causing you to be pulled in more, causing you to descend to more thicker atmosphere and slow down even more. This process is carefully planned out for re-entry but inevitably involves crashing through the air at high enough speeds for air compression to cause significant heat.
You can have a craft that briefly enters space at non-orbital speeds and have it re-enter with a lot lower velocity. The highest skydive was performed from an altitude close to what many would consider space. But in these cases, you aren’t coming from orbit.
Sure, that’s how the Space Shuttle works (worked). Even the modern capsules use this sort of aerobraking, albeit less elegantly than the Space Shuttle. Slowing down using the air is great, because the atmosphere is “free”, it’s already there. Slowing down before entering the air would require energy, like rocket fuel that has been lifted to the orbital region, which is very expensive.
The tyranny of rocket equations…
https://www.nasa.gov/mission_pages/station/expeditions/expedition30/tryanny.html
There’s a point of diminishing returns where hauling more fuel up takes more fuel, which in turn takes more fuel to launch that fuel, resulting in more fuel being needed, etc.
Results in like 90 plus percent of the rockets mass at takeoff being fuel.
In order to slow down enough for frictionless re-entry, you’re talking about taking so much fuel that the effective cargo carrying capability of the rocket nears zero.
It takes an entire rocket’s worth fuel and tricks like dropping stages of the rocket to get a single small satellite up to orbital speed. While there are some losses to gravity and air-resistance on the way up, the majority of the effort is just getting the satellite up to orbital velocity 9 kilometers per sec – faster if you want to go beyond low orbit.
If you wanted to slow that satellite down to 0 meters per second in orbit (or something low) you would roughly need an entire rockets work of fuel up there with it, requiring a truly massive (if not impossible) vehicle. Thus if you carry a heatshield thats ~10% of spacecraft, you don’t need 100x its weight in fuel.
Space Dragon Capsule with Payload ~10,000 kg
Falcon 9 Booster: ~544,600 kg
PICA-X Heatshield mass: ~1000kg
Rockets are expensive and heavy. Friction based deceleration is free as long as you can withstand the temperature.
Why don’t you put your car in reverse to slow down?
We routinely return spacecraft to Earth without them burning up. Mercury, Apollo, SRS, Soyuz, …
I think that you mean to ask why we return spacecraft to Earth without using engines to slow them down. The answer is: money.
Gravity’s cheap; stuff falls. Getting things down is no problem, it’s the getting them down in one piece that’s hard. We use parachutes or gliding because adding landing engines and shooting a giant fuel tank into space to power them means more complicated craft, and an enormous amount of added weight.
It costs $6,000 to $20,000 per kg to get something to space, you have a total limit of about 25,000 kg that our biggest rockets can carry, if you spent most of that on fuel to get down again, the per-kg cost of going to space would be much higher (because you’d have little weight left over for other stuff).
It’s just not necessary to use engines to descend, and the cost for adding it would make space flight even more impractical.
Latest Answers