Why do spacecraft have to go so fast when returning to earth?

Question

I have followed the space program from the start and workrd in the aerospace field and nobody has intellegently explained why spacecraft are required to re-enter the atmosphere at 17,000 mph when it seems to me that they could simply slow the vehicle down and 'fly ' it to it's landing site. I understand the extreme velosity needed to escape earths gravity but I just can't get my head around why coming back needs to be as suicidal as it is.This may sound really simpleminded,but the atmosphere is just air,and in the upper atmosphere it's thin air at that!In the early days of the space race NASA flew the X-Planes out of the stratosphere and back at speeds that were nowhere near the speeds used today. Please, I'm not stupid,help me to understand why we have to risk astronauts lives when it seems we could do the same job a lot safer.

Answer 1

In order to slow an orbiting vehicle to a sensible landing speed, say 300 km/hr takes the same energy as it did to get it to orbit in the first place. So, in the case of the space shuttle, two additional SRB's and a full external tank worth of propellant would be required to slow the shuttle to that extent. While it is technically possible to do that it should be pointed out that it is just as DANGEROUS to "unboost" a vehicle from orbit as it is to boost it there in the first place. Gradual slowing using the atmosphere is the only sensible means of returning vehicles from orbit. As for the x-planes they returned in precisely the same manner as the shuttle does today, but their initial velocities were much lower and it seemed safer. However, two near fatal X-15 crashes (and others in other planes) prove how dangerous even lower velocity spacecraft can be. And, just "flying out of the atmosphere" does not put an object into orbit, as your statment seems to imply...you still need that 17000 mph velocity to maintain a low earth orbit.

Answer 2

Well, for one the space shuttle is in orbit and is already doing those speeds. For example, the slinging around like a sling shot. So when they enter back, all they can do to slow down is use up energy and air braking. The shuttle is a glider so there is no power (engines) to slow it down, it is all on it's own. Air does have Resistance so it isn't like there is nothing there at all. The speeds the shuttle is doing, like you said, is almost in space where there is almost no atmosphere, just the very outer part of it, can't remember the name of it. So there is very little resistance out there. If they were to slow down to quickly, the shuttle would fall out of the sky, since it isn't powered. And, besides the astronauts would probably die from the deceleration. (G-Forces)..

Answer 3

They had to go that fast to get into orbit to begin with. They have to stay at that speed to remain in orbit. When they start to slow down, they fall out of orbit an re-enter, even then they are still moving like a bat out of hell.

Answer 4

They don't *have* to, they just do go fast already, and have no choice.
In orbit, any object is travelling at 4.8 miles per second. That is the speed that took so much fuel to achieve (most of the fuel a rocket burns is to go fast, getting high takes only a small percentage of the fule, and getting high is done only so that there is less air to drag you down, so it is easier to go fast).
If one was to slow down to much less than the 4.8 miles per second orbital velocity, one would require almost as much fuel to do so as was needed to go up before. And to get that much fuel in orbit would mean the rocket would have to be 10 times bigger at lift off.
Remember than, in space, in vacuum, the only way to speed up is the same as slowing down: rocket, and that takes enormous quantity of fuel.

Answer 5

I think maybe it is because of the atmosphere density, its just for safety, sorry im not a rocket scientist!

Answer 6

The bottom line, as far as I can reason, is that as long as we rely on rockets, we will never get past our solar system. Just sayin'.

Answer 7

so it can go through the atmosphere

Answer 8

that way if they do hit something on reentry they dont suffer.