OK, we all know about heat shields and stuff on the space shuttle and various other spacecraft that re-enter the atmosphere. I assume that the heat is so great because the vehicles are going nearly orbital velocity when they come back down, and that’s “pretty darn fast”.
My question is, what if something came straight in without being in orbit first? Like if the Apollo mission coming back from the moon just aimed straight at the Earth, instead of at the edge. Is the velocity imparted by gravity alone sufficient to still require heat shielding?
And since all the heat issues are caused by going through the atmosphere so fast, couldn’t they use some kind of powered descent instead of free fall?
The problem with a burn to slow the craft is that you would have to shoot all that extra weight of fuel up there with the ship and all that extra weight is gonna cost a lot.
I think if you were gonna go straight in you would have to do a burn to slow down or else you wouldn’t be able to slow yourself down enough to prevent making a cute little crater somewhere. By gong down at an angle you use a lot more atmosphere to slow yourself down.
That would require them to carry more fuel than they probably have room for. Remember, saving weight is critical when you’re sending things into space.
The speed of Apollo capsules coming in from the Moon was far greater than those coming in from Earth orbit.
Thinks of these things in reverse: You are just sitting above the atmosphere and you want to a. go into orbit, b. go to the Moon. Which is going to take a higher velocity?
The only only manned rocket flights that came into the atmosphere slower than from orbit were the Mercury suborbital flights of Shepard and Grissom.
What about Gagarin’s one-orbit flight? Was it truly an orbital flight, or just a very long sub-orbital (95%+) flight?
On the issue of using braking rockets, instead of atmospheric braking, and the amount of fuel. Nothing says the fuel has to go up with the crewed ship that will use it. Once we start mining Luna for fuel, it might be cost-effective to launch fuel tanks from Luna, for use by Earth-returning ships. TANSTAAFL.
As others have said, a heat shield is just a method for slowing down the spacecraft. It’s lighter and more reliable than using a rocket to slow you down.
There are rockets which go straight up and come straight down without going into orbit. These are called sounding rockets, and recovering these payloads are much easier than recovering stuff from orbit. By my quick calculation, a fall from a 300km altitude would generate less than 1/10 of the heat compared to re-entry from orbit.