Originally Posted by Flynx (Post 660366)

I see what you are saying. Use rockets to slow down before coming home and you don't need a heat shield.

This is true.

The problem is the balance of energy and the amount of fuel that would be required to slow down. Using the shuttle as an example, it would take nearly as much fuel as the shuttle burns at launch to slow it down enough.

Right now shuttle Atlantis is in orbit at an altitude of 220 miles and a speed of ~16,600 mph.

The energy associated with going 16,600 mph is MOST of the energy in the space shuttle. Getting to an altitude of 220 miles doesn't take much energy.

Lets see why....

The energy needed to get to an altitude is given by:

e(p) = m*g*h

where e(p) is potential energy, m is the mass, g is the acceleration of gravity and h is the altitude.

The energy needed to accelerate to a speed is given by:

e(k) = 0.5*m*v^2

where e(k) is kenetic energy, and v is velocity.

Now if we divide the two equations, we can get the ratio of the two energies (and mass conveniently cancels out)

e(k)/e(p) = (0.5*v^2)/(g*h)

using SI units of meters and seconds to fill in the numbers...

e(k)/e(p) = (0.5*7421^2)/(9.81*354000)

e(k)/e(p) = 27535621 / 3472740

e(k)/e(p) = 8

That means that only about 11% of the thrust from the main engines and solid rocket boosters was used to lift the shuttle to 220 miles. The other 89% of all that thrust was used to accelerate the shuttle to 16,600 mph. (Most of the shuttle's ascent flight is horizontal. Only the first little part is vertical - to quickly get out of the atmosphere so you can accelerate without friction like you mentioned.)

You would have to slow the shuttle back down to zero mph if you don't want to use a heat shield. Space Ship One has an apogee velocity of practically zero. Remember you will accelerate quickly as you fall.

So you wouldn't need an entire external tank full of fuel plus two solid rocket boosters. You would only need 90% of them.*

This is why it is so much cheaper to use tiles and reinforced-carbon-carbon composite panels (both of which are reusable) than try to burn fuel (which is not reusable) to slow down.

*Remember - if you have the fuel available to slow down, that means the fuel is also going 16,600 mph and you have to slow down the mass of the fuel as well.


EDIT: This also shows why SpaceShipOne was such an "easy" accomplishment. (I put easy in quotes because it was in no way easy. Burt spent roughly $50 million dollars in order to claim the $10 million dollar X-prize - a net loss of $40 million).

Point is - SpaceShipOne only had to reach altitude, which is only the first 11% of getting to orbit. And it launched from an airborne platform, not the ground, so that knocks off even more. SpaceShipOne itself probably only had about 9% of the energy needed to get to orbit. And even that little amount was difficult for them to control - as evidenced by the flight that corkscrewed. When you consider all the other vehicle systems, calling Space Ship One a spacecraft is like calling a fish that jumps out of water a bird.

Originally Posted by xomm (Post 660859)

*Clap Clap Clap.*

How about a space elevator? A rope of carbon fiber connected to a orbiting dock, held taut by centrifugal (sp?) force.

Obviously we don't have the means to create carbon fiber ropes, but meh.

Originally Posted by cyanith (Post 660884)

The elevators from Gundam 00.... yup... thats the way to go.