Why tiles for re-entry and not launch?

[Line]

I've pondered, why does The Shuttle use tiles for re-entry and not lauch. The tiles are used for landing cause the air friction is so high with the speed. Well when The Shuttle launches it's going at tremendous speed also. Going at many times the speed of sound it should cause friction with air molecules shouldn't it? Does The Shuttle gain a signifigant amount of speed after liftoff and before landing?

 

[dav2008]

What do you mean it doesn't use tiles for launch? As far as I know the tiles are always on the ship...

With that in mind, the angle at which the ship is traveling is different for take-off and landing, as well as the speed at which it's traveling. The high surface area of the bottom of the ship that's exposed to the atmosphere due to the angle of attack is the cause for the high temperatures. It's also a different type of heating that isn't directly related to friction with air molecules. From what I understand, the air around the shuttle is basically ionized.

 

[Line]

Well yes it becomes so heated thatit becomesplasma.

The tiles are on for launch but there's no massive fireball like during re-entry.

 

[russ_watters]

The shuttle is through the vast majority of the atmosphere in just a couple of minutes, but takes another 7 to accelerate to orbital speed. So frictional heating isn't that much of a factor.

According to http://www.cdli.ca/CITE/sts_ascent.htm" link, after two minutes, the shuttle is going 3,000 mph (orbital speed is around 17,000) and is 28 miles in altitude.

 

[Pengwuino]

I think even the common sense answer to this is that they do but when you're launching, you want to be as aerodynamic as possible in your ascent to orbit and face as little resistance as possible. When you come down however, you want to give that shuttle hell in order to slow it down as quickly as possible so it can land.

 

[Q_Goest]

The shuttle is up and out of the atmosphere before it ever picks up much speed. From the reference Russ gave:

Two minutes into the ascent, the shuttle is about 45 kilometres (28 miles) above the Earth's surface and is traveling nearly 5000 kilometers per hour (3,000 mph).

Long before the shuttle gets to an altitude of 28 miles, air frictional heating no longer becomes a concern because there is so little atmosphere at that altitude. No atmosphere - no frictional heating. Besides which, at that altitude, it is still only going 3000 mph. There'd be some frictional heating at that speed if there were any atmosphere, but nothing like the 17,000 mph you get during reentry.

Reentry is very different. It's coming into the atmosphere at 17,000 mph and uses the atmosphere to slow it down. All the kinetic energy the shuttle gained during ascent by burning fuel through the rocket engines now has to be absorbed somehow. All the kinetic and potential energy the shuttle engines put into the shuttle now has to be turned back into thermal energy as the shuttle now comes in for a landing. The tiles on the shuttle are analogous to brake pads, turning all that energy from the rockets into thermal energy (ie: heat).

 

[gschjetne]

Also, from what Iv'e heard the effects of friction are negligble. The heating is caused by the compression of air.

 

[Mech_Engineer]

I've pondered, why does The Shuttle use tiles for re-entry and not lauch. The tiles are used for landing cause the air friction is so high with the speed. Well when The Shuttle launches it's going at tremendous speed also. Going at many times the speed of sound it should cause friction with air molecules shouldn't it? Does The Shuttle gain a signifigant amount of speed after liftoff and before landing?

During re-entry, the Shuttle is going far faster than takeoff, re-entry is around mach 30 (initially anyway, it then begins to slow from air drag). The shockwave created by the supesonic speeds create a pressure (shock) wave across which the temperature jumps up by several multiples. For air that is 225K (-50C), the air would jump to 4039K across the shock, this why they need ceramic insulation, because even the toughest of metals like Tungsten would melt (plus Tungsten is incredibly heavy, duh).

When launching, the shuttle only gets up to about mach 2-3 before it has exited the majority of the atmosphere and no longer has to worry about frictional heating. Even so, a shock wave from this speed would jump the air temp up to 624K, really hot, but not really enough to push air into the plasma region.

 

[Malakai]

So if a space shuttle kept half it's fuel on board, and used it to do a powered deceleration, would this negate the need for such an extensive Thermal Protection System?

Or is there some threshold, where once you begin to decend into the atomsphere, slowing your decend by thrusting back out would cause you to skip off the atomsphere or some other limitation?

I gues I'm curious that given infinite energy can you land from space without high temperature thermal issues.

 

[Astronuc]

The shuttle would still be moving too fast in the atmosphere, even if it tried to use rockets to decelerate.

Another fact is that is costs big bucks (~$10,000 /kg) to put material into orbit, so the return fuel would be considerably expensive, and also displace that much mass of usable material in orbit.

 

[Malakai]

So there is no way to match speed with the atomsphere before you enter it?

How would a space elevator negate the need for a TPS barrier? If you used a space elevator by climbing it to reach oribital height, released from the cable and then fired thrusters to reach oribal velocity at that height, performed your mission, returned to the space elevator cable, used thrusters to slow down back in line with it's orbit, re-attached climbed down the cable, would you incounter the same re-entry heat/forces?

 

[Danger]

No. Disregarding winds, the elevator is stationary relative to the atmosphere.

 

[russ_watters]

However, that would defeat the purpose of the space elevator. The speed is the main energy cost of space flight. A space elevator would get around the speed issue only by being so tall that it lifts satellites to geosyncronous orbit.