In school, we were always told that space capsules and the space shuttle need thick heat shields on their undersides to prevent burning up on re-entry. The high velocity contact with air molecules supposedly causes so much friction that the heat would normally melt the metal portions of the craft without such shielding. It seems to me that, in order to achieve orbit, the spaceship has to go at least as fast on the way up. But the top part of the capsule has no heat shield. So, why doesn’t the capsule burn up on the way out from the same friction forces? Any insight will be much appreciated.
Think about how the acceleration works-- the spacecraft will go faster and faster as it descends towards earth (limited by the terminal velocity) and enters the atmosphere. As it launches into orbit, the higher speeds aren’t attained until it is essentially out of the atmosphere. The key is the speeds reached where it matters-- in the thin layer of the earth’s atmosphere.
At the point where the atmosphere is the thickest (the launch pad) the rocket is going the slowest. As the rocket gets higher the air gets thinner and due to the reduced weight the rocket is going faster. By the time the rocket gets going really really fast, it is out of the atmosphere.
I’m sure that Stranger on a Train will be by any moment now to explain much better than I did, but you get the idea 
The other thing to keep in mind is that a spacecraft in orbit has a tremendous amount of kinetic energy that was imparted to it by the large rockets at launch, and it takes just as much energy to slow the spacecraft down as it took to launch the spacecraft in the first place.
This could theoretically be accomplished by bringing large retro-rockets along, but this is not feasible as it would entail bringing along the same amount of fuel and boosters used at launch.
Instead, all spacecraft to date have used atmospheric braking. Instead of rapidly punching out of the atmosphere as during the launch phase, the spacecraft intentionally takes a shallow trajectory into the upper atmosphere designed to bleed off the orbital speed of the craft. All of the orbital kinetic and potential energy of the craft is then converted into heat. This technique requires heat shields.
I imagine rockets undergo some heating on the way up - even fighter jets that go Mach 2 have substantial heating from friction.
But yeah, by the time the rocket is going really fast, it’s out of the majority of the atmosphere.
Many ballistic missiles use a sacrificial ascent shroud to shield their reentry vehicles during the most aggressive part of the boost phase. After about 150km of ascent, the atmospheric friction effects are trivially small, and the missile jettisons the shroud using small explosives. The reentry vehicle is more or less the same temperature it was during launch preparations, but the shroud is considerably warmer. The dynamics of the situation keep the missile from burning up, but there are ascent heating effects.
The reentry vehicle is covered with ablative heat shielding and its structure is made of metals with high heat capacity, so placing it into its exoatmospheric trajectory at a colder temperature gives its heat shields more margin: the RV can absorb more heat on descent, which means it can take a more aggressive reentry angle.
If you want to know more about ascent heating, read about dynamic pressure, the term that the aerospace community uses to describe the combination of speed and air density that causes ascent heating. Here’s the slightly-more-technical article from Wikipedia. Enjoy!
There is heating on the way up, it’s just not as severe as reentry because the vehicle isn’t at orbital velocity. As the vehicle ascends it gains speed increasing the stagnation pressure and temperature. However at the same time the atmosphere is getting thinner, decreasing the stagnation pressure and temperature. These two effects combined produce a curve with a hump of maximum pressure and temperature at some point in the ascent. When the vehicle gets to orbital height it isn’t really going all that fast and there is practically no atmosphere. It is then turned to be aimed tangential to the orbital path and rockets are fired to bring it up to orbital speed.
On the way down, the vehicle is slowed slightly to put it on a descending path. It enters the “top” of the atmosphere at virtually orbital velocity and the only way to lose that velocity is by friction with the air.
This is the first part of the answer. The 2nd part is that rockets DO have heat shields on the cone, they just happen to be different from the necessarily more robust shields required for re-entry vehicles. Also note that modern rockets aren’t metal to begin with. They are made of stuff similar to the leading edges of hypersonic military aircraft.
… that material being what? carbon-fiber? ceramic? cheese?
Interestingly enough, the next generation of reusable launch vehicles is going to be using ablative shields for atmospheric braking, like all of the pre-shuttle craft.
Carbon fiber based composite, yes. I really hate this answer, so apologies, but I don’t think I can be more specific.
Here is the answer that first popped in my head when I read the question. IANA Rocket Scientist so I just waited for the experts to tackle it. Since none on the answers given so far have anything to do with what I thought, I am going to drop it here for the experts to tell me where I am wrong:
All the work the engines did to put the rocket in orbit, the atmosphere is doing now on the heat shield on its way down. So picture all the rocket burn that you would need to do a soft landing (the same as for takeoff? cut by efficiency?) done by friction on the heat shield.
Not exactly. That is like saying that if you pedal a bike up a hill, the brakes do the same amount of work on the way down. It doesn’t necessarily follow.
What you have is the return of some potential energy, with some ridiculously complex involvement of friction and heat.
Most modern solid rocket bodies are made of a glass- or Kevlar[sup]TM[/sup]-wound fiber composite. This is lighter, stronger, and stiffer than steel, and has far better structural properties at elevated temperatures than aluminum.
Get ready for this; much of the insulating material–which has the fancy moniker of TPS (Thermal Protection System)–is actually natural cork. This is often on top of a honeycomb fairing over the payload, and for silo-launched vehicles, the backwash or upwash from the plume. As previously noted, some have ejectable shields, which server to both protect the payload and vehicle from atmospheric heating and impact of flying objects. Rick, robby, Jurph, and David Simmons have really left me very little to add on the topic, so I’ll be uncharacteristically reserved and just say, “What they said.”
Ablative shielding technology is fairly mature and reliable–something that can’t quite be said for high temperature reusable systems on the Shuttle, which have always been problematic, even before Columbia’s ill-fated reentry. Ablative heat shields on a blunt-arsed cone only require relatively moderate temperature resistance properties, and from our experience with the Gemini and Apollo vehicles, we have a substantial body of emperical data and experience, along with an excellent reliablity record. (The Air Force’s Blue Gemini program actually reused a Gemini Program capsule in an unmanned test mission without refurbishment, and it worked fine, suggesting that the “ablative” shielding is actually seriously overengineered.) For a reliable, stopgap, moderate cost system, returning to a proven technology is the smart move. Unfortunately, the costs quoted are far from “low”, especially since most of the technology used is slightly modified off-the-shelf designs, but whatever; it’s probably a better decision at this point than an extensive conceptual design effort.
Stranger
Actually, it’s a good thought, except that some of the work that the engines did to put the spacecraft into orbit was lost due to frictional heating on the way up.
See my previous post.
I should have added the same caveat, to wit: "it takes just as much energy to slow the spacecraft down as it took to launch the spacecraft in the first place (less frictional losses incurred during launch).
Maybe I should have been paying more attention to what I was reading than to my own musings. :smack:
Or you’d have to what, kill us?
Not necessarily. You might get off with just a few short hours being pelted by a fire hose. Hey, I apologized. It was stupid to give a partial answer if I wasn’t willing to explain more fully. Sorry.
No, I’m kidding. Your answer is fine. I was always wondering what space-capsules and fighter jets were made of and had always assumed it was some form of aluminum. Now I know much better and you’ve fought ignorance.
I say avoid the heating issues altogether and use an orbital airship.
This is a very cool idea. You make a big-ass airship, and let it slowly ascend. As it gets closer to space, you eventually kick in an ion engine to keep it accelerating. Eventually, it gets to obital velocity - not in minutes, but in weeks. To come back down, you do the same thing in reverse. Kick in your ion engines, and slowly bring it back into the atmosphere. Eventually, atmospheric drag helps slow it down more and more, until it’s back flying like a regular airship. The acceleration and deceleration to/from orbit happen so gradually you never have heating issues.
The scale of such a beast would have to be immense, so the plan is to create a permanent floating ‘airship station’ way up above most of the atmosphere. The orbital ascender would actually be inflated up there, and would never come down to earth - it would be too weak to handle the stresses of the lower atmosphere. So you have other airships meant to ferry from the floating station to the ground.
It’s a very cool concept that just might work.
Okay, what if the rocket ship is on a treadmill? 