We’re all familiar with the role of heat shields to protect astronauts/cosmonauts as they reenter the earth’s atmosphere from their extraterrestrial adventures.
But if their spacecraft will burn up from atmospheric friction on the way down, what prevents the non-shielded craft from burning up on the way up as it accelerates to escape velocity?
I’m guessing that the craft doesn’t reach maximum speed until beyond the thick part of the atmosphere that would cause the most friction, but it seems to me that a speeding rocket is going plenty fast by the time it gets up even to suborbital heights. Which leads to a second, related question: what kind of temperatures are reached on a spacecraft’s outer surface before it hightails out of the atmospheric danger zone?
I’m not a rocket scientist, but I did take some aero engineering courses in the university before I switched major. I believe your guess is correct. When launching a spacecraft, the goal is to try to get out of the atmosphere as soon as practical before accelerating to orbital speed. Any drag is a waste of energy, and at our current level of technology, we need every little bit of energy we can muster. The spacecraft hasn’t really gone very fast yet when it first gets up there.
Now I’ll let the real experts step in and fill in the details…
There is significant heating during ascent. This page from the Spacecraft Thermal Control Handbook has some info - scroll down to p65 and you’ll see a graph showing the fairing temperature of an Atlas rocket. It goes up to about 200[sup]o[/sup]C. But of course this is nothing compared to the heat of reentry, for the reasons already mentioned by others.
Also keep in mind, the heating during reentry is intentional. The intent is to slow down the spacecraft by converting its kinetic energy into heat.
And in case anyone’s wondering, they could come down the same way they go up, using a big rocket motor to slow down and ultimately landing gently tail-down back on the launchpad. But the fuel needed to do so, and the fuel needed to launch that fuel, would be hideously expensive, so nobody does that.
lack of air pressure by the time it gets really going.
OTOH, I have been told by the designers that the nose of the external fuel tank of the shuttle has 1 inch of insulation-and the outer surface reaches 800F while the inner surface remains at -180 (liquid oxygen which is what is in the upper part of the fuel tank).
Just curious what your search term was that resulted in not finding the thread in question. I hadn’t seen it come up before, but went with “friction atmosphere” as those were both extremely likely terms to be included, and rare enough in other threads to exclude any noise. I came up with about 8 threads, one of which was this:
Now I can’t remember but I guess if I had remembered that I had actually posting in that thread it may have been easier for me to find. Sucks getting old.
I once read an article about the SR-71 spy plane, the fastest plane in the world (Mach 3). A civilian was being given a demonstration flight in a two-seater and at one point the pilot told him to touch the canopy. When he did he instantly pulled it back because even through his pressure suit glove the canopy was blazingly hot!
It sounds like amazing insulation but remember that it only has to slow down the transfer of heat. A few minutes after launch the shuttle is out of the atmosphere and the fuel is gone.
Isn’t this the explained by the same reason that if you jump into a pool belly-first it, the water can hurt like hell, but if you somehow jumped out of the water, belly-first, at the same velocity, the air wouldn’t hurt at all?
There must be something I’m not getting. Why does the OP assume that the craft is non-shielded on the way up? Isn’t it the same craft, with the same shielding, that makes the return trip?
No. In your example in one case you are hitting the water boundary at speed and in the other you are not. You are in the water already. This bears no relationship to the shuttle’s experience. There is no sharp atmosphere/vacuum boundary. The heat on the way down is simply a factor of speed through atmosphere. If it were doing the same speed in the atmosphere on the way up, it would heat up in precisely the same way.
Because the boosters, and the fuel tank, and the top side of the shuttle, all of which are fully exposed to atmospheric friction on the way up are not shielded. The heavy shielding used on the way down is only on the underside of the shuttle, which has to remain in a very careful attitude on the way down to avoid exposing upper surfaces.
