Main Frictionless Reentry Discussion

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07:47:58 PM Aug 12th 2012
With the exception of half of the Video Games Section and 1 Western Animation item, all of the "examples" are either aversions or justified by Techno Babble.

While there is a Trope here, the lack of straight examples is mindblowing.
04:52:45 PM Mar 6th 2013
I agree. Perhaps this trope should either be adjusted, or a second more relevant one could be made for aversions.
06:55:59 AM Mar 23rd 2013
Part of the deal seems to be that ships and all survive stuff that, under current understanding, would turn them into tiny melty bits. Justified/handwaved by relevant tropes in many cases. (Advanced future tech, phlebotinum, etc.)

Puts me in mind of the scorpion mecha episode of Gatchaman. The Scorpion, when reentering Earth's atmosphere, was unmanned and didn't have to worry about a chewy center being fried. It got up to such a heat as it went through re-entry that the God Phoenix's missiles were exploding before they could even damage it (Mark I Gatch missiles, and this was 1972. Science and tech march on.) The mecha then crashlanded. Yet it was still able to raise havoc until its wireless power source was trashed. (A submarine in an entirely different part of the world. That only transmitted the energy periodically.)

(I swear, Gatchaman had such fun tromping all over physics.)
10:36:50 AM Dec 10th 2011
A more accurate title for this trope would be "cold reentry". The heat generated during reentry isn't actually due to friction - it's shock and compression.
11:39:07 AM Mar 13th 2013
Yeah. (For a trope about inaccurate physics, an inaccurate title is grating.)
07:37:09 AM Mar 4th 2011
Is there a reason that any time you fall to earth you must be pulling ridiculously high speeds? It seems like tons of the examples are literally someone falling at their own terminal velocity from a great height. That's not the same as having-previously-been-in-orbit. If you're in outer space, sitting still, and decide to just let earth's gravity pull you in for a safe, fuel-free landing, you wouldn't exceed 9.8m/s right?
12:04:41 AM Dec 4th 2011
9.8 is the acceleration of anything in freefall, meaning your speed increases exponentially. Terminal velocity varies depending on the actual object but can get up to 700 mph for people.
10:31:01 AM Dec 10th 2011
The speed doesn't increase exponentially. It's a constant linear increase: v = g*t. An exponential increase would be v = g^t...

Also, terminal velocity depends on the density of the atmosphere. The higher you are, the faster you can fall.
01:07:27 PM Dec 27th 2011
It is true that the lion's share of the velocity re-entering from orbit is orbital velocity. This is why suborbital spaceflights are much easier, and often used as warm-up (John Glenn took part in one, and Space Ship One performed another), even if the peak is at an orbital altitude. The core issue is that the only reasonable way we know to get down from orbit is to do a relatively small burn to shift the orbit to one that passes through the atmosphere, then let the air slow us to the point that we can safely land on wheels, splashdown, etc. And that means getting rid of a lot of speed in a relatively short passage through air.
01:36:03 AM Feb 16th 2011
I've been thinking about this, and I actually think that the reentry physics of science fiction are not as inaccurate as one may assume. We are basing what we know about reentry from the Space Shuttle and other assorted space debris that enters the atmosphere. But what is happening in those cases is that the object is essentially just falling to the earth. So naturally the friction of the atmosphere heats up the object which is travelling at orbital speed. Why, then, is there no heat problem when *leaving* the atmosphere? The answer is because the shuttle is gradually increasing speed as the atmosphere gets thinner and thinner. If you were to reverse that process by having rockets constantly decelerate the Shuttle, you wouldn't have an issue with friction. Of course, it would use up a lot of fuel.

However, in science fiction, we typically have advanced anti-gravity technology, so in theory the ship should be able to decelerate and enter the atmosphere at a controlled speed, thereby eliminating any effects of air resistance.
06:50:52 AM Nov 29th 2010
12:49:13 PM Feb 16th 2011
He was beamed into the atmosphere acording to my geeky work colleague
12:44:45 AM Oct 6th 2010
Conversation in the Main Page. Discuss it here on the discussion page, and if you can reach an agreement and come up with a concise description, that can go back on the main page.

  • Star Trek (2009 film): Kirk, Sulu & Security Officer Redshirt do a skydive from low orbit around Vulcan, with no sign of atmospheric burn. Granted, one can assume their spacesuits are specially constructed to allow safe re-entry, but a line of Techno Babble along the lines of "Make sure the suit's heat shield projectors are fully charged" would have been welcome.
    • No, it wasn't necessary. Spacecraft in orbit travel at tremendous speeds relative to the ground (e.g. the Space Shuttle orbits at a speed of around 17-18,000 mph at an altitude of 200-250 miles) in order to remain in orbit. The lower the orbit, the faster they must go to keep up. A stationary orbit which doesn't require thrust to maintain has to be at an altitude where one orbit is equal to one local day; unless the planet revolves very quickly, this is likely to be pretty high up. It's far more likely that the ships over Vulcan were not in orbit at all. Instead they were actively thrusting against gravity to maintain position over a particular spot on the surface from a very low altitude. Given how powerful their ships are, this is no big deal for them, I'm sure. But it means that the skydivers were not traveling at speeds of tens of thousands of miles per hour when they hit the atmosphere, and so there was not a lot of heat from friction. Instead they were traveling at far more mundane speeds, relying on gravity to pull them down just like any other skydiver. We don't have enough information to know what their maximum speed was, but on Earth skydivers have gotten over 600 mph at high altitudes without the benefit of 'heat shield projectors.' Kirk's suit might have heated up but probably not so much that he needed technobabble.
      • IIRC, the Enterprise was not in orbit. When Kirk et al. jumped, they would have been in free fall with no tangential orbital velocity to get rid of. When Joe Kittinger jumped from 103,000 feet in 1960, his maximum speed was over 600 mph. He complained not of heat, but of cold. And it's not friction that causes the heat; it's compression of the air in front of the speeding object. As the gas is compressed, it heats up and turns from a gas into a plasma very, very quickly.
      • On the other hand, Alan Shepard's Mercury flight wasn't orbital either; it ascended straight up to 116 miles altitude, then fell right back down. On the way down, it attained a top speed of over five thousand miles per hour, and experienced aerodynamic forces that peaked at over eleven gees. The Freedom Seven capsule most definitely needed a heat shield, even though there was no orbit involved.
        • No, not straight up and down. Like most spacecraft, Freedom 7 pitched over at an angle a few seconds after launch, (listen carefully in Apollo 13 and you can hear them talking about pitching and rolling as they go up) and wound up traveling a significant distance east. It would've gone even further had the engine run for more than a couple of minutes, and the had the spacecraft not used retro rockets to descend early. Shepard wound up about 300 miles east of the launch pad as it was. It's this lateral speed that necessitated a heat shield. If you travel in an arc (like Shepard), accelerating to go both up and across before apogee, gravity will eventually overcome the upward part, pulling you back down at a steady 9.8m/s^2, but you'll be stuck with the across part left over from before. Go straight up, however, and gravity will cancel out the entire thing, leaving you with far less downward acceleration as you reenter the atmosphere. No one ever does this in real life because there's no point; any kind of orbit at all requires accelerating laterally quite a bit, and it's easier to use the atmosphere to slow down than to carry sufficiently powerful retrorockets to do the same thing (since, after all, it costs a lot of money to carry things up, and engines and fuel weigh a lot). Since Kirk & co. have artificial gravity, impulse engines, powerful reactors, etc., it's no trouble for them.

10:07:48 AM Mar 25th 2010
Re: Star Trek

The team that falls through the atmosphere is leaping from a shuttle. There's no reason to assume that the shuttle was at orbital velocity, as opposed to simply flying high and fast. I always interpreted the scene as more akin to the Project Manhigh drops — not falling from orbit, just from the very edge of the atmosphere.
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