It’s a common trope in sci-fi that atmospheric re-entry of a spacecraft must be very precisely controlled. If you enter at too steep an angle you’ll burn up (of course). If you enter at too shallow an angle you’ll “bounce off into space”, which is sometimes helpfully explained by the simile “like a stone skipping across a pond”.
How can you “bounce off” the atmosphere? There’s no distinct boundary like the surface of water; it gradually increases in density. And of course if it were possible to bounce clear into space, then NASA could gain extraordinary fuel efficiency in long-distance probes just by launching them into orbit, deorbiting at the wrong angle, and “bouncing them off into space” forever.
Would I be correct in inferring that they really mean you’d fail to reenter at the intended angle and location, instead entering a fractional orbit that would place your reentry somewhere far from the intended point and likely at a fatally steep angle?
I think you inadvertently answered your own question.
Think about how NASA/JPL use planets to slingshot probes further along.
They do exactly what you said; they aim at a planet but at “too shallow” a path.
The probe picks up speed but “bounces” away because the increased speed makes orbital capture impossible.
This is what they mean by too shallow an entry causing a craft to skip away.
Do you have any examples of this trope? I’m familiar with spaceships using a planet’s gravity to gain speed and slingshot away, and also using a planet’s atmosphere to slow down (atmospheric braking) but I can’t recall instances of ships “bouncing off” an atmosphere.
It seems to me the only things traveling fast enough to actually “bounce” off atmosphere are things like light and radio. I suppose a ship traveling FTL could bounce off an atmosphere, but could they survive it?
This isn’t just a science fiction trope, it is actually reality. While the atmosphere does indeed increase in density in gradual fashion (more or less in proportional function to the acceleration of gravity) aeroelastic behavior is not linear; at certain changes of density you go from one aerodynamic regime to another, as the behavior of air goes from being a disparate gas that just causes localized radiative heating at shock interaction zones to a turbulent medium that causes erosive heating and sinusoidal load to a compressible laminar flow that “sticks” to the wetted surfaces of the craft and conveys heat via conduction. For a given re-entry speed, to shallow of an angle will result in enough lift to prevent the vessel from re-entering in a controlled manner, and while it probably won’t bounce out of the Earth’s sphere of influence it may go into a shallow elliptical orbit that will re-enter at some future and difficult to predict interval. Too steep of an angle and the heating from ram pressure and erosion will cause the craft to overheat or ablate away.
There have actually been several sub-orbital shuttle/bomber and lifting body re-entry vehicle concepts that utilize the “waverider” effect of flying at the boundary in the mesosphere, including the German A-12 ‘Amerika’ bomber, the MiG-105 ‘Spiral 50/50’ shuttle, early concepts of the X-20 ‘Dyna-Soar’ mini-shuttle, and a number of maneuverable ICBM re-entry vehicles (a class known as MaRVs). DARPA recently flew a vehicle called the Hypersonic Technology Vehicle (HTV) to support the DARPA/Air Force FALCON program for a hypersonic cruise vehicle (HCV); however, the vehicle became oscillatory and was lost shortly after it re-entered the upper atmosphere.
The problem with all of these vehicles is twofold; one is that the small amount of control offered by any conventional aerodynamic control surfaces and the great degree of change of aeroelastic regime as the vehicle goes through different parts of the atmosphere makes it very difficult to develop effective controllers even with modern high speed real time embedded digital control systems, and the amount of heating and erosion the vehicle sees challenges the state of the art of material science. So far, although there are some RVs that are claimed to have a degree of maneuverability (to avoid terminal-phase countermeasures) no one has really demonstrated a really effective atmo-skipping MaRV, and certainly not in a manned vehicle; existing vehicles are either blunt body capsules or delta wing/modified lifting bodies that are not configured to develop hypersonic lift but instead cause enough drag to slow the vehicle down to more controllable regimes as quickly as possible after re-entry.
So, this isn’t just bad writers’ fiction; it is a technical reality.
You’re conflating two different things. Gravitational swing-by maneuvers put a vessel in a parabola in order to steal some of the planet’s orbital momentum to change the velocity (speed or direction) of the vessel. (Don’t worry; the enormous different in mass ensures that a planet will never notice what it has lost, and it won’t fall into a measurably lower orbit and threaten the Earth.) The craft would not interact with the atmosphere and in fact this would be detrimental to the effect. A waverider is in close orbit of a planet and will skip off the atmosphere repeatedly, allowing it to make more distance than a purely ballistic track at the same energy level would permit. Instead of being pulled by gravity, it is being pushed up by the elasticity of the atmosphere.
Stranger, I really need to get you a bottle of mead or three. Your technical answers on this board are the most concise, focused and “understandable by morons like me” I’ve ever seen.
Ok… so I was wrong about the bounce, but essentially right about the result being that you’d reenter somewhere other than where you intended, at an angle other than you intended.
As for where this trope occurs… it seems like it appear any time the climax of the story involves reentry of a spacecraft in distress. The first example that pops to mind is a cheesy 80s movie called “space camp” or something like that.
Not necessarily, depends on your initial velocity. The Apollo command modules returning from the moon relied on atmospheric braking, and for them coming in too shallow would’ve resulted (IIRC) in the CM skipping off into a solar orbit.
