Understanding the Principle of 'Skipping' Off the Atmosphere

[Shraa]

I'm researching this for application in a story. I've looked at a dozen or so websites, which all explain that the principle of skipping off the atmosphere (like skipping a stone on a pond) works, but they don't explain exactly how. For what reason does an object hitting the atmosphere at a shallow angle bounce off? The gaseous equivalent to surface tension? If so, what is that, and how and why does it work like that?

I'll be very, very grateful for any answers and links that you can provide.

 

[Filip Larsen]

Welcome to PF!

The skipping is an effect of inertia combined with the geometry of a curved Earth surface, that is, there is not necessarily any particular force as such that bounce the vehicle. If you regard the orbit of a vehicle on reentry without considering the atmosphere, you would see it make an elliptical orbit, that is, the height of the vehicle would initially drop but then flatten out at perigee (closest distance to earth) and then start to rise again as the vehicle would climb back up to apogee (farthest distance, assuming a closed orbit here). So, without atmosphere the vehicle would "skip out" every time (as long as it didn't actually hit the earth, of course).

To make a direct reentry with atmosphere, the reentry orbit of the vehicle must be made such that the vehicle enters sufficiently low into the atmosphere to break to low speed. There are then two extremes.

If the reentry is made too high (corresponding to a "too shallow" reentry angle) the vehicle will not encounter air dense enough to slow it down from orbital speed and it will leave the atmosphere again (only, a bit slower than it came in). If the initial speed was very high, as when entering directly from another planet (on a so-called hyperbolic entry), the loss in speed may not even be enough to place the vehicle in a closed (elliptical) orbit around Earth and it may in that case continue "out into interplanetary space". If the orbit after passing through the atmosphere is elliptical the vehicle will continue around almost one full orbit and then reentry the atmosphere again where it will slow even more, and so forth until it is finally captured in the atmosphere.

If, on the other hand, the reentry orbit is made too close to Earth (corresponding to a "too steep" angle), the vehicle may enter the denser part of the atmosphere too quickly where the subsequent release of energy from friction will overwhelm the thermal protection system of the vehicle (consider here that the low orbital energy of just 1 kg of mass is enough to make around 100 liters of 20 degree C water boil and all that energy has to be bleed off into the atmosphere in a controlled fashion in order not to "burn up").

So, depending on the speed and the ballistic characteristics of the vehicle, there is a usually small range of initial entry orbit perigees that will allow for a safe slow-down of the vehicle. By using a vehicle that is capable of generating aerodynamic lift (which is the norm for manned vehicles), the safe range can be extended a bit compare to a purely ballistic (drag-only) reentry.

 

[tiny-tim]

Welcome to PF!

Hi Shraa! Welcome to PF!

I've looked at a dozen or so websites, which all explain that the principle of skipping off the atmosphere (like skipping a stone on a pond) works, but they don't explain exactly how. For what reason does an object hitting the atmosphere at a shallow angle bounce off? …

I'll be very, very grateful for any answers and links that you can provide.

The idea that the atmosphere bends trajectories outward in the same way that it bends light seems to be an urban myth.

Skipping off the atmosphere "like a stone" (btw, "like a stone" seems to be the best phrase to google-search for ) is an aerodynamic effect, ie the shape and attitude of the object creates lift, which may make it skip.

See eg aerodynamic analyst http://www.aerospaceweb.org/about/bios/jeffscott.shtml", including …

… the skip entry trajectory. In this case, the vehicle first grazes the outer fringes of the atmosphere and generates drag that slows the craft down by a small amount. However, the vehicle also generates a lift-to-drag ratio between 1 and 4 and uses this lift to pitch up and leave the atmosphere again. This process is repeated several times as the craft skips along the upper reaches of the atmosphere much like a stone skipping across the surface of a lake. This skipping gradually slows the vehicle until it can safely re-enter the atmosphere and fly a more ballistic path to touchdown.
…
… significantly higher aerodynamic heating … As a result, skip entry has never been used for a manned spacecraft​

Also at http://www.satobs.org/re-entry.html" …

Even then the craft may skip along the upper atmosphere, like a stone skimming across a pond, due to aerodynamic interactions.​

So whether an object skips depends on how much aerodynamic lift is generated by the shape it presents to the atmosphere.

 

[Shraa]

Okay guys, thanks for the answers. So I have to look at aerodynamics? Hmm... would a large, cube-shaped object count as aerodynamic enough to bounce off, when presenting a fully flat side? I doubt it would work, but I may be wrong.

Again, thank you both for the answers, and the links. Very useful.

 

[Alex1995]

i am very new and only 16 so i am most probably wrong.
But is this the same, or close to the effect seen when a bullet is shot into water at high speed at a certain angle.
First it goes down into the water, then comes up and keeps traveling up.
sorry very new to physics so can't use any cool language like u do. lol.