# Bounce off the atmosphere at reentry?

I am just watching Space Race part 3 and the cosmonauts just got their first view of Wostok. The scientist who designed the retrorockets told the soon-to-be spacemen that the rockets would have to be fired precisely in order to not:

1. reenter too steep (hence to fast) and crash or
2. reenter too shallow and bounce off the atmosphere into a higher orbit

The first point is, of course, well taken (even though one has to wonder why the rockets should be designed with too much power in the first place), but the second point seems dubious to me: After playing KSP, I would suspect that the Wostok's orbit would decay nonetheless after even a minimal braking. How could you enter a higher orbit by touching the atmosphere? That would seem like a great way to get to the moon, after all.

• A capsule can actually skip off of the atmosphere the same way a stone skips off the surface of a pond. See this question for more explanation. Dec 7, 2016 at 5:05
• It is not necessary for the rockets to be overpowered in order to effect a re-entry that is too steep. Just aim closer to the center of the cosmological object (presumably, Earth) than an ideal reentry point as you approach, and you'll succeed in destroying your spacecraft. In fact, the more overpowered the rockets are, the better your chances of surviving in a too-steep re-entry; you can use the rockets to shed speed before your heat shield burns away, and try to slow to a speed that won't rip out your craft's parachutes (or blow your impact cushions, or whatever) as you reach the surface. Dec 7, 2016 at 5:23
• So, it's another power-to-weight tradeoff problem, best solved by coming into the descent just steeply enough to ensure the atmospheric drag slows your craft to the point where it is not going fast enough to re-emerge from the atmosphere, and not much more than that. Dec 7, 2016 at 5:25
• @MilesBudnek I was thinking a lot about the lifting body effect, lift could obviously alter the trajectory in a way that drag alone couldn't, the question is if lift could in any sense of the word "raise" the orbit, I get the feeling that an encounter with the atmosphere strictly lowers apoapsis. Lift would have a significant effect on how hard the capsule "bites" into the atmosphere, as depending on angle of attack lift effects can exert an upward or downwards force on the capsule, pushing it deeper into or lifting it out of the atmosphere and thus altering areobraking efficacy. Dec 7, 2016 at 9:20

Yes, a capsule cannot literally bounce off the atmosphere and its kinetic energy must be reduced by an encounter with the atmosphere, rather it would just pass through the atmosphere and back into space, having failed to lose enough velocity to stay in the atmosphere. After going partially around the planet it will reenter the atmosphere, that is actually where the real problem lies, the capsule will come down at the wrong location at the wrong time. There may also be problems with power / oxygen reserves running out or heat shielding failure, for example the design may require jettisoning the heat shield to get rid of the the accumulated heat, or rely on convection in the atmosphere to cool the vessel. So there are lots of things that would go wrong if the vessel goes back into space instead of deeper into the atmosphere.

With that said, there is something of a lack of precise, every day terminology to describe the problem in a pithy way and ordinary people do not understand the distinction between "in orbit" and "high up" anyway, the capsule would literally go back high up into space, and with the right framing (i.e. a graph of altitude vs time) the trajectory would kind of look like a bounce.

Energy conservation is valid for orbits too. You need energy to get from a lower orbit to a higher one.

But "bouncing" of the atmosphere does not add energy, some energy is lost to air friction. Finally you will leave the orbit anyway when entering the upper atmosphere even for short time due to an elliptical orbit. If not at the first time, then at the next times at the lowest point of the elliptical orbit. Each short pass of the atmosphere will take some of the orbit energy again. Removing some orbit energy results in a lower orbit. If the orbit is too low finally, reentry is unavoidable. But a delayed reentry may be dangerous for the crew, the reserves for electrical energy and oxygen for the capsule are very limited.

• While your answer was first, Blake's is more clear. No votes in a couple months you might want to delete this one. Mar 23, 2017 at 15:15
• +1 For me at least, this is a better explanation of what the often-heard phrase "bounding off of the atmosphere" really means.
– uhoh
Feb 23, 2018 at 1:03
• "is lost to air friction" as this happens somewhere around atmosphere border, and this question is not limited to Earth, when you write "air" it makes the answer very misleading. Nov 23, 2018 at 10:06

tl;dr -- an atmospheric entry always loses energy, but sometimes not enough to keep the entering vehicle/capsule/meteor trapped in the atmosphere. It's possible for the entry to change the orbit such that it pops back up above the atmosphere in a lower-energy orbit than before, which might re-intersect the atmosphere, or might still be above escape velocity and never come back.

From experience with spaceflight simulators, specifically Orbiter:

The space shuttle had a tendency to bounce. Everyone says that it is a "flying brick" meaning that it doesn't fly well. Actually it's a great aircraft in its design range... it's just that its design range is mach 10+. The nominal entry profile is to enter the atmosphere at 7000+ m/s horizontally and about -150m/s vertically, with a range to go of about 4000km, with wings level and an angle of attack of about 40deg nose up. If the vehicle held this wings level attitude, it would generate so much lift that it would quickly cancel out the downward speed and start going up. Since the spacecraft is still almost in orbit, it still has almost all of its horizontal speed and the effective acceleration of gravity is quite low and it doesn't take much lift to start flying back up. It literally is almost like a wall at 80km altitude. If this attitude continues, the spacecraft will start flying up, still at almost orbital speed, and arc far over the landing site. I never tried to stretch the glide as far as I could, but based on my experience, I would say that it would probably not enter the atmosphere again until it was thousands of km past the landing site, and bounce again. Each time energy would be lost, so the average orbit altitude (semimajor axis) would continually decrease. Eventually it wouldn't bounce high enough to get back into space and would finish entering, after skipping something like half way around the world.

Since this isn't desired, there are of course tricks to prevent skipping. The technical definition of lift is the portion of aerodynamic forces perpendicular to the direction of motion, just like drag is the portion anti-parallel to the motion. The English word "lift" implies "up", but the math definition does not -- the lift can be directed in any direction perpendicular to the flight path.

The lift could be changed by changing the angle of attack, but there are a lot of tradeoffs between the structure of the wings, the placement of the heat shield tiles, etc which tend to make it preferable to keep the 40deg nose-up angle of attack.

So what the shuttle did, is right after entry, it would bank almost 90deg, so that its lift vector is nearly horizontal. It kept just enough lift in the vertical direction so that the sink rate is controlled. If the ship needed more drag, it would bank more so that it would sink faster and dig into thicker air lower down. Conversely if it needed less drag, it would bank less. As the ship slowed, the total lift decreased (and the effective acceleration of gravity increased) so that it had to roll closer to wings-level to keep enough vertical lift.

Apollo, even though it doesn't have a wing, had an off-axis center of mass. This meant that the capsule would come in at an angle, and effectively the whole bottom of the heat shield would be one big circular wing. It is then controlled the same way, by banking to control the amount of vertical lift. This is called a lifting entry, and all modern manned capsules do something like this. Coming back from the moon, the spacecraft was flying at above escape velocity, so if it lifted back out of the atmosphere, it might go into a very high orbit, perhaps high enough that it would take hours or days to come back. (Considering that by this point the service module had already been separated and the power and life support remaining was only a few minutes, that would have been a Bad Day.) So, the Apollo guidance was designed to control drag as first priority until it got below the local circular orbit speed, at which time it considered itself captured and unable to bounce out. Then it shifted focus to targeting a particular splashdown site.

The alternative for a capsule is to not have any center-of-mass offset. This is referred to as a ballistic entry (as opposed to a lifting entry above). Some of the Mars landers used this, as well as the original Vostok and Venera spherical entry capsules. This design doesn't generate any lift and therefore is never in danger of skipping out.

Similarly, I would expect an ordinary natural meteor to not have any significant lift and not be in danger of skipping out. Meteors sometimes do pass through the upper atmosphere with their original perigee in the atmosphere but above the ground. Sometimes the perigee is high enough that while the meteor burns and glows, it doesn't lose enough speed to drop the perigee to ground level, and therefore heads back out into space in a lower-energy (but still perhaps escaping) orbit. I leave it to you to judge whether you consider this "bouncing off" the atmosphere, but I don't.

Manned capsules tend to use lifting entry, for a variety of reasons. First, it is more controllable and therefore easier/possible to target a precision landing site. Second, because it lifts, it spends more time higher in the atmosphere in thinner air than it would otherwise. This allows it to come in with less acceleration than it would otherwise. The space shuttle coming in from low-earth orbit rarely felt accelerations greater than 1.5g, while a ballistic entry on even the shallowest path from the lowest orbit will hit over 8g.

Soyuz is designed to use either mode -- it normally uses lifting entry, but it can fall back to ballistic entry under certain conditions (usually malfunctions). It isn't fun to enter with over 8g, and it means giving up controllability, but the ability of Soyuz to withstand a ballistic entry has been used four times, and has saved crews that otherwise would have been lost.

You can bounce off the atmosphere if your craft's speed is higher than escape velocity at the height of the bounce and the hyper aerodynamics of the craft allow you to hyper glide. This will occur if the craft is some form of hyper sonic waverider as a waverider generates lift.

If no lift is generated the craft will traverse the atmosphere, as long as the speed exceeds the escape velocity then the craft will leave Earth and may never return. That is not exactly a bounce, but will produce the same dismay in its passengers, crew, operators and their families.