Curious about re-entry physics

[ColdEthyl75]

l've just started playing Kerbal and as someone whose grasp of physics is based on a long-forgotten secondary school education in the 1980s, it took a while to sink in that the command module slows down on re-entry, even though I'd played tutorial 4 at least twice. It seems so counter-intuitive to me but now I've got that into my head my Kerbal survival rate has improved dramatically. I'm curious as to the reason why? Is it something to do with atmospheric pressure, the shape of the module, what?

It seems this game is going be a great way to learn all the stuff I should have been paying attention to back in the 80s

[Freshmeat]

Drag in the atmosphere goes like the square of the objects velocity, and linear with atmospheric density and cross section of the object. So high speed slows down rather quickly, once the atmosphere get dense; and once you deploy a parachute, you are suddenly very large compared to your mass.

Note that once you get to an airless body, there is no drag to slow you down. Prepare to use your engine to slow you down.

[The Aziz]

In atmo there are air particles that smash into your ship as you travel through them, thus slow you down.  And another important thing, you want to slow down as much as possible (because crashing into the ground at orbital velocity is no fun, maybe with an exception to Danny2462) and there goes simple math: more air hitting your craft means you lose velocity faster. How to do it? With facing the most flat side to direction you're going.

[Guest]

Good answers here already. One thing that nobody mentioned yet is terminal velocity. That's the point where you won't slow down anymore. What your terminal velocity is depends on how dense the air is and how much drag you generate compared to your mass.

In terms of real-world objects: when you throw a javelin and a feather pillow, the javelin will travel a longer distance because it's dense and pointy whereas the feather pillow is the opposite. Or, in more Kerbal terms, if you drop a feather pillow from an airplane it won't kill anyone when it lands, but if you drop a javelin it certainly might.

[Gargamel]

52 minutes ago, Brikoleur said:

Or, in more Kerbal terms, if you drop a feather pillow from an airplane it won't kill anyone when it lands, but if you drop a javelin it certainly might.

Apparently, you've never played KSP.  

[ColdEthyl75]

Great answers. Thanks everyone. It looks like I've got a lot to learn before making that giant leap for Kerbalkind

[Zeiss Ikon]

Yep, I've seen a Mark 1-2 with a single Kerbal aboard pull 5 G reentering from the Mun with a 35 km perapsis -- and that acceleration will be close to 30 km up (but the command pod is still traveling at or slightly above circular velocity for an 80 km orbit).  Don't forget your heat shield!  That energy has to go somewhere...

[ZootinZack]

Even simpler, think of throwing a rock in water, it slows down when it hits. Air is matter too, just like water. Slows stuff down.

[Vanamonde]

Since actual science is taking place, this thread has been moved to Science & Spaceflight. 

[YNM]

On 28/02/2018 at 8:37 PM, ColdEthyl75 said:

it took a while to sink in that the command module slows down on re-entry, even though I'd played tutorial 4 at least twice.

You've been living in a "soup" called Atmosphere (or, Air). "Thicker" than when there's none of it (in space, there's almost none of it).

While the reason is simple, getting it down to analytical expression is very bizzare.

Edited March 2, 2018 by YNM

[Zeiss Ikon]

48 minutes ago, YNM said:

While the reason is simple getting it down to analytical expression is very bizzare.

Yes.  Fluid Dynamics does not coexist well with the desire for an exact mathematical description.  Numeric methods are the rule, even at the theoretical end -- which is why Computational Fluid Dynamics is such a big deal (and why most of the supercomputers that aren't doing weather or crypto-stuff are doing aerodynamics).

[MaxwellsDemon]

  I'm embarrassed to admit how recent it was that I finally "got" why blunt-body reentry works better (heatwise, as well as brakingwise) than other shapes.  I had taken it on authority, of course, figuring that someone else knew this stuff better than I did, but it took a while before I found an explanation (in an old NASA book, no less) that finally allowed all the pieces to fall into place for me.   Many things about space flight are counter-intuitive on the surface (although once clearly explained, they then make sense), since we're so accustomed to the way things work at the bottom of a gravity well in an ocean of air.

[sevenperforce]

Russian capsules use a truncated sphere as their re-entry vehicles because this is the most efficient use of space. However, truncated spheres cannot be oriented in such a way as to produce list, and therefore must enter ballistically, which is rather hard on the occupants. US entry vehicles have been conical, because a conical capsule (while not as efficient in terms of space used or launch vehicle size) can be steered to produce lift and adjust the rate of descent. Steering can also direct them more accurately to a particular point.

Remember that it is compression, not friction, which generates 99% of heat on re-entry.

What we identify as temperature is based on the average speed of the individual molecules in the air around us. This is actually a pretty steady clip; the average speed of an oxygen molecule in the air around you right now is about 450 m/s. Each time an air molecule hits your skin, there's an exchange of kinetic energy; if you're warmer than your surroundings, the air molecule gains energy and so you feel cool; if you're cooler than your surroundings, the air molecule loses energy and you feel warm.

High in the atmosphere, air molecules are moving much faster since they receive more direct heat from the sun, but molecules are few and far between and so there is less opportunity for thermal transfer, which is what allows water vapor to condense into clouds or even ice.

When a re-entry capsule slams into the atmosphere, air molecules are pressed together faster than they can flow around the heat shield. This compression causes the molecules to collide with each other much more frequently, building up kinetic energy which eventually becomes so great that the molecules ionize and turn to plasma. The reason for the blunt-body shape of a capsule is not only to expose as much surface to the atmosphere as possible, making drag maximum, but to shape the flow of plasma such that most of the heat flows out around the capsule rather than being transferred to the heat shield. Most of the heat taken by the heat shield is actually due to radiative heating from the plasma (ionized air releasing photons which strike the heat shield) rather than thermal transfer.

[YNM]

4 hours ago, Zeiss Ikon said:

Fluid Dynamics does not coexist well with the desire for an exact mathematical description.  Numeric methods are the rule, even at the theoretical end ...

And even then the proof work can only be empiric.

39 minutes ago, sevenperforce said:

Russian capsules use a truncated sphere as their re-entry vehicles because this is the most efficient use of space. However, truncated spheres cannot be oriented in such a way as to produce lift ...

Wait, what about the current Soyuz ?

[sevenperforce]

7 minutes ago, YNM said:

Wait, what about the current Soyuz ?

The Soyuz re-entry capsule is almost completely spherical; there is a very very slight amount of conic extension to the truncated sphere, allowing a tiny amount of lift, but entry is far closer to ballistic than typical US entry capsules. From Wikipedia:

Quote

One of the design requirements for the Descent Module was for it to have the highest possible volumetric efficiency (internal volume divided by hull area). The best shape for this is a sphere — as the pioneering Vostok spacecraft's Descent Module used — but such a shape can provide no lift, which results in a purely ballistic reentry. Ballistic reentries are hard on the occupants due to high deceleration and cannot be steered beyond their initial deorbit burn. That is why it was decided to go with the "headlight" shape that the Soyuz uses—a hemispherical forward area joined by a barely angled (seven degrees) conical section to a classic spherical section heat shield. This shape allows a small amount of lift to be generated due to the unequal weight distribution. The nickname was thought up at a time when nearly every headlight was circular.

 

[wumpus]

2 hours ago, MaxwellsDemon said:

  I'm embarrassed to admit how recent it was that I finally "got" why blunt-body reentry works better (heatwise, as well as brakingwise) than other shapes.  I had taken it on authority, of course, figuring that someone else knew this stuff better than I did, but it took a while before I found an explanation (in an old NASA book, no less) that finally allowed all the pieces to fall into place for me.   Many things about space flight are counter-intuitive on the surface (although once clearly explained, they then make sense), since we're so accustomed to the way things work at the bottom of a gravity well in an ocean of air.

The short answer is pretty surprising.  All that heating doesn't come from friction (presumably some of it does, but it is a minor source), but from adiabatic compression of air.  The advantage of the blunt body is that the highest temperatures happen *away* from the blunt body.  The energy is converted to heat, the air around the capsule is heated and then routed around the capsule.  You still need a heat shield (in real life, sometimes you can get away with the built in heat shields of KSP), but in the end you only have to shield your capsule from a fraction of heat.

This is more "learned thanks to playing KSP" rather than "learned while playing".  The actual mechanism doesn't come up in play, you just aim your Pe at 30k of Kerbin or so and watch your capsule come home.

[MaxwellsDemon]

'Zackly, and through the same route... KSP inspired me to read about it.   

[ColdEthyl75]

Many thanks to all. The rock and water analogy really helped. It's so blindingly obvious once you get it.

[Gargamel]

On 3/1/2018 at 8:09 AM, WildLynx said:

What?! No one linked this: ?

 

 

This is amazing.  Of Course, I realized I still have the maturity of a 13 year old.   My first thought was this is just like the videos we used to watch in Health Class.  With that in my mind, I actually laughed when the second word on the screen was penetration. 

Good vid, should be required viewing for KSP players.