Sign in to follow this  
strider3

"Bouncing" off on re-entry?

Recommended Posts

I remember watching Apollo missions as a kid (yeah, I'm that old :P) and listening to the commentators talk about the angle of re-entry. Too steep and the capsule could burn up, too shallow and it could bounce off the atmosphere and be thrown back out into space. Every time I come in too shallow, in KSP, I do bounce back into orbit...but it's not like I'm headed to Kerbol after that. My apo is reduced every time this happens, on subsequent orbits, until I have a successful re-entry. I do try and avoid all that, of course, but was the whole "bouncing off into space, lost forever" a myth?

Share this post


Link to post
Share on other sites
13 minutes ago, strider3 said:

I do try and avoid all that, of course, but was the whole "bouncing off into space, lost forever" a myth?

You have unlimited life support and next-to-unlimited battery power in KSP.

A reentry vehicle does not.

Share this post


Link to post
Share on other sites
39 minutes ago, strider3 said:

I do try and avoid all that, of course, but was the whole "bouncing off into space, lost forever" a myth?

It is a myth. It's impossible to "bounce off" into solar orbit or indeed, into anything resembling a stable one. However, the Apollo capsule didn't have a whole lot of consumables (batteries, RCS prop, life support), and was designed for very specific reentry profile. It would eventually reenter, but it would likely be too late by that point, and even if it wasn't (Apollo crews could be quite resourceful), there's a good chance they'd end up over land, which is another thing Apollo wasn't designed to handle.

Share this post


Link to post
Share on other sites

Another thing Apollo didn't have was a button in mission control that automatically recovered the capsule instantly no matter where it was in the world. They actually had to land reasonably close to their target so that their recovery ship could get to them.

Share this post


Link to post
Share on other sites
5 minutes ago, Dragon01 said:

It is a myth. It's impossible to "bounce off" into solar orbit or indeed, into anything resembling a stable one. However, the Apollo capsule didn't have a whole lot of consumables (batteries, RCS prop, life support), and was designed for very specific reentry profile. It would eventually reenter, but it would likely be too late by that point, and even if it wasn't (Apollo crews could be quite resourceful), there's a good chance they'd end up over land, which is another thing Apollo wasn't designed to handle.

Yes, you will loose energy so you Ap would be lower on next orbit. But as you say they could easy run out of resources, they had dropped the service module so they only had the power and oxygen in the capsule.  
And as other say they wanted to land close to the recovery ship although this was not so critical. 

Share this post


Link to post
Share on other sites

Still, I, too, remember Cronkite talking about the too-shallow reentry as if it was a one-way trip to Earth escape.  Fifty years later, it occurs to me that Walter was a reporter, and probably knew next to nothing about physics and orbital mechanics...

Share this post


Link to post
Share on other sites

They never do. Even during the Moon Race, the public had no understanding of how things actually move in space. 

Landing close to the recovery ship wouldn't be as much of a problem as it seems. The trajectory, even if the capsule is completely uncontrolled, can be tracked, extrapolated, and a recovery team sent into the general area (enough to pick the crew up by helicopter). Of course, if the landing site ended up being somewhere inconvenient, like in the middle of Sahara, Chinese waters or in the USSR, this would complicate things immensely.

Share this post


Link to post
Share on other sites

Although the Apollo Capsule did in fact use a lifting re-entry profile, coming in too shallow would only look like it was skipping off on a graph of altitude vs time.

The actual path followed would still be elliptical. It's just that the apogee wouldn't lower enough to ensure a capture on the first pass.

You would get a second pass, and a pass after that and so on, but the consumables might not last. An orbit touching lunar altitude would take another week or more to complete (less depending on how much apogee gets lowered), and you'd probably run out of consumables.

Also, you would have very little control over your profile on subsequent passes and may come in too steep as a result. Or land somewhere not conducive to safe recovery, such as over land or a long way from friendly rescue teams.

Edited by RCgothic

Share this post


Link to post
Share on other sites

And maybe there exists a dangerous edge case between "captured safely" and "not captured this pass".

On a steep entry peak thermal loading is higher (and may overload the heat shield) but as it takes less time the capsule absorbs less energy through thermal transfer from the plasma sheath overall. And as the outer skin is hotter it actually re-radiates that heat quite efficiently. As a result inside temperatures remain bearable.

A shallow re-entry may cause the capsule to be exposed to the plasma sheath for longer, causing it to absorb more energy over time. And it's cooler, so it doesn't re-radiate as efficiently. The inside of the capsule heats up and the contents get fatally cooked.

Not sure if this mode exists for Apollo, but a certain combination of entry angle, capsule mass and lift to drag ratio could experience it.

Edited by RCgothic

Share this post


Link to post
Share on other sites
5 hours ago, Zeiss Ikon said:

Still, I, too, remember Cronkite talking about the too-shallow reentry as if it was a one-way trip to Earth escape.  Fifty years later, it occurs to me that Walter was a reporter, and probably knew next to nothing about physics and orbital mechanics...

This guy?

Spoiler

maxresdefault-29.jpgtimetraveller.png?itok=FIU_1_CA

He did this himself in KSP.
Before that d-d chronoclasm... :(

Share this post


Link to post
Share on other sites

@kerbiloid Pretty sure Walter Cronkite wasn't a time traveler.  If he were, he might have managed to convince von Braun to quit smoking, or done so himself.  And that guy in the photo doesn't actually have a cell phone, even though it might look that way.  What he has is probably a tiny one-tube radio receiver.  Miniature tubes were available before WWII, if you had some money, and were built with two or occasionally even three "tubes" inside a single envelope -- that would give you enough electronics to build a superheterodyne AM receiver that would fit in a shirt pocket (pack of cigarettes, pack or cards size, running on an external miniature 22 V B battery; a couple D cells for the filament would give several hours of operation with weeks of life on the B battery).

Share this post


Link to post
Share on other sites
On 12/13/2019 at 6:43 AM, Zeiss Ikon said:

Still, I, too, remember Cronkite talking about the too-shallow reentry as if it was a one-way trip to Earth escape.  Fifty years later, it occurs to me that Walter was a reporter, and probably knew next to nothing about physics and orbital mechanics...

To be honest, I think there was a lot we all didn't know about orbital mechanics before Filipe and co created this wonderful game.  Even Randal Monroe has admitted how much he didn't know, with perhaps only Scott Manley leading the way toward getting to orbit.

- that said, I remember digging back to the very start of his videos, and he seemed to imply a misconception I had before the game about escape velocity sending you "infinitely far" from your home planet.  Granted, this was before the Kerbal system was implemented, so Scott could argue that it was actually the case.  But now I know that Earth escape velocity just means you will vary between 0-2 AU from the Earth.

Share this post


Link to post
Share on other sites
On 12/13/2019 at 5:43 AM, Zeiss Ikon said:

Walter was a reporter, and probably knew next to nothing about physics and orbital mechanics...

DIng Ding Ding! Important thing to remember any time any of the new generation reporters talk about climate. 

Share this post


Link to post
Share on other sites

Or about anything else. Like new drugs, for instance. Hooray, scientists cured Alzheimer's! Hooray, scientists cured diabetes! Hooray, scientists cured cancer! Except... no, this must by this greedy corporation holding all the miracles back! And let's not even get into their grasp of economics and finance...

It's always like this. The recent generation of reporters doesn't even't know much about reporting and journalism in general, let alone hard sciences. Even "popular science" is often highly lacking in detail and somewhat prone to sensationalism.

Share this post


Link to post
Share on other sites
1 minute ago, Dragon01 said:

Or about anything else.

Yes I really should have said this too! The slightest effort to GO RESEARCH any topic will 9/10 times yield a different result than mass media/popular opinion is talking about! This is as political as I need to get here :)

Share this post


Link to post
Share on other sites
On 12/13/2019 at 6:43 AM, Zeiss Ikon said:

Still, I, too, remember Cronkite talking about the too-shallow reentry as if it was a one-way trip to Earth escape.  Fifty years later, it occurs to me that Walter was a reporter, and probably knew next to nothing about physics and orbital mechanics...

Yes, the scientific discovery that the press is composed of amoral greedy blowhards, willing to opine on any matter as though they were some sort of genius-demigods, has been a long time coming.

Share this post


Link to post
Share on other sites

People have some fundamental misunderstandings about journalists. Their job is not to be experts. Their job is to find experts and then relay to everyone else what those experts say. Some do it better than others. Some find better experts than others.

As for just researching things yourself -- well, yes, that's a great choice. But beware the Dunning-Kruger effect. It's very common for people to do a little bit of research and suddenly decide they are experts.

Quote

 

A little learning is a dangerous thing;

Drink deep, or taste not the Pierian spring.

There shallow draughts intoxicate the brain,

And drinking largely sobers us again.

 

 

Share this post


Link to post
Share on other sites
On 12/13/2019 at 11:44 AM, strider3 said:

I remember watching Apollo missions as a kid (yeah, I'm that old :P) and listening to the commentators talk about the angle of re-entry. Too steep and the capsule could burn up, too shallow and it could bounce off the atmosphere and be thrown back out into space. Every time I come in too shallow, in KSP, I do bounce back into orbit...but it's not like I'm headed to Kerbol after that. My apo is reduced every time this happens, on subsequent orbits, until I have a successful re-entry. I do try and avoid all that, of course, but was the whole "bouncing off into space, lost forever" a myth?

Simple physics should make clear it's a myth.

 

"Bouncing" doesn't add extra energy to the object that bounces (where would it come from), at most it is elastic and hence has equal energy. When you are in an orbit that is very clearly not going outside the earth's sphere of influence and the re-entry vehicle has too less energy than required for escape velocity, there's no way you can "suddenly gain energy".

 

To gain energy you have to put work in it. This is just a basic thermodynamic law.

Share this post


Link to post
Share on other sites

I suppose if you entered the atmosphere with greater than escape velocity it's possible the atmosphere wouldn't scrub off enough speed to capture you, and then you wouldn't come back to earth again.

That's still not really skipping off.

Edited by RCgothic

Share this post


Link to post
Share on other sites

As the difference between the lunar transfer and Earth escape velocities is about 100 m/s, it's ok with the energy balance if you are approaching to the Earth with a little excess of velocity (i.e. if your transfer orbit is slightly hyperbolic).

 

Share this post


Link to post
Share on other sites
15 hours ago, RCgothic said:

I suppose if you entered the atmosphere with greater than escape velocity it's possible the atmosphere wouldn't scrub off enough speed to capture you, and then you wouldn't come back to earth again.

That's still not really skipping off.

I don't know why that wouldn't be "skipping off". What's the definition of "skipping off" anyway?

14 hours ago, kerbiloid said:

As the difference between the lunar transfer and Earth escape velocities is about 100 m/s, it's ok with the energy balance if you are approaching to the Earth with a little excess of velocity (i.e. if your transfer orbit is slightly hyperbolic).

Yeah, there's really not much difference in velocity here. It wouldn't be all that hard to add enough velocity when coming back from the moon to put yourself in a situation where you could in fact end up going fast enough to bounce out into solar orbit.

Edited by mikegarrison

Share this post


Link to post
Share on other sites
13 hours ago, mikegarrison said:

I don't know why that wouldn't be "skipping off". What's the definition of "skipping off" anyway?

I'd assume it means much like a flat stone can be skipped across a pond/lake.  Any reader wouldn't expect such a skip to add energy, but it isn't clear that it would necessarily come back down.  Accelerating from lunar orbit is already quite close to escape velocity, but I'm sure the skip would slow any ship down enough to put it well under lunar orbit (but still using up all the life support before returning for a second try).

Apollo capsules were designed to allow a limited "skip", that is they were designed with the center of mass offset from the center of drag.  This let them rotate the spacecraft to correct their course and was apparently originally meant to come down deeper into the atmosphere, raise up (and cool off how?) and finally make a full descent.  This was not apparently used (but I doubt they added any ballast to move the CoM closer to the center.  Possibly just rearranged some gear).

I've heard this myth over and over (before KSP).  I can't recall *anyone* pointing out how wrong it is.  Does anybody remember Apollo 13 (movie)?  I seem to recall them making up a silly story about why the spacecraft had to continue on the way to the Moon instead of something like "as you know, Bob, once we made the burn for the Moon we were committed to at least TLI.  Even if we had a working rocket and all the fuel (+oxidizer) was available, we couldn't turn around right now".  They seemed to imply that the only reason they couldn't  do that is that the damage was in the rocket (oxidizer tank).  People aren't expected to know or ever understand orbital mechanics.  Perhaps the Smithsonian can add a small area to their "how things fly" exhibit showing the basic idea that you start with an usually only need to increase or decrease the altitude of the opposite side of your orbit.

- Edit: according to linked ESA blog above, they included that old myth in the Apollo 13 movie as well (read the book, might have missed a bit where they explain the real issue with "skipping").  Granted, if they were about to re-enter the pacing had to be far to fast for a break for a lesson on orbital mechanics.  But it could have easily fit in one of those planning tables at Houston.

Edited by wumpus

Share this post


Link to post
Share on other sites
13 hours ago, mikegarrison said:

Yeah, there's really not much difference in velocity here. It wouldn't be all that hard to add enough velocity when coming back from the moon to put yourself in a situation where you could in fact end up going fast enough to bounce out into solar orbit.

Actually, 100m/s is quite a bit of velocity in a real spacecraft. In KSP we have overpowered engines and a dinky system, but IRL, 100m/s can make or break a launch. This is quite a bit of delta V in RSS or in reality. Add the fact that the atmosphere will slow you down far more than that, there's no real way for a lunar spacecraft to do that by accident.

A "skip-out" would, at worst, put you in a very elliptical orbit with an apogee below the Moon. From astronauts' standpoint, they could well be in solar orbit, though, since it'd be very hard for them to survive six or so days in a CM after separation. If CO2 scrubbers lasted, they could perhaps pull through, though they'd be extremely dehydrated.

Share this post


Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this