Really cool vid on Apollo Re-entry

[Awass]

Really interesting. Much more goes into re-entry in real life than Kerbal Space Program would have you believe. So many factors involved.

[dharak1]

Shouldn't this be in the science labs?

[Awass]

Shouldn't this be in the science labs?

Apparently, yes, yes it should. But let not my error detract from the informative value of this vid.

[hugix]

Re entry phase is a really cool part of spaceflight. It is difficult to predict, there are lots of variables and it goes really fast.

It annoys me that KSP doesn't have a realistic atmosphere.

[softweir]

That was a most fascinating and informative video. Very dry and "1960s" in tone, but with a lot of interesting information!

It's going to make life tough for KSP players who want stage authentic Apollo reconstructions...

[lajoswinkler]

There are levels of realism one can achieve. The level described in the video would make gamers' lives a lot more difficult.

I can imagine how tough would it be to combine that with real effects of reentry, like fiery death if you don't rotate to a proper position.

[Mr Shifty]

Orbiter. TEI and re-entry in Orbiter are pretty realistic. You have to pre-dict the position of the landing site when your spacecraft reaches EI. If you have a spacecraft with damage and re-entry effects enabled (e.g. XR2, DG-IV, AMSO Apollo capsule) you can certainly burn up if you have the wrong re-entry angle. You have to account for latitude and cross range. It's all there.

[hugix]

Orbiter. TEI and re-entry in Orbiter are pretty realistic. You have to pre-dict the position of the landing site when your spacecraft reaches EI. If you have a spacecraft with damage and re-entry effects enabled (e.g. XR2, DG-IV, AMSO Apollo capsule) you can certainly burn up if you have the wrong re-entry angle. You have to account for latitude and cross range. It's all there.

I love re entry in the DG-IV. That lovely autopilot to manage your angle of attack. On the left MFD is surface. Than the board computer switching between the heat information and the AOA info. And on the right MFD the map in groundtrack mode.

[rkman]

The re-entry that the video describes was pretty much the only way to do it in KSP with the old Deadly Re-entry mod when coming back from the Mun.

Had to temporarily gain altitude at some point during descent to prevent the heatschield burning up, though not by means of lift but by putting initial periapsis at the correct altitude.

[Awass]

The re-entry that the video describes was pretty much the only way to do it in KSP with the old Deadly Re-entry mod when coming back from the Mun.

Had to temporarily gain altitude at some point during descent to prevent the heatschield burning up, though not by means of lift but by putting initial periapsis at the correct altitude.

That sounds incredibly difficult being that you can't control the skip once you start it, and it's kinda a guess as to where to put the periapsis.

[hugix]

also, the modules in KSP have their centre of gravity (CoM) in the middle, this makes it impossible to skip.

[goldenpeach]

Even if you off-set the CoM by modding, I'm not sure if you could skip.

It would be easier to off-set the CoM and adding lift capacity of the module by modding.

[Motokid600]

Wow so skipping is not possible in ksp? LOL.. well so much for my past efforts to try and "skip" the pod closer to KSC. I loved that video.. nice and retro. I didnt know why the CM changed its alignment in the reentry vids. And I always thought the CoM was in the center. Neat. Wouldn't this be possible with an off center CoM and FAR? Or hell.. maybe adding tiny wings? Its nice for Deadly Reentry to scrub off some heat, but as for the stock aerodynamics Id love to have more control in atmosphere without the use of engines. For command pods anyway. Was never much for ssto's.

Oh and question... Has anyone ever tried hitting KSC with a direct reentry from the Mun? That'd be **** hard. If im shooting for KSC I aerobrake into orbit first and THEN descend. So lets say in real life the Apollo craft did the same thing. Instead of a direct reentry they went into orbit first. Would this "double reentry" maneuver be necessary? Seeing is how your moving at less then half the speed.

Edited October 15, 2013 by Motokid600

[Brotoro]

You could make the center of mass of your capsule off center by hanging your parachutes or some battery packs asymmetrically on the sides of the capsule. But the aerodynamics of KSP is probably not going to model the forces properly to get a skip.

[Beowolf]

Really interesting. Much more goes into re-entry in real life than Kerbal Space Program would have you believe. So many factors involved.

A wonderful find, Awass! I'd read most of this before, but the video pulled everything together for the first time. Dry text didn't work well enough.

No wonder I could never manually land where I wanted in Orbiter.

[Beowolf]

So lets say in real life the Apollo craft did the same thing. Instead of a direct reentry they went into orbit first. Would this "double reentry" maneuver be necessary? Seeing is how your moving at less then half the speed.

In the Orbiter sim, which is a serious version of KSP that uses the real planets, you still have to do a lot of steering during orbital reentry to even get within hundreds of km of your planned landing. I never did get the hang of it, except using MechJeb-type tools. There must be a dozen different Orbiter mods that provide varying levels of automation or info for reentry. So I suspect it's a hard thing to do in real-world spaceflight as well.

IIRC, most of the tools I looked at did indeed use a "double reentry" maneuver, and I normally did one just for temperature control even if I didn't care where I landed. I was more successful when I'd go back up into thinner air to let the heat shield cool for a while. I don't know whether that was necessary or just easier for me, though.

[lajoswinkler]

One question - if you do the skipping in real life, can you get stuck in a higher orbit? Cause I think I remember hearing that in Apollo 13 movie. They were talking about skipping "into outer space if this goes wrong". I don't think that's possible.

[Nibb31]

One question - if you do the skipping in real life, can you get stuck in a higher orbit? Cause I think I remember hearing that in Apollo 13 movie. They were talking about skipping "into outer space if this goes wrong". I don't think that's possible.

No, you wouldn't simply bounce off into solar orbit. I think that when they refer to skipping away, what they really meant was a dip into the atmosphere that would be too shallow to slow down the CM enough to bring the apogee of the orbit into the atmosphere. What would happen is that they would go through the atmosphere in a straight line and come out on the other side for another orbit. Depending on the severity of the error, the next orbit could take from several hours to several days (potentially travelling back to the Moon's orbital distance). This would have been catastrophic, because once the SM was dumped, the CM had very limited batteries and life support, a couple of hours max. Another orbit would have brought back three dead astronauts.

[Kerbart]

also, the modules in KSP have their centre of gravity (CoM) in the middle, this makes it impossible to skip.

The off-center COM was used to allow a stable position without the axis of the capsule aligning with the axis of flight; hence generating lift. We don't have that issue in KSP as we have our magical SAS wheels to take care of that problem; it's perfectly possible to have the capsule in a lift-generating angle during re-entry, thus implementing skipping if the aerodynamics model in KSP would support itâ€â€that's the real reason you can't do the skipping maneuver in KSP.

[Mr Shifty]

Oh and question... Has anyone ever tried hitting KSC with a direct reentry from the Mun? That'd be **** hard. If im shooting for KSC I aerobrake into orbit first and THEN descend. So lets say in real life the Apollo craft did the same thing. Instead of a direct reentry they went into orbit first. Would this "double reentry" maneuver be necessary? Seeing is how your moving at less then half the speed.

Apollo wasn't designed for double re-entry at all. The heat shield was between the CM and the SM, so they had to jettison the SM -- with its engine -- before re-entry. The CM had no engines of its own, so no way to raise perigee after aerobrake, nor de-orbit again after that. Also, the CM heat shield was ablative; it's heat tiles would heat up and then detach and fly away from the craft. So it wasn't re-usable. A second re-entry would have burned up the people spaces.

IIRC, most of the tools I looked at did indeed use a "double reentry" maneuver, and I normally did one just for temperature control even if I didn't care where I landed. I was more successful when I'd go back up into thinner air to let the heat shield cool for a while. I don't know whether that was necessary or just easier for me, though.

You can plan direct re-entry with both of the two major interplanetary tools: IMFD and TransX. To do direct re-entry, you have to figure out how long an efficient moon-to-earth transit will take, then quicksave and time accelerate to that time. Then you have to rotate the earth (by time accelerating) until your desired entry spot on the Earth is directly opposite the moon and note that time. Then you quickload and adjust your TEI so that your transit takes that long (you burn more radially so that the orbit is "rounder" and takes longer.) That way when you reach perigee, you'll be directly above your desired entry spot. I imagine you could do something similar in KSP.

Once you've reached the entry interface in Orbiter, fire up Aerobrake MFD. It gives you all the info you need to fly a deadstick landing.

Note that re-entry latitude is tricky. Perigee from a lunar return is a fixed spot on the Earth's surface; all of the orbits that encircle both the moon and the Earth have perigee at the same spot. But, the Earth's axial tilt means that the latitude of perigee will vary throughout the month, so you can choose your re-entry latitude (within a ~ +/-20° band) by varying your departure time throughout the month. To reach Kennedy, though, you usually have to rely on your spacecraft's crossrange capability. In KSP, since Kerbin's ecliptic and equatorial plane are the same, the Mun orbits on that plane, and KSC is on the equator, this wouldn't be a problem.

[PakledHostage]

The off-center COM was used to allow a stable position without the axis of the capsule aligning with the axis of flight; hence generating lift. We don't have that issue in KSP as we have our magical SAS wheels to take care of that problem; it's perfectly possible to have the capsule in a lift-generating angle during re-entry, thus implementing skipping if the aerodynamics model in KSP would support itâ€â€that's the real reason you can't do the skipping maneuver in KSP.

But KSP has a lot of unrealized potential. Kerbin's atmospheric temperature profile was modelled based on the Earth's and, despite the fact that Kerbin's atmospheric properties aren't consistent with the ideal gas law, you can use Kerbin's atmospheric properties and elementary aerothermodynamics to approximate realistic re-entry effects in KSP. Indeed this is what my old re-entry heating mod did. When the mod's "hardcore" mode was turned on, it would even approximate lift effects on the command pod. Players could use that lift effect to cause a slight skip, steepen their re-entry (at the risk of structural failure), or apply some cross-track trajectory adjustments. The plot below illustrates the re-entry corridor from my old mod (with the mod's "hardcore" mode lift effects turned off):

The trouble is that the triggers for the visual re-entry effects aren't currently implemented very well in the game and they should really be fixed. For example, here's a plot of heat flux and velocity (among other things) for the Stardust sample return capsule's Earth re-entry:

Notice that maximum heat flux for Stardust occurred while the spacecraft was still in the upper atmosphere. The same was true for the Space Shuttle. In the game, we don't see the plasma effects that should coincide with peak heating until the re-entry is almost over. Even so, it wouldn't be hard for Squad to tweak the game's visual re-entry effects for consistency with real-world physics and then incorporate some basic re-entry heating and structural stress effects. If I could do it in a mod, then it is certainly possible. The foundations for those changes already exist in the game.

[Krupski]

A wonderful find, Awass! I'd read most of this before, but the video pulled everything together for the first time. Dry text didn't work well enough.

No wonder I could never manually land where I wanted in Orbiter.

<ComplaintText>

At least Orbiter has decent graphics..........

</ComplaintText>

Real spacecraft

How the MOON should look

How a typical GAS GIANT should look

(those little dots are MOONS!)

The beauty of Orbiter with the simplicity of KSP would be the ULTIMATE package!

[Krupski]

One question - if you do the skipping in real life, can you get stuck in a higher orbit? Cause I think I remember hearing that in Apollo 13 movie. They were talking about skipping "into outer space if this goes wrong". I don't think that's possible.

Getting into a higher orbit requires MORE energy. ANY contact with the atmosphere DRAGS AWAY energy.

Therefore, the answer is "no, you cannot get stuck in a higher orbit".

On actual manned missions (i.e. Apollo) the danger was hitting the entry corridor too shallow and not burning off enough velocity to be "captured" by Earth (i.e. slow down below orbital velocity). Of course, hitting it too steep was equally bad since the crushed astronaut bodies would be incinerated by the excessive heat.

If that happened, the CM (minus the SM and it's life support!!) would zing through the upper fringes of the atmosphere, loop around the Earth and head back towards the Moon (albeit with a much lower apogee than the mission itself used).

By the time the astronauts came back around for the next try, they would be long dead from lack of life support (no O2, no electrical power, no water, probably terrible buildup of CO2 due to the LiOH being used up, etc...).

A re-entry profile COULD, if necessary, do a "small skip" where they would climb up to the upper fringes of the atmosphere, fly further, then roll back into a "lift down" orientation and enter later (to change splashdown location).

To think we use quad core or better computers with gigabytes of ram to run KSP and MechJeb... Apollo did it for real and did it right with a computer built from a bunch of NOR gates, 2K of "RAM" and 38K of "ROM" made of wires laced through little ferrite cores!

I get shivers every time I think of what the Apollo program accomplished...

[Awass]

To think we use quad core or better computers with gigabytes of ram to run KSP and MechJeb... Apollo did it for real and did it right with a computer built from a bunch of NOR gates, 2K of "RAM" and 38K of "ROM" made of wires laced through little ferrite cores!

I get shivers every time I think of what the Apollo program accomplished...

Ikr. Didn't the LM have less computing power than a modern calculator? They did all their work the old fashioned way: with a paper and a pencil and a few logic gates. It truly is amazing.

[rkman]

That sounds incredibly difficult being that you can't control the skip once you start it, and it's kinda a guess as to where to put the periapsis.

It took some trial and error but at interplanetary (inter-Kerbin system) speeds there's actually a very small margin of altitude (around ~32km) below which descent will be fairly short and direct (irl it would induce to much g-force), and above which the descent will 'reverse' for a bit (gain altitude again). With Pe even higher it would amount to aerobreaking (Apollo crew would not survive due to lack of life support time - but not a problem in KSP), while within the sweetspot the descent is shallow but direct (no 'bounce') - but that would be to hot for the Apollo capsule, and to hot for the old DR mod.

The FAR mod did/does in principal allow some control over lift of the pod, but i never got around to trying that. Last time i tried FAR and DR didn't play nice together.