Ideal Reentry Trajectory

[heakhaek]

Playing KSP with stock system and I'm using the Trajectories mod. It made me wonder how NASA calculates the ideal angle and altitude of reentry considering how Earth is far more unforgiving than Kerbin since I want to be able to calculate the best angle and altitude of reentry to kinda "simulate" how it would be on Earth because where's the fun in not trying to give myself tight margins.

Btw new here and thanks for being really nice guys and being willing to help out with my questions, and thanks mods for doing your best to keep the forums up considering what's been happening  with ksp, really appreiate it. 

[Vanamonde]

Welcome new-ish person.

Your question has been moved to the science sub because it's about real spaceflight rather than the game. Someone here might know the answer, but I do not. 

[darthgently]

2 hours ago, heakhaek said:

Playing KSP with stock system and I'm using the Trajectories mod. It made me wonder how NASA calculates the ideal angle and altitude of reentry considering how Earth is far more unforgiving than Kerbin since I want to be able to calculate the best angle and altitude of reentry to kinda "simulate" how it would be on Earth because where's the fun in not trying to give myself tight margins.

Btw new here and thanks for being really nice guys and being willing to help out with my questions, and thanks mods for doing your best to keep the forums up considering what's been happening  with ksp, really appreiate it. 

The two main factors are going to be the max temperatures the various parts of the craft can withstand and the drag coefficient of the craft.

The first is fairly easy to nail down for the parts that will be taking the main brunt of reentry.

The second is very hard as it depends on the attitude of the craft, the density of the atmosphere, and the surface velocity and the math varies a lot across mach regimes and there are only numerical solutions to any predictions, no analytical solutions.

 So the ideal trajectory is going to vary a lot depending on the design of the craft.  The Space Shuttle has a very different target trajectory than a typical capsule, for example.

 Generally, design such that you can control the attitude from space to splash or landing and design such that in the chosen attitude(s) the parts taking the heat can take it.  And iterate until it works.  Smarter ppl do it in more involved ways but that is how I do it.

In a nutshell, for any given altitude and air density there is going to be a velocity you want to be under before dropping that far and it will vary by craft design and craft attitude 

[magnemoe]

Note that real capsules from Soyuz to Dragon are not on an pure ballistic trajectory like capsules in KSP they have a bit of lift and can adjust trajectory with this. 

This is also useful returning from places like the moon, this is that the Orion capsule used, idea is that you skip along the upper atmosphere shaving off velocity ending up going higher at the end before you then falls down again as you are slower than orbital velocity. You can get this in KSP too even without any lift. 

[darthgently]

3 hours ago, magnemoe said:

Note that real capsules from Soyuz to Dragon are not on an pure ballistic trajectory like capsules in KSP they have a bit of lift and can adjust trajectory with this. 

This is also useful returning from places like the moon, this is that the Orion capsule used, idea is that you skip along the upper atmosphere shaving off velocity ending up going higher at the end before you then falls down again as you are slower than orbital velocity. You can get this in KSP too even without any lift. 

KSP capsules can have off center CoM and can adjust trajectory via roll of that is what you want to do

[magnemoe]

23 minutes ago, darthgently said:

KSP capsules can have off center CoM and can adjust trajectory via roll of that is what you want to do

They have, had no idea  

[heakhaek]

7 hours ago, Vanamonde said:

Welcome new-ish person.

Your question has been moved to the science sub because it's about real spaceflight rather than the game. Someone here might know the answer, but I do not. 

No problem and thanks for having me.

7 hours ago, darthgently said:

The two main factors are going to be the max temperatures the various parts of the craft can withstand and the drag coefficient of the craft.

The first is fairly easy to nail down for the parts that will be taking the main brunt of reentry.

The second is very hard as it depends on the attitude of the craft, the density of the atmosphere, and the surface velocity and the math varies a lot across mach regimes and there are only numerical solutions to any predictions, no analytical solutions.

 So the ideal trajectory is going to vary a lot depending on the design of the craft.  The Space Shuttle has a very different target trajectory than a typical capsule, for example.

 Generally, design such that you can control the attitude from space to splash or landing and design such that in the chosen attitude(s) the parts taking the heat can take it.  And iterate until it works.  Smarter ppl do it in more involved ways but that is how I do it.

In a nutshell, for any given altitude and air density there is going to be a velocity you want to be under before dropping that far and it will vary by craft design and craft attitude 

6 hours ago, magnemoe said:

Note that real capsules from Soyuz to Dragon are not on an pure ballistic trajectory like capsules in KSP they have a bit of lift and can adjust trajectory with this. 

This is also useful returning from places like the moon, this is that the Orion capsule used, idea is that you skip along the upper atmosphere shaving off velocity ending up going higher at the end before you then falls down again as you are slower than orbital velocity. You can get this in KSP too even without any lift. 

Thanks for the help, I think I understand reentry a little better now.

[RCgothic]

You can get a "skip" re-entry without aerodynamic lift on a ballistic trajectory. Just put your track high enough that you'll pass periapsis before scrubbing enough velocity and altitude will start to rise again.

When Apollo commentaries talk about missing the entry corridor and skipping off into space, this is what they mean. The capsule will definitely be coming back to Earth in a later pass, just at a time that's potentially beyond the endurance of the capsule and/or at a very non-conducive-to-safe-landing location.

Aerodynamic lift allows better control over the trajectory.

It might be desirable to keep the path higher in the atmosphere than a pure ballistic territory would allow, e.g. if the heat shield can endure moderate heating indefinitely and your craft has features that don't enjoy g-forces and high drag, such as wings or fins, then it makes sense to scrub off as much velocity high up as possible.

But if the heat shield is ablative, a long re-entry can wear it away. So to keep the total heat absorbed low it might be desirable to come in steeper than ballistics would allow. This would have higher peak temperatures and g-forces, but would ablate less material.

Another reason to use aerodynamics during re-entry is if the apoapsis is high or interplanetary, there's a lot of velocity to scrub off. A craft might not be able to endure a low enough periapsis to scrub off enough velocity in one pass ballistically. Using aerodynamics can hold the craft down at a survivable periapsis long enough to scrub velocity instead of skipping off.

 

The heat-shield on the Orion capsule was apparently designed for an initial phase of heating, a skip to a higher altitude for a period of cooling, then a final re-entry. It sounds like that design had some flaws when gasses generated by the first phase of heating built up in the heat shield during the cooling phase, and the build up of excess of pressure of those gas pockets popped large chunks off the shield.

 

[darthgently]

5 hours ago, magnemoe said:

They have, had no idea  

Just attach a mass on ( or clip inside) the capsule off center, iirc, if there isn’t a proper way to do it in the latest version now (I may be thinking of a mod part where one could select off center CoM degree or similar).  But I do know that attitude of the capsule affects lift and so you get a  combo of pitch and yaw via roll with off-center CoM

[AckSed]

To cap it off, here's a relevant video from Scott Manley:

 

[heakhaek]

7 hours ago, AckSed said:

To cap it off, here's a relevant video from Scott Manley:

 

Scott Manley to the rescue education! Cus when in doubt Scott has a video on it.

[heakhaek]

14 hours ago, darthgently said:

Just attach a mass on ( or clip inside) the capsule off center, iirc, if there isn’t a proper way to do it in the latest version now (I may be thinking of a mod part where one could select off center CoM degree or similar).  But I do know that attitude of the capsule affects lift and so you get a  combo of pitch and yaw via roll with off-center CoM

Oh yeah when I offset CoM do I just offset it to give it slight instability? On an unstable plane the CoM is behind the CoL and that causes the aircraft to nose up without FBW.  Like which way should the CoM be have the Capsule generate lift?

Edited December 12 by heakhaek

[darthgently]

2 hours ago, heakhaek said:

Oh yeah when I offset CoM do I just offset it to give it slight instability? On an unstable plane the CoM is behind the CoL and that causes the aircraft to nose up without FBW.  Like which way should the CoM be have the Capsule generate lift?

You want the CoM shifted off the centerline but not fore or aft necessarily.  How much you shift it depends on how it performs.  I think you want the centerline to end up 10 to 30 degrees off from prograde during entry so whatever offset gets you what works for you

[DDE]

On 12/11/2024 at 1:01 PM, RCgothic said:

When Apollo commentaries talk about missing the entry corridor and skipping off into space, this is what they mean. The capsule will definitely be coming back to Earth in a later pass, just at a time that's potentially beyond the endurance of the capsule and/or at a very non-conducive-to-safe-landing location.

To be clear, this can be harnessed instead.