[0.17] Re-entry Heat Module and Mk-1 Pod Heat Shields

[Olsson]

PakledHostage:

What are the physics behind coming in too steep or too shallow? I realize that it's friction with the air particles that generates heat while coming in and that's the reason for temperature, doesn't the temperature also get higher when you come in too steep because the craft will be heavier and therefore the friction is higher = more heat. And if you come in too shallow is it because you don't slow down quickly enough so the heat just builds up and builds up?

Also maybe a drogue chute and some aeroshells for the future, maybe a drogue chute that can withstand openings at 800m/s but not really slow down the craft sufficiently for a landing? Because often while coming in for a duna landing you're coming in so quickly that you can't open your main chute before you're below 400m/s and that might be when you're only a couple 100m off the ground. Also, does anyone know why parachutes tend to get ripped off from time to time even though I'm below 400m/s? Is it better to pop the chute as early as possible when I'm below 400m/s to let the drag slow me down or should I wait until I really need the chute to be opened at around 500m?

[whatisthisidonteven]

PakledHostage:

What are the physics behind coming in too steep or too shallow? I realize that it's friction with the air particles that generates heat while coming in and that's the reason for temperature, doesn't the temperature also get higher when you come in too steep because the craft will be heavier and therefore the friction is higher = more heat. And if you come in too shallow is it because you don't slow down quickly enough so the heat just builds up and builds up?

Also maybe a drogue chute and some aeroshells for the future, maybe a drogue chute that can withstand openings at 800m/s but not really slow down the craft sufficiently for a landing? Because often while coming in for a duna landing you're coming in so quickly that you can't open your main chute before you're below 400m/s and that might be when you're only a couple 100m off the ground. Also, does anyone know why parachutes tend to get ripped off from time to time even though I'm below 400m/s? Is it better to pop the chute as early as possible when I'm below 400m/s to let the drag slow me down or should I wait until I really need the chute to be opened at around 500m?

In Kerbin I open my parachute at 250m/s and I seem to be fine

[PakledHostage]

The physics model that I use in this module estimates the temperature of the atmosphere behind the supersonic shock that the command pod creates as it re-enters. It is a compressibility effect rather than a friction effect (much like how the bottom of your bike pump heats up when you pump up your bike tire).

The model assumes that the supersonic shock is detached from the spacecraft (i.e. a normal or strong shock) rather than a weaker oblique shock. The equations that I use for this are really only valid for air and up to about Mach 5.0, but this is a game not a physics simulation so I'm taking some liberties. The results are in the right ballpark, relative to the expected values. I then calculate the temperature of the heat shield due to convective heating effects, again making some gross simplifications because this is after all just a game. You can read more about the physics model that I use here if you are interested.

As for why coming in more steeply causes less heating than coming in at a shallow angle: You are exposed to the highest temperatures for less time when you re-enter steeply, as compared to when you enter at a shallow angle. Think of it like fried ice cream or waving your hand through a flame. You can expose objects to very high temperatures for short periods without appreciable heating. When you do a very steep re-entry, the spacecraft slows down so fast due to atmospheric drag that it doesn't have time to heat up. In that case, the spacecraft is destroyed by g-forces, rather than heating. I regard this effect as another piece of evidence that the re-entry heating physics model is actually working the way it should.

In the case of the Duna entry, maybe try going for a more shallow entry angle? The limit for the parachute is Mach 2.0, so you can open it at greater than 400 m/s. Maybe I need to expose the parachute Mach limit in the part.cfg file? That way it could be set higher if it needs to be. That being said, I referenced Curiosity's parachute's Mach limit when I chose the Mach 2.0 value.

In real life Mars entry, speeds are considerably lower than for Earth, but the heat shield still needs to be very well designed. According to Table 1 in the JPL paper "2011 Mars Science Laboratory Mission Design Overview", Curiosity reached Mars with an atmosphere relative entry speed of about 5.5 km/s. That is roughly 2 km/s less than the shuttle's re-entry speed. Even so, radiation heat transfer effects are more significant during Mars entry than for Earth entry. My best guess as to why is that Mars' atmosphere (mostly CO₂) is a strong greenhouse gas that reflects heat rather than transmitting it.

Edited September 30, 2012 by PakledHostage

[kerbalboy2000]

Sir we have a presentation in our Science Option and I used this to simulate re-entry, I also mentioned your name, greetings from Canada.

[Subcidal]

I'm sorry if i missed it, But did you do something with all parts? I've got stuff blowing up (Discarded tanks and engines and stuff) on re-entry due to overheating, And it's thrilling.

[whatisthisidonteven]

I'm sorry if i missed it, But did you do something with all parts? I've got stuff blowing up (Discarded tanks and engines and stuff) on re-entry due to overheating, And it's thrilling.

Yeah, I noticed that. I really like how my debris is getting destroyed during reentry, as they are not affected by aerobraking and are stuck forever in a low-high orbit. So whether this was intentional or not, it's great!

[PakledHostage]

So whether this was intentional or not, it's great!

Yup, it was intentional.

[Subcidal]

Yeah you've done amazing things with this. All it really needs are some sound effects to go with it, Something during re-entry, Different explosions sound effects for the parts that makes it sound like they fell apart and didn't just randomly explode.

Other than that this is mod is pure class.

[ddavis425]

Coming in from a Mun return trajectory is pretty scary, it feels like real life where you have a very small window of success. Too steep and you'll burn up, too shallow and you'll skip off the atmosphere. And I agree about the sound effects, that would make this plugin so much better.

[PakledHostage]

And I agree about the sound effects, that would make this plugin so much better.

I was actually thinking of maybe changing the inherent stability function to give the re-entering command pod a slight angle of attack and adding a small lift force component. That way Apollo style re-entry trajectories could be flown:

The Apollo capsules' centre of mass wasn't located on the capsule's axis of symmetry, so they would naturally re-enter at a slight angle of attack. That slight angle gave them a bit of lift. I figure I could simulate this by offsetting the reference vector that I use in the inherent stability routines. The reference vector is fixed to the spacecraft and is always held in alignment with the retrograde vector by the inherent stability functions. The reference vector is currently pointed straight out the top of the capsule. I could rotate it so that it is still coming out of the top of the capsule, but adjusted a few degrees away from the hatch. I could then simulate a lift force, which would be oriented perpendicular to the direction of travel and towards the hatch. You could steer the capsule slightly during re-entry by rotating the crew hatch towards the direction you want to deflect your trajectory.

What do people think? Would that complicate things too much? Would it make re-entry too difficult? You could end up skipping and then burning up on the second re-entry (because you used up some of your shield integrity during the first dip into thicker atmosphere), and you wouldn't be able to "turn off" the lift force if you wanted a straight-in ballistic trajectory.

Having said all of this, part of me wants to leave things the way they are because I want this mod to add something to the Kerbal experience, but I don’t want it to make things so challenging that nobody uses it.

Edited October 3, 2012 by PakledHostage

[Crimson_Pride14]

If you do all that you could give the possibility of having a steering mechanic for re-entry, just like the real thing. Not sure how you would do it, but being able to do pinpoint landings would be cool for sim players.

[togfox]

Undecided. Im thinking that steering during reentry would give us something to do during this phase of the journey but steering would be pointless if we didnt have a visual guide telling us what to aim for. Steering for steerings sake wouldnt interest me. If i had a glide slope or optimal reentry gauge then steering would have a purpose. Without that, id be making random turns without know the effect.

Im not saying these are easy things to do.

The factoring of lift regardless of orientation might be a fun thing to include. We already need to think about heat and g force. Knowing that there will be some level of lift to factor in wouldnt be unreasonable.

[PakledHostage]

Thanks, everyone for your input. Having thought about it a bit more, I think I'm going to add a lift effect. I will make it so that it can be enabled with a "hardcore" flag in the part.cfg file. The default will be off, but people who want it can turn it on.

I haven't implemented it yet so I don't know how it will work, but I expect that the lift force will affect the map view trajectory in real time. That should give an indication of where you'll end up as a result of the steering.

I won't make any promises as to when I'll get it done though. I think I've used up my quota of tolerance for my Kerbal addiction... And it is Canadian thanksgiving this weekend so I won't have a chance to work on it until next week at the earliest.

[Moach]

if a lift-generating attitude is only a matter of pulling the CG from the axis a notch, can't we achieve this by simply adding "ballast" to one side?

no need to force those things, let them play out naturally from the physics, it's a lot more interesting too!

[PakledHostage]

The problem is that the vector sum of the lift and drag components shown in the "Earth Re-entry and Landing" figure above should be collinear with the velocity vector, unless real lift is being generated. I would have to experiment because I don't know how the game's physics engine simulates these forces.

My knowledge of hypersonic aerodynamics is limited but clearly, in real life, the asymmetric supersonic/hypersonic flow and shock pattern around the re-entry vehicle generates a significant lift force on the re-entry vehicle when it is given an angle of attack. I have no idea how this supersonic/hypersonic lift force compares to forces on a subsonic re-entry vehicle flown at the same angle of attack, however. If someone reading this has expertise in this area, I'd be happy to read an explanation!

Below is a plot produced by the Hypersonics CFD Group at the Indian Institute of Technology, Bombay. The plot shows a CFD prediction of Mach number distribution in the pitch plane of an Apollo-shaped capsule at Mach 4.9 and angle of attack of 14 deg.

Edited October 4, 2012 by PakledHostage

[whatisthisidonteven]

This feature certainly should add some more realism to an already great reentry plugin.

I have a question though; Is it possible to add sound effects for the duration of reentry? At the moment all we have is a little boom as it starts and then its completely silent after that. It feels weird screaming through an atmosphere with no sound.

[PakledHostage]

I have a question though; Is it possible to add sound effects for the duration of reentry? At the moment all we have is a little boom as it starts and then its completely silent after that. It feels weird screaming through an atmosphere with no sound.

Sorry, I've been avoiding answering this question... Sound effects can be tricky because you need a believable recording that you can then "play" and distort according to some physics model. I have only done it once in another application that I developed, but it was a royal pain in the rear to do... It isn't my highest priority at the moment, but I would be happy to integrate someone else's sound effects class library if they wanted to collaborate on this project.

[goldenpeach]

If you are afraid of taking this plugin because it could be really hard to land on earth( I was before I took this plugin),don't worry:I did two mission with it,all with succesful landing and no dead!

I'm acutally doing a 75 km orbit just to make an attemp of a FAILED reentry(seriously).

EDIT:I failed the faillure,the flammes started at 49 km and stopped at 21 km,I consumed about a half of the healt shield.nest attemp: a very very very shallow reentry.

EDIT2:succes!!!! I made the craft do a vertical launch(I used infinite fuel option),and made it return on kerbin with a kind of vertical drop!(see screenshots for details)

It's the first time I'm so happy about a faillure,wich was,in fact, a success!

Edited October 5, 2012 by goldenpeach

[whatisthisidonteven]

Sorry, I've been avoiding answering this question... Sound effects can be tricky because you need a believable recording that you can then "play" and distort according to some physics model. I have only done it once in another application that I developed, but it was a royal pain in the rear to do... It isn't my highest priority at the moment, but I would be happy to integrate someone else's sound effects class library if they wanted to collaborate on this project.

Maybe you could use something like the SRB sound until an alternative is found? I think there was another reentry part that did this.

Edited October 5, 2012 by whatisthisidonteven

[togfox]

So I thought I'd try the heat shield on a Duna re-entry just to see what would happen.

I set my AP/PE to 75km/20km (atmo starts at 40km) and activated the heat shield. At about 30km (wasn't really checking) the debris I jettison exploded as it does on Kerbin. Nice. The heat shield did NOT glow and the smoke did NOT appear. I got to about 2000m above ground level and popped the chute. Chutes trailed nicely. As soon as there was enough pressure to 'inflate' the chutes, I got an instant "killed by G forces" message as my craft exploded. I've never had that before - even on Kerbin - so not sure why that happened.

NOTE: I used a NON STANDARD configuration and this might make a world of difference.

I had the following 'stack' :

parachutes

remote tech command module (no crew)

remote tech controller

heat shield

So you see the heat shield wasn't attached to the pod directly. The integrity bar appeared but never decreased.

Anyway - not sure why I typed all that - might help!

[PakledHostage]

Thanks. I'll look into it. What if you try it without jettisoning the heat shield? It could also very well be because you are using an unmanned pod. I do a check in the program for whether or not the heild is connected to a commandable vessel. That may not work properly when used with an unmanned part.

It may also be because Duna's atmosphere is very thin. I have never actually flown there myself so I don't know anything about it. (I set myself the goal of reaching each of the new planets NASA style... That is, the first time I try, no pressing f9, and with the smallest rocket possible. That requires a bit of planning.) If it is because Duna's air is thin, then I will have to tweak the atmosphere specific constants for Duna.

I will also look into how hard it will be to add simple sound effects. And I'll add the effect that I discussed above. It won't be for a week or two though.

Edited October 6, 2012 by PakledHostage

[Baleur]

Your logo needed a better less jaggy font. Enjoy

[togfox]

Well, i wont spoil Duna for you by providing atmo details. Ill wait for you to get there yourself. . I suggest the formulas are NOT tweaked to suit Duna. The physics that apply to Kerbin should also apply to Duna. that is, if the shield does not glow on Duna, then it does not glow on Duna.

I did not jettison the shield, but i didnt attach it to a manned pod either. I was curious to see what would happen. Apart from the lack of glow, which may be physically correct, everything seemed to work fine. That excess g at the end was a surprise but i cant say its wrong. Cheers.

[Baleur]

Maybe there should be a minimum glow anyway, to signify that you entered the atmosphere, at least when it is thick enough to start aerobraking your craft (as duna's certainly is).

Perhaps it's not completely realistic, but it'd give more usability to the hard work put into the mod and plugin..

I mean look at the engine emissive glow itself, one might argue that is unrealistic as well, why would the nuclear engine for example start to glow after a second of 5% thrust?

Not realistic, but by gosh i am glad it isn't restricted by realism, because it's awesome and serves as a guide to see which engines are currently on or out of fuel (when you have several in a packed state).

[PakledHostage]

I suspect that it actually does work (i.e. glow and display a plasma trail) if it is attached to a vessel made of stock parts. For that matter, the bug may even be in the addon part that you had it attached to, Togfox... In my module, I check the game's built-in "vessel.isCommandable" flag to decide if certain things should happen. Maybe that flag isn't being set correctly in the unmanned part mod that you are using? This would explain what you're reporting. Has anyone used the heat shield for a Duna landing using stock parts?

And about setting different heat transfer coefficients for atmospheres other than Kerbin's: This actually does have some basis in reality. For example in the case of Mars atmospheric entry, I've read that radiation heat transfer causes heating of the re-entry vehicle, while it causes cooling of an Earth re-entry vehicle. This would make a significant difference on the amount that the re-entry vehicle heats up.