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[KSP 1.1.2] How do I re-enter a spaceplane without it burning up?


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Hi, fellow Kerbonauts!

I was developing my Voyager Shuttle: a reusable launch system capable of sending small payloads into orbit (there is a subasembly of a second stage for delivering payloads into GO, but that's irrelevant). I did quite a lot of tests and couldn't seem to re-enter without burning up. The only way I can survive re-entry is if I do a super-super-super ballistic re-entry, having the nose pointing down around 45 degrees.

I tried testing some systems (only one, actually). I had a large 2.5m nosecone (I think it's called the MK-76 nosecone) attached to a decoupler with some sepatrons, so it is detachable. Behind that I have a 2.5m heat shield to try to protect it from re-entry.

That did not work. I kept burning up, even with the heat shield. Not that it ran out of ablator, but it overheated. I added some A.I.R.B.R.A.K.E's, but no luck either. I need help. One thing that might be contributing to my failed re-entries is that the shuttle has over 1,000 units of fuel left over when the mission is complete. Not sure if that's relevant, but just something you might need to know.

Also, I can't upload my craft file because it says "The page you are trying to access is not available for your account" every time I click insert other media and insert existing attachment.

If anyone knows any techniques I could use, please let me know. Is it my craft that's wrong, or is it my technique that's wrong? If it's my technique (and you can re-enter fine) please let me know how to re-enter a spaceplane without it burning up. I'll try to upload my craft file as soon as I can.

Edit: Here's the craft file - https://drive.google.com/file/d/0ByFj3hLfFCqbdmJ4NWU1ZjhDNGc/view?usp=sharing

Thanks!

Edited by XpertKerbalKSP
added craft file
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Last question first.  To upload a craft file use a file sharing service such as dropbox.  Upload your file there and provide the sharing link here.  This is similar to sharing photos where you need to upload your pics to an image-sharing site such as imgur and then paste the link into your post.

As for reentry.  Try pitching your nose all the way up to 90 degrees.  Yes, come in with your belly directly facing the air flow.  If you have enough this can work.  If not, you could try alternating between pitching up and pitching down.  Heat shields and space planes usually do not go together in my experience.  Wings which are perpendicular to the air flow slow you down very fast and very effectively.

Happy landings!

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yea, you have to use a 3rd party site to host your craft file and please... pictures.

Btw, what is GO? are you talking about the kerbal equivalent of GSO? ie KeoStationary Orbit?

Are you running some mod like RSS? A heat shield overheating just from coming down from low kerbin orbit should not happen

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If you've got 1000 fuel left, it should be easy no matter what mods you've got. Come in with a high Pe (60km on Kerbin), with your nose at 90 degrees to scrub off as many m/s as you can. When you get down to an altitude where you can still just barely steer (55km on Kebin) turn around retrograde and burn every last drop of fuel. Then turn around prograde with your nose at 30 degrees and the rest should be cake. (Unless 1000 fuel is way too much to burn -- then stop burning when your surface speed gets down to something so safe that your granny could reenter at that speed.)

Edited by bewing
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There are two factors you need to consider when it comes to heat tolerance during reentry.

For starters, a part with be destroyed when either of its two temperature values (skin or internal) exceeds its maximum temperature limit. Even if it touches that limit for a mere microsecond - boom, the part is gone. This is your limit for instantaneous heat load, AKA how high shock heating is allowed to spike in an instant. It also governs how aggressive you can be during reentry.

The second factor is thermal mass. That's the value governing how much heat flux is required to raise a part's temperature by a given amount. Skin has very little thermal mass; only very little heat is required to raise its temperature a whole lot. However, skin is what is actually exposed to the shock heating, so how does it stay alive? Because a hot skin radiates and conducts heat away. Radiated heat is happily gone, but conducted heat goes into the part's internals. Those have a high thermal mass, and take a lot of heat to warm up. Therefore, your craft can "soak" heat for a while. But the speed of heat conduction is dependant on the temperature difference between the skin and the internals. As your internals heat up, the rate of heat conducted away from the skin decreases, and as a result the skin grows hotter. This continues until one of two cases happen: either the internals will reach their temperature limit (which is usually lower than that of the skin) and cause the part to explode, or the lack of heat conduction causes the skin to reach its temperature limit and cause the part to explode. In other words, thermal mass is your limit for sustained heat load. It also governs how careful you can be during reentry.

 

To explain the "careful" vs. "aggressive" part:

As atmospheric pressure increases, heating starts off at a fairly high value but scales up only slowly; meanwhile, drag starts really low, but scales up very quickly. Reading this should make it obvious to anyone: the deeper you can go into the atmosphere, the better, because you get lots of extra drag for only a little extra heat.

However, keep in mind the limits for instantaneous heat load. Going deeper may only increase absolute heat at a sedate rate, but if you exceed the maximum you're allowed, it doesn't matter how sedately. Too much is too much. For any given speed, you can only go so deep without simply disintegrating in an instant. That is why having a higher instantaneous heat load tolerance lets you be more aggressive - it lets you go deeper right from the start, where you slow down faster. If you don't have the tolerance, you need to stay further up, where you don't slow down as quickly.

This unfortunately presents another problem: because you don't slow down quickly, you spend a lot of time high up in the atmosphere. And remember, even in the faintest of atmospheres you're going to get a noticable heat load. Not as high as further down, but not much less, either. And when you keep hanging out there, your craft starts soaking heat. Minute after minute, internal temperature climbs higher, making the skin hotter, further impeding your ability to safely lose altitude, and gradually approaching the moment where your parts deplete their endurance. You may not disintegrate instantly, but you're slowly melting yourself to slag. That's why having a higher sustained heat load tolerance lets you be more careful - you don't have as much of a time limit pressuring you to slow down quickly or die, so you can stay further up for longer and take the scenic route.

 

Taking these trains of thought to their logical conclusion, you invariably arrive at the question: what if I don't have the instantaneous tolerance to go deep enough to avoid running out of sustained tolerance? And the answer is: you are dead. Or rather, you have just discovered a reentry scenario that your craft cannot surive, no matter what. With capsules, you encounter these only very rarely, because they are generally fitted with ablative heatshields. Not only do those have extremely high temperature limits, but the ablation process itself consumes a lot of heat - and the hotter the heatshield is, the faster it ablates, and the more of the incoming reentry heating is simply removed from the equation. In the early days of 1.0, Jool was known as a spaceship killer because its atmosphere was misconfigured, causing heatshield skin temperatures to spike so fast that the shields would explode before they could even really start ablating... but nowadays this is fixed, and it's hard to kill a heatshield before it runs out of ablator. You usually need to be intentionally trying to die to pull it off.

But spaceplanes don't have the luxury of ablative shields that conveniently handwave away a large portion of the incoming shock heating and are blessed with incredible instantaneous tolerances. As such, with a spaceplane, it's actually fairly easy to find yourself in a situation that is simply impossible to survive. And even in those cases where you can survive, the corridor between "too careful" and "too agressive" is a lot narrower than for shielded capsules.

When you determine - through trying various different approaches - that you have such a situation at your hands, the only choice you really have is to avoid that situation. For example, instead of coming in directly from your geostationary satellite release, first lower your orbit so that your speed at periapsis will be lower, thus allowing you to drop your periapsis deeper, where you slow down faster.

 

Edited by Streetwind
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Nice detailed reply Streetwind.   It seems to have changed very recently - since 1.1  or even since 1.1.2.

Previously I'd just pitch up to a high lift position - 15 or 20 degrees - and stay high as i can until the speed has bled off.   Heat soak , or some other factor has possibly been introduced.   I've found that minor differences in construction are making big differences in survivability, angle relative to airflow causing instantaneous heating.

Re-entry in 1.05 was much harder than in 1.04, everyone complained, but i found it easy due to my craft having high lift.

https://www.youtube.com/watch?v=TJGF-RF6eIc&index=4&list=PLF64Jfn9iJDEJVIsu50IRcz6g1JAiTgfV

Now it's an uncomfortable situation, though as a mark 1 craft with airliner wings, it is asking for trouble somewhat

https://www.youtube.com/watch?v=a8THkl98C-4&list=PLF64Jfn9iJDGPpBfLk01SZDmFCeaA0DB_&index=7

(embed link not working right now, shows 1st video in series for some reason but when opened in new window goes to re-entry one)

 

Edit - What would really help if i configured kerbal engineer to show more info. At the moment it just has Critical Thermal %, is not showing core vs skin or which part (though that's easy to guess, from the temp bar and red glow).  I need to do more testing, but what i've found with that Astrojet Citation design, is that there's a critical point on re-entry where temperature gets very close to the boom point , creeps up very slowly but hasn't yet exploded, gets within a few percent of doing so every time and with different re-entry techniques,

I have tried pitching up at 25 degrees AoA from 70km down to the ground, fell like a brick and ended up nowhere near ksc.   Then I tried pitching for best lift:drag ratio , this ended up arresting my descent a good 10km higher than what my periapse was on the re-entry burn, however when  the plane finally resumed its plunge temperatures reached similar values.  Instead of 35km periapse i burned for 50km.  This made me skip off the amtmosphere and do another couple orbits before going down for good. But temperatures on the final plunge were identical to the first.    So I hit my quicksave and tried treating it like a landing , and varied the point where i'd "flare" for max lift after an approach on L/D max.   I tried flaring at the point where lift stopped my descent,  increasing pitch angle every time my plane threatened to head down again, until i reached stall angle.     I tried flaring higher up.  I tried flaring deeper, allowing the plane to start to sink again, but increasing pitch to keep rate of descent reasonable as long as i could.  TRied flaring at whatever height my plane encountered max heat last time. No difference.

BTW the heat values i was getting one return from Minmus were very similar to what i was seeing when returning from a 75km orbit,  i just skipped off the atmosphere a lot more times coming back from Minmus.  But the final descent always began at a similar angle and velocity.

What I have noticed, the problem part changes with AoA.    Coming in at low AoA, the subsonic airliner wings overheat.  If i pitch up, the wings cool dramatically, but the inline cockpit and mk1 crew cabin become critical.

 

Edited by AeroGav
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My question, and I think this will help greatly with your problem.  Why are you bringing so much fuel back?  You've wasted mass hauling all that fuel around the whole trip just to bring it back.  That amount of fuel contributes a lot of mass, and that reduces your ships ability to slow due to drag.  Either redesign or empty some fuel.  Your efficiency will sky rocket.

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On 5/23/2016 at 6:29 PM, KerikBalm said:

yea, you have to use a 3rd party site to host your craft file and please... pictures.

Btw, what is GO? are you talking about the kerbal equivalent of GSO? ie KeoStationary Orbit?

Are you running some mod like RSS? A heat shield overheating just from coming down from low kerbin orbit should not happen

GO means geostationary orbit. Well, that's how SpaceX says it anyway. Sorry for the confusion :P. I'll try using less acronyms in the future.

On 5/23/2016 at 1:15 AM, ForScience6686 said:

My question, and I think this will help greatly with your problem.  Why are you bringing so much fuel back?  You've wasted mass hauling all that fuel around the whole trip just to bring it back.  That amount of fuel contributes a lot of mass, and that reduces your ships ability to slow due to drag.  Either redesign or empty some fuel.  Your efficiency will sky rocket.

Honestly, I don't know the answer to that. With all of my fuel tanks full, I can be reassured that my center of mass will always be in the right position compared to the center of mass. I don't know much about keeping your COM in control, so....

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On 5/23/2016 at 7:38 PM, Streetwind said:

For example, instead of coming in directly from your geostationary satellite release, first lower your orbit so that your speed at periapsis will be lower, thus allowing you to drop your periapsis deeper, where you slow down faster.

I actually don't deliver my payloads at geostationary with the shuttle itself. I use an upper stage (invented by me) to get me into GTO and GO. But still. Very detailed explanation. Although I get the first paragraph, I have a bit of trouble understanding the rest. Sorry that my English isn't very advanced. Is it okay if you put it a bit more briefly?

Thx anyway!

On 5/23/2016 at 4:27 PM, Starhawk said:

Last question first.  To upload a craft file use a file sharing service such as dropbox.  Upload your file there and provide the sharing link here.  This is similar to sharing photos where you need to upload your pics to an image-sharing site such as imgur and then paste the link into your post.

As for reentry.  Try pitching your nose all the way up to 90 degrees.  Yes, come in with your belly directly facing the air flow.  If you have enough this can work.  If not, you could try alternating between pitching up and pitching down.  Heat shields and space planes usually do not go together in my experience.  Wings which are perpendicular to the air flow slow you down very fast and very effectively.

Happy landings!

Well then, Here's the craft file: https://drive.google.com/file/d/0ByFj3hLfFCqbbm94VTBiOUt6Qm8/view?usp=sharing

I'll update the post!

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58 minutes ago, XpertKerbalKSP said:

I actually don't deliver my payloads at geostationary with the shuttle itself. I use an upper stage (invented by me) to get me into GTO and GO. But still. Very detailed explanation. Although I get the first paragraph, I have a bit of trouble understanding the rest. Sorry that my English isn't very advanced. Is it okay if you put it a bit more briefly?

The TL;DR is:

Because reasons, spaceplanes in particular often only have a narrow corridor where reentry works. Coming in too carefully will kill you just as surely as coming in too aggressively, because your plane soaks heat like a sponge and can only do this for a few minutes before melting from the inside out. In some cases, when you are going fairly fast, it's possible to encounter a scenario that is not survivable no matter what. It's recommended that spaceplanes reenter only from a fairly low orbit. Not too low, because as mentioned, being too careful can kill you, so don't try something silly like AP 70, PE 60. But definitely not from an apoapsis outside the "low in space" situation (>250 km on Kerbin). Try AP 90, and ask some spaceplane experts for good periapsis altitudes.

Also, you should employ any and all strategies you can find to maximize your drag, such as going in belly forward, fully perpendicular to the direction of travel ("pancaking into the atmosphere") and doing consecutive S-turns deeper down. In a capsule, reentry is a 'hands off and lean back' affair. But your spaceplane is extremely aerodynamic by design, and you have to actively fight it to make it lose speed. So, fight! Keep that nose up! Do not be afraid of the temperature that your parts accrue either. If the overheat bar is stable at 95%, that's the best place you can be to slow down. Just be sure to not forget the "stable" part... :wink:

Edited by Streetwind
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i'm a bit confused about the orbit from which you reenter. is the shuttle in a high orbit or in a low orbit?

shouldn't matter too much either way i guess. try different techniques. Streetwind explained it much better than i could, but in my experience, it's often a good idea to come down relatively steep. it's dangerous to put your periaps at 20 km when you come in from a trip to mun or minmus or an interplanetary transfer, but the heat resistance of spaceplane parts is actually more than enough to survive a failry aggressive reentry from a stable kerbin orbit.

assuming your shuttle is somewhere in a relatively low orbit (something like 70 to 150 km), it's probably safe to do a retro burn that puts your periaps below ground level (but not too far below - you don't want to drop down vertically like a stone). this way, you don't spend much time in the upper atmosphere where you just soak up lots of heat for very little drag. while you're still in the upper atmosphere (40km or higher), you can probably keep a radial attitude without too much work. the critiical part of the spaceplane is the cockpit - and by "pancaking" you make sure that the cockpit doesn't take all the heat. also, this means that you use the broad side of the wings as huge airbrakes. you still won't lose much speed so high up, but you won't heat up the cockpit and you will lose much more velocity than by going in nose first.

once you get lower, you probably wont be able to keep the nose pointed in radial attitude - aero forces will more or less try to force it into a "nose first" dive. you want to avoid that at all cost - in a nose dive, the cockpit takes all the heat and unfortunately, the cockpit has the worst heat resistance of all the parts. if you can't hold a radial attitude, start pitching the nose up and down. this way, the wings will still act as airbrakes at least for the few seconds before aerodynamics force you down/up again, and the cockpit will be out of the airstream for a bit and has a few seconds to transfer heat to adjacent parts.

 

to be honest, the easiest solution is probably to turn the spaceplane into a drone plane and get rid of the cockpit altogether. use a combination of adapter tanks (mk3 to 2.5 and then a 2.5 to 1.25 meters) and put a service bay and a shielded docking port in front (as the nose). put a probe core with some batteries in the service bay to control the plane. the shielded docking port is very heat resistant and if you somehow blow it up, the service bay below it is even more heat resistant.

 

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Plenty of good answers above me, thought I'd add my 0.02.

As said before, spaceplanes have a rather shallow reentry corridor because it's hard to fit aeroshields on them.

To get my spaceplanes back to KSC :

  1. I try to use spaceplane parts only or parts that have a good max heat (2000k is NOT enough)
  2. Before re-entry, I set myself on the lowest orbit possible (70x70km) either through orbital maneuvering or prior aerobraking maneuvers
  3. Rule of thumb : anything with a PE < 30km gets back to Kerbin. My orbital parameters just after the deorbit burn are usually around 70kmx30km. Note that there probably are better re-entry profiles, but this one usually works fine for me.
  4. In high atmo : the most painful part of the trip because you keep heating and don't brake that much. Plus your orbital speed is still increasing.
    What I do is keeping a very high AoA, sometimes as high as 90° (ie nose up) to generate some drag. I also activate everything on the plane that generates drag : opened cargo bay, airbrakes, etc. Note that the airbrakes will overheat, but I've re-entered so many planes with opened cargo bays...
    Note that the belly of the plane is exposed to the airflow and this is why I use heat resistant parts (mk2).
  5. When the air gets denser (50km, 45 km when I feel lucky), I get back to a saner 45° AoA.
    The goal is to keep only the belly of the plane exposed to the airflow, while generating drag (lower your speed) and a bit of lift (reducing vertical speed and thus avoiding hitting the lower atmo too soon and too fast)
  6. Critical altitude is 30 km : atmo density is going to climb fast under 30 km. This is bad if you are coming too hot (I'm usually around 2000 m/s IIRC) because you'll get fried. But if you slowed down enough in high atmo, you'll be slowed quickly through high drag. Heat will go up as well, but since the atmosphere is getting thicker, convective cooling will start working again. It's a nervewracking moment... :D
  7. Once you're confident enough with your plane speed and handling (I usually wait until I'm under Mach 2), you can start all the "plane" stuff : leveling your wings, relighting your engines, lining up with the runway, etc.

A cheesy trick I used for quite a long time was putting a radiator on the back of the plane. It's bad during ascent because of the drag, but during re-entry, it is protected from the airflow because of the 45° AoA attitude and can be deployed/activated to suck the heat from critical parts (usually the nose cone) and will dissipate it in the atmosphere. Not very realistic, I know. I guess the next step would be putting the radiator inside a cargo bay : shielded during ascent, exposed by the open cargo bay during re-entry (especially now that the flush radiators panels can turned on and off).

Hope this helped!

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I usually pitch my plane up to roughly 70-90 degrees AoA in the upper atmosphere (until I'm down to below 45km) and then vary my pitch depending on heating. If that doesn't work I just add radiators on the bottom of the craft to act as a heat shield of sorts. It actually works surprisingly well, and doesn't look bad if you place them right. :)  

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My approach is to maintain a high angle of attack, something like 30-50 degrees, and control my vertical speed. Initially I'll be flying "straight" but once my vertical speed gets to what I want I start making S-turns, banking hard while maintaining the AoA. On my current design a 50 m/s descent seems about right and it's handled some pretty aggressive re-entries like 600x30 (albeit only just). Other designs may differ.

EDIT: To clarify, I use FAR. I didn't realise things would be significantly different in stock.

Edited by cantab
FAR
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If you're playing around a lot with planes, I'd highly recommend FAR. I find it to be actually easier to fly planes there.

In stock, pitching up high in the atmosphere creates drag, but almost no lift. This causes you to rapidly lose altitude with no way to arrest your descent. In FAR, you have much more control over your lift, so you can either slow your descent (by pitching up), or create lots of drag and increase your descent (by doing S-turns).

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19 hours ago, mk1980 said:

 

i'm a bit confused about the orbit from which you reenter. is the shuttle in a high orbit or in a low orbit?

 

I'm coming in from a 100km orbit, not that low Also, This is an unmanned shuttle and I am using the 2.5m white nosecone to shield the front. (I think it's called the MK70 nosecone or something). Check out the craft file, it should help. I updated the post.

Edited by XpertKerbalKSP
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Here is my technique on 1.0.5: First start from a low orbit <100kms

The Cobra re-entry explained in a short vid. it should work on 1.1.2 but I did not try because all my aircraft are using Adjustable landing gear mod that is not updated....

 

Edited by gilflo
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actually i only wanted to test your craft, but then i couldn't help but redesign the whole thing (except for the orbiter/shuttle, that is mostly "intact" still.

i realize that this ir probably not what you were looking for, but i made a few images anyway. maybe they can serve as inspiration :)

 

 

 

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If you dont need a "single-turn" deorbiting, try to make an ecliptic orbit with is periapsis  <70km high....

 

this will decrease your apoapsis and your energy turn by turn....
(and your vessel will cool down during the space flight...)
 

Edited by Sereneti
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