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Lego_Prodigy

SpacePlane Reentry?

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Posted (edited)

So as some may now know all too well, I love building MK2 stock spaceplanes, but the recurring problem i have is extreme overheating. I have this so often I know what parts heat up first: the weakest fins, then the parachutes, then the engines and random fuel canisters, then Kaboom!. Is it better to dive and go as steep a descent as possible to minimize heating time or a shallow dive to reduce speed and therefore reduce the frictional heat?

 

Edited by Lego_Prodigy

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The choice is a bit more complicated than that.  Steep descents minimise total heat exposure but have a high thermal spike.  They can work if your parts have high skin temperature limits.  Shallow descents have lower instantaneous temperatures, but because you spend much longer in the fall, you generally get more total heat input.  That works best if you have low skin temperature tolerance but high internal temperature tolerance.

In actual use, most parts have the same skin and internal values, but since skin heats up quickly and the internals heat up slowly, shallow is usually the better choice.  If that isn't working for you, then you have at least four other options:

  1. Use different parts, or change the way you use existing parts.  There isn't much to be done for fins--by their nature, they have to be exposed to the air stream--but you can put the parachutes in a service or cargo bay.  If fuel tanks are blowing up, you should consider changing the fuel flow rules so that the tanks most exposed to the heat have fuel in them while you descend.  Fuel can take some--some--heat, as well.  Fixing the balance is something you'll need to figure out.
  2. Leave a lot of fuel in the tanks and try a powered descent.  This is wasteful and requires either forward-facing engines (seriously?) or descending backwards.
  3. Use radiators.  Obviously, they cannot be deployable, but remember that they need to be in the air, not a cargo bay.  This is something that can bite you--radiators, assuming I understand the behaviour, will take on heat as well as dispose of it.
  4. Try tumbling.  This does two things:  it increases drag, which slows you, and it spreads the heat around every part of the plane.  The drawbacks are the complete loss of control and attendant general concern over whether you'll get it back and the need to ensure that every part on the exterior of the plane has high enough heat tolerance.  After all, rapid deceleration due to drag is exactly what makes steep re-entries spike in temperature, so it will be hotter.  But no single part will take the full brunt of it.

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MK2 parts have a lot of drag. This is a hindrance on the ascent, but a big bonus on the way back down again. They also have a very nice heat tolerance, and are good at radiating away excess heat.

So the answer to your question is that you need to give your MK2 parts time to work. You don't really want to dive at all. Come down to a nice 59km Pe, and point your craft Radial Out for maximum drag. Then just let it sit there. After a thousand kilometers of that, it will start falling all by itself, without even the tiniest heating problem.

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Posted (edited)

My usual approach is to set Pe at.... whatever! (seriously, -100km to 30km) but then pitch up while descending to stay above 30km until bleeding off some speed, often a pitch of around 30 degrees is appropriate. It must be said you do yourself a massive favor if you can use a mod like Mechjeb or Atmospheric Autopilot to maintain an appropriate pitch without constant babysitting.

I use rocket parts (rather than aircraft parts) all the time with no problems when using this kind of reentry. It's also possible for a sufficiently light plane to "skip" across the upper atmosphere about halfway around Kerbin, at an altitude of around 40km, then once it is has skipped its way to the KSC continent just pitch down severely and glide to the runway.

Temperature tolerance is only really important for the parts of the plane that takes the brunt of the heat - the nose and leading wing surfaces - everything else is at least partially shielded by the stuff in front of it which makes a huge difference to heat loading. It's a bad idea to have anything with an external or internal temperature tolerance of 2000 or less taking the brunt of the heat at leading surfaces, but behind the leading surfaces you are far more free in your choice of parts. It is wise to never use the pointy cockpits (especially Mk1) because they are extremely prone to internal overheating only use the inline cockpits, and then for the nose make it something with temperature tolerance; shockcone intake or aerodynamic nose cone aren't ideal but do tend to suffice, fairing is ideal, small nose cone is bad it will burn off due to low thermal mass.

Edited by blakemw

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I find that if you do a big braking burn from LKO,  you reenter steeply,  but the parts so not get all that hot- because you've already lowered your orbital speed,  and if you keep your plane pitched up you should not regain much speed. But a shallow descent usually works too.

Where are you reentering from when youget this issue?  If coming from somewhere higher than LKO, you mat want to bring your apoapsis down first,  such as by making a few aerobraking passes.

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Ive created a newer pylon design meant to minimize heating, and my MK2 parts can't wick away the heat of OTHER parts that are vital, such as tail fins, engines, and fuel tanks.

it uses structural fuselages with RTG powered small TCS panels inside, giving it very good heat control for a fiery descent, and I've already tested the pylon with a reentry.

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Well, I still say: maximize your drag on reentry. Spaceplanes have wings and therefore can have immense amounts of drag if you just let them use it. Above a 30 degree AoA you get nearly full drag but you get some lift, too. Keep your nose between 30 and 90 degrees and give your plane time to slow down. The closer you go to prograde, the less likely that you will have enough drag to survive the heating.

 

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https://imgur.com/a/1PzRZ2e

I put a single pylon like that up into orbit with the debug menu, then decelerated it with detachable thrusters and added Swept Wing Bs for stability. It preformed very nicely.

It only broke off when I reopened them, without thinking about it, and ripped off all but two, after reentry.

Now testing a craft fitted with two of them, and the front of the pylons is a 2.5 heatshield.

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I'm with @bewing on this. Take a bone-stock Aeris 4 spaceplane. From a 70 - 100 km orbit, set your de-orbit burn for a Pe of about 35 km. Point radial out and stay that way, full 90 degrees pitch up, as long as you can hold it. Once drag makes that impossible, keep it pulled back as high as you can manage. Assuming you had an efficient ascent and aren't completely dry, you can shift fuel around the tanks to push your Cg aft a bit but it's not really necessary.

In any case, just stay as nose-up as possible, which will be about 45 degrees or more at peak drag, or at least in that range. You can get fancy and add alternating left- and right-banks to create what were known in shuttle days as "S-rolls". This will basically trade some lift for your kinetic energy and help get you pointed to your landing point, as well has help decelerate a bit faster. Anyway, f you have temp gauges enabled, you'll see some parts climb up into the red but nothing should explode. By the time you've dropped to about 30km, temps will begin dropping. If you've saved fuel, you'll be able re-start the turbojets if necessary to stretch your glide to landing.

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I'd be interested to see what your design looks like @Lego_Prodigy   

I never use the pointy cockpits (like the one in your avatar pic) because the part at the front gets most of the heat and the cockpit is the most sensitive to heating.

In 1.3 heating effects made pointy cockpits more or less useless for Spaceplanes,  in 1.4 heating levels have been reduced but still the first thing i'd try is to swap to the mk2 inline cockpit with a mk2 to mk1 adapter in front then a nose cone or intake on the front of that.     In fact you could even put a mk2 clamp o tron ahead of the cockpit.

For re-entry profile, I usually come in pitched up moderately with the nose 10 degrees above prograde.   That way you're not completely stalled and are still making lift, which keeps you in the thin air longer.   You also have more control that way and can fine tune your arrival point by pitching up for more drag (if overshooting) or lowering pitch to 5 degrees for best glide ratio (if coming up short).   This all works better when  you have a lot of wing.

I normally retro burn until my PE goes to ground level somewhere in the desert continent west of KSP,  my planes have good / lift drag ratios and when they encounter significant atmosphere the impact point inevitably starts moving downrange.

BTW, are you dumping your payload after launch and immediately re-entering,   or are you orbiting  in orbit a little while first?   It is harder to re-enter if the cockpit is still heat soaked from the launch process.

This video demonstrates the landing point aim process mostly - but you can see how even a mk1 design has no problems with heat when using inline cockpit.   Make sure you have subtitles / captions enabled for the text (think that only works on a pc tho)

 

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Cooling systems are a joke during re-entry.  Where exactly are they going to dump the heat?  Nowhere, they can't!

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Posted (edited)
1 hour ago, Corona688 said:

Cooling systems are a joke during re-entry.  Where exactly are they going to dump the heat?  Nowhere, they can't!

Passive radiation still works during reentry due to some quirks of the heating model (we're going to go down the rabbit hole here). Basically whatever isn't directly exposed to the oncoming air (is in a heat shadow of another part) is icy cold so parts (or the part of a part) which aren't exposed are still radiating away heat just fine. Now that would make radiators really good during reentry, and that would make it obvious that the heating model is borked. So there's a hack: active cooling for radiators/thermal control systems is basically just switched off during re-entry, it's not that the environment is too hot to radiate heat into, it just arbitrarily stops working (well not exactly arbitrarily, but the point is it's not the laws of thermodynamics that stops it working, it's a hack which goes "looks like re-entry! better switch off the active cooling of radiators!). Nevertheless, radiators thanks to their large surface area can still passively radiate away heat into the icy cold of the heat shadow, they just aren't actively sucking heat out of other parts, being limited to normal part-to-part conduction which is not as effective. Though more likely a radiator will actually have a greater effect from the extra drag it causes than passive heat radiation, they have a pretty extreme "drag to mass" ratio and if you want your plane to slow down better than it flies, add radiators (or for the best of both worlds, put them in a cargo bay).

Note that in KSP the plasma wake does not generate heat, IRL it bathes the vessel in infrared radiation but in KSP it's a purely visual effect.

Edited by blakemw

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This is the airplane 

nVUk1G5.jpg

and here is an exploded view of the mk1 fuselages attached either side of the main body

hBB00UN.jpg

On stock settings, unless you're re-entering at 5000 m/s from a Jool trajectory, only the Kerballed bits are at risk and the cockpit in particular.     Clipping stuff inside the structural fuselages does not hide them from drag or heat, the game still sees them as radially attached stuff on the outside of the ship, regardless of how you offset.     And that stuff will make huge drag btw.    

Just swap to an inline cockpit and if that's not enough,  increase wing area so you can make more lift and drag on re-entry.    Lift keeps you out of the thickest atmo till you've slowed up a good amount, and drag obviously reduces your speed and therefore heat too.

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K I swaped the pointed cockpit for the inline cockpit. I also took the fairing method, and I got it to look nice and streamlined :D I also hammered out some other minor details unrelated to reentry. 

https://imgur.com/a/SZBQJZn 

Preformed gracefully in normal atmospheric flight, but I didnt have enough time this morning to test it during reentry.

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