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DunaManiac

Spaceplane reentry?

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Posted (edited)

Help! I can't seem to do reentry without destroying the cockpit. I've tried to look at all the tutorials, but it doesn't seem to work. (I wish that the cockpits were more heat shielded).

Edited by DunaManiac

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Pics are helpful to help with this kind of issue.

General pieces of advice are using a cockpit with a higher heat tolerance (avoid Mk1 in favour of Mk2/3), reentering with a high angle of attack (>30° at least) to spread the heat load to the entire plane, and setting a high periapsis to reduce the peak heating on your craft.

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2 minutes ago, Gaarst said:

Pics are helpful to help with this kind of issue.

 

I have no screenshots, but I will try your advice. 

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If at all possible, reenter with a 90 degree AoA, and maintain that as long as you can -- make your wings generate as much drag as you possibly can. Try a very wide range of Pe values for your reentry -- make sure you try setting your Pe at 60 to 63km and see if you can make that work.

 

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The Mk1 nose cockpit is pretty fragile, but it isn't impossible to re-enter if you're very careful. I managed to return a stock Aeris 4a from Minmus orbit and managed to fly one of Thrimm's low-tech space planes to orbit and back, even with Ferram Aerospace Research loaded.

For the stock craft and that Minmus challenge, I used a Mun assist to lower my Kerbin Apoapsis from 44 000 km to 11 000 km, then chose a Periapsis of just over 49 km. Any lower and I'd have cooked Criskie Kerman (Crispy Kerman?) for sure. For the LTS Dove, I added an antenna to the nose so it would bear the brunt of the heating, but the cockpit was still a little toasty.

I seem to remember discussions about the re-entry shockwave being the major source of heat, and that any parts in contact with the shockwave would heat up. Flatter surfaces would bear that heat better because the shockwave would not be in direct contact with the surface, which is why the command pods re-enter retrograde without overheating too much. Sharper noses like that Mk1 cockpit's nose would easily contact the shockwave since they're nose-first and sharp.

Putting an antenna or another object ahead of the Mk1 cockpit would separate it from the re-entry shockwave a little bit. Take a look at the LTS Dove-F for an example.

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point the ship upwards towards the 45 degree angle so the bottom of the ship is dragging more than the cockpit and the result should be a lot better

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As you may notice there is a lot of opnions about how high/low the periapsis should be and what AoA to keep. But the idea is the same: expose a lot of area to avoid heat to build up in a single point and to slowdown quickly. Figuring out what works better for you may take some time .

Notice that some crafts can avoid overheating rushing to the lower altitude where you slow quickly toba safe velocity.

 

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I built a quite workable Mk2 space-plane built vertically, so it looked like the English Electric Lightening (pictured somewhere in the "what did you do today?" thread for anyone who really feels the vigor of youth to dig through and find it).  I flew it unmanned several times, then used it manned several times.  Then one re-entry the angle was just a little bit too wrong and-  poof.  Personally, I think the transition parts for the Mk2 body do *not* actually live up to the heat tolerance that's on the tin.

I now bedazzle my non-capsule re-entry vehicles with radiators.  Maybe it helps?  Not sure, but no-one has died yet...

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On 8/9/2017 at 8:02 PM, Gordon Fecyk said:

The Mk1 nose cockpit is pretty fragile, but it isn't impossible to re-enter if you're very careful.

Its not impossible, but IMO, its not worth the effort. I don't consider the mk1 cockpit to be rated for orbital operations around Kerbin in my KSP games.

Its fine as a suborbital spaceplane. Its fine for use on a Laythe or Duna spaceplane (that stays at Laythe/Duna) - but if I send them to orbit of Kerbin, I don't plan on bringing them back down, generally speaking.

There are also ways to design a craft to have other parts take the brunt of the heating, but again IMO its not worth the effort.

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On 8/9/2017 at 5:42 PM, Spricigo said:

As you may notice there is a lot of opnions about how high/low the periapsis should be and what AoA to keep. But the idea is the same: expose a lot of area to avoid heat to build up in a single point and to slowdown quickly. Figuring out what works better for you may take some time .

Notice that some crafts can avoid overheating rushing to the lower altitude where you slow quickly toba safe velocity.

 

This is my favorite method.  I burn retro until my trajectory intersects the ground at about the island runway,  then use a cobra descent to control speed.  Almost guarantees a runway landing for me.

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I seem to do OK with heat -- it's stability I have a problem with. My spaceplanes always go into an uncontrolled spin between 30,000 and 35,000 m, and I can never gain control of them again, because if they stabilize at all, they do so tail-first rather than cockpit-first owing to the mass of the engines.

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1 hour ago, Catbus said:

I seem to do OK with heat -- it's stability I have a problem with. My spaceplanes always go into an uncontrolled spin between 30,000 and 35,000 m, and I can never gain control of them again, because if they stabilize at all, they do so tail-first rather than cockpit-first owing to the mass of the engines.

Two most common causes:

-shifting CoM fall behind CoD as fuel is empty

-CoD in front of CoL because of unaccounted drag (either drag unrelated with lift or differences due AoA)

As always a picture of your craft (in SPH with CoM and CoL enabled) and a brief description of its purprose is the ideal to craft specific advice. Also point if there is some restriction (tech, mass, cost...) or particular requirement (%recovery, deltaV in orbit, RCS ....)

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I usually do 60-65k peri and hold the nose about 45 degrees up.

Works great even with the Mk1 cockpit barring any stability issues.

As @Spricigo said, you'll prolly be close to empty on your way down so be sure that your COL/COM is balanced with the fuel tanks empty. (Drain all the tanks in the SPH and watch the COL/COM indicators.)

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On 8/9/2017 at 6:33 PM, GarrisonChisholm said:

I now bedazzle my non-capsule re-entry vehicles with radiators.  Maybe it helps?  Not sure, but no-one has died yet...

I've tried this too, but it looks like the small static panels only operate at around 20-30% during the worst bit of reentry. Only when my speed drops to ~1km/s do they suddenly suck the heat up and then usually sit around 70-80% until touchdown or the heat dissipates.  Intuitively this doesn't seem right, but I'm not nearly versed in thermo enough to repudiate it. Something to do with the plasma sheath perhaps?

The panels are also really draggy so I try to avoid using them whenever possible.

As for stability, pairs of airbrakes at the rear of the vehicle can dramatically help during reentry.  If you attach them top and bottom but make the top ones stick further out, the resulting drag will tend to keep your nose up a few degrees.  They're also super useful if you end up in a flat spin - I've recovered several times from certain death by deploying them all (beware of over-gee!).  Their thermal tolerance is pretty low however, make sure to watch the gauges and retract them before they fry. 

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Posted (edited)

No one has mentioned this yet, the most important factor is how close the cockpit is to the front of the ship.    The mk1 cockpit may have a low heat tolerance, but this one often shows no heat bar at all on re-entry -

https://kerbalx.com/AeroGav/Stretch-Ray

KMDm74u.jpg

The mk3 actually gets quite close to exploding, because you've got no choice but to put it right at the front.

 

Another important factor is wing area relative to weight.   Glider-like designs like the above ship generate a lot of lift, and tend not to fall out of the upper atmosphere till you're below mach 4.   This reduces the heating rate.     If you re-enter this plane with a low AoA it will glide all the way around Kerbin, and willl briefly show a heat bar as it passes through 1200-1100 m/s due to the prolonged heat soak, but that's about it.

I also did a video explaining how to target KSC (need to have annotations turned on, there's no talking)

 

Edit - there is another advantage of pushing the cockpit back.     Sometimes,  we make only "good" landings instead of excellent ones.       To wit -

 

KSC landings are generally easier due to the thicker atmo (lower speeds) but if you're not perfectly lined up with the runway, an inline cockpit can lead to a better outcome -

 

Edited by AeroGav
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5 hours ago, AeroGav said:

No one has mentioned this yet, the most important factor is how close the cockpit is to the front of the ship.  

I didn't because in my experience that's not much relevant.

But my experience is limited mostly to fairly light, probe controlled crafts, coming from 80km orbits, with over 60degree pitch up.

Btw, welcome back. 

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