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How do I make a spaceplane that doesn't melt on reentry?


MAFman
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Using parts which can withstand high temperatures is critical.  Many of the aero parts are not suitable for spaceplanes, because their heat tolerance is not sufficient.

Another thing which can make a huge impact on survivability is your entry profile.

One common misconception is that a shallow entry into the atmosphere will be gentler on your space plane.  This is not correct.  A steeper entry often works better at managing heat.

With a shallow entry you end up spending longer at high speed, but producing little drag.   Even at high altitudes the high speed generates a lot of heat.  Eventually your ship generates more heat than it can safely handle.

A steeper entry, which gets you below ~30 km quickly, often works better.  Below ~30 km the atmosphere is thick enough to generate drag, slowing you down.   
So my suggestions are:

  • Make sure you are using parts with high heat tolerance, and
  • Perhaps try using a steeper entry profile.

As an example of an entry profile that normally works well for me, starting from an orbit of 80-120 km-  I burn retro-grade until my projected path intercepts the surface about 1/4 to 1/3 of the way around Kerbin.  I do not try to add drag by pitching up until I’m below ~45 km.  I’m trying to get to 30 km as quickly as I can.  Once I’m below 45 km I start pitching up to add drag.  But if it looks like I’ll level off above 30 km I reduce the pitch to continue the descent to 30 km or below.

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11 hours ago, 18Watt said:

Using parts which can withstand high temperatures is critical.  Many of the aero parts are not suitable for spaceplanes, because their heat tolerance is not sufficient.

Another thing which can make a huge impact on survivability is your entry profile.

One common misconception is that a shallow entry into the atmosphere will be gentler on your space plane.  This is not correct.  A steeper entry often works better at managing heat.

With a shallow entry you end up spending longer at high speed, but producing little drag.   Even at high altitudes the high speed generates a lot of heat.  Eventually your ship generates more heat than it can safely handle.

A steeper entry, which gets you below ~30 km quickly, often works better.  Below ~30 km the atmosphere is thick enough to generate drag, slowing you down.   
So my suggestions are:

  • Make sure you are using parts with high heat tolerance, and
  • Perhaps try using a steeper entry profile.

As an example of an entry profile that normally works well for me, starting from an orbit of 80-120 km-  I burn retro-grade until my projected path intercepts the surface about 1/4 to 1/3 of the way around Kerbin.  I do not try to add drag by pitching up until I’m below ~45 km.  I’m trying to get to 30 km as quickly as I can.  Once I’m below 45 km I start pitching up to add drag.  But if it looks like I’ll level off above 30 km I reduce the pitch to continue the descent to 30 km or below.

This will NOT work for higher speed reentries.

This only works for relatively low drag craft on low speed entries, where the limiting factor over a long entry is the internal temperature of a weak part getting too high due to heat bleed.

For high speed entries (well over 3000) you  are instead limited by the maximum skin temperature of the weakest part.  Often this entirely prevents you from dipping bellow 40km, as any lower and your craft turns into a meteor shower as parts instantly turn into flashpaper.

For these types of entries you are forced to stay high, and need to generate as much drag as possible using an extremely high AoA.  If my ssto design can't sustain at least 40 AoA from entry through to sub Mach 3, I redesign it.

As an example, on a recent mission I hit atmosphere at 4300 m/s.  Any lower than 44km and the craft burns up due to exceeding skin temp, so it has to scrub over 1000 m/s in a single pass at 44km at 90 degree AoA.

 

As a matter of fact, I have NEVER had a craft that dealt with problems with internal overheating on reentry.  The only time I have ever had an issue with that was with a craft that needed to do an extended hypersonic glide halfway around Kerbin post reentry, and one of the science parts (1200 internal temp max) got close to blowing up.

 

TL:DR - A properly designed spaceplane is going to be able to generate enough drag that it can scrub speed fast enough for skin temp to always be the limiting factor, not internal temp.

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