PSA: how to make your SSTO faster

[fourfa]

I've been spending time lately optimizing SSTOs for high speed and low drag.  I found out a couple of interesting things.

Top speed in the atmosphere on ascent is often limited by the temperature tolerance of the nose pieces.  No big newsflash there, I'm sure we've all had some cockpits blow up from overheating on ascent. (Assuming here that you have enough engine power, intake air, and minimum of drag, so that it's possible to overspeed the heat tolerances on your spaceplane.)  So naturally, we'd think to put a heatshield on the nose of the craft to take advantage of its 3300K limit, highest in the game.  

Then you'll find out something interesting about the heatshields - if both the inside and outside attachment nodes are occupied (like the configuration you'd find in an ordinary rocket stack), they are very low drag pieces.  But if the outside node is not occupied (like you'd have on a capsule re-entry) the heatshield drag is stupendous, totally out of line with other similar sized parts.  I assume this is one of the tweaks made to allow for safe and easy capsule reentry from a broad range of orbits.  So if you just slap a heatshield on the nose, you will likely never get to high speed at all.  So, hack the system - attach something to the outside, but use the build tools to offset that piece away from the front.  Even better, offset it into a service bay or fairing to hide it from drag entirely.  Doing this, I routinely find that the wings (all of which have the same 2400K limit) are the limiting factor.  Not much you can do about that short of using heatshields themselves for wings... 

This simple hack has me routinely seeing 1650m/s on just airbreathing RAPIERs, instead of the 1200-1400m/s I used to normally see.  Then the rocket equation reaps benefits - that extra speed on ascent means less rocket fuel needed to reach orbital speed and circularize.  It's a bigger assist than one might think, and directly leads to smaller fuel tanks, fewer parts, less mass, less drag, fewer engines, fewer intakes... it's good fun to downsize for better performance rather than spiral bigger!

While we're misusing fairings to hide things from drag - consider that a fairing only needs to surround the centerpoint of an item (ie where the three Offset tool handles converge) to occlude it.  This makes it very easy for a central 'spine' of fairing to occlude lots of parts (fuel tanks, fuselages, even cockpits) from drag... and thus also from heating.  Hax! Cheating!  Definitely but it makes it possible to build some very small pocket SSTOs, things that fit in cargo bays.  This craft has no drag or heating on the entire front half due to a hidden extension to the fairing seen at the nose:

https://kerbalx.com/fourfa/Starling-Science-SSTO

This craft hides the whole nacelle assemblies in interlocking fairings, plus the orbital engine in a tail fairing, plus attitude control RCS in the nose service bay, and I was surprised how small and simple it ended up for the payload capacity and ease of flight:

https://kerbalx.com/fourfa/Sundiver-SSTO

The node-occupied heatshield trick could make a circumnavigation-record craft a little easier, something I might work on soon

Hope someone finds this interesting.

[FleshJeb]

5 hours ago, fourfa said:

Doing this, I routinely find that the wings (all of which have the same 2400K limit) are the limiting factor.

Big-S parts transfer heat to other parts half as fast as structural wings. For my high-speed aero stuff, I use structural wing parts only, but you need them for fuel. The first thing I'd try is attaching them to the rear size adapters, because adapters tend to be much better radiators than straight-sided parts (check the cfg files). This will increase the rate at which heat fluxes through them.

I'm not a big fan of using node-occlusion exploits, so my standard Mk3 nose is 1 or 2 precoolers and a hemispherical nose cone. Offset stuff back into the cockpit so the nose looks right. Landing gear on the front precooler.

Approaching the upper thermal limits is also safer than you might think for a couple reasons:

• The hotter the part, the much more power you can radiate through it (proportional to T^4).
• I think there's a delay mechanism in the heat transfer code to save calculations. If you watch the parts in F11 overlay, a leading part will be glowing white hot and close to failure. In the next frame it will dump a boatload of heat into a cooler attached part (like an elevon on the trailing edge).

I post this image a lot, but this prototype survived by transferring thermal energy:

Spoiler

I think the main wings are attached to the precooler (a fantastic radiator).
The front E-wings are attached to the NCS adapter.
The rear E-wings are attached to the Small Nose Cone and the elevons are on the trailing edge of those. They're also in the heat shadow of the front E-wings.

 

Edited December 8, 2021 by FleshJeb

[fourfa]

Fascinating! Yeah for both those examples, I need the wings for fuel.  It's not THAT much fuel though, there might be another way with structural wings.  But it would be interesting to replace the dorsal aerospike intake with a dorsal precooler, and attach the wings to that...  I have seen that heat-dumping behavior, where one of the hot wing segments suddenly cools down massively, then starts to heat up again.

[FleshJeb]

1 hour ago, fourfa said:

But it would be interesting to replace the dorsal aerospike intake with a dorsal precooler, and attach the wings to that...

I would put it between the engine and the adapter, clip it in, then attach the wings to it. Engines also radiate at 95% and should be heat negative when you're high and fast. The bonus is you're also getting rid of a whole 1.25m drag face. (Unless you're into the dorsal intake for aesthetic reasons.)

EDIT: Are you also using a hemispherical nose on the rear of the Rapier? Worth it for the drag reduction (Also a 95% radiator). You should actually have enough thermal margin to just use a Mk1 Fuel tank and offset it into the front fairing.

Edited December 9, 2021 by FleshJeb

[fourfa]

Aesthetics yes up to a point; I don't want to have intakes that are totally invisible inside some other part, or on the ventral (bottom) side where real-world reentry ablation would destroy them (despite the fact that KSP doesn't care at all).  Everyone's got their own playstyle, I guess

I have played with how to protect a nose-mounted intake with  the front-most part being a node-occupied heatshield:

I personally don't currently see the node-occupied tweak as an exploit.  A thin, sideways, high-temp part like this should not have ten times the drag of the entire rest of the craft - that's how it is otherwise, just an odd side effect of gameplay tweaks that don't make sense for this alternate use.  

And hmm, so far the dorsal precooler experiment has been a bust.  No substantial change in cooling, and a little more drag.  There's something magic about the advanced nose cone + shock cone nacelle, it's super slippery.  Not what you'd expect from a whole additional 1.25m drag face ordinarily.

Edited December 9, 2021 by fourfa