FAR drag-reducing aerospike test

[Nixod321]

So, after reading about real world aerospikes on Wikipedia, I decided to give it a go in FAR. 

Basically, they work like this:  "A drag-reducing aerospike is a device used to reduce the forebody pressure aerodynamic drag of blunt bodies at supersonic speeds. The aerospike creates a detached shock ahead of the body. Between the shock and the forebody a zone of recirculating flow occurs which acts like a more streamlined forebody profile, reducing the drag."

Which is a fancy way of saying that it spreads the shock wave cone out so the air hitting the rocket body is slower.

^Real world, on a Trident I missile.

 

So, do they work with FAR?

As far as I've tested, they seem to. I'm not sure whether it's some other effect created by FAR or it's the real deal, I'm not sure how well FAR models shock waves as this was part of my testing.

The test involved two nearly identical craft, one with the aerospike, one without. I'm playing with some of the settings tweaked to be a bit harder (greater transonic drag, strict area ruling) so the default results might be different. The nosecones were built with the Procedural parts mod, but they are fully FAR compatible. They were both launched straight up at full throttle with SAS engaged until their fuel ran out to get them to experience as many mach numbers as possible. Once the engine cut, I hit F3 and recorded the maximum speed:

Aerospike testbed: avg Max speed across 5 launches: 2,000m/s

Non-aerospike testbed: avg Max speed across 5 launches: 1,800m/s

This obviously isn't extensive testing, so I highly encourage other FAR users to give this a go themselves.

Further notes: The Aerospike testbed also resisted shock heating far better than the Non-aerospike testbed, whose nose cone exploded several times from overheating. The Aerospike model suffered extreme heating only on the top plate of the Aerospike itself, with minor heating on the cone piece. This indicates to me that the airflow hitting the cone piece has been reduced in velocity. Additionally, the Aerospike testbed had marginally greater stability at high Mach numbers when compared to the Non-aerospike model. To me this indicates the effect of the SAS's over corrections is diminished by reducing the supersonic flow over the winglets; but I'm not entirely sure how valid that conclusion is.

 

Edited June 22, 2016 by Nixod321

[MKellerBR]

Great work!

 

[Rune]

59 minutes ago, Nixod321 said:

snip

Great sciencing! But I'm afraid you are seeing a stock game effect. Vanilla KSP already models detached/attached shock cones, and it does this by looking at the space of the foremost part, calculating a cone that envelops it, and deciding based upon the semi-angle of the cone whether you should get a detached or attached bow shock wave, and whether the rest of the ships fits within that cone. If it's detached, it's slightly cooler and draggier (blunt noses), and if it's attached it's slightly hotter and with less drag. Of course the angle of the cone also plays a part in calculating base drag. Long story short? You might be seeing FAR effects, or you might not. Stock KSP already does a mighty fine job of recreating the nuances of "aerospaceflight" with some simple rules.

 

Rune. Don't know enough about the innards of FAR to say it with authority, tough.

[Van Disaster]

@Rune: FAR replaces stock aero completely ( ie, literally disables stock aero ) per part. It also models shock cones and their change with mach number reasonably comprehensively ( has done for quite some time ), so unless the part doesn't have a FAR aero module somehow, then it *can't* be a stock aero effect ( and if it somehow failed to apply the partmodule, odd things usually happen ).

As for the actual test, did you check wave drag with & without the spike?

Edited June 22, 2016 by Van Disaster

[Hodo]

I know a few of us did some testing a while back with the shock spike on the front of some super sonic aircraft designs.  It actually works in FAR quite well, but there is a fine balance in the setup of it.  

FAR is by far, no pun intended, the best aerodynamic model for a game like this I have ever messed with.

[Foxster]

In stock you get the effect with a Communotron 16 aerial stuck on the front of just about any craft. Reduces heat and drag a lot. Dunno about FAR. 

[Van Disaster]

You would get a similar effect in FAR with a long pointy nose, it should show up as reduced wave drag. I don't know if there's a heat mod that takes shape into account that would make blunt bodies worthwhile?

[Hodo]

2 hours ago, Foxster said:

In stock you get the effect with a Communotron 16 aerial stuck on the front of just about any craft. Reduces heat and drag a lot. Dunno about FAR. 

It works in FAR, I have tried it.  It is what I usually use on the nose of some of my high speed interceptors, and SSTOs.

[Nixod321]

6 hours ago, Van Disaster said:

@Rune: FAR replaces stock aero completely ( ie, literally disables stock aero ) per part. It also models shock cones and their change with mach number reasonably comprehensively ( has done for quite some time ), so unless the part doesn't have a FAR aero module somehow, then it *can't* be a stock aero effect ( and if it somehow failed to apply the partmodule, odd things usually happen ).

As for the actual test, did you check wave drag with & without the spike?

Without Spike:

With Spike:

[Rune]

16 hours ago, Van Disaster said:

@Rune: FAR replaces stock aero completely ( ie, literally disables stock aero ) per part. It also models shock cones and their change with mach number reasonably comprehensively ( has done for quite some time ), so unless the part doesn't have a FAR aero module somehow, then it *can't* be a stock aero effect ( and if it somehow failed to apply the partmodule, odd things usually happen ).

As for the actual test, did you check wave drag with & without the spike?

I stand corrected, then.

 

Rune. Funny that both systems end up working the same way, right?

[Van Disaster]

Well, not really - different implementations of real laws are still using the same laws, let's just be thankful stock aero isn't old stock aero and you can take things from real life. Great that stock has got that sort of detail now ( also easier to handle that & FAR when molding. ..)

What I find interesting is just how much difference tiny tweaks to nose & tail can make quite significant changes to supersonic drag in this FAR. @OP, what level of area-ruling are you using?

[Nixod321]

1 hour ago, Van Disaster said:

Well, not really - different implementations of real laws are still using the same laws, let's just be thankful stock aero isn't old stock aero and you can take things from real life. Great that stock has got that sort of detail now ( also easier to handle that & FAR when molding. ..)

What I find interesting is just how much difference tiny tweaks to nose & tail can make quite significant changes to supersonic drag in this FAR. @OP, what level of area-ruling are you using?

Strict area ruling.