Why do they not fair engines nozzles?

[Helmetman]

I know the engine nozzles are under extremely high vibrations and temperatures and need to cool properly. It's probably the reason why this is no modern application as it is probably to hard to get done properly. Or not, you tell me?

While not the draggiest of shapes engine nozzles are quite the most draggiest parts on a rocket. Assuming one doesn't look like a brick.

What am I getting at? [v] This [v]

This doesn't work in KSP obviously. It doesn't calculate aerodynamics with the complexity to simulate any effect. This does look cool IMO.
 

Shouldn't such a design create more aerodynamic efficiency? Is it doable engineering wise? Are there sufficient cooling techniques? Or is it to dangerous?
Would it save Delta V to orbit considering the extra weight? 
Maybe one could decouple them from the fuselage once pretty high up there?
Or is the gain from such a design to minimal to care about?

I haven't seen such encapsulated nozzles, yet! So there's probably a clear straight answer as to why not. Probably the ones I already given. Maybe there's viable discussion about cooling and vibration absorbers. Ofcourse this will get harder with multiple engines or ones with high swivel. Maybe it's just not worth it for current rocket designs out there. It would probably save to little in the end.

This was just one of those intriguing questions from the top of my head.

 

 

 

 

[DDE]

I think SpaceShipOne had such a design... and they immediately ran into cooling issues.

[p1t1o]

I think you have answered your own question lol!

Aerodynamic drag is nowhere near as important for [space] rockets as it is for say, aircraft or cars. Mass on the other hand...

**edit**

Point of note, rockets designed for operation within the atmosphere (like missiles) almost always do have a faired over nozzle.  But these vehicles operate in regimes where aerodynamic drag is colossal.

 

 

Edited September 14, 2017 by p1t1o

[Bill Phil]

Aerodynamic losses for rockets are usually the smallest part of the Delta v cost (for orbital launch vehicles).

[Helmetman]

@Bill Phil Yeah I know that, but it wouldn't it deliver at all?  Is that the bottom line?

Short topic with short answer then 

 

[mikegarrison]

Weight. Complexity. Loss of radiative cooling. Constraint on gimbal freedom.

[sevenperforce]

51 minutes ago, mikegarrison said:

Weight. Complexity. Loss of radiative cooling. Constraint on gimbal freedom.

I was going to come and say this, but then mike said this.

Weight is probably the single biggest factor, though everything else is part of it. If the mass of the fairing hurts you more than the decreased drag might help you, then the whole idea is toast right from the start.

[NSEP]

Maybe on an SRB or a supercool science fiction SSTO plane. But i don't see the point in an engine fairing with a regular other than it looking cool. I don't think a rocket going upwards and going faster than the speed of sound needs a streamlined back.

[p1t1o]

53 minutes ago, NSEP said:

...a supercool science fiction SSTO plane...

Bing-Bang-Boom! I got your back!

[kerbiloid]

Would the drag economy compensate the mass growth?

(Oops, pre-ninja'd many times.)

Edited September 14, 2017 by kerbiloid

[Skylon]

On 14/09/2017 at 3:58 PM, p1t1o said:

Bing-Bang-Boom! I got your back!

 

But...I...I'm not science fiction...just need a bigger budget...

Back on topic, I'd agree with what's already been said. I guess that the weight is too much compared to the negligible aerodynamics improvements. And of course there's the cooling problems and such. Adeline was going to do this, but that's because it had to re-enter and land.

Edit: Huh, just noticed my rep is now 503...halfway there.

Edited September 17, 2017 by Skylon

[magnemoe]

On 14.9.2017 at 4:58 PM, p1t1o said:

Bing-Bang-Boom! I got your back!

No tapered fairing on it either. In fact it looks thicker on back than it needs to be, 

Now if the engine is burning it would reduce much of the issue with an flat back of rocket. Some artillery shells even has an small slow burning rocket engine at the flat rear, purpose is not to create trust but created pressure and negate the turbulence from the flat back of shell. 

[p1t1o]

16 hours ago, magnemoe said:

No tapered fairing on it either. In fact it looks thicker on back than it needs to be, 

Maybe not the best example. Its not exactly a "fairing", but the engine is "faired over". Surrounding the main 4-nozzle exhaust is a ring of ramjets which burn excess hydrogen generated by the intake cooling loops - More liquid hydrogen is used to cool incoming air than is required to combust said incoming air, so the excess is burned in these ramjets using uncooled air, which is thermodynamically unfavourable and so does not produce much thrust - but like the base-bleed artillery projectiles, the jets reduce drag around the engine I think.

[sevenperforce]

Also, note that terminal/wake drag on an engine is a different affair than wake drag on stuff NOT producing a ginormous cloud of hot expanding gas.

[Racescort666]

18 hours ago, magnemoe said:

No tapered fairing on it either. In fact it looks thicker on back than it needs to be, 

Now if the engine is burning it would reduce much of the issue with an flat back of rocket. Some artillery shells even has an small slow burning rocket engine at the flat rear, purpose is not to create trust but created pressure and negate the turbulence from the flat back of shell. 

I was going to say something about ballistic coefficient and boat-tail ammunition but then I came across this picture. Bullets have an interesting trade off between fitting in the cartridge, fitting in the gun (magazine), fitting in the barrel (on the rifling and sealing the bore), and aerodynamics. 

[Human Person]

I do this a lot. I found that I have less issues with engines overheating on reentry of SSTO rockets. I'm not sure though, need to do some more testing to confirm.

Edited September 18, 2017 by Physics Student

[sevenperforce]

1 hour ago, Racescort666 said:

I was going to say something about ballistic coefficient and boat-tail ammunition but then I came across this picture. Bullets have an interesting trade off between fitting in the cartridge, fitting in the gun (magazine), fitting in the barrel (on the rifling and sealing the bore), and aerodynamics. 

Another issue, and a reason why boat-tails are favored: you WANT tail drag, to keep the center of pressure behind the center of mass. The small amount of drag you get on the tail is far less than the amount of drag you'd get if you started to tumble. Not to mention the total loss of inertial guidance with trajectory flapping all over the place.

[p1t1o]

19 minutes ago, sevenperforce said:

Another issue, and a reason why boat-tails are favored: you WANT tail drag, to keep the center of pressure behind the center of mass. The small amount of drag you get on the tail is far less than the amount of drag you'd get if you started to tumble. Not to mention the total loss of inertial guidance with trajectory flapping all over the place.

This is a pet peeve of mine (for some reason there are a ton of misconceptions around it) - bullets are not aerodynamically stable.

Apart from their centre of mass not being optimal, their length/width ratio means that CoD/CoM placing is insignificant in terms of moments of inertia and corrective impulse. In fact, that slight narrowing at the tail (the "boat-tail") is there to reduce drag. Im not sure why they are not made with a fully pointed tail, like a "Sears-Haack" body, I suspect it has more to do with ease of manufacture.

This is the exact reason why they are made to spin about their axis - their length/width ratio makes this method of stabilisation more effective.

Bullets spin fast (on the order of ~300,000rpm for an average rifle), and are thus very well stabilised.

FunFact^TM: this is why long-rod penetrators are fin stabilised. Their length/width ratio is the opposite of a conventional "bullet", so spinning it on its axis risk DEstabilising the round, and CoD/CoM placing is more effective.

[sevenperforce]

1 minute ago, p1t1o said:

This is a pet peeve of mine (for some reason there are a ton of misconceptions around it) - bullets are not aerodynamically stable.

Apart from their centre of mass not being optimal, their length/width ratio means that CoD/CoM placing is insignificant in terms of moments of inertia and corrective impulse. In fact, that slight narrowing at the tail (the "boat-tail") is there to reduce drag. Im not sure why they are not made with a fully pointed tail, like a "Sears-Haack" body, I suspect it has more to do with ease of manufacture.

My understanding (which may be incorrect) is that spin-stabilization is the primary actor in ensuring attitude control, but the boat-tail helps to damp precession. A Sears-Haack body would be more likely to precess and tumble than a boat-tail.

[p1t1o]

2 hours ago, sevenperforce said:

My understanding (which may be incorrect) is that spin-stabilization is the primary actor in ensuring attitude control, but the boat-tail helps to damp precession. A Sears-Haack body would be more likely to precess and tumble than a boat-tail.

Any feature which increases the "long-thin" character of the body would increase the tendency to precess, so adding a "pointy" tail like on a sears-haack would do that too, that makes sense.

[magnemoe]

1 hour ago, p1t1o said:

This is a pet peeve of mine (for some reason there are a ton of misconceptions around it) - bullets are not aerodynamically stable.

Apart from their centre of mass not being optimal, their length/width ratio means that CoD/CoM placing is insignificant in terms of moments of inertia and corrective impulse. In fact, that slight narrowing at the tail (the "boat-tail") is there to reduce drag. Im not sure why they are not made with a fully pointed tail, like a "Sears-Haack" body, I suspect it has more to do with ease of manufacture.

This is the exact reason why they are made to spin about their axis - their length/width ratio makes this method of stabilisation more effective.

Bullets spin fast (on the order of ~300,000rpm for an average rifle), and are thus very well stabilised.

FunFact^TM: this is why long-rod penetrators are fin stabilised. Their length/width ratio is the opposite of a conventional "bullet", so spinning it on its axis risk DEstabilising the round, and CoD/CoM placing is more effective.

Having an pointed back of bullets probably cause problem inside the barrel, guess you need an flat back to put the high pressure against. 
You can get away with this with sabots but its increase the bullet length and complexity a lot, might also not work well for rifles as it would make sense for sniping. 
For artillery shells keeping size down is also important as you can carry more ammo with the gun. 

An interesting side note about bullet stability, An bullet shoot straight upward will loose aerodynamic stability and go into tumble, an arrow shot straight upward will regain it and come back almost as fast. 
(no you should never test this firing by hand its likely you shoot at an angle and bullet keep it horizontal vector and can hit and kill hundreds of meter away) 

[sevenperforce]

6 minutes ago, magnemoe said:

Having an pointed back of bullets probably cause problem inside the barrel, guess you need an flat back to put the high pressure against. 

Bullets expand under heat and pressure to seal the barrel, meaning that the geometry at the back of the bullet will not really affect the way the propellant gases push it down the barrel.

Other than the previously-noted problem with a lack of precession damping in a Sears-Haack body, another problem with a conical tail is that it cuts into the amount of available space for propellant inside the cartridge.

Some of the old Mauser rifle rounds were very long and very close to Sears-Haack, but even they invariably had a truncated tail.

[p1t1o]

For what its worth to the discussion, I found these with a little random googling:

Why a pure Sears-Haack bullet doesnt work:

 

Also:

 

 

But here is an interesting image (note the size):

[wumpus]

Do New Glen and ITS (whatever spacex is calling the Raptor-based rocket) have faired nozzles, or are the rockets simply that much bigger than the nozzles you can't really see them?  I think they shroud the nozzles for different reasons (mostly coming down), so don't expect the justifications to be remotely similar.

[sevenperforce]

30 minutes ago, wumpus said:

Do New Glen and ITS (whatever spacex is calling the Raptor-based rocket) have faired nozzles, or are the rockets simply that much bigger than the nozzles you can't really see them?  I think they shroud the nozzles for different reasons (mostly coming down), so don't expect the justifications to be remotely similar.

The New Glenn has an octaweb-like enclosure around the base of the engines so they are protected on entry and don't have to do a re-entry burn.

The BFR booster doesn't really have faired nozzles at all.