When is the best time to stage a fairing?

[FinalFan]

I've occasionally used fairings when the stuff I was putting on the top half of the rocket got too blatantly unaerodynamic (or had gained a forest of doohickeys), but I've never really been confident of when the "best" time to get rid of it is.  I mean, I guess I'm working on the underlying assumption that fairings are not always counterproductive.  But if that's true, then presumably at some point the equation changes to where future delta-v savings of being aerodynamic are outweighed by the future delta-v losses of the mass of the fairing.  Obviously this happens before you get to space, but is there a rule of thumb to follow here?  40km, lower, higher?  How much do variations in speed matter compared to altitude thresholds if you are more or less following the usual type of ascent path for a large rocket? 

P.S.  Do fairings help at all in reducing bending?  I've always sort of imagined that they do, but without concrete reason or evidence. 

Edited July 18, 2018 by FinalFan
Postscript

[Xd the great]

So usually i use it for unaerodynamic payloads only, like the ones with a disk shaped comm on top.  

For seperation, i usually position it to seperate at 70km for realism, but i figured it will be better to do so at 40 to 50 km. The aerodunamic forces is small enough to not cause drag and the fairings are essentially just dead weight.

[The Flying Kerbal]

I too don't pop mine until out of the atmosphere.  Maybe it takes a little more Delta V but a few gallons extra in the gas tank and it's all good.

[PrvDancer85]

I would say that depends on your acentprofil, if u speed up very fast and then many cruise to ap then i wouldnt pop the fairing when still in atmo, u have already spend the fuel to speed up. If u burn until ap i would pop them at about 50km to save some weight.

That's just how i would do it 

[bewing]

Almost no matter how you fly your ascent, there is a period where you coast up to your circularization burn. That almost always begins while you are still deep in the atmosphere. During that coasting phase, nothing is costing you anything. So the basic answer is that you always want to blow the fairings just a bit before you do the circularization burn, but after you are above the atmosphere.

And yes, fairings do have a value (sometimes significant) -- just so long as the cross-section of the fairing is not significantly bigger than the cross-sectional area of your payload. But if your payload is a few skinny girders sticking out 5 meters from your core, then putting a giant cylindrical fairing around that is going to be a big mistake. In any case, though, the smart thing is always just to try it with a fairing and see if you end up in orbit with more fuel.

 

 

[FinalFan]

3 hours ago, bewing said:

Almost no matter how you fly your ascent, there is a period where you coast up to your circularization burn. That almost always begins while you are still deep in the atmosphere. During that coasting phase, nothing is costing you anything. So the basic answer is that you always want to blow the fairings just a bit before you do the circularization burn, but after you are above the atmosphere.

And yes, fairings do have a value (sometimes significant) -- just so long as the cross-section of the fairing is not significantly bigger than the cross-sectional area of your payload. But if your payload is a few skinny girders sticking out 5 meters from your core, then putting a giant cylindrical fairing around that is going to be a big mistake. In any case, though, the smart thing is always just to try it with a fairing and see if you end up in orbit with more fuel.

That's very true:  if you're already coasting then never pop in atmosphere.  But I have doubts that I am coasting from "deep" in the atmosphere, and the question of the OP still applies in the case that you still plan on doing significant burning in atmosphere—I suppose the answer would depend on how much burning you have yet to do as well as how high and fast you are going.  I guess your final advice still applies—I could quicksave at a certain point of the ascent and blow it at different times and see where I end up fuel-wise.  It's a pretty straightforward idea, but one that hadn't occurred to me for whatever reason, so thanks. 

[XtraChrisP]

I guess you could conduct an experiment where you use the AeroGUI to record the drag pre and post fairing deployment and various speeds and various altitudes.  Then you can compare the dV loss due to the excess weight of the fairing to the dV loss due to drag.  There might be a point where you're still burning high in the atmosphere where you might gain a little by popping your fairing, but if you're still coasting, then, as mentioned in a few other posts, deploying your fairing is counter-productive.  You already traded the fuel for velocity, so increasing drag while decreasing mass both work against you if you're still in the atmosphere.

[Foxster]

It'll depend a bit on how massive the fairing is and how un-aerodynamic the payload inside is. 

Drag is getting fairly low above about 45km on Kerbin but if you have a really draggy payload then probably best to leave it until 60km+. 

[Zeiss Ikon]

The very general rule, if you're still under boost (which you likely are, if you're playing at "real scale" as with RSS or a 6.2x Kerbin), is to dump the fairing as soon as the dynamic pressure is low enough not to damage your payload.  On Earth, that's generally taken to be the Karmann Line, 100 km high -- but then, Earth's atmosphere is dense enough to cause orbital decay on a scale of years out to at least 250 km.  The general definition of the Karmann Line is the height at which orbital velocity is the same as the speed needed for wings to support the weight of a craft.  This is obviously based on a  number of assumptions (an airframe that can handle high Mach numbers is top of the list, since you're around Mach 22 equivalent to get an orbit), but effectively, it's the point beyond which the atmosphere doesn't matter in the short term.

Now, Kerbin's atmosphere is shallower than Earth's (half the height used in RSS), so one might make a first approximation of 50 km as the height at which the atmosphere doesn't matter for the short term, and in fact I've read a number of posts indicating people blow their fairings around 50 km if still under boost, when launching from Kerbin.  So, if you're launching from Kerbin, you might reasonably discard the fairing at the end of any coast phase that ends above 50 km.  This could vary depending on the payload -- that big dish antenna might be a good candidate to hold onto the fairings until 70 km (times your atmosphere scale factor), while a tank of ore or fuel might not even need a fairing, just a nose cone.

[wumpus]

On 7/21/2018 at 11:36 AM, Zeiss Ikon said:

Now, Kerbin's atmosphere is shallower than Earth's (half the height used in RSS), so one might make a first approximation of 50 km as the height at which the atmosphere doesn't matter for the short term, and in fact I've read a number of posts indicating people blow their fairings around 50 km if still under boost, when launching from Kerbin.  So, if you're launching from Kerbin, you might reasonably discard the fairing at the end of any coast phase that ends above 50 km.  This could vary depending on the payload -- that big dish antenna might be a good candidate to hold onto the fairings until 70 km (times your atmosphere scale factor), while a tank of ore or fuel might not even need a fairing, just a nose cone.

Except if you are playing on Kerbin you are almost certainly coasting while you exceed 70km (or reaching nearly 70km for an extremely low orbit) and Bewing's advice is true.  I think I've had some "never stop burning" launches on Kerbin, but they involved NV-Rs and aren't really intentional (and involve a dip back into the atmosphere).

[FinalFan]

12 hours ago, wumpus said:

Except if you are playing on Kerbin you are almost certainly coasting while you exceed 70km (or reaching nearly 70km for an extremely low orbit) and Bewing's advice is true.  I think I've had some "never stop burning" launches on Kerbin, but they involved NV-Rs and aren't really intentional (and involve a dip back into the atmosphere).

If I understand your post correctly you are saying that Zeiss's post in some way contradicts bewing's post and you agree with bewing.  What is the contradiction or disagreement that you see? 

I see bewing as saying "almost everybody coasts to circularization from "deep" in atmo and that costs no dV so hold onto it while coasting" and Zeiss as saying "blow if you are still accelerating at 50km, unless your stuff needs the fairing very much."  

[Mahnarch]

59,734 meters.

No exceptions!

 

Except that one time.

 

...and that one other time.

 

And then, there was that time with the Giant Sloar on the wing!

Many Shubs and Zulls knew what it was to be roasted in the depths of the Sloar that day, I can tell you.