• 0

# Optimum altitude to jettison fairings?

## Question

Is there a general rule of thumb as to the optimum altitude/velocity/pressure (Q, I think) to jettison fairings-- where the aerodynamic drag reduction is no longer worth the weight/gravity loss?

I don't really need the exact equations-- just a general "feel" for the right timing/altitude.

Thinking of firing off a series of sounding rockets to test, but I imagine someone else has done a lot more thinking about it already...

## Recommended Posts

• 2

Techcanilly, yes. :

thrust * fairing_mass / rocket_mass kN of thrust is lost accelerating the fairing

popping the fairing will increase drag, also measured in kN

Since these numbers are directly comparable, once the rocket is wasting more kN of thrust accelerating the fairing, than the fairing is reducing drag, it is time to pop the fairing.

However often in KSP you will complete the main burn to nearly gain orbital velocity, while still deep in the atmosphere ( < 40km) and as such there is no advantage in jettisoning the fairing until out of the atmosphere since you're just coasting.

If I am using a steep trajectory I'd usually pop the fairing at 50-55km - I have a hunch it'd often be profitable to do so earlier (depending on payload) but it's one of those "better safe than sorry" things, keeping the fairing on for longer than necessary is just a small penalty to deltaV, jettisoning it too early can cause serious problems.

##### Share on other sites
• 1

To a certain extent it depends on the craft, but somewhere after 45km is usually safe

##### Share on other sites
• 1
4 hours ago, MaxwellsDemon said:

Is there a general rule of thumb as to the optimum altitude/velocity/pressure (Q, I think) to jettison fairings-- where the aerodynamic drag reduction is no longer worth the weight/gravity loss?

Well, as usual: it depends... one very important thing to keep in mind, besides altitude, is how fast you already are and how much faster you want to get by the time you're out of the atmosphere.

If you're already going as fast as you're going to get, and are merely coasting to apoapsis (a pretty normal situation IMO), it's always worthwhile to cling to the fairings: the fuel to accelerate them has already been spent at that point, so there's nothing to be saved anymore.

• 1

##### Share on other sites
• 0

Once you are high enough for the drag not to matter, you should be at nearly orbital velocity sideways. Which means that jettisoning your fairings increases drag and does not get you any benefits for gravity losses. If I'm on a steep (inefficient) ascent, I tend to wait until about 55km.

• 2

##### Share on other sites
• 0

I agree with the others. I usually achieve Ap when I'm still around 45km, so there's no point in jettisoning the fairing. I usually wait to do that until just prior to the circ burn.

Best,
-Slashy

##### Share on other sites
• 0

Thanks!  Sounds like something I can quit worrying about-- roughly 50km is about the altitude that I pop them as it is, so sounds like there's no big adjustment to be made on my part.

##### Share on other sites
• 0

I agree with above -- if you've already cut the main engines, there's nothing to be gained by dumping the fairing before reaching 70 km.  If you're still boosting, I'd wait until the Mach effects have died down (between 40 and 60 km, depending how fast you're going).

##### Share on other sites
• 0

i usually do whenever i remember after 30km

##### Share on other sites
• 0

I was reading a tutorial here that said drag is based on weight of all parts regardless if there's any aerodynamics or not. So if you add a fairing it will cause more drag because it weighs more. Is that just old news or is it still true?

##### Share on other sites
• 0
2 minutes ago, CrashyMcCrashFace said:

I was reading a tutorial here that said drag is based on weight of all parts regardless if there's any aerodynamics or not. So if you add a fairing it will cause more drag because it weighs more. Is that just old news or is it still true?

CMcCF,

Old news, no longer applies.

Best,

-Slashy

• 2

##### Share on other sites
• 0

My rule of thumb is 2/3 of the way through the atmosphere if you're still under power. Then again, I tend to play in upscaled systems and try to do single burns to orbit, so I frequently don't have much of a coast phase.  As others have said, hang on to the fairing if you've already cut your engine and are coasting to Ap.

##### Share on other sites
• 0

Yes, fairings reduce drag but increase mass, and BOTH of these help keep your apoapsis up when coasting to it.

##### Share on other sites
• 0
On 17/4/2018 at 11:01 AM, Kryxal said:

Yes, fairings reduce drag but increase mass, and BOTH of these help keep your apoapsis up when coasting to it.

This is incorrect. Once you're coasting, you've already "paid" (in energy) to loft your entire mass to the Ap, so shedding mass at this point does not help keep your Ap up.

• 1

##### Share on other sites
• 0

No, I meant that NOT shedding the mass helps keep your apoapsis up.  Given equal force, the lighter object will be more affected by that force, so the more mass you're coasting with the better.

• 1