What's the minimum altitude I should jettison fairings at with FAR?

[smjjames]

I usually jettison procedural fairings somewhere around 40-60 km, but I'm wondering here, could I eject fairings around 35km and still be fine? Obviously the optimum would be when you're above the atmosphere.

Just to reiterate, I'm using FAR.

[KerbalEssences]

To give you a very scientific answer: Well.. it depends.

Giving a general answer to this would be just wrong in my opinion. It obviously depends on the weight, size of the fairings and also on the speed you are going. Using FAR you can switch on the aerodynamic pressure colors I believe. It will show you whether your fairings are exposed to high aerodynamic pressure or not. You can also use the temperature (wich deadly reentry) as an indicator.

[Van Disaster]

That's a little hard to tell without knowing how draggy the payload is; in older versions of FAR you almost didn't have any drag until 35km, but the latest ones have a fair amount of skin drag at high altitude. Only real way to tell is do some trial launches & test, really. You're likely to always be fine above 55km at least, I'd imagine.

[eddiew]

On the whole, I'd call 50km. This is the point at which most of my SSTOs can cut thrust and coast without losing more than a few hundred metres of apoapsis.

There IS still air drag past this point, but it's very small, usually in the order of a few fractions of a kN, unless your payload is very bulky and angular with a lot of exposed nodes. On the KSC screen, open FAR options, and turn on drag display for the right-click menu; this will help you assess how much difference fairings make, though you'll need to do a few trial runs first.

If the question relates to "can I dump the fairings while I'm still burning to save weight"... probably not. Unless you find you're still burning the rockets beyond 50km, you might as well wait until you clear the atmosphere completely.

[Jouni]

It depends on the rocket. Most of the time, the fairings are so lightweight that you don't really save anything by jettisoning them early. I usually take such fairings to space and jettison them before orbital insertion.

Sometimes I launch giant monsters, and the fairings may form a significant fraction of the payload. In such cases, I prefer to jettison the fairings as soon as dynamic pressure falls below a predetermined threshold (usually 500 Pa or 1000 Pa).

[Jovus]

Generally I'm cutting thrust low enough in atmo that by the time I'm out of the regime where FAR models significant skin drag, I'm coasting to space anyway.

[Yakky]

It depends on the rocket. Most of the time, the fairings are so lightweight that you don't really save anything by jettisoning them early. I usually take such fairings to space and jettison them before orbital insertion.

Sometimes I launch giant monsters, and the fairings may form a significant fraction of the payload. In such cases, I prefer to jettison the fairings as soon as dynamic pressure falls below a predetermined threshold (usually 500 Pa or 1000 Pa).

BTW dynamic pressure is called "Q" on your FAR data display. Just watch for that number to get low enough. You don't need fairings anymore when Q gets negligible.

It would actually be interesting to know what Q values are safe enough -- I just wait for it to get "really low" but I haven't run any experiments to see how low is low enough. Does anyone have any data points on what "Q" value causes extended solar panels to break off, for example? If the dynamic pressure is low enough that solar panels don't disintegrate, it's probably safe for most things.

Edited March 17, 2015 by Yakky

[InterCity]

As said before, throw them away at 50 km at least. 68 km is the height of atmosphere, so if you want to have max efficiency, then use 68 km.

[Drew Kerman]

KSA dumps fairings at 45-50km, as that's when our rockets are still under thrust and thus ensures the fairings are not only separated but completely removed from the vicinity of the craft

[smjjames]

@thread move: I thought it would have been fine where it was before, but okay.

[Jarin]

I tend to cut mine in the mid-30s, but that's not for drag purposes; it's for extending antennas for RemoteTech. Been bit too many times by lobbing a rocket over the horizon without putting up the main comm antenna, and losing signal because my geostationary network can't reach the tiny little basic antenna I have to use for launch. I'll even pop open solar panels at 40km without issue.

[Astrofox]

Scott Manley has fairing separation at around 40km, and that seems to do well for him.

[kerbiloid]

Above the height where the deployed solar panels start fire or crash.

Just launch one test probe to 100-200 km, extend its panels and on its re-entry check where that begins. Add 5 km.

[Axelord FTW]

If you intend to keep thrusting as you go up past 50km until your reach space, I'd say 55km would be a good point to drop them.

IF you are already coasting and don't intend on applying any more thrust until you reach space, keep the fairings as long as you like. Of course, you drop them before you plan on doing any sort of d/v changing maneuvers.

[Streetwind]

I usually wait until Kerbal Engineer Redux tells me that my atmospheric efficiency is very low. Like 3%-ish. That usually happens around 40km for the trajectories I fly (I am generally under thrust almost all the way up, often reaching apospasis with next to no coast phase and >2000 m/s velocity).

If I know that I have an exceptionally draggy payload - the flat Rockomax 1.25/2.5 adapter for example is a guaranteed culprit if facing forward - then I go another 5km beyond that or so. I definitely never carry fairings to or beyond 50km. Do note I am not using procedural fairings, but rather KW Rocketry's ones. They are certainly not of irrelevant mass, so it's worth ditching them sooner rather than later.

[Van Disaster]

I tend to cut mine in the mid-30s, but that's not for drag purposes; it's for extending antennas for RemoteTech. Been bit too many times by lobbing a rocket over the horizon without putting up the main comm antenna, and losing signal because my geostationary network can't reach the tiny little basic antenna I have to use for launch. I'll even pop open solar panels at 40km without issue.

You can extend them inside the fairings too, used to do that when I ran RT.

[John FX]

I usually wait until 40km, you may be OK lower but 40km is pretty safe.