[1.3.1] Ferram Aerospace Research: v0.15.9.1 "Liepmann" 4/2/18

[ExEvolution]

there are no fairings wide enough

[ferram4]

Procedural fairings are your friend.

[Camacha]

I'm having a real hard time launching large satellite dishes into orbit because I have nothing to cover them with and they're producing insane drag

Get procedural fairings, they will solve a lot of your aerodynamics problems Another option would be to get some fairings from modpacks (AIES, KW Rocketry, NovaPunch), but that generally means having to puzzle a bit more to get it all to fit.

[NathanKell]

Use procedural fairings. They're always wide enough. But what dish are we talking about here!?

[White Owl]

there are no fairings wide enough

Get Procedural Fairings and/or figure out to make your payload smaller.

[ExEvolution]

Procedural fairings is practically cheating in my pack though. Its the Reflectron KR-14 from RemoteTech

[NathanKell]

That's not even that big, though, right? Only like 3m? Even the expanded 2.5s from KW should cover it.

[Raven Coldheart]

@ExEvolution: Try Procedual Fairings, those will be big enough.

[Camacha]

Procedural fairings is practically cheating in my pack though. Its the Reflectron KR-14 from RemoteTech

Why would properly fitting fairings be cheating? It is not like NASA has to deal with a fixed set of fairings to mix and match.

[longhornchris]

Why would properly fitting fairings be cheating? It is not like NASA has to deal with a fixed set of fairings to mix and match.

Actually, they sort-of do unless they spend a lot of money to build a custom fairing. The fairing aerodynamics are important and thus rocket manufacturer's only offer a few sizes, so most of the time you fly the lightest one that will fit the payload.

Of course if the mission is expensive enough then a custom fairing may be merited BUT its the analysis that says it will work that costs a lot of money, not the actual fairing.

[NathanKell]

Ferram: unless you've already figured this out, the issue with the Novapunch parachutes appears to be that FAR grabs their deployed model instead of their undeployed model, so (for example) the reference area for a probe core with a (tiny-fied, so it'd be more for an unrescaled one) NP radial is 10m^2.

[ferram4]

I figured it was something like that. I can look into making it ignore the parachute shape itself, which should fix the issue.

[a.g.]

The curious thing here is what's the difference between stock parachutes and those, since the code doesn't appear to do anything special for parachutes. Before 0.11 parachutes were just ignored altogether and left stock.

Another small detail, is that I suspect that setting stowed drag to 0 in the MM cfg may not completely neutralise the stock drag because there is also the default 0.1 drag for the part itself, but I'm not sure. It's probably similar to intake drag.

[Hodo]

Procedural fairings is practically cheating in my pack though. Its the Reflectron KR-14 from RemoteTech

You have three options.

1- Procedural Fairings (like everyone has said)

2- KW pack has a fairing part in it. (but it is just like the Procedural Fairings)

3- B9 parts, and learn to use the large 3m cargo bay.

I have two of those three, and I can tell you now, when I launched my communications module for my space station I used the B9 pack cargo bay. But that was because I was using one of my SSTOs to do it. I launched all of my communications satellites in the first network I setup in RemoteTech2, with procedural fairings. Because even the little satellites are about as aerodynamic as a wall.

[Frederf]

A drag coefficient of ~0.002 is for very, very low skin friction drag only, with no significant pressure effects. A drag coefficient much higher than that would include pressure effects, like shockwaves, air being compressed, pressure much lower than the ambient, etc.

Odds are that if your DRE shield isn't making enough drag that the fairing is somehow picking it up by mistake... try putting some extra space between the fairing and the shield and see if that changes things.

I thought so. I mean the blunt end of the shield approaches definition of Cd=1.0 by simple geometry. Is reference area the cross sectional area or equivalent? (The number Cd is multiplied by or rather the flat plate the shape is divided by to get Cd).

[ferram4]

Reference area is approximately the total surface area; it makes handling skin friction drag easier. Also Cd = 1 only happens in incompressible flow; as Mach number increases, the Cd for that heads towards 1.86, most of the change happening near Mach 1.

[Selgald]

Are you attaching ONE radial chute? Or at least mirroring it with one on the other side?

I added two. I think its just something wrong with this part.

[Sparker]

I was using command pod from Kosmos pack and noticed that the command pod had excellent aerodynamic characteristics:

I could even build a plane with this pod instead of main wings:

Well, I changed the capsule a little. I set its mass to 2tons instead of 3tons. Anyway, its behaviour is auite strange.

[Camacha]

Actually, they sort-of do unless they spend a lot of money to build a custom fairing. The fairing aerodynamics are important and thus rocket manufacturer's only offer a few sizes, so most of the time you fly the lightest one that will fit the payload.

Of course if the mission is expensive enough then a custom fairing may be merited BUT its the analysis that says it will work that costs a lot of money, not the actual fairing.

Yes, the same goes for engines and stages. It is often more cost effective to use something that is already there than than to develop parts that are completely new. However, like you said, when dealing with an unusual payload, they will simply (have to) develop a new fairing. This is just like a procedural fairing. You will not catching NASA launching a payload without fairings because they could not find any appropiate ones Also, spending money is not yet modelled in KSP so if you want to use a new fairing shape each time you lauch, sure. It is not realistic nor unrealistic.

I do not see much reason to not sure procedural fairings. If you want to roleplay you could always use the fairings supplied in the common parts packs, unless you have some weirdly shaped payload and no other option than to make a custom one. That would resemble the real life approach.

Edited December 1, 2013 by Camacha

[Guest]

I figured it was something like that. I can look into making it ignore the parachute shape itself, which should fix the issue.

Hmm, I've been getting strange behavior on BobCat's Ares I pod. It seemed as if it's COM was offset, but it wasn't. However, after the chutes were deployed, the weirdness stopped. It's got a peculiar parachute arrangement, with two sets of canopies, one of them (drogue) asymmetrical. I'd really like to try a build that ignores parachute canopies in drag calculations. Ares I turned out to be very unstable and wobbly, so perhaps this would help.

[jrandom]

A question about TWR for launches: I've read in many places that you want a TWR of around 1.5 so you don't go too fast to make your gravity turn or too fast in the atmosphere (heading for instability, etc.).

My question is: Do I launch at 1.5 and leave the throttle where it is, or do I continuously throttle back as I burn fuel to keep the TWR at 1.5 throughout the entire first stage?

[Camacha]

A question about TWR for launches: I've read in many places that you want a TWR of around 1.5 so you don't go too fast to make your gravity turn or too fast in the atmosphere (heading for instability, etc.).

I think in real life the aim is to have a take-off TWR of between 1 and 1.2. If you have anything more, you could have used smaller engines.

[jrandom]

I think in real life the aim is to have a take-off TWR of between 1 and 1.2. If you have anything more, you could have used smaller engines.

So the best profile is to launch at around 1.2 and leave the throttle alone after that (for that stage)? Eg. TWR climbs upwards as the fuel tank empties.

[Senshi]

If stability is not an issue for you, you can pretty much throttle up as much as you want. If you use stock engines (or stock-alike ones) in terms of ISP, that usually is the most DeltaV-saving ascent.

If your rocket tends to flip/lose control at a certain altitude, this is usually caused by you going too fast at too low an altitude. In that case it makes sense to limit your throttle. A good and simple (if not entirely exhaustive) indicator for the stability issues is the MaxQ you can see in the FlightData window inflight.

The rule of thumb is: Throttle up as much as possible without losing control . A TWR of 1.3 is just a good starting point as it usually means a nice balance between stability and deltaV loss.

[ferram4]

@sparker: And let me guess, the attach nodes are much smaller than on the stock parts, right? FAR uses attach nodes to help determine drag, and when they are set wrong the calculations are often off. If that's not it, then the origin of the part might be placed in the wrong place, causing things to screw up. Basically, when mod authors make parts that aren't set up completely, bad things happen.

@Dragon01: I'll look into that. It's probably just mod parts designed to different guidelines.