FAR lightweight rocket flipping out, screens and ship included

[jospanner]

Hi there, I'm mostly getting the grips with rocket design with FAR, but I'm having a lot of trouble launching small payloads into orbit. This is the payload in question:

http://imgur.com/UhWFeyb

It's 0.4t, that's all. So first I tried installing it on a pretty boring rocket, with only procedural fairings to actually spice it up...

http://imgur.com/exG0Ho7

The TWR at launch is limited down to 1.2. It reaches 6-7km before it flips out, regardless of the profile of my gravity turn. OK, it should have plenty of thrust to counter the drag, the engine gimbals, so maybe the CoM is dropping too low? So I made several rockets along the lines of this...

http://imgur.com/x9xgJgD

Which does look silly, but the CoM is now right at the top of the rocket. It still flips out at about the same altitude.

Other things I have tried include winglets, both with and without control surfaces, at various points along the rocket. A set in the middle, a set at the bottom, or both. Similar performance each time. I've also tried adding more SAS units which, of course, does nothing. There's way too much torque on the rocket for those to counteract it. I've tried various profiles for the gravity turn, I've tried keeping the AoA as low as I can, no results. It will go straight upwards, though, for all the use that is. :/

Maybe the TWR is getting too high at too low an altitude as the fuel is burnt and that's making it flip, but how do I counteract that in a sensible way? I've already tried having a fuel tank acting as ballast in the third screenshot (just above the decoupler). I've spent about 2 hours on this (). I could build some incredibly over-engineered and expensive thing to do this eventually, but I'm looking for a more elegant (and realistic) solution. So, I've come to you fine people for help! Any suggestions are welcome.

I also wonder, are there any mods out there that can help me visualise the forces on the rocket? Obviously I know of the tools in the editor, and RCS Build Aid lets me see the dry CoM, but I'd like to be able to see the aerodynamic forces acting on the rocket in real time. I don't even know if such a thing is possible. As you might have gathered, I'm not a physicist. I'm more into photography and medicine. I don't know how to use the tools built into FAR properly for a start. But I love the challenge of building more realistic rockets and I'd love to learn more.

Here's the craft files, they require procedural fairings and that's all, AFAIK:

FARFail-1 is the first, very basic design.

FARFail-2 is the second, top-heavy design with the girder.

[Streetwind]

First thing you should do: in the VAB, clip your camera into the fairing and rightclick the top part of your payload. Note the "isShielded" property.

I've heard people mention before that Procedural Fairings isn't properly shielding their payloads under FAR. Other people have it work perfectly fine. Better check which group you belong to I have never had a properly shielded payload flip out at any speed when pointing reasonably prograde; it's the flat, unshielded surfaces like the top of your probe core that really don't want to fly straight.

And, failing everything else, have you tried doing it stock style, simply flying straight up for a while? Make a really late turn? It'll be inefficient but you're already using an overengineered rocket according to your own words.

Other things I can come up with, which may or may not actually matter:

- Payload may wobble inside the fairing due to the flimsy bottom attachment node of the ant engine

- You have nearly zero control authority besides the engine gimbal; the probe core's reaction wheels alone won't really let you steer something this big, especially when full of fuel

You can even test for most of the mentioned points simply by removing the fairing, attaching the payload directly to the rocket (ideally on a size 1 -> size 0 adapter) and installing a nosecone on top of the probe core. This way it will be properly shielded and you can observe it for any wobbling on the way up.

Edited November 7, 2014 by Streetwind

[jospanner]

Hi, thanks for the reply!

I've had a look, and with the fairing installed it shows "isShielded: False" for the probe body and "True" for the little fuel tanks. :/ This is strange because I'm sure the fairings were working fine on my other craft. Is there a fix for this?

So anyway, I tried what you said, and created this bizarre contraption, which I think is following your instructions! The payload has no wobble at all, and the parts beneath the comically oversized nosecone show "isShielded: False". It works about as well as the other designs tbh. I tried adding an SAS module to it, but it didn't help. The torque that flips the craft out happens suddenly and violently.

EDIT: I've also tried the old "Go straight up to 10km and start turning" thing. The rocket violently flips out as soon as I get anywhere off the velocity vector marker.

Edited November 7, 2014 by jospanner

[Streetwind]

Very odd. "Suddenly and violently" reminds me of the onset of wave drag when going supersonic, which FAR does model, but your rocket doesn't look like it should have to worry about that...

How is your heading in relation to the prograde marker at the point of flipout? What happens if you fly through the "danger zone" momentarily ignoring whether or not you're turning properly, and instead blindly stick to 100% prograde for a few kilometers?

[rodion]

i haven't personally used FAR for a while but i've heard that it doesn't play too nicely with PF. usually even with the more unwieldly shapes of rocket putting control surfaces at the bottom is enough to easily maintain control authority in the atmosphere. maybe try launching with an alternate set of fairings (KW's are nice, though obviously more limited than PF) and see if that's the issue.

[Alshain]

Make sure you remove the fairing shell, trash it, and put a new one on after resizing the base. There is an issue with FAR and PF combination where this doesn't create proper shielding.

It's a minor inconvenience, but really any time your take the fairing off or resize the base, just throw it away and grab a new one. Fortunately because it is PF, it's only a small inconvenience.

[Starman4308]

I also wonder, are there any mods out there that can help me visualise the forces on the rocket? Obviously I know of the tools in the editor, and RCS Build Aid lets me see the dry CoM, but I'd like to be able to see the aerodynamic forces acting on the rocket in real time. I don't even know if such a thing is possible. As you might have gathered, I'm not a physicist. I'm more into photography and medicine. I don't know how to use the tools built into FAR properly for a start. But I love the challenge of building more realistic rockets and I'd love to learn more.

All I know of is just to try visualizing CoM and CoP (Center of Pressure, AKA Center of Lift) while tilting your rocket in various ways. In the VAB, the airflow is coming from the top (in SPH, from the hangar doors). Keep CoP as far below CoM as possible to retain stability.

Be sure you are executing an actual gravity turn instead of what stock leads you to believe is a gravity turn. A real gravity turn is to make a 2-5 degree pitch when you're moving 60-100 m/s (higher TWR means an earlier, sharper turn), and then to ride prograde all the way down until you're in upper atmosphere and can, for yourself, decide what the proper pitch is. If properly constructed*, you can actually turn SAS off and go "Look Ma, no hands!", because aerodynamic stability will keep your rocket dead prograde. Technically speaking, if you ever touch the controls after the initial turn, it isn't a gravity turn, it is a pitch profile, though it should work if you always keep rocket orientation close to prograde. If it deviates too far from prograde, you go from aerodynamic stability to aerodynamic instability, and flip in all sorts of interesting, rocket-ending ways.

*Has fins to keep CoP below CoM. I loved this discovery.

EDIT: These are for RSS, and you'll have to tweak it for stock (stock planets just feel so dinky once you start playing RSS or 6.4x), but are still helpful: Ferram's guide to making an RSS launch vehicle, and his guide to launch profile.

Edited November 7, 2014 by Starman4308

[Hodo]

Hi, thanks for the reply!

I've had a look, and with the fairing installed it shows "isShielded: False" for the probe body and "True" for the little fuel tanks. :/ This is strange because I'm sure the fairings were working fine on my other craft. Is there a fix for this?

So anyway, I tried what you said, and created this bizarre contraption, which I think is following your instructions! The payload has no wobble at all, and the parts beneath the comically oversized nosecone show "isShielded: False". It works about as well as the other designs tbh. I tried adding an SAS module to it, but it didn't help. The torque that flips the craft out happens suddenly and violently.

EDIT: I've also tried the old "Go straight up to 10km and start turning" thing. The rocket violently flips out as soon as I get anywhere off the velocity vector marker.

Sounds like you are having a problem with the rocket losing stability after it turns off of the prograde. Well there is a simple fix for that, add winglets to the tail of the craft, this will help keep it stabile through the air at speed, there is a reason why lawn darts have them.

The issue you are running into is your rocket is reaching nearly supersonic speeds quite low, this is causing the pressure on the nose of the rocket to be higher then that at the rear of the rocket, making it want to flip around if there is any change in vector. So you need something to force the center of pressure back a bit, this is what those winglets are for at the bottom of many rocket designs. Once you start getting bigger rockets you will not have to worry about this as much as they will not accelerate as fast and run into the same problems, instead you will have whole new problems. But that is half the fun of KSP.

Generally with FAR I start my gravity turns at 100m/s or 1000m altitude, which ever is first. I dont turn more than 2-5 degrees at most, I gradually roll it into the direction I want it to go. So when I get to 10-15km altitude I am nearly 60deg. I will also throttle back to keep from over speeding, or going faster than mach 1 untill I hit 20km altitude, at that point I will throttle back up to full and let it burn itself out of fuel till I reach orbit or second stage.

[rhoark]

Another thing to try is disabling the forward fuel tank until after the turn.

[Hyomoto]

I'm going to second what most people have written, you have to be aware of terminal velocity. However, there is an alternative: start your gravity turn much later. In my pre-FAR days it was at 9k, but with FAR and depending on my payload, I may not start my turn until 15k. It may need to be a bit more aggressive than starting lower, but you get that back in stability. Besides, FAR requires a lower deltaV to reach orbit so your losses are minimal as it is. Once you hit that point the atmosphere is thin enough that even large changes won't introduce enough drag that the pure thrust of your engines cannot overcome it.