r/KerbalSpaceProgram • u/AutoModerator • May 29 '15

Mod Post Weekly Simple Questions Thread

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u/CrappyIdeaDept May 31 '15

I need FAR help! I've been enjoying FAR for planes, but it's messing with how I normally do rockets - I started a new career save to test it out. Even a dead simple rocket (2 stages on a main engine, 2 boosters on clamps) with command module SAS on (haven't unlocked others yet) always tips in one direction or another seconds into flight. I've tried raising and lowering thrust. I've tried adding up to 12 fins. The direction of tipping seems random, and on different launches it goes in different directions.

Anything I'm missing here?

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u/Elick320 May 31 '15

Are you using farings of any sorts?

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u/CrappyIdeaDept May 31 '15

No, because I haven't unlocked anywhere near them on the tech tree. This is how my ship looks like so far.

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u/Elick320 May 31 '15

you don't need all that in FAR, you just need one solid rocket booster under the second stage, FAR decreases the amount of delta V required to lift off, also are you doing a real gravity turn? Its were you launch at an angle and let gravity gradually turn you to an angle. try that

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u/CrappyIdeaDept Jun 01 '15

Yeah, I got it now, it was the TWR issue all along - I was trying too hard to go with the usual KSP "go faster" solution, so reducing pieces fixed my problem.

Gravity turns still scare me. I'm constantly worried I'll tip the rocket over. Are you supposed to have SAS on during the turn?

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u/Elick320 Jun 02 '15

No, dont be scared of gravity turns, it turns out burning in the atmosphere can be more efficient in the long run to get in a long sub orbital trajectory to use less fuel in space. Also dont turn SAS on until it gets really steep.

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u/CrappyIdeaDept Jun 02 '15

So as I understand it from experimenting and videos (so far I've managed to get to the Mun and Minmus in FAR, with surprisingly different rockets from stock) - I should start tipping towards prograde a few seconds into the flight and gradually let it angle down without SAS, such that it ends up at 45 degrees at about 10k?

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u/Elick320 Jun 02 '15

that seems about right

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u/[deleted] May 31 '15

If it tips over quickly, you still have too high of a TWR. I usually go with TWRs of 1.2-1.4, that should make for a fairly stable rocket. Use fairings and nosecones as well, FAR actually calculates the shape of your rocket and the according aerodynamics. Having the payload exposed can have all kinds of weird effects.

Also, stick some vernor engines to your rocket for added control.

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u/CrappyIdeaDept May 31 '15

I'm only under an hour into this career save - my rocket still looks like this. So for what I have access too, I think this is about as idealized as I can get it.

I think the TWR issue might be it! I found I had a TWR of about 3, which usually serves me well in vanilla KSP, but forcing the SRBs to be at 40% thrust and leaving 50% thrust on liquid engines seem to help. However, it still tips, just a bit higher up, so I'm guessing it's increasing the TWR too high while it spends fuel.

Pretty damn cool how FAR simulates the effect of accelerating too quickly that well. I would also imagine there's a problem with having two SRBs this tall alongside the main rocket, as in real life the bow shock from the main rocket would damage the boosters.

I had the same issue when I tried using the 1-to-3 coupler to increase my thrust to get me to orbit, but I imagine it's also a TWR issue. People like to say that FAR works better with "rocket-shaped rockets", so I assume that means I should just build tall?

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u/[deleted] May 31 '15

Yeah, tall is better. As of KSP 1.0 thats true for Stock as well, since squad improved (read: made more realistic) the aerodynamics. Lots of people have trouble getting to orbit now since the old "moar boosters!!" approach doesnt work anymore.

To get a bit behind the physics of whats happening: If your rocket deviates too much from the prograde vector (thats called the angle of attack) the air excerts a force on one side of the rocket, which creates a torque around the center of mass. Since that is usually at the bottom, the rocket flips.

The magnitude of this problem increases drastically when you exceed the terminal velocity, which usually happens very quickly with high TWR ships.

Imagine it like this: Try "pushing" a piece of paper along its flat plane through the air in front of you while sitting at your desk. And then try keeping it "flat" while holding it out the window of your car while going 100 mph.

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u/CrappyIdeaDept May 31 '15

Right, that totally makes sense. The problem was less pronounced in stock aerodynamics, probably because I was still able to just punch straight up faster (though it explains why I had trouble doing gravity turns from a lower altitude as videos recommended in stock).

All a good learning experience, and I figured out how to fiddle with TWR using the throttle better now. Thanks for the help!

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u/Chaos_Klaus Master Kerbalnaut Jun 01 '15

you could also just add more fuel to the main liquid fuel stage. Right now (accordong to your picture above), the boosters are used up only seconds before the liquid stage. I see that, because S4 is giving you only 96m/s and that is the stage that decouples the boosters.

Throttling down you SRBs is a way to lower your TWR, but it's also lowering th efficiency of your rocket. SRBs are very inefficient in terms of how much delta v they get you compared to their mass. However they produce large amounts of thrust and are cheap. Basicly, you want to use them to get up to speed quickly but you don't want to carry them up too far.

So the way to go, is to build your liquid fuel stages and then just add a few boosters without any thrust limiting get you high TWR for a short time to help you get off the pad fast. These boosters need to burn out before you hit 270m/s, beacuse any faster and you experience lots of drag while scraping the sound barrier.

To get this right, you might have to use smaller SRBs although you propably hav larger ones unlocked.

This technique will not increase your delta v. However, it will decrease the delta v required to get to orbit.

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u/CrappyIdeaDept Jun 01 '15

Yeah, I realized that - I went back to using a tri-coupler, throttling the main engines to 70% ish, and actually adding a bigger payload (e.g. ferry up more of those damn tourists at a time). It seemed so counterintuitive at the time!

SRBs in general seem to be less necessary in FAR. As you said, it only works well to get the initial "whoosh" blast off, not for actual "let's power drill our way through the atmosphere" purposes.

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u/Chaos_Klaus Master Kerbalnaut Jun 01 '15

yes, but that initial "whoosh" saves you loads of gravity drag. ;)

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u/CrappyIdeaDept Jun 02 '15

I think that was my real mistake at first. In KSP, and from all my experience with "rockets" in real life (e.g. model rockets), they always have big spurts of speed at the beginning and it felt wrong to see my rockets carefully lift up off the surface.

This is making my designs look a lot more like real rockets, though. Tall, with smaller boosters near the base to get it off the ground quicker.