TWR for each ascent stage

[rolypoly]

Has anyone done some research to find the best TWR for each part of your ascent? Like:
Starting TWR
After 10km - TWR
After 30km - TWR
and so on?

[Chibbity]

Gravity losses are greater than aero losses so really the faster the better; within reason of course.

2.5ish is pretty normal for atmo launch by most players standards I think.

I'm sure there is a detailed chart out there if that sort of nitpicking interests you.

[Enorats]

2.5 starting out might be a bit much for me, maybe 2.0 is normal. Too far above that and you start having to throttle down a lot, in which case you may as well add more fuel for extra delta-v or use a smaller engine.

My sustainer stage tends to be 1.0 to 1.5 starting out. You're less worried about losses at that point, it's just a matter of finishing the orbital burn before you start descending.

Space stages tend to be 0.5 to 1.0 starting out. I have no desire to split nodes into multiple passes.

[rolypoly]

So does your speed through the atmosphere not have any effect on the gravity turn? Or is that entirely based on how long it takes you to get through to a particular altitude where gravity would play a lesser role?

I'm trying to minimize attitude corrections during ascent and let gravity do the entire turn. Is that a possibility?

[Hate Bear]

You can get away with different values because you can bend the rules in KSP. I tend to keep things closer to realistic (despite not running RSS). I prefer to launch at 1.5 (with the LFO main engine in the center's thrust set to ZERO I check outside bottom-level booster and set THAT thrust limiter only for 1.5 TWR total on the vehicle). With the boosters set accordingly, then turn the thrust limiter on the main liquid engine back up to 100%.

Now, when you launch, you can run full-throttle until you get air resistance at Mach 1. I throttle the liquid engine down (exact performance varies by vehicle, but it is a good estimate). This tends to balance gravity versus drag up to about 10km-15km altitude with stock distances. At around that time, you will usually have emptied your boosters (and any possible extra LFO on top of them, lol) and can jettison them the moment they hit 0 fuel. Make sure your main engine throttle is up for this, though (you want to get away from them as soon as possible).

Any TWR above 1.2 will work adequately for the middle stages, but most stock mid-stage engines and most balanced mod mid-stage engines have a good fuel efficiency setup for rapid burning, so I tend to go for a higher TWR like 1.5 again. It is nice to be able to get up to suborbital altitude and velocity rapidly also, in and of itself (it gives you more time to calculate a circularization burn at Apoapsis).

The limiting factor with not having drained your middle stage completely for THAT (I usually overbuild missions at least a bit for safety) is that you generally will need to start the circularization burn a bit earlier than calculated (which can highly vary by craft). When in doubt (or especially if using a gigantic lower stage that you actually haven't jettisoned yet), set the thrust limiter on it DOWN a bit (even as low as 25%), and run the burn calculated off of that. That way, when you jettison it dry and switch to your vacuum stage(s), you will have more time to complete the burn, rather than shafting yourself into possibly falling back into the atmosphere with some highly efficient, weak, expensive engine.

[eragon8864]

As long as your TWR is 1/0 or greater throughout the entire burn and you aren't accelerating too quickly (early on and potentially overstressing your craft) and as long as you don't accelerate too slowly at the top of the turn (and thus either burn up or fail to achieve orbit), then you're doing pretty well. I generally have anywhere from 1 to 2.5+ TWR at any given point - just depends how crazy over-engineered I've done a stage/how efficiently I've done various burns etc. I know I actually had one rocket that no matter what I did, I couldn't get to have anything above a I believe it was a 0.97-0.99TWR on the launch pad so I finally said screw it - slapped on some launch clamps, raised the overall height the rocket rested at, adding a couple more launch clamps to support the middle sections, an djust launched it - ended up having to activate the engines and burn for about 30 seconds or so and once it hit 1.0+ I just staged the clamps and off it went, ever so slowly starting to climb away from blowing itself up on all those big try not to hit us sticks that had kept it off the ground before then :P Honestly, long as you get to space and stay there, you're doing good. Even better if you make it back down safely :P

[Enorats]

Originally posted by eragon8864:

As long as your TWR is 1/0 or greater throughout the entire burn and you aren't accelerating too quickly (early on and potentially overstressing your craft) and as long as you don't accelerate too slowly at the top of the turn (and thus either burn up or fail to achieve orbit), then you're doing pretty well. I generally have anywhere from 1 to 2.5+ TWR at any given point - just depends how crazy over-engineered I've done a stage/how efficiently I've done various burns etc. I know I actually had one rocket that no matter what I did, I couldn't get to have anything above a I believe it was a 0.97-0.99TWR on the launch pad so I finally said screw it - slapped on some launch clamps, raised the overall height the rocket rested at, adding a couple more launch clamps to support the middle sections, an djust launched it - ended up having to activate the engines and burn for about 30 seconds or so and once it hit 1.0+ I just staged the clamps and off it went, ever so slowly starting to climb away from blowing itself up on all those big try not to hit us sticks that had kept it off the ground before then :P Honestly, long as you get to space and stay there, you're doing good. Even better if you make it back down safely :P

If you start at anything below 1.2 or so on the pad you really should go back to the drawing board. At that point you're practically hovering on the pad spending most of your energy just fighting gravity.

Almost all of my launches start between 1.5 and 2.0.

[Yuki]

Originally posted by Enorats:

If you start at anything below 1.2 or so on the pad you really should go back to the drawing board. At that point you're practically hovering on the pad spending most of your energy just fighting gravity.

Almost all of my launches start between 1.5 and 2.0.

Craft dependent. There are designs that go even lower.
If you use SRB's exclusively for an extremely cheap first stage for example. They dont stay at 1.09 for long and it might be necessary to lower their thrust this much to prevent G-Force blackouts towards the end of their stage.
Starting these however is indeed not beginners material. And personally i would not call them "well designed", but they have their uses. For certain challanges e.g.

[RoofCat]

you need TWR above 1.5 to lift off without extra waste on gravity. Which is your main enemy.

The higher your TWR, the less REAL delta v you will waste during launch. Though

high TWR ~ low total delta v (theoretical limits)

It doesn't matter how fast you can launch in the end. Rather how far will you go. That's why TWR is not that relevant. So just have it at 1.5+ while going up. And once reaching orbit it is even less relevant. All you want is more delta v.
TWR issues there can be solved with split burns and higher orbits. You don't fall from orbits so your TWR can be anything. Actually since you are in microgravity, it will still be a number (not divided by a zero weight). Though no one cares about TWR on orbit. Rather acceleration per sec, but even that is not really important. TWR is important only launching and landing - closer to the surface.

During launch as you reach shallower trajectories you will need less power to achieve the same acceleration due to less waste on gravity and your TWR will grow on its own (less gravity=less weight). Also you burn away fuel (mass) which makes your TWR grow too. And then upper stages usually have much smaller TWR, because they are built for max delta v and don't have to lift much from high G surface in vacuum (except you go to Tylo).

btw to the right of NavBall there is the green starting, red ending G meter. While launching from Kerbin it actually equalls TWR meter for a while. Because Kerbin gravity is 1 G. It isn't perfect and for some reason never goes below 0 even though it could nor does it help with TWR on other bodies, but should be enough to build and launch properly your first rockets.
That's about beginners in KSP and simple visual tools at their disposal.

You really don't need math to start this game. And once you get good, you will understand the math too. Practice and have fun.

[rolypoly]

So you're saying that it doesn't matter how much thrust the rocket outputs, I'm always going to end up with the same gravity turn? I don't find this to be true.

If I pitch 5 degrees to East at 100m/s and let go, then depending on whether I have high or low TWR my gravity turn will be different.

Too high, and the turn is very shallow and I end up in vacuum without much horizontal velocity.
Too low, and it's possible to have your nose get pointed below the horizon and never even reach space.

That's why I'm trying to find the sweet spot. Or understand how to calculate it.

Originally posted by RoofCat:

You really don't need math to start this game. And once you get good, you will understand the math too. Practice and have fun.

Math is precisely what I'm asking for. I've already played this game for very long (500hrs+), now I want to understand the math that goes into it. I am actually using a mod called Kerbal Operating System to write programs to automate launches, and from my observations so far it seems like the most efficient way (least fuel spent) to get to orbit is to allow gravity to do your turn. However, either your acceleration or your TWR (and they are closely tied together of course) has a great affect on what kind of turn you're going to end up getting. I'm trying to understand how to achieve the "perfect" turn in terms of TWR.

[Yuki]

I doubt there is simple math suitable for a forum, its a university study subject for a reason. And it still keeps being very craft dependent. There is a reason for different launch profiles too.

BUT- for your specific problem: If you want to program your launchers beforehand you may want to take a look at the mod Gravity Turn Continued (Catchy name isnt it?) and its approach to it, especially its key values. Not "to use it instead", but to take a look at how it works, especially the reiteration over one and the same craft over several launches. Its Dev thread holds lots of information too, including math.

[RoofCat]

the question is what are you trying to achieve? To reach LKO with least delta v, you need high TWR. Quick, high "impulse" propulsion.
http://steamcommunity.com/sharedfiles/filedetails/?id=927437439
But once it reaches orbit, there is no fuel left for anything else!
Shallow launch curve (less wasted on ascent and gravity hovering) is cheaper. You also need stronger engines to be able to sustain that curve without nosediving. So you will also have to find the spot where your ship goes as fast as possible, yet not exploding nor reaching forbidding drag values. TWR (dynamic mass and gravity) vs. heat (atmosphere temperature changes with altitude as does density) vs. drag (grows as speed²).
And then to optimize that system. Also called rocket science :)

Most of your rockets have to go somewhere. Not just LKO. Again
high TWR ~ lov delta v

You can have high TWR on first stage and low on orbital one. Doable. Will reduce some delta v waste during launch. A bit. But it will be even cheaper if you use just regular TWR ~1.5 launch to LKO. You need a lot more power (engines, thus also fuel), to save just a bit on gravity waste. Slower launch stage will be less efficient in terms of some m/s waste, but more efficient in terms of price and simplicity. Also total delta v. And once you go below TWR 1.5, empty tanks will start to punish you with dry mass except you find a way to drop them all... which adds decouplers (launch mass and money). So ~1.5 it is.

Best theoretical ship has just enough fuel to complete the task. But most players wouldn't want to fly those. All your ships will have reserve, that's why the whole math thing loses any value except you just like to do it anyway. You don't go to orbits with 3200m/s ships. You have 4000m/s and chill.

[flying diomedea]

Originally posted by Hot Butter:

Math is precisely what I'm asking for. I've already played this game for very long (500hrs+), now I want to understand the math that goes into it. I am actually using a mod called Kerbal Operating System to write programs to automate launches, and from my observations so far it seems like the most efficient way (least fuel spent) to get to orbit is to allow gravity to do your turn. However, either your acceleration or your TWR (and they are closely tied together of course) has a great affect on what kind of turn you're going to end up getting. I'm trying to understand how to achieve the "perfect" turn in terms of TWR.

Math is actually pretty easy once you know what to look for. Which isn't TWR per se.
The best launch profile, beyond always being a gravity turn, is the one where the sum of gravity and drag losses, integrated over the whole profile, is the minimum. Gravity losses are pretty easy in math terms (sin(pitch)GMm/r^2); drag losses as well (1/2ρV^2ACd). Now, V (craft velocity) changes because of the sum of all forces applied to the craft (gravity, thrust, drag, lift) / mass of the craft; this is just a vectorial sum but real easy as well to put in math form.
What isn't easy at all is to compute a closed integral for the above, therefore you won't have one simple differential equation to compute: this is better done through numerical analysis. One reason is, velocity changes with those forces, but then influences drag. Position changes with the integral of velocity, and then changes gravity and air density.

In terms of TWR, an ideal craft would be built to have just the correct acceleration at full throttle during the whole ascent. This generally goes contrary with the increase of TWR (in any stage) due to mass ejected. Indeed, The minimum possible TWR is what just compensates gravity losses (therefore 1.0 at ground, 0.0 in orbit) so TWR profile goes contrary to our needs with any stage: we have to build different stages with decreasing TWR. Clearly we also have to exceed that minimum TWR for the craft to climb, but then (as some correctly posted) it depends on DV (or rather, the ISP with engines): choosing a lower TWR but higher ISP allows to burn longer: what will then be the final altitude? (it's just the integral in time of vertical velocity from the previous vectorial sum, sorry again it requires numerical analysis to compute). Whatever combination of engines gives the best final altitude, is the better choice.