Subortibal launches in new aerodynamics (and the ideal TWR for lift-off).

[PTNLemay]

Sorry for making a thread just for this, but please, can someone tell me what I'm doing wrong?  Before 1.0 I was really good at liftoff, I could eye-ball a rocket onto any biome on kerbin without mechjeb or anything.  Estimate how much fuel I'd need, start the turn just right, coast, soft land (sometimes even without any parachutes).  I have a good... 300 hours logged, most of it in pre-1.0.  But the new aerodynamics are totally killing me.  Every single rocket I build starts tumbling, even the most aerodynamic arrow.  Straightforward launches to orbit are okay, because I just keep the pointing straight up for most of the flight, by the time aerodynamics might create issues when turning, the air is too thin for it to matter.  But suborbital ones need to start their turn much earlier, otherwise you're just bouncing up and straight back down.  



Originally I was using a kickback to boost me up, but that thing wants desperately to flip out.  It's too tall and flimsy.  I was hoping that a stockier rocket with a heavy engine bell would stay more steady, but it's flipping out as well (at twice the cost of my earlier design).   I must be missing something obvious, because I've seen players on YouTube start their turns quite early, in 1.0.5, and they don't flip out.  So please, what am I missing?

Edited January 20, 2016 by PTNLemay

[Halo_003]

Reduce TWR, at liftoff you want to be around 1.1, use SAS, and if needed you can use Vernor thrusters at the top of the rocket to help balance.

Edited January 18, 2016 by Halo_003

[Chakat Firepaw]

25 minutes ago, PTNLemay said:

Originally I was using a kickback to boost me up, but that thing wants desperately to flip out.  It's too tall and flimsy.  I was hoping that a stockier rocket with a heavy engine bell would stay more steady, but it's flipping out as well (at twice the cost of my earlier design).

 

That sounds like you are making a common error:  Remember that your rocket will pivot around its centre of mass.  It's a weathervane, not a pendulum.

So something tall, with weight at the top and something to generate a bit of drag at the bottom.

[PTNLemay]

I think what I'm doing wrong is I'm waiting too long.  Once the rocket has built up enough momentum, it's locked on the prograde vector.  Trying to pull it away from that vector seems to be what's causing the flip-outs.  So for these sort of suborbital hops, you want to turn over right away, as soon as you leave the  ground.  Then you've got your prograde marker set on the side.

I'll test it out and get back with the results in a bit.

@Halo_003
I'm not sure, but I think that 1.1 is way too low for your liftoff TWR.  I always find that 2 is a better number to aim for.
Granted, I'm flipping out like a scrub, so maybe I don't know what I'm talking about.  lol

Edited January 18, 2016 by PTNLemay

[Temstar]

TWR of 2 is much too high. With a high TWR what happen is your max Q happen earlier in the fight, in worst case max Q happen right about as you start your gravity turn which puts vast amount of dynamic load on your rocket, causing it to flip around as soon as it shifts away from prograde by a tiny amount. A lower TWR means max Q happen later in the flight which means the absolute amount of dynamic pressure at max Q is lower.

High TWR is also bad because you're not getting enough use out of your engines. Remember engines are expensive and fuel are cheap, so for a given amount of delta-V it's always cheaper to use more fuel and less engine, up to the limit of your mission (ie TWR > 1 for launch vehicles).

Personally I favour a TWR at lift off of about 1.3

[KerikBalm]

4 hours ago, PTNLemay said:

Originally I was using a kickback to boost me up, but that thing wants desperately to flip out.  It's too tall and flimsy.  I was hoping that a stockier rocket with a heavy engine bell would stay more steady, but it's flipping out as well (at twice the cost of my earlier design).   I must be missing something obvious, because I've seen players on YouTube start their turns quite early, in 1.0.5, and they don't flip out.  So please, what am I missing?

The heavier the engine, the more likely it is to flip, because that mass is all the way at the back, you want your mass as far forward as possible.

Also, you want your fins as far back as possible (leverage), so a short and stout rocket doesn't help there either. The kickback's only drawback is that it lacks thrust vectoring, which can help point your rocket and allow SAS to compensate to some degree for aerodynamic instability.

But I think its likely that you have too much TWR and don't start your turn early enough, and then try to force it to deviate too much from prograde

[Jetski]

If you slap on a big swept wing as a fin, with a tail fin below it for steering, symmetry x4, pretty much anything will fly straight.  Probably not perfectly efficient, but you will not flip over.

[Frozen_Heart]

There is currently a glitch with fairings that is making rockets much more unstable than they should be. Rather than the centre of drag being in the middle of the fairing, its beyond the end of its nose making a huge turning force on the rocket if you don't face prograde.

[Guest]

TWR of @ 1.3 on the pad (1.2 is as low as I go, 1.5 is as high as I go).  Tilt 5 degrees at @ 50m/s, then follow the prograde marker.  I generally make sure I'm near 45 degrees by the third atmosphere band, plus or minus 5 deg.  Once you're in the third atmosphere band you can adjust your trajectory pretty freely.

[Snark]

On 1/18/2016 at 7:22 PM, PTNLemay said:

I was hoping that a stockier rocket with a heavy engine bell would stay more steady

Actually, it's the exact opposite.  You want the heavy stuff in front.  Putting something heavy on the back will make it unstable.

You want your CoM to be as far forward as possible.  For example, you have those two Science Jr. modules sitting right up at the top of the rocket.  This is the worst possible part to put up there:  it's very big and very light (just 0.2 tons) and is a recipe for instability.  (For that matter, why do you have two of them?  You only need one, just make sure you crew your ship with a scientist so you can reset it.  This will allow you to shrink that fairing by a lot, which ought to help aero stability significantly.)

The great big heavy orange tank isn't helping much, either.  It's far heavier than that very light, bulky lander you've got strapped to the front.  For example:  you may want to consider a lighter tank with less fuel, but have some radial SRB boosters down at the bottom to provide lift off the pad.  Since they're located down low, the CoM of your ship will move upward significantly as they burn their fuel.

Also, as others have said:  your TWR is far too high.

[RizzoTheRat]

On ‎18‎/‎01‎/‎2016 at 5:13 AM, PTNLemay said:

@Halo_003
I'm not sure, but I think that 1.1 is way too low for your liftoff TWR.  I always find that 2 is a better number to aim for.
Granted, I'm flipping out like a scrub, so maybe I don't know what I'm talking about.  lol

Interestingly Saturn V rockets launched at just over 1.1 (7.5M lb thrust, 6.5M lb craft)

Reading this thread I think I'm often launching with too high TWR too

[Vegetal]

On the matter of TWR: These are just general guidelines. I have some launch vehicles that go a little beyond 2 and the launches are very efficient.

The trick is to use launch clamps and give the vehicle some starting inclination on the VAB. Most of those vehicles are small shuttles launched with 2 kickbacks, and delta V wise, high TWR is the most efficient, and it allows me to launch those shuttles very cheaply using SRBs in my career game.

They follow a standard path during the first stage though, that is dictated by the starting conditions, so they aren't very flexible for going into specific orbits.

In that case, you want a standard rocket that can be easily steered, and high TWR is *not* your friend in that regard. The higher the TWR, the sooner you want to start your turn - that might be your main mistake here - so that you can keep close to prograde all the time.

[Guest]

2 hours ago, RizzoTheRat said:

Interestingly Saturn V rockets launched at just over 1.1 (7.5M lb thrust, 6.5M lb craft)

Reading this thread I think I'm often launching with too high TWR too

Conversely, something like the Minotaur V rockets off the pad at 1.8 TWR.  You have to adjust your launch profile to match performance for best efficiency.  If your typical launch profile favors craft with a certain TWR it's probably best to stick with that if you're all about efficiency.

Of course, launching craft with different TWRs can also be a good learning experience to help you understand launch profiles.  The stock/vanilla game doesn't do it well but when you're playing Realism Overhaul and your launches are anywhere from seven to fifteen minutes long you really start to notice these little things.  Try launching an actual Saturn V manually in an RO install; the early launch is fairly straightforward but the upper stages are anemic, so you almost have to use the first stage to loft the rest up to a decent apoapsis so it has time to circularize as it falls down.  I used basically the same technique with my Angara 5 replica, although the A5 was much easier to fly, for me, than the Saturn V.

[suicidejunkie]

What I usually do is have a rocket with a low starting TWR, but then hit it with a bag of Hammers on the bottom to bring it up to 250m/s straight off the launch pad.  The main stage then just acts as a sustainer through the thick air, and higher TWR arrives as the atmo and fuel remaining both thin out.

 

... I might be a little too paranoid about supersonic rockets after jackknifing a skylab at 10km up ...

[PTNLemay]

I remember reading somewhere that the more you idle near the gravity well (in the case of Kerbin, low in the atmo), the more you suffer from "gravitational drag".  It was on Atomic Rockets: http://www.projectrho.com/public_html/rocket/blastoff.php#id--Acceleration

That page says that ideally, we want 10 Gs.  Is the 1.1 G limit  you guys are suggesting primarily based on air drag?   On an airless world do we just burn as hard as we can?   And are you suggesting that you keep 1.1 G all the way up until you circularize?  Because... that feels really slow.  I haven't done a rigorous side-by-side comparison, but it feels like I'd be bleeding off a lot of fuel during the ascent.

@ the heavy engine bell
Yeah... I don't know where my brain was.  The arrow needs a stone on the front, feathers on the tail.  I must have been drunk when writing that sentence.

Edited January 20, 2016 by PTNLemay

[MarvinKitFox]

39 minutes ago, PTNLemay said:

That page says that ideally, we want 10 Gs.  Is the 1.1 G limit  you guys are suggesting primarily based on air drag?   On an airless world do we just burn as hard as we can?   And are you suggesting that you keep 1.1 G all the way up until you circularize?  Because... that feels really slow.  I haven't done a rigorous side-by-side comparison, but it feels like I'd be bleeding off a lot of fuel during the ascent.

Ideally, you want infinite Gs.  Fuel-wise ideal, that is. If you are flying in a vacuum.

Flying through the atmosphere at 10Gs would effectively turn the air in front of you into a granite cliff. Bad idea.

 

In Ksp, you want to get off as fast as you can, without forcing matters. If you see more than the faintest whiff of aero shockwave around your rocket on the way up, you are likely overspeeding.

If you see red flames flying past, and you are still in the first atmospheric band, then you need a straitjacket. * I suspect your initial design might fall in this class, due to the TWR at launch of 3.31 !

 

With a sensibly designed rocket (long & pointy, heavy in front), your optimal TWR can be as high as about 2.2 

However, you may find control a bit of a challenge.

The same rocket at TWR of 1.6 at takeoff will fly like a dream, and still only sacrifice 200m/s of deltav *at most* due to increased gravity loss.

 

 

* Tested from your design.

If you launch at full throttle, you have bigtime aero shockwave by 2500m alt, and red stuff flying by at 10200m.

This is way way way WAY way too much too fast.

 

Take that exact same ship, throw away the Mainsail engine. Instead strap in a Skipper.

Now you have a rocket that can go to orbit, fly to Duna, and enter orbit around Ike. But has insufficient Deltav to actually land on Ike.

Edited January 20, 2016 by MarvinKitFox

[Snark]

7 hours ago, PTNLemay said:

That page says that ideally, we want 10 Gs.  Is the 1.1 G limit  you guys are suggesting primarily based on air drag?   On an airless world do we just burn as hard as we can?   And are you suggesting that you keep 1.1 G all the way up until you circularize?  Because... that feels really slow.  I haven't done a rigorous side-by-side comparison, but it feels like I'd be bleeding off a lot of fuel during the ascent.

There are two reasons why 10 Gs is a Bad Idea™.

One reason is as folks have mentioned, that at that TWR the air is a solid wall.  So this reason wouldn't apply on a vacuum world.

The other reason, however, has nothing to do with air and would apply whether you're on a vacuum world or not:  If you're pulling 10 Gs, you're lugging around FAR too much engine mass.  Engines are dead weight, and carrying more of them than you need is counterproductive.

[PTNLemay]

@Snark

True... but think about how bada** that vehicle would look.  lol

I don't know if any of you are watching the new Expanse TV show, but in it they have strike crafts that can regularly go up to 12Gs.

[ExtremeSquared]

Keep in mind that fairings are still buggy to the point of being useless. Until fairings are implemented, it pays to over engineer the craft's deltav and keep atmospheric velocity low.

[RizzoTheRat]

12 hours ago, MarvinKitFox said:

In Ksp, you want to get off as fast as you can, without forcing matters. If you see more than the faintest whiff of aero shockwave around your rocket on the way up, you are likely overspeeding.

If you see red flames flying past, and you are still in the first atmospheric band, then you need a straitjacket. * I suspect your initial design might fall in this class, due to the TWR at launch of 3.31 !

According to this thread the flame effects aren't properly linked to the aero model so might not mean as much as you think

http://forum.kerbalspaceprogram.com/index.php?/topic/130137-whats-the-best-launch-trajectory-again/

13 hours ago, PTNLemay said:

Is the 1.1 G limit  you guys are suggesting primarily based on air drag?   On an airless world do we just burn as hard as we can?

Yes, it's a tradeoff between atmospheric drag and the weight of fuel needed to fight gravity (the quicker you go the less time you need to be burning against gravity, if you see what I mean).  On an airless world pin the throttle to stops and point at the horizon as soon as you know you're going to clear any inconvenient mountains.

[Streetwind]

14 hours ago, PTNLemay said:

I remember reading somewhere that the more you idle near the gravity well (in the case of Kerbin, low in the atmo), the more you suffer from "gravitational drag".  It was on Atomic Rockets: http://www.projectrho.com/public_html/rocket/blastoff.php#id--Acceleration

That page says that ideally, we want 10 Gs.  Is the 1.1 G limit  you guys are suggesting primarily based on air drag?   On an airless world do we just burn as hard as we can?   And are you suggesting that you keep 1.1 G all the way up until you circularize?  Because... that feels really slow.  I haven't done a rigorous side-by-side comparison, but it feels like I'd be bleeding off a lot of fuel during the ascent.

@ the heavy engine bell
Yeah... I don't know where my brain was.  The arrow needs a stone on the front, feathers on the tail.  I must have been drunk when writing that sentence.

The thing is this rule of the thumb: overloading your first stage is free.

What do I mean with this? Well, assume you have a rocket with a TWR of 2.0 on the pad. What, praytell, keeps you from adding fuel to the first stage of that rocket? Sure, launching with a higher TWR is more efficient in the sense that you need less dV to orbit. But, you know, you could simply add fuel and have more dV in the first place. Fuel is pretty cheap, after all. In fact, adding more fuel generally adds more dV than you lose to a less efficient launch, until you get down to somewhere between TWR 1.2 and 1.1.

And in a world where the ideal multistage launch vehicle has the same fuel mass fraction in every stage, and overloading one of the upper stages negatively impacts all of the stages below it, the first stage is the only one which can deviate safely from the ideal. So why not fill it up to the brim?

IRL there are a lot of factors that you don't generally encounter in KSP, because launching in KSP is orders of magnitude easier than it is IRL. But even then, this factor - overloading the first stage is basically free - is sometimes the best course of action, resulting in a rocket that lumbers ponderously off of the pad. Like the Saturn V did.

[Sovek]

That TWR is way way WAY too high. Its way too easy to get going way too fast too low, and ANY deviations from the direction of travel results in alot of drag along the side of the rocket, which will push it even further down and by then, its too late.

I never play with FAR, so I don't know too much about the stock aero model, but I've found that clustering engines with a low TWR will make your rocket pretty stable, and pretty steerable compared to slapping a couple fins on the back. I also never launch with that kind of TWR. 1.3 at the most for me (atmo), I get to space just fine even when on a 3x scale system. I do what @Streetwind suggested. I overload stages, even to the point of bringing it BELOW a 1 TWR, for this I'll stick some solids on it and get it off the pad much like the STS needs just in order to reach orbit.

Let me walk you through one of my launches in a 3x scale system. To demonstrate, I have a (simulated) launch of my newest rocket in my career mode, which can be found here.

http://forum.kerbalspaceprogram.com/index.php?/topic/130514-how-sovek-launches-rockets-in-a-scaled-up-solar-systems-and-rf/

 

[Waxing_Kibbous]

Try it without the fairing http://forum.kerbalspaceprogram.com/index.php?/topic/130267-are-fairings-useless/

 

[Levelord]

To the OP, this might help shed some light for you:

On 1/19/2016 at 8:51 PM, Warzouz said:

In 1.0.5 (not in 1.0.4) there is a but with fairings. The body lifting force is applied far in front of the fairing (outside of the rocket !), then the rocket would flip MUCH faster if you use a fairing than if you don't. As soon as you quit prograde, the rocket with start a unrecoverable flip.

I stop playing KSP mostly because of that bug. I'm waiting for 1.1

In light of this, there is also a fix that helps sort out the fairings from producing unnecessary lift in front of the craft::

Stock Bug Fix

Edited January 23, 2016 by Levelord

[Laie]

First off, I suspect your problem may be a matter of technique: don't just launch up then turn, but start your turn early and gently -- or at least, comparatively gently. By the time you're subject to serious aerodynamic forces, say going 100m/s+, you don't want to deviate much from prograde. Ideally not at all, but it's in the nature of ideals that they're pretty hard to achieve.

Much has been said already about how higher TWRs are energetically better, at least up to a point. Kerbin is a small world with little atmosphere, so TWRs in the game can be a lot higher than in real life. Looking at entrants in ascent challenges, the sweet spot seems to be somewhere around TWR 2-2.5. However, such a rocket makes for quite a frantic launch where you have to turn aggressively right off the pad. Besides, that's "efficiency" measured as "least dV to orbit"; you may find that if you optimize for other values (like... cost?), a lower TWR may well be better. In terms of dV, going from TWR=2 to TWR=1.4 will only require an additional 200m/s or so.

Lastly, you may want to have a look at the new "Gravity Turn" mod (this week's modding monday) -- it tries to guess a good ascent profile for any lifter and does so very well. Even if you don't want to use it in the long run, it's a great teaching tool in that it shows you what a good turn looks like.