TWR - a general question.

[Snark]

@paul23: Alshain has a point: a perfectly tweaked gravity turn that doesn't require any control input on the way up is a very finicky design problem and is really hard (read: almost impossible) to get right. So in practice, either you need to use some control authority on the way up to tweak it, or else you need to do many many trial-and-error launches to find the sweet spot. And different people have different patience thresholds for the trial-and-error method. Lack of gimbal on SRBs makes them a challenge this way.

Myself, I'm fine with using SRB-only first stages, though admittedly I'm fairly OCD and may have a higher patience threshold than most.

For little ships (things that take off with a single Hammer or Thumper), I can generally steer well enough with the torque from the command pod.

For slightly bigger ships up to a few dozen tons, adding a reaction wheel or two does the trick.

For the big guys-- the ones whose first stage is eight Kickbacks and the second stage is a Mainsail under a Big Orange Tank-- I add four steerable fins (AV-R8 Winglet is great for this) down at the bottom of the Kickbacks, and that works wonders. (The steerable fins are pricey, which is why I don't use them for small ships, but for a really big ship the cost of four winglets is a minor investment).

But all said and done, it's still a lot less torque authority than having a gimbaled engine would, which means a more finicky control task, which in turn means a lot more trial-and-error practice to develop a workable technique. So it may be better for people with a higher boredom threshold, like me.

[KerikBalm]

And actually, SRBs are good for stability for just this reason, in a somewhat subtle way:

An SRB is a very lightweight "can", full of fuel. It's a cylinder, and its CoM is right in the middle of the cylinder, and stays there the whole time it burns. In other words, its CoM stays stationary as it burns its fuel.

LF designs are even worse if you have two LF tanks stacked on top of each other, as the mass is lost from the top first, and the CoM shifts faster earlier on (unless you tweak it to drain out of the bottom tank, and then open the top tanks when the bottom is nearly empty)

[GoSlash27]

Oh geez, I would never ever do that no matter how much cost it saved. That's just painful to fly. No gimbal = no go for me, I'd never get an efficient gravity turn on those things and my cost savings would be swallowed.

Alshain,

There's no wrong way to play this game, and far be it from me to tell you how to run your railroad.

But for me *personally* I have no problem getting SRBs to behave just as well as LFBs. They're *definitely* not "painful to fly" and I have no problems getting efficient gravity turns out of them.

Best,

-Slashy

[Alshain]

Alshain,

There's no wrong way to play this game, and far be it from me to tell you how to run your railroad.

But for me *personally* I have no problem getting SRBs to behave just as well as LFBs. They're *definitely* not "painful to fly" and I have no problems getting efficient gravity turns out of them.

Best,

-Slashy

Don't mistake, I never said your way was wrong, just that I would never do that (I mean, I've tried it before and never will again)

[GoSlash27]

Don't mistake, I never said your way was wrong, just that I would never do that (I mean, I've tried it before and never will again)

Alshain,

Oh yeah. I know. It's just that you stated that they would be painful to fly and you'd never get a proper gravity turn out of them and neither statement is factually accurate. Or at least neither statement is factually accurate for anybody but you.

They actually fly just fine and have no problems executing a proper gravity turn so long as they're properly designed.

All I'm saying is what I said earlier: They are much cheaper for disposable booster stages than LFO, and dramatically reduce launch costs for multistage boosters. The fact that you *choose* not to use them is a completely different issue and doesn't make it any less true.

Best,

-Slashy

[Sovek]

I might have been one of those that mentioned that 1.15 TWR.

I like keeping mine in the 1.15-1.2 range. Any higher and its nigh impossible to get the craft to turn smoothly, and turning too soon/too low/too fast in FAR results in a craft that at best... goes level with the horizon, at worst.. points straight down. Keep in mind that I'm playing with KScale 2, and in lower funding career. Engines are not cheap, even more so when fueled with hypergolic fuels. I may have slightly more powerful engines that need to be toned down, but it allows me to pack more fuel in for heavier loads while using the same basic design.

For example... My Hoshiryuu rocket.

As you can see... this craft had a vacuum rated T:W of 1.3, ASL is about 1.15. It uses 6 of the KW Wildcat V engines, 2 on the center stack, 2 each on the boosters. The side booster engines were cut down on the thrust to allow for a 2 minute burn time. Any shorter and the core wouldnt have enough T:W and looses several HUNDRED m/s of dV fighting gravity. Sticking more powerful engines wouldn't be the answer either as the core stage burns out before the side boosters, or none are efficient enough at sea level. I didnt have the tech for wider fuel tanks either, so it was either this.... or a longer rocket that I couldn't launch because of the pad limitations.

This rocket can get about 20t into orbit, that includes a lander with a heatshield, 3 kerbals, a crap TON scientific equipment, a lander with more than enough dV for Minmus, and the return trip. Total cost of each launch I estimate at about 25K, the side boosters could be recovered for 9K each, the CM and its scientific equipment would be returned to kerbin, for about 40K.

So, long story short, I was limited in options and the lower T:W actually helped more than hurt. Yeah I might be loosing some, but its not bad. And when I can design every rocket around the same ratio's, it means I dont have to change how this one flies compared to the next generation equipment.

[Tyren]

That SRB issue:

I usually follow the Ariane concept, an efficient first stage with a low price/dV setup with additional SRB to keep right below terminal velocity. With a little bit of fiddling it is no problem to drop those SRBs right after getting a TWR of 1.1. Per se gravity pulls you down to a flattened ascneding vector for direct shots to othee planets leaving you more than enough flexibility in designing your upper stage(s). Almost everything works, just dont exaggerate