So in real life rockets (seem) to have very small TWRs, and I've seen playthroughs of KSP online that show them building rockets with like 1.1-1.3 TWR. Obviously I'm missing something... to me it seems like a high TWR is optimal to get out of the atmosphere asap. Additionally, why are SRBs preferred on first stage when they have far worse fuel efficiencies? Thanks
#TWR? / Is a high TWR worse or better than a low one? + SRBs vs. Liquid
rugged crown
junior token
So, with a thrust to weight of exactly 1, you can counteract gravity. Above one, and you start moving up.
brisk girder
Indeed, and Starship's booster (Superheavy) has around 1.5, wich seems optimal. Too much and you'll increase drag losses.
junior token
And solid rockets are cheap. That's why they're preferred.
fluid epoch
High TWR is better as long as your rocket is sufficiently aerodynamic and you are not carrying a ridiculous amount of engine mass.
Also if your drag is sufficiently low the optimal gravity turn is far lower/steeper than you would expect
serene crypt
All things being equal, more TWR (up to something like 2.5) is good. But adding propellant may get you enough more Δv to offset the additional gravity losses. (And there's a bit of wonkiness in that depending on your profile, drag losses in atmo might not be too bad, so you should pitch over more agressively)
rugged crown
So as a rule of thumb, the optimal TWR would drop as the ship gets heavier/less aerodynamic?
junior token
There's a balance to initial speed. Too slow, and you'll spend a lot of time in thick atmosphere. Too fast, and you waste fuel on drag.
On Kerbin, the limit in heavy atmo seems to be around 280 while you are in the the "light blue" atmo. After that, you can speed up.
brisk girder
About SRBs... They're just an option, not necessarily the preferred one, and in the first stage they shine. Why? Because they're compact and powerful. At low altitudes and speeds you need thrust, not Isp, and they make a non-huge booster compared to a hydrogen fuelled liquid booster.
Also, they're dirt cheap in comparison, perfect for expendable uses.
This said, there are real rockets that use many solid stages, and a liquid upper one (otherwise you'd have no control), like Minotaur, Vega, and Rokot
rugged crown
Okay ty all
brisk girder
Oh, and as they're solids, they have no control once ignited, and in real life rockets, if they fail, a huge explosion with incandescent shrapnel rail is guaranteed, both of wich limit uses in actual crewed rockets. As far as I know, NASA is the only one launching people with solids (shuttle and soon SLS)
fluid epoch
Atlas V/Starliner soon
brisk girder
True, and later Vulkan, I forgot about ULA
serene crypt
Imagine if the Ares I-X had happened
brisk girder
One odd rocket to see, with a compact first stage, then a large LH2 upper stage.