when is twr not important?

[deepspacecreeper]

hello, i hv heard from somewhere that the twr becomes less important after gravity turn, but i don't really know why and when the twr is not important at that time. could anyone answer?

[UmbralRaptor]

After you've pitched over, you're no longer directly fighting gravity, and gravity drag losses are much reduced if not entirely eliminated.

Once in orbit, you're under much less pressure, and can afford burn times up to a significant fraction of your orbital period. Longer if you split the burn.

[kurja]

There's no definite, specific moment afaik, TWR is just less important after you've cleared (most of) the atmosphere. While still in the atmosphere you'll want some more of that TWR so you can get out of it quicker... Ofc during the initial ascent (before the gravity turn) you must maintain >1 TWR to keep accelerating so you can get out of the gravity well at all, after your gravity turn and having reached an apoapsis above the atmosphere it's possible to circularize with a lower TWR. I'm pretty sure that's all there is to it.

[Lheim]

Just for shizzes I decided to see what would happen if I launched my interplanetary nuclear tug stage - TWR .31, with itself as the final boost stage.

My skipper engines were staged aside at about well inside the atmosphere - maybe about 37 klicks up, with my AP at about 60 klicks.

I had to throttle up nearly to overheating and point near vertical, but my time-to-apoapsis was still slowly ticking down. Finally it seemed to turn a corner and the time to my apoapsis stopped decreasing.. and started increasing, slowly. I nudged the nuke more horizontal.

Long story short, it was a harrowing approximate 5 minutes and I ended 146 kilometers up once I had achieved an orbit that wouldn't dip into the atmosphere. It felt horribly inefficient. If you're going to go that route I suggest trying to get your AP as high as possible before you start relying on such a low-twr stage in atmosphere. Give yourself time.

[allmhuran]

Be aware that a higher TWR always does have some advantages.

It means you can execute all required burns in less time. This makes manoeuvre planning much, much easier, and often more efficient.

If you have a very low TWR, then you will usually *not* be able to complete transfer burns in one go, you will need to do multiple burns at periapsis. This retains the efficiency, but means your predicted intercept will be less accurate, which means you will need to do more burns to line things up again.

But, having said all that, it's quite common to have a very low TWR on interplanetary craft, because the efficiency gained by using nuclear engines easily outweighs the low TWR provided.

[deepspacecreeper]

am i burning radially out or prograde after starting gravity turn?

[SRV Ron]

Design your orbital vehicle with enough power to get your apoapsis over 100 k and about 1,500m/sec orbital speed before those engines are staged. Your smaller one will then have sufficient power to get to orbital speed using the momentum from the initial launch to reach the apoapsis. This is the idea behind this small probe launcher which can also place the same probe into Mun or Minmus orbit. That will work with any design that has insufficient power to lift off from Kerban by itself. Even monopropellant thrusters will work as long as you have enough burn time to get to orbiting speed as you reach apoapsis.

[capi3101]

am i burning radially out or prograde after starting gravity turn?

It's more radially out than prograde, but you're doing a bit of both, actually. You want to burn more outward if you get close to your apoapsis during the ascent - say closer than thirty seconds or so. Otherwise follow the prograde vector on the navball. Go horizontal above 50k.

[Taki117]

As a general rule of thumb you should always try to keep your TWR at, or just above, 1. This reduces the chance that you will not make it out of the atmosphere after the gravity turn (I've had it happen) and it will greatly increase the eficency of all future maneuvers.

[capi3101]

Short answer - TWR always has some level of importance. It's most important when you're launching and landing; a TWR of less than one on the target body is always bad, whether it's taking off from Kerbin or landing on Tylo. In space, it's not as important; mainly it determines how quickly you can complete maneuvers.

[Specialist290]

Pretty much what capi said. It's less that TWR ever stops mattering and more that there's always a trade-off involved between having a high TWR and a high delta-v capacity. Many of the things you can do to increase one in a design will end up decreasing the other, by virtue of the fact that you're altering the craft's mass ratio with every part you add or remove.

[Starwaster]

In orbit. Interplanetary transfers where you don't care how long the burn takes.

It matters for takeoff or engine only tail landings where your TWR MUST be greater than 1 taking into account local gravity