Gravity turns

[The_FD]

Hi!

Green KSP-er with some gravity turn issues.. FYI no physics mods except FAR.

I read up and looked at videos about gravity turns and I think I grasped the basic concept, but mine seem to be inefficient. I can get into orbit, I’ve orbited and returned from the Mun, yet I still seem to get pretty inconsistent or inefficient results.

Playing career mode the rocket in question has 2 stages, starting with 4 BACCs, 3 T400’s and one LV-T45, followed by a Terrier with 2 T400 fuel tanks and two passenger cabins plus MK1 pod. The BACCs are throttled back to 80% for a stage 1 TWR of 2. I’ve had it into orbit, but it seems to be quite an inefficient gravity turn.

Either I:

- Turn relatively slowly, then end up with a high Apoapsis  (150k+), coast to it (whilst decelerating) and then fire the remains of the first stage + second stage while falling like a brick and taking forever to accelerate. This seems to work, but makes it a highly eccentric orbit, especially if I burn up a bit to stop the rate of the descent.

- Turn more aggressively, ending up at 45 degrees towards the 10k mark and an ~80k Apoapsis. I coast to it, losing all my orbital velocity and then when it’s time to get the Periapsis up it takes so long that I end up back in the atmosphere unless like above I burn up. This doesn’t always work either due to hitting the atmosphere.

Both options seem very inefficient and the second doesn’t always work.

The third option I just tried seems to be the least inefficient. I try to do a gravity turn as best I can, then when the Apoapsis reaches around 80k I keep on burning, but pitch down slightly towards Kerbin. This keep my orbital velocity increasing whilst maintain the Apoapsis where it is. This seems to give me a reasonably circular starting orbit.

So questions then..

- Is my gravity turn too inefficient, or is the 2nd stage underpowered for the payload thus needing another stage. The upper stage TWR is 1.76.

- If it’s the gravity turn, how do you work out on the fly (by dead reckoning?) the ideal angle for the most optimal trajectory. Or at least something that comes close!

- I sometimes get either white streaks on the rocket which presumably means parts approaching the speed of sound, or red streaks from heat. If this means I’m going too fast, should I throttle back or turn less? I thought a TWR of 2 was pretty reasonable.

A bit of a wall of text with questions but I’d really appreciate some guidance (pun intended).

Thanks!

Edited October 12, 2019 by The_FD
Formatting..

[bewing]

If you are feeling a desperate need to circularize, then yes the stage you are using to circularize is underpowered.

Having white streaks or red streaks in the atmosphere on ascent is normal, has very little effect on your ascent, and should either be ignored or celebrated. You need that velocity. Do not, under almost any circumstances, slow down to avoid aero effects (unless the aero makes your rocket unstable).

Once you get above 35km or so, it may be helpful to you to just point your rocket straight at the horizon by hand, and forget about pointing prograde.

Once your Ap gets to 80km, you can kill your thrust, flip to map mode, create a maneuver node at your Ap, and pull the prograde tab until it shows a >60km Pe. This maneuver node will do two things. It gives you a total burn time, and a marker on your navball. If you point at the marker (which will be slightly nose-down) and burn for the required time, you will circularize fairly efficiently -- even if your engine is underpowered. But the important point is that doing this will prevent you from waiting too long to burn, which is what causes you to reenter.

However, the most important thing of all is that circularizing is overrated, and mostly just gets you style points. In general, the reason to go to orbit is because you want to go further than that. So you are going to eventually need a high Ap anyway. So having a high Ap is a good thing, just so long as it's pointed in the right direction. Because of Oberth, you will want to do your next burn at the Pe of your orbit. Which means you want your Ap to be exactly on the opposite side of that. Or, in other words, you need to know in advance which direction you want your Ap to point -- and then you time your launch so that your Ap will point in that direction. And then it doesn't matter how high your Ap is, because it's already pointing in the right direction and all you need to do is add to it. So you burn and burn and burn prograde until you are almost in orbit, and then you stop. If your Ap ends up not pointing quite right, then burning prograde before your Ap pushes it forward, and burning after your Ap pushes it backward. Burning exactly at your Ap only raises your Pe, of course.

 

Edited October 13, 2019 by bewing

[Laie]

 

20 hours ago, The_FD said:

- Turn more aggressively, ending up at 45 degrees towards the 10k mark and an ~80k Apoapsis. I coast to it, losing all my orbital velocity and then when it’s time to get the Periapsis up it takes so long that I end up back in the atmosphere unless like above I burn up. This doesn’t always work either due to hitting the atmosphere.

That sounds quite right.

I've re-built your rocket as you describe it, and find that it has a rather high TWR and more than enough deltaV. It should get you into a Mun and back again.

Stage	TWR	  dV	Burn time
0	0.8	2410	3m45s
1	1.7	 670	35s
2	2.2	1620	52s
			----
			4700

• TWR=2 on the pad is a lot. You should turn even more aggressively, perhaps striking 35deg at 10km. I think that's still doable without burning up. In the end, you want to bring your rocket into a situation where AP is perhaps 1 minute away when the LVT-45 runs out -- that probably means you will be staging at 30km altitude with a 50km apaoapsis or somesuch.
• you may consider moving one of the FLT-400 tanks down, so that your upper stage only has single one (but higher TWR, and still plenty dV).
• Never mind the BACCs: if the goal is only to make orbit, much smaller boosters will do. As a side effect, you will have more time to dial in your turn and certainly won't burn up on ascent.

 

[CBase]

One of the best guide on manual gravity turns I have read was: Turn 10 degree east after reaching 100 m/s (which makes rocket stable) and push time to AP up until it is about 45s. Then throttle down and hold time to AP at 45s until AP is high enough (e.g. 80km), then stop eninges. Throttle back up at about 10s before AP and keep AP 5-10s ahead.

The learning effect is to keep AP in front, so you always raise PE and AP and get a feeling for balancing the split between AP and PE via time until AP.

Later on you can keep full throttle and vary the ascent angle to balance orbit eccentricity until 30km alt, then just boost straight and use bewing nav node methode to circulize.

When starting with KSP getting into a circular orbit is desireful, as it allows to split the task of ascent to space from hitting your destination. Placing a nav node in a highly eccentric orbit is a pain, while easy to drag around on circular orbits. After dozens of ascents you will slowly blend ascent timing and target navigation, but for now allow yourself a step in between.