Ascent profile?

[pandaman]

Has anyone figured out a workable 'optimum' ascent profile yet?

I expected things to behave differently to KSP1, but I'm generally either going way too shallow and failing to reach orbit, or way too steep and having to crank over sharply once high in, or out of, the atmosphere .

I can't seem to find the "ballpark guidelines" such as "turn 5 deg at 50ms off the pad" or "45deg attitude at 10km altitude"  that gives me a decent chance at consistency.

Also the 'follow prograde' SAS option doesn't seem to want to do a gravity turn as efficiently as KSP1, most likely 'pilot error' but a couple of hints would be appreciated 

[Streetwind]

2 hours ago, pandaman said:

I can't seem to find the "ballpark guidelines" such as "turn 5 deg at 50ms off the pad" or "45deg attitude at 10km altitude"  that gives me a decent chance at consistency.

That's because the amount of pitchover you need depends largely on the vehicle itself - how much TWR it has on the pad, how much TWR the second stage offers, and even other factors like the sizing of the first stage and the Isp gap between sea level and vacuum of the first stage engine. Experience can tell you a first guess you can work from, but even after over 1000 hours in KSP1, I still need to test-fly a new rocket a few times before I figure out how it best likes to turn.

I have a Minmus rocket that wants to pitch over just a tiny 5 degrees at 60m/s to hit 45 degrees by around 12-13 km of altitude, or it risks being too shallow. I also have a Mun rocket that wouldn't hit 45 degrees until 30km of altitude at that same cautious profile.

Your best bet is to always aim to send your rockets to the pad with roughly the same sea level TWR. That will not fully eliminate the variability between designs, but it will minimize it decently. A TWR of 1.3 is a good mark to aim for, but if you prefer it a bit higher, by all means. Just try to be consistent among your designs.

SAS Hold Prograde has worked flawlessly for me from day one, by the way. I've encountered a bevvy of bugs all over the place, but never with that.

 

Edited March 31, 2023 by Streetwind

[pandaman]

6 minutes ago, Streetwind said:

That's because the amount of pitchover you need depends largely on the vehicle itself - how much TWR it has on the pad, how much TWR the second stage offers, and even other factors like the sizing of the first stage and the Isp gap between sea level and vacuum of the first stage engine. Experience can tell you a first guess you can work from, but even after over 1000 hours in KSP1, I still need to test-fly a new rocket a few times before I figure out how it best likes to turn.

I have a Minmus rocket that wants to pitch over just a tiny 5 degrees at 60m/s to hit 45 degrees by around 12-13 km of altitude, or it risks being too shallow. I also have a Mun rocket that wouldn't hit 45 degrees until 30km of altitude at that same cautious profile.

Your best bet is to always aim to send your rockets to the pad with roughly the same sea level TWR. That will not fully eliminate the variability between designs, but it will minimize it decently. A TWR of 1.3 is a good mark to aim for, but if you prefer it a bit higher, by all means. Just try to be consistent among your designs.

SAS Hold Prograde has worked flawlessly for me from day one, by the way. I've encountered a bevvy of bugs all over the place, but never with that.

 

Thanks.   Yeah, realise it's  user error on my part and unfamiliarity with the little differences from KSP1.

Even with identical rockets each launch I can't seem to find a 'middle ground' I try swallowing it just a smidge and it's too much. 

Probably just me, but it seems less forgiving than KSP1, that's not a complaint though.   Better TWR and Dv indicators in VAB and flight would also help I think.

[NeoMorph]

I tend to pitch the rocket over just enough that it shows a solid direction (E usually) on the compass. Then I click prograde on the SAS. I then control the arc by how much throttle I use… if I’m going up too much vertical I slow down a little (unless I’m using my SRB launcher) and it begins to arc over quite nice.

A lot of it requires on building a decently balanced rocket. I’m launching parts up to my interplanetary ship and am making a lot of launches so getting it right time after time is key. I got so used to using MechJeb to launch my rockets that I forgot how to do it manually and had to learn it using the built in system. But now I’m really getting into flying my rockets again. If there was one complaint though it would be these weird fairings. I hope they get to them soonish. Other than that my experience with KSP2 0.1.1 has been pretty good.

[The Aziz]

"hold prograde"

Bahahahaha.

That's my rockets laughing as they refuse to stick to the prograde marker despite having gimballed engines, reaction wheels etc.

My perfectly symmetrical, strutted Mun rocket? Few seconds after liftoff it will start turning south without any input.

Others? Even after that gentle push eastwards, it will start veering off to the sides, often leading to flip.

Male cow excrement. There's no reliable ascent profile.

[Streetwind]

2 hours ago, pandaman said:

Even with identical rockets each launch I can't seem to find a 'middle ground' I try swallowing it just a smidge and it's too much.

If precision is a problem, remember that you can trade off speed and pitch magnitude.

If you pitch over later, when the rocket is going faster, you need a larger pitch. That may be easier to hit consistently than a very gentle one earlier on. So instead of doing 5 degrees shortly after launch, try and see if you can find the speed where 10° will do the trick. 10° is very easy to hit consistently because it's just the innermost circle around the center of the blue hemisphere of the navball.

 

Another thing you can do is build rockets that are partially self-correcting. I did that by accident with this Mun rocket here, where I was trying to go for minimal mass. I couldn't find a good single-engine solution that would push 22-odd tons off the pad in a 1.25m form factor, and there aren't any XS sized SRBs yet, so I ended up sticking a pair of Thuds to the bottom, dialed their throttle in to be just a bit too high, and designed them to be staged off halfway through the first stage burn. Both to reduce the then-unnecessary thrust, and to let a higher-Isp sustainer have the rest of the fuel.

What happened was that I suddenly had a rocket where, if I found myself going too shallow, I would just delay dropping the Thuds and let their excess thrust correct the problem. And if I found myself steeper than intended, I could get rid of them earlier and give  the rocket more time to fall sideways by itself.

You can try it out by downloading the workshop file and dropping it into C:\Users\YourNameHere\AppData\LocalLow\Intercept Games\Kerbal Space Program 2\Saves\SinglePlayer\YourSavegameHere\Workspaces.

After launching, start pitching over gently at 50 m/s and keep nudging it until you're at about the 10° ring by the time it hits 100m/s, perhaps just shy. Then turn on SAS Hold Prograde. That should get you a turn where you end up in the neighborhood of 10km by 45°. If you hit that mark, stage the Thuds off when your time to apoapsis hits 50 seconds. If you find yourself shallower, perhaps 9km by 45°, then hang onto them until 55secs to apoapsis. And at 8km at 45°, just keep them until the stage burns out. Conversely, if you end up at 11km by 45°, drop the Thuds by 45sec to apoapsis. Or even earlier for even steeper ascents, at your discretion.

 

[cocoscacao]

My standard way of doing this is to go straight up, until I reach 1km altitude. Keep speed to be about 1/10 of your altitude until you reach 3km. Between 3km and 17km, gradually rotate to 45deg. At 17km, atmosphere gets considerably thinner, so it's less risky to flip over. Slowly point to 60deg, and keep speed below 1500ms until you reach 30+ km altitude (to minimize heating effects). After that, try making AP as far away as possible, so circularization burn is as small as possible. I restrict myself not to leave garbage in orbit, so any stage that isn't going further into space (debris), cannot have PE bigger than 69 km.

[Streetwind]

@cocoscacao It's always great to see when players find themselves a reliable method that works for them and they are happy with. And far be it from my place to prescribe anyone how they should play.

However, because this subforum is a place where new players come looking for good advice, I feel compelled to reply and point out that this method is not objectively good. It priorizes the wrong things at the wrong time and, for all its manual effort, ends up spending more dV to orbit than a regular, passive gravity turn.

Concerning here is primarily your repeated advice to "limit speed". This is bad advice, because aerodynamic drag is a largely negligible part of the cost of reaching orbit on Kerbin, whereas gravity drag is substantial and is incurred in larger amounts the slower and steeper you go. Throttling down is an acceptable response to accidentally launching too steeply, but you should not build it into your standard procedures. Excess speed should be put either into flattening out your trajectory sooner, or into redesigning the rocket to let it carry more fuel/payload on the same set of engines, so it trades the unneeded speed for something more useful.

Pushing the apoapsis far away is also not minimizing your circularization burn, because it will result in a long coast phase, during which your altitude will increase and your speed will decrease. Remember, the goal of circularization is to lift the periapsis out of the atmosphere, and therefore, the measure for how large or small your circularization burn is going to be is how high you can get your periapsis before your apoapsis reaches your desired target altitude and you have to cut your engine and coast. And the best place to raise your periapsis is to burn directly on top of the apoapsis - yes, even during a launch. This, of course, would mean that your apoapsis itself won't rise anymore, so in practical application you'll always be pushing your apoapsis ahead of you by at least a minute or so, to ensure you maintain vertical speed until you leave the atmosphere. But trying to intentionally push it away as far as possible actually has the opposite effect that you think it has.

I also don't understand how you are first describing pitching over to 45 degrees, and after that, say "point to 60 degrees". The horizon is 0°, so 60° is steeper than 45° - therefore you would be going backwards, pulling the rocket back upright instead of letting it flatten out. Did you by any chance mean 30° instead of 60°?

[Periple]

My rule of thumb is to be pointing at 80 degrees when I'm going at 100 m/s and then just hold prograde. It's close enough for most of my designs which are about 1.4 TWR at the launchpad. If it's a slower rocket (less launchpad TWR or a short lifter stage followed by a longer-burn, less powerful sustainer stage) I point at 80 degrees a bit later, if it's a fast rocket, a bit earlier. I usually just hold prograde after the initial turn, and in fact design most of my rockets to be passively stable so once they're on their way I disable SAS until they're out of the atmosphere.

This has worked fine for me both in KSP 1 and KSP 2, except for the aerodynamics bugs in KSP 2 throwing things out of whack sometimes.

[cocoscacao]

13 hours ago, Streetwind said:

I feel compelled to reply and point out that this method is not objectively good

Okay, I'm actually curious what the best method is, plus I wasn't very clear about pushing AP away. I meant that AP should be at ~80 kms max, but "horizontally" as far away as possible. Maybe that's what you call flattening out trajectory?

14 hours ago, Streetwind said:

Did you by any chance mean 30° instead of 60°?

Yup. My bad.

[Streetwind]

The best method depends on your goal. Maximizing payload fraction to orbit, minimizing dV cost to orbit, minimizing funds cost to orbit (back in KSP1 where that mattered), or a number of other goals might require different strategies . For example, minimizing dV cost to orbit usually involves extremely high thrust, tipping over really hard really early, and watching your rocket be a fireball all the way to space. Very few people actually play like that, though, not least because that greatly limits you to very small, sleek payloads.

When considering the average use case, where a player wants to push a not-ridiculously-shaped but otherwise unspecific payload to orbit in a neat, clean, and efficient manner, without going hard into any particular optimization corner, then you arrive at something like this: your rocket has a TWR of around 1.3 to 1.4 when sitting on the pad; it will begin by ascending straight up for a short distance, until a certain speed is reached; it performs a single pitch maneuver that tilts it a certain amount of degrees over to the east; it activates SAS Hold Prograde; and then it flies completely without the player touching the keyboard. Due to the choice of pitchover speed and magnitude, it will hit 45° towards the horizon at around 10 km in altitude above sea level. It will eventually receive a staging command from the player to drop the spent first stage and light the second. It will continue flying hands-off and full throttle, like it has the entire time, until the target apoapsis altitude has been reached; at which point it will cut the engine and coast to apoapsis for orbital insertion.

Optionally, the player can intervene in the flight of the second stage by starting to throttle down at some point to prevent the apoapsis from running away from the rocket. Keeping the apoapsis close to the rocket more efficiently raises the periapsis, and because the rocket is nearly level to the horizon at that point, gravity drag is no longer a factor. However, this should only be done when the apoapsis is already close to leaving the atmosphere, something like 55-60 km up. Otherwise you risk stalling your ascent in the upper atmosphere, which is the opposite of what you want. (This is admittedly a bit advanced.)

Here's another thread where I discussed this with someone, and gave him a (different) sample rocket. He actually recorded his two attempts on video, so it may help you see what I mean. He ended up getting an orbit insertion burn of just 150 m/s for a 102 km orbit. Had he gone for 80km, it might have been closer to 100. Not bad for a flight that didn't touch WASD at any point after the initial pitchover, right?

[cocoscacao]

@Streetwind Thx. Pls, just tag me in the future. I almost missed your reply.

[LowSnow]

On 4/2/2023 at 11:41 AM, cocoscacao said:

My standard way of doing this is to go straight up, until I reach 1km altitude. Keep speed to be about 1/10 of your altitude until you reach 3km. Between 3km and 17km, gradually rotate to 45deg. At 17km, atmosphere gets considerably thinner, so it's less risky to flip over. Slowly point to 60deg, and keep speed below 1500ms until you reach 30+ km altitude (to minimize heating effects). After that, try making AP as far away as possible, so circularization burn is as small as possible. I restrict myself not to leave garbage in orbit, so any stage that isn't going further into space (debris), cannot have PE bigger than 69 km.

My standard way is... not doing a gravity turn. I am just completely unable to do so. TWR 1.3? Even then the slightest millisecond nudge on the D key makes any rocket flip entirely.

Sooo... My standard way is:

1. Overfueling every rocket;
2. Going straight up until 100-150km
3. Turning 90° east
4. Wait till you get orbit (whilst sometimes intermittenly stopping the burn because of only Ap increasing without any movement in Per)

I have a feeling that the SAS in the current state of the game (0.1.2.0) is AND underpowered AND overcorrecting. Devs should really fix this soon, cause I won't be able to get anywhere further than Eve/Duna if I need to keep overfueling my rockets like I have to now.

[18Watt]

48 minutes ago, LowSnow said:

My standard way is... not doing a gravity turn.

You will likely get a lot of responses telling you you're not doing it right.  But if it works for you...

 

[Streetwind]

7 hours ago, LowSnow said:

I have a feeling that the SAS in the current state of the game (0.1.2.0) is AND underpowered AND overcorrecting. Devs should really fix this soon, cause I won't be able to get anywhere further than Eve/Duna if I need to keep overfueling my rockets like I have to now.

There's no issue with the game that needs fixing. The problem you're having is that you're building passively unstable rockets.

[LowSnow]

9 hours ago, Streetwind said:

There's no issue with the game that needs fixing. The problem you're having is that you're building passively unstable rockets.

Okay this information was long, but really helpful. Only one more question: I guess when talking about "center of pressure" you're not talking about "center of lift". So the next question is: how (if at all) can you see where the center of pressure is in KSP2?

[Bej Kerman]

58 minutes ago, LowSnow said:

Okay this information was long, but really helpful. Only one more question: I guess when talking about "center of pressure" you're not talking about "center of lift". So the next question is: how (if at all) can you see where the center of pressure is in KSP2?

After looking at a few Google results, I can only conclude that the center of pressure is the center of lift.

[Streetwind]

14 hours ago, LowSnow said:

So the next question is: how (if at all) can you see where the center of pressure is in KSP2?

The editor should actually be able show you this in KSP2. There should be a button where you can activate the display.

 

13 hours ago, Bej Kerman said:

After looking at a few Google results, I can only conclude that the center of pressure is the center of lift.

It's not the same, no, because center of pressure is about drag, not about lift. So CoP includes drag from parts that do not produce lift, whereas CoL doesn't. In KSP1 this was a bit of an issue because the editor could only ever show center of lift, and in some cases that meant that the indicator you got was in a different enough place from the actual CoP that it mattered.

In KSP2, meanwhile, the editor shows center of pressure instead, even when building planes it seems. Though to be fair, I don't generally build planes. So I couldn't tell you if this was a big departure from the way KSP1 did it, or basically negligible.