MacLeod-Industries

Over 400 hours in KSP... and apparently I have never orbited correctly.

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My usual ascent profile is similar to @michaelhester07 but differs in that I'll hold 30 degrees until Ap reaches 70k then I'll cut power and coast uphill until the time to Ap is under 20 seconds, at which point I burn at 90 degrees, keeping the time to Ap close to 10s while bringing my Pe up to circularize.

My usual Kerbin deorbit burn is from 70km, targeting KCS, and I begin the burn when I am at zero degrees elevation to where I want to land (ie, when an observer on the surface at KCS would see my vessel rise over the horizon).  I continue to burn until the descent path reaches out to a point just past the island east of KSC (I actually use the mod Trajectories to get a more accurate targeting on top of KSC to get 100% cash back from the StageRecovery mod).  This usually ensures a close splash-down to KSC.

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11 minutes ago, MacLeod-Industries said:

Armed with this new knowledge, I go forth to them moon!

If my rocket doesn't fall apart due to staging glitches...

Ouch... you really need good gimbal with large rockets...

I've had to abort twice.

Fortunately I was using space shuttle engines, but I put major gimbals limits on it as I though I didn't need that much...

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43 minutes ago, Snark said:

Ouch.  Yes, that will be really inefficient and expensive in terms of dV.

Even in the old pre-1.0 aero days, that still would have been really inefficient.  The classic path to orbit, pre-1.0, was:  "go straight up to 10 km, then crank it over 45 degrees."

(I hasten to add, for the benefit of anyone new to KSP, in case it's not clear already, that's not how you do it now.)  :wink:

While this thread comments on an how to get an efficient gravity turn, I'd like to point out that I've had some success with rockets that leave the pad with TWR between 1.5 and 2.0.  With rockets this powerful, typically anyway you can wrangle them to roughly 45 degrees before transonic effects lock your controls is an effective means of getting to orbit.  It often looks more like a pre-1.0 KSP turn than a proper gravity turn.

35 minutes ago, Snark said:

The problem is that that's not how it works.  Reentry heat in KSP is not simply a matter of, "I have X amount of kinetic and potential energy, and it's all going to turn to heat in my ship, and the total amount of heat I get will be the same regardless of how I enter, so I should enter more slowly so the heat is spread out more over time."

That would make intuitive sense, on a certain level... but it's not how it works.  If you're zipping along at high speed in the extreme upper atmosphere, it generates a whole lot of heat but doesn't actually slow your ship down by a corresponding amount.  You end up generating a huge amount of heat.  You actually get less total heating if you enter more steeply.

So, just rip that band-aid right off.  :)

While I have no idea how it really works in real life and not sure how effectively KSP models aero (especially here), the reason capsules have those flattish bottoms is that the heat isn't coming from friction.  It is all generated by compressing the gasses as the capsule rushes through the atmosphere.  Also the point of maximum temperature isn't touching the capsule, it is a ways in front of the capsule and the overheated air slides around a pocket of slower-moving air near the capsule.  I suspect that at lower altitudes, the point of maximum temperature might be further away, but also suspect that Squad isn't overthinking re-entry heat.

Using a pe of 50km might work for aerobraking from Minmus/Mun (with multiple passes - don't drop your engines so you can still move your pe to 30km when needed), especially if you want to leave part of your craft in Kerbol orbit.  Warning: this is slow and tedious and not nearly as good as simply bringing more fuel along for the ride.

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My own method is pretty similar to what have been described here. I tend to like low TWR on the pad, 1.20/1.25 works fine for me. SRB first stage so it goes up quickly normally. I normally use control surface rather than gimbal at that point to give the initial nudge to the rocket, cependant on the rocket, sweet spot look like around 80m/s. 10 degree or so nudge and the i cut SAS. I try to make it so my SRB are good for at least getting me to 7.5km or so, then i want to have LFO take relay at something around 2.0 TWR. That allow me to adjust my TWR anywhere between 1.5 and 2.0 as soon as my engine goes on. Then, judging by my appoapsis height, actual altitude, speed and angle, i adjust my TWR till my AP is what i want. Circularize and it is done. Not 100% the most efficient way really, but quite easy to do and works for me most of the time, only exceptions are the really weird bases and station module payload.

First time i ever go on to my method, would be happy to get other players opinion!

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2 hours ago, Rocket In My Pocket said:

Right off the pad, do it slowly and gradually...basically just tilt the nose and let the rocket fall to 45 degrees as you climb. You should definitely complete it by 10km up.

Seriously?  So much fuel is wasted as atmospheric heating if you do the turn on the pad.  Waiting until at least 10-20km is traditional.  I do turn over 5 to 10 degrees from vertical so my boosters don't hit the pad on the way down.

How I do it:

I don't obsess over delta-v or twr, but if your rocket gets over 2 gees on the pad atmospheric friction is probably going to waste a lot of fuel.  Fly straight up at no more than 2 gees until you beat 10,000m, then begin your turn to 45 degrees and burn as hard as you like until your apogee is where you want, then cut thrust until you want to circularize.  Burn closer to 0 as you circularize, but watch the ETA on the apogee -- if the time is ticking down faster than you're moving it, you're not going to circularize in time and need to burn at a higher angle to compensate.

The lower your TWR, the higher the angle you circularize at.  Really lean-burning stages might circularize at 30 or even 45, though that's not ideal efficiency-wise,

Edited by Corona688

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14 minutes ago, Corona688 said:

Seriously?  So much fuel is wasted as atmospheric heating if you do the turn on the pad.  Waiting until at least 10-20km is traditional.  I do turn over 5 to 10 degrees from vertical so my boosters don't hit the pad on the way down.

How I do it:

I don't obsess over delta-v or twr, but if your rocket gets over 2 gees on the pad atmospheric friction is probably going to waste a lot of fuel.  Fly straight up at no more than 2 gees until you beat 10,000m, then begin your turn to 45 degrees and burn as hard as you like until your apogee is where you want, then cut thrust until you want to circularize.  Burn closer to 0 as you circularize, but watch the ETA on the apogee -- if the time is ticking down faster than you're moving it, you're not going to circularize in time and need to burn at a higher angle to compensate.

The lower your TWR, the higher the angle you circularize at.  Really lean-burning stages might circularize at 30 or even 45, though that's not ideal efficiency-wise,

What you said would be true in prior versions of the game.

It's not true any more.

I'll let the fact that Snark agreed with me speak for itself. Lol.

1 hour ago, Snark said:

^ This.  What R.i.m.P. said.  I generally wait until I've reached around 20 m/s, just so I have a reasonably stable prograde marker, but yeah, practically right off the pad.  Just a gentle eastward nudge, and then you can pretty much hands-off the controls (except for staging) and let it fly itself.

The tricky part is not when to do it, but estimating how much of a nudge to give it.  That's because the initial eastward nudge needs to be really small, and it's very sensitive.  Not enough, and you're still going too steep when you get up high.  Too much, and you pitch over too fast and end up going horizontally when you're still low in the atmospheric soup.

The classic path to orbit, pre-1.0, was:  "go straight up to 10 km, then crank it over 45 degrees."

(I hasten to add, for the benefit of anyone new to KSP, in case it's not clear already, that's not how you do it now.)  :wink:

 

Edited by Rocket In My Pocket

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Gravity losses are much higher than aero losses in KSP. The adage is that vertical velocity is temporary, but horizontal velocity is yours to keep. Maximizing horizontal velocity is the key to an efficient ascent.  To do this, the craft should tip to the east as soon as possible based on the TWR of the rocket. For a high-TWR rocket, this means starting the turn at no more than 40 m/s. For a lower TWR rocket, this means waiting until you're going 70+ m/s. Your rocket should never be pointing more than five degrees off the prograde marker as long as you're below 30 km in altitude.

Edited by Norcalplanner
Typos

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12 minutes ago, Norcalplanner said:

Your rocket should never be pointing more than five degrees off the prograde marker as long as you're below 30 km in altitude.

Now that's advice I can use.  Thanks.

Does this mean we're getting more boost for free now, being able to turn earlier?

Edited by Corona688

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Don't feel bad, I have been playing for about 16 months and I generally go up to 30-35km, turn to 45 degrees until Ap is above ~65 then burn horizontal.

(I'll let Ap get higher if I will be circularizing with a low thrust engine, generally I am lifting vehicles made with Mk3 parts and nuclear engines, so I am extra careful about the launch as the aerodynamics are not great(lots of things bolted on the outside of the air-shell, like Gigantors, drills, and heat sinks))

It is not a method I would advocate, but it is simple to use and avoids many of the flipping related issues(you are pretty much out of drag at 30km and above 35km you can't even use fins for steering any more.  Not a bad time to pop your fairing/nose cones if you do not need them later)

 

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8 minutes ago, Corona688 said:

 

Does this mean we're getting more boost for free now, being able to turn earlier?

Effectively, yes. It will take less delta V to get to orbit with a proper gravity turn and ascent profile.

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Since we're supposed to start vertical and gradually turn to 45° by 10 km altitude, I tend to treat the upper-eastern quadrant of the nav ball as a progress bar correlated to my altitude. If I'm X% of the way to 10 km, then both my prograde vector and my nosecone should be pointing about that far through the turn at that point. This gives me a way to get back on track if my initial tilt off the pad is too strong or too weak; I can switch to stability assist to get my prograde vector where it should be, then lock prograde.

ZAZT4Xs.png

(Note, nosecone and prograde vector are misaligned in this picture because I borrowed it from someone's blog before adding the numeric annotations.)

Edited by HebaruSan

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I can reach stable orbit in two flights in Hard campaign, but I too have never managed a decent gravity turn! I always thought my rockets looked pretty stable, but any attempt to turn them below 30km ended in a flipping disaster. Perhaps more importantly, until I read this thread I did not realise a gravity turn starting at 30km was considered wasteful!

Given the limitations of the components available on those early flights (back when you have only the most ineffiicient parts, and hit part count limits long before weight limits) one might be justified in taking a safer, albeit less efficient, flight - but I'm happy to churn few a few Jebs finding out!

Ditching science modules is a neat trick I too only hit on recently. Alternatively if you are bringing the science module back (and if you have Bob along for the ride) you can collect copies of the data and reuse the module multiple times to let you get the full science from MysteryGoo etc. in one journey (store a copy in each cockpit available and leave another in the science module itself).

Another recent discovery worth knowing: you can ditch heat shields after use. (You can even set them to appear in staging!) I guess this is useful for reducing weight and therefore the number of parachutes needed...

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The ideal gravity turn starts just above the pad and ends exactly at the altitude of your desired orbit.

My strategy for a gravity turn is usually to have a TWR of 1.5 for the bottom stage at the launch pad, then go straight up until about 50 m/s (so that aerodynamic forces get strong enough to stabilize the craft), and then give the rocket a (very) slight push towards east. From that point on, aero should keep the rocket nearly prograde, with it slightly tilting towards the horizon by itself. Stages above the bottom stage tend to have an initial TWR of 1.0 in my craft designs, so the rocket should already have a decent angle when decoupling the bottom stage, so that the dV of the upper stages isn't wasted in gravity losses.

This of course only works for aerodynamically stable rockets, meaning that their center of drag is behind the center of mass.

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1 hour ago, Corona688 said:

Now that's advice I can use.  Thanks.

Does this mean we're getting more boost for free now, being able to turn earlier?

One way of putting it is that before the v1.0 atmosphere/aerodynamics update, the recommended delta-v to orbit was 4500m/s. It is now more like 3200m/s.

They reduced the ISP values of (nearly) all the engines to counter-balance this, though. An aerodynamic rocket flown with an old-style "gravity" turn in versions up to v0.90 should get to orbit about as well with a true gravity turn in v1.0+

Disclaimer: there are some exceptions e.g. if you had a rocket that used a Rhino, Terrier or Poodle low in the atmosphere, it may struggle with lack of thrust.

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I just managed to get into orbit on my first flight* with a gravity turn. Only barely - 136 m/s dV left - but still, orbit is orbit. :D

(*Note "flight", not "mission"; I do scrounge some basic science off the launch pad first, but I do limit myself to the launch pad - Kerbals waddle far too slowly to bother going anywhere else at this stage!)

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Since we are talking about efficient launches...What about MaxQ and timing when to break the sound barrier? I've been throttling down at 300m/s until I pass the 7.5K mark and hit thinner atmosphere. Does this matter? is there a better approach?

Edited by tjt
I had my speed units scarily wrong

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17 minutes ago, tjt said:

Since we are talking about efficient launches...What about MaxQ and timing when to break the sound barrier? I've been throttling down at 300K/s until I pass the 7.5K mark and hit thinner atmosphere. Does this matter? is there a better approach?

MaxQ isn't much of an issue in stock KSP.  It's my understanding that this is more important in FAR.  In stock, the rockets are built so strongly that there's little point in throttling down.  I'm regularly going 700-800 m/s at 10 km.

One thing to keep in mind - ignore the heating visual effects on ascent.  Unless you actually have thermometers popping up on your parts, there's no need to slow down.  The benefit in reducing gravity losses far outweighs any minor increase in aero losses.

Here's a video of a good ascent profile done by Maccallo as part of my Cheap and Cheerful rocket payload challenge back in 1.0.5.  Not exactly a gravity turn per se, and shallower than most ascents, but it gives you an idea of speeds and angles for an efficient launch.  There's a significant difference between attitude and prograde vector after the SRBs are dropped because of the nature of the challenge (maximizing payload and minimizing cost) and the low TWR of the upper stage, but only after the craft is out of the worst of the atmosphere (above 30 km).

 

Edited by Norcalplanner

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300K/s in the atmosphere at 7500m would be a neon line from horizon to horizon, a sharp echoing noise, and a gentle rain of aluminum oxide flakes a minute or two later.

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MaxQ isn't really a problem in FAR either, because these parts are so enormously strong. Aero losses on Kerbin are tiny, don't worry about them. Aero losses in real life are also tiny ( 50m/s for Saturn V I think, about 250m/s for current satellite launchers ).

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Are there any mods out there that track gravity losses and aero losses during ascent? Would be interesting to see so I could refine my designs based upon first-hand experimental results. It would probably make efficient ascents more instinctual as well. Would be nice to see an accumulated total of dV lost to gravity and drag, as well as an "instantaneous" output of how much loss there was in the last second.

Edited by Clubbavich

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8 hours ago, Rocket In My Pocket said:

I once had a guy tell me the entire space program was fake because get this;

The shuttle's don't go straight up, they turn...so they couldn't possibly be going to space.

I was just like... :rolleyes:

He must have learned orbital mechanics by watching old episodes of Space Angel. :cool:

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3 hours ago, Corona688 said:

300K/s in the atmosphere at 7500m would be a neon line from horizon to horizon, a sharp echoing noise, and a gentle rain of aluminum oxide flakes a minute or two later.

Umm...yeah...haha...I caught that and fixed that in the op

4 hours ago, Norcalplanner said:

Here's a video of a good ascent profile done by Maccallo as part of my Cheap and Cheerful rocket payload challenge back in 1.0.5.  Not exactly a gravity turn per se, and shallower than most ascents, but it gives you an idea of speeds and angles for an efficient launch.  There's a significant difference between attitude and prograde vector after the SRBs are dropped because of the nature of the challenge (maximizing payload and minimizing cost) and the low TWR of the upper stage, but only after the craft is out of the worst of the atmosphere (above 30 km).

Thanks for sharing that. It looks like you're practically tipping off the launch pad. I've always held straight up for the first 1000m. This was very instructive and I'll have to give it a try.

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I always just use GravityTurn- two clicks, and I'm on my way to space. Usually. Especially good for me as I have a slow computer. No official 1.1.3 release, but there is a recompile on the last page of the thread. I'm still on 1.0.5 due to issues on my end, so I get to use an official release.

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