What's the best launch trajectory again?

[quasarrgames]

Normally it takes me about 3700m/s to get to a 75km orbit. I heard that i can shave 500m/s or more off this number, so i am curious how. could anyone tell me?

[Rodyle]

I'm actually writing a kOS autolaunch script at the moment, so I'm quite interested in the answer to this question.

My experiences so far: gravity turn is the key. When you reach about 100 m/s, turn over 10ish degrees to the east and turn off your SAS. Let the rocket get to 300 m/s and allow it to turn on it's own. It should have about a 45 degree angle at 10 kilometres. At around 12 to 15 km, start gaining speed, and a lot of it. You want a pretty shallow trajectory, so you can get plenty of speed before reaching apoapsis.

That's as far as I've gotten. If anyone can explain more on the subject, or even prove me wrong, please do so.

[Plusck]

I've just done a series of launches where KER was telling me I had 3700 m/s, and I was getting to orbit with over 800 m/s remaining. I suspect that KER's figures were a bit off due to it being unable to calculate the true benefits of asparagus staging and/or the immediate dropping of boosters and tanks. I should have taken screenshots at each stage to check the numbers, but I was a bit busy trying to keep things on track.

So, in 1.0.5. what works best?

1 - 100% thrust off launchpad. At about 200m/s reduce thrust so that speed rises slowly up to 300m/s* as you climb to about 8-10,000m altitude. If you're using SRBs, you might find that your LF engines are barely ticking over at this stage.

2 - start gravity turn straight off the launchpad. There is no point waiting to do this, except that you can't know in advance exactly how much to turn to get the right angle in point 3 below. Some people say that if you wait to get up to about 100 m/s then it becomes a bit more pedictable.

3 - follow prograde down to about 50° at 8,000m or 45° at 10-11,000m**, then slam thrust up to 100%. Be prepared for scary heating at about 28-30,000m.

* terminal velocity is about 180 m/s at sea level and about 300 m/s at 10 Km. Apparently you should stay exactly at terminal velocity in order to optimise your fuel vs gravity losses. 

** if you're already at 50° at 8,000m, you really need to throttle up fast or you'll start heading for the horizon before getting high enough to get out of the atmosphere.

The higher your TWR, and the better the aerodynamic profile, the more agressive you need to be. A few weeks ago I was testing a small and sleek craft and it actually gave me the most fuel remaining in orbit by slamming thrust back up to 100% at around 50° at just under 6000m... but it was getting extremely hot at around 25-30,000m.

And in any event you need a good aerodynamic profile, and preferably as little thrust vectoring as possible. If all goes well you can leave SAS off until you get to about 30,000m.

Edited January 17, 2016 by Plusck

[Padishar]

23 minutes ago, Plusck said:

I've just done a series of launches where KER was telling me I had 3700 m/s, and I was getting to orbit with over 800 m/s remaining.

I suspect that the 3700m/s was atmospheric (sea level) deltaV and the 800m/s remaining was (obviously, since you were in orbit) vacuum deltaV.  What does KER show for the deltaV when in the VAB (with KER not set to Atmospheric)?

[Plusck]

1 hour ago, Padishar said:

I suspect that the 3700m/s was atmospheric (sea level) deltaV and the 800m/s remaining was (obviously, since you were in orbit) vacuum deltaV.  What does KER show for the deltaV when in the VAB (with KER not set to Atmospheric)?

I don't think it was listing atmospheric stats, since on one of the ships the final stage had 5 LV-Ns and their atmospheric delta-v should have been dismal, no?* The idea was to get to orbit and maybe have to light them to circularise, but I ended up with more than 800 m/s left on the lifter stage. That's why I suspect the dropped SRBs and the drop-tanks were confusing KER more than anything else.

Unfortunately I can't tell because as a self-confessed KSP addict, I've now compressed my career save and put it on a USB stick so that I can't play it "just for 5 minutes" without going through a very slow and annoying process. I will however come back to this thread when I eventually crack...

 

*edit: in fact, it is quite possible that it was stuck on Duna atmo setting, since that's where I was sending it... Again, I'll check and get back on this when I crack.

Edited January 17, 2016 by Plusck

[Plusck]

However, I made an album for that "small and sleek craft" - showing what I found to be the best launch profile for that particular setup (single stack, SRB as first stage with thrust reduced, then two LFO stages). Available here. I've just added a couple of pics taken with KER showing that in fact, the best I could get out of it was 3250 m/s dv to low orbit. (and I could do that without unpacking my tempting career save...)

[Snark]

Agreed that a well-executed gravity turn is key.  The exact shape of the "ideal" curve for a given craft will depend on your ship design, primarily its TWR.

I like to use a launchpad TWR of about 1.5, which is probably a little on the high side.  With that TWR, I start my gravity turn much sooner than 100 m/s; I do the initial gentle nudge practically right off the pad, like when it's only going 20 m/s or so.

By the time I get to 10km altitude, I should be pointing no more than 45 degrees above the horizon, and going at least 300 m/s.

Then just follow prograde until Ap is raised to the desired level.

[Laie]

3 hours ago, Rodyle said:

My experiences so far: gravity turn is the key. [...] It should have about a 45 degree angle at 10 kilometres. At around 12 to 15 km, start gaining speed, and a lot of it. You want a pretty shallow trajectory, so you can get plenty of speed before reaching apoapsis.

That's about all there is to it. (Heh.) Your actually going to orbit happens mostly between 10-30km. The purpose of the gravity turn is to get you into a good position (altitude, velocity and direction) from where you can pull through in one smooth motion.

If you want to do it on as little dV as possible, you have to come in pretty shallow (like 30deg @10km) and with a pretty high TWR. That way you're going quite fast at comparatively low altitudes, but drag be damned, the gains are worth it. What makes it an art is that you need to stay just high enough so you don't burn up in the atmosphere.

By and large, this is challenge material (I've actually seen the 30deg@10km in a post-1.0.2 challenge somewhere); taking the scenic route is much easier on the nerves, and the lifters tend to be cheaper, too.

[Streetwind]

16 hours ago, Plusck said:

* terminal velocity is about 180 m/s at sea level and about 300 m/s at 10 Km. Apparently you should stay exactly at terminal velocity in order to optimise your fuel vs gravity losses. 

Careful - that was correct in the old souposphere, but it no longer is! You have less than one fifth of sea level pressure left at 10km; terminal velocity will be running away from you like racing horse hopped up on caffeine. Not to mention that there's no such thing as a "general terminal velocity" anymore. It depends completely on your rocket's size and shape, so every rocket will perform differently - as will returning vehicles. A launching rocket's terminal velocity is far, far higher than that of a single pod barreling through the air bottom end first. That is why getting mach effects or reentry flames on your way up doesn't have anythng to do with reaching/exceeding terminal velocity. These effects are simply computed based on speed vs. altitude, so a pointy rocket at 25% terminal velocity may produce more flames than a pod at 100%.

As far as my experience goes, if your TWR on the pad is lower than 3 and your launch vehicle is at least vaguely rocket-shaped, the only reason you ever need to throttle down on ascent is because you misjudged the pitchover and aren't turning sideways quickly enough. The lowest dV's to orbit are achieved with extremely high TWR rockets with extremely pointy shapes to maximize terminal velocity, which turn almost completely horizontal only a couple kilometers up and then pretend like the atmosphere isn't even there. I think there was a challenge somewhere in the respective subforum, had some pretty extreme screenshots/videos in it

 

Edited January 18, 2016 by Streetwind

[Plusck]

1 hour ago, Streetwind said:

Careful - that was correct in the old souposphere, but it no longer is! You have less than one fifth of sea level pressure left at 10km; terminal velocity will be running away from you like racing horse hopped up on caffeine. Not to mention that there's no such thing as a "general terminal velocity" anymore. It depends completely on your rocket's size and shape, so every rocket will perform differently - as will returning vehicles. A launching rocket's terminal velocity is far, far higher than that of a single pod barreling through the air bottom end first. That is why getting mach effects or reentry flames on your way up doesn't have anythng to do with reaching/exceeding terminal velocity. These effects are simply computed based on speed vs. altitude, so a pointy rocket at 25% terminal velocity may produce more flames than a pod at 100%.

As far as my experience goes, if your TWR on the pad is lower than 3 and your launch vehicle is at least vaguely rocket-shaped, the only reason you ever need to throttle down on ascent is because you misjudged the pitchover and aren't turning sideways quickly enough. The lowest dV's to orbit are achieved with extremely high TWR rockets with extremely pointy shapes to maximize terminal velocity, which turn almost completely horizontal only a couple kilometers up and then pretend like the atmosphere isn't even there. I think there was a challenge somewhere in the respective subforum, had some pretty extreme screenshots/videos in it

 

Yes, I agree and I should have said that that was a very broad generalisation. However, I felt I was already adding so many qualifications to what I was saying that yet another caveat seemed unnecessary.

So from now on I'll consider that "300 m/s at 10 km" to be an overly-cautious approach, the main benefit of which is that it tends to avoid losing control with a less-than-perfect-dart type of rocket. 

 

[Kerbal101]

Gravity turn. Real gravity turn - gravity turns you.
1) SAS off
2) Throttle to full (Z) and ignite (whitespace)
3) When its 40m/s - hit direction key twice in quick succession (introduces 5 degree "punch" that starts growing slowly)
4) When its 140m/s - again hit twice quickly in same direction (accelerates it just a bit; higher m/s and higher AoA - less effect)
stages, stages, stages; whitespace, whitespace, whitespace
When apoapsis extends to 70km, cut off the power (X) and drift further.

At t-minus 30 seconds, SAS on, align with horizon below prograde, burn until around 2200.

Perfect orbit with help of higher beings gravity.

This does not work, however, if rocket is not balanced... Then you have to resort to approximate, manual "gravity" turn. Which always looses in efficiency due to steering drag waste.

Edited January 18, 2016 by Kerbal101

[GoSlash27]

Like everyone else said, the gravity turn is going to be the most efficient.
 I like to keep my initial t/w in the neighborhood of 1.2-1.4 and limit it by the function 2sin(pitch).

Also, keep in mind that true "efficiency" isn't getting your payload to orbit with minimum DV expended, it's getting your payload to orbit with the least mass and cost.

Higher t/w helps reduce your total DV requirement, but it also reduces your payload fraction and increases your cost.

Best,

-Slashy

 

 

 

[RizzoTheRat]

22 hours ago, Streetwind said:

That is why getting mach effects or reentry flames on your way up doesn't have anythng to do with reaching/exceeding terminal velocity. These effects are simply computed based on speed vs. altitude, so a pointy rocket at 25% terminal velocity may produce more flames than a pod at 100%

I hadn't realised that, I've been using the flames  as an indication that I'm going too fast at that altitude and altering my profiles accordingly for future launches.  Sounds like I need to be to turning sooner and getting more horizontal speed lower down than I usually do. 

I'm generally needing about 3500 m/s dV on my launchers, is Plusck's 3250 m/s about right for an optimum launch or is anyone getting much lower than that?

[Nefrums]

To make an efficient launch the most important is to limit drag.   Having done that more twr is always better, never throttle your engines.

The angle of attack is the one thing that affect drag the most, therefore hold prograde as much as possible.

Make a small 5-10 degree turn right of the launchpad how much depend on your twr and drag. After that hold prograde for the rest of the launch.

If your AP gets out of the atmosphere before your PE is of the ground you need to turn more at the start. If you fail to keep the AP in front of you your initial turn where to big.

I get the most efficient launches with a twr of 2-2,5 for most of the launch.

This thread is full of examples of efficient launches:  Stock Payload Fraction Challenge: 1.0.5 Edition

Edited January 19, 2016 by Nefrums

[Warzouz]

On 17/1/2016 at 11:15 PM, Plusck said:

I don't think it was listing atmospheric stats, since on one of the ships the final stage had 5 LV-Ns and their atmospheric delta-v should have been dismal, no?* The idea was to get to orbit and maybe have to light them to circularise, but I ended up with more than 800 m/s left on the lifter stage. That's why I suspect the dropped SRBs and the drop-tanks were confusing KER more than anything else.

Here do you read you dV ? For example, on the launchpad, KER always gives dV in ASL (that's why dV sometime rises as your rocket take-off. In the VAB, dV is VAC or ASL, depending if you clikcked on the "atmoshpere" button.

I always use dV VAC value because ISP rises very fast with altitude. The real dV is much more near VAC value than near ASL value.

In 1.0.4, I managed to go to LKO for 3150m/s VAC, but I remembered doing a very nice curve I never succeed to redo again. I usually manage to get 3200m/s. In 1.0.5, I think I'm more around 3250 or 3300 when I (reasonably) mess up. As my rocket usually have 3400m/s I'm good. But I need fuel to deorbit the launch stage and do a power landing

 

To answer to the OP,

• In 1.0.4, I used to try crossing the 45° at 8km and start turning at 60m/s
• In 1.0.5, I'm back with 45° at 10km. I usually start turning around 100m/s. The longer you wait to turn, the harder it is to and you can burn more fuel (to force turning and to circularize)

In any case, there is no precise value. I tend to get always the same result more because I always build my rocket the same way, than because it's the smartest trajectory...

 

EDIT : and as other already said. DON'T USE SAS at take-off. it creates wobble and make turning harder. Reactivate SAS when your rocket trajectory becomes unstable (around 30km). Switch to prograde as soon as you turn your engines off.

Edited January 19, 2016 by Warzouz

[Streetwind]

2 hours ago, RizzoTheRat said:

I'm generally needing about 3500 m/s dV on my launchers, is Plusck's 3250 m/s about right for an optimum launch or is anyone getting much lower than that?

My rockets are usually in the neighbourhood of 3200-3300 too, though I'm not pushing it. If you really boost hard you can do it under 3000, but honestly I am not a fan of being shrouded in flames that much either. It breaks my immersion It also becomes a balancing act due to atmospheric heating.

 

1 hour ago, Nefrums said:

To make an efficient launch the most important is to limit drag.   Having done that more twr is always better, never throttle your engines.

Nnnope. Atmospheric losses account for maybe 5%-6% of the dV cost to orbit, while gravity losses are four times that. As long as you don't exceed terminal velocity, you always win when accepting more drag in return for extra TWR and a flatter trajectory. Drag should be your last concern, not your first; you address it primarily by making your vehicle look pointy and rocket-shaped, and not by piloting.

 

Edited January 19, 2016 by Streetwind

[Plusck]

3 hours ago, RizzoTheRat said:

I hadn't realised that, I've been using the flames  as an indication that I'm going too fast at that altitude and altering my profiles accordingly for future launches.  Sounds like I need to be to turning sooner and getting more horizontal speed lower down than I usually do. 

I'm generally needing about 3500 m/s dV on my launchers, is Plusck's 3250 m/s about right for an optimum launch or is anyone getting much lower than that?

Join the club! I only realised that when testing that 3250 m/s rocket, and I still tend to be overly cautious as virtually all of the above replies demonstrate...

For that particular rocket, I was testing it to answer a question from somebody in early career. Using an SRB as bottom stage is not optimal, nor is reducing the SRB's throttle in the VAB (but without doing that it was impossible to get a decent gravity turn on that set-up). So the 3250 m/s was optimal in testing for that particular set-up, but the set-up itself was certainly not optimal...

[Kerbal101]

6 hours ago, Warzouz said:

EDIT : and as other already said. DON'T USE SAS at take-off. it creates wobble and make turning harder. Reactivate SAS when your rocket trajectory becomes unstable (around 30km). Switch to prograde as soon as you turn your engines off.

A lot of people say "gravity turn" but mean - turn around planet, which is wrong. Like, "I do gravity turn, I first turn XX at xxx, then wait till altitude of XX" - this is not a gravity turn, this is a "manual gravity turn".

Gravity turn means - Gravity turns your space craft. For the whole time, since Launch <---> up to Orbit burn, you don't touch controls and there is no autopilot, you don't apply any rules after you figured few right points where to add some influence.

BUT the possibility of gravity on itself to turn your rocket correctly demands that your rocket is aerodynamically correct. That it has right CoM and CoL placement, right T/W.
Rockets which carry exposed equipment, or control surfaces in upper stages are gonna have it real hard though, for example - the stock shuttle is impossible to do gravity turn.

The speed distribution in gravity turn is - first vertical speed goes up fast, horizontal pretty slow. Again - it happens by itself, gravity is autopilot.
Over the whole trajectory the rocket continuously stalls - all by itself. At around 12km, the rocket falls into 45 degree and Vspeed and Hspeed match. Then Vspeed starts falling, Hspeed boosting. At around 35km the Vspeed is around 200-150 m/s, but Hspeed already at 1600+ m/s...
 

Personally I do engage stability assist only at T minus 30 before orbit burn to align with horizon below (incoming) prograde. Thats at 80 km near apoapsis.

If rocket gets unstable at 30km, it either has too bad aerodynamical qualities or too bad CoM/CoL or too weak T/W. This is not necessary bad.. sometimes its demanded by mission.

[Plusck]

33 minutes ago, Kerbal101 said:

A lot of people say "gravity turn" but mean - turn around planet, which is wrong. Like, "I do gravity turn, I first turn XX at xxx, then wait till altitude of XX" - this is not a gravity turn, this is a "manual gravity turn".

Gravity turn means - Gravity turns your space craft. For the whole time, since Launch <---> up to Orbit burn, you don't touch controls and there is no autopilot, you don't apply any rules after you figured few right points where to add some influence.

I don't think anybody in this thread means that at all.

You have to initiate the gravity turn manually because a rocket will just fly straight up. What people are consistently saying is that at a certain altitude of XX you should be at an angle of approximately YY°, to be sure to get a decent trajectory all the way to orbit. To get that angle you need to give a certain push to start with. And although you should just be able to leave the controls alone, draggy payloads and/or fairings often make a little manual help unavoidable.

[Kerbal101]

7 minutes ago, Plusck said:

 What people are consistently saying is that at a certain altitude of XX you should be at an angle of approximately YY°, to be sure to get a decent trajectory all the way to orbit. To get that angle you need to give a certain push to start with. And although you should just be able to leave the controls alone, draggy payloads and/or fairings often make a little manual help unavoidable.

Thats a manual "gravity turn", not *the* gravity turn.
*The* gravity turn says - gravity should turn you. Of course, you have to punch a direction at start. But thats it. See you in orbit with no auto-pilot.

Build a basic rocket with basic chute, MK1, decoupler, two FL-T800 and one Swivel. No boosters, nothing - and try to close 70/70 orbit, then do re-entry. If you succeed, then you do it right.

Edited January 19, 2016 by Kerbal101
Forgot about chute. But who needs them, right? =)

[Padishar]

1 hour ago, Kerbal101 said:

A lot of people say "gravity turn" but mean - turn around planet, which is wrong. Like, "I do gravity turn, I first turn XX at xxx, then wait till altitude of XX" - this is not a gravity turn, this is a "manual gravity turn".

Gravity turn means - Gravity turns your space craft. For the whole time, since Launch <---> up to Orbit burn, you don't touch controls and there is no autopilot, you don't apply any rules after you figured few right points where to add some influence.

BUT the possibility of gravity on itself to turn your rocket correctly demands that your rocket is aerodynamically correct. That it has right CoM and CoL placement, right T/W.
Rockets which carry exposed equipment, or control surfaces in upper stages are gonna have it real hard though, for example - the stock shuttle is impossible to do gravity turn.

Personally, I hate the name "gravity turn" because it's not actually gravity that "turns" the rocket.  Gravity pulls the direction of the rocket's velocity vector (prograde) down towards the horizon but doesn't cause any actual rotation of the vessel.  The rotation is caused by the lawn dart nature of the rocket's aerodynamics (i.e. the drag at the back makes it turn towards prograde).  Try a gravity turn on an airless body with SAS turned off and your ship will not automatically follow prograde but it will if there is enough of an atmosphere (and your rocket is designed "properly").

 

Edited January 19, 2016 by Padishar

[Plusck]

14 minutes ago, Kerbal101 said:

Thats a manual "gravity turn", not *the* gravity turn.
*The* gravity turn says - gravity should turn you. Of course, you have to punch a direction at start. But thats it. See you in orbit with no auto-pilot.

Um, that is exactly what I just said. And it's exactly what most people here are saying. I don't understand how you are misunderstanding the phrase "you should be at..." because you seem to be interpreting that to mean "turn your ship to...", which is not at all what anyone in this thread has said.

[Kerbal101]

2 hours ago, Padishar said:

Personally, I hate the name "gravity turn" because it's not actually gravity that "turns" the rocket.  Gravity pulls the direction of the rocket's velocity vector (prograde) down towards the horizon but doesn't cause any actual rotation of the vessel.  The rotation is caused by the lawn dart nature of the rocket's aerodynamics (i.e. the drag at the back makes it turn towards prograde).  Try a gravity turn on an airless body with SAS turned off and your ship will not automatically follow prograde but it will if there is enough of an atmosphere (and your rocket is designed "properly").

 

Yes, I know, otherwise adding wing area would give no effect - its a combined force. I mean, the steering - the steering drag waste is near zero.
And its pretty big difference between a) constantly checking altitude, apoapsis, current speeds, making turns, leaving SAS on, and  b) touching direction key four times for a total of 1 second before orbit, without any SAS.

 

2 hours ago, Plusck said:

Um, that is exactly what I just said. And it's exactly what most people here are saying. I don't understand how you are misunderstanding the phrase "you should be at..." because you seem to be interpreting that to mean "turn your ship to...", which is not at all what anyone in this thread has said.

No. No. Yes. No.

Edited January 19, 2016 by Kerbal101

[Streetwind]

10 hours ago, Kerbal101 said:

Thats a manual "gravity turn", not *the* gravity turn.
*The* gravity turn says - gravity should turn you. Of course, you have to punch a direction at start. But thats it. See you in orbit with no auto-pilot.

The technical term is "pitchover maneuver", by the way

[Nefrums]

20 hours ago, Streetwind said:

Nnnope. Atmospheric losses account for maybe 5%-6% of the dV cost to orbit, while gravity losses are four times that. As long as you don't exceed terminal velocity, you always win when accepting more drag in return for extra TWR and a flatter trajectory. Drag should be your last concern, not your first; you address it primarily by making your vehicle look pointy and rocket-shaped, and not by piloting.

 

That is true when you have a very low angle of attack and are flying slowly.  Drag increases rapidly with the angle of attack.

Even when exceeding terminal velocity you still suffer less losses from drag then what you gain in decreased gravity losses.  (For a low drag rocket pointing prograde.)

Fun thing about the gravity turn definition,   I was the only one suggesting "the" gravity turn as the best method, even without calling it that.    And using prograde lock in SAS helps to perform the gravity turn on less balanced rockets.